Anatomy of a Storm Drain

Familiarization with properly functioning drainage systems is key to being able to recognize when storm drain infrastructure is failing. Understanding the consequences of clogs and other maintenance problems is the first step in resolving them. Knowing the Anatomy of a Storm Drain, and the support available from R+R, can help keep your customers from stepping out of the car and into a pool of standing water.

Headwaters: Drain Inlets

The primary components of a storm drain, also known as a type 1 catch basin, include an inlet, riser, basin, and piping (type 2 catch basins have the same function, but are used when the system is deeper than five feet or the piping is larger than allowed for type 1). In combination, these components work to divert stormwater runoff and help prevent flooding. Each part can vary in design and size, and the inlet is no exception. Typically located at ground level, the inlet can be assessed with a quick visual inspection. Inlets work to allow water to enter the stormwater system while restricting entry to large objects, and debris that could catch in the basins. Grate inlets are frameworks of metal bars inlaid on the ground that open into a vault beneath them. The primary maintenance concern for inlets is blockage, since sediment buildup around the inlet or clogging with leaves or other debris can seriously impair the ability of water to move into the system. To avoid this, your storm drains should be checked at least once a month, with an increased frequency after storms and in the fall.

Inserts can also be added to the grates to assist with water quality improvements. Inlet inserts can be boxes, baskets, fabric, media filters, or screens. These can treat the water through screening, filtration, or even sedimentation depending on the design. Generally, they reduce total suspended solids and litter, while some proprietary designs address additional parameters such as dissolved metals. Inserts provide an additional line of defense against debris entering the basin, but are an additional maintenance consideration.

Body of Work: Form and Function

Once the stormwater passes into the storm drain through the inlet, it enters a sump. These varieties of basins are connected by storm pipes, and can have an inlet pipe and an outlet pipe. Often the outlet pipe is lower than the inlet, and the distance from the outlet pipe to the bottom of the sump.  Both pipes are typically above the midline of the chamber, allowing sediment to settle out before less contaminated water continues on; however, there are pass-through basins with no sumps where the outgoing pipe is at the very bottom of the basin. Most drainage pipes are connected to the larger network of storm piping and usually end in a single discharge point at a best management practice (BMP). In this way, the basins and pipes act as pretreatment for other BMPs, such as a bioswales or retention ponds. The pipes themselves may also contain pretreatment features such as waterfalls and stairways.


Image: Catch basin with over a foot of accumulated sediment.

The sump, like the inlet, needs to be regularly checked for debris. Inspecting the internal components of the catch basin requires the proper equipment and training, and partnering with a stormwater expert like R+R will let you rest assured that your storm drains are being taken care of with the utmost consideration for your business and the environment. To inspect the basin, the grate is removed and sediment accumulation in the vault is checked. Requirements for cleaning are often regulation driven and vary by municipality. For example, the State of Washington requires that catch basin cleanings occur if the layer of deposited debris and sediment comes within six inches of the lowest pipe or is over sixty percent of the vault depth, then the sediment needs to be removed (usually requiring the assistance of a vactor truck).