WMS:Orange County Unit Hydrograph

Unit hydrograph analysis is used for watersheds greater than 1 square mile (640 acres) in Orange County, California. Losses are accounted for by calculating an effective precipitation. Runoff hydrographs are computed using a unit hydrograph, which is developed using an S-graph. You can use a small area hydrograph instead of a full blown unit hydrograph analysis for watersheds with a Tc < 25 min. WMS creates the input files required for HEC-1 to compute hydrographs for the Orange County unit hydrograph analysis. Much of the interface for the Orange County unit hydrograph analysis is similar to the HEC-1 interface within WMS. Some of the processes specific to Orange County are described below:

• Basin data
• Effective precipitation – losses are included in the precipitation rather than defining a loss method
• Unit Hydrograph Method – unit hydrographs are developed using Orange County S-graphs and lag time derived from the time of concentration computed in an Orange County Rational Method analysis
• Routing
  1. Stream/channel routing – a convex routing method is included in HEC-1
  2. Flow-through detention – define using reservoirs or storage routing in HEC-1
  3. Flow-by detention – define using diversions in HEC-1

Saving a Simulation

Save only the HEC-1 input files using the Save Simulation command in the OC Rational menu. This will only save the hydrologic tree and the HEC-1 input parameters at each of the tree nodes. Terrain data, feature data, and GIS data will not be saved unless you save a WMS project file using the Save command in the File menu.

Related Topics

• Orage County Hydrology
• Editing HEC-1 Parameters
• Job Control
• Design Storms
• Small Area Hydrographs
• Using GIS Data
• Running a HEC-1 Analysis
• Saving a Simulation
• Post Processing