Downtown Corridor Streetscape Low Impact Development Improvements Pre-Design Report

Client—City of Longview, Washington

Gibbs & Olson was hired to provide the geotechnical investigation, stormwater modeling and pre-design report for the proposed Low Impact Development (LID) improvements to the City’s main downtown corridor. The modeling was completed for 33 bioretention basins, crosswalks and one parking facility. In preparing the stormwater modeling and pre-design report, Gibbs & Olson provided peer review of the proposed stormwater design and recommended improvements to the proposed stormwater layout. Utilizing a geotechnical subconsultant, the geotechnical investigation provided information on the infiltration capabilities of the project area, which was incorporated into the stormwater modeling. In addition, a groundwater mounding analysis was prepared to provide the City with the potential impacts of stormwater infiltration to existing businesses adjacent to the property.

The objective of this project was to showcase LID techniques, raising public awareness of the important connection between the built environment and water quality. Additional project goals included; reducing pollutants entering the downstream receiving waters, addressing drainage issues, and generally improving the overall appearance and character of the downtown area.

Bioretention planters were recommended as the primary stormwater treatment method for the LID retrofit project as these facilities are well suited for an urban environment, provide enhanced treatment for roadway runoff, and reduce pollution generating impermeable surfaces. Other treatment methods recommended included maximizing the area of permeability surfacing which will help to reduce the volume of street runoff.