How is Delta D calculated in PondPack?

  Product(s): Bentley PondPack
  Version(s): CONNECT Edition, 08.11.01.XX
  Area: Modeling

Problem

How is Delta D calculated in PondPack? What is Delta D?

Problem Number: #75356

Solution

DD (Delta D) is the duration of excess runoff (1 in.) used when
calculating an SCS unit hydrograph. The default curvilinear SCS unit
hydrograph suggested time interval (DD) is defined by using two related
equations:

1.7*Tp = Tc + DD (where receding limb inflection = 1.7Tp)
Tp = DD /2 + 0.6Tc (where Lag=0.6Tc)

Where Tp = Time to peak of the unit hydrograph

By setting these two equations equal, and solving for DD, you get:

DD = 0.1333Tc, which is the recommended duration of excess runoff.

Also, from this relationship, DD = 0.20Tp (or 1/5 time to peak).

NEH-4 states that a slight variation in DD is permissible, but should be
no greater than 0.25Tp (1/4 time to peak). Page 91 in Design Hydrology
and Sedimentology for Small Catchments (Haan, C.T., Barfield, B.J.,
Hayes, J.C. 1994) states that DD should be about 1/5 to 1/3Tp.

The selection of DD is important since it has significant impacts on the
shape and peak discharge of the computed runoff hydrograph.

Recommended
Related