West Plains Stormwater Management Study

Background

The West Plains area is one of the fastest growing areas in Washington State. To accommodate this growth, the West Plains Public Development Authority (PDA) is conducting a study that will result in an action plan for expanding and improving stormwater management in the West Plains.

What is Stormwater Management? Stormwater Management is the practice of controlling runoff from precipitation to prevent flooding and protect water bodies. This may include conveying runoff through pipes and ditches to "best management practices" (BMPs), which remove pollutants and either infiltrate runoff into the ground or discharge treated runoff to surface water bodies, such as rivers or lakes.

Several unique challenges and opportunities with managing stormwater in the West Plains PDA include:

  • Soil Conditions: Slow infiltrating soils and high groundwater result in limited options for BMPs and/or onsite BMPs may not be feasible on some parcels.
  • Wildlife Concerns: The West Plains is located near the Spokane International Airport and Fairchild Airforce Base. Wildlife habitat must be handled with care near these airfields to limit the potential for aircraft bird strikes. Because of this, BMPs with ponded water are not feasible.
  • Paleochannels: Within the West Plains, paleochannels are stream channels carved in basalt bedrock that were filled with sediment during the Glacial Lake Missoula floods of the most recent ice age. These channel sediments have significantly higher capacity for infiltration than the surrounding soils. Paleochannels are known to exist in the West Plains and further investigation is needed to determine their potential for infiltrating stormwater runoff.

Location

The West Plains PDA boundary is generally defined as I-90 to the southeast, Fairchild Airforce Base to the west, and the City of Airway Heights to the north. The boundary is shown below.

West Plains PDA Boundary Map

Goals & Approach

The goal of this study is to develop an action plan for expanding and improving Stormwater Management in the West Plains. The long-term vision is to construct new regional stormwater infiltration facilities (RSIFs) in the paleochannel areas and conveyance systems to route stormwater runoff to the RSIFs.

The study will be accomplished through the following general steps:

  • Collect Data: Collect data on contributing basin areas, runoff patterns, soils, groundwater, paleochannels, and drainage complaints.
  • Identify RSIF and Conveyance Options: Identify potential locations for the RSIFs in the paleochannel areas and routes for conveying runoff to the RSIFs including potential combined stormwater facility and open space / passive recreation opportunities within the PDA boundary.
  • Identify Guidance for Developers: Create stormwater management guidance to assist developers in evaluating whether onsite BMPs are feasible in specific areas.
  • Evaluate Conveyance Options: Calculate the expected runoff from developments, evaluate regional facilities, and determine whether gravity or pumped conveyance will be needed.
  • Evaluate Paleochannels: Study the capacity and feasibility for paleochannels to receive stormwater.
  • Develop a Capital Improvement Plan (CIP): Incorporate RSIF and conveyance options into a CIP, conduct a cost-benefit analysis to select and prioritize projects, and identify funding options to support the design and construction of projects.

Timeline

  • Summer 2019 – Collect Data
  • Fall 2019 – Identify Alternatives
  • Winter 2019 – Evaluate and Select Alternatives
  • Spring 2020 – Develop Capital Improvement Plan

Study Team

  • West Plains/Airport Area Public Development Authority
  • Spokane County
  • City of Spokane
  • Spokane International Airport
  • Osborn Consulting, Inc.
  • GeoEngineers
  • SPVV Landscape Architects
  • FCS Group