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r 3. The design of ` on-site sewage disposal systems for this development is contingent upon the results of site specific percolation tests. The San Joaquin County Ordinance governing on-site sewage disposal systems requires a minimum of one percolation test for each proposed leachfield or seepage pit area in this type of subdivision. Due to the relatively stiff soils encountered at the site and the potential for variations in soil permeability , the following mitigations are recommended : a'. A_„ minimum of three percolation tests abeper- .'rmed at, + each leachfield site. These tests arse to be,. ma`de; at ;the d pth of tf�e proposed leac`.hf.eld system. If these test resuhts do not comphy with County standards, an fal�te.r_nat-ive to -septic tanks/leachfield systems would be r required . � • b. High concentrations of nitrate-nitrogen may result from development at the proposed density . Preliminary F` analysis indicates that increasing lot sizes to two i acres would reduce this concentration from 6. 5 to 5. 0 Mg/l . Drainage and Flooding t . . r 1 . Environmental Sett 'ng The project site has relatively uniform mild slopes of about 0.5%. Elevations range approximately from 42 feet in the n northeast to 38 feet in the southwest. Linder existing conditions, storm runoff either percolates into the soil or flows to the southwest towards Highway 99 and the Calaveras River. There are no major drainage facilities on the project site and with the exception of minor ditches or swales, most surface runoff is primarily sheetflow. Mean annual rainfall is 13. 77 inches for 30 years of County records. The entire project site is outside the 100-year flood zone as defined by the Federal Emergency Management Agency (FEMA) and as shown in Figure 95 . The Calaveras River is located approximately one-half mile south of the project. and 'd-rains nearly 500 square ` miles upstream of the project. The 100-year--peak flow in this reach of the river is over 800 cubic feet per sec-and--Cefs) with } velocities less than 3 feet per second ( fps) , Lower Mosher ;Creek F is located approximately 1 mile north of the project .and drains approximately 10 square miles at Highway 99 . The peak discharge at this location is 2400 cfs with velocities less than 2 fps. Areas immediately north of the project are susceptible to 100-year flooding from lower Mosher Creek overflowing its banks. Flows in Calaveras Creek are generally contained within its banks during the 100-year event. Historically , the most damaging recent flood event was in December 1955 . During this storm, flows in the f Calaveras River were estimated to be a 70-year event. The San 45