UnaLinea is a state-of-the-art model that simulates the evolution of the plan shape of a beach. The beach plan shape is determined by the position of a single contour, therefore making UnaLinea a "one-line model". UnaLinea uses a formulation of total longshore transport rate based on the widely used CERC formula. The model changes the coastline every time step, allowing for the correct simulation of the changing drift rates with time.
A typical application of this model would be to study the impact that different interventions to the fluvial load will have in the adjacent coast in a medium to long term time.
The accompanying user manual provides a guide on how to use the model itself.
The predecessor tool had been derived by HR Wallingford as part of the FRMRC2 project, as a simplified, regional-scale one-line model of wave-driven beach plan-shape evolution, implemented in Fortran as a finite difference solution of the continuity equation for sediment (Stripling et al, 2011). The model assumes an average beach slope and does not consider short term changes in the beach profile. Offshore wave conditions are transformed into the position of breaking at each point along the beach. These breaking wave conditions are used to calculate the longshore drift at each of these points. The cross-shore change in position of the specified contour is calculated from longshore gradients in the wave induced longshore transport rate (see Figure 1).
Figure 1. Schematisation of the grid in UnaLinea
Due to the fact that the profile remains unchanged, the shoreline position moves forward or backward, according to the longshore drift gradients, translating the profile, as shown in Figure 2.
The effects of engineering structures and active beach management techniques, such as beach nourishment or losses to offshore, may be included in the model, as may a simple representation of cliff erosion (Stripling and Panzeri 2009, Stripling et al, 2011).
Figure 2. Profile schematisation in UnaLinea and symbolic erosion and accretion
UnaLinea models the following processes:
- Wave transformation:
- Fluvial interaction:
- sediment input from rivers
- sediment losses offshore
- Sediment transport:
- CERC formula
- Damgaard and Soulsby (1997) formula
- Active beach management techniques:
- beach renourishment
- beach mining
The beach plan shape is specified by the position of a single contour, usually either Mean Water Level or a particular high tide level. The model assumes an average beach slope and does not consider short term changes in the beach profile. Offshore wave conditions are refracted into the position of breaking at each point along the beach. These breaking wave conditions are used to calculate the longshore drift at each of these points. The change in position of the specified contour is calculated from differences in the wave induced longshore transport.
In the presence of groynes, the sediment transport is considered zero at the groyne position, acting as an impermeable barrier.
The UnaLinea model has been designed as a first-stage tool in understanding the behaviour of a coast and the impact of engineering works upon it. Its relative simplicity and ease of use allow the model to be used by non-specialist engineers with a minimum of data, as well as allowing more detailed investigations by more experienced users.
For iCOASST the model was made OpenMI compatible.
Application of Unalinea Model to iCOASST pilot sites and lessons learnt
UnaLinea has been applied to the Liverpool pilot site in two instances, one for the Blackpool frontage and another one for the Formby frontage, with the Ribble estuary in the middle of both being represented in ASMITA. The composition is described in detail here