Improved Surface Drainage of Pavements Final Report (1998) / Chapter Skim
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Chapter 3 Selection and Development of Models for PAVDRN
Chapter 3 Selection and Development of Models for PAVDRN
Pages 45-68
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From page 45... ...
A more comprehensive discussion of surface flow models and the development of values for Manmng's n can be found In Appendices B and C and the references cited therein. WATER lILM THICKNESS MODEL Before discussing the water film thickness model, the terms water fihn thickness (WFT)
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From page 46... ...
The WFT is the thickness of Me water fihn above the tops of the asperities on the pavement surface. The total depth of flow, y, is the thickness of the WFT plus the MTD.
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From page 47... ...
Gallaway's equation is based on an extensive set of water depth data for a variety of pavement types. The equation, however, does not contain a variable, such as M~nning's n, to describe the hydraulic resistance of the pavement surface.
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From page 48... ...
h h (6, where u, h, i, and f are the same as described In equation 5 and g - Acceleration due to gravity (32.2 ft/s 2 or 9.~1 m/s 2) Sax Sex vr Slope of the flow path In the x-direction Slope of the energy grade Ime In the Erection Terminal rainfall velocity Ox - Angle of rainfall input with respect to the x-axis The last term on the right-hand side of equation 6 represents the momentum due to the angle of incidence of rainfall velocity In the Indirection.
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From page 49... ...
(1 It = 305 mm) a = Friction loss coefficient m = Friction loss exponent u = Velocity of flow (ft/s)
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From page 50... ...
Stability and convergence are relatively easy to obtain with sets of linear equations, but difficult to acquire win ache non linear sets of equations implicit in the Zhang and Cundy model. The computational stability and convergence for nonlinear sets of equations are usually obtained by assuming linearity and using the linear solution as a conservative first approximation of the nonlinear case.
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From page 51... ...
For example, PAVDRN analyzes the topography of a section and determines the longest flow path length before applying the kinematic wave equation to determine water film thickness at points along the flow path. The longest flow path is determined from geometric conditions as the path from the point where Me water falls on the pavement to the pout where it exits the surface of the pavement.
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From page 52... ...
Subsurface Flow Mode} For porous asphalt surfaces, flow within the porous asphalt layer parallel to the pavement surface must be considered. Two options were explored for the subsurface flow model.
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From page 53... ...
Water film thickness versus distance along flow path for several pavement surfaces as calculated using PAVDRN.
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From page 54... ...
Therefore, the one-~nnensional surface flow model, which was modified as described in the following, was chosen to compute water film thicknesses on porous pavements. For dete~n~ng water film thickness on porous pavement sections it is necessary to modify equation 10 to account for the infiltration rate.
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From page 55... ...
Portland cement concrete · Porous asphalt . Dense-graded asphalt concrete 55
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From page 56... ...
However, additional experimental data were needed in order to extend values of Manning's n to rough Portland cement concrete surfaces and to porous asphalt surfaces. Manning's n must be determined through laboratory or field experimentation during which the water film thickness is observed as a function of rainfall intensity on different surfaces.
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From page 57... ...
flow. Flow is considered turbulent because of the impact of raindrops on relatively thin Dims of water flowing over pavement surfaces (34J.
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From page 58... ...
3. Porous asphalt concrete: 1.490 S 0 306 N 0.424 (19)
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From page 59... ...
HYDROPLANING SPEED MODEL The hydroplaning model selected for the study is based upon the work of Gallaway and his colleagues (4) and as further developed by others (37, 38)
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From page 60... ...
Manning's n versus length of flow path for various rainfall rates, Portland cement concrete, 500 < NR < 19000.
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From page 61... ...
Mannir g's n versus length of flow path for various rainfall rates, Portland cement concrete, NR < 500 61
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From page 62... ...
Rainfall intensity = ~ mm~h Rainfall intensity = JO mm/in Rainfall intensity = 20 mm/in Rainfall intensity = 40 mm/in Rainfall intensity = 60 mm/in ~ ~ ~ ~ ~a c I ~ _ _ __ ~ ~ 1 0 30 50 70 90 1 1 0 Length of Towpath, m Figure ~ ~ . Mar mr~g's n versus length of flow path for various rainfall rates, dense-graded asphalt concrete.
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From page 63... ...
Manning's n versus length of flow path for various rainfall rates, porous asphalt concrete.
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From page 64... ...
MID 0.14 (24) The model predicts the onset of hydroplaning on the basis of the water film thickness where water hen thickness is the thickness of the wafer film above the mean tenure depth, as presented In figure 1.
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From page 65... ...
Hydroplaning speed versus water film thickness.
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From page 66... ...
Rainfall Intensity The AASHTO highway design guides Include an equation for relating rainfall intensity, vehicle speed, and maximum allowable sight distance as follows: i - [80,000/(SV-Vi)
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From page 67... ...
As Intensity increases, sight distance decreases. Likewise, vehicle velocity decreases with increased intensity and decreasing sight distance.
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From page 68... ...
- -- - V ehicIe speed = 60 knn/h - -- V chicle speed = 80 krr`/h V chicle speed = 100 k m/h Vehicle speed = 120 krn/h -- -- Vehicle speed =160 k m/h 80 s ~ 60 ~n 0 ce ._ ce 40 20 100 200 300 Sight distance Sv, m 400 500 Figure 14. Rainfall ~ntemit~r versus sight distance for various vehicle speeds.
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