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HomeMy WebLinkAbout1986-05-20; City Council; 8634; Buena Vista Lagoon WatershedCIT OF CARLSBAD — AGEND BILL iv 8 j O OO ARtf XL»3Lf * MTG 05/20/86 DEPT. ENG TITLE: BUENA VISTA LAGOON WATERSHED SEDIMENT CONTROL PLAN nFPT H05>^^ CITY ATTY\jQS- CITY MGR._JA» RECOMMENDED ACTION: adopting the Buena Vista Lagoon the implementation of recommended By motion, adopt Resolution No. Watershed Sediment Control Plan and actions contained therein by approving, in concept, the report prepared by June Applegate and Associates. ITEM EXPLANATION Buena Vista Lagoon is an ecological reserve under the jurisdiction of the California Department of Fish and Game. The only freshwater lagoon located in Southern California, it is rapidly filling with sediment from the 19 square mile watershed draining into the lagoon. Carlsbad, Oceanside and Vista, the three (3) jurisdictions located within the boundaries of the Buena Vista Watershed, are naturally concerned with the rapid siltation of the lagoon and measures available to protect and preserve the lagoon. As a result, in 1982 the Buena Vista Lagoon Joint Powers Committee was formed as an advisory body to consider issues vital to the lagoon. At the current rate of sedimentation entering the lagoon, life of the Buena Vista Lagoon is estimated between ten and twenty years. In 1981, the easterly portion of the lagoon was dredged at a cost of $1,000,000. A Phase One study of the lagoon by Browne and Vogt, Civil Engineers, indicated several alternatives needed to be evaluated for sediment control. Consequently, in 1985, June Applegate and Associates prepared a report of the Phase Two study of sediment control for the Buena Vista Lagoon and Watershed for the State Coastal Conservancy and the State Water Resources Control Board. The purpose of the report by June Applegate was to explore alternatives to the continued dredging of the lagoon. Hydrology models of the watershed and hydraulic models of the lagoon were developed and evaluated by Applegate. Peak flows in Buena Vista Creek being responsible for a significant amount of sediment transported to the lagoon resulted in modelling of feasible locations for detention basins in the upper reaches of the watershed. Channel enhancement modelling was performed with the purpose of determining methods to slow velocity of water in the middle reach of Buena Vista Creek. Many alternatives were reviewed by Applegate for controlling sedimentation of the lagoon. Effective solutions to the problem were analyzed and the following recommendations presented in the report: 1. Eight (8) detention basins should be constructed; six (6) in Vista, one (1) each in Carlsbad and Oceanside. The Carlsbad detention basin would be located in Calavera Hills, near the location of the new elementary school. Page 2 of Agenda Bill No. /ffe. 2. Incorporate creek enhancement measures in Buena Vista Creek to reduce sediment entering the lagoon resulting from creekbed erosion. Estab- lish a maximum channel velocity criteria of six (6) feet per second (f.p.s.) for the 100-year flood event to allow riparian growth in the creek. 3. Construct two (2) sedimentation basins in Carlsbad, one (1) at the South Coast Asphalt Plant and one (1) in the Buena Vista Creek, just east of Jefferson Street, immediately north of the proposed North County Plaza Shopping Center. 4. Continue to provide erosion control education to developers, engineers and farmers. The report concluded that by implementing the recommendations, a reduction of sediment accumulation in the lagoon of approximately 50% could be achieved due to the reduction of flood peaks in the creek from runoff within the watershed and from enhancing the creek. The detention basin recommended for construction is intended to impound runoff from the upper reaches of the watershed and release the flow at a slower, controlled rate such that the released water reaches the Buena Vista Creek after the major flows in the creek have subsided, thereby lowering the peak flow. Also proposed are two (2) sedimentation basins to be constructed in Buena Vista Creek enabling the sediment in the water to settle to the bottom of the basin, thus reducing the amount of sediment carried into the lagoon. It is feasible that the developer of the South Coast Asphalt property would be expected to construct the sedimentation basin at their location. Creek enhancement measures include widening the creek, constructing drop structures to slow velocities and the reconfiguration and revegetation of the creek channel, as necessary. A low flow channel should be designed to accommodate the 2-year flows. The State Coastal Conservancy is willing to pursue funding for land acqui- sition and construction recommendations if the three (3) cities adopt resolutions that would implement the recommendations in the Applegate report. No money would be available for the operation and maintenance costs of the creek, detention or sedimentation basins. The reach of Buena Vista Creek in Carlsbad affected by the creek enhance- ment recommendations extends from approximately Jefferson Street easterly to the South Coast Asphalt Plant. At the present time, a portion of this channel is the responsibility of the May Company to maintain while the City of Carlsbad maintains a portion of the concrete lined channel near El Camino Real. The remainder of the channel is upon private property. Page 3 of Agenda Bill No. cT6>3 Staff has several comments regarding the creek enhancement recommendations. The criteria for a maximum 6 f.p.s. velocity in the Buena Vista Cireek would allow vegetation to become established in the creek as a result of the slower velocities. Therefore, it is possible that the Department of Fish and Game would declare the creek a vegetative riparian habitat and not allow maintenance of the channel. At this time, funding is unavailable to pay for the design and construction of the creek necessary to achieve the 6 f.p.s. velocity criteria. If the cities were to adopt a resolution requesting financial assistance for the construction of the recommended actions in the report, the State Coastal Conservancy would submit an application and pursue funding. Also being investigated by the Conservancy is the possibility of acquiring funds for the acquisition of property necessary to implement the plan. FISCAL IMPACT Implementation of the plan and ultimate construction of the improvements would result in the City of Carlsbad maintaining the two (2) sedimentation basins and the one (1) detention basin in Calavera Hills. Yearly maintenance costs to be incurred by the City for the sedimentation basins, detention basin and Buena Vista Creek have been estimated as $30,000, $15,000 and $24,000, respectively. It is assumed that only one-third of the channel would be dredged yearly, in order to preserve as much riparian area as possible. EXHIBITS 1. Sedimentation Basin Locations. 2. Detention Basin Location. 3. Buena Vista Lagoon Watershed Sediment Control Plan Report 4. Resolution No. /jfY^ adopting the Buena Vista Lagoon Watershed Sediment Control Plan. HIGHWAY BUENA VISTA 12 CREEK SOUTH COAST ASPHALT PROPERTY LEGEND SEDIMENTATION BASIN NOT TO SCAL€ LOCATIONS OF SEDIMENTATION BASINS EXHIBIT I FUTURE CALAVERA .HILLS ELMENTARY SCHOOL o<cr FUTURE ELM AVE. LEGEND DETENTION BASIN NOT TO SCALE LOCATION OF DETENTION BASIN EXHIBIT 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 RESOLUTION NO. 8569 A RESOLUTION OF THE CITY COUNCIL OF THE CITY OF CARLSBAD, CALIFORNIA, ADOPTING THE BUENA VISTA LAGOON WATERSHED SEDIMENT CONTROL PLAN. The City Council of the City of Carlsbad, resolves as follows: 1. The Buena Vista Lagoon Watershed Sediment Control Plan prepared by June Applegate and Associates September 3, 1985 indicated as Exhibit 3, attached hereto and incorporated herein, is adopted as the instrument to guide the control of sedimentation entering the Buena Vista Lagoon. 2. The Mayor is authorized to request funding from the State Coastal Conservancy to provide funding for construction of the recommendations contained in the report. 3. The Mayor is authorized to urge the members of the Buena Vista Joint Powers Committee to approve an equitable arrangement of costs for an operation and maintenance program prior to implementing recommendations of the report. PASSED, APPROVED AND ADOPTED at a regular meeting of the Carlsbad City Council held on the 20th day of May , 1986 by the following vote, to wit: AYES: Council Members Casler, Lewis, Kulchin and Pettine NOES: None ABSENT: Council Member Chick /l*.**^- <J ATTEST:MARY H.^CASLER, Mayor ALETHA L. RAUTENKRANZ, City Clerk (SEAL) BUENA VISTA LAGOON WATERSHED SEDIMENT CONTROL PLAN FINAL. / CALIFORNIA STATE COASTAL CONSERVANCY Prepared by June Applegate & Associates Philip Williams & Associates -7 JUNE APPLEGATE & ASSOC. CIVIL ENGINEERS BUENA VISTA LAGOON AND WATERSHED SEDIMENT CONTROL STUDY FOR THE CALIFORNIA COASTAL CONSERVANCY Coastal Conservancy 205 (J) grant Buena Vista Lagoon Sediment Management Phase II 83-058-81-48-C BY JUNE APPLEGATE, P.E, September 3, 1985 PBDJECT TEAM COASTAL OONSEBVANCT Peter Grenell, Executive Officer Alyse Jacobson, Enhancement Program Manager Laurie Marcus/ Project Manager JUNE APPLEGATE & ASSOCIATES June Applegate, Principal PPTT.TP WTT.T.TAMS & ASSOCIATES Philip wniiaras. Principal Jane Kerlinger, Hydrologist TABLE OF SUMMARY I. INTRODUCTION II. THE WATERSHED AND LAGOON SYSTEM Historic Changes Buena Vista Creek The Lagoon Sunnary III. SEDIMENT SOURCE CHARACTERIZATION IV. ANALYSIS The Watershed Models The Lagoon Model V. CONCLUSIONS VI. RECOMMENDATIONS VII. BIBLIOGRAPHY APPENDIX ONE APPENDIX TWO APPENDIX THREE APPENDIX FOUR Cost Effectiveness Calculations Upper Middle Reach Erosion Calculations Lagoon fl«»H •tjnorH-.a't- "*rir> Calculations LIST OF FIGURES AND TABLES PAGE 12 12 13 14 16 17 19 23 23 28 37 39 43 FIGliWs 1 FIGURE 2 FIGURE 3 FIGURE 4 FIGURE 5 FIGURE 6 FIGURE 7 TABLE 1 TABLE 2 TABLE 3 TABLE 4 TABLE 5 TABLE 6 TABLE 7 TABLE 8 TABLE 9 Regional Location Watershed Subareas Natural Conditions Detention Basin Locations Proposed Channel Cross Section Hydrographs Schematic Diagram of Buena Vista Lagoon Erosion Control Alternatives Sediment Size Distribution for Different Hydrographs Sediment Source Sumnary Sunmary of HEC-1 peak flows and time to peak for future conditions at the lagoon Projected Sediment Reductions Sediment Rating Curve Relationships Used in Analysis Buena Vista Lagoon Computer Model Runs Sumnary of Inflow Hydrographs Summary of Lagoon and Watershed Simulations 2 3 5 9 10 26 38 6 18 22 25 25 29 31 35 36 SUMMARY IOTPDDUCTION Due to the rapid sedimentation of Buena Vista Lagoon, the California State Coastal Conservancy funded this study of sediment control for the lagoon and its watershed. A Phase One study was performed by Brown & Vogt of Vista. Leedshill-Herkenhoff of San Francisco subconsulted and provided the hydrology models of the watershed. The findings of the Phase One Study indicated that several alternatives needed to be evaluated for sediment control. The California State Water Resources Control Board and the Coastal Conservancy then funded a Phase Two Study. June Applegate & Associates of Carlsbad was hired to perform the watershed modelling, project coordination, and alternatives evaluation. Philip Williams & Associates of San Francisco prepared the hydraulic analyses of the lagoon. The primary goal of this study is to formulate a prioritized list of sediment management procedures for the Buena Vista watershed and lagoon based on a cost-benefit comparison. Buena Vista Lagoon is the only freshwater lagoon in southern California. It is situated between Carlsbad and Oceanside. Its 19 square mile watershed includes the cities of Vista, Oceanside and Carlsbad (see Figure 1). The water level is maintained by a fixed weir at the mouth of the lagoon. The fills of the railroad, Hill Street, Interstate 5 and Jefferson Street cross the lagoon. Since the position and size of the mouth of the lagoon was made permanent and the flow restricted, there has been an increase in the sedimentation rate of the lagoon. Increased urbanization in the watershed further accelerated the sedimentation and necessitated the dredging of a portion of the lagoon. If projected future sediment rates materialize, the lifetime of Buena Vista Lagoon could be less than 10 years. THE LAGOON SYSTEM Buena Vista Lagoon was created by the rapid rise in sea level after the last ice age. In its natural state, it appears that the amount of sediment deposited in the lagoon was balanced by the rate of sea level rise. Two factors have upset this balance. One is the limitation of the lagoon's ability to flush sediment to the ocean by the placement of a weir at the mouth and road fills that cross and restrict the lagoon. The other is the increase in sediment delivery to the lagoon from the watershed. The cause for this increase is two-fold. Urbanization has increased flows to the lagoon. Encroachment upon the floodplains that once accepted sediment from the creek during large storm flows has eliminated most of this buffer and sediment now flows directly into the lagoon. Before the human-caused disturbances to the watershed, creek, marsh, and lagoon much of the sediment from the watershed was deposited before it reached the lagoon. Sediment was deposited on the broad floodplain in the middle reach. Then in the lower reach, a large marsh area spread and filtered the storm water, trapping more sediment before it entered the lagoon. , I 8o CO< LJ IT' <ca JCNJ CO 2? Q 52 3 5> ^r < 17 <fltr UJu5i fi ~! I ^ x.,1. a uj •-> — a •<fl*\- Ul Z <"" V. I x < < o «-Vs* w S u 2 Q•\\>..