The September 2008 floods on the River Coquet, Northumberland: Characteristics and immediate impact on river morphology

2008年9月6日至7日，诺森伯兰及其河流遭受强烈降水事件的重创，在24小时内带来了相当于一个月的降雨量(从80毫米到150毫米不等)。诺森伯兰向东流动的河流的流量反应迅速而壮观。万斯贝克河决堤，影响到Morpeth镇和更北边的1000多名居民，科奎特河切断了Rothbury集镇，造成进一步破坏，居民从被洪水淹没的财产中撤离。2008年9月的洪水是历史上影响万斯贝克河和科克特河水系的最大洪水，以前的所有高水位都被超过了。再往北，布雷米什河和泰尔河也受到严重影响。虽然对于有良好测量设施的城市化路段来说，洪水流量的计算相对容易，但集水区上游的监测往往不那么严格。然而，在高降雨量事件期间，集水区上部发生的事情对下游社区有直接影响。我们把重点放在科克特河，因为PI已经获得了大量洪水前基线数据，这将使2008年洪水的影响得到更清楚的定义。这条河发源于苏格兰边境的科克特黑德，集水面积为636平方英尺。从源头到河口，全长62公里，横贯切维奥特山脉南侧。它是一个在富含盐基的岩石上快速流动的高地河流的绝佳例子，沿着它的长度，在到达罗斯伯里镇之前，有许多主要的支流连接在一起，包括阿尔温河和里德利河、乌斯韦、霍斯通和雷赫伯恩斯。在罗斯伯里下游，这条河与其他流入支流汇合，然后在安布尔流入北海。我们将把重点放在罗斯伯里以上的河段(子集水区面积为225平方米)。公里)，因为这标志着科克特河明显活跃的山前地区，正在积极地重新开发其泛滥平原。我们正在根据NERC紧急计划申请量化这一段集水区的洪水影响，因为拟议的实地研究显然符合该计划的标准-这是一个意想不到的短暂科学机会，科学家需要对不断变化的事件做出快速反应。在这里，需要尽快开展实地工作，以调查瞬时洪水冲刷界限证据，并在随后的洪水开始之前绘制河道地貌图，这可能会掩盖2008年9月洪水的特征。由侵蚀边界线、泥沙淤积范围和碎屑垃圾线组成的洪水冲刷界限都是固有的瞬时性的，因为它们在洪水清理期间以及包括分解、水流作用和放牧在内的自然过程中都会被清除。尽管缺乏直接流量测量，但存在标准的水力学技术(统称为坡面法)，并将用于重建洪水水坡的洪水流量(根据界定最高水位或水流深度的泥石流冲刷界限重建)、渠道横截面积和流速(利用水力信息和渠道粗糙度知识重建)。

Northumberland and its rivers were hard hit by an intense precipitation event on 6th-7th September 2008 which delivered over one month's worth of rainfall (ranging in places from 80 mm to150 mm) within a 24 hour period. The discharge response from Northumberland's eastward flowing rivers was rapid and spectacular. The River Wansbeck burst its banks affecting over 1000 residents in the town of Morpeth and further north, the River Coquet cut off the market town of Rothbury resulting in further damage and evacuation of residents from inundated properties. The September 2008 floods were the largest to affect the Wansbeck and Coquet river systems in historic time with all previous high water marks being exceeded. Further to the north, the River Breamish and River Till were also heavily impacted. While calculation of the flood flow is relatively easy for urbanised sections with good gauging facilities, upper reaches of catchments often are less heavily monitored. Yet, what happens in the upper sectors of catchments during high precipitation events has direct implications for downstream communities. We are focusing on the River Coquet due to the large amount of pre-flood baseline data already obtained by the PI, which will allow the effects of the 2008 floods to be more clearly defined. The river rises at Coquet Head on the Scottish border, has a catchment area of 636 sq. km and runs eastwards for a length of 62 km from source to mouth, draning the southern flanks of the Cheviot Hills. It is an excellent example of a fast flowing upland river on base-rich rocks, and along its length, is joined by a number of major tributaries including the River Alwin and the Ridlees, Usway, Holystone and Wreigh Burns before reaching the town of Rothbury. Downstream of Rothbury, the river is joined by other inflow tributaries before discharging into the North Sea at Amble. We will be focusing on the reach above Rothbury (with a sub-catchment area of 225 sq. km) as this marks a piedmont region where the River Coquet is notably dynamic, actively re-working its floodplain. We are applying under the NERC Urgency scheme to quantify flood impacts in this section of the catchment as the proposed field-based research clearly meets the Scheme's criteria - it is an unexpected and transient scientific opportunity where scientists need to respond rapidly to changing events. Here, fieldwork needs to be undertaken as soon as possible to survey transient flood wash limit evidence and to map channel morphology before the onset of subsequent floods, which may mask the signature of the September 2008 flood. Flood wash limits consisting of erosional trimlines, silt deposition extent and debris trash lines are all inherently transient as they are subject to removal during the flood clean-up as well as by natural processes including decomposition, actions of water currents and grazing. Despite the lack of direct flow gauging, standard hydraulic techniques (collectively termed the Slope-Area Method) exist, and will be used to reconstruct flood discharge from flood water slopes (reconstructed from debris wash limits demarcating the highest stage or flow depth), channel cross section area and flow velocities (reconstructed using hydraulic information and knowledge of channel roughness).