Watershed Drainage System and Outfalls

The Thornton Creek Watershed has a number of drainage types within it; natural drainage, drainage ditches, piped storm drains, piped combined lines, and a piped sewer system. Within this network, the piped drainage network has 84 outfalls, all of which should be stormwater. Four of these outfalls are to Lake Washington, and 80 are to the creeks and ditches drainage system which, along with the other drainage types, are briefly described in structure and status below.

Natural Drainage

A natural drainage system is comprised of the naturally occurring creeks. Though the Thornton Creek system is called a natural drainage system and most of the larger creeks have been left above ground, the system is still highly modified and impacted by human inhabitation. The larger creeks are frequently interrupted by culverts and storm drains, and some of the smaller streams have been put into pipes, (see Drainage Network for details).

In this drainage system these stormwater flows have been shown to increase pollution, thereby decreasing water quality. Roads and other vehicle bearing surfaces are primary contributors to pollution. The impacts of development to the natural drainage system go beyond water quality problems, and include water quantity problems.

Drainage Ditches - Drainage ditches (not shown) are lined open channels that are usually adjacent to roadways. The lining can be of various materials such as asphalt, rock or vegetation. In the Thornton Creek Watershed, these ditches usually discharge to the creek system. The implications of this to the natural creek system are the same as those noted above.

Storm Drains - Storm drains are the traditional hard-infrastructure piped drainage network which is, in this section of the city, separate from the sewer network for the most part.

Combined Mainlines - The Combined Mainlines, which are dominant in the City of Seattle, in the Thornton Creek Watershed are present only in the most southerly part of the watershed. Combined Mainlines are those that are responsible for moving both stormwater and sewage.

Sewer Network - The Metro Sewer Mainlines are what carry sewage to the West Point Sewage Treatment Plant. The Sanitary Sewer Mainlines carry sewage from the lateral lines (not shown) to the Metro Sewer Mainlines.

Service Rates - For stormwater service, residential properties are charged a flat rate of about $81/ year in Seattle and $85/ year in Shoreline. Other properties pay by lot size and percent impervious service. The City of Seattle is considering incentives programs to encourage property owners to take measures that improve water quality and water quantity. The City of Shoreline already offers fee reductions to commercial property owners who maintain stormwater management facilities to a given set of standards.

Property Owner Responsibilities - The individual property owner is held responsible for any improvements to their local infrastructure.