< K E CC C Q^ g 2 2 51^ I 3 S S CMIO ;NCO ?> fl I The only remnant of this protection system is the area west of South Coast Asphalt and east of the Haymar Street cul-de-sac. This reach of the creek is currently absorbing very large quantities of sediment and is the last natural buffer to prevent sate of the sediment from entering the lagoon. The best functioning portion of this reach is the thick riparian area just east of the Haymar Street cul-de-sac. The other sediment buffer was the marsh above the lagoon. This marsh has been completely filled. The cattails in the channel between the shopping center and Highway 78 and on both sides of Monroe Street at Marron Road are all that are left of this marsh. Before the filling of the marsh, the water slowed and found its way through the tales to the lagoon dropping most of the finer sediment that was left after the riparian area upstream. After changes in the watershed, particularly urbanization in the last two decades, increased peak flow have dramatically transformed the natural hydrological system. The increased flows not only discharge more sediment from the watershed into the creek, but, more importantly, have caused the creek bed to start to erode (degrade) in portions of creek. The increase in stream flow energy has caused this stream downcutting. The process will continue until a new and lower equilibrium can be reached (see Figure 3). All of these factors contributed to the rapid filling of the lagoon from 1978 to 1981, the years that marked the end of a 13-year dry period in this region. In 1981 the eastern portion of the lagoon was dredged. The purpose of this study is to explore alternatives to continued dredging of the lagoon. THE STUDY Hydrology models of the watershed and hydraulic models of the lagoon were developed and evaluated. Maximizing the flushing action of the lagoon resulted in a small reduction in the sediment accumulation rate. Greater reduction in the sediment delivery rate can be produced by reducing the peaks of the storm flows into the lagoon. Reductions in the peak flows were modeled by optimizing feasible detention basins in the upper reaches and by modeling channel enhancement that would slow velocities in the middle reach. Regional sediment transport curves were integrated with the inflow hydrographs in the hydraulic model of the lagoon. Because the quantity of sediment transported is an exponential function of the quantity of water, the peak flows are responsible for a large portion of the sediment transported to the lagoon. Reducing these peak flows resulted in much larger reductions in sediment flows into the lagoon. The study reviewed nine alternative for controlling sedimentation of the lagoon (see Table 1). These include; LI) modify the lagoon flushing action by increasing the weir size, the size of the Hill Street Bridge and clearing the opening under 1-5; L2) dredging a flow-through channel through the lagoon from Jefferson to the weir; Wl) construction of stormwater detention basins in the upper watershed; WS) enhancement of the main channel to lower main channel Philip Williams K .*!»ocia««s Consultant'., in H'7'.!f>"".)V Figure 3 Natural Conditions ROSS SETT ION 'A-A' 3 U E N A VISTA C R ;-. K K Sediment TM exceeds ^ Sediment OUT i Y....... Nil lural .ChanncL l-'lood plain S 'Si.'::Y.Y-:X'v.: :::::: "x.-^ :::: :::::;Y::::::-. ::: '.".'. :.".".: • «* ~ _!^^ s li c T i o r: l.'f.1.rat! inn Stream OUT : cxcctds 3 U s. M A VIS T A C K I! K Y. Sediment TN Potentia' Erodiblc Wedge "'.' * i T " 1 ..... ''.''.'.•.- . /\ L i I1. V 1 II r •:•' • - ... y1 '•''' U111ma t c New Floodnlain TABLE CNE EROSION (JUPfi'rt3L ALTERNATIVES ALTERNATIVE ESTIMATED ESTIMATED PERCENT ANNUAL COST- INITIAL ANNUAL REDUC- COST BENEFIT COST O&M COST TION REDUC'N 1 RATIO 2 PHASE II LAGOON MODIFICATIONS: LI WEHUHILL ST+I-5 $570,000 $15,000 12% $168,000 3.0 L2 CHANNEL $4,030,000 $2,010,000 14% $197,000 .1 PHASE I WATERSHED MODIFICATIONS: Wl DET BASINS Const. $92,000 $139,000 20% $281,000 2.0 Land Acquisitions $750,000 WATERSHED MODIFICATION WITH SEDIMENT SOURCE CONTROL: WS CREEK ENHANC $800,000 $160,000 32% $440,000 2.8 SEDIMENT SOURCE CONTROL: 51 E.C.R. SIDE ARROYO $30,000 $6,000 1% $7,000 1.2 52 GRADED AREAS $6,000 $6,000 3% $37,000 6.3 53 AGRICULTURAL $6,000 $6,000 2% $33,000 5.6 54 S.COAST SED BASIN $12,000 $12,000 1% $18,000 1.6 55 JEFF. SED BASIN $5,400 $5,400 1% $12,000 2.2 NO MODIFICATION: $1,410,000 3 BASIS OF COMPARISON 1.0 1. Annual cost reduction is equal to the percent reduction tines $1,410,000 (the estimated annual cost of dredging with no modifications to the lagoon or watershed). Annual cost reduction is the annual benefit. 2. Estimated total annual cost divide by the annual benefit. 3. This figure is the annual cost of removing 191,500 cubic yards of estimated sediment delivered into the lagoon at $7.35 a cubic yard for dredging. Further details on the derivation of these figures is included in Appendix Two. velocities and help repair erosion damage; SI) side channel arroyo repair; S2) erosion control education for protection of graded lands; S3) erosion control education for protection of agricultural lands; S4) a sediment basin at South Coast Asphalt; and S5) a sediment basin at Jefferson Street. The nine alternatives were compared to no modifications and the consequential dredging of the lagoon at the anticipated rate of sediment accumulation in the future. EFFECTIVE SOLUTIONS Eight detention basins (Wl) in the upper reaches will produce an anticipated 20 percent reduction in annual sediment accumulation. A detention basin consists of a snail check structure (i.e.: a 5' high dam or a road crossing) and a restricted outlet like an 18" pipe. This allows the water to pond during storms and to be released at a greatly reduced rate. There are eight detention basins proposed in this alternative (see Figure 4 for detention basins in Vista). Creek enhancement (WS) proposes peak flow velocities to be less than six feet per second, allowing riparian growth in the creek. This can be accomplished with a 15 to 30 feet increase in creek width and drop structures which reduce the grade of the channel and thereby reduce the velocity of the water (see Figure 5). Creek enhancement was modelled with the detention basins in place. The combination of creek enhancement and the detention basins resulted in a 45 percent reduction in annual sediment accumulation because of the resulting reduction in storm flows. Since the detention model alone yielded a 20 percent reduction, it is assumed that the reduction in storm flows due to the creek enhancement alone can reduce sediment delivery by a minimum of 25 percent. Reducing the erosion of the main channel (which is the major sediment source in the watershed) by this enhancement will further reduce the sediment accumulation by an additional 7 percent, for a estimated total of 32 percent reduction. Additionally, the lower middle reach floodplain should be preserved. The combination of a movable and 80' wide weir, a 100* bridge opening at Hill Street and dredging under 1-5 (LI) produced an estimated annual sediment accumulation reduction of 13 percent. However, this alternative has major drawbacks that can not be measured by a dollars and cents comparison. lowering the weir will create other problems that are not related to sediment accumulation. Among those concerns are water quality, and salt water intrusion. This alternative increases the probability of lower water surface levels in the lagoon. Shallow water allows sunlight to penetrate the water, warming it and increasing algae growth. Lowering the weir also increases the probability of ocean waves overtopping the weir, introducing salt water to the lagoon. Another anticipated problem with this solution is that it is likely that the channel dredging under 1-5 will have to occur frequently. Spending a relatively small annual sum of money on education for sediment control on grading sites (S2) will produce an estimated two thirds reduction of this source. Reducing sediment sources will change the sediment rating curves, thereby resulting in a reduction of sediment accumulation in the lagoon. There is no data on the relationship between sediment source reduction and the reduction of sediment accumulation in Buena Vista lagoon. This study assumes n that for every cubic yard of erosion controlled, there is one half of a cubic yard of reduction of sediment accumulation in the lagoon. The anticipated two thirds reduction in the grading site sediment source multiplied by one half indicates that this improvement would mean a three percent reduction in annual sediment accumulation rate in the lagoon. Table One indicates that this has the highest rate of return on dollar invested. Similar results are anticipated for agricultural sites (S3). The budget for this program should reduce as the land for agriculture diminishes. Currently a reduction of 2 percent of the total sediment accumulation in the lagoon per year is anticipated. It is anticipated that 5,000 cubic yards of sediment per year could effectively be removed fron a sediment basin located at the South Coast Asphalt property (S4). It is estimated that this would result in an annual reduction of 2,500 cubic yards of sediment per year, using the source and accumulation relationship estimate described above. This would mean a one percent reduction in the annual sediment accumulation rate in the lagoon. Removing sediment at this site would reduce the sediment accumulation rate of the lagoon. Another 1,600 cubic yards of sediment per year could effectively be removed at the mouth of the creek, just east of Jefferson Street (S5). Since there is no dampening effect expected at this close proximity to the lagoon, it is assumed that this is a direct reduction in the sediment accumulation in the lagoon. This could result in a reduction in annual sediment accumulation in the lagoon of one percent. It is also effective to repair side channel arroyos (SI) in the manner described for the main channel. The example given is the channel which is on the west side of El Carnino Real (E.C.R.) from Hosp to Chestnut. Repairing this arroyo also yields a one percent reduction and is cost-effective. The alternative that proved to be far too costly was to maintain a deep channel in the lagoon from Jefferson Street to the weir (L2). It is anticipated that large amounts of material would have to be dredged from this channel, because the channel would act as a sediment trap for the lagoon. The high sediment rate of the lagoon means that the channel would fill in at a rapid rate. It is estimated to cost over two million dollars per year to maintain such a channel because of the large volumes of material to be removed. Refining the estimates with data from a watershed, creek, and lagoon monitoring program, and with more precise cost estimates may or may not improve the cost-effectiveness ratio for this alternative. CONCLUSICNS In the order of effectiveness the following solutions are recommended: creek enhancement (including floodplain preservation in the lower reach), detention basins, sediment control education, sediment basins, and side channel arroyo repair. TO COLLEGE LEGEND PHASE 1 DETENTION BASIN FOR STORM ATTENUATION (4 LOC'NS) PHASE 2 DETENTION BASIN FOR STORM ATTENUATION (2 LOC'NS) CHANNEL ENHANCEMENT DOWNSTREAM OF BRENGLE TERRACE PARK TO COLLEGE AVENUE. MODELED IN PHASE 1, BUT NOT PHASE 2 MODELED IN PHASE 2, BUT NOT EFFECTIVE Figure 4 STORM ATTENUATION BASIN & CHANNEL ENHANCEMENT MAP WRTH I LOW FLOW r* • CHANNEL 100-YEAR, 6 FEET PER SECOND CHANNEL Figure 5 Cross Section of Proposed Channel O The study found some lagoon modifications to be cost-effective, however there are major concerns for the ecology of the lagoon if these are implemented. The cost-effective modifications included a moveable and widened weir, enlarging the Hill Street Bridge to be 100' long, and keeping the opening under 1-5 dredged. 