Seattle Public Utilities

Typical Private Drainage System

Typical Private Drainage System

Your drainage system is composed of several parts, including roof drains, pipes and catch basins, which all typically flow to a detention or infiltration system. The City regulates the maintenance of drainage systems through the Stormwater Code (SMC 22.800-808). In general, a detention system consists of three parts:

  1. Flow Control Maintenance Hole: A concrete structure used to gain access to the detention system. It is usually recognized by a round metal maintenance hole marked “DRAIN.”
  2. Detention Storage: Usually a large underground pipe or vault connected to the flow control maintenance hole. The pipe or vault collects and stores stormwater during rainfall events. This water is then metered out by the flow control structure or pump.
  3. Flow Control Structure or Pump: This structure is located inside the flow control maintenance hole and regulates the flow of water being discharged from the detention pipe or vault. Flow regulation is especially important during peak flow events, which sometimes lead to combined sewer overflows. The flow control structure is a T shaped pipe with an orifice plate on the bottom and open on the top. The water gets metered out through a small hole in the orifice plate to the City’s drainage system or by a pump system.

Regular inspection and maintenance are necessary for these systems to function as designed. Properly maintained systems prevent flooding, reduce combined sewer overflow events and protect surface water quality. Seattle Public Utilities Environmental Compliance Inspectors evaluate the level of sediment in the system, verify that all necessary components are in place and functioning properly and check to be sure there are no illicit connections to the system.

If your system is in need of maintenance, the Environmental Compliance Inspector will provide you with a map of the property, a list of contractors who perform drainage system maintenance and a list of the required elements in need of repair or maintenance. As the property owner, you are responsible for completing the necessary maintenance within 30 days.