Edge effects (Water Overlay)
When the Water Module calculates the flow of water, it can "freely" flow across the entirety of the map, until it reaches the edge of the project area. Water cannot flow in or out of the project edge. It behaves as an impenetrable barrier and causes water to bounce back. Edge effects include both the lack of external water flow and the behavior of water bouncing off the boundary.
All hydrological models require some forethought on the existence of edge effects when performing hydrological calculations and analyses. Depending on your use-case, you can consider doing any or all of the following to account for edge effects.
Note that there is no clear-cut single method for perfectly resolving these limitations. For every situation and use-case you will need to make your own judgement call as to which solution is most appropriate.
Larger project area
By creating a larger project area, the water which flows out of your area of interest does not immediately reflect back in. Instead, it has the room to flow further, continuing onward to the edge. Additionally, when dealing with water sources such as rain, water from just outside the area of interest can flow in naturally as well, creating a more accurate representation of the amount and movement of water in your model.
The degree to which this lessens the edge effect depends on the increase in size and the impulse of the water.
Boundary conditions
Inlet, outlets and Breaches can be used to add or remove water at the boundary when it reaches a particular level. This can be relevant when modelling a steadily flowing river. Deep waterways or ditches allow you to store water that reaches the project's boundary, keeping it inside the project.
The degree to which this lessens the edge effects depends on the amount of boundary conditions added to the project, their capacities relative to the amount of water they are to process, and their placement.
Limit Areas
Limit Areas can be added to a Water Overlay to control which part of the project area is simulated. Parts that are not simulated act as a boundary.
Accept artifacts
Edge effects may not be an issue for all use cases or calculations. Edge effects may be negligible if the required detail is low or if existing dataset inaccuracies already outweigh them. In these cases, it is important to be aware of the presence of such an artifact.
How-to's
See also
- Models
- Surface • Ground • Rain • Evapotranspiration • Infiltration • Elevation • Breach • Sewer • Storage • Tracer flow • Border • Order
- Model formulas
- Timestep • Season • Surface water level • Groundwater level
- Flow formulas
- Surface flow • Surface infiltration • Ground flow • Ground infiltration • Surface evaporation • Ground evaporation • Ground bottom flow
- Hydraulic structure formulas
- Culvert • Weir • Breach growth • Breach flow • Pump • Sewer Overflow • Inlet • Drainage
- Related