Overland flow simulates the movement of ponded surface water across the topography. It can be used for calculating flow on a flood plain or runoff to streams.
You can run the Overland flow module separately, or you can combine it with any of the other modules. However, overland flow is required when you are using MIKE Hydro River in MIKE SHE, as the overland flow module provides lateral runoff to the rivers.
The Simplified Overland Flow Routing (V1 p. 476) method can be used for regional applications when detailed flow is not required. This method assumes that ponded water in the upland areas of a subcatchment flows into the flood plain areas of the subcatchment, which in turn discharges uniformly into the stream network located in the subcatchment.
The Finite Difference Method (V1 p. 459) uses the diffusive wave approximation and should be used when you are interested in calculating local overland flow and runoff. There are two solution methods available.
· Successive Over-Relaxation (SOR) Numerical Solution (V1 p. 463)
· Explict Numerical Solution (V1 p. 464)
The choice of method is a tradeoff between accuracy and solution time. The SOR solver is generally faster because it can run with larger time steps. The Explict method is generally more accurate than the SOR method, but is often constrained to smaller time steps. The time step constraint prevents flow from crossing a cell in a single time step. The time step constraint is determined by the cell with the highest velocity and applied to the entire model in the current time step.
The Explicit method is generally used when the river is allowed to spill from the river onto the flood plain. Alternatively, you can use Flood codes (V1 p. 264) to inundated flood plain areas based on the water level in the river.
The Multi-grid overland flow option allows you to take advantage of detailed DEM information if it is available. The multi-grid method, sub-divides the overland flow cell into an even number of sub-cells. The gradients between the cells and the flow area between cells water surface elevation in the cell is then calculated based on the volume of water and the detailed topography information.
In MIKE SHE, the calculation of 2D overland flow can become a very time consuming part of the simulation. So, you need to be very careful when setting up your model to minimize the calculation of overland flow between cells when it is unnecessary.
Detailed information on Overland Flow, the coupling between MIKE Hydro River and MIKE SHE and the overbank spilling options, and ways to optimize the calculation of overland flow can be found in the chapters:
· Working with Overland Flow and Ponding- User Guide (V1 p. 483)
· Working with Rivers and Streams - User Guide (V1 p. 519)
The Manning M is equivalent to the Stickler roughness coefficient, the use of which is described in Overland Flow - Technical Reference (V1 p. 459).
The Manning M is the inverse of the more conventional Manning’s n. The value of n is typically in the range of 0.01 (smooth channels) to 0.10 (thickly vegetated channels). This corresponds to values of M between 100 and 10, respectively. Generally, lower values of Manning’s M are used for overland flow compared to channel flow.
If you don’t want to simulate overland flow in an area, a Manning’s M of 0 will disable overland flow. However, this will also prevent overland flow from entering into the cell.
Detention Storage is used to limit the amount of water that can flow over the ground surface. The depth of ponded water must exceed the detention storage before water will flow as sheet flow to the adjacent cell. For example, if the detention storage is set equal to 2mm, then the depth of water on the surface must exceed 2mm before it will be able to flow as overland flow. This is equivalent to the trapping of surface water in small ponds or depressions within a grid cell.
If you have static ponded water in an area and you do not want to calculate overland flow between adjacent cells (can be slow), then you can set the detention storage to a value greater than the depth of ponding.
Water trapped in detention storage continues to be available for infiltration to the unsaturated zone and to evapotranspiration.
Initial and Boundary Conditions
In most cases it is best to start your simulation with a dry surface and let the depressions fill up during a run in period. However, if you have significant wetlands or lakes this may not be feasible. However, be aware that stagnant ponded water in wetlands may be a significant source of numerical instabilities or long run times.
The outer boundary condition for overland flow is a specified head, based on the initial water depth in the outer cells of the model domain. Normally, the initial depth of water in a model is zero. During the simulation, the water depth on the boundary can increase and the flow will discharge across the boundary. However, if a non-zero value is used on the boundary, then water will flow into the model as long as the internal water level is lower than the boundary water depth. The boundary will act as an infinite source of water.
If you need to specify time varying overland flow boundary conditions, you can use the Extra Parameter option Time-varying Overland Flow Boundary Conditions (V1 p. 740).
Separated flow areas
The Separated Flow Areas (V1 p. 273) are typically used to prevent overland flow from flowing between cells that are separated by topographic features, such as dikes, that cannot be resolved within a the grid cell.
If you define the separated flow areas along the intersection of the inner and outer boundary areas, MIKE SHE will keep all overland flow inside of the model - making the boundary a no-flow boundary for overland flow.