In designing the river restoration, it is desirable to create a channel that is as close as possible to that stream type’s most stable (equilibrium) condition in dimension and channel composition so as to reduce the potential for future erosion within the channel and along the stream banks.

To accomplish this objective, NCD employs the approach of identifying and measuring the characteristics of what is called a reference reach.

This is typically a relatively undisturbed segment of the same river either upstream or downstream of the project area. Alternatively, a segment of a different river with an identical classification located in the same landform can be used for this purpose. Because the area downstream of the Mill River dam is tidally influenced, measurements were taken of the channel’s morphology (form) in the reach immediately upstream to better identify various dimensions (e.g., channel width, depth, and gradient) and characteristics (e.g., size of stones occurring in the channel) applicable to the restoration design.

Upstream measurements were combined with flow data from both the Mill River watershed and the New England region to determine anticipated flood flows and flood elevations for a variety of storm events. This information then was used to design the channel dimensions for the restoration reach.

One of the goals of the river restoration design was to minimize the use of streambank armoring as a means of preventing bank erosion. Instead, the goal was to use bioengineering techniques wherever possible to promote bank stabilization with vegetation. The design team felt this would increase both the natural aesthetics and wildlife habitat value of the project. Toward this end, the project will make extensive use of livestakes, branch layering (fascines), and container grown woody vegetation installed within coconut fiber (coir) blankets and logs to provide both immediate and long-term bank stabilization. In some areas where floodplain is limited and banks will be steeper, the design incorporates the use of vegetated geogrids, permanent soil retention structures that provide a footing for vegetation establishment.

An additional consideration in the channel design was that the restoration reach lies within an urban park; and although streams are naturally dynamic and tend to shift positions within their floodplains, this was not a desirable outcome within the park setting. Therefore, the channel includes frequent use of various types of rock vane structures (e.g., cross vanes, j-hook vanes) that control both the elevation and location of the channel, and reduce bank erosion on meander bends by forcing higher flows toward the center of the channel. The vanes also maintain the structure of pools within the channel and thus enhance habitat for aquatic biota, including fish. Lastly, several of the vanes will provide foot access to the river itself for park visitors.

Superimposed on the bank stabilization techniques are a palette of floodplain adapted native plant materials, including trees, shrubs, and a seed mix, and herbaceous plugs.