1、DEPARTMENT OF THE ARMY ETL 1110-2-120 Office of the Chief of Engineers ENGCW-EH Washington, D.C. 20314 Engineer Technical Letter No. 1110-2-120 14 May 1971 ENGINEERING AND DESIGN Additional Guidance for Riprap Channel Protection 1. The purpose of this ETL is to discuss detailed factors which must be
2、 considered in the application of criteria presented in paragraph 14 of EM 1110-2-1601, “Hydraulic Design of Flood Control Channels“, to the design of graded stone riprap channel protection, present additional hydraulic design guidance, and suggest a coordinated office design procedure which should
3、be adopted. Purpose. * 2. Applicability. This letter is applicable to all divisions and districts having the responsibility for the design and construction of Civil Works projects. 3. Discussion. a. Riprap Physical Characteristics - Design Phase. The stone shape *(paragraph 14c), gradation limits (p
4、aragraph 14h) and layer - thickness (paragraph 14i) are riprap material parameters for which equations (33) and (34), using 0.040 for coefficient “avf indicate the ability of the riprap to resist boundary shear. sufficiently broad to encompass the usual riprap material produced from many acceptable
5、quarries. A grizzly or similar type processing operation may be required to obtain acceptable riprap material from some quarries. Riprap material which fails to meet the criteria for gradation limits or stone shape is not capable of resisting the same amount of shear as indicated by the value of 0.0
6、4 for “a“ in equation (33). an adjustment of a for such material, sufficient samples should be tested during the design stage to determine its strength, gradation, and stone-shape limits within which material can be produced without undue waste. layer thicknesses to establish the layer thickness req
7、uired to produce an acceptable protective blanket, giving consideration to the following: The criteria are As sufficient information is not available on which to base Sample test installations should be made in varying (1) All stones should be contained reasonably well within the layer thickness. (2
8、) Surface irregularities should be minimal. *Subparagraphs and equations indicated in the text, unless otherwise noted, are found in paragraph 14, EM 1110-2-1601. 1 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ETL 1110-2-120 14 May 1971 (3) If poc
9、kets of smaller stones are present, the strength of the riprap material in these pockets may be determined by using the spherical diameter of the average size stones in equation (33). (4) A gradation which is deficient in smaller stones will have an excessively irregular surface and openings may tra
10、verse the layer from top to bottom. coarser bedding material which would not pass through the voids should be used. using a stone size diameter somewhat less than the average size, based on judgment, in equation (33). In this case, an increase in layer thickness and The strength of the riprap materi
11、al may be determined by (5) Careful examination should be made of the in place position of elongated and/or slabby stones which fail to meet the shape criteria. If these stones are well keyed rather than lying loosely on the surface and the provisions of subparagraphs (1) and (2) above are satisfact
12、ory, the material should be acceptable. Elongated and/or slabby stones which lie on the surface should not be considered as contributing to riprap strength. (6) Other reasonable assumptions should be made based on experience 1 and judgment. correlated with previously tested and accepted materials fr
13、om similar type quarries, it may be possible to establish reasonable design strength and specification requirements for the material. In summary, the discussion in this paragraph emphasizes the need to supplement the guidance given in EM 1110-2-1601 with the considerations, outlined ahove, in order
14、to specify riprap shape factors, gradation limits and layer thickness, as well as to determine riprap strength. This discussion also applies to the use of “quarry run“ material for riprap protect ion. For example, if riprap material from a quarry can be b. Design Aids. Inclosures 1 through 3 are tab
15、les showing riprap gradation limits for various specific weights (SSD), layer thicknesses and design factors as follows: (i) Inclosure 1 corresponds to design factors indicated in paragraphs 14h and i, except 14i(3). (2) Inclosure 2 corresponds to the design factors indicated in subparagraph 14i (3)
16、. (3) Inclosure 3 duplicates inclosure 2 except that the W50 maxi- mum theoretical stone diameter is one-half the increased layer thickness and the W1s maximum limit is increased correspondingly. Riprap material corresponding to these gradation limits should be con- sidered when placed in the dry at
