Water flowing in a confined stream or channel contains large erosive forced and severe scour potential. These characteristics define the "energy" of a stream which, if left unchecked, causes serious damage. This energy can wash out roads, undermine foundations, and cut gaping canyons.
Locations where this energy is generated include the end of a discharge pipe, where streams converge, and along an excessively steep channel. Because damage potential at these locations is great, a significant amount of research has been devoted to the design and construction of hydraulic structures called "energy dissipaters."
Storm water flows at different velocities depending on the roughness of the channel lining. Water flows faster and at a lower depth in a concrete-lined channel than in a rip rap-lined channel. Storm water in a channel lined with small rip rap flows faster and at a lower depth than in a channel lined with large rip rap. Energy must be dissipated to make the water go slower. The roughness of the channel lining causes turbulence that dissipates energy thereby slowing the water.
The simplest use of energy dissipation is the installation of rip rap at the outlet end of a storm sewer pipe. Storm water exiting a culvert or storm sewer pipe travels at erosive velocities. As this water is discharged, it enters a rip rap lined channel causing turbulence which slows the water. The rip rap-lined channel is continued to the point where the velocity in the stream is no longer erosive. Although dependent on the slope and soil type in the natural channel, this distance is typically 20 to 50 feet.
Construction of a rip rap energy dissipater is relatively simple. One or more truck loads of limestone are dumped at the discharge point of the pipe or culvert. A backhoe then places rip rap along the bottom and up the sides of the channel as needed.
A second use of energy dissipation is the construction of a stilling basin. Storm water exiting a pipe or culvert is discharged into an existing basin of water which absorbs its energy. As water overflows the basin, it is discharged into the natural channel. Because of the large amount of energy released in a small, confined area, the basin must be constructed of large limestone rip rap or concrete. Care must be taken to design the outlet of the basin to avoid downstream erosion.
A third simple method to dissipate energy is to install headwalls in the path of discharging water. As storm water leaves a pipe or culvert, it hits a solid impediment causing the water to slow, dissipate energy, and either go around or over the impediment. On spillways and large outlet structures, rows of large concrete blocks are securely anchored in the path of discharging water. Water appears to boil, dissipating energy as it passes through rows of concrete blocks.
An alternative to concrete blocks is to use large limestone blocks in the storm water's path (as shown in Figure 18). The receiving area must be lined with rip rap capable of surviving the large mount of energy being released.
Figure 18 Limestone Headwalls
Drop structures in open channels change the channel slope. A long steep channel is converted to a series of gentle slopes and vertical drops. Flows are reduced to non-erosive velocities. The energy of a stream is reduced as water flows over the crest of each spillway and is dissipated by an apron or stilling basin.
Although the design of drop structures in large streams and rivers is highly complex, simple drop structures can be used in smaller channels. By planning, the flow line of an existing stream is modified into a short, level stretch followed by a vertical drop. Locations of stream modification are chosen with care to avoid removing trees and other vegetation. Where possible, use existing rock outcroppings for further erosion protection.
The crest of the vertical drop is constructed of large limestone blocks or reinforced concrete (as shown in Figure 19). Below the drop, the channel bottom is excavated several feet allowing the placement of large rip rap. Rip rap is placed up both sides of the bank and extended downstream a minimum of 20 times the height of the drop. In instances where a drop is severe, the crest and splash area should be made of reinforced concrete.
Figure 19 Drop Structure