Report of Data Pertinent to Lake San Cristobal

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-4- Clayey sand was encountered in Borings 2 and 3 in layers ranging from 5 to 22 feet thick. Penetration resistance values range from 4 to 50 blows per foot. Materials having penetration resistance values in this range are classified as loose to dense. " Based on the field penetration resistance test results an allowable bearing pressure of 2,000 psf was recommended by Chen- Northern, Inc. This value appears appropriate for the medium dense sands and gravels on which the bridge peirs were founded, but is considered an upper limit based on the variability of the soil profile with depth The clay layer on the other hand encountered in the bottom of the streambed, boring No. 2, would have an allowable bearing capacity of about 1,000 psf for a saturated condition. The above described subsurface profile reveals two factors which should be considered in selecting a repair alternative. First, the silty, sandy gravel and the clayey sand layers must be protected if exposed because of their erosive nature. Second, the bearing capacity of the clay layer in the channel bottom will dictate minimum foundation pressures for any type structure placed across the channel to curb erosion. . IR •.t1 In order to ascertain the level of flooding the current outlet channel can sustain without bank overflow, a hydrologic model of a synthetic storm was evaluated. The HEC -1 Flood Hydrograph Model was used to compute the flood hydrograph and simulate routing of the flood through Lake San Cristobal. The hyrologic aspects of Lake San Cristobal and the tributary drainage area are not unlike situations confronting evaluation of man -made dams constructed on live streams. Therefore, setup of the model was anal ogous to a dam with an uncontrolled, open channel type spillway. The outflow channel from the lake was modeled as a spillway and the overbank area on each side as the crest of a dam Although, several different magnitudes of floods could be evaluated, the 100 -year, 24 -hour storm was selected to determine the outflow channel capacity. For instance, a dam at this location the size of the Slumgullion slide mass would require evaluation of a more extreme event.