Storm Drainage Study in Downtown

EDMOND, OKLAHOMA

The City of Edmond contracted with Cabbiness Engineering to prepare a detailed engineering analysis of the existing downtown storm sewer systems, particularly the area adjacent to 100 North Broadway (northeast corner of Hurd and Broadway).

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The City of Edmond provided Cabbiness Engineering with the latest city wide contour data, as-built construction plans, aerial images, and storm sewer information from the city’s GIS system. In addition, a specific topographical survey was conducted on the existing downtown storm sewer system that also serves the area adjacent to 100 North Broadway.

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The existing storm sewer in downtown Edmond included approximately 8,800-feet of pipe and 81 storm inlets. During the topographical survey, nearly all pipes and inlets were located. Additional pipe and inlet locations that were not found by topographical survey were verified via aerial photography, City of Edmond as-built plans or actual field reconnaissance. The lengths and grades on these pipes and inlet properties were determined based on connecting structures and City of Edmond contours. Furthermore, pipes with reverse or flat grades had to be assumed with a positive grade for the design software to model the storm sewer system. It should be noted that some of the grade issues were due to silted in inlets or pipes and the survey crews not being able to obtain completely accurate data.

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Based upon our engineering analysis of the existing storm sewer system and conceptual design solutions, the following conclusions were reached:

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The existing storm sewer system was undersized using the current City of Edmond design guidelines. Inlet capacities were much less than what was required to convey current storm water runoff for 2, 5, 10, 25, 50, and 100 year storm events and contributed to the currently observed water spread widths and depths. Pipe capacities were also less than what was required to handle the current predicted storm water runoff.

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Without increasing the pipe or inlet capacities, any other measures taken either diverted the same drainage and flooding issues further downstream or provided a very limited improvement. So, adding a concrete flume through the sidewalk at the northeast corner of Broadway and Hurd was predicted to reduce the water spread and depth in the street.