PERMFILE106431

• Permit No: C1980005
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: HYMO MODEL
• Section_Exhibit Name: TAB 07 APPENDIX 7-6 ATTACHMENT 7-6-1
• Media Type: D
• Archive: No

The main program reads the command table
and then calls the HONDO subroutine (2) to
read a program data card. Subroutine HONDO
determines the command name and number by
comparing columns 1 through 20 of the program
data cazd with the command table. Then
HONDO determines individual data items by
comparing columns 21 through 80 of the pro-
gram data card with the ZALFA array. The data
are placed in an array and returned to the main
program. Based on the command number, the
main program calls the proper subroutine to do
the desired calculations. When the calculations
aze complete, control is returned to the main
program, and HONDO is called again to read the
next program card. .
HYDROLOGIC PROCEDURES USED IN HYMO
~'
~,
The procedures used in HYMO were selected
because of their accuracy, general applicability,
practicality of inputs, and computational effi-
ciency. For most watersheds the input is easy to
obtain, and the procedures produce reasonably
accurate results without excessive computer
time.
Hydrograph Computation
When flood routing is performed, a watershed
is divided into many small azeas according to its
hydraulic chazacteristics. The hydrographs from
these azeas must be estimated, since streamflow
measurements are seldom available. A procedure
for computing unit hydrographs was developed
previously (4). A modification of this procedure
is used in I~YMO. Unit hydrographs are divided
into three parts for computation (fig. 1}. From
the beginning of rise to the inflection point, tp,
the hydrograpil is computed by the two-
parameter gamma distribution equation
where q =flow rate in cubic feet per second et [ime r,
qp =peak flow rate in cubic feet per second,
tp time to peak in hours,
and n dimensionless parameter.
From tp to rl (rl = tp + 2K} the hydrograph
is computed by the recession depletion equation
ro-r -
K
9 ' 4oe (2)
where qo = flow rate at the inflectioo point,
to =time at the inflection point,
and K =recession constant in hour.
From rl to m the recession depletion equa-
tion becomes
tl-t
Kt
(3)
r (n-1) (1-n) (t/tp-1) where q~ flow rate at tt,
q = gpft/tp~ a (l) and Kt 3K=semndrecession constant.
i.o
e
s
~~nrcnw ran
_,.~
o i z e • a e ,
ti,r
Figure 1. Dunensionless unit hydtograph.
J~
The dimensionless shape parameter, n, is a
function of K/rp, as shown in figure 2. The peak
flow rate is computed by the equation
9 ' qte
BAQ
qP - tP (4)
where B = a watershed pazameter, a function of n as
shown in Fgure 3,
A =watershed azea in square miles,
and Q = volume of runoff in inches.
.Therefore, the entire unit hydrograph can be
computed if K and $ aze known. K and tp
can be determined by hydrograph analysis (4)
for gaged watersheds. To compute K and tp for
ungaged watersheds, HYMO uses the equations
K = 27.OAO.231SLp-0.777~~/ly/0.124 (5)

7-6-1-3