City of Leavenworth, Kansas

2017

Runoff from an adjacent undeveloped hillside led to both residential flooding and erosion in an open channel behind homes in this Leavenworth neighborhood. The existing stormwater system consisted of an aging 54-inch corrugated metal pipe connecting several curb and area inlets before discharging into the channel behind the homes — all in tight proximity to the nearby residences.

Water Resources Solutions' preliminary feasibility study first analyzed the expected existing conditions runoff from specific design storm events using USDA-NRCS's Technical Release 55 methodology. For the future developed condition, the method outlined in APWA 5600 using the average channel slope and associated velocity was used to calculate the time of concentration and expected runoff. Runoff time-series graphs for each of six identified drainage areas were calculated using the U.S. Army Corps of Engineers' Hydrologic Modeling System. Two additional drainage areas were analyzed for the open channel. Capacity of the existing and proposed pipe systems was evaluated using the EPA's Storm Water Management Model running the 10-year storm event.

Results found the flooding was attributable to a combination of the system being sized at only 75% of necessary capacity to convey the design event, along with inlets sized too small to collect the runoff. Three alternative solutions were developed to improve system capacity:

• Increasing the capacity of the existing system by increasing pipe size, changing pipe material to reduce the friction factor, and adding and improving collection structures.

• Increasing the capacity of the existing system by constructing a detention basin just upstream, to detain the 10-year storm event and reduce the peak flowrate into the enclosed stormwater system.

• A combination of the two.

In addition, WRS's preliminary study also identified seven potential future detention sites, strategically located within the impacted drainage area and based on their connectivity to the natural drainage patterns of the currently undeveloped land upstream. Those locations can be built out as needed to accommodate future anticipated development in the area.

WRS completed the final design of the detention basin utilizing the hydrologic analysis and results from the preliminary engineering study and using HEC-HMS to size the detention basin and determine the outfall rates into the system.