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HomeMy WebLinkAbout3190; San Marcos Creek; San Marcos Creek; 1971-04-01FLOOD PLAIN INFORMATION SAN MARCOS CREEK VICINITY OF SAN MARCOS SAN DIEGO COUNTY CALIFORNIA O PRKPAKED FOR SAN DIEGO COUNTY- HY COUPS OF ENGINEEKS. U- S. ARMY IX)8 ANGELES DISTRICT. CALIFORNIA APRIL 1971 CONTENTS Page PREFACE ' BACKGROUND INFORMATION Settlement •^ The Streams and Their Valleys ^ Developments on the Flood Plain FLOOD SITUATION ^ Data Sources and Records Flood Season and Flood Characteristics ^ Factors Affecting Flooding ^ Flood damage prevention measures ^ Obstructions to floodflows ^ Other factors affecting flooding ^2 Flood Warnings and Forecasting Flood Fighting and Emergency Evacuation Plans 13 PAST FLOODS Summary of Historical Floods 15 Flood Records 15 Flood Descriptions FUTURE FLOODS 1Q intermediate Regional Flood Standard Project Flood ( PLATE I BACKGROUND INFORMATION Settlement The San Marcos area lies In the northwest section of San Diego County and is about 35 miles north of the City of San Diego. Inhabited lands in the study area include the City of San Marcos and several small communities in unincorporated areas, such as Lake San fVlarcos, La Costa, and Twin Oaks. In 1940, the area around and including San Marcos and its neighboring city to the east, Escondido, was primarily rural and had about 9500 residents. This area has experienced rapid growth since 1940 and its population had increased to about 60,000 by 1970. Future population growth is projected to much higher levels. For example, the La Costa development alone has an ultimate planned population of 25,000, which is projected to be reached within 10 to 15 years. Planners of the City of San Marcos project the population of that city to be four times Its 1970 total of 10,000 by the ^ year 1990. They also project the Escondido-San Marcos area to increase In population to 140,000 by 1990. Flat developable farmland, much of which is in the flood plain, will come under increasing demand for residential use. The Streams and Their Valleys San Marcos Creek, which drains an area lying just north of the San Dieguito River basin originates In the coastal range of mountains north and east of San Marcos and empties Into the Batiquitos Lagoon about 2.6 miles east of the Pacific Ocean. Encinitas Creek, a major tributary, originates In the mountains southwest of San Marcos and joins San Marcos Creek as It empties Into Batiquitos Lagoon. The East Branch of San Marcos Creek originates In the mountains east of San Marcos and joins San Marcos Creek in the City of San Marcos. The entire drainage area totals 46 square miles and ranges In elevation from sea level to slightly over 1700 feet in the Merriam Mountain range northeast of San Marcos. Natural vegetation Is abundant and consists mostly of native grasses, brush, and trees. The climate varies from warm summers to cool winters; extremes in temperature are rare. Mean seasonal precipitation for the drainage area ranges from about 10 inches near the coast to about 16 inches In higher mountains and averages about 13.5 Inches over the total drainage area. Most of the precipitation occurs during the months of December through March. Snow Is not considered an Important contributing factor to runoff. Drainage areas upstream from several points along San Marcos Creek and its tributaries are shown In table 1. TABLE 1 DRAINAGE AREAS Location Drainage area sq. miles San Marcos Creek At mouth into Pacific Ocean 52.3 Upstream from El Camlno Real 38.3 Upstream from Rancho Santa Fe Road 29.4 Upstream from Discovery Street 23.0 Upstream from State Hwy. 78 17.4 Upstream from East Branch of San Marcos Creek 11.3 Upstream from Olive Street 5.5 Encinitas Creek Upstream from confluence with San Marcos Creek 7.4 East Branch of San Marcos Creek Upstream from confluence with San Marcos Creek 6.1 The stream gradient ranges from an average of 10 feet per mile near the mouth to about 600 feet per mile in the headwaters. San Marcos Creek has a small and not well defined channel upstream from Lake San Marcos. Below that point, the channel is well defined, steep, and rocky to the La Costa development where It becomes small and not well defined again to Batiquitos Lagoon. The flood plain Is broad upstream from Lake San Marcos. From Lake San Marcos Dam to the La Costa development, there Is virtually no flood plain due to the steep, well defined channel. Through the La Costa development to the Batiquitos Lagoon, the flood plain is again broad. During large floods, flows exceed the channel capacity and inundate parts of the broad flood plain. Batiquitos Lagoon extends from the downstream limit of San Marcos Creek, namely the Pacific Ocean, as