A couple of recent 2D models have been seen where a tidal boundary is being represented and results on the 2D mesh show water surface elevations greater than the tidal level…this is due to poor model schematisation and in particular the setting of initial conditions, or lack of! The models in question represented a coastal area in 2D with some bathymetry data representing the tidal zone. A tidal boundary was then set up with the first tidal level value being several metres (~10m) above the mesh element elevations. By default the 2D domain is considered to be dry at the first timestep and therefore a tidal level several metres above the element elevation would be considered to be a wall of water. The simulation engine then estimates velocities at these boundaries. As you can imagine, a 10m wall of water, similar to some kind of tsunami will provide some large velocities which due to momentum can cause some large waves which can manifest themselves in the model as re-circulations, high velocities and water levels which are much higher than anticipated. Catastrophic to the model and it’s results as such a situation would be in reality.
The key is to use the appropriate 2D Initial Conditions. This functionality allows the modeller to set the initial 2D conditions to be wet at time zero. Therefore, if the tide level is set to say 0.262m AD, all of the 2D surface under 1m AD (accounting for any defences) is inundated at the beginning of the simulation.
The 2D Initial conditions takes on two parts, firstly there is the network 2D initial conditions polygon which defines the area of the 2D initial condition. These can be used to define hydraulic, water quality or sediment initial conditions. This blog will focus on the hydraulic initial conditions. This is digitised as a simple polygon and should cover the area which is inundated at the start of the simulation. Once digitised, the 2D zone will need to be remeshed. It is also possible to use the 2D zone and set a 2D initial condition for the whole 2D zone, however bear in mind that there may be some mesh elements lower than the tidal elevation which are disconnected from the tide via embankments or other high ground.
Once the network has been setup correctly, a 2D Initial Conditions database object is created. This references the network 2D Initial Conditions Polygon and is where the starting water surface elevation (or depth) is specified. The water level in all elements which lie within the 2D Initial Conditions Polygon will then have their initial water level set accordingly. If the mesh element elevation is greater than the assigned elevation then the element will be dry. If it is lower, then it will be designated to be wet.
The 2D initial conditions database object should then be used in the relevant location in the run dialogue.
The subsequent simulations will use the initial condition, the specified mesh elements will be wet at the initial timestep, the wall of water avoided and the boundary will be simulated as a tidal boundary gradually rising and dropping rather than a sudden input of a wall of water similar to a tsunami.