31 Mar 95
Figure 3-14. Modified Puls storage-outflow plot
For channel routing, the degree of attenuation depends on the
(b) A ditch rating or discharge-elevation relationship may be
number of routing steps used. The number of routing steps is a
entered for a conveyance channel connecting the ponding area
calibration parameter and represents the number of subreaches
to the gravity outlet and/or pump. It is required if the flow is
into which the total channel reach should be divided. The
controlled from the ponding area to the primary outlets.
Muskingum method is defined by three parameters: number of
routing steps, Muskingum K coefficient (which is the travel time
(4) GRAVITY module.
through the reach), and Muskingum X (which is a weighting
factor). The Muskingum-Cunge method is a nonlinear routing
(a) Gravity outlets through the line-of-protection are
technique that is defined by channel length, channel invert slope,
normally the most cost-effective means of evacuating interior
channel roughness coefficient, and channel shape. A trapezoid,
flood waters when the interior stage is greater than the exteriors.
a circular cross section, or a maximum eight-point cross section
Analysis of culvert hydraulics is complex because inlet or outlet
are the allowable channel shapes. This method is only available
controls may govern. The GRAVITY module produces a family
of outlet rating curves based on different exterior stage
(3) POND module.
(b) HEC-IFH performs gravity outlet analysis by direct entry
(a) Elevation-area relationships for the ponding area
of the outlet rating or by enabling the user to define the outlet
adjacent to line-of-protection should be developed using 15-20
characteristics and a range of computation elevations and
points to define the relationship. HEC-IFH automatically
intervals for computing the outlet rating curve. Exterior and
generates the storage values. The minimum value should define
interior invert elevations define the lower bound of the rating.
the pond bottom (zero storage) and must be at the same
No flow can occur until the interior ponding elevation exceeds
elevation or below the lowest outlet invert elevation. The
the invert elevation. The interior water elevation must also be
maximum value should exceed the highest stage anticipated in
greater than the exterior for flow to occur. Figures 3-17 and
3-18 depict the basic data entry screen for the gravity outlet
these maximum or minimum elevations. Figure 3-15 illustrates
the ponding area data entry screen and Figure 3-16 shows
table, respectively. Instead of using only the limited data shown
typical elevation-area-storage for a ponding area.
in Figure 3-18, the program uses a computed 50x50 matrix of