Erwan Garel

Recommendations for the sustainable exploitation of tidal sandbanks

A basic requirement for allowing marine aggregate (sand) extraction on the Belgian Continental Shelf (which takes place on sandbanks) is that it should not result in major environmental changes. However, a tidal sandbank (Kwinte Bank, Flemish Banks), exploited intensively since the 1970’s, has shown evidence of significant morphological changes with the development of a 5 m deep depression in its middle section; thus, since February 2003, sand extraction has ceased in this area in order to study the environmental impacts and the regeneration potential of the seabed. The present contribution synthesises the results of the multidisciplinary research, which has taken place in the area and, on the basis of these findings, considers the need for an efficient management framework, in both the planning and monitoring stages of the extraction. The investigation has shown that extraction has had significant impacts on the seabed sedimentary character and ecology and the local hydro-and sediment dynamic regime. Under these conditions, regeneration of the seabed is not likely in the short-term and, although modelling exercises have indicated possible recovery in the medium- and long-term, this is likely to be inhibited by the lack of appropriate sediments in the area. The results have provided the basis of the identification of a ‘suite’ of criteria, which can assist in the strategic planning/design of marine aggregate concession zones, the efficient management of marine aggregate extraction and the planning of effective environmental monitoring; these criteria are related to considerations on resource location, the nature/thickness of the targeted deposits, morphodynamics and sediment dynamics, biology and ecology and extraction practices. The Kwinte Bank investigation has demonstrated also the need for intensive monitoring schemes in order to identify the morphological, sedimentary and ecological impacts, related to the dredging activities. A critical part of these schemes should be the evaluation of the dredging-related effects, against the background of the natural dynamics of the seabed; thus, baseline information is crucial, as, in its absence, impact assessments are likely to remain inconclusive.

Effects of Tidal-Forcing Variations on Tidal Properties Along a Narrow Convergent Estuary

A 1D analytical framework is implemented in a narrow convergent estuary that is 78 km in length (the Guadiana, Southern Iberia) to evaluate the tidal dynamics along the channel, including the effects of neap-spring amplitude variations at the mouth. The close match between the observations (damping from the mouth to ∼ 30 km, shoaling upstream) and outputs from semi-closed channel solutions indicates that the M2 tide is reflected at the estuary head. The model is used to determine the contribution of reflection to the dynamics of the propagating wave. This contribution is mainly confined to the upper one third of the estuary. The relatively constant mean wave height along the channel (< 10% variations) partly results from reflection effects that also modify significantly the wave celerity and the phase difference between tidal velocity and elevation (contradicting the definition of an “ideal” estuary). Furthermore, from the mouth to ∼ 50 km, the variable friction experienced by the incident wave at neap and spring tides produces wave shoaling and damping, respectively. As a result, the wave celerity is largest at neap tide along this lower reach, although the mean water level is highest in spring. Overall, the presented analytical framework is useful for describing the main tidal properties along estuaries considering various forcings (amplitude, period) at the estuary mouth and the proposed method could be applicable to other estuaries with small tidal amplitude to depth ratio and negligible river discharge.

Collection and dissemination of data from environmental monitoring systems in estuaries

Environmental monitoring stations providing high frequency data over a multiyear time frame are not common in estuaries. These systems are designed to record extended time-series at high frequency that are of great value for decision makers and the scientific community. However, the continuous acquisition of good quality data at estuaries is generally challenged by harsh environmental conditions. This contribution describes the main issues for continuous valid data (water quality and currents) acquisition in 2008–2014 with a monitoring station deployed at the Guadiana Estuary and how both near real-time and post-processed data were disseminated using web interfaces.

Wave-induced sand re-suspension at dredged gravel pits based upon hydrodynamic measurements (Tromper Wiek, Baltic Sea)

ABSTRACT Gravel pits created by anchor hopper dredging may affect regional sediment transport patterns by trapping sediments. In turn, this may cause -or enhance- erosion at the adjacent coastline. Reliable assessment of such impacts requires a good understanding of the hydro-sediment dynamic processes acting at dredged pits. This paper examines the processes for sand re-suspension from pressure, current and turbidity data collected inside and outside a single dredged pit, in a non-tidal environment (Tromper Wiek, Baltic Sea). The data confirm the generally weak sediment dynamics in the area, with waves being the main hydrodynamic agent for sediment re-mobilization. Comparisons with historical data indicate a small number of sediment re-suspension events (<15%), over a 37 months-long period, without significant difference inside and outside the pit. Suspended sediment concentration profiles are predicted inside the studied pit by a simplistic model, tuned to over-estimate sediment re-suspension. The results suggest that the depth of the excavation should be very shallow (<1 m) for the bed material to be frequently extracted out by waves, and redistributed over the area. With pits up to 7 m-deep within the extraction zone, we conclude that a significant fraction of sediment is trapped over the long-term period (years).

HYDRODYNAMICS AND SEDIMENT TRANSPORT DURING A TIDAL CYCLE ON THE KWINTE BANK (SOUTHERN NORTH SEA)

Short-term dynamics of the sandy Kwinte bank is examined in terms of water circulation, sediment transport and bedforms mobility during a tidal cycle using a hull- mounted ADCP and towed side-scan sonar, along a profile across the bank. Tidal rectification of the flow over the bank has been observed, together with clockwise rotation of the tidally-averaged currents. The computed net sediment transport, from a 1D sediment transport model, is generally in the direction of the ebb currents, towards the SW. In details, a convergence of the sand transport is predicted towards the top of the west flank. SSS images reveal reversal of megaripples asymmetry during the tide. These results match with the stability model for sand banks formation and maintenance, and suggest that bank dynamics have to be studied as a whole system which includes the adjacent swales and banks.

The Guadiana River Delta

The Guadiana Estuary is a good example of rock bounded estuary which consists of a single narrow estuarine channel with a meandering morphology imposed by faults systems affecting the hard geology of the substrate. Only along the last kilometers of the estuarine channel, the valley opens when Cenozoic Guadalquivir Basin formations appear. In this area, the Guadiana develops a prograding coastal system constituted by successive sandy barriers separated by salt marshes which configure a wave dominated delta. This progradation is possible thanks an interaction of the coastal agents which enhanced the silting in addition to a good availability of sediments. This chapter explains an explanation of the dynamic functioning of the open coastal environment, so as the resulting facies model.