Fine sediment dynamics in a shallow lake and implication for design of hydraulic works

Abstract

Lake Markermeer is a large (680 km 2 ), shallow body of water in the middle of the Netherlands, with a mean water depth of 3.6 m. One of the major problems in the lake is its decreasing ecological value which is, among other reasons, caused by a gradual increase of suspended sediment concentration and associated increase of light attenuation in the water column. A thorough understanding of fine sediment dynamics in the lake is a prerequisite for solving this problem. This paper addresses the 3D nature of near-bed sediment dynamics in Lake Markermeer, based on data sampled from a 1-month field experiment in autumn 2007. The campaign involved the collection of 71 bed samples across the lake. At each location, dual-frequency echo soundings were carried out to assess the thickness of the silt layer, and sediment concentration throughout the water column was measured with an Optical Backscatter Sensor (OBS). Moreover, 2-week time series of wave height, water level, current velocities, and near-bed sediment concentration were collected at a single location. The time series of sediment concentration were measured with a regular OBS and an Argus Surface Meter IV (ASM). During the measurement period, flow velocities ranged between 2 and 15 cm/s, wave heights up to 1.2 m were observed and turbidity levels varied between 40 mg/l to more than 300 mg/l. The ASM data generally showed uniform concentration profiles. However, profiles with steep concentration gradients near the bed were found for wave heights above 0.5 m. The field experiments further revealed pronounced 3D structures near the bed during discrete storms. The results are generalized for a wider range of conditions and across the full water depth through application of a 1DV point model, using a two-fraction representation of the grain size distribution. The fine and coarse fractions are found to resuspend rapidly for wind speeds above 5 m/s and 10-12 m/s, respectively, forming a uniform concentration profile if these wind conditions persists. High-concentration (∼1 g/l) layers near the bed, containing the coarse sediment fraction, only occur at the onset and towards the end of a storm, when wind speed changes rapidly. It is under these conditions that horizontal gradients in layer density or thickness can transport considerable fine sediment. This transport provides an additional mechanism for the infill of, for instance, silt traps and navigation channels.

Construction of asymmetric copulas and its application in two-dimensional reliability modelling

“…Such applications can be found in other disciplines. For example, the proposed method can be used to model the flow velocity and the wind speed [23].…”

Effect of algae on flocculation of suspended bed sediments in a large shallow lake. Consequences for ecology and sediment transport processes

“…The uppermost layer is a thin oxic sediment layer, highly bioturbated and, due to its low erosion threshold, frequently re-suspended (de Lucas Pardo et al 2013). Below, a thicker and stronger anoxic layer can be found, which is eroded only during storm events (de Lucas Pardo et al 2013;Vijverberg et al 2011). Finally, the bottom layer consists of old deposits from the period in which the whole region was still an estuary.…”

Effects of Suspended Sediments on Seston Food Quality for Zebra Mussels in Lake Markermeer, The Netherlands

“…3). The overall inorganic suspended sediments (ISS) concentration, averaged across depth, in Lake Markermeer has remained approximately constant but is high compared to the surrounding lakes (Vijverberg et al 2010 and references therein). In Lake Markermeer, the average total suspended matter (TSM) concentration is 55 mg L −1 and is composed of 70% inorganic clay particles (<10 µm), depending on the season and meteorological conditions.…”

“…For comparison, in Lake Ijsselmeer the average TSM concentration is 30 mg L −1 , composed of 50% ISS (Centre for Water Management, Lelystad, The Netherlands, unpublished monitoring data). The fetch in Lake Markermeer is 40 km and this, combined with an exposed, shallow clay-silt bottom (mean depth 3.6 m), causes a constant load of resuspended sediment into the water column (Vijverberg et al 2010); however, information is lacking on the quality and exact composition of this TSM (e.g., potential changes in particle size classes, organic/inorganic ratios, or phytoplankton composition), which may affect the filtration capabilities of the zebra mussels and hence their biomass in the lake over time.…”