Muskingum-Cunge Prismatic Channel Routing

The Muskingum-Cunge Prismatic Channel method ("Figure: Muskingum-Cunge Prismatic Channel Routing") is the same basic method as the Muskingum-Cunge 8-pt Channel method but it uses one of two shapes to describe the representative cross section—a trapezoid or a circle. Use this method for reaches that have no overbanks and whose channel cross section can be approximated by a trapezoidal shape or for routing through pipes or culverts.

The parameters for defining the Muskingum-Cunge Prismatic Channel method include:

• Channel Length — The length of the routing reach, measured along the channel centerline.
• Channel Slope — The average bed slope of the routing reach, $//<![CDATA[ \begin{array}{l}\Delta y/\Delta x\end{array} //]]>$ ("rise over run").
• Manning's n — The roughness coefficient to be used in the normal depth calculations of the cross section.
• Channel Shape—Select from the following channel cross section shapes:

  • Prismatic—the representative cross section is shaped like a trapezoid (whose bottom width is less than or equal to its top width — see "Figure: Prismatic Channel"). The trapezoidal shape is defined by two parameters: a bottom width and a side slope; depth of the trapezoid is not required, ResSim will assume the channel is infinitely deep.
    

    Figure: Prismatic Channel

    
    

    • Bottom Width—b, the width of the bottom of the trapezoidal section.

    • Side Slope—z, the slope of the sides of the trapezoid entered as $//<![CDATA[ \begin{array}{l}\Delta x/\Delta y\end{array} //]]>$. ("run over rise". Yes, this is the inverse of the bed slope.) If the sides of your trapezoid are vertical, the slope is 0.0.

  • Circular—This cross section shape is a simple circle. Use this shape to represent a free-flowing pipe or culvert. This method and shape is valid for flow depths up to 0.77*diameter. For depths greater than this, the depth used by the method is held to 0.77*diameter and a warning message is generated. If you choose Circular as the Channel Shape, you also need to specify the Channel Diameter.

    • Channel Diameter—Define the size of the circle.

• Muskingum-Cunge Stability Control—Enter the default reference flow and the routing timestep subintervals.

  • Default Reference Flow—The flow value is used to compute the celerity (and thus the travel time) for the reach. If the travel time is greater than the timestep size, the reach is divided into subreaches for the computation. Travel time (and thus the number of subreaches) is greater for low flow; therefore, the reference flow value should approximate the base flow for the reach.

  • Routing Timestep Subintervals—As its name suggest, this parameter is the number of increments to break the timestep into for purposes of numerical stability of the Muskingum-Cunge solution. Select the number of subintervals as a whole number value between 1 and 12.