Chapter 3 - Natural Resources - Marine Corps Base Camp Pendleton

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MCB Camp Pendleton, California 1954 and Leedshill-Herkenhoff in 1989. The USGS reports determined basic hydrogeologic information for the Base, and the Leedshill-Herkenhoff report increased the USGS safe yield figures. Safe yield volumes from the Leedshill-Herkenhoff study were utilized for the current Base drinking water permit for the Santa Margarita (7,640 acrefeet/year) and Las Pulgas (700 acre-feet/year) groundwater basins (Khan & Teason pers. comm. 2009). 3.1.4.2. Precipitation and Runoff Over one hundred and thirty-two years of precipitation records (initiated July 1876) for the lower area of the Base (Lake O’Neill weather station) reveal an average of 13.84 inches of precipitation per year, with a minimum of 4.51 inches (in 1960-61) and a maximum of 38.23 inches (1992-93). In the mountains at Case Springs (at 2,300 ft. elevation), the 1965 to 2005 records indicate an average precipitation of 21.86 inches, with a minimum of 6.08 (2001-02) and a maximum of 50.42 inches (1968-69). Table 3-2 shows precipitation data from weather stations located throughout the Base. TABLE 3-2. PRECIPITATION DATA FROM BASE WEATHER STATIONS Weather Station Maximum Precipitation Year Amount in Inches Minimum Precipitation Year Amount in Inches Years of Record Year of First Record Lake O’Neill 1992-93 38.23 1960-61 4.51 132 1876 Case Springs 1968-69 50.42 2001-02 6.08 43 1965 San Mateo 2004-05 39.15 1960-61 5.38 51 1957 Cristianitos 1997-98 33.75 2001-02 4.87 26 1982 Las Flores 2004-05 20.54 2001-02 3.46 24 1984 Ammo Dump 2004-05 29.78 2003-04 7.51 6 2002 Target Range 408 1997-98 26.51 2001-02 3.39 13 1995 Talega 2004-05 26.20 2003-04 7.46 6 2002 a Precipitation Year runs from 1 July to 30 June of the succeeding year -- OWR. The potential for large floods on Camp Pendleton is particularly high because of the extreme variability of precipitation and runoff. Successive soil-saturating storms in early 1993, combined with intense rainfall (6.8 inches in 24 hours) in the upper watershed, led to record flooding in the Santa Margarita River on January 16th. At the damaged gauging station at Ysidora, the estimated peak discharge of 44,000 cubic feet per second (cfs) was the highest in 68 years of record keeping, exceeding the previous record (16 February 1927) by about 12,000 cfs or 34% (Bowers 1993). The variability in annual runoff for the major streams on the Base can be seen by reviewing the minimum, maximum and average flows recorded in the annual hydrologic records 3-6 Chapter 3 – Natural Resources
Integrated Natural Resources Management Plan [March 2012 - Update] maintained for each gauging station on or near Camp Pendleton and presented in Appendix H. A discontinuous collection of flow data, however, hinders the accuracy of some of these historical records. Peak discharges will likely increase in future years due to the effects of expanded urbanization in the upper watershed. Since the Leedshill-Herkenhoff 1989 study, the Base reexamined these 100-year flow computations, particularly during the design of the Levee and Bridge project that was constructed from 1998 to 2001 to protect MCAS Camp Pendleton and the 22-Area Industrial Complex. The 100-year design was initially computed to be 57,000 cfs but a severe storm in 1998 caused a revaluation of the river’s hydrologic characteristics resulting in an increase in the estimation of the 100-year flow to 64,000 cfs. During the summer months and periods of extreme drought, the frequency of extremely low-flows within unregulated streams is particularly high throughout Camp Pendleton. It is not unusual for the San Mateo, San Onofre, and Los Flores Creeks to be dry from July through October. Historical data shows that the Santa Margarita River fails to flow to the ocean approximately 25% of the time (Leedshill & Herkenhoff 1989). 3.1.4.3. Floodplains and Surface Waters Camp Pendleton recently completed several hydrologic and hydraulic studies of the major stream systems on Base. The Santa Margarita River study was completed in July 2000 and the Las Flores, Horno, Aliso, San Mateo and San Onofre Creek studies were completed during 2004. These studies determined the flow rate that would predict 100-year flood conditions for each of the major streams on Base (Table 3-3). TABLE 3-3. FLOW RATE PREDICTING 100-YEAR FLOOD CONDITIONS FOR MAJOR DRAINAGES ON MCBCP Drainage 100-year flow (cfs) Author/Date Santa WEST Consultants 64,000 Margarita 2000 Aliso 2,659 WEST Consultants 2004 Las Flores 7,803 WEST Consultants 2004 Horno 1,404 WEST Consultants 2004 San Onofre 14,158 URS 2004 San Mateo 56,697 URS 2004 Chapter 3 – Natural Resources 3-7
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