Pipe and riser structures are used as the principal spillway for many of the dams in the State of Ohio. The proper operation of these spillways is an important part of maintaining the overall safety of the dam. Pipe and riser spillways are susceptible to obstruction and damage by floating debris such as leaves, branches, and logs. One device used to ensure that these spillways operate correctly is a trashrack. Trashracks are designed to keep trash and other debris from entering the conduit and causing damage or fouling of the riser pipe.
Trashracks usually become plugged because the openings are too small or the head loss at the inlet causes material and sediment to settle out and accumulate. Small openings will stop debris such as twigs and leaves, which in turn cause a progression of larger items to build up, eventually completely blocking the inlet.
Pipe and riser spillways can also become blocked by a build up of debris in the bottom of the riser. This type of blockage occurs when no trashrack is in place, or if the openings are too large.
In most pipe and riser spillway systems, the diameter of the outlet pipe is smaller than the diameter of the riser. Therefore, it is incorrect to assume that debris which passes through the riser will not obstruct the flow through the outlet pipe. Large debris, such as logs and tree limbs, can become lodged in the 90° transition from the riser to the outlet pipe. This reduces the capacity of the spillway and could damage the riser pipe. An obstructed outlet pipe can be a major problem because removal of large debris from the bottom of a riser can be very difficult.
A partially blocked pipe and riser spillway reduces the capacity of the spillway, and may also create a higher than normal pool level. The combination of these two factors can dramatically reduce the discharge/storage capacity of the dam which then increases the potential for overtopping. Overtopping for even a short period of time can cause damage to the embankment and possibly failure of the dam. If the dam has an emergency spillway, a blocked principal spillway will cause more frequent and greater flows in the emergency spillway. Since emergency spillways are usually grass lined channels designed for infrequent flows of short duration, serious damage is likely to result.
A well-designed trashrack will stop large debris that could plug the pipe but allow unrestricted passage of water and smaller debris. The larger the outlet pipe, the larger the trashrack opening should be. In the design of a trashrack the maximum openings should be no larger than one-half the size of the outlet pipe. For example, if the outlet pipe is 18 inches in diameter, the trashrack openings should be no larger than 9 inches by 9 inches. This prevents debris from passing through the riser and blocking the outlet pipe. This rule applies up to a maximum trashrack opening of two feet. For smaller spillway systems, trashrack openings should be at least 6 inches by 6 inches regardless of the size of the outlet pipe. Another important design criteria is that the trashrack should be securely fastened to the top of the riser. The connection should be strong enough to withstand the loads which it will experience during periods of high flow.
Many owners are concerned about losing fish through trashracks that have large openings. If this is a concern, a metal plate surrounding the riser inlet which extends above and below the normal pool level should be installed. See Figure on back of sheet. On the bottom of the plate, a metal screen should be attached and connected to the riser pipe. The solid plate at the water level will prevent the fish and floating debris from passing over the crest of the riser. The underwater screen will keep the fish from moving under the metal plate and through the spillway. The underwater screen will not become blocked because most of the debris floats on the water surface. If this design is used, the area between the inside of the cylinder and the outside of the riser must be equal to or greater than the area inside the riser.
An anti-vortex plate can easily be incorporated into most trashrack designs. An anti vortex plate is a flat metal plate which is placed on edge and attached to the inlet of the riser pipe. See Figure below. The capacity of a pipe and riser spillway will be increased by equipping the trashrack with an anti-vortex plate. The anti-vortex plate increases capacity by preventing the formation of a flow inhibiting vortex which may form during periods of high flow.
Maintenance should include periodic checking of the trashrack for rusted and broken sections and repairing as needed. Trashracks should be checked frequently during and after storm events to ensure they are functioning properly and to remove accumulated debris. Extreme caution should be used when attempting to remove accumulated debris during periods of high flow.
The benefits of a properly designed and maintained trashrack include the following:
1. Efficient use of the existing pipe and riser system that will maintain the design discharge/storage capacity of the dam and prevent overtopping.
2. Prevention of costly maintenance items such as, the removal of debris from the pipe and riser, repair or replacement of damaged pipes, and the repair of erosion in the emergency spillway.
3. A reduction in the amount of fish lost through the pipe and riser system.