Effects of mud, Spartina anglica and Zostera marina on large scale morphodynamics in a tide-dominated estuary

Abstract

Estuaries are morphodynamically active regions where tidal and river currents together with (cohesive) sediment and vegetation create patterns of channels, shoals, mud flats and salt marshes. The effects of mud and vegetation have mainly been studied on a local scale and their effects on the large scale morphological development of estuaries on the long term is unknown. Classic numerical models are unable to produce realistic vegetation patterns and cannot take into account their influence on estuary morphodynamics. Recent advances in modelling, however, made it possible to integrate dynamic vegetation models with advanced numerical morphodynamic models. The objective of this study is to use a computer model to assess effects of mud and vegetation on estuary hydro-morphodynamics. A numerical model which combines dynamic intertidal and submerged vegetation with morphodynamics is developed. The dynamic vegetation module incorporates vegetation colonization, mortality due to scour, uprooting, burial, dessication, flooding and light attenuation and it adjusts the hydrodynamics through flow resistance and thus affects morphodynamics as well. The development of two vegetation species, Spartina anglica and Zostera marina, and their effect on estuary morphodynamics, has been modelled for 50 years in a hypothetical tide-dominated estuary with sand and mud. When mud is modelled without vegetation it accumulates predominantly on the edges of the estuary. Due to its cohesiveness it limits channel movements and decreases the braiding index in the estuary. Spartina anglica colonizes shoals and the intertidal area on the edges of the estuary, where it drives rapid sedimentation. Because Spartina increases the flow resistance in the intertidal area it decreases flow velocity and stabilizes the intertidal area if it is naturally prone to erosion. When the intertidal area is not prone to erosion, however, Spartina might actually cause erosion of the intertidal area because it increases the flow velocity on its patch edges. The decreased flow velocity in the Spartina marshes causes an increase in flow velocity in the deeper parts of the estuary. Dependent on the initial estuary bathymetry the increased flow velocity in the deeper parts of the estuary might drive channel deepening. Zostera marina is difficult to model properly due to its flexible nature and the need for a specific colonization module. Advances have been made in modelling Zostera but it has to be tested comprehensively. The first results indicate that Zostera might deepen channels and that it does not elevate subtidal areas into the intertidal regime. Spartina enhances the deposition of mud and redistributes the mud over the estuary. The mud accumulation pattern follows the vegetation distribution and through this Spartina elevates itself into a higher part of the intertidal zone. Through elevating itself Spartina decreases its physical stress and Spartina concentrations in the estuary increase. The enhanced sedimentation of predominantly mud under influence of Spartina enhances the morphological development induced by Spartina. In the future this knowledge of the interaction between Spartina and mud might be used in managing hyper-turbid estuaries like the Ems-Dollard.