SU0013451 - Laserfiche WebLink

Home Browse Search

4.6 WATER RESOURCES Flooding Impacts Impact 4.6-4 All of the proposed homesites, and most of the access road,are within the 100-year floodplain and would be inundated during a 100-year flood. Flooding is a significant issue for the project since most of the site is within the 100 year flood plain of the Mokelumne River. There are two possible sources of flooding on the project site; Jahant Slough and the Mokelumne River. The proposed project will place 26 homesites within the 100-year floodplain. Nearly the entire site is �t ithin the 100-year floodplain. Depths of flooding during a 100-year event will vary with the elevations within the site. The 100-year flood velocities throughoutthe site will generally be less than one foot-per- second except in Jahant Slough or other large drainages. The large amount of floodplain storage in the Mokelumne River floodplain function to keep the flood Mages from substantially increasing by allowing the flood water to spread out into the floodplain in the form of shallow overbank flooding. The majority of the project site provides floodplain storage during the 100-year flood. Without this floodplain storage at the site, the flood elevations in the Mokelumne River would be higher. To evaluate the relative impact of the project site on the Mokelumne River floodplain,the EIR consultant used the National Weather Service dynamic wave hydraulic routing model, known as DWOPER (Dynamic Wave Operational Model). Unlike standard water surface profile models, DWOPER has the capability to account for the varying effects of floodplain storage. For the EIR analysis, a base conditions model was created by using cross-section data for the Mokelumne River floodplain provided by the Sacramento District Crops of Engineers. Following completion of the base conditions model, a post project conditions model was created by modifying the base conditions model. The modifications incorporated into the post project model included the elimination of about 100-acre-feet of floodplain storage from the Mokelumne River floodplain in the vicinity of the project site. The estimated loss of 100 acre-feet of floodplain storage due to the project was based on the estimated volume of fill at the project site along with the estimated management of the Tracy Lakes. A comparison of the results of both models indicates that the Mokelumne River floodplain is not very sensitive to the loss of this volume of flood storage. The difference in the 100-year water surface elevations computed by-the two models indicated an increase of 0.06 feet near the project site, diminishing to 0.00 about two to three miles away from the site. This significant impact can be mitigated to a less-than-significant level. 4.6-19