Predicting sediment flows in steep streams

Posted on March 28, 2013

The sediment that’s continually being eroded from the upper parts of a catchment and washed downstream can have a major influence on channel conditions and habitat quality in the lower reaches.  For this reason mathematical models have been developed to predict rates of sediment transport.   However, when it comes to estimating sediment flows in steep streams, such as the headwaters of hilly or mountainous catchments, the standard predictive equations developed for use under relatively low-gradient conditions can be highly inaccurate.   Most of these equations assume that all of the bottom sediment is potentially mobile.  However, the beds of steep channels are typically composed of large boulders and finer, more mobile gravel.  The larger particles interlock and form relatively immobile steps, which means that the stress needed to move the sediment is higher than in flatter streams.  Another standard assumption, namely that the supply of sediment is unlimited, is also problematic, because in steep streams many of the smaller grains are “hidden” in deep pockets behind the larger particles and only a proportion of the sediment is available for transport.  To address these issues, the standard transport equations have been modified to deal with the conditions in steep streams.  The new model partitions the mobile and immobile sediment fractions and derives key parameters (median grain size, shear stress, availability of sediment) by reference to the mobile fraction only.  The  model was tested using field measurements from three streams (the Erlen in Switzerland, Rio Cordon in Italy and Fox Creek in California).  While the standard mode overestimated sediment flows in Rio Cordon by several orders of magnitude in all cases, the new equations gave predicted flows that were within an order of magnitude of the measured values in 83% of cases.

Reference:  Yager, E.M., Dietrich, W.E., Kirchner, J.W. & McArdell, B.W.  2012.  Prediction of sediment transport in step-pool channels.  Water Resources Research 48 W01541, doi:10.1029/2011WR010829, 2012.