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SPM response to tide and river flow in the hyper-turbid Ems River

机译：SPM对高浊度Ems河中潮汐和河流流量的响应

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摘要

In this paper, we analyse the behaviour of fine sediments in the hyper-turbid Lower Ems River, with focus on the river's upper reaches, a stretch of about 25 km up-estuary of Terborg. Our analysis is based on long records of suspended particulate matter (SPM) from optical backscatter (OBS) measurements close to the bed at seven stations along the river, records of salinity and water level measurements at these stations, acoustic measurements on the vertical mud structure just up-estuary of Terborg and oxygen profiles in the lower 3 m of the water column close to Leerort and Terborg. Further, we use cross-sectionally averaged velocities computed with a calibrated numerical model. Distinction is made between four timescales, i.e. the semi-diurnal tidal timescale, the spring-neap tidal timescale, a timescale around an isolated peak in river flow (i.e. about 3 weeks) and a seasonal timescale. The data suggest that a pool of fluid/soft mud is present in these upper reaches, from up-estuary of Papenburg to a bit down-estuary of Terborg. Between Terborg and Gandersum, SPM values drop rapidly but remain high at a few gram per litre. The pool of fluid/soft mud is entrained/mobilized at the onset of flood, yielding SPM values of many tens gram per litre. This suspension is transported up-estuary with the flood. Around high water slack, part of the suspension settles, being remixed during ebb, while migrating down-estuary, but likely not much further than Terborg. Around low water slack, a large fraction of the sediment settles, reforming the pool of fluid mud. The rapid entrainment from the fluid mud layer after low water slack is only possible when the peak flood velocity exceeds a critical value of around 1 m/s, i.e. when the stratified water column seems to become internally supercritical. If the peak flood velocity does not reach this critical value, f.i. during neap tide, fluid mud is not entrained up to the OBS sensors. Thus, it is not classical tidal asymmetry, but the peak flood velocity itself which governs the hyper-turbid state in the Lower Ems River. The crucial role of river flow and river floods is in reducing these peak flood velocities. During elongated periods of high river flow, in e.g. wintertime, SPM concentrations reduce, and the soft mud deposits consolidate and possibly become locally armoured as well by sand washed in from the river. We have no observations that sediments are washed out of the hyper-turbid zone. Down-estuary of Terborg, where SPM values do not reach hyper-turbid conditions, the SPM dynamics are governed by classical tidal asymmetry and estuarine circulation. Hence, nowhere in the river, sediments are flushed from the upper reaches of the river into the Ems-Dollard estuary during high river flow events. However, exchange of sediment between river and estuary should occur because of tide-induced dispersion.
机译：在本文中，我们分析了高浊度的下埃姆斯河中细沙的行为，重点是该河的上游河段（特堡河口上游约25公里）。我们的分析基于长期沿河七个站附近河床的光学反向散射（OBS）测量得出的悬浮颗粒物（SPM）记录，这些站的盐度和水位测量记录，垂直泥浆结构的声学测量结果仅在Terborg的上河口和靠近Leerort和Terborg的水柱下部3 m中的氧气剖面。此外，我们使用通过校准数值模型计算得到的横截面平均速度。在四个时标之间进行区分，即半日潮汐时标，春季-潮汐时标，围绕河流流量的孤立峰值的时标（即约3周）和季节性时标。数据表明，从帕彭堡的上河口到特堡的下河口，这些上游都有大量的软泥。在特堡和甘德苏姆之间，SPM值迅速下降，但仍保持在每升几克的高水平。洪水开始时，流体/软泥池被夹带/移动，产生的SPM值为每升数十克。该悬架随洪水被输送到河口。在高水位松弛附近，部分悬浮物沉降，在退潮期间重新混合，同时向河口下迁移，但可能不会比特堡更远。在低水位松弛附近，很大一部分沉积物沉淀下来，从而改变了流体泥浆池。仅当峰值洪流速度超过大约1 m / s的临界值时，即当分层水柱内部似乎变得超临界时，才可能从低含水量的泥浆层中快速夹带。如果峰值洪水速度未达到此临界值，则f.i。在潮汐期间，泥浆不会被夹带至OBS传感器。因此，不是经典的潮汐不对称性，而是洪峰速度本身决定着下埃姆斯河的超浑浊状态。河流流量和洪水洪水的关键作用在于降低洪水的峰值速度。在延长的高流量期间，例如冬季，SPM浓度降低，软泥沉积物巩固，并有可能被从河中冲刷的沙子当地铠装。我们没有观察到沉积物被冲出超浑浊带。特堡（Serborg）的河口下游，SPM值未达到超浑浊的条件，SPM动力学受经典的潮汐不对称性和河口环流控制。因此，在河流流量大的时候，河中的任何地方都没有沉淀物从河的上游冲入Ems-Dollard河口。但是，由于潮汐引起的分散，河流和河口之间的沉积物交换应该发生。

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