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HomeMy WebLinkAbout9053-5884-E; Agua Hedionda Creek - Flood Plain Study; Agua Hedionda Creek - Flood Plain Study; 1987-02-01r•w r •tt f COUNTY OFIS4N DIEGO COMMUNITY SERVICES AGENCY Department of Sanitation & Flood Control C. J. HOUSON Director FINAL REPORT M\J1 , '.•••••: *-'• ".;• ' ',' ..,:•'.•'•••"' * •' ' ' "*' "•" -»JJ"- -•*••••- - •»--.-• BEST ORIGINAL Contract No.9053-5884-D FEBRUARY, 1978 GEORGE S. tfOLTE AND ASSOCIATES 6333 Clairemont Mesa Boulevard San Diego, California 92111 D520.B.35 TABLE OF CONTENTS 1.0 INTRODUCTION 1 . 1 Purpose of the Study 1.2 Acknowledgements 1.3 Description of Results 2.0 AREA STUDIED 2.1 Stream and Basin Characteristics 2.2 Developments on the Flood Plain 2.3 Flood History 2.4 Previous Studies 3.0 METHODOLOGY 3-1 Hydrology 3.2 Topography and Mapping 4.0 HYDRAULIC ANALYSIS 4.1 Use of HEC-2 and Field Observations 4.2 Effective Flow Areas 4.3 Roughness Coefficients 4.4 Supercritical Flow 4.5 Bridges and Culverts 4.6 Debris 5.0 DESCRIPTION AND RESULTS OF THE HYDRAULIC ANALYSIS 5 5 5 5 5 Reach 1 (Lower Agua Hedionda) Reach 2 (Buena Tributary) Reach 3 (Upper Agua Hedionda) Floodway Analysis Hydraulic Capacities of Existing Struct ures 1 1 2 3 5 6 7 8 9 1 1 1 1 12 12 12 13 14 15 16 16 18 FIGURE 1 APPENDIX A APPENDIX B 1.0 INTRODUCTION 1.1 Purpose of the Study San Diego County has adopted a program of careful flood plain management, in order to minimize the risk of flood damage to new developments, and to avoid increasing flood hazards currently existing in the City. In order to implement the flood plain management program the elevation and the boundaries of the 1% flood must be determined, together with the limits of the regulatory floodway. This study was undertaken to provide flood boundary, flood ele- vation, and floodway limit information for Agua Hedionda for use in conjunction with flood plain management measures. The Federal Government also recognizes the importance of sound flood plain management, and through the Federal Insurance Administration (F.I.A.), a branch of the Depart- ment of Housing and Urban Development, offers federally subsidized flood insurance to individuals in communities adopting programs designed to minimize the possibility of flood damages. San Diego County's flood plain management program complies with the federal criteria for flood plain management measures. In addition to the information developed for the 100-year flood, the 10-year flood was also studied in order to determine which properties are subject to frequent flood- ing. Although the 10-year flood limits are not shown on the final maps resulting from this study, the data is available through the County Department of Sanitation and Flood Control. 1.2 Acknowledgements This study was conducted under the direction of the Direc- tor of Sanitation and Flood Control, County of 'San Diego. The Department of Sanitation and Flood Control coordinated and reviewed all of the work performed by consultants, and conducted a review of the maps with representatives of the City of Carlsbad. The majority of the work w=s performed by three consulting firms. The Redding, California office of CH2M Hill prepared the orthophoto topography used for the base maps. Field con- trol for all of the photogrammetric topography was provided by the San Diego County Department of Transportation. Ml J San Lo Aerial Surveys (the Photogrammetrist) provided digitized cross-sections, plotted the location of the 100 year and 10-year flood plains, and prepared the final maps for the study. Hydrology was provided by the San Diego County Department of Sanitation and Flood Control. George S. Nolte and Associates (the Consultant) performed the hydraulic analysis, provided working drawings for the final maps, and prepared the text of the study report. Bridge and channel construction plans were provided by Caltrans and the City of Carlsbad. 1.3 Description of Results The results are shown on the County of San Diego Floodplain Maps - Agua Hedionda, sheets 358-1701, 366-1701, 362-1701, 366-1707, 370-1707, 362-1707, 354-1671, 358-1671, 354-1677 and 358-1677. I 31 2.0 AREA STUDIED 2. 