11 i. This is a report of the Phase Two study of sediment control for Buena Vista lagoon and watershed for the California Coastal Conservancy and the State Water Resources Control Board. Findings of the Phase One Study indicated that several alternatives needed to be evaluated for sediment control. June Applegate and Associates, Civil Engineers of Carlsbad was selected by the Coastal Conservancy to perform this study. Philip Williams & Associates of San Francisco was hired to subconsult and provide the hydraulic analyses of the lagoon. The watershed modelling, project co-ordination, and alternative evaluation were performed by June Applegate and Associates and are summarized in this report. The Phase Two study relied upon the findings of the Phase One study and has greatly expanded and revised their recommendaitons. This degree of in-depth research into the Phase One recommendations has resulted in the divergent findings of the Phase Two report. It is the primary goal of this study to report a prioritized list of sediment management procedures for the Buena Vista watershed and lagoon based on a cost-benefit comparison. The Buena Vista watershed is an ungauged watershed. All of the predictions in this study are based on synthetic hydrographs, synthetic mathematical models of the lagoon, and sediment rating curves extracted from other similar small coastal watersheds. Sediment monitoring using detailed bathometric surveys of the lagoon after major storm events, stream flow sediment monitoring, and rain gauge information during storm events will yield more accurate predictions of sediment accumulation, sediment flows and sources. Due to limited budget, almost no field data, and a range of uncertainties, the sediment predictions in this report have a wide range of error changing the true cost-effectiveness analysis dramatically. Consistent techniques were applied uniformly for each category of modification. The estimate of reduction of annual sediment accumulation in the lagoon provided a relative ranking of the effectiveness of each alternative. The categories of alternatives in which the relative cost-effectiveness ranking is valid is: maximization of the flushing ability of the lagoon with lagoon modifications (LI and L2), minimization of the sediment delivery to the lagoon with watershed modifications (Wl and WS), and minimization of erosion in the watershed (WS, and SI through S5). II. THE WATERSHED AND LftGOON SYSTEM Buena Vista Lagoon was formed during the rapid rise in sea level at the end of the last ice age, ten to fifteen thousand years age. During the last five to seven thousand years sea level rise has been more gradual, apparently about 1/2 foot per century. The slow rate of rise was sufficient to compensate for natural sedimentation rates in the lagoon, allowing it to survive for thousands of years. Wave action caused the littoral transport of sand along the coast, sealing off the entrance to the lagoon with a barrier beach. Except possibly during an 12 early stage in its evolution, the tidal prism of the lagoon was insufficient to scour a channel across the beach. Tidal action occurred for a short period in the lagoon, and only when winter floods opened up a channel. Seme sediment carried into the lagoon during floods discharged directly to the ocean through the opening. Some sediment deposited in the lagoon would later be resuspended by wave and tidal action, and would be flushed out of the lagoon during ebb tide. Because of the barrier beach across its mouth, the character of the lagoon varied greatly from season to season, and from year to year. In normal winters, storm runoff filled the lagoon with freshwater until at some point it overtopped the barrier beach. In the spring, after the beach was reestablished, the lagoon level dropped, fed only by springs and the base flow of Buena Vista Creek. In summer and fall the inflow decreased further until evaporation exceeded inflow. Water salinities increased and large areas of mud flats or salt pans would be exposed. During drought periods it is likely that the lagoon almost completely dried out, and was fed mainly by salt water seeping through the beach. In its natural state the watershed of Buena Vista Lagoon was covered with native plant vegetation. The plants generally provided a higher resistance to erosion than many of the introduced, new species. They not only protected against direct erosion from rain drops but allowed the infiltration of storm runoff into the soil, reducing peak runoff rates downstream and providing greater base flows in the creek later in the year. These flows supported dense riparian vegetation along the creek banks and on its floodplain. In its lower section the creek would have discharged into a tule freshwater marsh at the upper end of the lagoon. The floodplain and marsh vegetation acted as sediment traps during high flood flows, building up the alluvial floodplain and reducing the amount of sediment discharging to the lagoon. Historic Changes In the two centuries since Europeans settled the area, man has made major modifications to the natural hydrologic system. The hyraulics of the lagoon have been completely altered since 1940, when outlet culverts were installed at the mouth to regulate maximum water levels. This eliminated tidal action until the big flood of 1969, when the culverts were washed out, re-creating the natural entrance for a short while until the existing fixed outlet weir was installed in 1970. Since that time tidal flows have been excluded and the lagoon has been converted to a freshwater lake. The construction, first of the Hill Street road embankment and then of the 1-5 freeway embankment across the lagoon, has also affected the hydraulics by segregating the lagoon into three distinct basins. These changes have greatly limited the lagoon's ability to flush sediment out to the ocean. Urbanized runoff pollutants discharged directly into the lagoon degraded water quality. In addition, until the 1960s, sewage was discharged directly into the 13 lagoon. The watershed has also changed dramatically. Extensive grazing and, later, farming, removed soil cover, increasing erosion and sedimentation in the lagoon. Urbanization, which has been particularly rapid since the 1970s, increases flood peaks, causing gullying and creating arroyos. This process greatly accelerates erosion and downstream sedimentation. In addition, the floodplain of Buena Vista Creek has been filled in several locations, reducing the filtering effect of the riparian vegetation. The floodplain in the middle section of Buena Vista Creek has evolved as a result of sediment from the surrounding hillslopes being transported as bedload in the creek. During flood flows, these flows would overtop the creek banks and deposit sediment on the adjacent flocdplain. The floodplain and the creek built up over time. Buena Vista Creek was aggrading in this manner throughout the middle and lower reaches in ancient times (see Figure 3). Only a fraction of the total sediment eroded in the watershed actually reached the lagoon. After changes in the watershed, particularly in the last two decades, the increased peak flows have dramatically transformed the natural hydrological system. The increased flows not only discharge more sediment from the watershed into the creek, but, more importantly have caused the creek bed to start to erode (degrade) in portions of the creek. The increases in stream flew energy have caused the stream to downcut. This process will continue until a new and lower equilibrium can be reached. At the same time one of the largest sediment buffers in the watershed has been filled in. The marsh above the lagoon was the spreading area for storm flows. Here the water slowed and found its way through the tules to the lagoon, dropping much of its sediment load. All that is left of that marsh area is a small area where cattails grow in the channel between the shopping center and Highway 78. Buena Vista Creek Water flawing into Buena Vista Lagoon comes primarily through Buena Vista Creek. The 19 square mile watershed that drains into the creek and lagoon covers areas of Oceanside, Carlsbad and Vista. Vista is in the upper reach of the watershed, covers over half of the watershed and has the major impact on the flow characteristics of the system and yet it does not border the lagoon. The creek can be identified by reach. The upper reach is the reach above Highway 78. The middle reach is from Highway 78 to El Camino Real. The lower reach is from El Camino Real to Jefferson Street (see Figure 2). The upper reach is in its natural channel to Brengle Terrace Park. Above Wildwood Park the creek has been stabilized by check structures. They were built in the 1930's and it appears that only one of them has failed. This channel now appears to have the potential to overflow and flood the surrounding area, probably because of increases in runoff from the urbanization of the watershed. The creek has been channeled into concrete structures through the downtown area of Vista to Melrose Drive. These structures were also designed 14 and built before the major urbanization of the area. The middle reach was a wide aggrading floodplain. It has been divided into two by the falls just downstream of College Avenue. The two sections functioned very similiarly before the impact of urbanization occurred. Now the falls mark the location of the hydraulic division of this reach. In the upper middle reach, from Highway 78 to College Avenue, the creek has dramatically changed from trapping sediment in its broad floodplain (aggrading) to a down-cutting creek. This difference is marked by the large gully which has cut into the ancient floodplain deposits in the area of the old sewage treatment plant in Vista. Instead of absorbing much of the sediment it receives as it had done in the past, this reach is now sending that sediment plus the eroded material downstream. Currently the lower middle reach (College Avenue westerly to El Camino Real) is still aggrading. In fact, it is aggrading at an alarming rate of up to four feet per season. This indicates that the lower middl/? reach is absorbing much of the sediment before it gets to the lagoon. The cause of down-cutting in the upper middle reach is the increase in the scour action of the creek. Lowering the flow line of the channel at various road crossings has contributed to this degradation, but this is a long-term phenomenon. If the creek still had aggrading characteristics it would fill in these crossings with sediment as the middle reach of Lama Alta Creek has. Typically during urbanization downstream channels start to erode. This is in response to higher peak flows of water from the developing area. The existing and future hydrographs modeled in the phase one study indicate that the peak flows have the potential to double in this reach. This means that the sediment transport capacity of the creek more than doubles. If the sediment transport capacity increases to the point that it is greater than the sediment entering the reach, the creek then has enough energy to cut into the bed material. When it had too much sediment to transport it dropped its load on the wide floodplain. Now it is hungry for sediment and erodes into these ancient fluvial deposits and carries them and its original load downstream (see Figure 3). The increase in peak flow explains the cause of the erosion in the upper middle reach. It is also a warning. Channelizing the upper middle reach will cause the peaks in the lower middle reach to increase dramatically and could trigger down-cutting in the lower middle reach. Since there is very little buffer between the lower middle reach and the lagoon, the lagoon will experience an even greater sedimentation accumulation rate than we have previously seen. In the lower reach, above the lagoon there was a wide flat marsh. At the location of El Camino Real the creek spread over the marshland. The flatness of the land and the thick growth of the marsh plants greatly reduced the velocities of the water. Here the water slowed and found its way through the tules to the lagoon, dropping much of its sediment load. The marsh acted as a filter for the water entering the lagoon. 