far as El Camino Real Road, some 2.6 miles upstream. It comprises about 600 acres of area and would provide considerable storage during large floods. Developments on the Flood Plain Except for the steep canyon reach of San Marcos Creek from the La Costa development upstream to the Lake San Marcos Dam, most of the land in the flood plain is developed. At present, the bulk of this development is limited to agricultural uses, such as crops, orchards, and grazing. Limited heavy agricultural uses-chicken farms and dairy operations—and limited residential uses exist on the edges of the flood plain. State Highway 78 (the Anza Freeway) and the Atchison, Topeka and Santa Fe Railroad cross the flood plain in San Marcos. Other developments existing on the flood plain include: (1) several local street bridges and culverts, (2) golf courses at the La Costa and Lake San Marcos developments, (3) several foot and golf cart bridges on these golf courses, (4) corrals and horse training facilities at the La Costa development, (5) a restaurant and shopping center at the Lake San Marcos development, and (6) an inactive sewage treatment plant downstream from Rancho Santa Fe Road. Planned uses for the flood plain in the San Marcos area include recreational, industrial, retail commercial, agricultural, and four classes of residential use, namely: very low density, low density, medium density, and high density. Uses for the flood plain In the La Costa and Lake San Marcos developments include recreational, horse training, and limited residential and commercial. There are several soil conservation (earth-graded) channels through flat farmlands north of the City of San Marcos. These channels are smail and are of very little significance 1^ during either of the floods being considered. Private developers have constructed about 1,000 feet of Improved earth channel just upstream from Discovery Street, in conjunction with a mobile home development. The channel is adequate to convey the Intermediate Regional Flood; the bank futl capacity of the channel Is approximately equal to the Standard Project Flood. There are also about 2,000 feet of Improved earth channel downstream of Discovery Street through Lake San Marcos Golf Course. This channel has been constructed and Is maintained by the Lake San Marcos Development. As shown on Plate 9, either the Intermediate Regional Flood or the Standard Project Flood would overflow the channel banks. Scour would occur during either the Intermediate Regional or Standard Project floods In both of these channels since average velocities of about 12 feet per second would be attained. There is a concrete arch dam (shown in figure 8) forming Lake San Marcos southwest of the City of San Marcos. Created in 1952, the lake is used for recreational purposes. It is assumed that this reservoir would be full to the spillway crest during the occurrence of the Intermediate Regional Flood and the Standard Project Flood. The dam would therefore have no effect in reducing the magnitude of the study floods in the San Marcos Creek area. Obstructions to floodflows - Natural obstructions to floodflow include brush and other vegetation growing along the streambanks In floodway areas. Man-made obstructions include all bridges and culverts listed in table 3 on page 23 plus all bridges and culverts within the study area along the East Branch of San Marcos Creek and along Encinitas Creek. There are also several foot and golf cart bridges In the La Costa and Lake San Marcos developments. During floods, brush growing In floodways impedes floodflows, thus creating backwater and Increased flood heights. Brush may be washed out and carried downstream to collect on bridges and on other obstructions to flow, creating a damming effect and a raised water surface. As floodflow Increases, masses of debris break loose and a wall of water and debris surges downstream until another obstruction is encountered Debris may collect against a bridge until the load exceeds Its structural capability and the bridge Is destroyed. Limited capacity of obstructive culverts, a debris plug at the culvert mouth, or a combination of these factors could retard floodflows and resutt In flooding upstream, in erosion around the culvert entrance, and In possible damage to the overlying roadbed. In general, obstructions restrict floodflows and cause overbank flows, unpredictable areas of flooding, destruction of, or damage to, bridges and culverts, and increased velocity of flow immediately downstream. Representative obstructions to floodflows are shown In figures 1 through 12. Table 3 on page 23 lists major obstructions to floodflows on San Marcos Creek. FLOOD SITUATION Data Sources and Records There are no