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iCOASST

ASMITA

Introduction

ASMITA (Aggregated Scale Morphological Interaction between Tidal inlets and the Adjacent coast) is a behaviour-orientated model for predicting the large-scale evolution of estuaries over decades to centuries. Within ASMITA the estuary must be schematised into morphological elements, such as channels, tidal flats and ebb-tidal deltas. For each element a morphological equilibrium is defined relating the morphology to the hydrodynamic forcing (usually tidal prism). The volumes of the schematised elements are predicted through time, based on sediment exchange between elements which is driven by difference between current volume and equilibrium volume (Figure 1).

Figure 1 Typical three element schematisation of an estuary as used in ASMITA

Predecessor version

ASMITA was originally developed by Marcel Stive (TU Delft) and ZB Wang (TU Delft/Delft Hydraulics) as an aggregation of their ESTMORPH model (Stive et al 1998).

It was further developed and an user interface was added as part of the joint Defra/EA Flood and Coastal Erosion Risk Management R&D Programme (project FD2119). This version of ASMITA (V1.3) can be downloaded from http://www.estuary-guide.net/software_downloads.asp and was the basis for the iCOASST model development.

The Environment Agency project "Characterisation and prediction of large-scale, long-term change of coastal geomorphological behaviours" (SC060074) added additional coastal elements within the ASMITA code and used the Kraus reservoir model (Kraus, 2000) to move sediment from the updrift coastal element, to the ebb tidal delta and on to the downdrift coastal element. This version of ASMITA allowed the effect of longshore drift at the coast on the estuary to be included, but did not allow for feedback between the estuary and the adjacent coast.

iCOASST objectives/conceptual vision for the tool

For iCOASST the object for ASMITA was to allow coastal models to be linked in such a way that they could supply or receive sediment from ASMITA, making the feedback between models bi-directional.

Summary of the revised tool

ASMITA V1.3 has been modified to allow littoral drift from the adjacent coast to be included and to structure the ASMITA code to allow openMI wrapping.

Inclusion of littoral drift

The inclusion of littoral drift in ASMITA requires an ebb-tidal delta to be included in the model schematisation. It is assumed that ASMITA will be linked with two coastal models which will have a littoral drift (Q, m3/s) either into (supply) or out of (demand) the ebb-tidal delta. In keeping with the proposed coastal model (UnaLinea), positive fluxes are directed from left to right (Figure 2).

Figure 2 Definition sketch for estuarine elements, coasts and littoral fluxes (Q)

Littoral drift could be directed towards the delta on both sides, away from the delta on both sides or towards the delta on onside and away on the other (bypassing) (see Table 1). It is important to consider the rate of littoral drift predicted by the coastal model is POTENTIAL littoral drift (the littoral drift that will occur if unlimited sediment is available) and therefore when it is directed away from the ebb-tidal delta it will be controlled by the sediment availability in the delta. For this reason, potential drifts away from the ebb-tidal delta were considered. Demands and the ability of the estuary/delta system to meet the demand were calculated by modifying the Kraus (2000) model of sand bypassing at deltas such that:

Where Vi is the sediment volume of the ebb-tidal delta, VEi is the equilibrium volume and n is the sediment transport exponent from ASMITA.

The volume change of the delta resulting from littoral drift is then

In the ASMITA code, Qsin and qsout are used to calculate the net littoral drift volume and this is included in ASMITAs internal volume change calculations and therefore feeds back into equation 1 on the next time step.

How has the ASMITA Model been applied to the pilot sites and lessons learnt

Table 1 Potential combinations of Q in and Qout for ASMITA
Qslft_in Qsrht_in Qsin Qsout
Positive (in) Positive (out) qslft_in Calculate and assign to qsrht_out
Negative (out) Positive (out) 0 Calculate and assign proportionally
Positive (in) Negative (in) qslft_in-qsrht_in 0
Negative (out) Negative (in) -qsrht_in calculate and assign to qslft_out

Changes to code

Several changes to the ASMITA code were required to handle the inclusion of littoral drift. These included:

  • Equilibrium volume of ebb-tidal delta can take into account long-term littoral drift rate (supplied as an input argument in pipistelle). This is handled in the same way as river flows and tidal pumping, where correcting the equilibrium volume prevents these fluxes from acting as a disturbance on the estuary (i.e. the system is in equilibrium with these fluxes). Using the long-term littoral drift rate means that the delta is in equilibrium with littoral drift, but CHANGES to the littoral drift rate would be disturbances to the system and would therefore influence the morphology (e.g. reduced littoral drift due to a seawall).

  • Addition of global variables

    • LITQS.qslft_in (the input drift rate from the left hand coast)
    • LITQS.qsrht_in (the input drift rate from the right hand coast)
    • LITQS.qslft_out (the output drift rate from the left hand coast)
    • LITQS.qsrht_out (the output drift rate from the right hand coast)
    • LITQS.qslt (long-term rate of littoral drift, used to estimate equilibrium volume of delta)

  • New subroutines

    • Lit_qs2.m

      • checks directions qslft_in and qsrht_in
      • calculates Qin

    • Delta_qs2

      • Calculates Qout based on equation 1 and assigns to relevant coast (if there is a drift divergence at the estuary and Q is directed away from the estuary along both coasts, Qout is assigned proportional to the demand on each side.
      • Returns Qout for calculation of volume change of ebb-tidal delta

  • ASMITA has been applied to the Ribble Estuary as part of the Liverpool Bay composition. It was linked with two instances of the one-line model, Unalinea, which supplied sediment to the coast.

Evaluation for end-users

As part of the Environment Agency project "Embedding iCOASST into practice", HR Wallingford have undertaken an independent evaluation of the usability of the iCOASST models. Each of the models have been downloaded from this website, compiled and run using (i) the documentation and (ii) the site-specific data sets on which they have been developed (also provided on this website). The outcome of this evaluation for this model can be found here and should be referenced by anyone interested in using or developing the model further.