Morphological evolution of the Dee Estuary, Eastern Irish Sea, UK: A tidal asymmetry approach

Abstract

Asymmetry in the tide (unequal ebb and flood duration) is a dominant factor in causing residual sediment transport and morphological changes in estuaries. The evolution of estuarine morphology is a process of dynamic equilibrium in the shortterm, while these features are ephemeral in the long-term. In this study we investigate the spatial distribution of tidal distortion and asymmetry of the Dee estuary, UK, by 3-dimensional numerical modelling methods. High resolution LIDAR surveys are used to underpin and explain our numerical modelling results in terms of basin hypsometry and areas of recent erosion and deposition. Harmonic analysis of the numerical modelling results showed that the shallower intertidal areas (sand and mud banks) were the most tidally asymmetric, showing flood dominance. The main navigation channels showed some ebb dominance but the tides here were relatively undistorted. This overall flood dominance is likely to induce net sediment import to the Dee, which explains known historical morphological changes (large scale accretion over the last 2 centuries) and also recent morphological changes as seen from the LIDAR surveys (which show predominantly net accretion between 2003 and 2006). Hypsometrical analysis suggests the Dee may be approaching equilibrium, and that the flood dominance and sedimentation rate may therefore decrease in the future. In an infilling estuary, an increase in the area and elevation of tidal flats can eventually shift an estuary towards ebb dominance, as shown by previous research and by 'idealised estuary' modelling results presented in this study. The large tidal amplitude to hydraulic depth ratio of the Dee, however, suggests that the tidal flats would have to be very extensive indeed for this to occur. 1

Effective energy controls on flocculation under various wave-current regimes

“…The levels of accretion in the estuary indicate the Dee is a depository of sediments (Moore et al, 2009) with the sediment identified to mostly be of marine origin (Turner et al, 1994) which is in agreement with observations and modelling results that show bottom currents and sediment transport from the Liverpool Bay to the estuary entrance (e.g. Halliwell,…”

“…In addition, results by Moore et al (2009) show a decrease in accretion rates which means the estuary is nearly in geomorphological equilibrium. The sediment transport in the estuary is not well known and is further complicated because of the presence of fine sediments leading to cohesive behaviour.…”

“…Tides are significantly distorted due to the shallow nature of the estuary and tidal asymmetry results in flood dominance on sandy and muddy shallow areas, and weaker ebb dominance in the channels (Moore et al, 2009). In spite of the low river discharge,…”

Modelling-based assessment of suspended sediment dynamics in a hypertidal estuarine channel

“…Although the Dee imports sediment, a hypsometrical analysis suggests that it may be approaching equilibrium with an associated decrease in accretion rates (Moore et al, 2009). However, more detailed studies are still required to better understand the sediment transport patterns and budgets.…”

Field deployment and evaluation of a prototype autonomous two dimensional acoustic backscatter instrument: The Bedform And Suspended Sediment Imager (BASSI)

“…The tidal range at the mouth is 8 to 9 m on spring tides with tidal currents reaching 1.5 m/s in the nearby Hilbre channel, though in the intertidal area utilised in this study tidal currents rarely exceed 0.7 m/s. The mean freshwater river discharge into the estuary is 31 m 3 /s, equating to only ~0.35 % of the tidal prism over a typical tidal cycle (Moore et al, 2009). Sediments in the Dee include both sand and mud sized deposits with compositional analysis suggesting that the sediments in the lower estuary are predominantly supplied from the adjacent Irish Sea (Turner et al, 1994).…”