Software Version

An unreleased version of HEC-HMS was used to create this tutorial. This tutorial is intended to show a future capability for setting diversion area.

Overview

In HEC-HMS a Depth-Area Analysis uses the drainage area at an analysis point to set the storm area for a Frequency Storm or Hypothetical Storm at the analysis point.  When there is a diversion in the stream network, the default behavior is to assign all the drainage area to the downstream branch, and no area is assigned to the diversion branch. When an analysis point is at or below the node where the diversion branch reconnects, the analysis point’s drainage area will be underestimated. The resulting storm will overestimate precipitation and stream flow at the analysis point. To correct this behavior a new feature has been added to HEC-HMS to allow assigning drainage area to a diversion branch.

Demo

Consider the stream network defined in this diagram:

Streamflow from Junction-1 enters Diversion-1 and is split between two branches, downstream to Reach-1 and diversion to Junction-2.

A new field has been added to the component editor for a Diversion. This field specifies an Area Divert Method. The choices are None or Specified Area.

If the Area Diversion method is None, all the area is assigned to the downstream branch and none to the diversion branch. When the diversion branch reconnects to the stream network at Junction-2, no area is added to the drainage area at Junction-2. The drainage area for Reach-1 is the same as Diversion-1.

 If the Area Diversion method is Specified Area, a new field is added to the Divert editor. This field specifies the area to be assigned to the diversion branch. The specified area is assigned to the diversion branch, and the remainder is assigned to the downstream branch. When the diversion branch reconnects to the stream network at Junction-2, the area assigned to the branch is added to the drainage area at Junction-2.

In this example, the diversion area is 12 sq.mi. The Element Inventory shows that 12  sq.mi was added to the drainage area at Juncion-2, and that 12 sq.mi. was subtracted from the drainage area for Reach-1.