1 Stream and Basin Characteristics About one-half of the study reach of Agua Hedionda Creek lies within the City of Carlsbad; the remaining area lies within unincorporated San Diego County. The study reach of its main tributary, Buena Creek, lies entirely within unin- corporated San Diego County. Most of the land area in the vicinity of the study reaches has been used for agricul- tural purposes in the past, although ranching was the first major use of the land with the establishment of the large Spanis.h land grants in the early nineteenth century. Ranching, which was introduced by the Spanish founders of California, never did lead to self-sufficiency; so crop- raising and other agricultural uses gradually replaced ranching as the significant land use. The self-sufficiency of agriculture, mixed with light ranching, had already been demonstrated at the various missions established in the San Diego area. With some exceptions, the land continues in agricultural use today, obviously because water is readily available and there is extensive arable land on the flood plain. Agua Hedionda Creek and a major tributary, Buena Creek, drain an area of 29 square miles. The drainage area is bounded on the north by a range of hills just south of State Highway 78; on the south by Palomar Airport Road; on the northeast by San Marcos Mountains; and on the west by the Gulf of Santa Catalina in the Pacific Ocean. Agua Hedionda Creek originates in the hills south of the San Marcos Mountains and flows in a general southwest direc- tion, confluencing with Buena Creek about 3 miles down- stream. The stream gradients along Agua Hedionda Creek range from about 43 feet per mile in the upper reaches of the study to about 35 feet per mile in the lower reaches approaching the lagoon. There is an extremely steep canyon reach between mile 5.2 and mile 6.0 along Agua Hedionda Creek which is called Los Monos Canyon. Stream gradients in this reach average 180 feet per mile. Buena Creek, with a drainage area of about 6 square miles, originates in the San Marcos Mountains and flows about 5 miles in a southwest direction before it confluences with Agua Hedionda Creek. The stream gradient of Buena Creek through the study reach is about 51 feet per mile. All of Agua Hedionda Creek described above was not included as part of the study. Three stream reaches were included as follows: 1. Lower Agua Hedionda - beginning at the lagoon and con- tinuing upstream and ending just upstream of the Rancho Carlsbad Trailer Park. 2. Buena Tributary - beginning at confluence with Upper Agua Hedionda and continuing upstream to mile 11.415. 3. Upper Agua Hedionda - beginning at confluence with Buena Tributary and continuing upstream to mile 9-733 (at the S.F. and A.T. railroad culvert). Agua Hedionda Creek, from its origin in the hills south of the San Marcos Mountains to near the upper limits of this study at mile 9.733, is a relatively small but well defined natural channel with steep slopes and virtually no flood plain. From the upstream study limit to where the stream enters Los Monos Canyon, the channel remains well defined, though on a gentler slope, with a relatively narrow flood plain. For its entire distance of more than a mile through Los Monos Canyon. Agua Hedionda Creek becomes a very steep narrow channel with no flood plain. Downstream from Los Monos Canyon, it broadens out into a shallow, wider chan- nel, with gently rolling overbanks, transitioning into an improved channel through Rancho Carlsbad Mobile Park and golf course. Downstream from Rancho Carlsbad Mobile Park, from El Camino Real to Agua Hedionda Lagoon, the natural channel becomes shallow and indistinct, with a very bro^d flood plain. During large floods, flows exceed the channel capacity and inundate the flood plain before entering Agua Hedionda Lagoon where considerable flood storage occurs. Attenuated peak floodflows then discharge into the Pacific Ocean, the lower limits of this study. Natural vegetation pa, I is abundant, consisting mostly of native grasses, brush, |,| and trees. Relatively dense chapparal predominates in the "1 higher reaches, yielding to sage in the lower reaches of I tideland flats along the coast.m I The stream reach excluded from the study was determined to be too steep and undevelopable. This reach lies between the Rancho Carlsbad Trailer Park development and the con- fluence of Buena Creek and Agua Hedionda Creek. The climate varies from warm summers to cool winters. Extremes in temperature are rare. Mean seasonal precipita- tion of the drainage area ranges from about 11 inches near the coast to about 18 inches in higher mountains and aver- ages about 13 inches over the total drainage area. Most of the precipitation occurs during December through March. Snow is not considered an important contributing factor to runoff. * 2.2 Developments on the Flood Plain Except for the area around Agua Hedionda Lagoon and for a short reach upstream from El Camino Real, the land in the flood plain of Agua Hedionda Creek is relatively undevel- oped. Only about five percent of the approximately 700 acres in the flood plains in this study can be considered as developed. There is a small number