圣海伦斯火山爆发对图特河北汊河床演变的影响

Response of the North Fork Toutle River following eruption of Mount St. Helens

受1980年美国华盛顿州圣海伦斯火山爆发的影响,图特河北汊的流域特性、水沙条件和河床边界条件均发生了巨大变化,河流的调整和响应十分迅速,为研究河流受突发剧烈扰动后的河床再调整过程提供了一个很好的范例。本文选取图特河北汊上中游14个测量断面为研究对象,收集了近30年来的水文泥沙和断面资料,对河道的垂向、横向及纵向的形态调整过程进行了全面分析。结果表明,图特河北汊在火山爆发后前3年内的调整最为迅速,后期调整逐渐减缓,并向平衡状态发展,显示了河流受到扰动后调整速率逐渐衰减的重要特征,这一特点是冲积河流自动调整的反映。图特河北汊河床的调整在空间上呈现上冲下淤的特点,河流正是通过河床上冲下淤的调整方式,使得河道比降整体不断变缓,水流能量降低,上游河道冲刷减弱,进而使河流逐渐向平衡状态发展。此外,河岸崩塌展宽也是图特河北汊调整的重要方面,上游源区段和峡谷段的河岸侵蚀主要由河床持续冲刷下切导致,而宽谷段河岸的崩塌主要是由于河道的横向摆动引起。

The North Fork Toutle River （NFTR） was severely affected by the 1980 eruption of Mount St. Helens located at Washington State, and its watershed characteristics, water and sediment conditions, channel morphology and boundary conditions have undergone tremendous changes. Its fast and high-magnitude readjustment provided an excellent chance for investigating channel response to extreme disturbances. In this study, we collected geometry data and hydrological data of ihe last 30 years for 14 cross sections in the upper and middle reaches of this river and comprehensively analyzed the vertical, lateral and longitudinal variations of channel geometry. The results show that the river adjusted most rapidly during the first three years after the eruption, but the adjustment rate decayed with time and the channel gradually tended to a stable or equilibrium state, reflecting the auto-conditioning of natural fluvial system. The channel bed was severely degraded in the upper reach while aggraded in the lower reach, which jointly decreased the slope of channel bed and lowered the stream power. This was the way that the river conditioned itself and relaxed to a relatively stable state. Bank collapse was another significant aspect of channel response to the eruption and all the cross sections in the studied reach experienced bank erosion in different degrees. Bank collapse in the head reach and the narrow channel reaches was induced by consistent channel bed erosion while that in the wide channel reaches downstream was caused by lateral shift of main channel.