Abstract
PurposeA thorough understanding of mechanisms controlling sedimentation and erosion is vital for a proper assessment of the effectiveness of delta restoration. Only few field-based studies have been undertaken in freshwater tidal wetlands. Furthermore, studies that measured sediment deposition in newly created wetlands are also sparse. This paper aims to identify the factors controlling the sediment trapping of two newly created freshwater tidal wetlands.Materials and methodsTwo recently re-opened polder areas in the Biesbosch, The Netherlands are used as study area. Field measurements of water levels, flow velocities, and turbidity at both the in- and outlet of the areas were carried out to determine the sediment budgets and trapping efficiencies under varying conditions of river discharge, tide, and wind in the period 2014–2016.Results and discussionShort-term sediment fluxes of the two study areas varied due to river discharge, tide, and wind. A positive sediment budget and trapping efficiency was found for the first study area, which has a continuing supply of river water and sediment. Sediment was lost from the second study area which lies further from the river and had a lower sediment supply. The daily sediment budget is positively related to upstream river discharge, and in general, export takes place during ebb and import during flood. However, strong wind events overrule this pattern, and trapping efficiencies decrease for increasing wind strengths at mid-range river discharges and for the highest river discharges due to increased shear stress.ConclusionsDelta restoration, based on sedimentation to compensate for sea-level rise and soil subsidence, could only be effective when there is a sufficient supply of water and sediment. Management to enhance the trapping efficiency of the incoming sediment should focus on directing sufficient river flow into the wetland, ensuring the supply of water and sediment within the system during a tidal cycle, creating sufficiently large residence time of water within the polder areas for sediment settling, and decreasing wave shear stress by the establishment of vegetation or topographic irregularities.
Highlights
IntroductionDeltas are dynamic, riverdominated systems, which drown or aggradate in response to changes in environmental conditions (Giosan et al 2013)
Responsible editor: Jose Andres Lopez TarazonElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Under natural conditions, deltas are dynamic, riverdominated systems, which drown or aggradate in response to changes in environmental conditions (Giosan et al 2013)
We use two recently re-opened polder areas in the Biesbosch, The Netherlands, as study area, taking advantage of the differences in geographical setting and boundary conditions of the two nearby wetland areas that are similar in climate conditions and tidal range
Summary
Deltas are dynamic, riverdominated systems, which drown or aggradate in response to changes in environmental conditions (Giosan et al 2013) Both sedimentation and organic soil formation by vegetation contribute to elevation gain and delta aggradation (Reddy and DeLaune 2008; Calvo-Cubero et al 2013; Kirwan and Megonigal 2013; Schile et al 2014). Paola et al (2011) defined river delta restoration as “diverting sediment and water from major channels into adjoining drowned areas, where the sediment can build new land and provide a platform for regenerating wetland ecosystems.” This type of river delta restoration is currently considered or implemented in the Tidal River Management project in Bangladesh (Khadim et al 2013), the diversion projects in the Mississippi deltaic plain (DeLaune et al 2003; Day et al 2007; Paola et al 2011) or in the Atchafalaya subdelta (Roberts et al 2015; DeLaune et al 2016), and the Plan Integrale de Proteccion del Delta Ebro for the Ebro Delta (Calvo-Cubero et al 2013) Paola et al (2011) defined river delta restoration as “diverting sediment and water from major channels into adjoining drowned areas, where the sediment can build new land and provide a platform for regenerating wetland ecosystems.” This type of river delta restoration is currently considered or implemented in the Tidal River Management project in Bangladesh (Khadim et al 2013), the diversion projects in the Mississippi deltaic plain (DeLaune et al 2003; Day et al 2007; Paola et al 2011) or in the Atchafalaya subdelta (Roberts et al 2015; DeLaune et al 2016), and the Plan Integrale de Proteccion del Delta Ebro for the Ebro Delta (Calvo-Cubero et al 2013)
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