Many streams in Ohio have been modified to improve their capacity to convey stormwater. This has most often been achieved by straightening the channel alignment and lowering the streambed to increase channel slope and flow velocity. Although these practices greatly improve the efficiency with which stormwater can be carried away, this kind of channel modification degrades the stream's water quality and destroys aquatic habitat.
Reducing channel roughness removes much of the in-stream structure that provides habitat for fish and other aquatic life. Restoring some of this structure through biotechnical practices (methods that use vegetation and natural materials to restore channel roughness) provides additional hiding, spawning and feeding areas for fish. This also increases the substrate suitable for benthic (bottom dwelling) organisms to become established which supports a healthier, more diverse aquatic ecosystem. Other biotechnical practices, such as establishing streamside vegetation to enhance in-stream structure, can vastly improve a stream's value as an aquatic habitat while maintaining much of its ability to convey runoff.
The purpose of this Ohio Stream Management Guide is to describe the site conditions suitable for gravel riffles, their design and installation. The guidelines listed herein are a compilation of specifications from agencies in other states and from field experience here in Ohio. As with any construction project that takes place within a stream, the Ohio Department of Natural Resources recommends you consult with the applicable local, state, and federal authorities listed in Guide 06, Permit Checklist for Stream Modification Projects, prior to construction. The extent of permit requirements will depend on the location and design of your project
Where to Use Gravel Riffles
The use of gravel riffles should be considered if a coarse gravel substrate was an original characteristic of the stream, but has been removed. This situation is common in deepened, modified, or relocated channels. It may also occur where the bedload sediment supply has been interrupted by the construction of an instream pond or by the enclosure of the upstream channels in a storm drain system. Constructed gravel riffles are usually of greatest value in small channels. The bedload of larger streams is more likely to supply adequate coarse material to maintain natural riffles. A stream's natural condition can further be enhanced when gravel riffles are supplemented with the addition of forested riparian corridors.
Gravel riffles should be used to augment natural channel formation. Supplies of gravel and cobble introduced into a channel will not force a channel into a desired shape but can mimic what will eventually accumulate from natural deposition of bedload sediment.
Design and Installation
The length of gravel riffles should range from one to two times the channel width. The thickness of the gravel should generally be less than one foot and not more than the depth of the water at normal stage flow so that it doesn't extend out of the water and back up a significant pool or otherwise act as a dam. To concentrate lower stream flows, the gravel should be placed so that it is slightly lower in the middle of the channel and higher along the streambanks as illustrated in figure 1.
Gravel size is best determined by examining the substrate and any gravel bars in the existing stream channel. Gravel should be sized so that it is stable at low and medium flows but erodible at high or bankfull flows. This typically ranges from one to four inches in diameter. Several methods (or criteria) can be used to determine the appropriate placement of gravel riffles as shown if figure 2 including: placing them where existing riffles are already forming; placing them to match the spacing of riffles in a similar undisturbed portion of the stream (not illustrated); placing them at the crossovers between meanders; or placing them spaced approximately five to seven stream widths apart.