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.
