Overland flowpaths (Overflow routes)
These are surface paths along which flows can occur. There are two types:
paths followed by stormwater runoff from a catchment on its way to an inlet to a pipe or channel system,
paths for bypasses and overflows from inlet pits, that may travel to another pit or inlet, or may flow out of the system being modelled.
Commonly these paths will be along street gutters and road surfaces, but they may go through properties and areas such as car parks.
The paths followed by runoff on its way to pits and inlets can be described by the facilities in the Sub-catchment property sheet. The time of travel is the main feature of interest here, and this is described as a sum of constant times, times calculated by the kinematic wave equation, and possibly, gutter flow times.
DRAINS originally specified overflow paths simply, with users only having to provide a fixed overland flow time. It can still run older data files specified this way. However, it now requires users to select a cross-section from the Overflow route data base. An overland flow time is still required when performing Lite hydraulic model calculations as DRAINS does not calculate flow times. (In the Full Unsteady hydraulic model, routing effects and equivalent time delays are calculated, so it is not necessary to specify times of flow along overflow routes.)
The following diagram shows the sub-catchments in which the flow paths are located.
The bypass flow from upper Sub-Catchment 1 will combine with the runoff from Sub-Catchment 2, through which it flows. In DRAINS a percentage of the downstream sub-catchment flow can be added to the bypass flow when calculating flow characteristics, and when designing piped drainage systems using the method in DRAINS. This allows users to select points along the overflow route, calculating hydraulic conditions at these points.
When using storage routing models, DRAINS can perform routing calculations along the stream routing reaches.