Stream-bed stability: differing effects of floods

Posted on December 18, 2015

Floodwaters are often released from dams to rejuvenate channel form and aquatic habitats, but in the process, large volumes of sediment can be transferred downstream. Therefore, especially in the case of rivers where the sediment supply is limited, it’s useful to be able to predict how different flows affect particle mobilisation and retention. In a study carried out on a reach downstream of the Lewiston Dam on the Trinity River in northern California, motion-sensing radio transmitters with a long battery life were inserted into rocks on the stream bed. Transmitter size (81 x 23 mm) limited tracking to the larger particles (90 -128 mm diameter). A flow model was used to compute the distributions of velocity and boundary shear stress in the study reach. Model inputs included data on water discharge, channel topography and roughness. Grain size distributions and vegetation patches were mapped with high spatial resolution. Three bed-mobilizing floods occurred during the 2-year study. Most tagged rocks moved less than 1m, but a few moved as far as 1128 m. The tagged particles moved relatively easily during a dam release flood that came immediately after a strong flood generated by tributaries downstream of the dam, which suggested that strong floods can reduce bed strength by disrupting the armour layer. In contrast, other results implied that lower-intensity floods tend to increase bed strength by selectively winnowing small grains and arranging particles into more stable structures. The researchers concluded that sediment movement doesn’t depend just on particle size and flow discharge: the recent history of flood events is also important. These events can either disrupt or strengthen the bed and thus affect the force required to entrain bed particles. Dam-release floods may be more effective when large floods occur more frequently or when smaller dam-release floods occur shortly after tributary-generated floods.

Reference: May, C.L. & Pryor, B.S. 2014. Initial motion and bedload transport distance determined by particle tracking in a large regulated river. River Research and Applications 30, 508–520.