中国西南山区北江支流望谟河流域，域内地形起伏大，短时强降水多，山洪泥石流活跃。历史上多次发生山洪，如2011年造成37人死亡的特大山洪事件。近年来，该流域内新修建了纳坝水库、新屯水库等多处防洪工程设施。本文基于ALOS卫星数字高程数据(12.5m)以及5种年遇型(5年一遇、20年一遇、50年一遇、200年一遇、500年一遇)暴雨情景，借助FLO-2D模型进行望谟河流域山洪危险性分析，并根据望谟县城房屋暴露及文献中脆弱性曲线，对比水库有无条件下县城房屋的潜在损失，并对防洪水库进行经济效益评价，以期为望谟河流域山洪灾害防治工作提供科学依据。本研究主要工作与结论如下：

(1)计算了望谟河流域出水口的清水流量及洪峰流量，并进行了山洪演进数值模拟。结果显示：“20110606山洪事件”模拟水深高值区位于县城中部宁波桥附近河段，而县城南部望谟桥附近山洪冲击力和流速最大。这与当年洪痕吻合，较好验证了模拟方法和参数的准确性。分别对重现周期5a、20a、50a、200a及500a降水情景触发的山洪灾害进行模拟，山洪淹没县城面积约为77-123万m²。“20110606山洪事件”打易降水约为200年一遇，县城淹没面积约99万m²。

(2)评估了无水库条件下不同重现期情景下县城房屋损失风险。重现周期5a-500a降水情景下，县城房屋淹没范围约9-20万m²，若按当地重置价格1200元/m²计算，损失为2577-7734万元，占2018年全县GDP的0.4%-1.1%及财政收入的3.2%-9.5%。高风险区域主要分布在县城北部王母桥附近。

(3)开展了水库正常运行条件下县城山洪灾害风险评估。当纳过、纳坝、新屯和祥乐4个水库正常运行时，望谟河下游县城山洪灾害风险整体降低，高风险区面积显著减少。重现周期5a-500a降水情景下，山洪淹没范围缩小了8.6%-12.7%，特别是县城东南部淹没面积明显减少。县城房屋淹水面积减少12.9%-30.2%，损失减少19.7%-45.7%。

(4) 完成了流域水库工程经济效益评价。基于望谟县当地社会经济发展情况以及水库工程折损率，在50年正常运行期内，纳过、纳坝、新屯和祥乐4个水库多年平均净效益为7.64亿元，相当于2018年GDP的10.7%及财政收入的93.9%。其中，防洪、供水、灌溉净效益分别占2018年GDP的2.6%、7.9%、0.3%，供水和防洪效益十分显著。

本研究基于水库工程防洪效益考虑，对望谟河流域典型多情景下山洪暴发过程进行了数值模拟，评估了山洪导致的县城房屋损失风险，并对流域内水库工程进行经济效益评价。科学合理估算流域内水库工程对山洪灾害风险的影响有利于提高望谟河流域山洪灾害风险现状评估的准确性，可以为流域内防洪减灾工作，尤其是防灾工程建设提供决策支持依据，对于保障山丘区小流域居民生活和社会经济发展具有重要的现实意义。

Wangmo River is a tributary of the Beijiang River. Repeated flash flood events occurred in this small basin because of its upstream catchment area with rugged terrain, which shorten the confluence time of river flows into a few hours. Floods has frequently caused casualties and property damage to house and public facilities, most notably in 2011 with 37 deaths. In the recent decades, several reservoirs, such as Naba reservoir and Xintun reservoir, have been built for flood mitigation. Consequently, a detailed analysis was carried out to quantify the hazard of flash floods under the multiple scenarios of presipitation and reservoir in Wangmo. To determine the extent and characteristics of the flooded area, 2D hydraulic model (named FLO-2D) was applied with the ALOS (12.5m) topographic data. Additionally, five rainstorm scenarios were considered, including the return periods of 5-year, 20-year, 50-year, 200-year, 500-year, and two flood mitigation conditions, with and without reservoirs. Stage-damge function from the literature on similar study area was selected for housing loss evaluation and flood mitigation benefit assessment. The major work and conclusion can be summarized as:

(1) Flood flow and peak flow at the outlet of Wangmo River basin were calculated using the rainstorm data, and put into the flash flood simulation model. The result shows that in the 20110606 event, the high value area of inundation depth is located near Ningbo Bridge in the central part of the urban area, while the area with maximum of impact force and flow velocity of flash flood is distributed near Wangmo bridge in the southern part, which matchs the flood marks well. Flash floods triggered by rainstorm scenario with return period of 5-500 years inundated about 0.77-1.23 million m² of land. The 20110606 flood inundated 0.99 million m² of the urban area with a return period of precipitation of about 200-year.

 (2) The risk of housing loss caused by flash flood in different return periods without reservoir was evaluated. About 0.09-0.20 million m² of houses flooded under the rainstorm scenarios with a return period of 5-500 years. The house losses were 25.77-77.34 million RMB, according to the local replacement price of house, 1200 RMB/ m², accounting for 0.4% - 1.1% of GDP and 3.2% - 9.1% of the fiscal revenuec in 2018. High risk areas are mainly distributed near Wangmu Bridge in the north.

(3) The risk of flash flood inundations and housing losses in different return periods with reservoir was evaluated. When Naguo reservoir, Naba reservoir, Xintun reservoir and Xiangle reservoir all work, the risk of flash flood in Wangmo river basin is significantly reduced: the inundation area is reduced by 8.6%-12.7%, with the highest decreasing value in the southeast of the urban area, and the area of houses flooded by flash flood decreased by about 12.9%-30.2%, resulting in a reduction of 19.7% - 45.7% in housing losses.

(4) The benefit evaluation of flood-control reservoir was completed. Considering the local social and economic development, in the 50 year operation period, the annual average net benefit of Naguo reservoir, Naba reservoir, Xintun reservoir and Xiangle reservoir projects is 7.64 billion RMB, equivalent to 10.7% of the GDP and 93.9% of the financial revenue of Wangmo County in 2018. Among them, the annual average net benefits of flood mitigation, water supply and irrigation account for 2.6%, 7.9% and 0.3% of GDP in 2018, respectively. The benefits of water supply and flood mitigation of thses projects are of great significance.

In this study, the numerical simulation of flash floods and housing losses assessment under the multiple scenarios in Wangmo River Basin was implemented. Considering the flood mitigation benefits of the reservoir project, it is proved that the reservoirs are helpful to improve the accuracy of the current risk assessment of flash floods. They also provide the reference for the development of flood mitigation and disaster reduction work in Wangmo River basin to protect the life and property.