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HomeMy WebLinkAbout; Buena Vista Creek Flood Plain Delineation; Hydrology Report; 1967-01-01BUENA VISTA CREEK FINAL REPORT 1.0 INTRODUCTION 1.1 Purpose of the Study. 1.2 Acknowledgements 1.3 Description of Results 2.0 AREA STUDIED 2.2 Developments on the Flood Plain 2.1 Stream and Basin Characteristics 2.3 Flood History 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 5.1 Reach 1 5.2 Reach 2 5.3 Floodway Analysis 5.4 Hydraulic Capacities of Channel Improvements FIGURE 1 APPENDIX A APP2:MDJ.X B ENGINEERING DEPT. LIBRARY City of Carlsbad 2075 Las Palmas Drive Carlsbad. CA 92009-4859 1 1 2 3 4 6 a 8 10 10 11 11 11 12 13 14 15 17 16 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 County. 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 elevation, and floodway limit information for Buena Vista Creek 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 cities of Vista, Oceanside and Carlsbad. .The majority of the work was 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. 1 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 cities of Carlsbad and Vista. 1.3 Description of Results Maps - Buena Vista Creek, sheets 366-1677,366-1671, The results are shown on the County.of San Diego Floodplain and 366-1665. Maps 370-1665 and 366-1665 do not have topo- graphic contours. 370-1671, 370-1677, 370-1683, 374-1683, 374-1689, 370-1665, 2 2.3 Flood History the study basin in 1862, 1884, 1895, 1916, 1927, 1932, ,1938 Damaging floods have occurred in the region which includes and 1942. Little information is available, but indications are that significant inundation occurred in the basin, blocking roads and flooding out farmhouses and crops. Flood damage from such floods have been relatively light since virtually no high-value developments existed on the flood plain during these floods. Nine years of record are available from the stream gage at Wildwood Park but no major floods have been recorded at this gage within such a short period of time. Information on historical floods is based on research of newspaper accounts and on interviews with local residents and offi- cials. 1No flow or stage hydrographs are available for the study area. Very few descriptions of past floods that have directly were found of flood damage in the Buena Vista drainage affected the study area are available and no photographs area. However, the following excerpts from several news- papers in the area regarding past storms should serve to depict the type of flood damage which has occurred in the area. Excerpt from the Oceanside Blade, January 22, 1916. "For a week San Diego County has been struggling in the grip of a storm and floods that have extended all over the state and that for severity and in point of damage done exceed anything experienced for years past." pally the destruction of the Bonsall bridge which is "The damage up the river from late accounts is princi- entirely gone. The San Luis Rey bridge is unhurt but north side which is passable for teams. The roadway is there is a considerable cut in the approach on the badly damaged at the Monserrate Ranch and at the Rich- man place at Bonsall. No reports are made of any damage at Pala and Supervisor Westfall who got down Thursday afternoon reports that Fallbrook was unhurt 6 nothing being damaged but small culverts. The rainfall for the storm at Fallbrook was 10.50." "The road on the north side of the river can now be used as far as Monserrate from Oceanside." Excerpt from the Carlsbad Journal, December 8, 1966. years, fell in Carlsbad in a major storm which started "Four inches of rain, half a season's supply in some 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." Oceanside Community Hospital for treatment." "In one wet-weather mishap, two men were taken to 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.'l "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." 7 3.0 METHODOLOGY 3.1 Hydrology Hydrology reports for Buena Vista Creek are available from San Diego County Department of Sanitation and Flood Con- trol. The flow rates used for this study are listed in Table 1. 3.2 Topography and Mapping graphic maps were prepared by the Redding Office of CH2M Initially, five foot contour interval, 200 scale topo- Hill, from photographs taken on December 1'1, 1974. Hori- Diego County Department of Transporation. The Consultant zontal and vertical control points were provided by San 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 kook 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 the 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 were marked on the plan view plots of the cross-sections. The photogrammetrist then located the intersection of the water surfaces with the ground and located the flood plain lines using the stereo plotter. The Consultant reviewed the photogrammetrically plotted flood plain lines in the field and modified them to include construction or changes which had occurred subsequent to the date of photography. 8 Concen- tration Point No. 2 1 3 5 6 7 FLOW RATES FOR BUENA VISTA CREEK TABLE 1 Location .Terrace Dr. tributary upstream of conflu- ence with Buena Vista Creek just downstream of Frances Drive Buena Vista Creek just upstream of conflu- ence with Terrace Dr. tributary Buena Vista Creek just downstream of confluence with Terrace Dr. tributary Buena Vista Creek just upstream of Melrose Drive Buena Vista Creek at Sunset Dr. Buena Vista Creek 0.6 miles upstream from El Camino Real and at a point directly across from Avenue de Laura Buena Vista Creek at El Camino Real Buena Vista Creek just downstream of Jeffer- son Street at entrance to lagoon Buena Vista Creek just upstream of Interstate 5 overcrossing in lagoon 9 Drainage Area SO. Mi. 1.7 2.7 4.1, 9.8 12.7 15.9 18.0 19.9 20.8 1 00-Year Discharge C.F.S. 1 , 500 2,500 4,000 7,300 8,500. .. . . .. . , ,- , :' \. 