15 The marsh has been filled. Presently the cnly evidence of that marsh.area is a small area where cattails grew in the channel between the shopping center and Highway 78. This channel is now the lower reach of Buena Vista Creek. The Lagoon Because of the changes in the lagoon hydraulics and the greatly increased rates of sedimentation the lagoon is no longer in equilibrium with natrual hydrologic processes and is rapidly silting in. (tost sediment deposited into the lagoon is discharged during the peak flows of large storms, such as those in 1969, 1978 and 1980. Sediment discharge is exponentially related to the flow velocity by a power of two to three. Consequently, though peak flows may last only a few hours, they can carry tens of thousands of tons of sediment into the lagoon. It appears that sediment carried into the lagoon is predominantly silt. The typical size distribution of suspended sediment for different sotrms is shown in Table 2. The Table is based on sediment sampling on other San Diego County coastal streams (see Appendix 4) and synthetic flood hydrographs generated for the Buena Vista Creek watershed. As the floodflow approaches the lagoon, backwater reduces the velocity and carrying capacity of the flow. Mich of the bed load, consisting of coarser sands, appears to be deposited on the floodplain and in the channel between the South Coast Asphalt quarry and Jefferson Street, while most of the suspended sediment, consisting of sands, silts, and clays, are discharged into the lagoon. As the flood flows enter the lagoon, flow velocities drop to a fraction of a foot per second. These velocities are insufficient to keep the sediment in suspension, and particles start to settle out. The settling velocity of sands if very rapid, so they tend to settle out immediately. Silts settle out more gradually but rapidly enough for most of them to be deposited upstream of 1-5. Clays settle out very slowly, but because velocities through the lagoon are so low, most are deposited in the lagoon and only a fraction are discharged to the ocean. The roadfills of 1-5 and Hill Street and the outlet weir have reduced flow velocities through the lagoon, increasing sedimentation. Buena Vista Lagoon now acts as a very efficient sediment trap. Based on the very limited boring information available, it appears that, prior to 1940, the lagoon bed consisted of fine sands at an elevation of about -1.5 ft NGVD. By 1961 approximately 2.5 ft of organic rich mud had accumulated in the lagoon in the vicinity of 1-5, and by 1982 an additional 2.5 ft of organic rich silty clay had accumulated. In the last 42 years, presuming the same siltation rate over the 200 acre lagoon, this amounts to about one and a half million cubic yards or tons (for these sediments, a cubic yard weighs roughly a ton), or about 35,000 tons/year. For the 19 square mile watershed this amounts to 1840 tons/square mile/year which is comparable to an earlier estimate of 1,000 to 2,000 tons/square mile/year (Inman 1976). After initial deposition during and after a flood, sediments can be resuspended by wave action and redistributed in the lagoon. Water depths are fairly constant, deepening to two to three feet in areas of high wave action. The thick growth of tales under the 1-5 bridge prevents all except the finest sediments from circulating into the western segments of the lagoon. Consequently, sediments accumulating in the eastern segment are mainly sands, silts and muds, whereas to the west of 1-5 sediments are mainly organic ituds. In the wind-protected area between the railroad and the beach, sediments are highly organic and appear to contain sewage sludge. Water levels in the lagoon are maintained at a minimum elevation of 5.8 ft NGVD by the outlet weir crest or more commonly between about 5.8 ft and 6.5 ft NGVD by the barrier beach forming across the outlet. Water depths in the eastern segment are typically 1.5 to 2 ft, and in the western segments 2 to 2.5 ft. Summary Before the human-caused disturbances to the watershed, creek, marsh, and lagoon, much of the sediment from the watershed was deposited before it reached the lagoon. Sediment was deposited on the broad floodplain in the middle reach. Then, in the lower reach, a large marsh area spread and filtered the storm water, trapping more sediment before it entered the lagoon. The only remnant of this protection system is the area west of South Coast Asphalt and east of the Haymar Street cul-de-sac. This reach of the creek is currently absorbing very large quantities of sediment and is the last natural buffer to prevent some of the sediment from entering the lagoon. The best functioning portion of this reach is the thick riparian area east of the Haymar Street cul-de-sac. 17 -7'-? TABLE TWO. Sediment Size Distribution for Storm 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr Different Hydrocjraphs Peak Flow (cfs) 1371 1979 2589 3659 5832 7200 Sediment Load at Peak Q (tons/day) 5 1.2x10 5 3.2x10 5 6.8x10 6 1.7x10 6 6.7x10 7 1.6x10 <.004 25 23 20 18 15 13 % by .004-. 6 5 4 3 2 2 Particle Size at 016 .016-. 064 23 26 28 31 32 33 Peak Flow .064-. 25 . 33 38 40 44 47 48 25-1. OOnm 11 9 8 6 4 4 18 III. SEDIMENT SOURCE CHARACTERIZATION Channel Erosion It appears that there is an estimated 'total of 173,000 cubic yards of erosion in the main channel of the creek in the middle reach between Melrose Avenue and College Boulevard. This was calculated by using a real topography flown in the spring of 1985. Based on newspaper clippings, most of this erosion has occurred since 1978. 1978 marked the end of a a 13 year dry period in this area. Land use had changed frcm primarily rural and agricultural to urbanization in that time. The combination of the increase of rainfall and urbanization created a dramatic increase in the amount of runoff experienced. What had been an aggrading section of creek where sediment was deposited upon a broad floodplain became a degrading section. The down-cutting is evident in a gully that is in excess of 15 feet deep in some places in this reach. Currently Graded Areas Presently there is approximately 600 acres of graded area in the watershed. Vista, Carlsbad and Oceanside adopted sediment control ordinances as part of their grading ordinances. Sediment control plans must be filed in September and approved by October. The sediment control is supposed to be in place in November, and remain effective until March. Generally, on-site sediment control is still not effective. There appears to be a lack of understanding as to what it takes to keep sediment frcm leaving the site. Much of the work done in the creek in Vista during the winter of 1984 had no protection what so ever. Much of the sediment control as installed in the other cities was not effective. Sometimes sediment is directed into the storm drain system in the form of muddy water. Another common practice is to use sand bags that were not sealed, but just folded over. Some of these bags do not make it through the winter season before spilling their contents and adding to the sediment leaving frcm the site. Education of the contractors, inspectors and design professionals who submit and those who review the plans is desperately needed. The ordinances are good, but they are not yet being fully implemented. The local citizens are concerned and would be excellent watchdogs, if educated. The Buena Vista Lagoon Foundation has a hotline that is available for the reporting of sediment problems. The local citizens are dedicated to the saving of the lagoon, but do not yet recognize some of these problems. Sediment yields from these graded areas can vary frcm one cubic yard per acre per year to 30 cubic yards per acre per year. This is why good sediment control is so important. Sediment yields are still high from this source, so using an estimate of 25 cubic yards per acre per year over the 600 acres currently being graded gives a sediment estimate of 15,000 cubic yards of sediment per year. 19 Agriculture The following is a summary of interviews with Howard Mieller, who is currently a consultant and has worked for the Soil Conservation Service. While working for the Soil Conservation Service, he compiled the "Important Farmland Map" for the Buena Vista Watershed. The amount of farmland in the Buena Vista Watershed has been a constant of approximately 5 percent (approximately 600 acres) avocado groves and 5 percent truck crops. However, with the various economic and social pressures on the farmers these lands will decrease and become an insignificant sediment source within 5 to 10 years and will become completely urbanized within 20 years. Avocado groves of three years old or younger (of which there is 10 to 15 percent of all groves) produce 15 to 20 tons of sediment per acre per year. The remaining 85 to 90 percent of the groves have sufficient canopy and leaf litter to reduce the sediment yield to one to two tons of sediment per acre per year. Well managed truck crops also have a sediment yield of one to two tons of sediment per acre per year. However, poorly managed truck crops produce as much as 20 to 30 tons of sediment per acre per year. Historically, the poorly managed truck crop land constitutes about 1 percent of the watershed, and the well managed truck crop land constitutes the remaining 4 percent. Presently, the well managed farms only constitute one and a half percent of the 5 percent total. Historically, the agricultural land produced an estimated 5800 tons of sediment per year in the Buena Vista watershed. Presently they are producing an estimated 13,000 tons of sediment per year and within the next five to 10 years this source of sediment will be negligible. By comparison, the natural areas, which had periodically burned, yielded one to two tons of sediment per acre per year, average. Future Land Uses The following estimates for future land uses were made by each of the three cities in the watershed (in acres): Carlsbad Vista Oceanside Developed area: 1700 5760 3000 Underdeveloped: 300 1500 500 Vacant land: Area with approved plans: 250 2750 134 Area without approved plans: 250 765 2000 Area currently being graded: 190 325 85 20 The underdeveloped land is part of the total of the developed area, however it is likely to be redeveloped to a higher use. Natural Erodible Areas These numbers indicate that there is approximately 6,000 acres of natural erodible area. Using an average of two tons of sediment per acre per year indicates an estimate of 12,000 tons of sediment from this source per year. South Coast Asphalt South Coast Asphalt has a rock quarry just west of College Avenue. The excavations extend approximately 50 feet below the streambed. They have maintained the falls by keeping the banks and adding levees. In the flatter portion of the creek, there is runoff from the plant site and there is potential for erosion. Since the quarry area is granitic rock, the exposed faces do not have much erosion potential. The erosion potential on the site is in the reach with loose dirt banks. The plant has supplied detailed areal photography of the land for this study. Existing channel velocities in all but the smallest flows are in excess of 6 feet per second. These are erosive velocities, however since the creek has been carrying so much sediment in recent years. This section of the creek continues to aggrade. Higher peak flows could reverse this and this reach could start to degrade. In the first large flow in 1979 a portion of a southerly stockpile was eroded. To prevent this loss of material, South Coast Asphalt now protects the tockpiles with riprap at the toe. However this site has the potential for erosion because of above mentioned velocities, little bank protection, and fill-dirt stream banks that are vulnerable to erosion. The rock quarry is estimated to close in 1990. Plans for the development of the property are now being prepared and reviewed. The owners plan to develop the creek portion between 1990 and 1995. Ultimately the owners would like to have a natural looking channel, use the creek as a visual amenity to the project and maintain the falls. During the time between now and the ultimate development, the potential for erosion in this reach can be reduced by widening and adding revetment to the channel. Adding 30 feet to the width of the dirt channel will reduce the velocities by approximately 20 percent. This will be beneficial in three ways. Lowering the velocities will decrease the sediment transport capabilities of the stream and thereby decrease the potential for creek erosion. It will also make a small contribution to reducing the peak flows at the lagoon. Revetment of the creek banks will reduce the potential for erosion. 