Rock Grade Stabilizers

Specially designed sections of Structure made of rocks installed in ephemeral channels at the grade of the channel to prevent downcutting.

Purpose: The purpose of rock grade stabilizers is the same as log grade stabilizers, except that they are made of rock. The emphasis is on stabilizing the channel gradient rather than trapping sediment although some sediment will be trapped by these structures.

Relative Effectiveness: Excellent-0% Good-33% Fair-67% Poor-0% (Replies = 3).  Only a few interviewees commented on rock grade stabilizers. They rated this technique as “good” to “fair.” There were not many comments about this technique.

Implementation and Environmental Factors: Many comments on implementation and environmental factors pertaining to log grade stabilizers, apply to rock grade stabilizers. Proper design, adequate planning, and experienced crews often make the difference between “good” and “fair” effectiveness. Like log grade stabilizers, this technique is expensive and time consuming. A key implementation factor is the availability of rock for the grade stabilizers. A couple of important implementation factors that affect effectiveness are: (1) the use of rocks that are large enough to resist transport during runoff events, and (2) placement of organic debris or sediment screening on the upstream side of the grade stabilizer.

Rock Grade Stabilizer Implementation

OBJECTIVE

To prevent sediment from entering perennial streams during the first winter following a wildfire.

To trap and slowly meter sediment release through the system.

To establish grade control, and decrease water velocity and reduce accelerated stream channel downcutting.

SITE SELECTION

General area where straw bale check dams can be effective
In intensively burned areas
Locations of highly erodible and sensitive soils.
Areas of de-stabilized channels
Specific individual dam site locations
In ephemeral and some small intermittent channels.
Areas where logs and branches have created natural dam and were subsequently burned out.
In areas where rock is abundant.

CONSTRUCTION AND EQUIPMENT

Placement of the dam is critical to the success of the structure. As in the straw bale structure the rocks are placed in a horseshoe shape.

Smooth the ground where the structure will be placed making sure that all the ash is cleared. This insures that the rock seats properly to the ground, lessening the potential for water to flow under the structure.

Rock structures REQUIRE small branches, small woody debris and/or needles to be placed in the channel upstream of the structure. Without the woody debris the structures will not seal properly.

Rock structures are built without an energy dissipater as shown in diagram 1 and diagram 2.

The crest of the dam must be lower than the ground level of the rocks at each end of the dam. When the dam fills, the water will now be forced through the spillway, instead of around the sides.

Rock may also be used to build headcut structures and for bank armoring. Remember that burned area emergency stabilization and rehabilitation (ESR) is an emergency procedure and structures are not meant to be permanent.

The cost of rock dams varies with accessibility to the area, availability of rock near the site and efficiency of the crew.