8.3.1 Closed Depressions

The analysis of closed depressions requires careful assessment of the existing hydrologic performance in order to evaluate the impacts a proposed project will have.

Closed depressions generally facilitate infiltration of runoff.

If a closed depression is classified as a wetland, then Minimum Requirement #8 applies. Review Vol I–4.2 Minimum Requirements to determine which other Minimum Requirements apply to the site.

An Ecology approved continuous simulation hydrologic model must be used for closed depression analysis and design of mitigation facilities. If a closed depression is not classified as a wetland, model the ponding area at the bottom of the closed depression as an infiltration pond using an approved continuous simulation hydrologic model.

Infiltration shall be addressed where appropriate. Refer to Volume I, Chapter 2 for site planning of infiltration facilities and Volume II, Chapter 5 for design of infiltration BMPs.

If a proposed project will discharge runoff to an existing closed depression, the following requirements must be met:

• Case 1 – For closed depressions located entirely on site, and where no runoff occurs in the pre-developed condition, no runoff may leave the site in the developed condition. If the modeling indicates the facility will overflow in the developed condition, the closed depression may be modified to provide the required storage, or may be modeled as a combination infiltration/detention facility with control structure and emergency overflow weir, access road, etc., in accordance with Volume II, Chapter 5. The required performance shall meet the flow duration frequency standards per Minimum Requirement #7. To determine whether runoff occurs in the pre-developed condition, the pre-development runoff time series from the drainage basin tributary to the onsite closed depression shall be routed to the closed depression using only infiltration as outflow.

• Case 2 – For closed depressions located entirely on site, where runoff occurs in the pre-developed condition, the closed depression shall then be analyzed as a detention/infiltration pond. The flow duration frequency standards per Minimum Requirement #7 shall be met.

• Case 3 – If the closed depression is located partially or completely off site, impacts to adjacent properties shall be evaluated and appropriate mitigation provided including any downstream easements. If offsite easements can be obtained, the closed depression may be modified to meet the required performance standard as in Case 1. If offsite easements cannot be obtained, then the total volume of runoff discharged from the project site may not be increased above the total pre-development runoff volume.