21 TABLE THREE SEDIMENT SOURCE SUMMARY Approximate annual average erosion rates from each source in cubic yards per year: Main channel erosion: 25,000 Graded areas: 15,000 Agricultural: 13,000 (currently) Natural erodible areas: 12,000 Side channel erosion: 11,000 22 IV. ANALYSIS The watershed analysis was two-fold. Hydrologic watershed modifications aimed at reducing the peak flows fran storm runoff were analyzed. Sediment source reduction was evaluated. Reducing peak flows will reduce the sediment carrying capacity of the stream. There is an exponential relationship between the stream's sediment carrying capacity and the water flow. Consequently a reduction in the storm peaks results in a much higher reduction in the sediment delivery to the lagoon. Reducing peak flows also has the side benefit of reducing flooding. Specific areas identified as areas of concern that will be benefited are in the low lying areas around the lagoon and areas in Vista. Sediment source reduction will reduce the sediment rating curves of a watershed. The sediment rating curves are a plot of the relationship of the water flow to sediment transport. The estimates of sediment accumulation reduction for sediment control could be an order of magnitude off. However, the cost for greatly improving the source control is relatively low, return on the dollar spent is very high. THE WATERSHED MCDELS Encouraged by the peak flow reductions modeled in the Phase One study, the data from those original watershed models were re-entered into the Army Corps of Engineers Hydraulic Engineering Center's H.E.C - 1 hydrology model and rerun on a mainframe computer in order to print hydrographs for the spectrum of six storms. These were the 2-year, the 5-year, the 10-year, the 25-year, the 50-year and the 100-year storms. Those six storms were run for the Phase One's "Existing Condition", "Future Condition" and "Existing Condition with Detention". Very late in the Phase Two analysis some problems in some of the assumptions for these models were noted and the watershed had to be remodeled. The Phase One input data was used as a skeleton and a new watershed model was created for the Future Condition. Since this is an ungauged watershed, the model could not be calibrated. Appendix One describes the calculations for the Future Conditions. These include a summary of the soil types, future development types, Soil Conservation Service Tag, and SCS curve number for each of the Phase One subbasins. The 2-year and the 100-year hypothetical storms were run on this model. (Note: The future condition watershed models for Future Condition, Future Condition with Detention and Future Condition with Detention and Channel Enhancement are copyrighted c 1985. Funding was not allocated for this re-modelling.) The main channel beginning below Brengle Terrace Park to College Avenue (a portion of the upper and the entire upper middle reach) is modeled as a trapezoidal channel having a bottom width of 20 feet, side slopes of 1.5 to 1 and a Manning coefficient of 0.02. This is typical of the Tri-lock. channel that is being placed at Breeze Hill. 23 Detention basin model (Wl) Because the future condition would have the major long range impact on the lagoon, the future condition was then modeled with eight detention basins. The basin locations had all been modeled in the Phase One study. Two of them were re-sized based upon more detailed topographic information. The two re-sized basins were at Brengle Terrace Park and at Monte Vista School. Detention Basin Locations The detention basins in Vista (shown in Figure 4) are located at: 1. Creek crossing Warmlands Avenue, approximately 200 feet Northerly of ELm Drive 2. Creek Crossing Warmlands Avenue, approximately 200 feet Northerly of Suemark Terrace 3. Creek Crossing Warmlands Avenue, approximately 100 feet Northerly of Calle Simoloa 4. Creek Crossing Stephanie Lane on the north side of Vale Terrace Drive 5. Brengle Terrace Park 6. Monte Vista School The other Phase One detention basins are located at: 7. The canyon north of Mira Costa College in Oceanside 8. The canyon to the east of the extension of Elm Avenue Creek Enhancement Model (WS) To model the effects of an enhanced channel, with vegetation, the detention basins were kept in place and the above mentioned channel was widened. The channel in this third model had a bottom width of 15' above Santa Fe Drive and a bottom width of 40 feet wide below Santa Fe Drive with side slopes of 2 to 1 and a Manning coefficient of 0.04. The energy slope is about one quarter of a percent. This was the most effective improvement to the model. Table 4 gives the summary of the results of these new models. The 6 feet per second maxinua allowable velocity is attainable with 0.25 to 0.35 percent slope, a Manning coefficient of 0.040, 2:1 side slopes and a 40 foot bottom in the upper middle reach reducing in width up the creek until it is about 15 feet wide. Table 4 lists the summary of the peak flows and lag times at the lagoon for each model. Figure 6 shows graphs for the 2 and 100-year storms in the future condition, future condition with detention basins, and future condition with creek enhancement for a location in the downtown area of Vista, in the middle reach, and at the lagoon. Comparison hydrographs are shown in Figure 6 for a point at the lagoon and a point at Breeze Hill. 24 A TABLE FOUR Suimary of H.E.C.- 1 peak flows and time to peak for future conditions at the lagoon. 100 year: Future condition With detention With enhancement 13,734 cfs 11,431 cfs 9,231 cfs 2.94 hours 2.95 hours 3.28 hours 2 year: Future condition With detention With enhancement 3,213 cfs 2,590 cfs 2,130 cfs 3.27 hours 3.28 hours 3.82 hours TABLE FIVE PROJECTED SEDIMENT REDUCTIONS ALT WATERSHED LAGOON ANNUAL COND COND SEDIMENT REDUCT. NO FUTURE EXISTING BASIS LI FUTURE COMB 12% FUTURE COMB4CHAN 26% L2 FUTURE CHAN (XNTRIBUTICN 14% Wl F W/DET EXISTING 20% DET+ENH EXIT 52% ** WS ENHANC EXIST 32% * TRIBUTARY REDUCTICN OF THIS ALTERNATIVE ** 45% V«ICH IS DUE TO STORM FLOW ATTENUATION PLUS 6.5% REDUCTION IN ANNUAL SEDIMENT ACCUMULATION DUE TO THE REDUCTION OF THIS SEDIMENT SOURCE 25 CFS 15,000 10,000 5.000. LAGOON FC 3 4 HOURS 100-YEAR STORM 2-YEAR STORM CFS 10,000 5,000 FC ABBREVIATION FC FO FE BREEZE HILL HOURS 100-YEAR STORM FUTURE CHANNELIZED CONDITION FUTURE DETENTION MODEL FUTURE WITH DETENTION AND ENHAflCEHEO CHANNEL MODEL 3 4 HOURS 2-YEAR STORK Figure 6 Hydrographs Sediment Source Control Main Channel (WS): The average of 25,000 cubic yards of sediment per year eroded from the main channel primarily occurred during the 5 years from 1978 to 1983. Although this source will not be completely eliminated with the channel modifications, the reach of the recommended enhancement is at least three times the length of the badly degraded section from where the 25,000 cubic yards came. Protecting the Main Channel will reduce the sediment source to the lagoon by 25,000 cubic yards per year. For the comparative analysis half of this quantity was added to the reduction in sediment delivery for the "Channel Enhancement" alternative, because it is assumed that for every cubic yard sediment source reduction there is a corresponding reduction of a half of a cubic yard of reduction in the sediment accumulation in the lagoon. Graded Areas (S2): The adopted sediment control ordinances for graded areas are good and money is being expended by the developers and the cities for the design construction and review of sediment control plans. However, there is still an estimated 15,000 cubic yards of sediment escaping from from these sites. Misplaced or broken sand and gravel bags are allowing sediment flow into the storm drains and channels. Sometimes the projects are caught with their gravel bags down by a surprise rainstorm. Education of the engineers, inspectors and contractors involved will probably reduce this source to one third of the volume it is today. Agricultural (S3): A similar education program for agriculture could be as effective as education for grading. This would reduce the sediment source by an additional 5 percent. Natural credible areas: This source has such a low concentration very little improvement can be made. Side Channel Erosion (SI): Three side channels that need reconstruction and enhancement have been identified. They are just west of Pamelo Drive, on the westerly side of El Camino Peal between Elm and Chestnut and next to Monroe, south of Marron. According to the Vista City staff and the Carlsbad City Staff the side channel arroyos at Pamelo and Monroe are in areas that are approved for development. The requirements for the developments include repairing and preventing these arroyos. The side channel arroyo on the westerly side of El Camino Real between Elm and Chestnut locations could effectively be repaired using drop structures in the same manner as the recommended main channel repair in the upper middle reach. This will reduce the total watershed sediment source by an estimated one percent. 27 THE IAGOCN MGDEL To determine the most effective means to reduce sediment accumulation in the lagoon, its sediment budget must be estimated under different conditions. A sediment budget is simply an accounting of the inflow, outflow and stroage of sediment in the lagoon for a particular time period. Sediment inflow is determined by empirical relationships between the variation of sediment discharge and flow rate during flood events. The sequence of flow rates, known as a hydrograph, are computed for particular storms using a standard computer model simulation referred to as HBC-1. When the sediment enters the lagoon, some settles out, some is discharged to the ocean, and some remains in suspension for a particular time period. The amount settling out is calculated using settling velocity relationships for each particle size. The amount remaining is suspension is determined by the difference in sediment inflow and the amount settling out in a particular time period. The amount discharged to the ocean is the sediment concentration remaining in suspension at the outlet, multiplied by the discharge volume. Most sediment is carried into the lagoon by a few large, infrequent floods. Consequently, the sediment budgets must be averaged statistically over a long period of time to estimate an average annual sediment budget. The average annual sediment budget can be calculated for different inflow and lagoon conditions to provide a comparison of the average annual sediment accumulation in the lagoon under different conditions. Because of the enormous number of calculations required to estimate average annual sediment accumulation, it is best done using a computer model that simulates the movement of both water and sediment through the lagoon system. Such a computer model was developed specifically for this study and is described in succeeding sections. It should be noted that there are a great many uncertainties in most of the calculations involved in determining a sediment budget. Consequently sediment budgets of this type should be used for comparative purposes only. Sediment Inflow The best method for determining sediment discharge to the lagoon is to develop a sediment rating curve for Buena Vista Creek. A sediment rating curve plots suspended sediment against flow discharge measured at a particular point on the stream. Unfortunately, neither suspended sediment nor discharge data exists for Buena Vista Creek. Therefore, a sediment rating curve was constructed, based on sample data from other streams. Stream gauge data from 11 gauging stations on coastal streams south of Dana Point were examined (see Appendix 4). The results showed that streams with watersheds greater than 100 square miles had different sediment rating curves than those with smaller watersheds. Consequently only data from the five smaller watersheds were used. 28 A TABLE SIX. SEDIMENT RATING CURVE RELATIONSHIPS USED IN ANALYSIS Sediment Particle Size qs (tons/day) Fal Velocity (ft/sec) -4 2.4 -5 Clay <.004 mm 9 x 10 Q 1.31 x 10 Silts Sands .004 - .016 - .064 - .25 - 1 .016 ram .064 mm .25 mm .10 mm -3 2 4 x 10 Q -5 3 1 x 10 Q -5 3 1.6 x 10 Q -3 2.1 3.7 x 10 Q 1.15 x 2.3 x 3.28 x 2.3 x -4 10 -3 10 -2 10 -1 10 The suspended sediment data was broken down into five different size fractions representing muds, fine silt, coarse silt, fine sand and coarse sand. The plots