Abstract

Rivers and estuaries are flanked by floodplains built by mud and vegetation. Floodplains affect channel dynamics and the overall system's pattern through apparent cohesion in the channel banks and through filling of accommodation space and hydraulic resistance. For rivers, effects of mud, vegetation and the combination are thought to stabilise the banks and narrow the channel. However, the thinness of mudflats and salt marsh in estuaries compared to channel depth raises questions about the effects of floodplain as constraints on estuary dimensions. To test these effects, we created three estuaries in a tidal flume: one with mud, one with recruitment events of two live vegetation species and a control with neither. Both mud and vegetation reduced channel migration and bank erosion and stabilised channels and bars. Effects of vegetation include local flow velocity reduction and concentration of flow into the channels, while flow velocities remained higher over mudflats. On the other hand, the lower reach of the muddy estuary showed more reduced channel migration than the vegetated estuary. The main system-wide effect of mudflats and salt marsh is to reduce the tidal prism over time from upstream to downstream. The landward reach of the estuary narrows and fills progressively, particularly for the muddy estuary, which effectively shortens the tidally influenced reach and also reduces the tidal energy in the seaward reach and mouth area.

Highlights

  • The size and shape of natural estuaries are potentially modified by life, as they are often flanked by mudflats and salt marsh 15 (Whitfield et al, 2012)

  • All three estuaries widened by bank erosion from the initial, monotonously converging estuary to form channels and bars

  • The channel-bar patterns and bed elevation distributions were caused by the morphodynamics of channel erosion and migration, estuarine bank erosion, and bar formation and accretion

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Summary

Introduction

The size and shape of natural estuaries are potentially modified by life, as they are often flanked by mudflats and salt marsh 15 (Whitfield et al, 2012). In analogy with rivers, where floodplains formed by vegetation and mud deposition affect the channel dimensions and channel-bar pattern (see for review Kleinhans, 2010), it is plausible that floodplains in estuaries have similar effects. There is in principle no limit to the available flow discharge from the sea, unlike in rivers where long-term discharge frequency and magnitude are determined by hinterland characteristics and climate. This raises the question how vegetation and mud sedimentation affect the channel pattern in estuaries as far as such estuaries are not laterally constrained by 20 valley walls. Two alternative hypotheses for effects of floodplains on the dimensions of estuaries are proposed here from observations and mechanisms in rivers

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