One of the purposes of a storm water best management practice (BMP), such as a wet pond, is to protect streams below the BMP from erosion and bank scour. Natural channel design can play a major role in achieving this BMP when trying to improve older urban watersheds that were "built out" prior to today's stringent regulations.
An example is Little Falls Branch, located in suburban Maryland adjacent to the District of Columbia. During the last century, single-family homes, condominiums, shopping centers, roads, and federal government facilities proliferated within the 4.1-square-mile watershed. One-third of the watershed is paved or roofed.
This growth occurred without significant storm water management, so as the infrastructure evolved, the streams degraded. Stream banks destabilized, exposing sewer lines. Aquatic life diminished. Consequently, the Little Falls Mall and Main Stem streams, both tributaries to the Potomac River, were severely impacted.
The Montgomery County Department of Environmental Protection had one primary goal for these two streams: to restore stability.
"When you're dealing with this level of imperviousness and stream slope, stream stability is the overriding objective," said John Hollister, stream restoration engineer with the DEP.
Tim Schueler, PE, was commissioned to manage the projects while working with his previous employer. He now works with McKim & Creed as a senior project manager specializing in stream restoration.
For this assignment, Schueler was charged with evaluating the situation and generating a plan to allow safe flow conveyance through these steeply-banked streams by giving them a stable, non-eroding geometry.
These projects were a "big exercise in grade control," says Schueler. "We had to step down storm water runoff as quickly as possible, minimizing the potential for erosion." To that end, he developed a plan that combined utility protection, natural channel design and bio-engineered vegetative bank treatment.
A key element in the project was the hydraulic modeling. The Main Stem stream flows swiftly enough to move boulders as large as a dishwasher.
"It generates the most erosive power of any stream I've modeled to date, and there was a lot riding on getting it fixed correctly," says Schueler.
Adds Hollister, "Tim did an excellent job with the hydraulics. We're trying to make better use of that type of data so we can improve our success rates and improve the performance of our installations."
Construction for the Little Falls Mall tributary work was completed in 2005 and the Main Stem work was completed this spring. The vegetation is growing in, the pipelines are safely buried, the banks are stable and aquatic habitat is improving. Streambank erosion has been decreased by 99 percent for the restored portion of the Mall tributary. "The frogs are crazy about it," says Schueler.
Editor's note: Reprinted with permission from McKim & Creed Client Newsletter, Summer 2007.