are shown in Appendix 4. There is a large amount of scatter in the data. Up to an order of magnitude of scatter is typical of sediment rating curves. However, the rating curves for each size fraction can be simplified as the straight line relationships shows in Table 6. These are used in calculating sediment inflow to the lagoon. A portion of the coarser sediment discharge, often estimated to be an additional 20%, is carried along the channel bed as "bedload". This was not included in the sediment inflow because most of the coarser material appears to be deposited in the Buena Vista Channel upstream of the lagoon, and because it represents only a small portion of the total sediment load discharged to the lagoon. The sediment inflow for a particular storm is calculated by integrating the sediment rating curve with the inflow hydrograph for successive time increments. • Lagoon Hydraulics In order to calculate the sediment accululation in the lagoon for a particular flood, a flood routing calculation must be carried out to determine the water surface elevations, volumes and flow velocities at different times, as the flood flows enter the lagoon. For Buena Vista Lagoon this is complicated, because hydraulically the lagoon acts as three distinct cells. The eastern cell includes the area from the inflow point at Jefferson to the 1-5 embankment. The embankment constricts outflow to the central cell that extends from 1-5 to Hill Street. Sediment 29 accumulation further constricts outflows under the 1-5 bridge to approximately the existing lagoon level. The central cell discharges to the western cell through a constricted culvert tinder Hill Street, or over the top of the roadway, if the water level rises high enough. The western cell discharges to the ocean over the existing sharp crested weir. The railway bridge crossing does not significantly constrict flood flows in the western cell. A sketch of the lagoon is shown in Figure 7. A flood routing is a sequential calculation that calculates outflow, water surface elevation, change in storage and average flow velocity for a given inflow and initial lagoon conditions. Unfortunately, no detailed survey of the bathymetry of Buena Vista Lagoon exists. Therefore, the water elevation/ storage relationship for each of the cells is estimated based on few point soundings and available topoghraphic survey. These are shown in Appendix 4. Outflow from each cell is determined by standard weir flow or culvert flow formulae. Average velocities are simply calculated as the outflow divided by the average cross-sectioned area for a given instant in time. Sediment Accumulation Sediment inflow to the lagoon is assumed to be uniformly vertically mixed in the flow. When it reaches the lagoon, flow velocities drop considerably and sediment particles settle out. The rate at which they settle out is determined by the particle settling velocity. For the median diameter of each of the five size fractions, this value is shown in Table Two. It can be seen that there is roughly an order of magnitude difference between the settling rates of each size fraction. Dividing the sediment inflow into five discrete size fractions can therefore introduce another source of error in the sediment budget. Sediment accumulation is» determined by the fraction of sediment that settles out while the sediment-laden flow travels from the inflow to the outflow end of each cell. It is assumed that flow travelling through the 1-5 bridge and the Hill Street culvert completely mixes the sediment, so that sediment discharged to the next cell is uniformly mixed. The model for sediment deposition described above is strictly valid only for quiescent uniform flows in stilling basins. In Buena Vista Lagoon flood flows entering the lagoon will be highly turbulent, which tends to hinder settling of sediment particles until the turbulence dies out. Unfortunately there is no feasible way to model this process, and calculating deposition based strictly on time of travel may overestimate sediment accumulation particularly for the finer sediment fractions. The velocity of the flow in the lagoon itself can generate turbulence that can keep particles suspended. There is this lower limit of sediment concentration in the lagoon for a particular average lagoon velocity. There are many alternative methods of calculating this lower limit; and as is cannon in the field of sediment hydraulics, there are significant differences in different estimates. For these calculations the relationship developed by Bagnold was used (Yalin 1971). 30 TABLE 7 . BUENA VISTA LAGOON COMPUTER MODEL RUNS RUN NO. INFLOW MYDKuCMPH LAGOON CONDITIONS SEDIMENT ACCUMULATION RESULTS 1-3 Frequency Watershed Wtir Condition* Eltv ft 1 2-yr Exi*ting 2 5-yr Existing 3 23-yr Existing 4 100-yr Existing 5 2-yr Futurt 6 5-yr Futurt 7 25-yr Futurt 8 100-yr Futurt 9 .2-yr Futurt 10 S-yr Futurt 11 25-yr Futurt 12 100-yr Futurt 13 2-yr Fucurt 1* S-yr Futurt IS 23-yr Futurt 16 100-yr Futurt 17 2-yr Futurt 18 S-yr Futurt 19 2S-yr Futurt 20 100-yr Futurt 21 2-yr Futurt 24 100-yr Futurt 29 2-yr Futurt 30 100-yr Future 5.8 3.8 5.6 5.8 5.8 5.8 5.6 5.8 1.3 1.5 1.3 1.3 3.8 5.8 5.8 3.8 3.6 3.6 3.8 3.8 1.3 1.3 3.8 5.8 Hill St. initial wattr Inflow Outltt Weir surface ton* Deposited [bnt *" Dis- charged Couswnts Width invert length eltv NCVD ft culvert ft ft ft tit* ft 15 13 15 13 15 13 15 IS IS 15 15 IS 100 100 100 too 13 IS IS 15 100 100 IS IS 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 4.0 2.0 2.0 80 80 80 80 80 80 80 80 80 80 80 80 SO SO SO SO 60 60 8V 80 80 80 SO SO 3.8 5.8 5.6 5.6 5.8 5.6 5.8 3.6 5.8 5.8 5. 8 5.8 5.8 5.8 5.8 5.8 5.8 3.6 S.« 3.8 5.8 5.8 5.8 5.6 3.8 3.8 3.8 3.6 3.8 3.6 5.6 5.8 1.8 5.6 1.6 3.8 1.8 5.8 1.8 5.6 1.8 5.8 1.6 5.8 5.6 5.8 5.8 5.8 10.915 26.580 1*2,770 672.440 20.050 51.635 235.3*0 1.1*7.620 20.163 31.920 233.3*0 1.193,070 20.013 31,163 — 1.1*3,265 20.060 51,6*0 235.963 1.1*5.875 20.060 1.1*7,830 2U.073 1.1*9,200 10,095 24.125 126.510 7*7.303 16,370 46.6JS 208, 723 1,011,465 18,510 46.985 206,593 1.012.000 19.240 46,510 — 1.012.315 . 17.543 44.943 204.323 9,4.993 15.605 963.4*0 18,273 I.021.3U3 92 91 89 88 92 90 89 88 ft 90 89 65 86 91 - - 86 67 67 87 87 78 84 90 69 8 Existing watershed 4 laff.«<>n conditionaiouclet vtir-tu' 9 Existing watershed 6 lexuoncondition*; autltt wtir-au' 11 Existing watershed 4 lagoon conditions; outltt wtir on' 12 Existing watershed t lagoonconditions; outlet wtlr*)ii>' 8 Existing lagoon 4 tut. water'shed tend.; autltt weir-W 10 Existing lagoon 6 ful.wat«r sned conaV. ; outltt weir-so' 11 Exiatlng lagoon 4 fuc. watershed cond.; outlet welr»t>n' 12 Existing lagoon 6 fut. watt- shed cond. ;out Jet wttr-R»' 8 1-5 weir lowered; no othvi lagoon change* 10 I-S weir lowered; no othvi lagoon changes 11 1-3 weir lowered; no othvr lagoon chance* IS 1-5 weir lowered, no otherlagoon change* 14 Hill St. opened; outlet weir at SO1 9 Hill St. opened; outletweir at 50' — Hill St. opened; outlet weir st 50* 12 Hill St. opened; outlet weir at 30' 13 Outltt weir lowered to 1.8' NCVD 13 Outlet weir lowered to 1.6' NCVD 13 Outlet weir levered to 1.8' NCVD 13 Outlet weir lowered to 1.8' NCVD 22 Combination run: 1-3. Niil St. » 1-3 opened up 16 Combination run: 1-5, Hill St., » 1-5 opened up 10 Outlet weir at original length:Cxittint lagoon con4. II Outlet weir at original TABLE 7, page 2 RW NO. INfUW LAGOON CONDITIONS SK01NENT ACCUMULATION RESULTS Initial Hill water St. Outlet Weir eurfaceI-S Inflow ten* Dti- DepoaUejl charged cone ""~^aT i Frequency Waterehed Weir Width Invert length tie* NCVO Conditlena tie* ft culvert it ft (t (t ele» (I 11 100-yr Future 12 100-yr future 33 12-yr Future I.J 100 2.0 »•>!•• !•• 1.} 100 1.8 BO I.B 1.8 l.S 1UU 1.8 t» l.S US 34 l-yr Future l.S 100 1.7 to I.B !.• 1) 100-yr Eilat. w/dec. 1.} 200 1.7 BO I.B I.B 3b 100-yr Future 1.5 100 1.7 BO I.B I.B • 17 100-yr E*i*t.w/det. J.B 1} 2.0 SO S.8 S.B IB 100-yr Future l.S 100 I.B BO 1.8 I.B 3» 100-yr Future l.S 100 I.B 80 I.B I.B • 40 100-yr Future • 41 2-yr Future • 44 2-yr Olet w/dat. t 4S 2-yr Future • *» 100-yr Future • 47 2-yr Fut.w/det. • 48 100-yr Ful.w/det. » 4» 2-yr Fut.v/eet. enh. channel • SO 100-yr Fut.v/det. enh. channel -B.-3 100 -B.O -8.S 100 8.0 80 80 1.8 1.8 I.* I.ft I.I4», JIO »7*.B*() 20.0*0 Ib.UlO 34*. 13S 282.125 1,141.470 HS.flS 347.4eS 30J, 710 I.I49.3IU 948.SSO 1.141.470 tOf.ltO 1,141,»BJ 8*2,«40 run: Initial •: FATAL EMOR ->lo« of netl. no. BS IS Lowered Mill St. rulwrt Invert to avoid abnv* err. — — No (low out of Baaln 2 or 3 Note, pro*ran did run 80 20 Lowered Hill St. to 1.7' '• avert above probleai 82 IB CF Kan «32 • 30 (for eclating lagoon condition-; 84 U Channel aloMlatlon: width «f are«*l/IO original coni !>•"»«.• tlun v-lu» 87 13 iKlaclitc lagoon condttlmi>- 10. l)I4.21S 83 17 80 20 78 22 71 2* S«M aa 13* but depth Increaaed by S ft 10* channel. SO'wlde. ar*.i aealed down by ratio ot wldtha (aee SCU.DAT (I In 10' deep channel, SO'wIdr: thla la beat caae »c»n*ii- 10* deep chann»l. M* will, thla la beat caae aeenari -8.S -8.S S.B S.B S.B S.B S.B S.B 100 100 IS IS IS IS IS 13 -8.0 -8.0 2.0 2.0 2.0 2.0 2.0 2.0 80 80 SO SO SO SO so so 1.8 1.8 S.B S.B S.B S.B S.B S.B I.e I.B S.B S.B S.B S.B S.B S.B M8 « • el 2.SI4 48 l.fOI 33 .MO .JIM ,3» .«* ,77" ,S»S .002 .«20 Ml b 8 Sb 2.22S 43 1.472 31 ,»4S .3*0 .bSO .S»2 .230 ,»8I .570 .280 7b M »3 90 88 11 88 12 14 32 7 10 12 * 12 8 S.B IS 2.0 SO Future la«ie> with channel Future U«eon w/ channel blatlnf laiomi condlilon- Kevlaed nydrolofy Revlavd hydrology Kevleed hydrolocy Mevtaed hydraloRy Bevlaed hydrology 5.8 S.B 1.361.VI4 1.1*0,35» 87 13 Revtaed hydrolocy q = pV*lOn(0.17 +2.01 Qn/w) s where * Un/v* - 2.5*n{3.32 V*h/y) where: qs is sediment discharge p is the fluid density V* is the shear velocity On is the mean velocity w is the particle fall velocity h is the depth is the kinematic viscosity This lower limit is particularly important in estimating net accumulation of the finer sediment fractions. The total sediment accumulation for a particular flood hydrograph for each cell can be estimated by sunning the accumulation in each time period. The Computer Model A computer model was developed to simulate sediment accumulation in the three cells of Buena Vista Lagoon for a given inflow flood hydrograph. This model, identified as MPOND c PWA 1985, was an adaption of an earlier lagoon flood routing model with the addition of sediment routing subroutines. The input data required is the inflow flood hydrograph, sediment rating curves for each sediment size selected, lagoon geometry, and weir and culvert . characteristics. The output is a computation for each hour of the sediment inflow, sediment accumulation, and sediment discharge, for each fraction for each basin in the lagoon. The computer model also calculates inflow, outflow, water surface elevation and velocities of each basin for each hour. Over the entire hydrograph period the model calculates the total sediment accumulation and discharge for each basin. Results A total of 5(7 computer runs were carried out to examine the combinations of different scenarios of watershed conditions and lagoon hydraulics. A complete summary of these runs is shown in Table Seven. Forty-four of the runs were carried out based on HBC-1 inflow hydrographs representing three different watershed conditions that were developed in the Phase One study. These were: 33 1. Existing conditions 2. Future conditions with no action 3. Existing conditions with detention basins. For each condition the 2-year and 100-year flood hydrograph was analyzed, and in sane instances the 5- and 25-year hydrograhs as well. Late in the study, errors were found in these HBC-1 inflow hydrographs. Accordingly they were recalculated and six additional computer runs based on three new sets of hydrographs were carried out: 4. Recalculated future conditions with no action 5. Recalculated future conditions with detention basins 6. Recalculated future conditions with detention basins and preserving the f loodplain upstream. A summary of the inflow hydrographs used is given in Table Eight. Although the earlier HEC-1 hydrographs were not accurate, the computer modelling results can still be used as a basis for comparing the effectiveness of different modifications to the hydraulics of the lagoon. Five different lagoon hydraulics conditions were examined, either individually or in combination: 1. Existing conditions 2. Removing the sediment accumulated under the 1-5 bridge 3. Enlarging the Hill Street culvert 4. Increasing the capacity of the outlet weir by widening and lowering its crest 5. Excavating a deep channel through the lagoon from Jefferson Street to the outlet weir. The average annual sediment accumulation for selected scenarios is shown in Table Nine. These values are determined by plotting a sediment accumulation probability curve using the 2-year and 100-year computer run results for total sediment accumulation and then calculating the average annual sediment accumulation by the method described in the ASCE Sedimentation Engineering Handbook Table 4.18. Table Nine shows that average annual sediment accumulation in about 33,000 tons/year under existing conditions (scenario I). This amounts to about 20 acre feet per year which, averaged over the whole lagoon, is about 0.1 ft/year. At this rate the lagoon would fill in completely in twenty to thirty years. Sediment accumulation of 33,000 tons/year, assuming a trap efficiency 34 TABLE EIGHT. SUMMARY OF INFLOW HXDFOGRAPHS Source Phase I study Phase I study Phase I study Phase I study Phase I study Phase I study Phase I study Phase I study Phase 1 study Phase I study Phase II study Phase II study Phase II study Phase II study Phase II study Phase II study Watershed Condition Existing Existing Existing Existing Future Future Future Future Existing w/det Existing w/det Future Future Future w/det Future w/det Future w/det & fp Future w/det & fp Flood Return Frequency 2 5 25 100 2 5 25 100 2 100 2 100 2 100 2 100 Peak Flow cfs 1371 1979 3659 7200 1712 2485 4412 8000 1227 5044 3213 13734 2590 11431 2130 9231 Flood Volume acre ft. 429 608 1067 1942 539 741 1245 2145 414 1553 428 1583 1483 314 1351 35 TABLE NINE. 51M4AFY CF LflGOCN PND WATERSHED SIMULATIONS Avg. Annual Simulation Watershed Hydrograph Lagoon Corputer Sed. Accu. % No. Conditions Source Cond. Puns (tons) reduct. 