Hydrograph Asymmetry Drives Bedload Transport Hysteresis: Evidence from Fluvial Seismology

Abstract:

Bedload transport rates are frequently different for an equivalent stage or discharge on the rising and falling limbs of a hydrograph. This asymmetry in transport results in hysteresis in sediment rating curves, which although widely documented, is not well understood mechanistically. A major factor contributing to the uncertain origins of hysteresis is the difficulty associated with making continuous bedload transport measurements in large rivers during floods. However, recent work demonstrates that seismic signals from rivers can provide quantitative constraints on the evolution of bedload transport during storms on large rivers, when direct measurements would otherwise be impossible. Here, we analyze patterns in the amplitude of seismic shaking along the Cho-Shui River in Taiwan and along the Erlenbach Stream in Switzerland during a total of 10 storm events. The storms exhibit varying amounts of hysteresis in the amplitude of seismic shaking as a function of flood discharge (as measured from nearby gages). However, the amount of hysteresis, which we quantify with a non-dimensional metric, correlates strongly with the degree of asymmetry in the flood hydrograph. Specifically, the degree of hysteresis observed during an event scales inversely with the ratio of the rising limb time to the recessional limb time. Flume experiments and field observations indicate that vertical sorting and, hence, armor formation in gravel-bedded river channels is more pronounced during longer storms. Although we lack direct constraints on the bed surface during the observed events, our results support the possibility that differential gravel sorting on the rising and falling limbs is responsible for the observed hysteresis. Specifically, our results imply that for a given stage, armor will be better developed on a slow recessional limb, resulting in lower gravel transport rates and a pattern of clockwise hysteresis.