Effects of estuarine mudflat formation on tidal prism and large‐scale morphology in experiments

Abstract

Human interference in estuaries has led to increasing problems of mud, such as hyper‐turbidity with adverse ecological effects and siltation of navigation channels and harbours. To deal with this mud sustainably, it is important to understand its long‐term effects on the morphology and dynamics of estuaries. The aim of this study is to understand how mud affects the morphological evolution of estuaries. We focus on the effects of fluvial mud supply on the spatial distribution of mudflats and on how this influences estuary width, depth, surface area and dynamics over time. Three physical experiments with self‐forming channels and shoals were conducted in a new flume type suitable for tidal experiments: the Metronome. In two of the experiments, we added nutshell grains as mud simulant, which is transported in suspension. Time‐lapse images of every tidal cycle and digital elevation models for every 500 cycles were analysed for the three experiments. Mud settles in distinct locations, forming mudflats on bars and sides of the estuary, where the bed elevation is higher. Two important effects of mud were observed: the first is the slight cohesiveness of mud that causes stability on bars limiting vertical erosion, although the bank erosion rate by migrating channels is unaffected. Secondly, mud fills inactive areas and deposits at higher elevations up to the high‐water level and therefore decreases the tidal prism. These combined effects cause a decrease in dynamics in the estuary and lead to near‐equilibrium planforms that are smaller in volume and especially narrower upstream, with increased bar heights and no channel deepening. This trend is in contrast to channel deepening in rivers by muddier floodplain formation. These results imply large consequences for long‐term morphodynamics in estuaries that become muddier due to management practices, which deteriorate ecological quality of intertidal habitats but may create potential area for marshes. © 2018 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.

Growing Forced Bars Determine Nonideal Estuary Planform

“…Pilot experiments showed that tilting with a simple sine function results in net exporting systems (supporting information in Braat et al, 2018), which means that the system could be classified as a delta sensu Dalrymple et al (1992). However, we here refer to the system as an estuary, because the relative contribution of river discharge to the tidal volume is too low (≈1%) while ebb and flood currents are much larger and approximately equal (Kleinhans, van der Vegt, et al, 2017).…”

“…The experiment was run for 15,000 tidal cycles, which corresponds to approximately 20 years of natural tidal cycles assuming a semidiurnal tide. The experimental settings were selected based on a set of approximately 30 pilot experiments in which boundary conditions have been varied systematically, which are reported in the supporting information of Braat et al (2018). The settings were selected to ensure that sediment was well above the threshold for motion and that the tidal excursion length, which is the distance a water particle travels in half a tidal cycle, was shorter than the flume length.…”

“…This sediment mixture was selected to prevent the occurrence of scour holes as much as possible . Another set of experiments were conducted with the addition of crushed walnut shell to simulate the effect of cohesive material, which are reported in Braat et al (2018). We will summarize the effect of this as far as relevant for bar growth and estuary widening in the discussion.…”

Experimental delta evolution in tidal environments: Morphologic response to relative sea‐level rise and net deposition

“…funnel-shaped tidal channels, tidal mouth bars, etc.) Physical experiments can help bridge this knowledge gap, allowing one to capture delta evolution at a spatial and temporal resolution otherwise impractical in the field (Malverti et al, 2008;Paola et al, 2009;Kleinhans et al, 2012Kleinhans et al, , 2014aStefanon et al, 2012;Braat et al, 2018;Lentsch et al, 2018;Leuven et al, 2018). Physical experiments can help bridge this knowledge gap, allowing one to capture delta evolution at a spatial and temporal resolution otherwise impractical in the field (Malverti et al, 2008;Paola et al, 2009;Kleinhans et al, 2012Kleinhans et al, , 2014aStefanon et al, 2012;Braat et al, 2018;Lentsch et al, 2018;Leuven et al, 2018).…”

“…are typically not well preserved (Dalrymple and Choi, 2007). Physical experiments can help bridge this knowledge gap, allowing one to capture delta evolution at a spatial and temporal resolution otherwise impractical in the field (Malverti et al, 2008;Paola et al, 2009;Kleinhans et al, 2012Kleinhans et al, , 2014aStefanon et al, 2012;Braat et al, 2018;Lentsch et al, 2018;Leuven et al, 2018). Recent studies demonstrated that physical experiments are capable of reproducing several typical tidal features, such as ebb deltas (Kleinhans et al, 2012), tidal channel networks (Stefanon et al, 2010;Vlaswinkel and Cantelli, 2011) and tidal estuaries (Kleinhans et al, 2014a).…”