streamflow records for San Marcos Creek. Information on past floods was obtained primarily from newspaper files and from Interviews with local officials and residents. Other streamflow data were devetoped from records of basins with characteristics similar to those of San Marcos Creek. The stream gage Installed at Discovery Street in 1967 has no recorded data yet. Maps and tables prepared for this report are based on topographic maps that vvere supplied by the San Diego County Department of Sanitation and Flood Control. These topographic maps were produced from 1958 and 1961 aerial photographs and were modified to reflect existing topographic conditions. Structural and survey data on bridges and culverts were supplied by the California Division of Highways and the San Dlego County Department of Sanitation and Flood Control. Flood Season and Flood Characteristics Most of the precipitation in the San Marcos Creek drainage basin occurs during the months of December through March. Rainless periods of several months during the summer are not uncommon. Three types of storms produce precipitation in the area: general winter storms, thunderstorms, and tropical hurricanes. Little streamflow occurs, except during and Immediately after rains, because climatic and drainage area characteristics are not conducive to continuous runoff. Streamflow increases rapidly in response to effective rainfall. High intensity rainfall, In combination with steep gradients and the effects of preceding fires, results In intense debris-laden floods on the flood plain. Factors Affecting Flooding Flood damage prevention measures - At present there are no Federal flood control projects In the study reach. FUTURE FLOODS Although floods of the same magnitude as those that have occurred In the past could recur In the future, discussion of the future floods In the report Is limited primarily to those that have been designated as the Intermediate Regional and Standard Project Floods. The Standard Project Flood would be larger and would occur less frequently than the Intermediate Regional Flood, and both of these floods would be greater than any known to have occurred In the past. A Standard Project Flood would be a rare event, but could reasonably be expected to occur in the future. Selection of the Intermediate Regional and Standard Project Floods was based on hydrologic computations (correlation of records of similar drainage basins) and consideration of pertinent meteorologic and physiographic conditions. As previously Indicated, floods caused by severe local storms can affect large areas of the flood plain, result In greater flood damage, and create greater hazards to people than general winter storms. Therefore, the future floods discussed herein are of the severe local storm type. During floods, debris collecting on bridges and culverts could decrease their carrying capacity and cause greater water depths (backwater effect) upstream from these structures. The occurrence and amount of debris are indeterminate factors; however, a limited amount of debris was considered In preparing profiles of the Intermediate Regional and Standard Project Floods. Similarly, the maps of flooded areas show the backwater effect of obstructive bridges and culverts, and reflect increased water surface elevation that could be caused by limited amounts of debris collecting against the structures. Intermediate Regional Flood The Intermediate Regional Flood is one that could occur about once In 100 years on the average, although it may occur In any year or more than once in one year. Usually 19 the peak flow of such a flood is developed from statistical analyses of streamflow and precipitation records and the runoff characteristics of the stream basin. Since there are no streamflow records for San Marcos Creek, it was necessary to analyze precipitation and streamflow records of another stream basin having hydrologic, meteorologic, and physiographic characteristics similar to those of the San Marcos Creek basin. Studies were made to transpose the information thus derived to the San Marcos Creek basin and to compute peak flows for the Intermediate Regional Flood. Peak flows thus developed for the Intermediate Regional Flood at selected points In the study area are shown In table 2. The stream-gaging station for De Luz Creek near Fallbrook in north San Dlego County yielded the data for transposition to San Marcos Creek. Standard Project Flood For the purpose of determining peak flows of the Standard Project Flood, a standard project storm'^' was derived for the San Marcos Creek basin from statistical analyses of other stream basins having similar meteorologic characteristics. Studies were made to transpose