Basic Concepts
The Normal Depth method is very similar to the aforementioned Modified Puls method. Specifically, storage within a reach is assumed to be primarily dependent upon outflow. However, the Normal Depth method automatically develops storage vs. discharge relationships using Manning’s equation, a normal depth assumption, and user-defined channel properties. This method allows for more efficient parameterization, but also loses the ability to simulate backwater effects and the impacts of hydraulic structures since hydraulic simulations are no longer used to develop storage vs. discharge relationships.
Required Parameters
The parameters that are required to utilize this method within HEC-HMS are the initial condition, the reach length [ft or m], bottom slope [ft/ft or m/m], Manning’s n roughness coefficient, index flow [ft3/s or m3/s], and 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 index flow should represent the expected maximum flow within the reach. Storage-discharge for the reach will be created ranging from zero to 1.5 times the index flow. The index flow is also used in combination with the reach length and channel geometry to compute the travel time through the reach. The number of subreaches is then computed by dividing the travel time by the simulation time interval.
Five options are provided for specifying the cross-section shape: circle, eight-point, 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. Depending upon 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.
When using the eight-point shape, a simplified cross-section (which is a paired data object) with eight station vs. elevation values must be selected. The cross-section is typically configured to represent the main channel plus left and right overbank areas. As such, separate Manning's n values are required for each overbank. Storage vs. discharge relationships (ranging from zero to 1.5 times the index flow) will be automatically generated for the given channel properties using Manning's equation and a normal depth assumption.
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.