The impact of channel deepening and dredging on estuarine sediment concentration

Abstract

a b s t r a c tMany estuaries worldwide are becoming more urbanised with heavier traffic in the waterways, requiring continuous channel deepening and larger ports, and increasing suspended sediment concentration (SSC). An example of a heavily impacted estuary where SSC levels are rising is the Ems Estuary, located between the Netherlands and Germany. In order to provide larger and larger ships access to three ports and a shipyard, the tidal channels in the Ems Estuary have been substantially deepened by dredging over the past decades. This has led to tidal amplification and hyper concentrated sediment conditions in the upstream tidal river. In the middle and outer reaches of the Ems Estuary, the tidal amplification is limited, and mechanisms responsible for increasing SSC are poorly understood. Most likely, channel and port deepening lead to larger SSC levels because of resulting enhanced siltation rates and therefore an increase in maintenance dredging. Additionally, channel deepening may increase up-estuary suspended sediment transport due to enhanced salinity-induced estuarine circulation.The effect of channel deepening and port construction on SSC levels is investigated using a numerical model of suspended sediment transport forced by tides, waves and salinity. The model satisfactorily reproduces observed water levels, velocity, sediment concentration and port deposition in the estuary, and therefore is subsequently applied to test the impact of channel deepening, historical dredging strategy and port construction on SSCs in the Estuary. These model scenarios suggest that: (1) channel deepening appears to be a main factor for enhancing the transport of sediments up-estuary, due to increased salinity-driven estuarine circulation; (2) sediment extraction strategies from the ports have a large impact on estuarine SSC; and (3) maintenance dredging and disposal influences the spatial distribution of SSC but has a limited effect on average SSC levels.

Surficial sediment erodibility from time-series measurements of suspended sediment concentrations: development and validation

“…Additional lines of evidence that support the presence of the fluff layer are summarized in the Appendix. Similar sediment dynamics have also been observed in other tidal and estuarine systems (Maa et al 1998;Van Maren et al 2015;Van Kessel et al 2011;El Ganaoui et al 2004;Wang 2003;Bedford et al 1987). Using the intra-tidal range in depthaverage SSC of ∼10 to ∼75 mg/L within the salt wedge, with tidally averaged water depth of 6 m, and assuming a dry density of 200 kg/m 3 (for the sediments comprising the fluff layer) results in an estimated fluff layer thickness of about 2 mm.…”

“…Three approaches that have been reported in the literature for parameterizing the erosion properties of the fluff layer include (1) direct measurements using an apparatus such as the Gust microcosm, Sea Carousel, etc. (Gust and Mueller 1997;Parchure and Mehta 1985;Amos et al 1992;Maa et al 1993), (2) model calibration to SSC measurements (Van Kessel et al 2011;Van Maren et al 2015), and (3) using field measurements (Wang 2003;Sanford et al 1991;deVries 1992). The entrainment flux method is an additional approach using field measurements that can serve as a useful alternative in situations where direct measurements of erosion may be difficult or cost-prohibitive.…”

“…The concept of a fluff layer has also been implemented in numerical sediment transport model applications at a few sites, most notably by Van Maren et al (2015) and Van Kessel et al (2011). However, some relevant information about the fluff layer such as its spatial distribution, composition, structure, and density are currently not very well understood.…”

“…Equation (1) and its variants have been implemented in numerical sediment transport models for numerous applications around the world. Such applications typically rely on either parameterization derived from physical measurements of the erosion properties, achieved using a number of different devices and approaches such as the Gust microcosm (Gust and Mueller 1997), Sedflume (McNeil et al 1996), carousels (Parchure and Mehta 1985;Amos et al 1992;Maa et al 1993), soil mechanical properties (Winterwerp et al 2012), etc., or rely on model calibration to suspended sediment concentration (SSC) measurements (Van Kessel et al 2011;Van Maren et al 2015). The experimental methods vary in complexity in terms of sample requirements (for example, undisturbed cores versus grab samples), ex situ versus in situ measurements, laboratory equipment (for example, specialized flumes versus conventional soil mechanical measurements), sediment strata sampled (for example, thin layers near the bed surface versus larger depth intervals but deeper in the bed), etc.…”

Cross-Scale Baroclinic Simulation of the Effect of Channel Dredging in an Estuarine Setting

“…Accordingly, water resources managers and engineers need to assess the impacts of any proposed activity prior to its inception. Numerical models are frequently required in understanding the effects of existing projects and predict the changes under future plans [5][6][7][8]. This demands good accuracy, efficiency and robustness from the modeling tool, which can be challenging in an estuarine setting because: (1) human activities often involve small-scale alterations on natural conditions, such as the construction of hydraulic structures or localized changes in shoreline and bathymetry features, which require a high-resolution model grid to resolve; (2) the spatial scales of such systems are intrinsically linked, which can range from kilometer-scale shelf dynamics to meter-scale local dynamics at the project site.…”