Principal parameters

The main overland flow parameters are the surface roughness which controls the rate of flow, the depth of detention storage which controls the amount of water available for flow, plus the intial and boundary conditions.

Surface Roughness

The Stickler roughness coefficient is equivalent to the Manning M. The Man­ning M is the inverse of the commonly used Manning’s n. The value of n is typically in the range of 0.01 (smooth channels) to 0.10 (thickly vegetated channels), which correspond to values of M between 100 and 10, respec­tively.

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

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.

Water trapped in detention storage continues to be available for infiltration to the unsaturated zone and to evapotranspiration.

Detention storage also affects the exchange with MIKE Hydro River. Only ponded water in excess of the detention storage will flow to MIKE Hydro River. Also, flooding from MIKE Hydro River will only happen when the water level in the river link is above the topography plus detention storage.

The OL Drainage is also linked to the detention storage. Only the available ponded water will be routed to the OL Drainage network - that is the ponded depth above the detention storage. If you want to route all of the ponded water to the OL Drainage network, then you can use the Extra Parameter: OL Drainage Options (V1 p. 746).

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 initial condition 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.

Time varying OL boundary conditions

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).