WMS:Routing HEC-1 Cards

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Routing Data...

Outlet points are used to define locations where hydrographs are combined and then routed downstream. The appropriate combined hydrograph (HC cards) stations are generated automatically when writing a HEC-1 file. However, routing data must be entered in order to simulate the movement of a flood wave through the river reaches or reservoirs. The effects of storage and flow resistance are accounted for in the shape and timing of the flood wave. In addition to these changes, volume may be lost due to channel infiltration. Most of the routing methods available in HEC-1 are based on the continuity equation and some relationship between flow and storage or stage.

Routing data is entered by selecting an outlet and then selecting the Routing Data button from the Edit HEC-1 Parameters dialog.

Outlet Names (KK)

Since outlets are used for both types (combining and routing) of hydrograph stations in the HEC-1 input file, a separate name for each type of hydrograph must be entered. The name should be six characters or less and correspond to the name used on the KK card to represent the appropriate hydrograph station. By default WMS uses the basin ID number followed by a "R" for the name, but a descriptive name is generally more useful.

Channel Losses (RL)

Constant channel losses may be defined by defining values for the RL record. These parameters include the following:

• QLOSS – Constant channel loss in entire routing in cfs (cms). This value is subtracted from every ordinate of the inflow hydrograph.
• CLOSS – Ratio of remaining flow (after QLOSS) which is lost for entire routing. After subtracting QLOSS each inflow hydrograph ordinate is multiplied by (1-CLOSS).
• PERCRT – Percolation rate cfs/acre (cu m/sec-acre) for wetted surface area of channel. This option is used in conjunction with storage routing and requires SA or SV/SE records to be defined.
• ELVINV – Average invert elevation of channel L used to compute flow surface area for PERCRT.

Kinematic Wave (RK), Muskingum Cunge (RD) and Convex (RV)

The Kinematic wave, Muskingum Cunge, and Convex are defined with essentially the same parameters. A brief description follows.

• L – Channel length.
• S – Channel slope.
• N – Manning's roughness.
• Shape – Characteristic channel shape.
• WD – Base width of the channel.
• Z – Side slope of channel.

If the Muskingum-Cunge method is selected, define the channel geometry using an eight point cross-section by specifying the appropriate radio button and selecting the Define RC Record button.

Muskingum (RM)

The Muskingum method is dependent primarily upon an input weighting factor. The parameters along with short descriptions of their meanings follow:

• NSTPS – The number of integer steps for the Muskingum routing.
• AMSKK – Muskingum K coefficient in hours for entire reach.
• X – Muskingum x coefficient.

Using the basin data computed by WMS when a TIN or DEM is used to delineate the watershed, the AMSKK and NSTPS coefficients can easily be estimated. AMSKK is essentially the travel time for the reach, which can be estimated by noting the length of the stream segment (displayed in the Muskingum Cunge dialog even though it is dimmed) 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 AMSKK edit field. The NSTPS value is the number of time steps the flood wave is in the channel and can be determined by dividing AMSKK by the computational time step found in the Job Control dialog (again, be sure that units are consistent).

Straddler Stagger (RT)

Parameters used to define the Straddler/Stagger or Tatum routing method are defined below.

• NSTPS – Should be one for Straddler/Stagger method or integer number of routing steps to be used for Tatum method.
• NSTDL – Integer number of intervals hydrograph is to be lagged in the Straddler/Stagger method or 0 if using Tatum method.
• LAG – Integer number of ordinates to be averaged in the Straddler/Stagger method or 2 if using Tatum method.

Storage Routing (RS)

Storage-discharge routing can be used to define either channel or reservoir routing. When this routing option is specified the appropriate data items are dimmed and additional radio buttons are used to determine whether channel or reservoir routing is to be used.

The following parameters must be defined regardless of the storage routing option specified.

