Spokane’s South Hill is a desirable residential neighborhood in a growing metropolitan area. As the area’s residential, commercial and retail development grows, managing the increasing stormwater runoff continues to be a challenge, especially during the rainy months.
The City of Spokane and Spokane County recognized the need to add stormwater management capacity with new storage, conveyance and disposal facilities that would keep pace with regional development and minimize major storm and snowmelt impacts. The Hazel’s Creek Stormwater Facility that currently handles South Hill’s stormwater is located in a natural drainageway and supports a short stream that emerges on one part of the South Hill site and infiltrates into permeable sediments in another part of the site. These permeable sediments are unusual within the South Hill area, which generally is underlain by low-permeability soil and rock. The City engaged GeoEngineers to perform an in-depth hydrologic investigation of the facility to determine where the infiltrated water was flowing and whether running more stormwater through this the facility would adversely affect nearby residents.
Using an interdisciplinary approach of drilling exploration, surface geophysics and three-dimensional modeling, GeoEngineers’ project team located and delineated a previously undiscovered sand-and-gravel aquifer that occupies an ancestral drainageway and extends about 1.25 miles before discharging to a larger aquifer system. This newly discovered aquifer had potential to handle increased stormwater runoff from future development. However, GeoEngineers identified a significant flow-limiting constriction a short distance downhill from the facility.
GeoEngineers used GIS tools to create a three-dimensional “fly-through” of the subject aquifer that enabled the City and other stakeholders to visualize how well the aquifer would handle increased stormwater flow. The team also constructed a numerical groundwater flow model to simulate the impacts of various development scenarios on aquifer conditions. Modeling results indicated that the aquifer could support additional stormwater infiltration at the facility without flooding impacts. Unfortunately, because of the aquifer constriction GeoEngineers had pinpointed, this additional capacity is finite and the facility alone cannot serve as the sole long-term solution to the South Hill’s stormwater needs.
Working with City stormwater engineers, GeoEngineers developed and evaluated alternative scenarios for extending stormwater pipe from the facility to highly permeable sand and gravel located nearby, which would enhance the stormwater capacity of the system as a whole. This one-of-a-kind stormwater evaluation helped the City understand its subsurface resources for stormwater disposal and, importantly, helped avert potential adverse impacts to South Hill residences that may have resulted from unrestrained use of the facility.
Using hydrologic instrumentation, GIS analysis and groundwater flow modeling, GeoEngineers discovered a subsurface drainageway that can be a part of the solution to the area’s stormwater disposal challenges.
Susan King, Planning Specialist
City of Spokane