VOLUME-BASED RUNOFF CONTROLS – THE KEY TO EFFECTIVE REGIONAL STORMWATER MANAGEMENT

Many communities across the nation have adopted stormwater regulations with the objective of controlling the increase in peak flow associated with new development. These regulations require the use of management practices or controls to limit the peak flow leaving the development. Typically, these controls limit the post-development peak flows to pre-development conditions. Controlling post-development peak flows to predevelopment conditions is effective in preventing increases in the “local” peak flows for adjacent properties or developments. However, local peak flow management alone is not always effective in limiting the increase in peak flow at points further downstream of the development; that is, runoff controls that limit local peak flows to pre-development peak flows do not provide the same effective control of downstream or regional peak flows. Over the long term, this can result in unanticipated increases in peak flow in the downstream watershed even with full implementation of local peak flow management techniques for all new development. The increases in flow downstream in the watershed are caused by ineffective management of the increase in runoff volume associated with development.The increase in runoff volume associated with new development is significant, but often overlooked in stormwater management planning. Even with local peak flow controls, increased runoff volumes produce small increases in flow that can cumulatively result in an increase in peak flow in the downstream watershed.In the Milwaukee metropolitan area, a volume-based approach is utilized to effectively manage increases in stormwater peak flow and runoff volume from new development on a watershed or regional basis. This volume-based approach provides the appropriate control of local peak flows to prevent increases in flow associated with increased runoff volume downstream in the watershed.The Milwaukee Metropolitan Sewerage District (MMSD) provides wastewater services for 28 municipalities with about 1.2 million people in the greater Milwaukee area. In January 2002, MMSD implemented its Chapter 13 Surface Water and Storm Water Rules (Rules) to effectively manage the increases in stormwater peak flow and runoff volume associated with new development. The volume-based Rules became effective for all the governmental units in the 420-square mile service area and are in their third year of application.This paper describes the technical basis for the Rules, the development of the performance requirements in the Rules to achieve effective control of the increase in runoff volume, and the process for implementation of the Rules. Also presented are the lessons-learned from the first two years of implementation of the set of new and relatively restrictive development requirements administered by 28 municipalities.