WMS:HEC-HMS Reach Routing

Reach routing will lag and attenuate the hydrograph computed (combined) at an outlet according to the reach properties defined. To define reach parameters, you should choose the Edit Parameters command from the HEC-HMS menu to obtain the HMS Properties dialog, shown below, and then select the HMS Property type to be Reaches. You can either show/edit properties for only the selected reach(es), or for all reaches in the model. You select a reach in WMS by selecting the upstream outlet of the reach since it is from this point hydrographs are routed.

You can choose to Display properties of the reach and then show one or more methods in the properties table. The method that HMS will use and the associated parameters are all edited from the properties table.

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Lag

The Lag method simply lags the hydrograph without any attenuation. The only parameter for this method is the lag time with its accompanying units (minutes or hours).

Muskingum

The Muskingum method is dependent primarily upon an input weighting factor for the reach. The required parameters are as follows:

• Number of sub-reaches (time steps that hydrograph will be in the reach).
• Muskingum K coefficient in hours for entire reach.
• Muskingum x (weighting) coefficient.

Using the basin data computed by WMS when a TIN or DEM is used to delineate the watershed, the K coefficient and number of sub reaches can easily be estimated. K is essentially the travel time for the reach, which can be estimated by noting the length of the stream segment (you can see this by displaying in the Muskingum Cunge method) and multiplying by an assumed channel velocity (1-5 ft/s would be appropriate for most natural channels). Of course you will need to convert the estimated travel times from seconds to hours before entering it into the K property field. The sub-reaches value is the number of time steps the flood wave is in the channel and can be determined by dividing K by the computational time step found in the Job Control dialog (again be sure that units are consistent). A button exists in the Muskingum K property field so that these computations can be done directly within WMS.

Modified Puls

The modified Puls method uses a storage routing technique, or level-pool routing. You must enter the storage, outflow relationship as well as the number of sub-reaches and initial condition.

Muskingum Cunge Std., Muskingum Cunge 8 Point, or Kinematic Wave

The Muskingum-Cunge and Kinematic Wave methods are defined with essentially the same parameters.

• Channel length.
• Energy grade slop (generally you can use channel slope).
• Manning's roughness.
• Characteristic channel shape.
  • Type, base width and side slope of a prismatic cross section or.
  • 8 point cross section defining the right over bank, center, and left over bank.

Straddle Stagger

This is a seldom used method which requires the number of ordinates to lag and the duration.

Related Topics

• Editing HMS Properties
• Sub-Basins
• Reservoirs
• Junctions
• Diversions