AQUA TERRA Consultants
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Otay River Watershed Management Plan (WMP) and Special Area Management Plan (SAMP)

Clients: Aspen Environmental Group, Agoura Hills, CA

In 2003 the San Diego Regional Water Quality Board issued a new National Pollutant Discharge Elimination system general stormwater permit for the San Diego Region. This permit requires that a watershed planning approach be implemented for the Region. In order to implement this strategy a number of watershed-focused management plans are under development, including the Otay River Watershed Management Plan (WMP). As a component of the WMP, AQUA TERRA developed a locally based application of the PLOAD tool contained in USEPA’s BASINS modeling system. To overcome a limitation of PLOAD that requires the use of a single value for precipitation in all subbasins, AQUA TERRA reprogrammed the PLOAD calculations within an Excel spreadsheet to allow the precipitation to vary by subbasin throughout the watershed. We refer to this spreadsheet as the Otay Watershed Pollutant Loading (OWPL) Tool.

The model calculates annual nonpoint loads using the EPA’s Simple Method approach1, which employs runoff coefficients and event mean concentrations (EMCs) to calculate the annual loads. Point sources and best management practices (BMPs), which can reduce nonpoint and/or point source loads in a specified area, may optionally be included in the analysis. Model output includes the constituent loads and loading rates for each land use within each subbasin, along with subbasin total loads, average loading rates, and annual mean concentrations.

The objective of this effort was to quantify pollutant loadings from the contributing watershed and to explore the impacts that implementing alternative management strategies might have on future loadings and resulting water quality. The OWPL model was developed for continued use by the County and the County’s team of consultants, and the OWPL results were incorporated into a GIS-based (ArcInfo) watershed decision support tool that was concurrently developed by the consulting team.

The Simple Method is an empirical approach developed for estimating pollutant export from urban development sites. The Simple Method has been endorsed by EPA as a viable screening tool for NPDES stormwater projects.

AQUA TERRA’s effort included using OWPL to assess loading impacts for up to five watershed scenarios developed by the consulting team in conjunction with the County; providing written input on the OWPL approach and application for inclusion in the WMP; providing input on the assessment results to support the development of watershed protection, restoration and management strategies; providing input to support the development of a water quality monitoring strategy; participating in public meetings; and finalizing the WMP report. The County of San Diego Board of Supervisors adopted the Otay River Watershed Management Plan in May 2006.

The Los Angeles District Corps of Engineers is developing a Special Area Management Plan (SAMP) for the Otay River Basin. The purpose of the SAMP is to develop and implement a watershed-wide aquatic resource management plan and implementation program, which will include preservation, enhancement and restoration of aquatic resources, while allowing reasonable and responsible economic development and activities. In support of the supplementary studies for the SAMP, a Hydrological Simulation Program – FORTRAN (HSPF) model is being developed and applied by the Corps (ERDC). It is anticipated that the Corp’s modeling effort can provide information useful to the County in the context of flood control, planning, erosion and sediment transport assessment, point and nonpoint source pollution assessment, and development of best management practices (BMPs) and TMDLs. AQUA TERRA is providing technical assistance to the County’s consultant team and the County in evaluating the Corp’s hydrologic and water quality modeling efforts. This evaluation includes preparation for and participation in meetings, assistance in preparing and reviewing meeting notes, and preparing technical memoranda and reports.

1: The Simple Method is an empirical approach developed for estimating pollutant export from urban development sites. The Simple Method has been endorsed by EPA as a viable screening tool for NPDES stormwater projects.

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