The principle of Concentrations of Flow appears in many aspects of drainage. Understanding it is critical to proper drainage design. It simply states that as water moves from larger areas through smaller areas, flow rates increase.
We see this every day on our golf courses when water runs off of hillsides. Not only is the bottom of the slope the wettest, but areas that run between mounds or valleys where the flow travels will be the wettest. These areas have to handle not only the water that is falling on the flow area, but the water that has fallen on the watershed above it. If we don’t collect this water before it concentrates in these areas, the heavy Concentrations of Flow will inundate an area to a point where it cannot be mowed. However, if we recognize these areas as opportunities to collect the water in the most concentrated part of the stream, we can become highly efficient with our placement of basins.
In the pictures shown, the placement of a basin in the narrow areas of flow allows us to collect large amounts of water inexpensively before it saturates the area around the basin at the bottom of the collection area. Thus, the objective here is to reduce the Concentrations of Flow by placing surface inlets higher in the stream’s elevation. In the next picture, we have water running across a cart path in a sheet. In this case, we want to increase the Concentrations of Flow by placing curbing in combination with an inlet.
Recognizing Concentrations of Flow during the planning of your drainage projects will allow you to collect the largest amounts of water for the least number of dollars.