Indian Trail Stormwater Study

When it rains, it pours. But when it really pours, Varela determines how to handle the stormwater.

On May 2, 2016, the Indian Trail residential neighborhood in northwest Spokane experienced an unusually heavy rainfall that overwhelmed the City’s drainage system, resulting in extensive flooding and severe localized erosion to residential properties. The heavy rainfall and lack of adequate drainage caused property damage including loss of backyard property and landscaping, basement flooding, damage to a backyard pool as well as damage to City streets and the drainage system. Neighborhood residents met with the City to develop an action plan.

Recommendations based on observation and modeling

Varela was hired to determine if the existing drainage systems were designed to meet the conveyance and flow control requirements for the 10-year design storm, use field observations and hydraulic modeling to identify possible causes of the flooding; and provide options to prevent, moderate, or mitigate damage due to future stormwater events.

Cause and effect

Modeling of the drainage system was performed using Autodesk Storm and Sanitary Analysis, an SWMM-based program. The model was run using a number of design storms, including the 6-month water quality storm, a 10-year storm used by the City for design, and the May 21, 2016 storm event.

Following completion of the observations and modeling, an engineering report for each area was prepared. The Technical Memos (TMs) included a description of the reason and purpose for the work, a description of the field observations, modeling results, the probable cause of the flooding, and recommendations to address future flooding.

The analysis showed that the May storm event was a short duration, high-intensity storm with a peak rainfall approximately twice the intensity of a 100-year storm event. Both the hydraulic model and the real-life event showed that the City’s drainage system was not designed to collect and convey stormwater of this intensity, but was adequate to convey the six-month design storm. 

Making it right

Varela’s recommendations included grinding asphalt catch basins to increase inlet capacity, installation of more drywells to increase infiltration, and in one location, to re-establish the pre-development drainage path. 

In 2018, the City began working on improving the drainage system. Staff indicated that the recommendation to increase maintenance has been helpful for their efforts to properly maintain the system.