of scattered ranches in the upper middle reaches of Agua Hedionda Creek, exclud- ing the virtually undevelopable steep reach of Los Monos Canyon. Crop cultivation and grazing constitute the bulk of the development on the ranches in this flood plain area. Just upstream from El Camino Real, Rancho Carlsbad, a mobile park and golf course, has recently been developed where Sinforosa Country Club was previously located. Horse training facilities are present at Rancho Carlsbad. A small private dam, with a concrete spillway at Oak Lake Ranch just upstream from the mobile park, provides private recreational opportunities in addition to a source of water, which is needed for ranching activities. Agua Hedionda Lagoon has been improved for public fishing at the downstream end of the study reach near Carlsbad Boulevard. There is a YMCA aquatic park at Agua Hedionda Lagoon between the Atchison, Topeka and Santa Fe Railway and Interstate Highway 5. This park has boating, picnic, water skiing, and other recreational facilities and is used through the courtesy of the San Diego Gas and Electric Company, who owns the property. In general, Agua Hedionda Lagoon is one of the area's outstanding recreational assets which has gained national fame as a water sports area. It has two boat landings, swimming, fishing, sailing and water skiing. The area surrounding the lagoon abounds with pri- vate residences, but because of the steepness of the lagoon banks, all these^residences are outside of the flood plain. Privately owned, operated, and maintained boat launching and docking facilities are upstream from Interstate Highway 5. In addition, several apartment buildings and condo- miniums have been built; and the Carlsbad Chamber of Com- merce projects that many more will be constructed in the near future. The entire lagoon area immediately outside of Agua Hedionda Creek flood plain is a highly developed area. In addition to heavy residential development, there are commercial and limited industrial developments. These include service stations, restaurants, grocery stores, banks, an electrical substation, storage facilities, and utility distribution facilities owned and operated by San Diego Gas and Electric Company. C "Four inches of rain, half a seaons's supply in some years, fell in Carlsbad in a major storm which started Saturday." "Heavy winds and rains set off a series of minor car accidents, flooded streets and gutters and caused some erosion at construction sites." "Buena Vista Lagoon and its eastern flood plain reached full capacity, forcing Carlsbad and Oceanside city crews to open an outlet channel into the ocean." "In one wet-weather mishap, two men were taken to Oceanside Community Hospital for treatment." Excerpt from the Vista Press, December 9, 1966. Vista Press, Friday December 9, 1966. "Vista's rainfall total for the recent four-day storm is less than one inch short of the greatest continuous rainfall total recorded here in 36 years, 7.29 inches recorded from February 27 to March 4, 1938." "City crews are now clearing mud from dozens of streets and remove felled trees in several locations including on Vale Terrace near the Optimist Club that left a^ portion of the community without power for nearly an hour Tuesday night." 2.4 Previous Studies Agua Hedionda Creek has been studied previously and pub- lished as Flood Plain Information. The report was titled Agua Hedionda Creek-Pacific^ Ocean to Buena-San Diego County. California, prepared by the Army Corps of Engineers in July 1973 at the request of the San Diego County Depart- ment of Sanitation and Flood Control with the endorsement of the Department of Water Resources, State of California. • Much of the information used in this report was taken from the previous report. i i i 3 i 3.0 METHODOLOGY 3.1 Hydrology Hydrology reports for Agua Hedionda are avilable from San Diego County Department of Sanitation and Flood Control. The following flow rates were used for this study. Agua Hedionda Creek Peak Flood Flows for Flood Plain Mapping Concen- Drainage 100-Year 10-Year tration Area Discharge Discharge Point No. Location Sq. Mi. C.F.S. C.F.S. !