In natural systems, runoff does not travel far as sheet flow. Rather it drains into natural and man-made drainage features in the landscape, such as creeks and ditches. Then, it generally discharges into streams, rivers or other surface water features. In urban areas, it may discharge into storm water retention basins designed to capture runoff.
The amount of runoff generated by MIKE SHE depends on the density of the stream network defined in MIKE Hydro River. If the stream network is dense, then you will get more runoff in the model because the lateral travel time to the streams will be short, and the infiltration and evaporation losses will be smaller.
Fine-scale, natural and man-made drainage systems are difficult to simulate in MIKE SHE using the traditional finite difference Overland Flow method. This is largely because the drainage features are often smaller than the grid scale, and shallow overland flow will not travel far laterally before it infiltrates or evaporates.
In large catchments, it may be impractical to defined the detailed natural drainage network. In urban catchments, the detail of the drainage network may be either unknown or impractical to define.
The Ponded Drainage (OL Drainage) option was introduced in the 2017 Release to alleviate some of these issues. Conceptually, the OL Drainage is the similar to the SZ Drainage in that a drainage network is calculated based on a downhill flow path from each node until it reaches a stream, a boundary, or a local depression.
However, as in the SZ Drainage, the pathway is only a reference system. It does not actually contain any water. Drainage from a particular cell is released directly to the destination cell. If the destination is a boundary cell, the water is added directly to the boundary. If the destination is a river link, then the water is added to the cell beside the river link.
A key difference from the SZ Drainage feature is that the OL Drainage includes an intermediate storage. The SZ Drainage is instantly added to the destination, whereas the OL Drainage is first added to an internal storage. Technically, this internal storage is “on the cell”, not “in the drain”. Thus, any cell that is generating OL Drainage will also have an amount of OL Drain Storage. The amount of OL Drain Storage retained on the cell depends on the difference between the Inflow and Outflow time constants. This allows you to simulate a rapid drainage of rainfall, but a slower discharge to the receiving destination. Effectively, this is the same as having storage in a drainage network, but the drainage from different cells are not summed together, and their is no physical drainage network to interact with while the water is in storage.
The Ponded Drainage function has been designed to facilitate a flexible definition of storm water drainage in both natural and urbanized catchments. In particular, the Ponded Drainage function supports the following features:
· In urban areas, paving and surface sealing (eg roof tops) limits infiltration and enhances runoff.
· Ponded water is routed to user-defined locations (boundaries, depressions, manholes and streams).
· The time varying storage in the drainage system is accounted for by different inflow and outflow rates.
· Drainage rates can be controlled by drain levels, drainage time constants and maximum outflow rates.
· Drainage is restricted if the destination water level is higher than the source water level.
· Urban development over time can be simulated by time varying drainage parameters.
· Overland flow is calculated normally for ponded water that does not drain in the current time step.
Conceptually, the Ponded Drainage function is shown in Figure 25.10.
Figure 25.10 Conceptualization of the Ponded Drainage function
Calculation order
The Ponded Drainage is calculate explicitly in a particular order with respect to the other hydrologic processes.
1. Rainfall is added to the ponded depth in the cell.
2. Then, OL Drainage will be immediately calculated.
In this way, the OL Drainage essentially acts on rainfall and removes rainfall before anything else happens.
If there is any ponded water remaining on the cell after the drainage has been removed, then it will be removed in the following order until no ponded water remains.
1. Evaporation is removed from the ponded water;
2. Then, Infiltration to the unsaturated zone will be calculated; and
3. Finally, Lateral Overland Flow will be calculated to the adjacent cells.
The Ponded Drainage is calculated for every cell independently. That is, the drainage that is calculated is not added to the destination cells until the end of the calculation for all the cells. This prevents circular drainage, and means that the order in which the calculation occurs is not relevant. After the ponded drainage is calculated, the depths in all the cells are updated - both sources and destinations. The calculation of ET, infiltration and lateral OL flow is based on the updated ponded depths.
