FLOOD01090

• County: Statewide
• Title: Spillway Rating and Flood Routing - Users Manual
• Date: 10/1/1966
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

4. METHODS OF CUMPUTATION

Step 1 - The input data consist or a description or the spillway,
a description or the flood routing, the elevation-capacity table, the
tailwater rating curve in the river channel, and the inf'lows or the
spillway design flood.

Step 2 - The maximum elevation ("ELTOP") ror which a discharge will
be ccmputed is determined by ,the minimum or either (a) the maximum
elevation of the elevation-capacity table or (b) one hundred and fifty
percent of the design head assumed ("DESlID").

;

r

Step 3 - The energy head over the weir is next ccmputed ror the
current elevation in the elevation table. This energy head ("HE") is
equal to the current elevation (ELEV(I)) minus the spillway crest eleva-
tion (EISPI) minus the spillway approach loss at this elevation. This
approach l08S is simply a ratio of the current energy head to the design
head times the approach 10&8 at the design head which was read in.
(APLOOS * HEjDESlID).

Step 4 - If variable "ISRCD" is 10, a criticaJ. depth rating for the
spillway will be made as described in step 5, if it is "0" a rating for
the "OGEE" spillway will be made as follows"! The general method of com-
puting the rating is described in the Corps or Engineers Engineering
Manual on Hydraulic Design or Spillways (EM 1ll0-2-l603). The generaJ.
equation of the rating is:

Q = C(L' - 2(NxKp + KA) HE) HE3/2

The coefficient of discharge (C) is determined :from WEB Hydraulic Design
Chart 122-iE Revised 1-64, by the use of the following equation: COFQ, =
XCC * XPHD C, where XPHD = the p/lID calculated from the approach depth
and design head, and XCC and XEC represent variables dependent on the
value of HE/lID. The values or XCC and XEC ror various values of HE/lID
are shown on Table 3. The "KP" is :from Plate 11 of EM 1110-2-1603 and
is determined by table look up of the values shown in Table 3. The "KA"
is determined :from WEB Hydraulic Design Chart 111-3/1 (See Table 3) or
111-3/2 (Revised 1-64). The :freeflow discharge for the "OUEE" spillway
is then calculated by the general equation shown at the first or this
step. The rreeflow discharge is then checked for submergence by calcu-
lating lID/HE and (lID + D)/HE and reducing the rreeflow discharge accord-
ll\<.sto 1:,beHydraulic Design Chart 111-4. Two hundred and fifty-two (252)
values of submergence dependent on lID/HE and (lID + D)/HE are used for
this computation (see Table 4). This reduction in discharge is a trial
and error solution since the tailwater is dependent upon the discharge
which is dependent on the submergence. The tailwater is determined
either by the specific energy equation (variable "ISPITW" . 10) as shown
on Page 17 of EM 1110-2-1603 or by a table lookup. Step No.6 is next
for the "OGEE" spillway.

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