^P ^m Unnamed tributary near Canyon 0.9 1,000 250 Drive just upstream of H confluence with Buena Creek l| 2 Buena Creek just upstream of 1.6 2,000 550 Ora Ava Dr. 3 Buena Creek just upstream of 2.3 2,500 600 confluence with unnamed tri- butary near Canyon Dr. 4 Buena Creek just down- p stream of Canyon Dr. 3.1 3,300 700 ta 5,6 Buena Creek at Highway 78 5.1 4,900 1,200 f* 7 Buena Creek just upstream 6.5 5,000 1,200 • of confluence with Agua Hedionda Creek 8,9 Agua Hedionda Creek at 2.0 2,100 400 Highway 78 10 Agua Hedionda Creek just 2.1 2,100 400 downstream of Highway 78 11 Agua Hedionda Creek just 3.0 2,700 550 upstream of confluence with Buena Creek 12 Agua Hedionda Creek just 9.4 7,000 1,600 downstream of confluence with Buena Creek at Green Oaks Ranch Concen- tration Point No. 13 14 15 [ 16 17 18 19 20 21 22 23 Location Drainage Area Sq. Mi. Agua Hedionda Creek 1.33 miles 11.6 downstream of Green Oak Ranch '•••",£•* Agua Hedionda Creek just upstream of confluence unnamed tributary at elevation 200 unnamed Tributary at elevation 200 just upstream of con- fluence with Agua Hedionda Creek Agua,;Hedionda Creek -, just'downstream of ..« confluence with ,tary AguaHedionda Creek just upstream of L C&layerj£Dam tributary Calayerailtem tributary just upstream of reservoir^; feC* '. : Calavera .-Dan;- tribu-tary justidownstream of reservoiri;i Ca: tfy^...,of confLi... Agua HeEl 12.5 3.3 15.8 17.6 3.7 3.7 5.8 23.4 Agua H entrance to 100-Year 10-Year Digarge WB^ 7 ?nn. 1,800 1,700 1,700 350 1,900 -x-v'IS 7,909' 1,900 2,300 900 1,400 9,300 550 250 300 2,100 10.500/ 2,100 2,100 8£ST ORIGINAL Concen- tration Point No. 24 Location Agua Hedionda Creek total inflow into lagoon Drainage Area Sq. ML. 30.3 30.3 100-Year Discharge Cj-.S. 10,500 10-Year Discharge C.F.S. 2,100 i 1,100 3.2 Topography and Mapping Initially, five foot contour interval, 200 scale topo- graphic maps were prepared by the Redding Office of CH2M Hill, from photographs taken on December 11, 1974. Hori- zontal and vertical control points were provided by San Diego County Department of Transportation. The Consultant marked the location of the cross-sections and top-of-road profiles required for the hydraulic analysis on copies of the topographic maps. San Lo Aerial Surveys then produced digitized cross-sections at the desired locations, and provided them to the Department of Sanitation and Flood Control, who re-organized the raw data in a format suitable for hydraulic analysis, checked the data for consistency, and produced cross-section and streambed profile plots, as well as plan view plots. The Consultant measured the significant dimensions and took photographs of all bridges and culverts. The topographic data was reviewed by the Consultant for consistency with field observations. Modifications were made to accurately model the true cross-section opening at bridges and cul- verts, and obvious errors were corrected based on field measurements. Some of die cross-sections used in the hydraulic analysis were derived from composites of informa- tion included in several of the digitized cross-sections. Following the completion of the hydraulic analysis, the water surface 'elevations of the 10-year and 100-year floods _ovUa/4 mi t-V»« nlflrt \H«J nlnfe nf *-U« *._• water surface elevations ot tne lu-year and 100-year floe marked on the plan view plots of the cross-sections, nhotoeramaetrist then located the inters«-Hnn of t-hpwerewere uicu.r«c<a «« ~~ r~.. ,~~~ r-^v-o v±. u.le uross -sect ions. The photcjgranraetrist then located the intersection of the water surfaces with the ground and located the flood plain lines using the stereo plotter. p • - '.- •, " '(• / --' ' The Consultant reviewed the photogranmetrically plottedflood plain lines, £j^ field and TOdified ^^ ^ construction or changes which had occurred subsequent the date of photography. M to 10 "-0 HYDRAULIC ANALYSIS J4-1 Use of HEC-2 and Field Observations Flooding on Telegraph Canyon Creek was analyzed using the Army Corps of Engineers' Water Surface Profile Calculation Program, HEC-2, as well as hand calculations and field investigation. The final hydraulic calculations submitted with this study were performed with the latest version of the program, dated November 1976 and updated February 1977, which include Error Corrections 1 and 2, and Modifications 50 - 53. George S. Nolte and Associates has made certain modifications to the program as released by HEC, in order to reduce the main core storage (and the cost) required to run the program. Although some subroutines are not available within this version of the program, the test data has been successfully reproduced. There have been changes in the way the program calculates Method 4 encroachments, hence, the old and the new (November 1976) versions may give slightly different results with the same data. During the study, the flood plain was inspected in the field to correct errors which are inherent in computer analysis. The photogrammetric cross-sections were compared with the actual ground surface, and adjusted to accurately represent a reach of stream. The final flood plains are based on engineering Judgment together with field observations. During the study, the flood plain was inspected in the field to correct errors which are inherent in computer analysis. The photogrammetric cross-sections were compared with the actual ground surface, and adjusted to accurately represent a reach of stream. The final flood plains are based on engineering Judgment together with field observations* 4.2 Effective Flow Areas EncroachmentsK'Were used to remove areas which were assumed to be ineffective; for the purposes of conveying flood flows.