Coastal sediment supply

Résumé

Coastal sediment supply is the transport of sediment to the beach environment by both fluvial and aeolian transport. While aeolian transport plays a role in the overall sedimentary budget for the coastal environment, it is paled in comparison to the fluvial supply which makes up 95% of sediment entering the ocean. When sediment reaches the coast it is then entrained by longshore drift and littoral cells until it is accreted upon the beach or dunes. While it is acknowledged that storm systems are the driver behind coastal erosion. There is a general consensus that human activity, mainly dam and reservoir impoundments on rivers are the cause of indirect human related coastal erosion, along with other local scale effects such as: land use change, irrigation, gravel extraction and river re-alignment. Rate of supply Worldwide, rivers discharge approximately 35x103 km3 of freshwater into the ocean annually. Transported in this freshwater is 15 to 2

Source officielle

https://en.wikipedia.org/wiki/Coastal_sediment_supply

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Water reservoirs used to produce electricity have an impact on the environment and durability by : • stopping the natural sediment flux • changing the flow regime downstream of the reservoirs • storing (fine) sediments that reduce the storage volume Those 3 issues have been leading to the development of strategies in order to improve the equilibrium of the ecosystem downstream such structures, with solution such as sediment flushing and simple water flushing reproducing flood events. Those operations produce excessive sediment inflow on certain river sections, which can lead to the clogging of the gravel bed. River construction work, soil erosion, emergency actions and natural river bank erosion can also bring similar problematic. For example, a dramatic event happened in 2013 on the Spöl River in Eastern Switzerland. Due to some operations on a Punt dal Gall dam, important amountsof fine sediments were flushed downstream the reservoir and resulted in significant damages to the river ecosystem1. A strong clogging of the river bed was noted. A clean water flushing a few months later led to a cleaning of the clogged areas.

2019

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Hybrid modeling of sediment management during drawdown of Räterichsboden reservoir

Milad Daneshvari, Giovanni De Cesare

The Swiss utility Kraftwerke Oberhasli AG (KWO) currently implements "KWO plus", an upgrading program involving a large number of economical and ecological improvements of their hydropower schemes. A major project, the modification of the Handeck 2 power plant intake, requires total emptying of the Räterichsboden reservoir having an active volume of 25 million m3. In order to define the most efficient and ecological emptying process for these circumstances, KWO commissioned a hybrid modelling program of the sediment remobilisation processes during the planned reservoir drawdown. Hydraulic model tests were conducted at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) by means of a 1:35 scaled Froude model. Numerical simulations of sediment transport were performed by the Laboratory of Hydraulic Constructions (LCH) using the software FLOW-3D. A good agreement between both models was found for total values of the sediment concentration and its variation in time. Both the physical and the numerical model were validated by prototype data obtained during an annual functional test of the bottom outlet. These measurements were conducted at three downstream positions in the Aare River. The involved solution procedures provide the (1) sediment flux during pressure flushing, (2) influence of outlet gate opening height and opening velocity on sediment concentration, and (3) time-dependent distribution of sediment concentration along the Aare River.

CRC Press, Taylor & Francis Group2010

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Landscape evolution in tidal embayments: Modeling the interplay of erosion, sedimentation, and vegetation dynamics

Andrea Rinaldo

We propose an ecomorphodynamic model which conceptualizes the chief land-forming processes operating on the intertwined, long-term evolution of marsh platforms and embedded tidal networks. The rapid network incision (previously addressed by the authors) is decoupled from the geomorphological dynamics of intertidal areas, governed by sediment erosion and deposition and crucially affected by the presence of vegetation. This allows us to investigate the response of tidal morphologies to different scenarios of sediment supply, colonization by halophytes, and changing sea level. Different morphological evolutionary regimes are shown to depend on marsh ecology. Marsh accretion rates, enhanced by vegetation growth, and the related platform elevations tend to decrease with distance from the creek, measured along suitably defined flow paths. The negative feedback between surface elevation and its inorganic accretion rate is reinforced by the relation between plant productivity and soil elevation in Spartina-dominated marshes and counteracted by positive feedbacks in multispecies-vegetated marshes. When evolving under constant sea level, unvegetated and Spartina-dominated marshes asymptotically tend to mean high water level (MHWL), different from multiple vegetation species marshes, which can make the evolutionary transition to upland. Equilibrium configurations below MHWL can be reached under constant rates of sea level rise, depending on sediment supply and vegetation productivity. Our analyses on marine regressions and transgressions show that when the system is in a supply-limited regime, network retreat and expansion (associated with regressions and transgressions, respectively) tend to be cyclic. Conversely, in a transport-limited regime, network reexpansion following a regression tends to take on a new configuration, showing a hysteretic behavior. Copyright 2007 by the American Geophysical Union.

2007

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Landscape evolution in tidal embayments: Modeling the interplay of erosion, sedimentation, and vegetation dynamics

Andrea Rinaldo

2007