Abstract: Session E 1:50 pm (Back to Session E)
Down the Drain and Back up Again: Urban Stream Restoration in Pittsburgh’s Four Mile Run Watershed (aka M29 Sewer Shed)
Civil & Environmental Consultants, Inc.
Authors: Stewart, R. L., Nuttle, T.J., and Ober, N.S.,
The Four Mile Run Watershed in Pittsburgh is an urbanized area comprised of mixed land use including highly impervious university campuses and forested parks, all 2,332 acres of which eventually flows through the combined sewer system. During urbanization most of the streams in Four Mile Run where converted to combined sewers and buried underground with the exception of streams located in the historic 456 acre Schenley Park. Combined sewer overflows, basement backups, and loss of park trail systems necessitated stream restoration/daylighting to help preserve cultural resources and create healthy flood prepared communities. Challenges that faced restoration included a network of roads and railroads along with utilities ranging from a 50” water main to 10” buried electric lines and other infrastructure to numerous to list. This presentation illustrates the design methodology and unique solutions used to restore Four Mile Run within the constraints of a highly urbanized and altered watershed (Aka M29 Sewer shed).
Restored stream networks in the park system were designed to function with existing stream flow and to accommodate additional storm water to be disconnected from the sewer system. A multi-stage channel approach was utilized to reduce shear during floods while increasing flow depth during low flow for aquatic habitat. Stream and trail restoration designs work symbiotically to reduce sediment inputs, attenuate peak flows, and accommodate multiple competing uses for space within the project area.
In high gradient narrow valleys, trail grading is placed along the floodplain extents to buttress landslide prone slopes and modulate floodplain expansions/contractions thereby reducing valley slope sediment inputs and instream scour potential.
In low gradient wide valleys, trail grading is strategically placed in the valley bottom to create constrictions and off channel wetlands to detain floodwaters during high flow events while creating an intimate interaction between park visitors and the restored ecosystem. A through trail was designed along the low gradient valley wall to reduce landslide potential and facilitate commuter traffic through the project reach.
The Four Mile Run project consists of many thousands of feet of restored and day lighted streams in a highly urban watershed that negotiated a gauntlet of infrastructure.