Forecasting Costs and Comprehensive Benefits of Stormwater Quality Improvements
- Alan Davis - Hazen and Sawyer
Understanding the costs associated with improving stormwater quality and conveying the benefits provided by these expenditures to the public are among the key challenges MS4s face within the Chesapeake Bay watershed. Further adding complexity to these efforts are the nature of changing construction and maintenance costs and variable water quality improvement credits over the life of implementation efforts that may span decades and include dozens or hundreds of stormwater controls. An online computer tool was developed to track local construction and maintenance costs for stormwater controls along with water quality benefits and environmental, social, and economic co-benefits. The tool utilizes tracking information on implementation costs and benefits to forecast a range of future program costs based upon water quality improvement needs.
Specific data entry requirements vary based upon the type of stormwater control, but include basic information regarding contributing drainage, system sizing, design components, and site characteristics. User interface elements and data validation rules guide and standardize data entry to facilitate analysis and forecasting. Assessments of water quality benefits account for pollutant and volume reductions utilizing procedures and efficiencies consistent with the Chesapeake Bay Program, with options to augment results based upon local performance monitoring efforts. Similarly, co-benefits evaluations are based upon literature review results, but can be modified to reflect locally relevant findings and priorities.
Utilizing recorded tracking information, the tool forecasts anticipated costs and benefits for an individual stormwater control or group of stormwater controls. A weighting scheme prioritizes the use of projects with similar characteristics in forecasting costs and benefits to improve the relevance of results, while also providing an illustration of the full range of potential variability. These evaluations have demonstrated that individual project and overall program costs vary substantially based upon the use of typical cost estimates versus actual costs, as well as other factors like stormwater control selection, design configuration, and site characteristics. In parallel to cost forecasts, quantifying the co-benefits of stormwater controls has provided valuable information to engage the public and justify the value of increased stormwater expenditures.
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