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STONE DRAIN OUTLETS, FIXED ENERGY DISSIPATORS, Sheet 722-1 to 722-3

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HYDRAULIC DESIGN CRITERIA
SHEET 722-1 TO 722-3
STONE DRAIN OUTLETS
FIXED ENERGY DISSIPATORS

1. Purpose.

Storm drains frequently terminate in unstable channels and gullies. Under these conditions dissipation of the energy of the out-flow is required to prevent serious erosion and potential undermining and subsequent failure of the storm drains. Adequate energy dissipation can be accomplished by extensive riprap protection(1,2) or by construction of specially designed fixed energy dissipators(3,4,5,6).

2. Hydraulic Design Charts (HDC¡¯S) 722-1 to -3 present design criteria for three types of laboratory tested energy dissipators(3). Each type has its advantages and limitations. Selection of the optimum type and size is dependent upon local tailwater conditions, maximum expected discharge, and economic considerations.

3. Stilling Wells.  

The stilling well energy dissipator shown in HDC 722-1 was developed at the U. S. Army Engineer Waterways Experiment Station (WES)(3). Energy dissipation in this stilling well is relatively independent of tailwater and is accomplished by flow expansion in the well, by impact of the fluid on the base and wall of the well, and by the change in momentum resulting from redirection of the flow to vertically upward. WES laboratory tests(3) indicated that the structure performs satisfactorily for flow-pipe diameter ratios (Q/Do^2.5) up to 10 with a well-pipe diameter ratio of 5.

4. HDC 722-1 shows the relation between storm drain diameter, well diameter, and discharge. Designing for operation beyond the limits shown in HDC 722-1 is not recommended. Intermediate ratios of stilling well-drain pipe diameters within the limits shown in HDC 722-1 can be computed using the equation given in this chart.

5. Impact Energy Dissipators.  

The U. S. Bureau of Reclamation (USBR)(5) has developed an impact energy dissipator which is an effective stilling device even with deficient tailwater. The dimensions of this energy dissipator in terms of its width are shown in HDC 722-2. Energy dissipation in the basin is accomplished by the impact of the entering jet on the vertically hanging baffle and by the eddies that are formed following impact on the baffle.

6. HDC 722-2 shows the relation between storm drain diameters, basin width, and discharge. WES laboratory tests(3) showed that this structure properly designed performs satisfactorily for Q/Do^2.5 ratios up to 21. Intermediate ratios of basin widths within the limits shown in HDC 722-2 can be computed using the equation given in this chart. Design for operation beyond these limits is not recommended. The WES tests also showed that optimum energy dissipation for the design flow occurs with the tailwater midway up the hanging baffle. Excessive tailwater should be avoided as this causes flow over the top of the baffle.

7. Hydraulic Jump Energy Dissipators. The St. Anthony Falls Hydraulic Laboratory (SAFHL)(6) has developed the hydraulic jump energy dissipator shown in HDC 722-3. Design equations for dimensionalizing the structure in terms of the square of the Froude number of the flow entering the dissipator are also given in the chart. WES laboratory tests(3) showed that this type of stilling basin performs satisfactorily for ratios of Q/Do^2.5 up to 9.5 with a basin width three times the storm drain diameter. WES tests were limited to basin widths of 1, 2, and 3 times the drain diameter with drops (drain invert to stilling basin) of 0.5 and 2 times the drain diameter. Parallel stilling basin walls were used for basin width-drain diameter ratios of 1 and 2. The transition wall flare was continued through the basin for W = 3Do . Parallel basin sidewalls are generally recommended for best performance. Transition sidewall flare (l:D¡¯) during the WES tests was fixed at 1 on 8. The invert transition to the stilling basin should conform to the geometry of the trajectory of a flow not less than 1.25 times the drain outlet portal design velocity.

8. HDC 722-3 shows the relation between storm drain diameter and discharge for stilling basin widths up to 3 times the drain diameter which results in satisfactory performance. WES tests have been restricted to the limits shown in HDC 722-3, and the equation given in the chart can be used to compute intermediate basin width-drain diameter ratios within those limits. General WES model tests of outlet works indicate that this equation also applies to ratios greater than the maximum shown in the chart. However, outlet portal velocities exceeding 60 fps are not recommended for designs containing chute blocks. This chart does not reflect the outlet invert transition effects on basin performance. The design of the basin itself (HDC 722-3) is dependent upon the depth and velocity of the flow as it enters the basin. The values should be computed taking into account the drain outlet transition geometry.

9. Riprap Protection.  

Riprap protection in the immediate vicinity of the energy dissipator is recommended. Preliminary, unpublished WES test results(3) on riprap protection below energy dissipators indicates the following average diameter (D50) stone size should result in adequate erosion protection.

D50 = D*(V/(g*D)^(1/2))^3

where

D50 = the minimum average size of stone, ft, where by 50 percent by weight of the graded mixture is larger than D50 size
D = depth of flow in outlet channel, ft
V = average velocity in outlet channel, ft/sec
g = gravitational acceleration, ft/sec^2

10. References.

(1) U. S. Army Engineer Waterways Experiment Station, CE, Erosion and Riprap Requirements at Culvert and Storm-Drain Outlets; Hydraulic Laboratory Model Investigation, by J. P. Bohan. Research Report H-70-2, Vicksburg, Miss., January 1970.

(2) _________, Practical Guidance for Estimating and Controlling Erosion at Culvert Outlets, by B. P. Fletcher and J. L. Grace, Jr. Miscellaneous Paper H-72-5, Vicksburg, Miss., May 1972.

(3) _________, Evaluation of Three Energy Dissipators for Storm-Drain Outlets; Hydraulic Laboratory Investigation, by J. L. Grace, Jr., and G. A. Pickering. Research Report H-71-1, Vicksburg, Miss., April 1971.

(4) _________, Impact-Type Energy Dissipator for Storm-Drainage Outfalls Stilling Well Design; Hydraulic Model Investigation, by J. L. Grace, Jr. Technical Report No. 2-620, Vicksburg, Miss., March 1963.

(5) Beichley, G. L., progress Report No. XIII - Research Study on Stilling Basins, Energy Dissipators and Associated Appurtenances - Section 14, Modification of Section 6 (Stilling Basin for Pipe or Open Channel Outlets - Basin VI. Report No HYD-572, Hydraulics Branch, Division of Research, U. S. Bureau of Reclamation, Denver, Colo. , June 1969.

(6) Blaisdell, F. W., The SAF Stilling Basin. Agricultural Handbook No. 156, Agricultural Research Service and St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, Minn., April 1959.

*** Âü°í¹®Çå[References] ***

USACE, Hydraulic Design Criteria, SHEET 722-1 TO 722-3, STONE DRAIN OUTLETS, FIXED ENERGY DISSIPATORS
D50 = D*(V/(g*D)^(1/2))^3
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