1 Existing 2 Future 3 Future 4 Future 5 Existing w/det 6. Existing w/det 7 Future 8 Future w.det 9 Future w det of floodolain Phase I Phase I Phase I Phase I Phase I Phase I Phase II Phase II . Phase II Existing Existing Future Conb. Future Comb. w/channel Existing Future Conb. w/channel Existing Existing Existina 1,2,3,4 29,30 32,34 40,41 37,44 42,43 45,46 47,48 49.50 32,970 73,550 64,230 56,280 25,490 19,840 191,500 153,600 105.100 N/A 0 13 23 65 73 0 20 45 36 of about 90%, corresponds to a watershed sediment yield of 1500 tons/squaremile/year, a value fairly typical of this area. It also corresponds closely to the observed historic rate of accumulation of approximately 35,000 tons/year. However, such a correspondence should be regarded as fortuitous because of the large potential errors inherent in analyses of this type. Sediment accumulation under future watershed conditions with increased urbanization and no sediment control is projected to increase by a multiple of six to about 200,000 tons/year (scenario 7). This is due to the increase inpeak flows from storm drains, "H™** channels and paved surfaces, and the exponential relationship between sediment delivery and flow rate. The resulting predicted watershed sediment yield of 7,500 tons/square mile/year is not uncuiiuun in urbanizing watersheds of this type. A sediment accumulation of 200,000 tons/year if distributed equally in the lagoon, could fill it in four to five years. V. COKUUSICNS The Future of the Lagoon If no action is taken and the existing sedimentation rates continue, most of the lagoon will probably fill with sediment within the next 20 to 40 years. However, because of additional urbanization in the watershed (underway or planned), and the filling of the Buena Vista Creek floodplain, most of the lagoon might fill in in the next 10 to 20 years. This sedimentation would not occur gradually but more likely as a result of 2 or 3 major floods. Consequently, the rate of filling will depend on the sequence of wet and dry years. There are two strategies for reducing sedimentation in the lagoon: either reduce sediment inflow or improve the flushing ability of the lagoon. Table One shows the relative successes of these two strategies. The most effective alternative for mitigating increased sediment flows to the lagoon is channel enhancement (Alternative WS). Channel enhancement was modeled from downstream of Brengle Terrace Park to South Coast Asphalt. Various channel configurations will accomplish this goal but a maximum velocity of 6 feet per second must be maintained. The key to keeping the velocities at this level will be to install check structures along the creek and recontour the channel as necessary. This velocity will allow for the growth of riparian vegetation; however, in many areas the existing "natural" channel will have to be redesigned, drop structures installed, and creek banks revegetated to meet this velocity goal. Fortunately we have a model to follow. Gully restoration was accomplished in the Tahoe Basin using check structures. These have been in place for several years. Not only does channel enhancement decrease peak flood flows to the lagoon more effectively it also decreases the erosion hazard for the major source of sediment to the lagoon. Almost as effective in decreasing peak flood flows is the design, installation and maintenance of eight storm attenuation basins (Alternative WI). The location of these and the channel enhancement are shown on Figure 4. All of the attenuation locations were modeled in the Phase One study. Two were remodeled, based on more detailed topography. 37 .,'"7 1-5, It is evident that there is a potential for decreasing lagoon sedimentation by about 45% by reducing flood peaks under future conditions by constructing detention basins and preserving and enhancing the floodplain upstream (Alternatives W and VB). The third most effective method of reducing sediment to the lagoon is stopping the soil loss frcm grading and agricultural operations. The grading ordinances for the three cities adequately require sediment control for grading operations. An education process still needs to occur for the methods that are effective to comply with the ordinances. Agriculture is still a major source of sediment and there appears to be no ordinances for this source. Agriculture will probably dwindle to be very small within 20 years, as the watershed is urbanized. This study envisions a program of education for the City personnel who check grading plans and the operators and bin.] ding inspectors who are responsible for the outcome of these plans. The education program would give a review of the requirements of the erosion control ordinance and specific recommendations for cost-effective design and implementation of the ordinance. Similiarly the agricultural education program would focus on agricultural operators and the Soil Conservation Service and emphasize cost-effective implementation of best management practices in the watershed. Small amounts of the total sediment can be removed in two sediment basins located at South Coast Asphalt and the mouth of the lagoon (Alternatives S4 and S5). The one at South Coast Asphalt will require less costly maintenance, as the material could probably be used on the site. The advantage to these sediment basins is that it is less costly to remove the material at these locations than after it reaches the lagoon. The nature of the material removed will be sandier than the total sediment accumulating in the lagoon, therefore a more usable material. The sediment frcm side channel arroyos can be effectively controlled by repairing the arroyo and preventing further erosion (Alternatives SI). These would be repaired in the same fashion as the main creek channel. Sediment inflow can be further reduced by control of sediment sources in the watershed. The releative success of these sediment control measures is not easily quantifiable. However, it is not unreasonable to suggest that vigorous application of such measures could reduce the sediment load by about 50%. This, combined with reduction of the flood peaks, would mean that the sediment inflow would be reduced to about 30% of its expected future value. In contrast, modifying the lagoon hydraulics is not nearly as effective in reducing sediment accumulation. With all the reasonably feasible modificaitons — clearing the 1-5 bridge, widening the Hill Street culvert and lowering and widening the outlet weir, the sediment accumulation reduces only by 13%. Even with these modifications, maintaining a deep channel through the entire lagoon reduces sediment accumulation by 23% (Alternative L2). A deep channel would require constant maintenance as it would tend to fill with sediment resuspended by wave action in the summer, and is not a very practical option. 39 The effectiveness of individual modifications in the lagoon is negligible. They only achieve the 13% reduction when implemented together. This can be seen by comparing runs 9 and 12, 13 and 16, 17 and 20, with 29 and 30, and with 32 and 34, as shown in Table Seven. In its present state, managed as a fresh water lake, Buena Vista Lagoon is a very efficient sediment trap. It traps about 90% of all sediment which enters it. The only hydraulically feasible way of greatly increasing the flushing of sediment from the lagoon is to restore it to Hdal action. However, because the sediment discharge to the lagoon is now so large, even with tidal action the lagoon would probably silt up to a fraction of its original size. Even with the best possible scenario of watershed improvements, considerable amounts of sediment will have to be removed in order to maintain the lagoon in its existing state. To predict the amount of dredging required, flow mesurements and sediment samples will have to be taken on Buena Vista Creek and at the outflow weir in order to calculate a more accurate sediment budget. In addition, periodic bathymetric surveys will need to be made of the lagoon to measure actual sediment accumulation. Based on the existing analysis using data from other watersheds, the average dredging requirement probably will be in the range of 10,000 to 100,000 tons/year, assuming the watershed modifications are implemented. Probably between five and twenty percent of this sediment will be of suitable size for beach replenishment. VI. RBOOEflENDftTICNS 1. The Lagoon's Future In its existing state Buena Vista Lagoon acts as a very effective sediment trap. Under existing watershed conditions it appears that the entire lagoon will silt in over the next twenty or thirty years, with most of the siltation occurring during a few large storms. Under future watershed conditions the rate of siltation will increase substantially, reducing the expected lifetime of the lagoon to less than ten years. The most effective means of reducing sediment accumulation in the lagoon is to reduce sediment inflow from the watershed. Sediment accumulation in the lagoon can be reduced by about 50% by reducing flood peaks in the watershed and enhancing the creek, thereby reducing sediment inflow. 2. Creek Enhancement (WS) To reduce sediment entering the lagoon from creekbed erosion, a maximum channel velocity criteria of six feet per second is required. This is effective and was modelled as the maximum velocity that vegetation could conceivably withstand. Lower velocities are recommended and will be more effective. All initial creek design should check flood control requirements, and utilize the maximum velocities using Manning roughness coefficients of 0.030 to 0.050, which is the range of the vegetation growth roughness in the channel. So the velocity of the 100 year storm should not exceed six 40 feet per second using a Manning roughness coefficient of 0.030 and the finished floor elevations should be one foot higher than the anticipated water surface elevation using a Manning coefficient of 0.050 for 100-year storms. A low flow channel should be designed to acccranodate the 2-year storm flows. This low flow channel should be configured to be in shade wherever possible. Drop structures should be placed in the channel to lower velocities and banks revegetated to prevent erosion. A typical stream cross section is shown in Figure 5. The lower middle reach is absorbing large quantities of sediment and is presently offering a significant buffer for the lagoon by accomadating great quantities of sediment being transported in the creek. This section of the floodplain most be preserved in its present state. 3. Detention Basins (Wl) The eight detention basins outlined on Figure 4 should be built and maintained. Maximum peak attenuation for the 2 to 100-year storms should be the design criteria for these basins. 