Basic Concepts and Equations
The derivation of the kinematic wave routing method is detailed within the Kinematic Wave Transform Model section. In summary, this method approximates the full unsteady flow equations by neglecting inertial and pressure forces. Specifically, the pressure gradient, convective acceleration, and local acceleration terms within the momentum equation are ignored. This results in the following simplification:
As shown within the previous equation, the energy gradient is assumed to be equal to the bottom slope.
As such, this method is only appropriate for use in steep channels (i.e. 10 ft/mi or greater) and does not recreate backwater effects.
Required Parameters
The parameters that are required to utilize this method within HEC-HMS are the initial condition, the reach length [ft or m], the bottom slope [ft/ft or m/m], Manning’s n roughness coefficient, the number of subreaches, an index method and value, and a cross-section shape and parameters/dimensions. An optional invert can also be specified.
Two options for specifying the initial condition are included: outflow equals inflow and specified discharge [ft3/sec or m3/sec]. The first option assumes that the initial outflow is the same as the initial inflow to the reach from the upstream elements which is equivalent to the assumption of a steady-state initial condition. The second option is most appropriate when there is observed streamflow data at the end of the reach.
The reach length should be set as the total length of the reach element while the bed slope should be set as the average bed slope for the entire reach. If the slope varies significantly throughout the stream represented by the reach, it may be necessary to use multiple reaches with different slopes. The Manning's n roughness coefficient should be set as the average value for the whole reach. This value can be estimated using “reference” streams with established roughness coefficients or through calibration.
The number of subreaches is used in concert with the index method to determine the minimum distance step to use during routing calculations. Two options for specifying an index method are included: flow [ft3/s or m3/s] and celerity [ft/s or m/s]. When index flow is selected, the user-entered flow rate is converted to an equivalent celerity using the cross-section shape of the reach. When the index celerity method is selected, the travel time is computed directly from the specified value. The distance step is first estimated from the travel time. If the distance step is greater than the reach length divided by the number of subreaches, then the distance step is decreased.
Five options are provided for specifying the cross-section shape: circle, deep, rectangle, trapezoid, and triangle. The circle shape is not meant to be used for pressure flow or pipe networks, but is suitable for representing a free surface inside a pipe. The deep shape should only be used for flow conditions where the flow depth is approximately equal to the flow width. Depending on the shape you choose, additional information will have to be entered to describe the size of the cross-section shape. This information may include a diameter (circle) [ft or m], bottom width (deep, rectangle, and/or trapezoid) [ft or m], or side slope (trapezoid and triangle) [ft/ft or m/m]. In all cases, cross-section shapes must be defined in such a way that all possible flow depths that will be simulated will be completely confined within the defined shape.
Many of the aforementioned parameters are typically estimated using GIS. However, field survey data may be necessary to accurately determine reach lengths, bed slopes, and/or cross-section shape parameters.
