Erosion Hazards Mapping in the Stevens Creek Watershed

Stevens Creek and its tributaries drain 85 square kilometers of largely agricultural land facing imminent development. The City of Lincoln and the Lower Platte South Natural Resource District (NRD) requested a plan to protect the incoming development from flooding and erosion hazards, while protecting the watershed's natural resources. The commissioned master plan included a geomorphic analysis, as well as a hydrologic and hydraulic model. Quantitative information on stream behavior is critical for any watershed plan. Recent advances in GIS, GPS, remote sensing and field-rugged computers now enable quantitative analyses at a manageable cost. Using the City's GIS layers of aerial photographs, topography and soil types, we performed a drainage network, planform and profile analyses of Stevens Creek and its tributaries. Geomorphic data gathered in the field was collected directly onto a series of GIS layers allowing real-time analysis and presentation. The geomorphic analysis, coupled with hydrologic and hydraulic modeling, enabled the generation of an erosion hazards map, analogous to a flood map. We correlated anomalies in meander geometry or profile slope with soil properties. From the analysis of stream shape and material strength, we determined resistance of the stream to channel adjustment, associated with changes in land use. By comparing the future-condition hydraulic stresses from a HEC-RAS model against the existing channel strength, we were able to infer the likely locations, extent and mechanism of channel adjustment. The results provided the City and Lower Platte South NRD with a defensible basis for planning decisions such as riparian buffer zones, required stormwater controls and stream protection strategies.