- Theselareaa occur near bridges where the channel contains;the majority of the flow, or where buildings which were not'included in the cross-section cause portions of the sectionsJtOjbe ineffective flow areas. Near major contraction8;|and:;«xpansions, certain portions of the chan- nel or overbankpay,$b«,outside the limits of flow separation.IjJJhjaa. areas, rather than providing additionalflood carrying^capacity, become low velocity eddies and cause add1*10™*,?"?.,1,0"6?' T*e limits of flow separa-tion are generally.assumed to follow a 1:1 taper at sudden 11 v&fr contractions, and a 4:1 taper at expansions. These ineffective flow areas were eliminated through the use of Method 1 encroachments, in order to maintain the proper channel velocities. In many places, the cross-section has low areas in the left or right overbanks which are sepa- rated from flood plain by artificial or natural barriers. These low spots would not provide any flood carrying capa- city unless the barrier is overtopped. Method 1 encroach- ments were used to eliminate these areas from the cross-section. 4.3 Roughness Coefficients Mannings wn" values, for the channel and the overbanks were based upon extensive field observation. The 200 Scale Orthophotography provided by the County was used to locate the limits of the various conditions affecting the "n" value in the overbanks. The values obtained by this method were quite similar to the coefficients used in the previous study of the stream by the Corps of Engineers. 4.4 Supercritical Flow It was assumed that flow would be subcritical throughout the study area. Where there was a potential for super- critical flow the water surface elevation was based on the minimum specific energy at that location (critical depth). This assumption is based on the belief that natural chan- nels cannot maintain supercritical flow for a significant distance. 4.5 Bridges and Culverts Bridges were coded in accordance with the guidelines given in the HEC-2 Users Manual, and HEC Training Document No. 6, "Application of the HEC-2 Bridge Routines". A minimum of four cross-sections were used to model each bridge; one section a short distance downstream of the bridge, two more at the downstream and upstream faces of the bridge or cul- vert, and a last section a short distance upstream of the bridge. The cross-sections at the bridge faces include only the actual flow area within the structure. Expansion and contraction losses were accounted for with variables CEHV and CCHV on the NC card. Their values were estab- lished according to the criteria in Appendix A. The entrance loss included in the orifice flow coefficient (XKOR) was calculated by a formula which accounted for the effects of entrance rounding and wingwalls. 12 In some instances, the channel has a considerably greater capacity than the bridges and structures which cross it. When these situations are encountered in the process of computer analysis, strange results can occur. The computer calculates the water surface profile working upstream, and finds that it can contain the entire flow within the chan- nel section at the downstream face of a bridge or culvert, however, due to the restriction caused by the structure, the flow is forced to weir over the road, and may be spread out over a great distance. Despite the face that the chan- nel downstream has adequate capacity, the weir flow will not re-enter the channel immediately. It will continue to move freely downstream as sheet flow until the topography or some barrier forces it back into the channel. When this situation occurs, the flow rate at the downstream face of the bridge or culvert has been reduced to the expected amount of in-channel flow; the remaining flow is not affec- ted by the channel water surface, and treated as sheet flow. The Special Bridge Routine was used for coding all of the bridges. When the Special Bridge Routine is used for bridges or culverts with piers, the computer program will choose either the Yarnell or the momentum equations to calculate losses for low flow conditions. These equations use data coded onto the SB card to describe the opening of the bridge, therefore, piers were not coded onto the GR^ cards when the Special Bridge Routine