BG Consultants conducted the Eastside Drainage Study for the City of Manhattan following completion of work on the City’s Stormwater Management Master Plan and Northview Drainage Study. This drainage study focused on 630 acres of commercial and industrial land in the Southeast quadrant of the City that was experiencing rapid commercial development and redevelopment. Due to the growth and relatively flat topography, the area had multiple existing drainage issues. The purpose of this study was to identify problems in the existing system and predict the system’s ability to handle further development in the area.
The stormwater system consists primarily of open channel ditch sections with culverts. Using survey-grade GPS equipment, BG surveyed culverts, open channels, curb inlets, area inlets, and piping of the existing system. For each of the five major watersheds in the area, BG created hydrologic models using HEC-HMS software. The peak flows were routed through the existing system model for 2, 5, 10, 25, 50 and 100 year design storms using frequency-based rainfall intensity tables. Existing deficiencies were identified. The model was adjusted for full future development and peak flows through the system were determined for the same design storms. Using the model, BG was able to indicate additional deficiencies that would result from increased flows. With this information, BG made prioritized recommendations for existing and future improvements, complete with estimates of probable cost. The study serves as an important planning tool for the City and developers now and into the future.