Massive Riverbed Erosion Induced by Inappropriate Grade Control: A Case Study in a Large-scale Compound Channel

Sediment flux in rivers has dropped globally in the past decades due to the increasing anthropogenic stressors, leading to significant riverbed degradation that may endanger the instream structures, collapse the banks and affect the riverine environment. To stabilise riverbeds, grade control structures (GCSs) have been widely applied in degrading channels. However, further understanding of the impacts of GCS on large-scale compound channels (with a main channel and floodplains) is needed. This paper reports a failure case of grade control in a large-scale compound channel (Shi-ting River, China), including a comparison of the bed profile measured using the structure from motion (SfM) technique before and after the flood season. The impacts of two improperly designed GCS on the bed morphology of the studied river reach are analysed and the reasons for failure of the present case are discussed based on numerical flow modelling. The results of numerical flow modelling and the measured bed profile indicate that the skewed mainstream flow direction in a curved main channel and the local scour downstream of a high drop GCS (3 ∼ 8 m) led to massive erosion on the floodplain near the GCS, significantly increasing the width and cross-sectional area of the main channel near the GCS. Consequently, the flow bypassed the GCS through the eroded area of the floodplain, leading to the dysfunction of the GCS. Due to the GCS dysfunction, a 5-year return period flood caused over 1.3 million m3 of erosion in the studied river reach (length ≈ 3.1 km), inducing severe bed degradation (up to 10 m) and reducing the width/depth ratio of the main channel upstream of the GCS. Through comparing the present study with the documented cases from literature, it is found that the bed degradation induced by GCS failure in compound channels develops much faster than those induced by sediment mining, weir or dam construction and reafforestation. Based on the analysed failure reasons, recommendations are provided for GCS design in large-scale compound channels.