RES 06035 - Laserfiche WebLink

Draft Environmental Impact Report Bernal Property Phase II Specific Plan oped, vegetated lands (the existing condition) generally have lower runoff coefficients than developed lands, meaning that less water leaves the undeveloped site as runoff during normal storm events. Much of the precipitation infiltrates into the subsurface, ponds on the surface, or clings to vegetation to be evaporated later. Impervious surfaces yield nearly all rainfall as runoff because infiltration and surface storage is greatly reduced. Increased runoff volume and the increased rate of delivery of runoff to stormwater conveyances could exceed the capacity of downstream drainage ways, causing localized flooding. If not mitigated, peak discharges from the site would increase under the proposed project, potentially causing flooding at drainage inlets where culverts are undersized and overbank flooding at creek locations that are flood-prone. Therefore, the proposed project may result in two types of flooding impacts: 1) exceedance of capacity of the individual culverts draining the site and 2) cumulative flooding impacts in downstream creeks that already are prone to flooding. In addition, alteration of drainage patterns on the site could result in hydromodification impacts on downstream creeks. Hydromodification is defined as the alteration ofthe hydro- logic characteristics of coastal and noncoastal waters that, in turn, could cause degradation of water resources. In the case of a stream channel, this is the process whereby a stream bank is eroded by flowing water. This typically results in the suspension of sediments in the water course. Provision C.3.f of the County NPDES Permit specifies the enhanced require- ments for limiting "the increase of peak stormwater runoff rates." Provision C.3.f.i states that member agencies will: Manage increases in peak runoff flow and increased runoff volume for all Group 1 Projects where such increased flow and/or volume is likely to cause increased erosion of creek beds and banks, silt pollutant generation, or other impacts on beneficial uses. Such manage- ment shall be through implementation of a Hydrograph Modification Plan (HMP). The HMP, once approved by the Regional Board, shall be implemented so that post-project runoff shall not exceed estimated pre-project rates and/or durations, where the increased stormwater discharge rates and/or durations will result in increased potential for erosion or other significant adverse impacts on beneficial uses, attributable to changes in the amount and timing of runoff. The term duration in this Provision is defined as the period that flows are above a threshold that causes significant sediment transport and may cause excessive erosion damage to creeks and streams. The Alameda Countywide Clean Water Program (ACCWP) has prepared and submitted a draft Hydrograph Modification Plan (HMP), but the HMP had not been approved by the RWQCB as of the date of publication of this DEIR. Therefore, projects that could result in hydromodification impacts that are approved prior to the approval ofthe final HMP must be evaluated and impacts mitigated, as needed, on a case-by-case basis. Even though the County HMP has not yet been approved, the concepts included in the draft HMP provide an approach to mitigation of potential hydromodification impacts and can be required as part of the CEQA process in advance of the RWQCB approval of the HMP. The following is an excerpt from the ACCWP's Draft Hydrograph Modification Management Plan Part A: General Provisions For Hydromodification Management: Stormwater management facilities are often designed to maintain peak flow rates at their pre-development levels to prevent increases in the frequency of flooding due to new 88