Control of the stream channel gradient is essential to stop further incision or downcutting of the streambed and to prevent streambank: undercutting and erosion. This section gives some examples of structures which reduce and maintain channel gradients and/or reduce the excess energy of flowing water. By reducing the channel gradient, a reduction in water velocity is achieved which prevents bank: erosion and streambed scouring, especially during peak flow periods. Likewise, energy dissipation structures may be installed temporarily or permanently as culvert appurtenances or at the foot of steep slopes to dissipate the energy of peak flows.
1. Check dams: These structures may be used to stabilize grades and control headward erosion in either natural or artificial channels (Figure 20). The existing check dams along Strawberry Creek have significantly reduced streambed degradation on the central campus. Check dams prevent bank erosion by reducing flow velocities and dissipating excess energy through the formation of a waterfall. Sediment accumulates above these small dams to some extent, helping to produce slightly gentler upstream gradients. Aquatic habitat is also improved because of the formation of pools upstream of the dams. Check dams may be constructed of logs, planks, rocks, staked brush, or staked hay bales. Rock weirs with accompanying rock sills have also been successfully employed. Rehabilitation of the approximately thirty deteriorating check dams on the central campus can be easily and economically accomplished in most cases. This should be done as soon as possible to control further streambed incision.
2. Drop structures: These are either inlets or weirs in which flow passes over or through an opening and then drops to an approximately level basin or apron and thence into the downstream channel. These structures may be used to control steam gradients in either artificial or natural channels. They may be more costly than some other types of structures where the required discharge capacity is less than 100 cfs and the total head/drop is greater than 8-10 feet (Amimoto, 1978). Drop inlets are already located at the entrance to the Little Inch bypass culvert above Haas and at the entrance to the city tunnel above Highland Avenue in North Berkeley.
3. Energy dissipators: Examples of strictly energy dissipating devices include:
A. Hydraulic jump: This is a permanent concrete structure used to dissipate the energy of concentrated runoff flowing down very steep slopes. This structure occupies a rather small area, but unless continually maintained, sediment deposits inside the structure will decrease its effectiveness. Formal engineering design is required. Several of these are in place in the North Fork drainage system at the LBL complex.
B. Discharge aprons: These are used to reduce the velocity of water flowing from a culvert or pipe or in other situations where flow is concentrated on a small area. Riprap or smaller hand-placed rocks are often used. Individual rocks must be of sufficient size to remain in place under a heavy flow of water. At least two layers of rocks are often needed to provide sufficient depth to prevent undercutting. Very heavy or continuous flows should be discharged onto a concrete or prefabricated metal apron.