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

for HEA.

of outlet rating curves based on different exterior stage

conditions.

(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

the analysis. No extrapolation is performed above or below

3-18 depict the basic data entry screen for the gravity outlet

these maximum or minimum elevations. Figure 3-15 illustrates

rating computations and the corresponding computed rating

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