Improved Surface Drainage of Pavements Final Report (1998) / Chapter Skim
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Appendix C Determination of Manning's n
Appendix C Determination of Manning's n
Pages 183-202
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From page 183... ...
Values for M~nning's n must be determined experimentally by measuring water flow depths under either natural or artificial rainfall. During the course of this study, the hydraulic resistance of three different types of pavement surface were determined: Portland cement concrete Porous asphalt Dense-graded asphalt concrete A great deal of experimental data that could be used to determine Manning's n values was available in the literature, and these data were used during this project to verify values of Mann~ng's n established by previous researchers (29-31, 36, 399.
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From page 184... ...
Variables that were not statistically significant were eliminated during the regression analyses. Separate regression equations for y were established for each of the pavement surfaces.
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From page 185... ...
S = Drainage surface slope (minim) Since MaTm~g's n is surface specific, the regression analyses must be considered separately for each of the surfaces of interest, in this case Portland cement concrete, dense graded asphalt concrete, and porous asphalt.
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From page 186... ...
. A total of 141 observations were eliminated from the data set based on this criterion.
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From page 187... ...
Elimination of these 141 data points ensured fully rough conditions and agreement with the Thematic wave assumption. The second criterion that must be satisfied for Me kinematic wave equation to be valid is that the energy gradeline slope, Sf be approximately equal to the surface slope, S
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From page 188... ...
the relationship between the flow depth and the experimental variables. The flow depth for the 1, 124 valid data points was regressed versus Me flow rate, q, Me mean texture depth, MID, and the surface slope, S
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From page 189... ...
In equation C-2, S is raised to a very small power and, according to equation C-2, appears to not be very important In determining the Manning's n value over a range of Wpical drainage slope values. Therefore, the smallest and largest slopes from the data set, 0.005 and 0.025 respectively, were used to calculate an average value of S00Q5, 0.90.
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From page 190... ...
Mann~ng's n for Reynold's Numbers Less Than 500 Equation C-9 underestimates Manning's n as the Reynold's number approaches 1,000, as shown in figure Cal. This was a major reason for the experimentation that was done on Portland cement concrete surfaces dunug this study.
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From page 191... ...
~ ~ 01 10 100 1000 Reynold's Number, NR Figure C-1. Mann~ng's n on PCC surfaces for Reynold's numbers less than 1,000 (Eq.C-9)
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From page 192... ...
The first step was to reduce the data set so that it contained only Reynold's numbers less than 500. A total of 1,070 of I, 124 observations had Reynold's numbers less than 500 and were used In the subsequent analysis.
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From page 193... ...
1 - ~ I o.1 ce 0.01 _ ~ _ 1 1 1 !
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From page 194... ...
The entire data set was first reduced to a data set of 940 observations with Reynold's numbers less than 240. A regression equation for hydraulic flow depth was completed and, with systematic substitution and elimination of variables, produced n= 0.388 N0.s3s win an R2 of 0.887.
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From page 196... ...
Since data with higher Reynold's numbers were eliminated, an increased emphasis on lower Reynold's numbers was Inevitable. Data with lower Reynold's numbers or hydraulic flow depths were prone to have more error due to the precision of the point gauge.
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From page 197... ...
o o - So ~ me o l 1 ~I 1 ° 1 1 1 ~ 1 1 o - -I · l so o l o o K/ / 1 7 o o l o 8 0 0 o so ooo O ~ ~ Z ,_ O ~ v, ~ Z O lo: lo 1^ o o o . ~ ~ 0 o ~ 0 0 c 0 o A lo lo U S,8UlUUB V' g lo Figure Cat.
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From page 198... ...
MANNING'S N FOR POROUS PAVEMENTS First, the criterion for fully rough flow was tested as described earlier, and of the 1,495 data points, only two sets did not satisfy this criterion. It was observed that the fully rough conditions did not prevail at high depth values with a tow rainfall intensity.
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From page 199... ...
These data sets included low depths on shallow slopes with a high rainfall intensity, possibly reflecting the difficulty In measuring the smaller depths. Upon examination of the data, a total of 49 data sets were eliminated from Me total of i,493 data sets leaving a total of 1,4421 data sets remaining for further analysis.
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From page 200... ...
With the coefficient of the air void content appearing to render the parameter negligible, a subsequent regression was performed and showed: y = 0.~995 MTDO-~29 ~ ~0345 S-O-~1 with an R2 _ 0.797 A subsequent regression of the data showed: y = 0.2080 (I L) 0.346 s-0.~6 with an R2 0.790, a negligible decrease in the correlation.
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From page 201... ...
The slopes ranged from 0.5 percent to 4 percent, with rain rates applied up to 150 mm/in and drainage lengths up to 7.3 m. A regression analysis of the data was performed and combined with the kinematic wave approximation, as described earlier for the PCC and porous pavement surfaces.
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From page 202... ...
Manning's n for porous asphalt surfaces.
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