storm data to the San Marcos Creek basin and compute peak flows for the Standard Project Flood. Peak flows thus developed for the Standard Project Flood at selected points In the study area are shown In table 2. Hazards of Large Floods The amount and extent of damage caused by any flood depends on the topography of the area flooded, depth and duration of flooding, velocity of flow, and developments on the ftood plain. An Intermediate Regional or Standard Project Flood on San Marcos Creek would result In inundation of residential, commercial, agricultural, recreational, and industrial sections in the San Marcos Creek basin. iai "^he most seve'e combmation of -pe+e-o'ogic cona^'ions -easonaoly cra-acie'-.si'c o' rne geograph.c -egion excluding exTemely -a'e como national 20 TABLE 2 PEAK FLOWS FOR INTERMEDIATE REGIONAL AND STANDARD PROJECT FLOODS Stream and Location Miles upstream from mouth Drainage area sq. mi. 1 ntermedlate Standard Regional Flood Project Flood cfs cfs San Marcos Creek At mouth into Pacific Ocean ao 52.3 15,000 23,000 Upstream from El Camino Real 2.7 38.3 12,000 19,000 Upstream from Rancho Santa Fe Road 6.3 29.4 13,000 20,000 Upstream from Discovery Street 9.5 23.0 12,000 18,000 Upstream from State Hwy. 78 11.7 17.4 10,500 16,000 Upstream from East Branch of San Marcos Creek 11.8 11.3 7,400 11,000 Upstream from Olive Street 14.9 5.5 5,500 8,000 Encinitas Creek Upstream from confluence with San Marcos Creek ao 7.4 4,500 7,000 East Branch of San Marcos Creek Upstream from confluence with San Marcos Creek ao 6.1 5,500 8,000 21 Deep floodwater flowing at a high velocity and carrying floating debris would create conditions hazardous to persons and vehicles attempting to cross flooded areas. In general, floodwater 3 or more feet deep and flowing at a velocity of 3 or more feet per second could sweep a person off his feet, thus creating definite dangers of injury or drowning. Rapidly rising and swiftly flowing floodwater may trap persons in homes that are ultimately destroyed or in vehicles that are ultimately submerged. Decaying flood-deposited garbage or other organic materials could create health hazards. Isolation of areas by floodwater could create hazards in terms of medical, fire, or law enforcement emergencies. Flooded areas and flood damage - The areas along San Marcos Creek, along the East Branch of San Marcos Creek, and along Encinitas Creek that would be flooded by a Standard Project Flood are shown on plate 2, which is also an index map to plates 3 through 17. Areas that would be flooded by the Intermediate Regional and Standard Project Floods are shown In detail on plates 3 through 17. These areas include recreational, industrial, agricultural, and residential sections and the associated streets, roads, and public utilities In the San Marcos Creek basin. Moderate damage to homes, mobile homes, recreational and commercial structures, and a railroad bridge would occur during an Intermediate Regional Flood. Due to the wider flood plain, a greater depth of flooding, a higher velocity flow, and a longer duration of flooding during a Standard Project Flood, damage would be more severe than during an Intermediate Regional Flood, Streets, bridges, culverts, and public utilities would be severely damaged by high velocity floodflows. Extensive deposits of silt and debris would occur in many parts of the flooded areas. Plates 18 through 2 7show water surface profiles of the Intermediate Regional and Standard Project Floods. Depth of flow in the channel can be estimated from these illustrations. Typical cross sections of the flood plain at selected locations, together with the water surface elevation and lateral extent of ^ the Intermediate Regional and Standard Project Floods, are shown on plates 28 and 29, Limtts of overflow indicated on plates 3 through 17 may vary from actual locations on the ground due to map scale, deposition, erosion, inaccuracy of original topographic maps, or other causes. As shown on Plates 10 and 11, flood water ponds behind the State Highway 78 culvert causing a separation of flow down Encinitas Road, The divergent flows join again at Las Posas Road. Obstructions - Numerous bridges and culverts cross the study reaches of San Marcos Creek and its tributaries: these Include several foot bridges and golf cart crossings in the La Costa development, two railroad bridges, one major Interstate freeway bridge, two major highway bridges and numerous other small bridges and culverts. Table 3 shows the water surface elevations at some of these crossings for the Intermediate Regional and Standard Project Floods. 