Response of a fixed sinuous channel to a doubling of sediment supply

Sediment supply is a key driver of river morphodynamics but the effects of supply changes on the morphology of meandering rivers are uncertain. This uncertainty makes it difficult to assess the impacts of land use and climate change on river dynamics and aquatic habitat. To investigate how sediment supply increases influence meander morphology, we doubled the bedload supply to a 45 cm wide experimental fixed wall sinuous channel with a D50 of 0.83 mm while holding the discharge constant at 2.6 l/s. The 22.1-m long channel had 5 study bends, a water surface slope of 0.0046, depth of 2.8 cm, and a sinuosity of 1.5. Prior to the feed increase the morphology and flux had been stable for over 40 hours. The doubling of sediment supply caused the channel to increase its slope to 0.0064 by building a wedge of sediment that tapered downstream. Facies mapping showed that the extent of fine facies declined after the feed increase, but was largely similar to the initial facies distribution at the end of the experiment. Aggradation of the bends ranged from 3 cm for the bend 5 m downstream of the flume inlet to 0.5 cm for the downstream-most bend, 20 m downstream of the inlet. The bar platform expanded in the 2 study bends where the in-channel supply changed most rapidly. One of the bars grew upstream and the other grew downstream. The bar platform in the three other study bends did not change significantly, despite total aggradation up to 72% of the channel depth. Downstream of the bar platform, 4 of the 5 pools experienced transient shoaling sometimes after the local sediment flux equilibrated. The morphological response was not a function of the total aggradation or total increase in supply. Our results suggest that for the case of doubling of supply, the downstream end of pools are the most likely response area, while the deeper portions of the pool near the bend apex are relatively stable, even as they aggrade. Aggradation similar to that observed in our experiments would lead to avulsion in floodplain bound channels.