Module # 8 Channel Evolution Implications & Drivers of Instability Iowa’s River Restoration Toolbox Level 1 / Base Training

Channel Evolution Models Key Tool Used for Stream Assessments Predictor of Current & Future Stream Stability Useful for Prioritization of Restoration Activities Useful to Inform Design

Channel Evolution Models Simon’s Modification of Schumm’s Model

Shear Stresses in Streams  Discharge Return Interval Shear Stress Natural Stream Channelized Stream 2-Year Storm 100-Year Storm D100 D2 Channelized Stream 100-Year Storm D100 2-Year Storm D2 Natural Stream

Use of Rosgen’s Classification System to Predict Channel Evolution Shear stress drives the evolution process. If the depth of the 1st E cross section is d and the depth to the top of bank of the G cross section is 2d then shear stress doubles of the slope does not change at all. On the original E channel the same flood even would spread out on the floodplain and minimally increase shear stress….If we were considering restoring the F stream here, can use see how we could restore the stream and reduce flooding? Excavate a floodplain and construct a C that will evolve to an E stream at the existing elevation. E  G  F  C  E

Use of Rosgen’s Classification System to Predict Channel Evolution Often it is helpful to understand how the stream reached the state that it is in. Why would a stream go from a “stable C” to a G?...lowering of base control/elevation causing a head cut. Actual raising of the banks by and impoundment such as a mill dam. Alavulsion or channelizing that reduces the stream length and then effectively increases the stream slope, which increases the shear stress and stream power. More shear stress likely means bed and bank erosion if no controls are in place. Would like to add in profile with bend removed to illustrate increase in slope.

Use of Rosgen’s Classification System to Predict Channel Evolution Source: D. L. Rosgen

Use of Rosgen’s Classification System to Predict Channel Evolution Source: D. L. Rosgen

Use of Channel Evolution to Inform Design Indicator of Risk of Instability Future Sediment Loadings Predictions Are Other Nearby Streams Headed for Instability When Can Streams Stabilize on Their Own? Temporary control has stopped the headcut. Insert headcut picture from Kleber?

Other Drivers of Instability Floodplain Access Bank Height Ratio Entrenchment Bed Stability/Headcuts Planform Stability Radius of Curvature Meander Width Ratio Pool to Pool Spacing Riparian Corridor Sediment Supply

Other Key Drivers Width to Depth Ratio Channel Length Channel Slope Stream Type Geomorphic Region/Geology Bed Materials Constraints

Module # 8 Channel Evolution Implications & Drivers of Instability Iowa’s River Restoration Toolbox Level 1 / Base Training