After a DRAINS run has been made, and peak flowrates and hydraulic grade line levels are displayed in the Main Window, it is possible to examine the flow characteristics of overflows from pits and detention basins in the page of the Overflow Route property sheet named Cross Section Data.  As shown below, this gives an indication of flow depth on the graphical cross-section, and presents a set of flow characteristics, including depth, width and velocity.

The numbers in the two boxes to the bottom left can be used to determine flow conditions at a number of points along the flow path.  As these are changed, DRAINS will recalculate the flow statistics.  It is also possible to specify different cross-sections and compare their flow characteristics.

To understand what these numbers mean, consider the diagram below, which shows flows bypassing Pit 1 and flowing to Pit 2.  Along the flow path, the flows will increase from Pit 1 to Pit 2, due to runoff from Catchment 2 collecting in the gutter.

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The flow conditions just downstream of Pit 1 can be determined by setting the box labelled % of downstream catchment flow carried by this channel to zero, indicating that no part of Catchment 2 contributes to the gutter flow.  For the flow conditions just upstream of Pit 2, the percentage should be set to 70%, which is the part of Catchment 2 that contributes runoff to the gutter that carries the bypass flow. The gutter flow used in calculating the statistics will then increase from the Pit 1 peak overflow to (Pit 1 Peak Overflow + 70% of Catchment 2 Peak Flow), and the statistics will be re-calculated. (See also the example in the topic for Overflow routes from pits.)

The longitudinal slope of the gutter can also be varied, so that allowance can be made for different slopes along the gutter profile. Flow conditions can be defined at any point between the two pits, by inserting appropriate percentages and slopes. Further information is given at the end of the topic describing the Overflow Route property sheet.

This calculator does not involve any repetition of the main DRAINS calculations. It merely uses the calculated hydrographs from Pit 1 and Catchment 2 to estimate a peak flowrate that can be used in calculations. The flow calculations are normal depth calculations based on Manning's equation, which are sufficiently accurate to define flow characteristics and to determine whether a safety hazard exists. If the safety limits set for the cross-section are exceeded, certain statistics will be labelled UNSAFE, and warning messages may appear.

This facility is also used in DRAINS as the basis of an automatic design procedure that will determine the pit and pipe sizes needed to ensure that under both minor and major flow conditions,  the surface flows at the nominated flow points for each overflow path will meet the safety requirements established in the Overflow route data base property sheet.