City of Kansas City, Missouri
When the City of Kansas City notified Water Resources Solutions on June 9, 2020, of a collapse in an existing, undersized multi-plate arch stormwater pipe installed in 1972 beneath a parking lot at the intersection of East 85th Street and Holmes Road, WRS engineers went to work immediately to study and recommend solutions under an emergency timeframe. As part of its certified Small Local Business Enterprise indefinite-delivery/indefinite-quantity on-call contract to provide a range of engineering, survey, technical and project-management services for the city, Water Resources Solutions studied options to achieve a stable channel, even as the city was daylighting the old pipe and removing the parking lot above it to create that open channel.
The project was complicated by numerous factors typical of emergency stormwater repairs, including lack of thorough survey data, discrepancies in the existing survey data, and grade line limitations caused by an existing sanitary sewer crossing. Because the HEC-RAS model provided by the city included the original culvert under the parking lot and thus was unusable, WRS created a new HEC-RAS model to guide design of the channel improvements.
Accounting for expected water velocity, roughness coefficient of the culvert, as-built slope, and peak modeled flow-rates, WRS first designed a 25-foot long by 2-foot deep D50=30” riprap structure to dissipate the energy of the water coming out of the remaining culvert at the upstream end of the channel.
Next, to control the inevitable erosion in the now open, 10-foot-deep by 16-foot-wide channel, WRS considered several stabilizing measures, including:
- A series of grade controls, one located at the sanitary sewer crossing and the second near the outfall of an existing arch pipe entering the channel on the right bank. The goal of using grade controls was to reduce the channel velocities enough that additional riprap would be unneeded.
- Increase in the grade control height to 3 feet. Because placement of the grade controls was constricted by the existing sanitary sewer crossing and the existing arch pipe, WRS considered achieving velocity control and channel stabilization by making the grade controls taller.
- Complete channel lining. Because the ideal grade control solutions involved introducing riprap, WRS also studied simply lining the channel in entirety, extending 5 feet up on each bank.
- Complete channel lining with reduced side slopes. The rock-lined channel option was further analyzed to consider the feasibility and effect of laying the side slopes back to 4-to-1 instead of 3-to-1. Above the riprap, WRS recommended planting native seed mix, trees, and shrubs up to 10 feet for further stabilization.
WRS worked with the city to design a final hybrid solution incorporating elements of several of its recommendations, to fit the limited emergency budget while recognizing future maintenance implications.WRS supplied construction drawings within just seven calendar days of the first contact, and construction was completed in less than six weeks after the original culvert’s removal. The engineering design cost was $6,518.