River Recovery at Mount Pinatubo, Philippines

[010668 . 81]蒙哥马利自从1991年皮纳图博火山喷发以来，从排水斜坡的流域产生的沉积物量达到了有记录以来的最高水平。尽管有如此巨大的沉积物负荷，但到1996年，被火山碎屑流沉积物淹没的河流有效地将几乎一半的初始沉积物运送到下坡冲积/碎屑扇。这种运输大部分是由泥流和高浓度水流发生的，但在低至中等水流时，河道提供了对高泥沙供应响应的末端成员例子。1997年和1998年的一个现场测量项目记录了即使在低流量下，由于颗粒在光滑、细粒度的床层上移动的流动性增强，也有极高的床层输运率。对帕西格-波特雷罗河和Sacobia河流速和深度、河道性质以及流动和稳定碎屑大小的测量表明，与粒度无关的无量纲临界剪切应力为0.015至0.038，曼宁n值为0.017至0.024，远低于先前报道的陡峭山区河道的值。在这些极其富沙的河道中，剧烈的河床移动和选择性输运表明，泥沙粒度的变化和河床粗糙度可以增加输运能力，以应对高泥沙供应。在相同的无量纲剪切应力下，帕西格-波特雷罗河的无量纲河床输运率大大大于以前报道的温带河流的无量纲河床输运率。在三个野外季节的过程中，我们建议使用这个正在进行的自然实验来测试一系列关于河流对沉积物供应变化的反应的假设。具体来说，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)由于山坡和山谷壁的泥沙输送减少，河床变粗和装甲化正在发生，并将导致河床输运的特征从选择性迁移到阈值迁移；（ii）由于流域恢复和相对运输能力的差异，帕西格-波特雷罗河的装甲、门槛流动床的恢复将比Sacobia河更快；（iii）流经皮纳图博山侧翼的河流河床表面结构变化的幅度和速率追踪了火山碎屑流沉积物对其流域初始泥沙负荷的差异；(4)卵石团簇的形成是河道响应泥沙负荷下降的一种机制，泥沙负荷下降反过来又会导致河床移动方式和河床输沙率的变化。

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.