8,500 8,500 1 0-Year Discharge C.F.S. 300 500 800 1,500 1 , 700 1,700 2,000 2,000 2,000 4.0 HYDRAULIC ANALYSIS 4.1 Use of HEC-2 and Field Observations Flooding on Buena Vista Creek was analyzed using the Army Corps of Engineers Water Surface Profile Calculation Pro- gram, HEC-2, as well as hand calculations and field with this study were performed with the latest version of investigation. The final hydraulic calculations submitted the program, dated November 1976 and updated February 1977, which include Error Correction 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 avail- able within our version of the program, the test data has been successfully reporduced. There have been changes in hence, the old and the new (November 1976) versions may the way the program calculates Method 4 encroachments, 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. 4.2 Effective Flow Areas to be ineffective for the purposes of conveying flood Encroachments were used to remove areas which were assumed flows. These areas 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 sections to be ineffective flow areas. Near major contractions and expansions, certain portions of the chan- nel or overbank may be outside the limits of flow separation. These areas, rather than providing additional flood carrying capacity, become low velocity eddies and cause additional head losses. The limits of flow separa- tion are generally assumed to follow a 1:l taper at sudden contractions, and a 4:l 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- barriers. These low spots would not provide any flood rated from the flood plain by artificial or natural 10 carrying capacity unless the barrier is overtopped. Method 1 encroachments were used to eliminate these areas from the cross-section. 4.3 Roughness Coefficients Mannings ttn" 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 value in the overbanks. The values obtained by this method the limits of the various conditions affecting the nnn 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 or earthen channels cannot maintain supercritical flow for a significant distance. 4.5 Bridges and Culverts Bridges were coded in accordance with the guidelines gi;en 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. 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- however, due to the restriction caused by the structure, ne1 section at the downstream face of a bridge or culvert, 11 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 top0 raph or some barrier forces it back into the channel. Wfien ttis situation occurs, we have reduced the flow rate at the downstream face of the bridge or culvert 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 1ow.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 tvo or pipe. When the Special Bridge Routine was used, debris feet of debris will accumulate on either side of each pier 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". 12 5.0 DESCRIPTION OF THE HEC-2 ANALYSIS The analysis of Buena Vista Creek was divided into two approximately 2000 feet downstream of El Camino Real Bridge separate stream reaches. Reach 1 begins at a section and continues upstream to stream station 4.727. The down- stream limit of work for the analysis is El Camino Real Bridge. However, the additional 2000 feet downstream was analyzed to insure a more accurate water surface elevation within the study area. Reach 2 began at stream station 4.727 and continued upstream to a section at stream station 7.375. The actual limit of work for this analysis is the bridge,at Highway 78 (which is approximately 400 feet down- stream from section 7.375). The additional 400 feet was analyzed to insure that all floodplains near the limit of work were plotted accurately. 5.1 Reach 1 Reach 1 begins at stream station 1.707. The starting water surface elevation at this section was critical depth. Critical depth was used at this section after making critical through this section). Since the limits of the several hand calculations (that indicated flow was super- study were 2000 feet upstream (and critical depth occurred several times between the starting section and the down- was not extremely important. stream limit of work), the starting water surface elevation Reach 1 continues upstream to a section just downstream of the bridge at Thunder Drive (stream station 4.727). A small portion of Reach 1 is channelized with an approximate top width of 45 feet. This channelized portion is located at El Camino Real. The remaining portion of the creek several hundred feet upstream and downstream of the bridge flows between the embankment of Highway 78 and natural terrain near the end of Reach 1 where the channel turns away from the highway and flows between natural constraints (small hills) on both sides. As suggested earlier in this report streamlining was initiated at areas such as bridges where rapid expansions and contractions occurred in order to eliminate ineffective flow areas. Numerous other areas were restrained with Method 1 encroachments to eliminate ineffective areas usually caused by one of two reasons: (1) the computer automatically assuming that flow is carried by a portion of the cross section (from which the flow is restrained by existing ground) simply because that portion of the cross section is lower in elevation than the calculated water surface (2) the stream suddenly widens, however, the upstream and down- banks that would allow said overbank to convey the water stream sections do not possess the large areas in the over- fictiously indicated at the widened section. The most extensive modeling change for Reach 1 occurred between stations 3.919 and 4.075. In this area, water drained through a very narrow space between two hills at very high velocities. From these higher elevations the stream bed dropped approximately 55 feet very quickly (in approximately 800 feet). At this lower elevation the over- bank is characterized by a very deep pit (probably a former quarry). This deep pit area required streamlining vertic- ally as well as horizontally. The streamlining criteria used was the same as that previously discussed in this report. 