11.5 Sustainable stormwater management  (Page 27/26)

Learning objectives

After reading this module, students should be able to

• describe how stormwater runoff affects water quality in urban watersheds
• explain how stormwater is currently managed in the United States
• analyze some of the conventional and innovative techniques that have been developed to address the water pollution and flood risks associated with urban stormwater runoff

Introduction

This module reviews some of the complex issues of urban stormwater management. It first examines the hydrological issues affecting the discharge of stormwater runoff to our urban rivers and streams, and then provides an overview of how urban stormwater is managed under the Clean Water Act. After describing the conventional approaches to urban stormwater management, the final section provides an overview of various "sustainable" strategies, especially the use of "green infrastructure," that can be considered to reduce the water pollution and flooding risks generated by urban stormwater runoff.

The hydrological context of urban stormwater

Stormwater runoff (or overland flow) is the portion of precipitation reaching the ground that does not infiltrate into soils, is not taken up and transpirated by plants, nor is it evaporated into the atmosphere. It is an especially important component of the hydrological cycle in urban areas, since it can cause both pollution and flooding risks to nearby waterways and their adjacent communities. It should also be noted that many of the current models of global climate change predict changes in the hydrological cycle in the future. They predict many more severe storms likely in parts of the Midwest as a result of the moisture and energy in the atmosphere increasing over the next century because of increasingly higher concentrations of greenhouse gases. Higher frequencies of more severe storms are likely to further increase the pollution and flooding risks posed by stormwater runoff, especially in urban areas ( USGCRP, 2009 ).

Current strategies to manage these risks employ the concept of a watershed    – the variations in natural topography that cause both surface water and surficial ground water to flow downhill towards lower-lying areas or points of discharge, usually to a stream or river. Watershed boundaries are defined topographically by mapping variations in land elevations around waterways that create hydrologic divides between adjacent watersheds and between sub-watersheds. The amount of stormwater that ends up as runoff within a watershed not only depends on the intensity and amount of precipitation reaching the ground in the form of rain or snow, but also on the characteristics of the watershed itself. State and federal environmental protection agencies have developed a number of sophisticated hydrological simulation models that enable the amount and characteristics of stormwater runoff (in terms of its volume and the pollutant load that would be carried by the stormwater to rivers and streams within the watershed) to be forecasted. They forecast this based on historical estimates of the amount of precipitation entering the watershed, the characteristics of a watershed's terrain and soils, the amount and location of impermeable surfaces associated with the development of the watershed, and the extent and types of ground cover within the watershed's drainage area ( NRC 2008, Appendix D ). A change in any of these factors will affect the amount and extent of flooding and water pollution attributable to the discharge of stormwater runoff into a river or stream.