Laserfiche WebLink
Water Quality Management Plan (WQMP) <br />Mainplace Mall Specific Plan <br />CEQA Significance Thresholds <br />Two of the CEQA thresholds being analyzed as part of the MainPlace Mall Specific Plan <br />relate to the analysis in this Conceptual WQMP: <br />Threshold (c) Would the project substantially alter the existing drainage pattern of the site or <br />area, including through the alteration of the course of a stream or river, in a manner which would <br />result in substantial erosion or siltation on- or off -site? (CEQA Environmental Checklist: <br />Hydrology and Water Quality) <br />Santiago Creek is the receiving water for a significant portion of the Mall site and is <br />susceptible to hydromodification. Per the Orange County Technical Guidance Document <br />for the Preparation of WQMPs, <br />"The physical response of stream channels to changes in catchment runoff and sediment <br />yield caused by land use modifications is referred to as hydromodification. Unless <br />managed, hydromodfication can cause channel erosion, migration, or sedimentation, as <br />well as biologic impacts to streams. Such impacts may be associated with impairment of <br />beneficial uses and degradation of stream condition. <br />Control approaches have evolved over time, with efforts first focused on managing peak <br />flows and then on matching the peak, volume, and timing of an event hydrograph. The <br />current understanding is that the long-term frequency, magnitude, and durations of the <br />range of sediment transporting flows needs to be managed. This can be accomplished <br />using structural BMPs designed to control the duration, frequency, and magnitude of the <br />entire hydrograph from the project (i.e., flow duration control)." <br />Projects that increase the flow rates and volumes for selected storm events could negatively <br />impact downstream waterbodies that are subject to hydromodification. The TGD states: <br />"In the North Orange County permit area, HCOCs are considered to exist if any streams <br />located downstream from the project are determined to be potentially susceptible to <br />hydromodification impacts and either of the following conditions exists: <br />• Post -development runoff volume for the 2-yr, 24-hr storm exceeds the pre-development7 <br />runoff volume for the 2-yr, 24-hr storm by more than 5 percent <br />OR <br />• Time of concentration of post -development runoff for the 2-yr, 24-hr storm event <br />exceeds the time of concentration of the pre -development condition for the 2-yr, 24-hr <br />storm event by more than percent" <br />Centennial Section II <br />Conceptual Specific Plan WQMP Page 9 <br />