4. Lagoon Modifications (LI) The most effective method of decreasing the sediment accumulation in the lagoon with lagoon enhancement is a combination of a movable crest weir with a width to 80 feet, enlarging the opening at Hill Street to 100 feet wide, excavating the material under the freeway, and breaching the barrier beach during storms. This scenario of improvements reduces sediment accumulation by only 13% and has many environmental drawbacks. 5. Continuing Erosion Control Education (S2 and S3) The first erosion control workshop was well attended. There seems to be a community commitment to erosion control. Efficient erosion control both for grading and agriculture will pay off well in terms of reducing sediment accumulation in the lagoon be beneficial. 6. Sediment Basins (S4 and S5) The two feasible locations for sediment basins are at South Coast Asphalt and just upstream of Jefferson Street. 7. Dredging Even with all of the effective methods outlined herein, the lagoon will have to continue to be dredged periodically. The alternatives were all compared to this inevitable and ongoing solution. 41 8. Monitoring The feet that this is a completely ungauged watershed gives the findings of this study an amount of uncertainty. There is a need for monitoring so more accurate estimates of flood flows, sediment transport and sediment accumulation rates can be observed. The models used in this study then can be calibrated and the accuracy improved greatly. Pain gauge and stream flow monitoring are needed to calibrate watershed models. These vail also help to answer many of the local questions regarding storm flow values. Present estimates of these storm flow values vary wildly. Sediment sampling in the creek will provide more definite correlation between the sediment flows and the flood flows. This will help develop a sediment rating curve for the Buena Vista watershed. Bathymetric surveys of the lagoon need to be taken, especially before and after major storm events in order to monitor the sediment accumulation . 42 VII. BTW.TQGRAPHY ASCE Manuals and Reports an Engineering Practice, No. 54, 1975. Sedimentation and Engineering, V. Vanoni, ed. Hoffman, J. S., et al. 1983. Projecting Future Seal Level Rise, Methodology, Estimates to the Year 2100, and Research Needs. A Report of the U.S. Environmental Protection Agency. Simons, Li & Associates, Fort Collins, 00. 1984. Effect of the Santa Margarita Project on Beach Sand Replenishment. Prepared under contract with the Bureau of Reclamation. U.S. Army Corps of Engineers, 1973. Flood Plain Information, Buena Vista Creek, Pacific Ocean to Vista, San Diego County, CA. Prepared for San Diego County. Yalin, M. S., 1972. Mechanics of Sediment Transport. Perganmon Press, Oxford. United States Geological Survey Water Resources Data for California, 1975-82. United States Geological Survey, Menlo Park, CA. 43 APPENDIX ONE SUBBASINS FUTURE HYDROLOGICAL CONDITIONS CALCULATIONS WATERSHED SUBBASIN FUTURE CHARATERISTICS 0.38 LAG - 24n(L X Lc/SQRT s) SUB- LU SOIL BASIN 1 COMM D HDR MDR D LDR FMST D 2 COMM D HDR D MDR D LDR FMST 3 COMM HDR MDR LDR FMST D 4 COMM D HDR D MDR D LDR FMST D 5 COMM D HDR D MDR LDR FMST 6 COMM D HDR D MDR D LDR FMST % 40% 20% 40% 40% 40% 20% 100% 30% 30% 20% 20% 50% 50% 50% 30% 20% SCS CN 92 88 36 92 90- 88 86 92 90 88 86 92 90 92 90 88 CN ELEV LI X % DIF L 36.8 220 0.0 17.6 0.0 COMPOSIT 34.4 CN 0.0 89 36.8 150 36.0 17.6 0.0 COMPOSIT 0.0 CN 0.0 90 0.0 700 0.0 0.0 0.0 COMPOSIT 86.0 CN 0.0 86 27.6 340 27.0 17.6 0.0 COMPOSIT 17.2 CN 0.0 89 • 46.0 50 45.0 0.0 0.0 COMPOSIT 0.0 CN 0. 0 91 46.0 150 27.0 17.6 0.0 COMPOSIT 0.0 CN 0.0 91 :NGTH PC-CNT BASIN SLOPE (FT) Lc (FT) n (FT/MI) 3500 1800 0.05 332 LAG (HR) 0.2263 3500 1400 0.045 226 LAG (HR) 0.1991 4000 2200 0.05 924 LAG (HR) 0.2115 4700 2200 0.045 382 LAG (HR) 0.2394 3000 1500 0.03 LAG (HR) 0.1537 2500 1000 0.03 317 LAG (HR) 0.0964 A-1 COMM 0.0 650 6000 3500 0.05 572 HDR 0.0 MDR ' 0.0 LOR 0.0 COMPOSIT FMST D .100% 86 86.0 CN LAG (HR) 0.0 86 0.3224 8 COMM . 0.0 900 4000 2500 0.05 1188 HDR 0.0 MDR 0.0 LDR 0.0 COMPOSIT FMST D 100% 86 86.0 CN LAG (HR) 0.0 86 0.2117 COMM 0.0 450 4000 2000 0.05 594 HDR 0.0 MDR 0.0 LDR 0.0 COMPOSIT FMST D 100% 86 86.0 CN LAG (HR) 0.0 86 0.2218 10 COMM 0.0 650 6000 2500 0.05 572 HDR 0.0 MDR 0.0 LDR C 50% 84 42.0 COMPOSIT FMST C 50% 82 41.0 CM'^ LAG (HR) ").0 83 0.2837 11 COMM 0.0 650 5000 2500 0.05 686 HDR 0.0 MDR 0.0 LDR 0.0 COMPOSIT FMST D 100% 86 86.0 CN LAG (HR) 0.0 86 0.2557 12 COMM 0.0 150 5000 2500 0.035 158 HDR 0.0 MDR C 40% 86 34.4 LDR C 40% 84 33.6 COMPOSIT FMST C 20% 82 16.4 CN LAG (HR) 0.0 84 0.2365 13 COMM 0.0 200 3000 1500 0.035 352 HDR 0.0 MDR 0.0 LDR D 30% 87 26.1 COMPOSIT FMST 0.0 CN LAG (HR) PARK D 70% 82 57.4 84 0.1378 14 COMM 0.0 600 10000 4000 0.05 317 HDR 0.0 MDR 0.0 LDR C 30% 84 25.2 COMPOSIT FMST C 70% 82 57.4 CN LAG (HR) 0.0 83 0.4609 15 COMM 0.0 250 3000 1600 0.035 440 HDR 0.0 MDR 0.0 LDR C 40% 84 33.6 COMPOSIT FMST 0.0 CN LAG (HR) PARK C 60% 77 46.2 80 0.1354 16 COMM B 40% 90 36.0 250 3200 1500 0.03 413 HOR B 40% 82 32.8 MDR 0.0 LDR B 20% 78 15.6 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 84 0.1175 17 COMM B 60% 90 54.0 250 3000 1000 0.03 440 HDR 0.0 MDR B 40% 30 32.0 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 86 0.0970 18 COMM C 40% 91 36.4 250 9000 5000 0.028 147 HDR C 20% 88 17.6 MDR C 20% 86 17.2 LDR C 20% 84 16.8 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 88 0.3124 19 COMM 0.0 500 6000 3000 0.05 440 HDR 0.0 MDR 0.0 LDR C 30% 84 25.2 COMPOSIT FMST C 70% 82 57.4 CN LAG (HR) 0.0 83 0.3196 20 COMM C 30% 91 27.3 250 9500 5000 0.035 139 HDR 0.0 MDR C 30% 86 25.8 LDR C 30% 84 25.2 COMPOSIT FMST C 10% 82 8.2 CN LAG (HR) 0.0 87 0.4027 A-3 21 COMM D 30% 92 27.6 100 10000 6000 0.03 53 HDR D 30% 90 27.0 MDR D 30% 88 26.4 LDR D 10% 87 8.7 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 90 0.4534 22 COMM C 50% 91 45.5 80 2000 1000 0.028 211 HDR B 50% 82 41.0 MDR 0.0 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 87 0.0893 23 COMM D 40% 92 36.8 220 9000 4000 0.028 129 HDR D 20% 90 18.0 MDR D 30% 88 26.4 LDR D 10% 87 8.7 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 90 0.2941 24 COMM C 60% 91 54.6 85 4000 2000 0.028 112 HDR C 30% 88 26.4 MOR C 10% 86 8.6 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 90 0.1705 25 COMM C 40% 91 36.4 180 5000 2500 0.03 190 HDR C 20% 88 17.6 MDR C 30% 86 25.8 LDR C 10% 84 8.4 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 88 0.1958 26 COMM 0.0 170 5000 2500 0.035 180 HDR 0.0 MDR D 100% 88 88.0 LDR 0.0 COMPOSIT FMST 0.0 CN 'LAG (HR) 0.0 88 0.2310 27 COMM C 20% 91 18.2 200 6000 3000 0.035 176 HDR C 20% 88 17.6 MDR C 60% 86 51.6 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 87 0.2663 28 COMM C HDR D MDR D LDR FMST 29 com HDR MDR D LDR FMST 30 COMM D HDR 0 MDR D LDR FMST 31 COMM C HDR D MDR D LDR FMST 32 COMM D HDR D MDR D LDR FMST 33 COMM HDR MDR D LDR FMST 34 COMM D HDR D MDR D LDR FMST 20% 40% 40% 100% 20% 30% 50% 20% 50% 30% 40% 30% 30% 91 18.2 180 90 36.0 88 35.2 0.0 COMPOSIT 0.0 CN 0.0 89 0.0 1200 0v • W 88 88. 0 0.0 COMPOSIT 0.0 CN 0.0 88 92 18.4 150 90 27.0 88 44. 0 0.0 COMPOSIT 0.0 CN 0.0 89 91 18 2 ??n i•» i u. f. CtU J 90 45.0 88 26.4 0.0 COMPOSIT 0.0 CN 0.0 90 4500 2500 0.03E LAG (HR) 0.2151 4000 1800 0.035 LAG (HR) 0.1918 5000 2500 0.035 LAG (HR) 0.2365 !400 1500 0.035 LAG (HR) 0.1454 92 36.8 260 7000 3500 0.035 88 26!4 0.0 COMPOSIT 0.0 CN 0.0 90 LAG (HR) 0.2933 0.0 180 4000 2200 0.035 100% 50% 30% 20% 88 88. 0 0.0 COMPOSIT 0.0 CN 0.0 88 92 46.0 180 47C 90 27.0 88 17.6 0.0 COMPOSIT 0.0 CN 0.0 91 LAG (HR) 0.1916 )0 2500 0.035 LAG (HR) 0.2205 211 158 158 342 196 238 202 A-5 35 COMM D 10% 92 9.2 200 5000 2000 0.035 211 HOR 0 20% 90 18.0 MDR D 70% 88 61.6 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 89 0.2057 36 COMM C 10% 91 9.1 200 4000 2000 0.035 264 HDR D 20% 90 18.0 MDR D 70% 88 61.6 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 89 0.1811 37 COMM 0.0 370 4500 2200 0.035 434 HDR 0.0 MDR D 100% 88 88.0 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 88 0.1787 38 COMM 0.0 340 5500 2500 0.035 326 HDR C 40% 88 35.2 MDR D 60% 88 52.8 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 88 0.2138 39 0.0 340 4000 2000 0.035 449 MDR A 10% 73 7.3 MDR C 20% 86 17.2 MDR D 70% 88 61.6 COMPOSIT 0.0 CN LAG (HR) 0.0 86 0.1638 40 0.0 320 3500 1500 0.035 483 MDR C 30% 86 25.8 MDR D 70% 88 61.6 0.0 COMPOSIT 0.0 CN LAG (HR) 0.0 87 0.1376 41 0.0 280 6000 3500 0.035 246 MDR C 60% 86 51.6 MDR D 40% 88 35.2 0.0 COMPOSIT 0.0 CN LAG (HR) 0.0 87 0.2649 A r 42 MDR C MDR D 43 MDR C MDR D 44 MDR C MDR D 45 COMM HDR MDR C LDR FMST PARK C 46 com c HDR MDR C LDR FMST OPEN C 47 MDR C MDR C 48 COMM D COMM C HDR D MDR D FMST 60% 40% 50% 50% 40% 60% 30% 70% 10% 80% 10% 30% 70% 40% 40% 10% 10% 86 88 86 88 86 88 77 86 91 88 71 86 88 92 91 90 88 0.0 240 51.6 35.2 0.0 COMPOSIT 0.0 CN 0.0 87 0.0 220 43.0 44.0 0.0 COMPOSIT 0.0 CN 0.0 87 0.0 240 34.4 52.8 0.0 COMPOSIT 0.0 CN 0.0 87 0.0 280 0.0 23.1 0.0 COMPOSIT 0.0 CN 60.2 83 9. 1 240 0.0 70.4 0.0 COMPOSIT 0.0 CN 7.1 87 0.0 260 25.8 61.6 0.0 COMPOSIT 0.0 CN 0.0 87 36.8 220 36.4 9.0 8.8 COMPOSIT 0.0 CN 0.0 91 3000 1500 0.035 422 LAG (HR) 0.7331 2500 1200 0.035 465 LAG (HR) 0.1121 4500 2000 0.035 282 LAG (HR) 0.1871 4000 1500 0.035 370 LAG (HR) 0.1523 4500 2000 0.035 282 LAG (HR) 0.1871 3700 1500 0.035 371 LAG (HR) 0.1478 4500 3000 0.035 258 LAG (HR) 0.2220 A-7 49 COMM C 40% 91 36.4 280 6500 2200 0.035 227 HDR D 20% 90 18.0 MDR D 40% 88 35.2 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 90 0.2324 50 COMM 0.0 330 7000 3000 0.035 249 HDR 0.0 MDR A 100% 73 73.0 LDR 0.0 COMPOSIT FMST 0.0 CN LAG (HR) 0.0 73 0.2644 A-8 APPENDIX TWO COST-EFFECTIVENESS ANALYSIS ANNUAL COSTS DREDGING: LAGOON IN EXISTING CONDITION WATERSHED IN FUTURE CONDITION 191.500 CY SEDIMENT DELIVERY X $7.35 /CY -$1.407,525 LAGOON MODIFICATIONS: LAGOON WITH 80' LOWERING WEIR, 100' HILL ST. BRIDGE & DREDGING AT 1-5 WATERSHED IN FUTURE CONDITION WEIR: $400,000 INITIAL COSTS LIFE: 20 YEARS - $20,000 OPERATION AND HAINTANENCE: $20,000 HILL ST: $140,000 INITIAL COSTS LIFE: 50 YEARS - $2,800 1-5 $30,000 INITIAL COSTS LIFE: 2 YEARS - $15,000 TOTAL ANNUAL COSTS: $57,800 LAGOON CHANNEL DREDGED 10' DEEP 100' WIDE • WATERSHED IN FUTURE CONDITION 268,500 CY X -$15 PER CY - $4,027.500 INITIAL COSTS LIFE: 2 YEARS - $2,013.750 DETENTION BASINS LAGOON IN EXISTING CONDITION WATERSHED IN FUTURE CONDITION WITH 8 DETENTION BASINS LAND: 19 ACRES X $130.000 PER ACRE - $2.470.000 LIFE: 50 YEARS - $49,400 CONST'N: $11,500 EACH X 8 - $92,000 LIFE: 10 YEARS - $9,200 MAINT: $130,000 TOTAL ANNUAL COSTS: $188,600 CREEK ENHANCEMENT LAGOON IN EXISTING CONDITION WATERSHED IN FUTURE CONDITION WITH 8 DETENTION BASINS LAND: 1,050,000 SQ FT X $4 PER SQ FT -$4,200,000 LIFE: 50 YEARS - $84,000 DROP STR $5,000 EACH X 160 - $800,000 LIFE: 5 YEARS - $160.000 TOTAL ANNUAL COSTS: $244,000 SIDE'CHANNEL REPAIR DROP STR $3,000 EACH X 10 - $30.000 LIFE: 5 YEARS - $6,000 TOTAL ANNUAL COSTS: $6,000 SOUTH COAST SEDIMENT BASIN 5,000 CY SEDIMENT DELIVERY X $2.35 */CY » $11,750 ANNUAL COSTS JEFFERSON SEDIMENT BASIN 1,600 CY SEDIMENT DELIVERY X $3.35 */CY - $5,360 ANNUAL COSTS *If.'CLUDES REDUCTION IN PRICE OF $4/CY FOR THE VALUE OF THE MATERIAL A-10 PROJECTED SEDIMENT REDUCTIONS WATERSHED LAGOON % REDUCT ANNUAL ANNUAL COND COND SEDIMENT SEDIMENT ACCUM REDUCT. FUTURE EXISTING BASIS 191,500 FUTURE CCMB 12% 168,520 22,980 FUTURE COMB&CHANN 26% 141,710 49,790 FUTURE CHANN EFFECT 26,810 MINIMUM F W/DET EXISTING 20% 153,600 37,900 DET&ENH EXIT 45% 105,100 86,400 ENHANC EXIST 48,500 MINIMUM ALTERNATIVE ANNUAL ANNUAL REDUCTION COST- COST SEDIMENT X BENEFIT REDUCTION $7.35 RATIO NO MODIFICATION $1,407,525 - - 1.00 WEIR-HULL ST+I-5 $57,800 22,980 $168,903 3.0 CHANNEL $2,013,750 26,810 $197,054 .1 DET BASINS $139,200 37,900 $278,565 2.0 CREEK ENHANC $160,000 61,000 $448,350 2.8 E.C.R SIDE ARROYO $6,000 1,000 $7,350 1.2 GRADED AREAS $6,000 5,000 $36,750 6.3 AGRICULTURAL $6,000 4,500 $33,075 5.6 S.COAST SED BASIN $11,750 2,500 $18,375 1.6 JEFF. SED BASIN $5,360 1,600 $11,760 . 2.2 A-11 APPENDIX THREE MIDDLE REACH EROSION CALCULATIONS SECTION NUMBER 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 CROSS SECTION AREA (SF) 655 243 139 519 526 645 520 581 384 1895 519 412 85 54 400 374 92.5 DISTANCE BETWEEN SECTIONS (FEET) 840 510 440 1.460 980 275 780 640 590. 200 710 420 no 460 640 300 VOLUME (CY) 13,963 3,612 5,361 28,242 21,252 5,930 15,893 11,435 24,900 8,941 12,238 3,864 284 3,869 9,170 2,590 171,542 TOTAL EROSION IN THE UPPER MIDDLE REACH •••> 1 1 $_ oJ_ XMa 3 1<.*:*?'i;»uii;*4llifl'litinf* Si i ' I H I I I I i - t , f ^l? i z i ! : I I A-13 ^ X 4 I S 3 SJ to VI•a I<n 2 _ . 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