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Designing to Account for Natural Variability

Lucas Babbitt, PE, CFM
WaterVation
Salida, Colorado

As stream restoration practitioners we often design our projects around the channel forming flow and various larger flood flows.  The estimations and calculations performed to derive each of these discharges inherently contains a large error band.  Subsequent hydraulic and sediment transport analyses heavily depend on these discharge estimates and contain error of their own.  With the current data and modeling software available to practitioners, there is no reason we shouldn’t start evaluating a range of potential scenarios when preparing restoration plans.

The variability and error associated with the analyses, calculations, and models we use presents an infinite range of conditions that, when evaluated individually, could result in a drastically different restoration design.  WaterVation has developed a limiting factor analysis that accounts for the variability associated with hydrologic, hydraulic, and sediment inputs into our restoration designs.  This limiting factor analysis is used to ensure that our restoration designs efficiently convey sediment and provide systemic stability for the most probable combination of natural variables.  This presentation will demonstrate the application of this analysis by using our Fountain Creek Restoration projects as a case study.

Fountain Creek is located in southern Colorado and is comprised of small gravel and sand alluvium with 100-year flows in excess of 35,000 cfs.  The upstream watershed has experienced ongoing hydrologic modification and is on the threshold of being a multi-thread channel.  WaterVation developed a restoration design based on the principles of natural channel design combined with embedded “insurance policies” to accommodate natural variability.  These “insurance policies” consist of log jams to encourage activation of secondary and tertiary overflow channels, planned sediment deposition zones, and floodplain storage.  This design approach was applied to the Colorado Stream Quantification Tool to show positive uplift and avoid an Individual Permit.

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About Lucas Babbitt, PE, CFM

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