Using Stream Assessments to Incorporate Natural Channel Design and Minimize Erosion
Managing Urban Streams
Managing urban streams involves reducing floods and erosion and improving water quality. Flood and erosion control dominate most urban stream improvement projects, but often there are opportunities to simultaneously incorporate natural channel design features and improve water quality, urban habitat and aesthetics. The urban stream environment often prevents the design of a truly “natural” solution, but proper assessment of stream restoration activities can help reduce erosion and provide opportunities to improve a stream or creek’s natural setting and functions.
What is a Stream Assessment?
A stream assessment helps evaluate the existing and long term stability of an urban stream channel using standard methodologies of fluvial geomorphology and helps determine appropriate natural channel design techniques. Steps include:
- Evaluating the Soil, Land Use, Channel and Landscape Slope, and Geology of a Basin – This evaluation is based on field and map data.
- Calculating Channel Forming Discharge Using Regression Equations and Routed Flood Flows Obtained from Unit Hydrograph Models for the Watershed – The channel forming discharge (or bankfull flow) is often equated between a six-month and two-year return frequency event and varies with the degree of urbanization within a watershed.
- Characterizing Bed Material to Determine the Varying Degree of Soil Particle Sizes Contained in the Stream Bed – This is integral to proper assessment of channel equilibrium slope as well as planform and vertical channel stability. A bed material movement assessment is done to determine the potential for aggradation or degradation of the channel. By using bed transport equations, one can assess the probability of bed material movement under different channel slopes and discharge.
- Estimating the Limiting, Equilibrium or “Ultimate” Stable Channel Slope – This is accomplished by combining bed material gradation with channel forming discharge. Various methods can be used to calculate the stable slope, depending on site conditions and the applicability of the equations with the design reach.
- Understanding Increased Erosion – This is key in a stream assessment. It must be factored into the development of restoration objectives and management alternatives.
A Note on Erosion
Accelerated bank erosion may often be attributed to increases in peak runoff to a stream when the surrounding watershed undergoes land use changes. Additional erosion factors can include loss of bank vegetation, which increases the vulnerability of the bank to erosion or man-made structures in the stream that redirect water into the bank. Critical failures in streams often occur on a local scale, and a single break in continuity can start a chain reaction that affects the entire stream corridor.
Putting Your Stream Assessment to Use
Stream assessment parameters, new channel dimensions, proposed drop structure locations and sizes, bridge alterations, and modifications to channel roughness all can be modeled to assess impacts to the stream and provide for adjustments. Stream corridor restoration/stabilization should include an assessment of current as well as future land use, projected development trends and landscape concerns in the watershed. Accommodating future land use and development patterns can further help prevent or reduce stream