岷江河谷锄头沟震后泥石流致灾模式

New Geo-Disaster Forming Pattern of Post-Shock Debris Flow:A Case Study of the Chutou Gully in the Minjiang River Valley,China

锄头沟位于岷江河谷汶川县绵虒镇羌锋村,是汶川地震震后大型活跃型泥石流沟,震后大规模泥石流活动造成锄头沟泥石流拦挡工程失效,多次淤积损毁都汶高速公路G317和国道G213。震后泥石流治理工程对泥石流的发展起到一定遏制作用,也改变了泥石流的致灾模式。本文通过实地调查与数据分析,探讨了2019年8.20锄头沟泥石流的成因及对工程设施的致灾模式,提出了拦挡坝对翻坝泥石流能力的放大机理。研究表明:(1)锄头沟8.20泥石流灾害是由各支沟暴发、主沟铲刮而形成的大规模泥石流,受侵蚀铲刮效应影响其冲出规模放大约44.2%;(2)锄头沟泥石流致灾模式主要为:翻坝加速、侵蚀铲刮、侧蚀强烈、弯道雍高、磨蚀槽底、堵断桥涵、冲淤民房;(3)锄头沟泥石流现仍处于发展壮年期,在未来10~20年内遇强降雨仍会暴发大规模泥石流;(4)翻坝泥石流的破坏能力和拦砂坝坝高及过坝运动速度密切相关,当初速度为2 m/s、坝高25 m时,泥石流入射速度是初速度的11.1倍,故有效坝高较高、速度较低的翻坝泥石流对坝下的沟道仍具有很强的冲蚀能力;(5)当过坝泥石流初速度一定时,有效坝高对翻坝泥石流的水平距离的增幅呈下降趋势,这可为坝下副坝位置的选取或护坦尺寸的设定提供科学依据。

The Chutou gully in Qiangfeng Village,Wenchuan County,China has become a large active debris flow gully after the 2008 Wenchuan earthquake.Large-scale debris flows at the Chutou gully has caused engineering failures in control facilities and multiple damages to highway G317 and national road G213.Engineered measuring in debris flow control facilities not only restrains debris flow development,but also reduce the extent of earthquake-induced debris flow.This paper explored the cause of a debris flow event on August 20,2019 at the Chutou gully and its driving mechanism for engineering control purposes based on detailed field investigation.The factors that amplify the energy during debris flow over dams were also discussed.The results show that the August 20,2019 disasters were large-scale debris flow that formed by the outbreak of each branch ditch and scraping by the main ditch.The scale of the debris flow was magnified by approximately 44.2%owing to erosion linked to debris flow scraping.The debris flows disasters at the Chutou gully mainly involved the acceleration of dam wall breaches,erosion linked to scraping,intensified lateral erosion,bend heightening,slot base abrasion,bridge and culvert blockages,and the scouring and silting of civilian buildings.The debris flow at the Chutou gully remains its development stage,and large-scale debris flow disasters are expected to continue to occur over the upcoming 10~20 years in cases of heavy rainfall.The scale of debris-flow destruction in dam wall breaches was closely related to the debris dam height and debris flow velocity while overtopping the dam.For an initial velocity of 2 m/s and dam height of 25 m,the debris flow incident velocity was 11.1 times higher than that of the initial velocity.The dam breaching debris flow,therefore,has strong ability to erode the channel beneath the dam under conditions of higher effective dam heights and lower velocities.For a given initial debris flow velocity over a dam,higher effective dam heights can reduce the increase of horizontal displacement,thus providing a scientific basis for the location selection of an auxiliary dam under a dam or the size setting of a protective dam.