Grade Control Measures

Description

Culverts, channel straightening and the removal of riparian vegetation can cause undesirable changes in stream channel slope. Usually, this problem will manifest itself as an incised channel (characterized by bed erosion) or an aggrading channel with no pool or riffle sequences and a shallow, wide streambed and silty bottom (Riley 1998).

Grade control measures include a variety of rock, wood and earth structures placed across the channel and anchored in the streambanks to provide a "hard point" in the streambed that resists the erosion forces of the degradational zone and maintains a streambed elevation. They either raise the stream invert or maintain the channel invert at its current elevation. Nearly all stream restoration projects incorporate some form of grade control in the project design. The two main types of grade control utilize logs and rock (Schueler). Examples of grade control structures include Rock Vortex Weirs, Rock Cross Vanes, Step Pools, Log Drops and V-log Drops. The Stormwater Manager’s Resource Center (http://www.stormwatercenter.net) provides information on each of these practices.

Applicability

If a stable channel bed is essential to the design, grade control should be considered as a first step before any restoration measures are implemented (FISRWG 1998). As with other in-stream work, it is critically important to seek assistance from a qualified river engineer.

Effectiveness

The effectiveness of instream practices are a function of the appropriateness of their application. Sometimes, these practices can cause unforeseen changes in the channel, as the stream might meander around the structures. The structures may be taken out by constricted stream flows (Riley 1998) or the structures may catch gravels. If effective, they can help create structural and hydraulic diversity in uniform channels (FISRWG 1998). It should be taken into consideration that log weirs will eventually decompose (FISWRG 1998). Proper design and installation are essential.

Costs

Select grade control measures, specifically Vortex Rock Weirs and Cross Vanes, are estimated at about $1200 per structure (MDE 2000).

Maintenance

These structures should be monitored to ensure that their orientation and geometry do not hinder fish migration. They should also be inspected for deposited sediment, and to bank instabilities or undesirable lateral stream movement (MDE 2002).

Design Considerations

Before instream practices are employed, the stream should be evaluated by a professional.
The design of grade control structures should be performed by an experienced river engineer.
These structures have the potential to become low flow migration barriers.
This practice can be designed to allow fish passage and may help restore fish habitat if properly designed to fit the stream’s characteristics.
The placement of the structures is crucial. One common strategy is to place weirs in upstream "V’s" or "U’s" to create scour pools below the weir for fish habitat and resting.

References

FISRWG. 1998. Stream Corridor Restoration: Principles, Processes, and Practices. Federal Interagency Stream Restoration Working Group, http://www.usda.gov/wps/portal/usdahome

Garrett, P. Stream Restoration, A Photo Essay. Federal Highway Administration. Accessed 7/11/02.

http://www.fhwa.dot.gov/environment/greenerroadsides/sum01_4.htm

MDE. 2000. Maryland’s Waterway Construction Guidelines. The Maryland Department of the Environment Water Management Administration, http://www.mde.state.md.us/wetlands/guide.html

Riley, A. L. 1998. Restoring Streams in Cities: A Guide for Planners, Policymakers and Citizens. Island Press. Washington, DC.

Recommended Fact Sheets And Resources

Heaton, M. G., R. Grillmayer and J. G. Imhof. 2002. Ontario’s Stream Rehabilitation Manual. Ontario Streams, Belfountain, Ontario. http://www.ontariostreams.on.ca/

Schueler, T. Stormwater Manager’s Resource Center. Center for Watershed Protection, Inc.,. Accessed 3/7/2002. http://www.stormwatercenter.net

USEPA. Restoration Bi-Weekly Update. United States Environmental Protection Agency, Office of Water. Accessed 7/03/02. http://www.epa.gov/owow/wetlands/restore/update/