MESO_i is an aggregate landform behaviour model that is capable of simulating the morphodynamic evolution of a tidal inlet subject to ebb delta breaching and periodic sediment bypassing. In contrast to reductionist modelling, aggregate models approach the simulation of morphological change at a higher level, usually by dividing the system into a number of geomorphic features or units which are characterized by a small number of attributes. These attributes (e.g. their area or volume) are then tracked through time by defining appropriate sediment transport pathways and exchanges between the geomorphic units.
MESO_i has been developed from scratch within iCOASST and thus does not have a direct predecessor. The key concepts and approach adopted for MESO_i, however, show similarities to models such as ASMITA (Stive et al., 1998) and the 'Reservoir Model' of Kraus (2000). In particular, the reservoir model has inspired the development of various elements within MESO_i. This reservoir model calculates sediment exchanges between geomorphic units by analogy to a reservoir system, where each reservoir can fill to a maximum equilibrium volume. More specifically, the transport of sediment out of a unit was defined by Kraus (2000) as the product of the incoming transport and the ratio of the actual and equilibrium volumes of that unit. Overall, the reservoir model and several extensions of the original version have provided valuable insights into the dynamics of tidal inlets and sediment bypassing patterns, and it has been successfully applied in various scientific and engineering studies.
iCOASST objectives/conceptual vision for the tool
A key goal of iCOASST is to develop predictions of coastal change over large spatial and temporal scales (van Maanen et al., 2016). This includes the interaction between estuaries and open coast and MESO_i was developed to simulate the processes and range of behaviours at this specific interface. MESO_i therefore needed to allow for the exchange of information in terms of both hydrodynamic and sediment transport processes between the estuarine and open coast models.
Summary of the revised tool
MESO_i simulates the evolution of tidal inlets that show cyclic behaviour including ebb delta breaching and sediment bypassing events. According to the aggregated modelling approach, it divides such a system in different geomorphic units (Fig. 1) and sediment exchanges between these units are defined (Fig. 2). The geomorphic units closest to the updrift and downdrift coast interact with these open coast sections. Sediment fluxes between units are driven by alongshore wave-driven transports or governed by diffusion processes. Equilibrium volumes of geomorphic units are based on literature (Burningham and French, 2006), and some of these are made dependent on estuarine tidal prism. The outcomes of MESO_i consist of simulated changes in the actual volume of each geomorphic unit over time (Fig. 3).
Figure 1. Geomorphic units that comprise the MESO_i model for the Deben inlet
Figure 2. Parameterizations of sediment fluxes between geomorphic units
Figure 3. Typical output of MESO_i showing simulated changes in the actual volume of each geomorphic unit over time
How has the MESO_i Model been applied to the pilot sites and lessons learnt
MESO_i was developed to be applied to the inlet and associated ebb-tidal delta shoals at the mouth of the Deben estuary, Suffolk. When building the model, however, an effort was made to keep the underlying concepts as generic as possible so that the model can easily be adjusted for other inlet systems. The ebb-tidal shoals of the Deben estuary show a broadly cyclic behaviour on a 10 to 30 year timescale, whereby the updrift ebb-tidal shoals progressively extend, causing downdrift migration of the main estuarine channel, followed by the breaching of the updrift shoal and relocation of the ebb-jet (Burningham and French, 2006). This sequence of events is accompanied by periodic sediment bypassing across the inlet. MESO_i reproduces this cyclic behaviour and is coupled with SCAPE+ (open coast component) and ESTEEM (estuary component) to explore the interaction between different landform complexes. In this composition, MESO_i exchanges information with two SCAPE+ components (one SCAPE+ component on each side of the MESO_i inlet component). The information exchange between MESO_i and SCAPE+ consists of sediment fluxes. That is, MESO_i receives sediment from or supplies sediment to the updrift and downdrift coast represented by the SCAPE+ models. MESO_i interacts with ESTEEM through the exchange of hydrodynamic information regarding tidal prism. A critical lesson which was learnt by building this composition is related to sediment fluxes along the open coast and sediment dynamics of the various shoals of the inlet system. That is, the magnitude of the alongshore drift was confirmed to be significantly smaller than previously reported and this highlights the importance of sediment recirculation within the inlet. MESO_i was adjusted to allow for this recirculation by incorporating diffusion terms to describe sediment fluxes from one geomorphic unit to the other. The diffusion coefficients determining the magnitude of these fluxes are calibrated so that the duration of bypassing cycles matches that of the observed cyclicity.
This experience shows the complexity of modelling inlets and in the final analysis, the approach adopted was in part data driven. The production of generic inlet model remains a fundamental research task.