River Recovery at Mount Pinatubo, Philippines

0106681蒙哥马利自1991年皮纳图博火山爆发以来，从其斜坡排水的流域产生的沉积物量是有史以来最高的。 尽管这压倒性的泥沙负荷，河流淹没火山碎屑流沉积物有效地提供了几乎一半的初始沉积物到1996年下坡冲积/碎屑扇。 这种运输大部分发生的泥流和高浓缩流，但在低到中等流量的渠道提供端员的例子，通道响应高沉积物供应。 在1997年和1998年的现场测量程序记录了极高的推移质输运率，即使在低流量，由于增强流动性的颗粒移动在一个光滑的，细粒床。 流速和深度，通道特性，和大小的移动的和稳定的碎屑在Pasig-Potrero和Sacobia河流的测量表明粒度独立的无量纲临界剪切应力为0.015至0.038，曼宁的n值为0.017至0.024，远低于以前报道的陡峭的山区通道的值。 在这些沉积物极其丰富的渠道，戏剧性的床的流动性和选择性运输表明，粒度的变化，和床粗糙度可以增加运输能力，以应对高沉积物供应。 在Pasig-Potrero河的无量纲推移质输沙率大大大于以前报道的温带河流在相同的无量纲剪切应力。 在三个实地季节的过程中，我们建议使用这个正在进行的自然实验来测试一系列的假设河流对沉积物供应变化的响应。 具体而言，我们将监测河道对泥沙供应逐渐减少的响应，以评估以下假设：（i）河床粗化和粗化是对来自山坡和谷壁的泥沙输送减少的响应，并将导致推移质输运特征从选择性流动性到阈值流动性的变化;（ii）由于流域恢复和相对运输能力的差异，Pasig-Potrero河的装甲门槛流动床的恢复将比Sacobia河更快;（iii）流经皮纳图博山两侧的河流河床表面结构的变化幅度和速度，反映了火山碎屑流沉积物在流域初始沉积物负荷方面的差异;卵石簇的形成是河道对输沙量下降的一种响应机制，进而导致河床运动和推移质输沙率的变化。

0106681MontgomerySince the 1991 eruption of Mount Pinatubo, the sediment yields from watersheds draining its slopes have been the highest ever recorded. In spite of this overwhelming sediment load, rivers inundated by pyroclastic flow deposits efficiently delivered almost half of the initial deposits to downslope alluvial/debris fans by 1996. Much of this transport occurs by lahars and hyperconcentrated flow, but at low to moderate flow the channels provide end-member examples of channel response to high sediment supply. A program of field measurements in 1997 and 1998 documented extremely high bedload transport rates even at low flow due to enhanced mobility of particles moving over a smooth, fine-grained bed. Measurement of flow velocities and depths, channel properties, and the size of both mobile and stable clasts in the Pasig-Potrero and Sacobia Rivers indicates grain-size-independent dimensionless critical shear stresses of 0.015 to 0.038, and Manning's n values of 0.017 to 0.024, well below values previously reported for steep mountain channels. The dramatic bed mobility and selective transport in these extremely sediment-rich channels indicate that changes in grain size, and bed roughness can increase transport capacity in response to high sediment supply. Dimensionless bedload transport rates in the Pasig-Potrero River are substantially greater than those previously reported for temperate rivers at the same dimensionless shear stress. Over the course of three field seasons, we propose to test a series of hypotheses for river response to changes in sediment supply using this ongoing natural experiment. Specifically, we will monitor channel response to the progressive decline in sediment supply to evaluate the hypotheses that: (i) bed coarsening and armoring is occurring in response to decreased sediment delivery from hillslopes and valley walls and will lead to a change in the character of bedload transport from selective mobility to threshold mobility; (ii) that recovery of an armored, threshold mobility bed in the Pasig-Potrero River will occur faster than in the Sacobia River due to differences in watershed recovery and relative transport capacity; (iii) that the magnitude and rates of changes in the bed surface texture in rivers draining the flanks of Mount Pinatubo track differences in initial sediment loading of their watersheds by pyroclastic flow deposits; and (iv) that pebble cluster formation is a mechanism of channel response to declining sediment load that, in turn, leads to changes in the style of bed mobility and bedload transport rates.