Walnut Creek Stormwater Outfall and Pump Station

Project Name:Walnut Creek Stormwater Outfall and Pump Station
Location: Des Moines, IA
Client: City of West Des Moines, Partnered with Foth

Project Status: Construction to be completed in April 2018

Challenges V. Solutions

Challenge Rapid growth in the city of West Des Moines, Iowa resulted in substantial increases in stormwater runoff in most of the community’s watersheds. Such growth made it necessary to mitigate flooding on several major roadways to ensure emergency services were not impacted by closed roadways, as well as to take steps to minimize flooding commercial districts and residential neighborhoods.

Solution Borton Lawson teamed with Foth to provide a comprehensive solution for the City of West Des Moines. The mitigation solutions include construction of a new 12’ by 5’ reinforced concrete box culvert to intercept and convey interior stormwater flows, and a new 200,000 gallons per minute (gpm) stormwater pump station to discharge runoff to Walnut Creek during elevated flood stages. The stormwater pump station is proposed to be situated along Walnut Creek in an existing U.S. Army Corps of Engineers flood control project – the Des Moines, Iowa levee system. As a result, a U.S. Army Corps of Engineers Section 408 Permit was required for the modifications to the levee.

Hydrologic and hydraulic modeling was completed using XPSWMM 2014 SP1 to evaluate the required total capacity and optimal configuration for the proposed stormwater pump station, to handle both low and high flow precipitation events, using Grand Avenue as the key control elevation to manage flooding in the drainage area. The complex urban drainage area, with multiple collection and conveyance systems, as well as the evaluation taking into account storage in the proposed concrete box culvert, required the sophisticated software package to accurately model the system. The analysis resulted in approximately 200,000 gallons per minute (gpm) of pumping capacity was required to keep interior water elevations to acceptable levels.

The stormwater pump station structure required a complex structural design. A three dimensional analytical model of the pump station was created using finite element analysis. Each component of the structure (walls, slabs, foundations, etc.) was represented by a mesh of 1 ft. x 1 ft. elements that are interconnected to transfer shear, moment, and axial forces. Wall and floor slab thicknesses were computed, floatation stability of the station was determined, and foundation requirements were provided. In addition to the structural design, mechanical design was required for sluice gate intakes to the station, sluice gate outfalls to the Walnut Creek, influent trash racks, and for the pump configuration set points and discharge configuration.

The stormwater pump station was modeled with Revit software, allowing for 3-dimensional views of the pump station proper and various components. The use of Revit also allowed for continuous updating of quantities of materials as the design progressed, yielding very accurate construction items and quantities for cost estimating. Additionally, considerable attention was given to the visual appearance of the pump station. The use of the Revit modeling software allowed for various architectural treatments to be applied to the views in a rendering fashion, assisting the client in making decisions with respect to the pump station aesthetics.