City of Spokane Hazel’s Creek Stormwater Facility

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.

Approach

In-depth evaluation and analysis

• Reviewed results of previous site investigation, including hydrologic data, geophysical survey results and drilling exploration data
• Monitored site hydrologic conditions, including groundwater and surface water elevations as well as surface water, stormwater and spring flow
• Sampled surface water, groundwater and stormwater to define water sources and zones of mixing
• Evaluated physical and chemical aspects of groundwater/surface water interaction between Hazel’s Creek and the underlying aquifer and the hydrologic response to precipitation and snowmelt events
• Estimated the infiltration capacity of the facility
• Defined how groundwater from the facility would travel through the aquifer

Numerical groundwater flow model

• Created a numerical groundwater flow model that estimated the capacity of the facility and proposed supplemental downgradient infiltration structures
• Identified potential impacts to nearby residents and future developments from enhanced facility operation

Commitment to a solution

• Worked with the City to evaluate a number of scenarios for extending a conveyance system from the facility to a portion of the aquifer better able to accommodate increased stormwater

Open communication with public and stakeholders

• Presented a three-dimensional animation to the community, sharing findings and results gained from the evaluation

Results

• Provided information that enables the City to fully evaluate the Facility’s hydrogeologic setting and stormwater disposal capacity
• Helped the City maximize the facility’s capacity to safely dispose of stormwater
• Created a sustainable solution to increase recharge to the area aquifer system while enabling future development