Creating an Effective GIS Regional Integrated Watershed Planning Tool for County-Wide MS4 Compliance
A key element of the current MS4 permit requirements is the development of a model regional Watershed Master Plan to illustrate the process for implementation of the various control measures that new development will utilize through low impact development (LID), hydromodification control, and treatment control to address hydrologic conditions of concern (HCOC), and also support long-term watershed masterplanning goals. This watershed planning process includes: (1) identifying areas within the watershed that are susceptible to hydromodification including downstream erosion, and impacts to riparian and aquatic habitats, (2) identifying areas within the watershed where stormwater infiltration is not feasible based on different subsurface conditions, (3) providing a hydromodification model/tool to enable proposed land development projects to select the appropriate stormwater control measures, (4) specifying hydromodification standards for each sub-watershed based on different watershed characteristics and constraints, and (5) identifying potential locations for sub-regional and regional LID and hydromodification control facilities where site controls are hydrologically infeasible. The technical planning program involves a multi-step process which initially required development of a significant mapping database for characterizing the watershed and subsurface conditions with the aid of GIS mapping tools. The different areas within the watershed can be characterized and grouped into different categories so that the most appropriate mitigation control measures can be applied to the different categories. This provides the basis for developing a conceptual regional watershed control “strategy” based on control needs and feasibility, including use of regional control measures. An interactive GIS mapping tool was developed to assist the general public located within a watershed to be able to utilize the information database about their specific site and readily provide information in a format that would (1) answer specific questions about their site related to MS4 permit requirements and applying the WQMP, (2) assist in providing the required design information for BMP facilities, and (3) identify the regional BMP which could be utilized to mitigate their particular site. A case study of developing the Watershed Infiltration Hydromodification Management Plan (WIHMP) for the County of Orange as part of the current MS4 compliance is reviewed. The WIHMP generated illustrates how the County MS4 criteria can be applied on a watershed basis. A detailed GIS database and analytical mapping techniques were developed and various specialized tools were created to assess applicable control measures. The process of how the local public would utilize the WHIMP for projects located within a specific watershed and implementation strategy is reviewed.