The Muskingum-Cunge 8-pt Channel routing method is a non-linear hydrologic routing method that uses a representative cross section to apply a physical or hydraulic influence to the method.
The edit panel for the Muskingum-Cunge 8-pt Channel Routing method is shown in "Figure: Muskingum-Cunge 8-pt Channel Routing Edit Panel". This panel includes a generalized illustration of an 8 point cross section to provide a guide for defining the cross section. Below the illustration is a thumbnail plot to show your cross section as defined in the Cross Section Table. Use the thumbnail plot to verify that your cross section points were entered correctly and adequately reflects the cross section template shown in the illustration.
The cross section illustration in the edit panel for the Muskingum-Cunge 8pt Channel routing method shows the numbering of the eight cross section points and how they relate to the partitioning of the cross section into a Left Overbank, Channel, and Right Overbank. This illustration indicates that points 3 and 6 are the bank stations of the cross section, these points the mark the boundary between the channel and the overbanks.
Be sure that when you enter your cross section into the Cross Section Table that its bank stations are at points 3 and 6.
The parameters for defining the Muskingum-Cunge 8-pt Channel routing method include:
Channel Length—The length of the routing reach, measured along the channel centerline. This value is not computed by ResSim based on the stream alignment.
Channel Slope—The average bed slope of the routing reach, \Delta y/\Delta x.
Eight Station and Elevation pairs—The horizontal and vertical position (respectively) of each point used to describe the representative cross section for the reach.
Manning's n Values—The roughness coefficients for the Left Overbank, Main Channel, and Right Overbank are used in the normal depth calculations of the cross section.
Default Reference Flow—The reference flow is used to compute the celerity of a flood wave through the reach which is then used to compute the travel time for the reach. If the travel time is greater than the size of the computation interval (timestep), the reach is divided into subreaches for the computations. Travel time is usually greater for low flow; therefore, start with a default reference flow value that approximates the base flow for the reach.
Routing Timestep Subintervals—As its name suggests, this parameter is the number of increments to break the timestep into for purposes of numerical stability of the Muskingum-Cunge solution. Although some implementations of the Muskingum-Cunge method include a strategy for determination of the subintervals based on a review of the inflow hydrograph, this option is not feasible for ResSim because the flow hydrographs into the reaches downstream of the reservoirs are not "known" at the start of a ResSim simulation. As a result, ResSim requires you to select the number of subintervals as a whole number value between 1 and 12.
A Rule of Thumb…
Use fewer subintervals for constant or gradually varying flow.
Use more subintervals for rapidly varying flow.
