Time of Entry

The time of entry te can be regarded as representing the delay and attenuation of the flow over the ground surface.

It has the effect of reducing the calculated discharge, since an increase in the time of entry, and consequently in the time of concentration, reduces the corresponding rainfall intensity. Recent research has shown that times of entry of 8 to 12 minutes are common amongst frequently occurring storms. The following values have been recommended for the time of entry.

Return Period	Time of Entry (Minutes)
5 years	3 - 6
2 years	4 - 7
1 year	4 - 8
1 month	5 - 10

For each return period the larger times of entry are applicable to large, flat sub-catchments (area greater than 400m2, slope less than 1 in 50) and the smaller values to small, steep sub-catchments (area less than 200m2, slope greater than 1 in 30). Note that these values of area and slope refer to the sub-catchments contributing to each pipe length.

The time of entry is calculated according to the following formula.

te = 7.44 LENGTH 0.133 SLOPE -0.274

Where:

LENGTH is the sub-catchment overland flow length (m).
SLOPE is the sub-catchment slope (percent).

Procedure

The Time of Entry Editor displays.

The following data can be entered.

Option	Description
Overland Flow Length	The sub-catchment overland flow length in metres.
Slope	The average slope of the sub-catchment area expressed in percentage.
Time of Entry	The calculated value is updated as you change the values of the above items.

Click OK to accept and transfer the calculated value into the time of entry control in the Wallingford Rational Runoff page.