长江下游崩岸预警模型水动力指标阈值研究

Study on hydrodynamic index threshold of early warning model for bank collapse in lower reaches of Yangtze River

长江下游河道崩岸灾害频发,多指标崩岸预警模型是基于层次分析法建立的岸坡稳定性评价方法。水动力作为关键影响因素,对于其指标阈值的确定缺少理论和系统的研究。本研究基于理论推导、数模计算和统计方法,确定了对应不同风险等级岸坡稳定性的水动力预警指标阈值,具体包括造床流量当量、近岸流速、水位变幅和汊道分流比变化率等指标。其中造床流量当量和水位变幅指标阈值确定考虑了感潮河段潮流参与造床和周期性潮位波动的作用。近岸流速指标引入了河床稳定性计算公式。汊道分流比变化率指标阈值计算区分了主支汊,得出当变化率大于8%时,主汊岸坡处于不稳定状态。研究结果可支撑长江下游多指标崩岸预警模型的建立,并应用于崩岸灾害防治。

Previous multi-index bank-collapse prediction models are built on a river bank stability evaluation method that relies on the analytic hierarchy developed in recent years.As a key factor affecting the stability of river banks,determining the threshold values of hydrodynamic factors responding to different collapse risk levels is often empirical and lacks theoretical,systematic or practical research support.In this study,the thresholds of the key hydrodynamic factors-such as dominant discharge,flow velocity,water level variation,and channel diversion ratio change rate-are determined by integrating data statistics,theoretical derivation,numerical model calculation,and the previous results in literature.Determination of the dominant discharge threshold of a river and its water level variation threshold takes into account river-tide interactions in its tidal reach;a flow velocity factor is introduced in a formula for calculating riverbed stability.For a branched reach,the threshold of the diversion ratio change rate is calculated to distinguish its major branch channel from the minor ones,and we conclude the bank of the major branch is unstable at the diversion ratio change rate greater than 8%.The results support the development and application of an analytical bank-collapse-prediction hierarchy model for the lower Yangtze,and help prevent bank collapse disasters.