was used. 4.6 Debris It is the County's policy to account for additional flood- ing which may occur because of debris accumulation on piers or along the sides of pipe culverts by assuming that two feet of debris will accumulate on either side of each pier or pipe. When the Special Bridge Routine was used, debris was accounted for by an increase in the pressure flow coefficient (XKOR) and by increasing the width of the piers on the SB card. In those places where the Normal Bridge Routine was used, a ground section was coded which included the widened pier at the upstream face of the bridge. This methodology is explained in greater detail in Appendix B, "Application of Debris Compensation at Bridges and Culverts". 13 •J . 5.0 DESCRIPTION OF THE HEC-2 ANALYSIS The analysis of Agua Hedionda Creek was divided into three separate stream reaches. Reach 1 begins at the outfall of the stream at Agua Hedionda Lagoon. The beginning station was 1.977. The ending station for Reach 1 is station 3.609. Reach 1 is refered to as Lower Agua Hedionda throughout the accompanying computer outputs. Included with Reach 1 is Calaveras Tributary. Confluence of Cala- veras Tributary and Lower Agua Hedionda (main channel) occurs at station 3-068. For the 100-year occurence the two streams (main stream and Calaveras Tributary) are analyzed as one (a more detailed explanation can be found in Section 5.1 which follows). Reach 2 is referred to in the computer outputs as Buena Tributary. Buena Tributary begins at station 7-688 which is just downstream of confluence with Reach 3 which is referred to as Upper Agua Hedionda. Reach 2 continues to station 11.415 which is the upstream limit of work for this project. Reach 3 is referred to as Upper Agua Hedionda in the accom- panying computer outputs. Reach 3 also begins just down- stream of confluence with Reach 2 (Buena Creek) at station 7.688. Reach 3 continues upstream from the confluence with Upper Agua Hedionda main channel to station 9.733, justv upstream of the AT and SF Railroad bridge (the railroad bridge being the upstream limit of work for this project). 5.1 Reach 1 (Lower Agua Hedionda) >'*'" The starting water surface elevation was determined by using the HEC-2 normal depth routine. The starting slope of S = 0.005-.was the average for the first 1000'. Normal depth beingja.reasonably good assumption for shallow, freely mOV.ingitfflOW in the overbanks. The starting station was 1.977/ll?rom\this Starting station upstream to El Camino Real|Bpi.dgej;the channel is natural ground with grass covered|oy^|||nkflj^nd::main channel. In general, the flood- plain gen^erftfctq^,^?yery wide and slow moving. No unusual modelinglpjgg^f^jpjfurred in this area. i I Just upstij changesfre^*. Calaveras re=rrfor!(5a«^»i^rlbutiry: the area Hedionda ***} Bridge some modeling station 3-068 confluence of gUa Hedionda (main channel) u creating a separate Photogrammetric data for BEST ORIGINAL the Rancho Carlsbad Trailer park grading plans (as there were several descrepancies between the photogrametry and the grading plans). The HEC-2 analysis indicated that all of the 100-year flow could be carried in the respective channels throughout the trailer court area with only minor sheet flooding at some locations. The channel was originally designed for the 50-year occurrence, however, freeboard allowed the 100-year occurrence to pass without substantial flooding. The HEC-2 analysis assumed that all of the water enters the mobile home park within the channels, however, due to inadequate channels upsteam, some water will spill into the park itself. These flows will be shallow and confined within the streets in most cases. Although this shallow flooding is a nuisance, the depths should not exceed one foot, and no structural damage is expected to occur. 5.2 Reach 2 (Buena Tributary) Critical depth was used for the starting water surface elevation for Buena Tributary. Since downstream conditions were very steep (in fact so steep as to remove the reach immediately downstream from the study area) and trial com- putations indicated that flow was critical at confluence (the starting station 7.688), the assumption that the water surface is at critical depth at the starting station seems quite valid. At the starting station (7.688) confluence with Upper Agua Hedionda occurred. Reach 2 continued along Buena Tributary upstream through grass covered overbanks with occasional trees lining the channel. The first serious modeling prob- lem occurred bet/ween station 8.827 and station 8.970. The problem arose from the new construction of a K-Mart shop- ping center facility. The new facility added an earthen channel and two additional box culverts. The cross sec- tions were revised to reflect the new construction. Another modeling problem occurred at station 9-538. The cross sections were actually drawn the wrong direction during the initial work. These cross sections were then warped so as to be perpendicular to the direction of flow. The needed elevations for these revised sections were established by interpolating upstream and downstream eleva- tions from known points. 