暴雨山洪、堰塞湖溃决是山区河流灾害性洪水事件的两种典型模式。两类洪水的演进、致灾过程常伴随着强烈的河床冲淤与泥沙输移。由于水沙运动特征复杂，当前对泥沙运动在山区河流洪水致灾作用中的认识较为有限，较难满足防灾减灾工作的实际需求。本文建立了能够表征沟道山洪演进、堰塞坝体溃决的水沙模型，使用灾害调查数据及水槽试验数据对其进行了检验。综合采用数值计算、野外量测等方法，研究了山洪演进、堰塞湖溃决过程中泥沙的运动特征及其致灾作用，分析评估了常见工程措施的防灾减灾效果，提出了可供实践参考的防灾减灾措施。 山洪过程中，泥沙通过改变沟道形态、水流阻力影响洪水水位与流速的时空分布，呈现“抬高河段整体水位、放大局部受灾风险”的致灾效应。单场山洪中，泥沙致灾作用的大小依赖于洪水的规模，具有“临界性”特征：重现期小于临界值的常遇洪水过程中，可近似忽略泥沙运动的致灾效应；规模较大的灾害性洪水中，断面泥沙致灾风险增幅随洪水重现期增大而提升。连续多场山洪中，泥沙的致灾作用具有“累积性”：前序洪水过程中的泥沙运动将对后续致灾过程产生持续影响，此情形下，洪灾风险分布具有空间动态性。 影响山洪水沙运动过程特征的工程措施能显著改变灾害风险的时空分布。拦沙坎（群）上淤下冲，沉沙池承接上游来沙、有效降低水位，应根据保护对象空间分布情况因地制宜布置；移除沟床表层粗颗粒后输沙强度增加，可能加剧中下游淤沙段风险，应谨慎使用。设计山洪沟整治方案时，可根据村镇、人员空间分布特征选取“重点保护”或“风险均摊”的风险调控方式。 本文建立的模型成功实现了“10.17”、“10.29”雅鲁藏布江加拉村堰塞湖溃决洪水应急预报，亦能较好复演易贡藏布堰塞湖、金沙江白格堰塞湖的溃决过程，具有较强的工程实用价值。溃决洪水洪峰流量、峰现时间、洪峰形态主要受到堰塞坝级配、形态参数的影响。多数滑坡堰塞坝溃决洪水呈现陡涨缓落的峰型，白格堰塞湖溃决洪水近似对称的尖瘦峰型可能与堰塞体级配偏细有关。在分析各类模型特点的基础上，结合参与应急救灾工作的实践经验，提出了溃决洪水应急预报综合方法。

Flash floods and the barrier lake floods are two typical modes of mountainous extreme floods, the flood routing and hazard initiating of which are often accompanied with intense bed deformation and sediment transport. Due to the complexity of water and sediment transport, the effects of sediment transport on flash flood hazard in mountain rivers are not well understood until now, and the actual demand of disaster mitigation can hardly be fulfilled. In this paper, we established a hydrodynamic-morphodynamic model which can describe the process of flood routing as well as barrier dam breach. The model is validated against data from field survey and flume experiment. Using the method of numerical modeling and field survey, we study the effects of sediment transport on flash flood routing and barrier lake breach. The effects of typical engineering works on disaster mitigation are analyzed and appropriate measures are proposed for reference.During the flash flood, sediment transport can affect the spatiotemporal distribution of water level and flow velocity by changing channel morphology and flow resistance, with typical characteristatics of raising water level, blocking local channel reach, and amplifying flood hazard. With the transport of sediment, water level and flow discharge are no longer synchronized. In a single event of flash flood, the effect of sediment tranposrt depends on the magnitude of flood, and with a characteristic of criticality. That is, for floods smaller than a critical value, the effects of sediment transport on flood hazard can be neglected, whereas for floods larger than the critical value, the increase of flood hazards due to sediment transport is positively correlated with the magnitude of flood. For successive flash flood events, the effects of sediment transport on flood hazard has a cumulative character, with the sediment transport in a previous even having influence on the following event, thus leading to a dynamic spatial distribution of flood hazard.Engineering works which affect water and sediment transport during flash flood can significantly alter the spatiotemporal distribution of flood hazard. A check dam can lead to bed aggradation in the upstream and bed degradation in the downstream. A sediment trap can deposit sediment from upstream thus lowering the water level effectively. Both the check dam and sediment trap should be placed upstream of the protected section. Removing the armor layer on bed surface can lead to an increase of sediment transport, which might increase the flood hazard risk in the downstream reach, and therefore should be implemented with caution. When designing the engineering management plan of mountain rivers, either a “key area protection” method or a “risk equalization” method can be choosed based on the spatial distribution of people and property. The numerical model established in this paper has been suscessfully implemented to predict the “10.17” and “10.29” barrier dam breach in the Yarlung Tsangpo River. The model can also well reproduced the breach process of barrier dams in Yigong Tsangpo and Jinsha River. The peak discharge, peak time, and breach flood hydrograph are affected by the morphology of barrier dam as well as the grain size distribution of dam material. In most cases, the breach flood hydrograph of barrier dam is assymetrical with a sharp rising limb and a gentle falling limb. The relatively symmetrical breach hydrograph of the Baige Barrier Dam might be related with the fine grain size distribution of sediment. Finally, based on the characteristics of various models, a comprehensive method for emergency prediction is proposed with the combination of practical experience of disaster relief efforts.