An Analysis of Flow Attenuation Provided by Stream-Buffer Ordinances in Johnson County, Kansas

Abstract

Many communities in the United States have enacted ordinances requiring that areas surrounding natural channels be preserved in a natural state. These areas are commonly referred to as "stream-buffers". One of the goals of the stream-buffer is to preserve dense overbank vegetation. This has the effect of attenuating peak flows during flood events which inundate the channel overbanks. The goal of this study is to use state-of-the-practice hydrologic and hydraulic models to estimate peak-flow attenuation provided by stream-buffers using vertical variation in Manning's n values. Accounting for vertical variation in Manning's n values in overbanks allows for simulation of the roughness of the overbank provided by zones of vegetation. Typical zones include dense grasses and undergrowth at low overbank depths as well as heavily treed zones at higher depths. An existing 2.6 miles stream reach was evaluated for this study. Hydrologic modeling was completed using HEC-HMS and hydraulic modeling was completed using HEC-RAS. Existing models completed for the Blue River Watershed Study in Johnson County, Kansas, were modified for use in this study. A maximum peak-flow attenuation of 20% was observed for the 2-year and 50-year events over 3,110 feet. The highest maximum peak-flow reductions were observed for events ranging from the 2-year and 100-year events, and a smaller maximum reduction was observed for the 500-year event. Another goal of this study was to compare the results to stream-buffer ordinances in Johnson County to evaluate if they provide the maximum attenuation of peak flows possible at the case study site. The results showed that maximum attenuation is achieved by the ordinances for events ranging from the 2- to 10-year events.