5.2 Reach 2 The downstream limit of Reach 2 is just downstream of the bridge at Thunder Drive at stream station 4.727 which is the upstream limit of Reach 1. Reach 2 continues upstream from station 4.727 to stream station 7.375 which is the upstream limit of the study. The streambed is character- ized by natural terrain, except at the station just upstream and downstream of the bridge at Melrose Drive' way 78. where the stream is constrained by the embankment of High- A small series of high points (islands) cause divided flow between sections at stream station 5.586 and 5.965. comes back together. Because the length of channel (where Through these areas the water surface divides and quickly ded flow with equal water surface elevations was an accur- flow is divided) is so short, it was decided that the divi- ate representation of the hydraulic characteristics for these sections. Therefore, the flow was simply allowed to divide and encroachments were not required. Streamlining occurred at many of the bridges throughout the upper reach. In several bridge locations the cross sec- tions had to be revised to accurately reflect existing conditions. 14 5.3 Floodway Analysis Encroachment on flood plains, such as artificial fill, reduces the flood-carrying capacity and increases flood elevations, thus increasing flood hazards in areas beyond ment involves balancing the economic gain from flood plain the encroachment itself. One aspect of flood plain manage- 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 encomp.asses the portion of the flood plain that could be completely obstructed without increasing the water at any point. Typical relatinships between the floodway surface elevation of the 100-year flood more than one foot and the floodway fringe and their significance to flood plain development are shown in Figure 1. within the channel and the flood plain, and thus tends to Encroachment also reduces the valley storage contained. 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, maps allows encroachment within the 10-year flood plain. the floodway computed and shown on the final 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. <. 15 t \ FIGURE 1 5.4 Hydraulic Capacities of Channel Improvements are reflected by the summarized information in Table 2. The capacities of the many bridges within the study area sing the ten year occurrence without flooding over the In general, all bridges hydraulically are capable of pas- bridge decks. However, many of the bridges are submerged during the one hundred year occurrence. 17 TABLE 2 Estimated Capacity Flood Results Flood 1 0-Year Frequency Results 0 100-Year Bridne Name Stations Lower AKua Hedionda West Haymar 2.018 Bridge El Camino 2.130 Real Bridge Haymar Exten- 2.500 sion Bridge College Blvd. .4.226 Bridge Thunder Dr. 4.733 Bridge Sunset Dr. 5.274 Bridge Private Rd. 6.315 (End of Hacienda Dr. ) Breeze Hill 6.748 Drive Bridge Frontage Rd. 6.789 (Hacienda Dr. Bridge) Private 7 079 Bridge Melrose 7.281 Drive Inadequate (Weir Flow) Adequate Adequate Inadequate (Weir Flow) Inadequate (Weir Flow) Adequate Inadequate (Weir Flow) Adequate Adequate Adequate Adequate Adequate Adequate Adequate 10 100 100 50 5 O+ 100 10 Inadequate (Weir Flow) Almost Adequate 10- (23 C.F.S. Weir Flow) Weir 3531 Press 3770 Adequate 5 O+ Weir 1650 Adequate Press 5666 100- Weir 665 Press 6610. Adequate 100- 18 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. OLOSS = C-HV I (HV1 - HV2 )I . 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 +s 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 ttntt 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 loo will increase losses. CEHV = 0.20 "CYLINDER QUADRANT" CCHV = 0.20 CEHV = 0.30 "WARPED WINGWALLS" e = 220 e = 450 e= 680 CCHV = 0.25 0.30 0.35 CEHV = 0.40 0.50 0.60 "VERTICAL WINGWALLS" e = 220 CCHV = 0.30 0.35 0.40 0 = 45O 8 = 68O - CEHV = 0.50 0.55 0.60 "MITRELI TYPE WALL" CCHV = 0.40 CEHV = 0.60 "HEADWALL" - " " - - - - - - - - - - - - T Case 1 Case 2 Box width % Box width < base width CCHV = 0.50 CCHV = 0.60 CCEV = 0.70 CCEV = 0.80 _"" - "_ c Cakl c.ag q....::: base width ""L ; ""_ - - - - - - - - Appendix B Compensation for, Debris .a.t~ Bridges and Culver~cs It is San Diego County's normal policy to reduce the flow carry- debris accumulation. Two feet of debris are assumed to accumu- ing capacity of bridges and culverts to account for potential 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 Lo~.~F.low 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 ', f.o.r_-each~"pi_er. These changes increase the losses calculated by either the Yar- ne11 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- additional loss due to debris was assumed to be equal to half the ent, Kd, was added to the orfice flow coefficient (XKOR). The difference in velocity head between the obstructed and unobstruc- ted bridge areas. Kd was calculated as follows: where Ao and Au are the obstructed and unobstructed areas, respectively.