Ponded drainage and paving

Ponded drainage (OL Drainage) is a special boundary condition in MIKE SHE used to defined natural and artificial drainage systems that cannot be defined by the River Network. This is an effective means to specify urban drainage networks and large scale upland drainage areas.

OL Drainage is removed from ponded water on the cell. Water that is removed is routed to local surface water bodies, local topographic depres­sions, or out of the model. The amount of drainage is calculated based on the depth of ponding and the drain level using a linear reservoir formulation.

When water is removed from a drain, it is immediately moved to the OL Drain Storage. In other words, the drain module assumes that the time step is longer than the time required for the drainage water to move to the storage. From the OL Drain Storage, the water is released to the recipient at a rate based on the discharge time constant.

Each cell requires a drain level, inflow time constant (which is the same as a leakage factor), and an outflow time constant. Both drain levels and time con­stants can be spatially defined. The default drainage level is the topography. A typical time constant may be between 1e-4 and 1 e-5 1/s.

Drainage reference system

MIKE SHE requires a reference system for linking the drainage to a recipient node or cell. There are four different options for setting up the drainage source-recipient reference system

·         Drain Levels The drainage recipient is calculated based on the drain levels in all the down gradient cells. That is, the location of the recipient cell is calculated as if the drain water was flowing downhill (based on the drain levels). This is the most common method of specifying drainage routing and the default setting.

·         Drain Codes The drainage recipient is specified by the user based on a distribution map of integer code values.

·         Distributed option With this option there are several different drainage possibilities, including a combination of Codes and Levels. The Distrib­uted option can also be used to define a specific MIKE Hydro River H-point or MIKE Urban manhole as a recipient.

·         Removed The fourth option is simply a head dependent boundary that removes the drainage water from the model.