The traditional method of stream engineering in an urbanized area was to “remove large woody debris from the channel, straighten that channel, confine the stream in a new concrete channel, armor the banks with rip-rap, or enclose the stream in a pipe” (Brown, 2000). The aim was to move water as quickly and safely out of the immediate area to prevent “local urban flooding” (Brown, 2000). Due to this distorted form, urban streams ecologically malfunction and the wildlife that the stream and stream banks would normally support cannot survive (Brown, 2000). The physical effects of urbanization on natural water processes can be grouped into four categories: impervious surface cover (ISC), drainage density, temperature, and chemical and biological contamination (Paul, 2001).
“The most consistent and pervasive effect of urbanization is an increase in impervious surface cover within urban catchments, which alters the hydrology and geomorphology of streams” (Paul, 2001). When there is rainfall in areas with high ISC, water runs swiftly from roof tops and pavement with little to no absorption into the ground. This has two rather devastating effects both on the function of the stream and potentially to human settlement around those watercourses. Since the water runs more quickly, the time between the precipitation event to the “center of the runoff volume shortens within the urban catchments” (Paul, 2001). This results not only in more frequent flooding, but “floods that peak more rapidly” (Paul, 2001). In addition, since less water is absorbed into the ground, there is no water to provide baseflow discharge in the urban stream. The situation becomes one where there is little to no flow in what would normally have been an ephemeral stream followed by episodes of dangerously high flooding (Paul, 2001). Flooding in itself is as well a natural and much needed occurrence, however when the type and amplitude of flooding is not responsive to the climate, soil, and geology of the area, important amounts of land and vegetation can be lost from an area that normally would not have experienced those events.
It is important to address other water strategies in conjunction with stream restoration. There may be a question as to the detrimental impact of high density development or redevelopment of former industrial sites on the newly restored stream (Richards, 2000). If more ISC is introduced to an area using traditional grading and drainage plans there is no doubt that there will be a negative impact on the watershed (Richards, 2002). Lynn Richards, a policy analyst for the EPA sited two important studies conducted in 2000 that suggest that stormwater conservation redevelopment is more desirable condition compared with low density
development or traditional redevelopment. The first is a study from Purdue University which “estimated that placing a hypothetical low-density development at the Chicago fringe area would produce 10 times more runoff than a mixed-uses development in the urban core” (Richards, 2000). The second, conducted at Jordan Cove, found that when “compared to conservation lot design, the large lot development produce 95% more runoff” (Richards, 2000). It only stands to reason that if there is a better and an often time cost effective (both in short and long term profit margins) means of constructing new, or retrofitting old neighborhoods it should be the preferable solution. greenfield
Brown, Kenneth. Urban Stream Restoration Practices: An Initial Assessment.
: US EPA, Office of Wetlands, Ocean, and Watersheds, 2000. Ellicott City, MD
Paul, Michael J., and Judy L. Meyer. "Streams in Urban Landscape."
, Institute of Ecology (2001): 333-356. University of Georgia
. Is Density Good for Water Quality? Lynn EPA--OPEI APA Conference. US 17 Apr. 2002. 19 Dec. 2005 <http://www.asu.edu/caed/ proceedings02/RICHARDS/richard1.htm>.