怒江大峡谷（云南段）对高密度泥石流堰塞坝群的河流地貌响应

Nujiang River can be divided into upper canyon section and lower wide valley section according to its and river geomorphology. The canyon section is deeply incised and many debris flows developed. Some debris flow fans block the river and form barrier lakes. There is almost one debris flow dam every 1-5 Km along the Gaoligongshan Canyon. The river channel is dominated by recurring barrier lakes (with subcritical flow) and barrier dams (with supercritical flow), which forms the high-density dams zone (Nickel Zone) in the edge of Tibetan Plateau. The broad valley downstream is mainly alluvial fan. The number and scale of the alluvial fans is much smaller than that of debris flow fans. The alluvial fan has little influence on the evolution of Nujiang river due to insufficient resistance. Debris flow fans are the main driving force for the river evolution because of its large coarse particles and high resistance. The dams block a large amount of sediment in the river and form beaches and deltas. The water energy dissipation by the dams slows down the river incision. The trend of downcutting of debris flow gullies has been changed, and the lower reaches of debris flow gullies have been changed from overall downcutting to local sedimentation. This project studies the response of the bed and sand evolution to the high-density barrier dams in the process of river downcutting through field surveys and mathematical statistics. The influence of debris flow dams on the stepped river can reveal the interaction mechanism between debris flow hazards and large rivers on the edge of the Qinghai-Tibet Plateau. This study can provide scientific support for the hydroelectric cascade development along the Nujiang River.

怒江（云南段）从河流地貌形态上可分成上游峡谷段和下游宽谷段。峡谷段为高山深切河谷，两岸泥石流众多，许多泥石流扇体堵塞怒江河道形成了堰塞湖。高黎贡山峡谷河段每隔1-5公里就有一个泥石流堰塞坝，峡谷河段形成一段段顺直河谷连接堰塞湖缓流+堰塞坝急流的不断重复的河道，构成青藏高原边缘密度极高的堰塞坝群分布带（尼克带）。下游宽谷段主要是洪积扇，洪积扇的数量和规模远远小于泥石流扇，洪积扇由于阻力不足对怒江河床演变的影响较小。泥石流扇体大石块含量多，阻力大，是影响怒江河道演变的主要动力。堰塞坝群的存在使得怒江峡谷大量泥沙淤积，形成边滩和三角洲，同时堰塞坝的消能作用使得怒江干流下切减缓，改变了泥石流沟群下游整体下切的趋势，沟口由下切变为局部淤积。本项目通过野外测量和数理统计，研究怒江在河道下切过程中对高密度堰塞坝群的床沙响应机制。研究堰塞坝群对梯级河流地貌的影响，可以揭示青藏高原边缘河流下切区泥石流灾害体与大江大河的相互作用机制，为怒江流域梯级水电开发提供科学支撑。

怒江是我国西南地区具有重要影响的国际河流。怒江大峡谷为深切河谷，泥石流等地质灾害频发，这些泥石流携带大量的泥沙堵塞河道，形成青藏高原边缘密度极高的泥石流堰塞坝群。研究发现怒江大峡谷（云南段）发育了187条泥石流沟，其中148条泥石流沟道的扇体延伸入怒江河道，53条泥石流沟的扇体完全堵塞怒江主河形成堰塞坝。每一个泥石流堰塞坝都是泥石流多次爆发冲进河道堵溃形成的，不同场次泥石流堆积物分层明显，具有明显不同于洪水形成的洪积扇的建造特征。泥石流堰塞坝经过洪水长期的冲刷下切、溃决和粗化，形成的尼克点阻力较高。洪积扇由于抗冲刷能力弱，阻力低，对怒江河道的影响小。高密度的泥石流堰塞坝群使得大峡谷呈现一段段正常河道连接堰塞湖缓流加堰塞坝激流的不断重复的河道，改变了泥沙在河道的运动和分布。项目采用无人机+Lidar的方式采集了泥石流堰塞坝的地形数据，利用无人船携带测深仪的方式采集了堰塞湖的水深数据，利用地球物理勘探方法（EH4）测量了堰塞坝和堰塞湖内泥沙淤积厚度。分析了泥石流堰塞坝的建造特征和堰塞湖内泥沙的淤积速率和泥沙淤积总量。结果表明：高密度的泥石流堰塞坝群使得大峡谷河道梯级化，泥沙淤积速率大大提高，极大的制衡了怒江大峡谷整体下切的趋势。现在的泥石流堰塞坝是泥石流多次爆发堆积河道和江水冲刷的结果，因此扇体边缘粗化且分层明显，具有不同于洪积扇的沉积构造，是影响怒江河道演变的主要动力。泥石流携带的大石块起到重塑河流地貌的主要作用，0.2 mm以下的泥沙颗粒不起作用，作为悬移质运动到下游。陡峭指数作为反映河流尼克点的特征参数与泥石流堰塞坝位置有很好的对应关系，泥石流堰塞坝和地质构造断裂带都会使得陡峭指数增大，但是泥石流堰塞坝由于数量众多，对怒江河流地貌改变的影响更大。泥石流堰塞坝群使得河道梯级化，造成堰塞坝上游水流能量集中，局部河段沉积物变厚，河床存在从泥质、粉细沙、粗沙、卵石夹砂到大石块过渡和交替分布的规律。这些高密度的堰塞湖沉积河段占到整个大峡谷长度的21%，拦截了70.8%的泥沙，抬高了侵蚀基准面，使得峡谷两岸新的崩滑体大大减少，扭转了怒江大峡谷整体下切的趋势。