• NSTPS – Number of steps to be used in the storage routing. Typically this is approximately equal to (reach length) / (average velocity * time interval (NMIN) ). NSTPS is usually equal to 1 for a reservoir.
• ITYP – The next parameter RSVRIC can be entered in one of three different ways:
  • STOR – Storage in acre-feet (1000 cu m).
  • FLOW – Discharge in cfs (cms).
  • ELEV – Elevation in feet (m).
• RSVRIC – Storage, discharge, or elevation as defined by ITYP corresponding to the desired starting condition at the beginning of the first time period as specified on the IT record in the Job Control dialog.

HEC-1 No Routing (RN)

By default there is no routing at an outlet point. This allows for hydrographs to be combined without considering routing effects.

Output Control...

See Output Control... section under Basin HEC-1 Cards.

Resevoir Data...

Reservoirs in HEC-1 can be defined in a few different ways, depending on the storage routing techniques that need to be modeled. The tutorial on creating topologic trees outlines the different methods that can be used to represent reservoirs. The parameters required to define the reservoir are the same in all cases. The main difference is whether the reservoir stands alone by itself or whether the routing option of the outlet is used to define the reservoir.

Reservoir Routing

If the reservoir routing option is specified then one method for volume and one method for outflow must be defined.

• Known volume – Define a known volume (SV) record using the XY Series Editor. Optionally, you can define the elevations (SE) which correspond to the known volumes.
• Computed volume – By defining an area (SA) elevation (SE) relationship the volume can be computed automatically by HEC-1. Both * records are defined using the XY Series Editor.

If a TIN has been used to define the watershed a storage capacity curve can be generated and the information used to set up the volume/elevation (SV, SE) or area/elevation (SA, SE) data. See Storage Capacity Curves for more information.

The available methods for outflow include:

• Known outflow – Define a known outflow (SQ) record using the XY Series Editor.
• Computed Weir Spillway – A combination of data records are used to define spillway characteristics (SL, SS, ST). Parameter description for these different records are as follows:

Low-Level Outlet (SL)

• ELEVL – Center line elevation of downstream end of low-level outlet.
• CAREA – Cross-sectional area in square feet (square m) in the low-level outlet orifice equation.
• COQL – Discharge coefficient in orifice outlet equation.
• EXPL – Exponent of head in orifice equation.

Spillway Characteristics (SS)

• CREL – Spillway crest elevation. This value must be less than the highest elevation on the SE card for HEC-1 to run properly.
• SPWID – Spillway length.
• COQW – Discharge coefficient in the spillway weir flow equation.
• EXPW – Exponent of head in the weir spillway flow equation, usually equals 1.5.

Dam Overtopping (ST)

• TOPEL – Elevation of the top of the dam at which overtopping begins.
• DAMWID – Length of the top-of-dam which is actively being overtopped.
• COQD – Discharge coefficient in the weir equation.
• EXPD – Exponent of head in the weir equation.

Channel Data...

Constant channel losses may be defined by defining values for the RL record. These parameters include the following:

• QLOSS – Constant channel loss in entire routing in cfs (cms). This value is subtracted from every ordinate of the inflow hydrograph.
• CLOSS – Ratio of remaining flow (after QLOSS) which is lost for entire routing. After subtracting QLOSS each inflow hydrograph ordinate is multiplied by (1-CLOSS).
• PERCRT – Percolation rate cfs/acre (cu m/sec-acre) for wetted surface area of channel. This option is used in conjunction with storage routing and requires SA or SV/SE records to be defined.
• ELVINV – Average invert elevation of channel L used to compute flow surface area for PERCRT.

Channel Routing

If the channel routing option is specified, the Define button will access a dialog which allows you to choose between Normal depth and Modified-Puls methods. If Normal depth is specified, the following parameters must be defined for the RC record.

• Manning's coefficients – Manning roughness coefficients for the channel and left and right overbanks.
• Length – The length of the river reach.
• Slope – The slope of the river reach.
• Max Elevation – The maximum elevation for which storage and outflow values are to be computed.

In addition to these parameters an eight point cross section must be defined using the XY Series Editor. The first two points define the left overbank, the third point defines the left bank, the fourth and fifth points define the channel itself, the sixth point defines the right bank, and the last two points define the right overbank.

If the modified-Puls method is chosen the volume (SV) outflow (SQ) relationship must be defined. Both records are defined using the XY Series Editor.