Spatial and temporal evolution characteristics and mechanisms of flood level at identical discharges in the Lower Yellow River

Driven by continuous channel degradation, the water level-discharge relationship has undergone significant changes in the Lower Yellow River (LYR). Based on the hydrological and topographic data from six hydrometric stations in the LYR from 1986 to 2023, this study investigated the characteristics and mechanisms of flood-level changes in the LYR, and quantified the effects of channel morphological adjustment and channel resistance variation on flood levels at a given discharge. Results show that: ①Before the operation of the Xiaolangdi (XLD) Reservoir (1986—1999), flood levels at all six stations showed an overall upward trend, with the values at a discharge of 4000 m³/s increasing by 1.43—1.80 m. After the XLD Reservoir operation (1999—2023), flood levels at 4000 m³/s decreased by 1.51—3.40 m; ②The decline in flood levels at identical discharges was positively correlated with channel scour volumes. For each 100 million m of scour in the braided, transitional and meandering reaches, flood levels at 4000 m³/s dropped by 0.17—0.85 m at the corresponding stations;③Numerical experiments were used to quantify the contributions of different factors on flood levels at 4000 m³/s between 1986 and 2020. It revealed that increased channel resistance raised flood levels of 0.45—0.91 m, whereas channel morphological changes lowered them by 0.59—1.58 m. Downstream of the Sunkou Station, these two effects largely offset each other, resulting in little net change. While in the upstream reach, the flood-level rise attributable to increased channel resistance was less pronounced than the decline caused by riverbed incision, but it still counteracted 24.3% of the total flood-level decrease.