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

In these equations subscripts 1 and 2 refer to
the beginning and end of the time interval or the
units aze cubic feet per second for flow, hour;
for time, feet per second for velocity, and feet
.for length and depth. The symbols aze defined as
follows:
I =inflow rate.
O =outflow rate.
!1 +/2
!p = 2 =average inflow rate.
C = .storage coefficient.
T =travel time through the reach.
L =reach length.
V =velocity.
SLPp =normal slope.
D =depth.
Since TZ and CZ aze dependent upon O2, an
iterative technique is required to solve the
routing equations. In equation 7, !Q and O 1 are
known, and CI can be computed from equation
9. This leaves only OZ and CZ as unknowns. OI
can be used as~a first approximation of OZ. The
normal depth and velocity for the approximate
value of OZ are entered into equation 11 for
computing TZ. Then equation 8 is used to
compute C~. The second approximation of OZ is
then obtained from equation 7. This iterative
process continues anti) the difference between
successive O ~ values is acceptable. HYMO is set
to accept differences of 0.1 percent or less.
Usually about four iterations aze required.
Reservoirs
HYMO uses the storage-indication method (3)
to route floods through reservoirs. This method
has been widely used and accepted because it is
practical and accurate. The SCS National Engi-
neering Handbook gives detailed instructions for
using the method.
Rating Curves
Rating curves must be available at enough
locations along a valley to adequately descrihe
the hydraulics of the stream and valley. Most of
these rating curves must be computed because
tl•ere aze never enough measured rating curves.
HYMO uses Alanning's equation to compute
the normal flow-rating curves that aze used in
the VSC flood-routing method. The normal
flood-plain slope is determined for each valley
section by plotting a profile of the flood plain.
The normal channel slope is determined by
plotting a profile of the flood plain with channel
distances.
Sediment Yields
The universal soil loss equation (8) was
modified to compute the sediment yield for
individual storms on watersheds. The modified
equation is .
S=95(gpXQ)C1.56xKxCxPxLS, {72)
where SI = sediment yield in Iona,
q peak Flow rate in cubic feet per second,
~: = volume of runoff in acre-feet,
K: = the soil-erodibility factor,
C~= the cropping-management [actor,
P~,= the emsion control practice factor,
and LSD= the slope length and gradient Cactor.
Detailed instructions for determining K, C, P,
and LS are given by Wischmeier and Smith (8).
Since equation 12 was designed to compute
sediment yield fiom watersheds, a delivery ratio
is not needed. The delivery ratio is built into
equation 12 by including the peak flow rate.
Many of the watershed chazacteristics that in-
fluence the peak flow rate also affect the
delivery ratio. Equation 12 has performed well
under limited testing, but future refinements are
expected.
RULES FOR USING HYMO
The reader should refer to the table, "Ex-
ample Input for HYMO Commands," as he
follows the narrative description of the rules.
The example HYMO program that is prese.rted
neaz the end of the manual may also be helpful.
General Rules
HYbiO commands aze expressed in the first
20 colurri_~s of the data cazd, and columns 21
through 79 are used for numeric data and
keywords. Column 80 is reserved for apage-
change code (an asterisk in column 80 causes the
card to be printed on a new page). Continuation
cards aze allowed when 59 characters aze insuf-
ficient to express the data.
The data can be written in any format, but at
least one blank space must be left between data
items. A decimal is required For numbers con-
7-6-1-5
u
•