PERMFILE114378

• Permit No: C1994082
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: Dodson & Associates Hydraulic Programs ‎(Trap, Pipe)‎
• Section_Exhibit Name: TAB 13 ATTACHMENT 13-1 ADDENDUM 2
• Media Type: D
• Archive: No

TRAPEZOIDAL AND CIRCULAR CHANNEL ANALYSIS HYDROCALC HYDRAULICS MANUAL. PAGE 14
i
For a trapezoidal channel, the equation is re-arranged in terms of the depth of flaw. An initial flow
depth esttmate of 1 foot Is substituted Into the equation, and a new approximation is computed '
for the flow depth. The newvalue Is comparedwith the previous approadmation, and ffthe difference ~
Ls less than 0.001 feet, the depth is assumed to be the Normal Depth. If not, a new appraadmation
for Normal Depth is computed as the geometric mean of the previous two approximations. This
method quickly gives a very precLSe and reliable value forthe Normal Depth of flow in a trapezoidal
channel.
2.1.2.2 Normal Depth in a Circular Channel
The first step in determining the Normal Depth in a ciroulaz channel 1s to compute the maximum
IIow capacity of the channel. This capacity occurs at a flow depth of 0.9382 times the channel
diameter. [Note: one of the example problems for the Rating Curve Procedure demonstrates that
this is the case.) If this capacity is less than the required flow rate, then it fs not possible to
compute a Normal Depth of flow, and the program gives you a message to that effect.
Assuming that the channel has sufficient capacity to convey the required flow rate, this program
computes Normal Depth using the same method as the Trapezoidal Channel procedure except
that the initial flow depth is assumed to be one-half of the channel diameter.
This method quicklygives very precise and reLable values forthe Normal Depth of flow ffi a circular
channel, except incases in which the Normal Depth Is above about 8896 of the channel diameter.
In such cases, the programmay complete the maxtmum3O iteratlonswlthout reachingtherequired
accuracy.
2.1.2.3 Flow Velocity at Normal Depth
Atter the program computes the Normal Depth, the flow velocity in the channel 1s computed as
simply the flow rate divided by the cross-sectional area of the flow. The velocity is assumed to be
constant throughout the cross-section.
The flow velocity is an Important consideration in many channel design situations. The allowable
IIow velocity maybe limited by local drainage criteria. `
2.1.2.4 Froude Number at Normal Depth '
The Froude Number is the ratio of the inertial forces to the gravitational forces in a flowing fluid. ~
It is computed using this formula:
Froude Number =
r
1n which: F
V =Flow Velocity (fps)
g =Acceleration due to gravity = 32.2 feet/sec/sec 4
A =Cross-sectional Area of Flow (square feet)
T =Top Width of Flow (feet)
If the Froude Number is greater than one (1.00), then flow in the channel is "super-critical". A
Froude Number less than one Is more common, indicating "sub-critical" flow. Section 2.2 of this
manual contains more infomiatton on critical flow and Critical Depth.

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