Cascading breaching of the Tangjiashan landslide dam and two smaller downstream landslide dams

Abstract

The 2008 Mw 7.9 Wenchuan earthquake triggered at least 257 landslide dams, most of which formed in series along rivers. The failure of an upstream dam could cause the failure of dams downstream one after another. The cascading dam breaching may result in a sharp increase in the flood and more serious damage downstream. This paper aims to simulate the cascading breaching of landslide dams with a case study of the Tangjiashan landslide dam and two smaller dams downstream. A modified DABA model is applied for this purpose, in which the water–soil interactions are simulated with erosion and shallow water flow theory. This paper first introduces the methodology of the cascading dam breach simulation, emphasizing modifications to the DABA model. Then, the cascading breaching of the Tangjiashan landslide dam and two smaller downstream dams is simulated using the modified DABA model. Finally, the influences of several key factors (e.g., dam height, dam width, dam erodibility) on cascading dam breaching are analyzed to identify the critical conditions of overlapping effect in breaching floods. The modified DABA model is shown to be a reliable tool for simulating the cascading dam breaching. The flood from the Tangjiashan dam was not significantly amplified by the breaching of the two small dams downstream, because the two smaller dams breached before the Tangjiashan dam. Some parameters, such as dam crest width and initiation water level, may not influence the peak outflow rate and breach size of a single dam obviously, but may influence the cascading breaching of several dams significantly through the amplification effect. The cascading breaching of several dams is likely to generate multi-peak floods. A critical condition for the overlapping effects of cascading dam breaching is that large and increasing inflow erodes the dam breach downstream in the development phase. If cascade breaching of several landslide dams is inevitable, one principle to mitigate the risks is to lower the breach flood of the largest dam with engineering measures. If significant overlapping effect is predicted, engineering measures may also be taken to breach the smaller dams before the breaching of the large dam upstream or delay the breach of those downstream dams by reinforcement measures.