There are various structures and design techniques that can lower the runoff velocity to prevent erosion and encourage sedimentation.1 Since runoff can carry a lot of sediment and other pollutants, urban areas can install different devices to allow runoff to flow at slower rates so that these materials can be absorbed into the soil, a process known as infiltration.2
Some of these devices may include concrete structures or porous barriers.3 Drop structures are concrete designs that can be used to stabilize waterways, serve as outlets for water flow, or act as sediment traps. Riprap is an example of a porous barrier that consists of a layer of stones or cobbles lining a channel for the purpose of increasing surface roughness and slowing runoff velocity.4
In addition, grassed waterways can reduce runoff velocity and remove excess runoff by absorbing sediments and other substances.5,6 This solution can divert water while reducing both soil erosion and flooding.
How it Helps
Implementing
Flooding Potential:
Although reducing runoff velocity has several positive impacts, including preventing soil erosion and removing polluted runoff, it may not be applicable in certain areas where higher velocities are required to maintain drainage rates.1 Some highways and roadside surfaces may need to be drained quickly to prevent flooding and other roadway safety hazards. Therefore, planners and drainage engineers should consider these factors when determining where to implement devices to slow runoff velocity.
Design Characteristics:
Depending on the surrounding environment, engineers and other experts will have to choose which structures and designs to reduce runoff velocity are best suited for a particular area. Drop structures, ripraps, grassed waterways, and other strategies each have advantages and disadvantages for certain situations that should be carefully analyzed prior to construction.
Examples
1) Drop Structure Design
There are many examples of drop structure, riprap, and other possible designs available for drainage engineers and planners to utilize when determining how to construct one of these structures. Many of these publications or reference manuals provide comparisons between the various subtypes of structures, details regarding the necessary supplies, and other design considerations.