Hydraulic resistance in mixed bedrock-alluvial meandering channels

We present an experimental analysis of hydraulic roughness variations due to changes in alluvial cover in a mixed bedrock-alluvial meandering channel with larger bedrock roughness than alluvial roughness. Three sets of experiments were conducted in a highly sinuous flume: one with bare bedrock, and two with enough sediment to cover 21% and 78% of the bedrock respectively. We compare our results with data from experiments previously conducted in the same flume with flat- and smooth-bed, and data from experiments with fully alluvial conditions. Our results show that: (1) hydraulic resistance in a mixed bedrock-alluvial channel changes with the degree of alluviation; (2) hydraulic resistance is greater for bare-bedrock conditions, and decreases as sediment supply increases; (3) if bedforms appear, hydraulic resistance may be larger than for bare bedrock conditions due to form drag; (4) fluctuations in alluvial cover due to freely-migrating bars lead to instantaneous changes in hydraulic resistance.

本研究针对基岩糙率大于冲积糙率的混合基岩-冲积蜿蜒河道，开展了由冲积覆盖层变化引发的水力糙率变化实验分析。实验在高蜿蜒度水槽中共开展三组工况：一组为裸露基岩床面，另外两组分别以足够沉积物覆盖21%与78%的基岩床面。本研究将实验结果与此前在同一水槽中开展的平坦光滑床面实验数据，以及全冲积床面条件下的实验数据进行对比。研究结果显示：
（1）混合基岩-冲积河道的水力阻力随冲积覆盖程度发生变化；
（2）裸露基岩床面条件下的水力阻力更大，且随沉积物补给量增加而降低；
（3）当出现床面形态时，由于形状阻力的存在，水力阻力可能大于裸露基岩床面条件；
（4）由自由迁移沙坝引发的冲积覆盖层波动，会导致水力阻力发生瞬时变化。