Innovative Modeling Techniques for Watershed Planning

The Fairfax County Virginia Stormwater Planning Division is in the process of developing watershed plans for all 30 watersheds in the County. Early on in the project, Camp Dresser and McKee (CDM) reviewed the available GIS data, modeling needs, and available models to develop modeling protocols to be used in the preparation of the watershed plans. The goal was to develop accurate models appropriate for countywide watershed management planning within the budgets available to do this work. This paper presents the comprehensive GIS data that the County has available to do the modeling and the procedures that were developed for converting the GIS data to model inputs. Hydrologic, water quality, and stream models, GIS interface programs, and graphic display programs were reviewed. Based on this review, the County is using a combination of SWMM-RUNOFF, SWMM-TRANSPORT, and HEC-RAS to model the watersheds. Procedures were developed to model the benefits provided by the hundreds of regional and onsite detention ponds and BMPs in the County at a resolution and accuracy appropriate for watershedwide planning. Finally, procedures and protocols were developed to store the modeling results in GIS and database files for future use by the County. The goal is to provide an Copyright ASCE 2005 Watershed 2005 2 innovative model along with input and output datasets in a format that the County can use and update in the future. Introduction Fairfax County includes the largely developed urban and suburban portions of northern Virginia west of Washington, DC. Fairfax County is in the process of developing watershed management plans for all 30 major watersheds in the County. There are various reasons for performing these plans as discussed below. The Watershed Plans will improve existing stream conditions and prevent further degradation in the County streams. The Stormwater Planning Division completed a Stream Physical Assessment Study in the spring of 2002 where all streams in the County were inventoried and stream conditions were documented. The stream assessment comprised a habitat assessment and an inventory of physical stream features based on protocols developed specifically for this project. The habitat assessment (scoring of various habitat parameters) and the inventory (characterization of physical features such as pipelines, utilities, and buffers) together provide a baseline of overall stream conditions, which can be further extrapolated to indicate the condition of the associated watershed. Based on this assessment, 70% of the streams in Fairfax County were found to be in fair to poor condition. The Watershed Plans will assist the County in meeting its commitments under the Chesapeake Bay 2000 Agreement. The Chesapeake Bay 2000 Agreement defines minimum requirements to be met by jurisdictions in the Chesapeake Bay watershed. The agreement commits the member jurisdictions to develop and implement watershed management plans in two-thirds of the bay watershed by the year 2010. The agreement also commits the member jurisdictions to reduce nutrient loads with a goal of removing the Chesapeake Bay from the list of impaired waters by the year 2010. The Watershed Plans will assist the County and the Commonwealth of Virginia to meet the requirements of the Clean Water Act. Section 303(d) of the Clean Water Act requires each state to submit a Total Maximum Daily Load (TMDL) Priority List to EPA. This list, contained in the 303(d) Report on Impaired Waters in Virginia, is a compilation of waters in the state that do not meet water quality standards. Most impaired waters require the development of TMDLs. TMDL studies identify the maximum loadings that will allow the impaired water body to meet the designated water quality standards. These studies must be completed by 2011. The Virginia Department of Environmental Quality (DEQ) samples streams and lists those Copyright ASCE 2005 Watershed 2005 3 that do not meet the designated water quality criteria. Currently, nine streams in Fairfax County are listed as being impaired, primarily because the criterion for fecal coliform concentrations for recreational use is exceeded. The watershed plans will address flooding, facility maintenance, and other watershed issues. Finally, the watershed plans will update current watershed plans performed in the 1980s and provide a comprehensive approach for addressing multiple regulations, commitments, and community needs. Fairfax County selected five watershed planning consultants to perform the watershed plans. Detailed watershed models were developed to support the watershed planning program. The models developed for the watershed plans models will be updated and supported by the County for use in other County programs. As such, the models will be a long-term asset for the Fairfax County Stormwater Planning Division. Early in the planning process, CDM was given the task to develop modeling protocols to be used by all five watershed management plan consultants for the stormwater modeling and delivery of modeling data and results. The modeling protocols meet the following criteria: • Ensure consistency in the model development, simulation results, and delivery of the model data and results for the various watershed plans. • Provide modeling accuracy appropriate for countywide watershed management planning. • Identify simulation modeling programs for use in the plans • Develop efficient procedures to perform the modeling within the available budgets. • Maximize use of available GIS data. As described in the remainder of this paper, modeling procedures and protocols were developed to meet these criteria.