17、 locations subject to turbulent flow 2 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3535789 O023604 425 THIS CONSTITUTES C-GE 1-TO ETL 1110-2-120,. 14 May 1971 ETL 1110-2-120 14 May 1971 r. i caused by energy dissipators, bridge piers and abutment
18、s, etc. This table modifies instructions contained in USWES, HDC, SHEET 712-1, Rev. 9-70. The tables show stone weights to the nearest pound which may be rounded for design and specification purposes. c. Riprap Physical Characteristics - Construction Stage. During construction, the riprap material s
19、hould be inspected using paragraph 14m for guidance. sample testing by the Contractor. Payment for testing should be made only for those samples which meet the specification requirements. In spite of extreme care in the design of riprap protection, the Con- tractor may experience difficulty in meeti
20、ng the specifications for gradation limits and/or stone shape due to particular quarry and/or production problems. negotiate a change in the riprap specifications a careful review of the specifications, considering the additional guidance given in paragraph 3a above, should be made by District desig
21、n personnel. The specifications should include provisions for If the Contracting Officer should desire to d. Additional Hydraulic Design Guidance. The following additional guidance, which supplements the guidance presented in EM 1110-2-1601 is applicable to the hydraulic design of riprap channel pro
22、tection: (i) Initially water surface computations involving the determina- tion of design flow depths and velocities should be made on the basis of friction coefficients which correspond to a trial riprap ipstallation. Appenpix IV of EM 1110-2-1601 and USWES, HDC, SHEETS 631-4 anpl 631-4/1 descqibe
23、procedures and provide design aids for determining composite channel roughness coefficients. i2) As roughness coefficients cannot be determined precisely, some hat low rather than high roughness values will produce conservative riprdp protection. Accordingly, the effective or equivalent riprap rougH
24、ness, k, corresponding to the minimum DS0 stone size should be used in Hydraulic Design Chart 631-1 to determine roughness coefficients for use in water surface computations. Similarly, expansion, contraction, bend and other losses which are included in the water surface computations should be estim
25、ated conservatively low. (3) Next, the design flow depths and velocities resulting from the trial riprap installation should be used in equation (32) to determine local boundary shear values. on the maximum limit D50 stone size to obtain conservative local boundary shear values for design. When velo
26、cities near the riprap boundaries are available from model tests, prototype measurements or other means, the following equation 8-13 from Chows “Open Channel The boundary roughness should be based 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-m 3
27、535789 0023605 3hL m -. - - . -. . . . . _. . I -. a ETL 1110-2- 120 1 Change 1 26 May _- 1971 _. . - _ .- .: .* -.- Hydraulics“, 1959 edition, may be used to determine the local boundary shear: I* .I V 7 - . 32.6 Loglo 30 Y . * O k -_ - - - - - . .- - . _ - -I I_ - -_i. - . . where: * - local bound
28、ary shear, lbs per sq ft TO y - sp wt of water lbs per CU ft v = local velocity, fps y = depth above boundary corresponding to v k = equivalent roughness, ft Inclosure 4 is a graphical aid for solution of this equation. (4) Equation (32) is based on the assumptions of fully rough flow conditions and
29、 normal logarithmic vertical velocity distribution produced by uniform channel flow. Fully rough flow conditions, in the range indicated on Hydraulic Design Chart 631, normally occur in channels which require riprap protection, but significant deviations from the normal logarithmic vertical velocity
30、 distribution occur in channels which have non-uniform cross sections, varying slopes and different bed and bank roughness coefficients. Thus, unless a uniform channel cross section with identical bed and bank riprap material occurs on a constant slope over a sufficient distance to produce uniform c
31、hannel flow at normal depth and velocity, maximum local boundary shear values will be greater than indicated by equation (32), due to greater localized velocities and pressure pulsations. As the effects of contributing factors to deviations from normal logarithmic vertical velocity distribution have
32、 not been established, values of local boundary shear computed from equation (32) should be increased by a factor of 1.5, except when flow is at or near normal depth in a channel with uniform cross section and equal bed and side roughness. The local boundary shear should be increased additionally in
33、 bends as indicated in EM 1110-2-1601, Plate 34. (5) Finally, the amount of shear that the trial riprap protection can withstand, called the riprap design shear, should be determined by equations (33) and (34) for the minimum DS0 size stone. boundary shear is within a reasonable range of being equal