22 As shown in table 3, all of the bridges except the Interstate Highway 5 bridge, the railroad bridge over Batiquitos Lagoon, the two major highway bridges and a privately owned bridge over Lake San Marcos would be overtopped by either the Intermediate Regional Flood or the Standard Project Flood. The effects of the obstructions to flow are shown on plates 18 through 27. TABLE 3 CULVERTS AND BRIDGES ACROSS SAN MARCOS CREEK Identification Location Pacific Coast Hwy bridge (southbound) Elevation (b) Streambed Low Chord Roadway IRF* SPF (c) (d) (e) (e) Ol ao 11.8 14.2 4.3 6.1 Pacific Coast Hwy bridge (northbound) ai ao IOI 12.9 4.4 6.2 Atchison, Topeka, and Santa Fe railroad bridge 02 ao 15.0 20.0 5.4 7.5 Interstate Hwy 5 bridge 07 ao 21.7 26.2 6.0 8.3 El Camlno Real bridge 2.6 2.9 9.8 11.5 13.2 13.7 Rancho Santa Fe Road bridge 6.3 323.0 335.0 337.5 338.5 339.3 Lake San Marcos bridge (privately owned) 8.9 484.4 504.3 507.9 501.0 502.8 Discovery Street bridge 9.4 5005 509.6 510.9 514.4 516.2 Las Posas Road (culvert) 103 527.4 532.9 534.9 536.8 537.3 State Hwy 78 (culvert) 11.2 543.3 551.3 559.3 56a8 561.3 Atchison, Topeka, and Santa Fe railroad bridge 12.0 567.5 575.0 579.3 5809 581.4 Olive Street bridge 14.9 6907 695.7 697.0 699.3 700.5 Sycamore Drive bridge 15.2 708.8 714.5 716.0 716.2 716.9 (a) Miles from Pacific Ocean. (b) All elevations are In feet, mean sea level datum. (c) Elevation of bottom of bridge structure or top of culvert. (d) Average elevation. (e) Computed water surface elevation based on estimated flow and existing channel and structures. * Intermediate Regional Fiood. Standard Project Flood. 23 Velocities of flow - Occurrence of the Intermediate Regional or Standard Project Flood would result in the flows shown in table 2. During an Intermediate Regional Flood, the average velocity of main channel flow upstream from Lake San Marcos would range from 5 to 8 feet per second. Water flowing at this rate Is capable of causing erosion to streambanks and transporting large rocks. Through the La Costa Golf Course, the average velocity of main channel flow would be about 6 feet per second. In the steep canyon section between the La Costa and Lake San Marcos developments, velocities could reach 30 feet per second; this would cause severe eroston and would dislodge and move large boulders. It is expected that velocity of flow during a Standard Project Flood would be slightly higher than that during an Intermediate Regional Flood. In the broad flood plain areas, the velocity of flow in the overbanks would range from 1 to 2 feet per second during both floods. Water flowing at about 2 feet per second or less would deposit debris and silt. Table 4 below shows average velocities at several points in the basin. Stream and location TABLE 4 VELOCITY OF FLOW I ntermediate Standard »/liles upstream Regional Flood Project Flood from mouth Channel Overbank Channel Overbank San Marcos Creek Upstream from El Camino Real bridge 3.1 5 2 6 2 Downstream from Lake San Marcos Dam 5.7 20 5 24 8 Upstream from Discovery Street 9.7 8 1 10 2 Upstream from Mission Road 12-5 8 1 9 1 Downstream from Olive Street 14.6 5 2 6 2 Near upstream limit of study 154 6 1 7 2 Encinitas Creek Upstream from La Costa Avenue 07 8 2 9 2 East Branch of San Marcos Creek Downstream from Valprede Road as 7 1 9 2 24 Rates of rise and duration of flooding — Intense rainfall from severe local storms and general winter storms centered over the upper reaches of the basin collects rapidly as surface runoff and reaches the vicinity of San Marcos within a short time after the beginning of the storm. The channel of San Marcos Creek has a small cross sectional area, and any large flow causes the stream to rise rapidly and to overflow its banks at several locations In the study area. Most local storms last for only a few hours. Consequently, the total volume of runoff Is relatively small, and the duration of overbank flows Is short. For the Intermediate Regional and Standard Project Floods at the Discovery Street bridge, table 5 gives the height of rise (from critical stage level to maximum floodflow level), time of rise (time period corresponding to height of rise), and duration of critical stage (period of time flooding is above critical stage level). TABLE 5 RATES OF RISE AND DURATION (at Discovery Street bridge) Height Time Duration of Flood of rise of rise critical stage ft hrs hrs Intermediate Regional 3.5 1.0 2.0 Standard Project 5.3 1.5 3.0 On the average, the water surface elevation of the Standard Project Flood on San Marcos Creek would be about 1 foot higher than that of the Intermediate Regional Flood. On the tributaries of San Marcos Creek, the water surface elevation of the Standard Project Flood would also average about 1 foot higher than that of the Intermediate Regional Flood. Photographs, future flood heights — The levels that the Intermediate Regional and Standard Project Floods are expected to reach at various locations on the San Marcos Creek flood plain are Indicated on the following photographs. 