Detention vs. Compensatory Storage: Know the Difference
By Charlie Penland, PE, LEED AP/ Director of Civil Engineering
While both storm water detention and floodplain compensatory storage are based on the same basic concept—storing water while keeping the overall drainage system in balance—the terms are not synonymous. The first element that should be considered is that at any given point during a rainfall event, there is a specific volume of water that has fallen. Drainage systems are designed to determine where, how, and to what extent that water will be directed.
Consider natural, undeveloped areas. Nature designs organic methods of drainage management such as ponds, waterways, wetlands, and vegetation that absorbs moisture trapped in the soil. While flooding still occurs in these areas, natural features are quicker to adapt to changes in moisture, often creating breathtaking landscapes in the process.
As humans evolved, we worked to manage nature, changing how and where rainfall is captured and routed so that the areas we inhabit don’t mirror those landscapes; such as the Grand Canyon or the Great Lakes. Early development drainage was designed to remove water as quickly as possible from inhabited areas. However, by simply redirecting the water from one location to another, secondary inhabitants were forced to handle the excess runoff created by the new development and “improved” drainage systems. This was especially burdening if a new development drained to an older development which may not have been designed to manage the increasing volume of water.
Flood control reservoirs have long been a flood control strategy for development; however, smaller “detention” reservoirs became popular in the 1970s and even more widespread in the 1980s. By design, storm water detention ensures that water leaving the area is controlled and limited to predevelopment flow rates. Therefore, the excess development runoff is captured and “detained” long enough and in sufficient volume to allow for controlled release protecting areas downstream.
Humans also started developing near rivers and streams for access to water supply and transportation since the proximity to water was important. However, the waterways near the developments and their overbanks were formed to carry extreme event rainfall runoff from the contributing watershed. Flooding of development areas became a problem. These waterfront developments proved to be too close to the waterways in what we have defined as the floodplain. Newer developments next to streams and rivers were filled to raise the area above the floodplain allowing for continued development with less risk of flooding. The fill displaced the floodplain storage pushing water to places it was not previously stored, causing flooding elsewhere. To address the loss of floodplain storage, the concept of floodplain compensatory storage was developed requiring any loss of floodplain storage to be compensated for in the adjacent floodplain.
These two storage concepts are separate and additive based on the impacts resulting from new development. It is imperative that storm water detention be able to capture the intended runoff before it leaves the development and reaches the receiving stream. It is also essential to account for floodplain storage in the floodplain to protect the downstream areas along the stream. These compensatory storage systems must allow floodwaters to freely flow into the storage area in the same manner that it did pre-development, as well as being able to empty at the same rate as the receiving stream water surface falls so that storage will be available for the next storm. As engineers, we should glean lessons from nature in order to create and maintain drainage systems that are successful, efficient, and yield low impact.