Flow Control Terminology in King County
Level 1 Flow Control
Level 1 Flow Control is designed to control flood flows at their current levels and to maintain peak flows within the capacity of the conveyance system for most storm events. Specifically, Level 1 flow control requires maintaining the predevelopment peak flow rates for the 2-year and 10-year runoff events.
The Level 1 flow control standard is typically applied to basins where studies have shown additional flow attenuation provides no significant benefit to the receiving waters.
Level 2 Flow Control
Level 2 flow control is designed to control the durations of geomorphically significant flows and thereby maintain or, in some applications, reduce existing channel and streambank erosion rates. A geomorphically significant flow is one which moves channel bedload sediments. The flow that initiates transport of channel sediments varies from channel to channel, but one-half of the 2-year flow is considered a good general estimate of the erosion-initiating flow. More specifically, Level 2 flow control requires maintaining the durations of high flows at their predevelopment levels for all flows greater than one-half of the 2-year peak flow up to the 50-year peak flow.
The predevelopment peak flow rates for the 2-year and 10-year runoff events are also intended to be maintained when applying Level 2 flow control.
Level 3 Flow Control
Level 3 flow control is intended to mitigate water level changes in certain volume-sensitive water bodies such as lakes, wetlands, or closed depressions where severe flooding problems have been documented. It is the most stringent standard applied in the Design Manual. Because such water bodies act as natural flow dampeners, it is difficult to detain collected stormwater beyond the natural residence time of these systems. The Level 3 flow control standard provides additional storage and increases the detention time to minimize these downstream impacts.
This standard requires maintaining the durations of high flows at their predevelopment levels for all flows greater than one-half of the 2-year flow up to the 50-year flow and holding the 100-year peak flow rate at its predevelopment level.
This standard is primarily applied in the contributing areas of specific water bodies with severe flooding problems, and which are known to be sensitive to flow volume changes.
This information was excerpted from Section 3.1.2. of the 2009 King County Surface Water Design Manual.