25 GLOSSARY Critical Stage. The point at which floodflows overtop the natural or artificial banks along any reach of a stream. Critical Stage Level. The elevation that corresponds to critical stage. Flood. An overflow of lands not normally covered by water and that are used or usable by man. Floods have two essential characteristics: The inundation of land Is temporary; and the land is adjacent to and inundated by overflow from a river or a stream, an ocean or a lake or other body of standing water. Normally, a "flood" is considered as any temporary rise in streamflow or stage (not the ponding of surface water) that results In significant adverse effects in the vicinity. Adverse effects may include damages from overflow of land areas, temporary backwater effects in sewers and local drainage channels, creation of unsanitary conditions or other unfavorable situations by deposition of materials In stream channels during flood recessions, rise of ground water coincident with Increased streamflow, and other problems. Flood Crest. The maximum stage or elevation reached by the waters of a flood at a given location. Flood Peak. The maximum instantaneous discharge of a flood at a given location. It usually occurs at or near the time of the flood crest. Flood Plain. The relatively flat area or lowlands adjoining the channel of a river, a stream, or a watercourse, an ocean, or a lake or other body of standing water that have been or may be covered by floodwater. Flood Profile. A graph showing the relationship of water surface elevation to location, the latter generally expressed as distance above mouth for a stream of water flowing In an open channel. It is generally drawn to show surface elevation for the crest of a specific flood, but may be prepared for conditions at a given time or stage. Flood Stage. The stage or elevation at which overflow of the natural banks of a stream or body of water begins in the reach or area in which the elevation is measured. Floodway. The channel of the stream and that part of the flood plain that would be used to carry floodflows. 29 General Winter Storm. A widespread storm usually occurring in the months of December through March, characterized by heavy and prolonged rainfall over a large area. Thunderstorm. A high intensity, convective type rainstorm of short duration that Is characterized by extremely heavy rainfall. As used In this report, "severe local storm" and "thunderstorm" are essentially synonymous. Intermediate Regional Flood. A flood having an average frequency of occurrence of once in 100 years, although the flood may occur In any year or more than once In one year. It is based on statistical analyses of streamflow records available for the watershed and analyses of rainfall and runoff characteristics in the general region of the watershed. Standard Project Flood. The flood that may be expected from the most severe combination of meteorologic and hydrologic conditions that is considered reasonably characteristic of the geographic area in which the drainage basin is located, excluding extremely rare combinations. Peak discharges for these floods are generally about 40 to 60 percent of the Probable Maximum Floods for the same basins. As used by the Corps of Engineers, Standard Project Floods are intended as practicable expressions of the degree of protection that should be sought In the design of flood control works, the failure of which might be disastrous. Probable Maximum Flood. A hypothetical flood representing the most severe flood with respect to volume, concentration of runoff, and peak discharge that may be expected from a combination of the most severe meteorologic and hydrologic conditions In the region. 30 LEGEND: OVIRROW UMITS IIKTERMEDIATC I REGIONAL R.OOD STANDARD PRO JEO aooo 2.50 ASOVE MOUTH [T] CROSS SECTION 400 GROUND ELEVATION IN FEET ASOVf MEAN SEA LEVB. CONTOUR IKTERVAC 'i FEET NOTES: UMrrs OF OVERFLOW INDICATED MAY VARY FROM ACTUAL LOCATIONS ON GROUND, AS EXPLAINED IN THE REPORT. DATE OF TOPOGRAPWY 1961 BASE TOPOGRAPHIC MAPS FURNISHED RY DEPARTMENT OF SANITATION & FLOOD CONTROL SAN DIEGO COUNTY, CALIFORNIA. CORPS OF ENGINEERS, U. S. ARMY LOS ANGELES DISTRia, CALIFORNIA FIOODED AREAS SAN MARCOS CREEK SAN DIEGO COUNTY, CALIFORNIA 400 FEET APRIL 1971 PLATE 7 520 480 440 aoo MUES AftOVE MOUTH 8.75 LEGEND Standord Protect Hood btvrnMdiot* Regional Hood CORPS OF ENGINEERS, U. S, ARMY LOS ANGELES DISTRia, CAUFORNIA FLOOD PROFILES SAN MARCOS CREEK SAN DIEGO COUNTY, CALIFORNIA APRIL 1971 PLATE