15 c;tti 5.3 Reach 3 (Upper Agua Hedionda) As mentioned earlier, Reach 3 begins at station 7.688 where the confluence with Buena Tributary occurs. As previously mentioned in 5.2, the starting water surface elevation was at minimum specific energy (critical depth). The streambed is characterized by grass covered overbanks Ij and channel bottom. Much of this reach is very steep, especially the upper portion between the AT and SF Railroad and Route 78. The only situation where modeling was a serious problem was the most upstream station (9.733) of the analysis. If the County's debris criteria is used, no flow would be allowed to pass through the M foot diameter culvert. Thereby, causing a depth of 30 feet behind the railroad fill. Unfortunately, it is highly questionable whether or not such an earthen structure could capacitate such depths of water without washing out. This location may well justify further investigation. 5.U Floodway Analysis Encroachment on flood plains, such as artificial fill, reduces the flood-carrying capacity and increases floodx elevations, thus increasing flood hazards in areas beyond the encroachment itself. One aspect of flood plain manage- ment involves balancing the economic gain from flood plain development against the resulting increase in flood hazard, the concept of a floodway is used as a tool to assist local communities in this aspect of flood plain management. Under this concept, the area of the 100-year flood is divi- ded into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent flood plain areas that must be kept free of encroachment in order that the 100-year flood be carried without substantial increases in flood heights. The area between the floodway and the boundary of the 100-year flood is termed the floodway fringe. The floodway fringe thus encompasses the portion of the flood plain that could be completely obstructed without increasing the water surface elevation of the 100-year flood more than one foot at any point. Typical relatinships between the floodway and the floodway fringe and their significance to flood plain development are shown in Figure 1. 16 CO oo o o ~ o "8 cc< FIGURE 1 Encroachment also reduces the valley storage contained within the channel and the flood plain, and thus tends to increase flood flow rates. This increase in flow varies from stream to stream, depending on the shape of the flood hydrograph and the actual amount of channel and overbank storage available. Storage attenuates the smaller, more frequent, floods more than large events like the 100 year flood, thus development in the flood plain has a more seri- ous effect on the frequent events. San Diego County recognizes the significance of this effect, and normally allows no encroachment on the 10-year flood plain. How- ever, at the request of the cities of Carlsbad and Vista, the flbodway computed and shown on the final flood plain maps allows encroachment within the 10-year flood plain. The floodway for this study was computed by reducing the flood carrying capability (conveyance) equally within both overbanks. HEC-2 Method 4 encroachments were used for a series of trials until a 1.0 foot rise was obtained. A final computer run using Method 1 encroachments was made to smooth the floodway boundary. 5.5 Hydraulic Capacities of Channel Improvements The capacities of the many bridges within the study area are reflected by the summarized information in Table "A". In general, all bridges hydraulically are capable of pas- sing the ten year occurrence without flooding over the bridge decks. However, many of the bridges are submerged during the one hundred year occurrence. 