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Rethinking Watersheds to Reduce Flooding June 25, 2010

Posted by Wapello Warbler in Environment, People.
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Last night, I had my concept of watersheds and wetlands turned upside down.

Most of our infrastructure for handling rain water assumes that the problem is to get excess water from one place to another. Usually that means getting water off the streets or out of our fields, into a river, and on its way to the ocean. According to Jim Patchett, a hydrologist and natural landscape designer, there are better ways to handle rain water.

How Watersheds Work Today

When I lived in the St. Louis area, I used to cross the River Des Peres each day on the way to work. One of the seven wonders of civil engineering, the River Des Peres is a huge ditch, armored with concrete, that was built to carry excess rain water away from the city. It’s ugly, but it has done it’s job well–except in 1993 when the Mississippi was so high there was no place for the water to go. The River Des Peres couldn’t hold any more rain water and the result was extensive flooding in south St. Louis.

The gutter to storm sewer to river model formed my thinking. Most of the creeks and rivers I had ever seen fit the model. With the exception of the Mississippi, they all seemed to begin at some eroded hill-side.

My mental model worked like this: Rain falls, the ground absorbs what it can, and the rest makes ponds, marshes, and rivers.

While that’s actually the way things work in many places, it’s not how they worked when Iowa was covered with Prairie.

How Watersheds Worked in Native Prairies

Patchett explained that natural prairies absorb and retain moisture like giant sponges. That’s due in large part to the tremendous root systems that our native prairie plants put down. The result is that runoff only occurs when the ground is frozen. The rest of the year the rain is absorbed where it falls and is either used by the plants or replenishes the ground water.

Here’s the curious bit: in this sort of ecosystem, wetlands aren’t places where the water sits while it’s waiting to be absorbed. They are places were ground water percolates out of the ground after it has been filtered and its temperature has been normalized.

Some of the springs produce streams of flowing water. These gain water as they go and as they merge along the way they become rivers.

The dynamics of the system are such that water levels in the marshes and streams change seasonally rather than daily and the water temperature changes just as slowly, if at all.

Why The Difference is Important

I took away two important reasons why change is needed:

  1. Increasing our ability to absorb water where it falls rather than sending it down the river could dramatically reduce flooding. That’s especially true of the summer floods we’ve been experiencing in Louisa County. On the other hand, if runoff continues to increase, so will the flooding.
  2. A lot of native plants, insects, and amphibians do poorly in water-collection wetlands. They were designed for relatively constant levels of cool, clean water.

Towards a Solution

Patchett does more work in urban areas than in rural ones, so most of the models he presented were for preventing runoff from business buildings, streets, and homes. He showed us how pavement and landscapes could be modified to greatly reduce runoff. Some of his projects with more buildings and less land area also incorporate various strategies for capturing rain water for later use.

Bridgestone Bandag in Muscatine, our hosts for the conference, have been applying some of the principles to their corporate properties. I encourage you to stop by their learning center behind the Muscatine Mall, where they are in the process of replacing 71 acres of turf grass with native prairie. The project still has a few years to go, but there are already numerous birds, insects and other critters that you normally don’t see in the vicinity of corporate offices and shopping malls.

If you’d like to find out more about Patchett and his associates at Conservation Design Forum, try their web site, www.cdfinc.com

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