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SECTIONTHREE Background Information 3.6 FLOOD HYDROLOGY The flood hydrology developed for the 2003 Study was used for this study and was developed ~ased on SEO 1987 Rules. The flood hydrology will need to be updated for final design based on SEO 2007 Rules. The inflow peak and volume for the inflow design flood (IDF} should be less based on the revisions made to the 2007 Rules for the development of the IDF. The assumptions and parameters used to develop the IDF for the 2003 Smith Study were not presented in their study so the accuracy, of the hydrology could not be confirmed. 3.7 FIELD INVESTIGATIONS Boring logs from the 2003 Study generally indicated that subsurface material was a soft to very stiff, sandy clay underlain by a slightly to severely weatfiered shale bedrock. Bedrock appeared to be deeper to the east across the north dam. A single boring located in the south dam encountered bedrock at a depth of 24 feet. Anticipated bedrock depths, estimated from the boring logs, are 35 feet at the north dam and 30 feet at the south dam. The geotechnical data developed for the 2003 Study is presented in Appendia~ F. URS performed an additional field investigation program from April 2 to April 4, 2007 and a summary of the data is presented in Appendix F. The locations of the borings are presented on Figure 1 presented in Appendix F. Spectrum Explorati~n, Inc. of Colorado Springs, Colorado, drilled the borings under subcontract to URS. URS observed the drilling, visually classified samples, and collected samples for laboratory testing. The borings were drilled by a CME 75 truck-mounted drill rig using 4-inch diameter hollow stem augers. Each boring was completed as an open-well piezometer. Samples were collected and visually classified at 5-foot intervals. All borings except DHS were sampled by alternating use of standard split spoon and California samplers. Boring DHS was sampled using only a split spoon sampler. The samplers were advanced using a 140-pound automatic hammer falling 30 inches. Penetration resistance was recorded as the number of hainmer blows required to advance the sampler six inches. SPT resistance, or blow count, is the total number of blow required to advance a split spoon sampler from 6 to 18 inches of penetration or a California sampler from 0 ~S 3-6