Grade Control Practices

Stream Restoration: Grade Control Practices

Grade control practices are installed to maintain a desired streambed elevation. These practices are used either to raise the stream invert (i.e., to reverse past channel incision), or to maintain the channel invert at a current elevation (i.e., to prevent channel incision). Nearly all stream restoration projects incorporate some form of grade control practice in the project design. Grade control practices create a �hardpoint� along the channel, preventing the streambed from degrading below the top elevation of the structure. The two main types of grade control practices are those that utilize logs for construction materials and those that utilize rock.

A rock vortex weir is a structure designed to serve as grade control and create a diversity of flow velocities, while still maintaining the bed load sediment transport regime of the stream.� The weir points upstream with the legs angling downstream at anywhere from a 15to 30 degree angle relative to the stream bank.� The legs are carried up the streambank to just above the bankfull elevation. The key component of the rock vortex weir is that the weir stones do not touch each other. �Most design details call for a distance of between 1/3 and � the stone diameter separating each stone.� An additional key design feature is that the weir stones do not rise above the channel invert more than 10 to 15% of the bankfull height.�

During baseflow conditions water is forced to flow around and between the stones creating a greater diversity of flow velocities and depths.� During high flows the water rises over the weir stones creating a scour pool below the structure but allowing bed load sediments to move through. Built in this way, the weir will not cause significant sedimentation upstream or reduce the channel cross section to the point of causing the channel to widen or erode around the structure, as is sometimes the case with structures that span the stream above the invert (e.g., log drop structures).

The rock vortex weir is constructed first by placing a foundation of boulders two to three feet in size in a trench excavated along the stream bottom.� Large stones are then placed in the trench behind and against the footer stones so that they extend up to the desired elevation.� A distance of 1/3 to � the stone width should be maintained between each stone. The rocks should extend up no more than 10 to 15% of the bankfull channel depth (Figures 14 - 16).

During baseflow, the interaction of the stream and rocks creates differing flow velocities, with higher flows creating a scour pool below the structure.� By shifting the apex of the structure toward one bank or the other it can be used to direct flows into or out of a meander bend or away from an eroding bank.� This device also works best as a grade control structure. Although, this must be judged against the amount of channel degradation expected.� If a large nick point is migrating upstream toward the structure, measures must be taken to insure that the migrating nick point does not undermine the structure.� In such cases a different type of structure such as a step pool should be utilized to halt the advance of the nick point. Rock vortex weir structures are more effective at preventing grade adjustments than halting them.

Rock vortex weirs have a moderate potential to enhance stream habitat.� Correctly sited and constructed, they tend form scour pools downstream of the structure and increase the diversity of flow velocities above and within the structure.

�A rock cross vane is similar to a rock vortex weir, but differs in that the stones extend little if at all above the stream invert.� These structures are predominately used to provide grade control and to narrow the base flow channel.� If they are designed to narrow the channel sufficiently, they work to create scour pools downstream.� Often a cross vane or a vortex weir will be placed at the top and bottom of a meander bend to establish invert elevations for pool/riffle formation.�

The rock cross vane consists of a rock sill perpendicular to the stream flow located at the invert elevation of the stream.� Two arms of the sill extend downstream along the banks, rising in elevation to the bankfull height as they extend downstream.�

The rock sill is constructed by first excavating a trench below the stream invert.� The width of this trench should be 2/3 to 3/4 of the channel width.� The width is based upon the desired characteristics of the channel.� Large flat rectangular boulders are placed in the trench so that they are touching.� The number of stones and their size will depend on the size of the channel and the erosive capacity of the stream. The trench should be three times as deep as the stones are high and just wide enough to place the stones. Once these stones are in place, the trench is extended upstream of the placed stones so that a second layer of stones can be placed, half on substrate and half overlapping the first set of stones. A third set of stones is then placed so that 2/3 overlap the second course and 1/3 lie on the substrate, with there tops even or slightly above the desired invert.� In smaller streams, only two courses of stone may be necessary. The number of courses and the size of the stone will depend on the size of the stream, the potential for scouring, and the composition of the substrate (Figures 17 - 19).

Rock cross vanes have a modest potential to enhance stream habitat.� Unlike the rock vortex weir, rock cross vanes interact little with baseflow but can promote pool formation downstream and the narrowing/deepening of the baseflow channel.

Step pools consist of a series of structures designed to dissipate energy in steep gradient sections of a stream.� They are often used where a large nick point has formed and is migrating headward or where a channel has degraded below a culvert or outfall. They are made of large rock in alternating short steep drops and longer low or reverse grade sections. The number of steps is determined by the extent of the drop in invert of the stream.� There are various configurations and arrangements of rock that can be utilized.� The requirement is that whatever the design configuration chosen it must be stable at all flows, the rock must be large enough to be essentially immobile, and the drops should be low enough to allow aquatic life to migrate upstream (Figure 20).

Step pools are not generally considered a habitat enhancement practice.� The enhancement potential is in the form of maintaining fish passage and expanding the total amount of habitat available for fish.

�A log drop is a simple pool forming and grade control structure.� Log drops mimic the influence of large woody debris (trees) that fall into the stream.� Most log drops are formed of two 16" or greater diameter logs.� The first log is laid in a trench perpendicular to the flow so that the top of the log is at or slightly below the stream invert and the ends of the log extend several feet into the streambank.� A second log is placed atop the first until the logs rise in height to just above the baseflow level of the stream.� Once the desired elevation is achieved, a weir notch is cut in the top log.� The notch serves to concentrate the baseflow.� Higher flows will form a scour pool below the log drop.� It is important that the logs be keyed into the stream banks far enough to prevent them from being scoured out at high flows.� The log/streambank interface must also be sufficiently stabilized with rip-rap or boulders to prevent washout around the sides (Figures 21 and 22).

Logs drops are little used today because they can become fish barriers even if installed carefully and they tend to cause upstream sedimentation and channel widening due to the reduction in bankfull cross sectional area.� During flows that exceed the capacity of the weir notch, there is no flow concentration and consequently the flows tend to spread over the whole length of the log promoting erosion at the streambanks.

If grade control, and not pool formation, is the primary function of a log drop, the footer log should be placed lower in the streambed and the top log should not rise above the invert of the stream.� In this way they can provide grade control without the potential negative impacts when constructed as pool-forming structures.

A variation of the standard log drop structure is the V-log structure.� Rather than having a single log that extends straight across the channel, two logs are used that form a V pointing upstream. The logs are lowest (at or below the stream invert) at the apex and rise into the stream banks.� This structure has the advantage of not potentially creating a fish barrier and is more effective at concentrating flows and creating scour pools below the structure.� Since it concentrates larger flows toward the middle of the channel, it is not likely to cause channel widening and bank erosion or deposition upstream (Figure 23).

Both the standard log drop structure and the V-log drop structure have a significant potential to enhance stream habitat through pool formation downstream of the structure.