Park West CC(18)-CS 900305 (2) DONALD E. DAVIS PAGE 1 OF °-3 ~
CONSULTING ENGINEERS
5506 COCKRELL HILL ROAD
DALLAS, TEXAS 75236
(214) 337-2332
MARCH
RLISSELL
C I TY ENC~EER
73:2~EF OREST
~PELL ~ TEXAS 7.J017
RE: F'ARK WEST COMMERCE CENTER
SITE 1 CONSTRUCTION
C058~081. ARD
THE CALCULATIONS FOR THE LENGTH OF OPEN TRENCH THAT IS NECESSARY TO DISSIPATE
A VELOCITY, CONSIDERS THAT THE WATER FLOWS OUT OF PIPES AT AN INITIAL HIGH
VELOCITY (13.8 fps IN THIS CASE) AND LOSES ENERGY GRADUALLY DUE TO CHANNEL
FRICTION. THIS FLOW IS SUPER-CRITICAL RESULTING IN A CORRESPONDING INCREASE IN
DEPTH. THE FLOW CAN BE COMPUTED WITH REASONABLE ACCURACY BY USING THE CHEZY
EQUATION:
2/3 1/2
V=Q/A = 1.47 R S
n
n = ROUGHNESS COEFFICIENT
R = HYDRAULIC RADIUS
S = SLOPE
AS THE DEPTH OF FLOW INCREASES - IT WILL REACH A LEVEL AT WHICH THE GIVEN
DISCHARGE "@" MAY FLOW IN THE GIVEN CHANNEL - DEFINED BY ITS SLOPE "$" AND
ROUGHNESS "n"
BY THE LAW OF MINIMUM ENERGY - NATURE WILL SELECT THE FLOW CONDITION AT WHICH
THE RATE OF ENERGY LOSS IS LESS. A HYDRAULIC JUMP MUST OCCUR.
EQUATING THE CHEZY FORMULA TO THE LOSS OF ENERGY IS DEFINED BY THE FOLLOWING
EQUATIONS:
2/3 1/2
Q 1.486 R S
V~vg= ---- =
A
~IARL-:H 5, 1S~?~0 PAGE 2 OF 3
C0587081. ARD
ANOTHER METHOD OF CACULATING THE LENGTH OF RIPRAP REQUIRED IS TAKEN FROM THE
DONALD E. DAVIS CONSULTING ENGINEERING MANUAL. THIS METHOD USES AN EMPIRICAL
TABLE BASED ON DATA AND RECOMMENDATIONS OF THE U.S. BUREAU OF RECLAMATION.
THIS METHOD IS MORE CONSERVATIVE THAT THE MATHEMATICAL METHOD BASED ON THE
CHEZ¥ FORMULA. THIS METHOD IS BASED ON THE FROUDE NUMBER.
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MARCH 5, 17~0 PAGE 3 OF 3
C058~C)81. ARD
LENGTH REQUIRED = 10.7' - THE MINIMUM RECOMMENDED LENGTH IN THE DGNALD E.
DAVIS CONSULTING ENGINEERING MANUAL IS 25 FEET. IN THIS CASE - 50 FEET IS
SELECTED BECAUSE OF THE NEWLY CUT DITCH AND EASILY ERODABLE SOIL.
DONALD E. DAVIS
CONSULT I NG ENG I NEERS