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OVERFLOW SPILLWAYS, DISCHARGE COEFFICIENTS, DESIGN HEAD, Sheet 122-1/2

작성자 Uploader : ezcivil 작성일 Upload Date: 2017-02-19변경일 Update Date: 2017-02-19조회수 View : 788

HYDRAULIC DESIGN CRITERIA
SHEET 122-1/2
OVERFLOW SPILLWAYS
DISCHARGE COEFFICIENTS
DESIGN HEAD

1. Purpose.

Hydraulic Design Chart 122-1/2 can be used for selecting the spillway design head for upstream spillway face slope and approach depth conditions not covered by other Hydraulic Design Criteria charts. The chart, used in conjunction with the USBR(1) extensive tables of lower and upper nappe surface coordinate data for weirs sloping downstream and for vertical weirs, should permit optimization of spillway crest shape design for free overfall spillways having upstream face slope ratios from 3V on 12H to vertical and approach depths from 0.15 to 10 times the design head (Hd).

2. Background.

The USBR tests were made in a 2-ft-wide, 9-ft-deep rectangular flume. Approach depths varied from less than 0.1 ft to 5 ft. Heads on the sharp-crested weirs ranged from about 0.1 to 1.0 ft. Discharge coefficients have been converted into values applicable to heads on the rounded crest. USBR coordinates of the upper and lower nappes of the weir overflow are in terms of the head on the sharp-crested weir. Comparable presentations of Bazin′s(2) data for weirs sloping downstream (3V on 6H and 3V on 12H) are also included on the chart and in the USBR tabulation. A plot of the WES experimental data (Chart 122-1) for H/Hd = 1 and P/Hd = 0.3 to 1 is included for comparison. The WES discharge coefficients for rounded crests have consistently been about 3 percent higher than comparable USBR coefficients for sharp-crested weirs (HDC 122-4). The use of the USBR coefficients for the design of unmodeled spillways should result in conservative design.

3. Application.

The spillway design flow Qd is computed using an appropriate coefficient from Chart 122-1/2 and pier and abutment contraction coefficients from other applicable charts (see Charts 111-3/1, 111-3/2, 111-5, 116-6, and 122-2). The spillway design head Hd and the computed design discharge Qd are then used with Chart 111-3/3 to develop a spillway rating curve for uncontrolled flow as described in paragraph 5a of Sheet 111-3/3.

4. References.

(1) U. S. Bureau of Reclamation, Studies of Crests for Overfall Dams; Hydraulic Investigation. Bulletin 3, Part VI, Boulder Canyon Project Final Reports, Denver, Colo., 1948.

(2) Bazin, M., ″Recent experiments on the flow of water over weirs.″ Annales des Ponts et Chaussees, October 1888.

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Unit discharge qd for the sharp-crested weir can be calculated by following formula.

qd = Cd * Hd^(3/2)

where

Cd = Spillway design head discharge coefficient
Hd = Spillway design head (ft)

*** 참고문헌[References] ***

USACE, HYDRAULIC DESIGN CRITERIA, SHEET 122-1/2, OVERFLOW SPILLWAYS, DISCHARGE COEFFICIENTS, DESIGN HEAD.
qd = Cd * Hd^(3/2)
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