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August 26, 2006: Volunteers backfill a cedar revetment with gravel and mulch to provide a planting medium and to help bring the toe of the streambank out from the exposed tree roots and back toward the center of the flowing water. The bottom roll of cedar trees is wrapped in coconut husks which degrade more slowly and are more dense than the upper rolls wrapped in jute, a burlap-like fabric.

Background: The Harpeth River Watershed Association began working with the Brentwood Parks Department in March of 2003. Since then, we have helped to stabilize over 400 feet of eroding river bank. Techniques have included cedar revetments, rock gabions, coir rolls and streambank planting. These techniques help to protect the bank from erosion during high water and also protect water quality by helping to keep soil from getting into the water. There have been many projects in these two parks along the Little Harpeth River. Taken together, these projects constitute a large amount of restoration work along a single stretch of river. While not necessarily contiguous, the projects are implemented in park locations picked for the severity of the erosion as well as for treatability.

A tree on the bank is losing ground and will eventually fall into the water, bringing soil (and part of the bank) with it.

The Little Harpeth River's watershed in this area has a large amount of pavement, rooftops and streets (called impervious surfaces). Because rainwater cannot soak into a roof or a street, it runs into the river almost as soon as it hits the ground. This results in a large quantity of rainwater running into the river almost at the same time - something that didn't happen before development.

This large amount of fast moving water in the river easily erodes the banks on either side and also erodes the bottom of river, called downcutting. As the river bottom is downcut, the banks of the river become steeper and thus more easily erodable.

Funding:

The Brentwood Parks Department provides funding for restoration supplies like rock and wire baskets for rock gabions, selectively-harvested cedar trees and jute for cedar revetments. They also provide funding for some HRWA staff time to manage the projects.

Before: The Little Harpeth has trees along much of its banks in these two parks and these trees are losing the soil that holds them in place. As the soil washes away from the tree roots, the trees lose their grip on the bank and fall into the river, increasing the pace of erosion even more and also exposing the water to more sunlight which increases water temperature and potentially decreases water quality.

Photo by Gardens of Babylon.

The Project:

The goal of each restoration project is to help stabilize a section of river or stream bank toe.

When the toe (or bottom) of a bank erodes away, the support for the upper bank is removed. When this upper bank gets saturated it slumps down to fill the place of the lost toe. In this way, river banks erode farther away from the center of the river.

Another way that rivers erode is when high water washes away soil from around tree roots. The trees need this soil and will lose their grip on the land without it.

Cedar revetments and coir rolls provide a barrier for the toe of the bank against the water. Cedar revetments also help to improve lost bank toe by trapping sediment from the water. When the sediment (soil) is trapped in the revetment, it falls to the bottom. This can help to actually rebuild lost streambank and, over time, provide a footing for new plants to grow and hold the new soil in place as the revetment degrades.

August/September 2006

In 2006, we continued our work with two additional projects in River Park.

Cedar Revetments (cedars rolled in jute) waiting to be installed. Photo by Gardens of Babylon.

Cedar Revetments and Coir Rolls, installed. A volunteer backfills with a mixture of gravel and seeded mulch. The coir roll is made of coconut husks which degrade more slowly than jute. Photo by Gardens of Babylon.

Volunteers:

All our projects have been done by volunteers under the direction of trained HRWA staff and partners. Our Volunteer River Restoration Corps (VRRC) is ma

The Tower Park project, March 15, 2003 Girl Scout Troop 1168, Amy Green’s 5th grade juniors and Sarah Bass' Troop 1704, 3rd grade brownies, each with a parent, planted close to 1000 trees in several areas to recreate a buffer zone along the Little Harpeth River.  It was done in 2 hours!!

After this project, our focus moved across Concord Road to River Park which is used and enjoyed by many of the local residents.

---More to come.....