Selecting Design Conditions as Part of a Watershed Approach to Water Quality Control

by David W. Dilks,
Kathryn A. Sweet,

Document Type: Proceeding Paper

Part of: North American Water and Environment Congress & Destructive Water


EPA water quality guidance recommends that water quality criteria be exceeded no more than once in three years. Standard approaches, based on the use of critical design conditions, have been developed for calculating wasteload allocations for individual continuous sources that are consistent with this guidance. Designing controls to meet water quality objectives on a watershed scale has been problematic because of complexities added by consideration of 1) multiple discharges and/or 2) episodic sources. Application of existing procedures to multiple discharges typically results in a compounding of safety factors and can lead to extremely overprotective limits when applied on a basinwide basis. The standard procedures for episodic sources consist of either a critical condition or continuous simulation approach. Statistically selected critical environmental conditions (e.g. drought stream flows, design storms) are easily applied, but represent an unknown level of protection. Continuous simulation provides rigorous results, but is often too resource intensive to apply on a watershed basis. This paper presents a range of analytical tools that consider the frequency of water quality standard exceedances more directly than design storms, with less extensive resource requirements than continuous simulation.

Subject Headings: Water quality | Water shortage | Watersheds | Quality control | Water discharge | Storms | Standards and codes | Environmental Protection Agency

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