18 i TABLE A Bridge Name Stations 100-Year Flood Results 10-Year Flood Results Estimated Capacity Frequency (Years') Lower Agua Hedionda El Camino 2.983 Real Bridge to 3.010 Rancho 3.491 Carlsbad to Drive 3.509 Upper_Agua Hedionda Sycamore Avenue Culvert Private Road (42 Inch RCP) Private Access D/S Hwy 78 Bridge at Hwy 78 Sereno Way Culvert Santa Fe/ Mission Road-Bridge Access Road -200 Ft. U/S Santa Fe/Mission 7.725 to 7.740 8.243 to 8.248 8.844 to 8.860 9.035 to 9.069 9.151 to 9.156 9.325 to 9.335 9.383 to Adequate Adequate Inadequate (Weir Flow) Inadequate (Weir Flow) Inadequate (Weir Flow) Inadequate (Weir Flow) Inadequate (Weir Flow) Inadequate (Weir Flow) Inadequate (Weir Flow) Adequate Adequate Inadequate (Weir Flow) Inadequate (Weir Flow) Adequate Adequate Inadequate (Weir Flow) Inadequate (Weir Flow) Inadequate (Weir Flow) 100 100 5 or Less 5 or Less 10 50 5 or Less 5 or Less 5 or Less 19 1mm m 90 •2 J Bridge Name AT & SF RR Bridge Stations 9-703 to 9.733 TABLE A 100-Year Flood Results Inadequate (Weir Flow) 10-Year Flood Results Inadequate (Weir Flow) Estimated Capacity Frequency {Years) 5 or Less B Buena Tributarym d&ijjj m^ ImrL - tw M. MC- 1• j. . •. Sycamore Avenue Bridge Bridges B New K-Mart Facility Bridge B Hwy 78 Robelini Drive Culvert Santa Fe/ Mission Road Bridge AT 4 SF RR-Bridge Private Dam-U/S of RR Lakeside Road Bridge Private Dam U/S of Lakeside Rd . 7.725 to • 7.740 8.827 to 8.923 8.994 to 9.100 9.539 to 9.549 9.696 to 9.706 9.790 to 9.794 10.359 to 10.367 10.374 to 10.375 Inadequate (Weir Flow) Very Near Adequate Inadequate (Weir Flow) Inadequate (Weir Flow) Inadequate (Weir Flow) Adequate Weir Flow Inadequate (Weir Flow) Weir Flow Inadequate (Weir Flow) Adequate Adequate Inadequate (Weir Flow) Inadequate (Weir Flow) Adequate Weir Flow Inadequate (Weir Flow) Weir Flow 5 or Less 100- 50+ 5 or^Less 5 of Less 100+ N/A 5 or Less N/A 20 Appendix A Contraction and Expansion Coefficients for Typical Situations Contraction = CCHV NC Card, FIELD 4 Expansion = CEHV NC Card, FIELD 5 These coefficients are multiplied by the absolute difference in the Head due to Velocity (HV) between two sections to determine the transition loss. OLdSS = C_HV • | (EV^ - HV2)j. The com- puter automatically determines whether an expansion or contrac- tion has occurred, and selects the proper coefficient. The coefficients measure the energy lost in converting from elevation to velocity and back again. When the change in channel section Js small and distributed relatively smoothly, the coefficients should be small. When change is large and rapid, use higher coefficients. Practical analysis values for natural and improved channel are given below. CCHV CEHV Topwidth * 0. 1 0.2 - 0.2 - 0.4 < 12.5 < 20° The topwidth angle shown is the greater of the angles made by the channel banks. Topwidth angles greater than 20° require detailed analysis. Small irregularities in the channel section are accounted for by an increase in the "n" value. In general, CCHV and CEHV are a measure of the turbulence associ- ated with a transition. The following page shows the most common types of transitions and the coefficients associated with them. Skew in excess of 10° will increase losses. OJ 'IJ IJ IJ [Jfi T T T 1 I FT rr tr "WARPED" CCHV =0.10 CEHV =0.20 "CYLINDER QUADRANT" CCHV = 0.20 CEHV = 0.30 TIT I -I I e "WARPED WINGWALLS"e =0 = 45° 9= 68° CCHV = 0.25 0.30 0.35 CEHV = 0.40 0.50 0.60 T T 1=1 Wall I I T / "VERTICAL WINGWALLS1 0 = 22« CCHV = 0.30 CEHV = 0.50 0= 450 9 = 68C 0.35 "0.40 0.55 0.60 "MITRED TYPE WALL" CCHV =0.40 CEHV = 0.60 "HEADWALL" Case 1 Box width « base width Case 2 Box width < base width CCHV =0.50 CCEV =0.70 CCHV =0.60 CCEV =0.80 •T Appendix B Compensation for Debris at Bridges and Culverts It is San Diego County's normal policy to reduce the flow carry- ing capacity of bridges and culverts to account for potential debris accumulation. Two feet of debris are assumed to accumu- late on each side of all piers. For pipes, two feet are assumed to accumulate on either side of the pipe. Debris affects only the entrance conditions at a structure. Where the "Normal Bridge Routine" was used for bridges with piers, the pier was enlarged by four feet and coded on the GR cards at the upstream face of the structure. For pipes operating under low flow, the upstream face was coded on the GR and BT cards with 2' removed from either side of the pipe. Where the Special Bridge Routine was used for Low Flow condi- tions, the following variables are affected on the SB card: 1. The pier shape coefficient (XK) was set equal to 1.25. 2. Both the channel basewidth (BWC) and the pier width (BWP) were increased by 4' for each pier. These changes increase the losses calculated by either the Yar- nell Equation (Class A Low Flow) or the momentum equation (Class B Low Flow), by increasing the ratio of obstructed to unobstruc- ted area. For pressure flow conditions, an additional debris loss coeffici- ent, Kd, was added to the orfice flow coefficient (XKOR). The additional loss due to debris was assumed to be equal to half the difference in velocity head between the obstructed and unobstruc- ted bridge areas. Kd was calculated as follows: Kd = 1/2 (Au2 - Ao2).(757* where Ao and Au are the obstructed and unobstructed areas, respectively.