FLOOD06423

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Preparation of Input Data for Automatic Computation of State-Discharge Relations at Culverts
• Date: 1/1/2000
• Prepared By: Matthai, Stull, Davidian
• Floodplain - Doc Type: Educational/Technical/Reference Information

AUTOMATIC COMPUTATION OF STAGE-DISCHARGE RELATIONS AT CULVERTS 3

identification as listed in columns 1-40
of card 2).

The next line will print out the entire
card on which the error occurs, followed by
a message describing the error. Prior
cards on which no errors we-re detected in
that culvert problem will not be printed out.
However, all subsequent cards in that set.
whether or not they contain errors. are
printed out. They should be carefully exam-
ined because an earlier fatal error message
for that culvert 'problem may preclude a
thorough internal error check of the succeed-
ing cards.

The last message for a culvert problem
in which a fatal error has been detected will
give an error count:

THERE ARE xx ERRORS IN THIS
SET.

The edit program then examines the next
culvert problem in the input deck.

Part 2

Approach- section properties are com-
puted by standard procedures used by the
U. S. Geological Survey. Up to four subareas
can be used with individual roughness coeffi-
cients that can vary with stage between as-
signed values at selected elevations. Up to
25 points define the cross section, and prop-
erties can be computed for up to 25 selected
elevations. The output is a tabulation of
area. conveyance, velocity- head coefficient
(alpha), and top width at each elevation.
Also computed and tabulated is the critical
discharge for each elevation at the approach
section.

The next step is the computation of section
properties of the Gulvert barrel in increments
of D/25 from d = 0 to d = D. where d is the
depth of flow measured from the lowest point
in the cross section of the culvert, and D is
the inside vertical dimension of the culvert
barrel. The output for the culvert barrel is
a tabulation of area, conveyance, and top
width for each incremental depth.

This part of the program can be used in-
dependently of the culvert-flow computations.
Thus, computations can be made only for
approach- section properties, or for both
approach and culvert-barrel section prop-

erties, by selecting options 1 and 2, respec-
tively, on card 2, column 66, If, however,
section properties are desired for cross
sections having five to 20 subareas. or 26 to
200 ground elevations, the computer program
C374 for slope-area measurements must be
used (see Lara and Davidian, 1970).

Part 3

The first of 12 selected discharges is
used to compute the headwater elevation as-
sociated with each tailwater elevation speci-
fied.

The critical depth of flow in the culvert
barrel is determined from the general ex-
pression

Q2

A 3
c

g ---;y.

in which Q is discharge. A is cross- section
C
area at critical depth, g is the gravitational
constant, and T is top width.. The convey-
ance at critical depth, K . is also computed,
c
and the critical slope, S , is obtained from
the equation c

Sc = (~ ) .
c

The culvert slope, S . is equal to zl L
o
where z is the difference in invert elevations
between the inlet and outlet, and L is the
length of the culvert barrel. If S ~ S ,
c 0
type 1 flow is indicated; if S > S , type 2
c 0
flow is indicated.

The computations are made for hI' the

piezometric head at the approach section
(section 1>, referred to the elevation of the
invert at the culvert outlet. For each dis-
charge the following computations are made
in sequence:

1. Solve for hi by either type 1 or type 2
flow equations as indicated.
2. Solve for hi by the type 3 flow equation
for each of the first nine or fewer tailwater
elevations.

3. Solve for hi by the type 4 flow equation