34、 to this design shear, the trial riprap stone sizes are satisfactory for design. However, if the local boundary shear is appreciably greater or less If the local O 4 I I 1117 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3535789 O023606 2T8 ETL 111
35、0-2-120 14 May 1971 than the design shear, then the computations should be repeated for a second trial riprap installation, with the stone sizes being greater or smaller, respectively, than the sizes used in the first trial computations. local boundary and design shears are reasonably equal and furt
36、her refinements would not result in a significant change in stone sizes and riprap layer thickness. correspond to 3-inch modules in riprap layer thickness, such as 12, 15, 18, 21, etc. inches. The trial-and-error computations should be repeated until Trial-and-error stone sizes should 4. Suggested D
37、istrict Office Procedure, riprap protection, submitted in accordance with EM 1110-2-1601 guidance, has in some instances indicated that proper distribution of the design effort between engineering disciplines was not accomplished. general, the following procedure should be adopted: OCE review of the
38、 design of In (1) Hydraulic design personnel should make sufficient preliminary hydraulic computations and analyses to determine areas which may require riprap protection and establish preliminary riprap material requirements. (2) Foundation and materials personnel should assess the physical charact
39、eristics of available material, as discussed in subparagraph 3a above. (3) the available material for use in providing the required riprap protection. Hydraulic design personnel should check the adequacy of (4) Finally, a coordinated review of the riprap design should be made by both hydraulic desig
40、n and foundation and materials personnel and modifications accomplished in the riprap design where indicated. 5. 1 on 2.5 side slopes for a design discharge of 7,500 cfs is to be provided in natural sand and gravel materials having an estimated effective roughness range of 0.5 to 0.8 ft. As a first
41、trial, it is assumed that the sides only will be riprapped and the following rip- rap material (Sp. wt. = 165 lbs per CU ft SSD) will be placed in an 18 inch layer. Design Example. A trapezoidal channel with 50 ft bottom width and PERCENT LIGHTER LIMITS OF STONE BY WEIGHT (SSD) WEIGHT-LBS 100 290 -
42、120 50 90 - 60 15 so - 20 S I118 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3515789 0021b07 134 ETL 1110-2-120 14 May 1971 From EM 1110-2-1601, Plate 30, the il50 size of the trial riprap material ranges from 0.88 to 1.0 ft. Friction coefficient
43、s for use in the water surface computations should be determined for the range of expected design depths by the procedures described in EM 1110-2- 1601, Appendix IV. for the determination of the friction coefficient based on boundary roughness for a 10 ft depth. computations, based on the determined
44、 friction coefficients and conservative estimates of other losses, result in a depth of 10 ft for the design discharge of 7,500 cfs, the corresponding average velocity in the channel section is 10 fps. smoother than the riprapped sides, the average velocity in that part of the channel section vertic
45、ally above the bed, which will be greater than the average velocity in the entire section, should be used in determining the local boundary shear at the toe of the riprap revetment. is 11.5 fps. size equal to 1.0 ft and applying the 1.5 non-uniform flow factor, the local boundary shear at the toe of
46、 the riprap revetment is com- puted as follows: Inclosure 5 illustrates the computational procedure Assuming that the water surface As the channel bed is The computations in Inclosure 5 indicate that this velocity From equation (32) and Plate 32 for the maximum DS0 stone = 10/1 = 10 and K2 = 0.0135
47、yD50 T = 1.5 x 0.0135 x (11.5)2 = 2.68 lbs per sq ft O The design shear, using the minimum D50 stone size = 0.88 ft, for the trial riprap gradation, as determined from Plates 35 and 36, is: * 7 = 3.60 lbs for sq ft and Ky = 0.82 7 = 0.82 x 3.60 = 2.95 lbs per sq ft As the design shear exceeds the lo
48、cal boundary shear by a significant amount, the possibility of using a 15-inch riprap layer thickness should be checked. EM 1110-2-1601, the 15-inch layer thickness would require the following riprap gradat ion : In accordance with paragraphs 14h and i of PERCENT LIGHTER LIMITS OF STONE BY WEIGHT (S
49、SD) WEIGHT - LBS 100 50 15 170 - 70 so - 35 25 - 10 For this gradation, the minimum D50 stone size is 0.74 ft and the corresponding design shear is 0.82 x 3.0 = 2.46 lbs per sq ft. value is less than the local boundary shear determined for the first trial riprap gradation and the smaller size stone in the 15-inch layer As this -_ I tfi, 4 6 .- Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3535789 00
