Quantitative technique for assessing the geomorphic thresholds for floodplain instability and braiding in the semi-arid environment

For the semi-arid environment of southern California, a reach-scale classification system and conceptual model were created to synthesize the observed floodplain forms into three basic floodplain continuums: armored, non-armored, and active-regional alluvial fan. These continuums are comprised of three to five alluvial floodplain forms (cascade, step-pool, plane-coarse-bed, plane-mixed-bed, plane-fine-bed, pool-riffle, braided, and dune-ripple). For the non-armored floodplain continuum, a floodplain state diagram has been generated to quantitatively describe the natural downstream progression of floodplain forms and geometry, using specific stream power and the width-to-depth ratio as the state and shape metrics, respectively. Based on field data for 91 cross sections along 14 watercourses, this floodplain state diagram provided the basis to define geomorphic thresholds for floodplain instability and braiding using logistic regression analyses. This method for defining floodplain instability thresholds provides a unique management tool by providing both a systematic and a quantitative means to assess the stability state of watercourses and, thereby, provide a means for assessing the potential for the hazards associated with watercourse instability. Copyright Springer Science+Business Media B.V. 2010