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CHAPTER 2
LITERATURE REVIEW AND CURRENT PRACTICE AND
TECHNIQUES FOR IMPROVED SURFACE DRAINAGE
Water films develop on the pavement surface during natural rainfall and tend to
increase in thickness along the water drainage or flow path. At the onset of rainfall, the water
first occupies the macrotexture on He pavement surface and is contained within the
macrotexture of the pavement surface or is drained from Be surface through grooves or
interns drainage (porous asphalt surfaces). With increasing rainfall, a film of water forms
above He macrotexture. The flow of water on the pavement surface under these conditions is
referred to as sheet flow, the depth of the sheet flow tends to increase in the direction of the
drainage path. The kept of the sheet flow is of critical importance because the depth of this
flow controls He skid resistance of He pavement and the tendency for hydroplaning. The
vehicle speed at which hydroplaning occurs is inversely proportional to the depth of the sheet
flow.
The pavement design engineer must be able to identify any points on the pavement
where sheet flow is sufficient to cause hydroplaning and must provide alternative or
complementary strategies for reducing the kept of the water film Sickness. The models
identified during this study provide He tools needed to calculate the depth of sheet flow as a
function of four general pavement characteristics: pavement geometry, location and capacity of
drainage appurtenances, surface texture of the pavement surface, and any internal drainage
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offered by open-graded asphalt concrete (OGAC) surfaces or grooved Portland cement
concrete pavements. The term open-graded asphalt concrete is used in this study to indicate
either open-graded asphalt friction courses (OGAFC) or porous asphalt. Both types of mixes
provide internal drainage; OGAFC is typical of U.S. practice, porous asphalt is typical of
European practice. By varying any one or any combination of these characteristics, the
Pavement design engineer can predict the effect of the characteristics on the water film
thickness and, in turn, the propensity for hydroplaning. As part of this study, an interactive
computer program, PAVDRN, was developed to predict water film thickness and tile potential
for hydroplaning. The program is described in Appendix A.
SI)MMARY OF MODELS NEEDED TO DEVELOP GUIDELINES
The one~imension~, steady-state, kinematic wave equation was selected for
calculating water film thickness in He computer-based design program, PAVDRN. The
selection of a one-dimension~ flow equation was based on computations stability and
efficiency. The major advantage of the one~imensional, kinematic wave mode} is that it is
easy to apply and is computationally stable. A full description of this mode} is given in
Chapter 3, where the development and rationale for choosing the various models used within
PAVDRN are discussed.
A number of other models were needed to develop PAVDRN. These include models
for:
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.
.
.
Predicting the flow through porous pavement surface layers: For this purpose, the
water film thickness mode] for impervious surfaces was modified to account for
internal flow.
Relating sight distance and vehicle speed to rainfall intensity i: a model from the
AASHTO design guide (~) was selected for this purpose.
· Predicting hydroplaning speed (HPS): A model first proposed by GalIaway (3) was
used for this purpose. The HPS is a function of water film thickness and pavement
macrotexture (MTD).
Determining the hydraulic roughness coefficient, Manning's n: This is an empirical
parameter (see equation 18) that depends on the type of surface and the Reynold's
number, NR. The Reynold's number is a dimensionless parameter Hat is used to
identify flow as laminar or turbulent (see equation 20~. Relationships were developed
on the basis of data in He literature and new data collected as part of this study for
three cases; Portland cement concrete pavements, dense-graded asphalt surfaces, and
open-graded asphalt concrete surfaces.
A full description of the rationale used in selecting these models and in their development is
given in Chapter 3 and in Appendices B Trough D.
METHODS FOR CONTROLLING WATER FILM THICKNESS
A literature survey and a questionnaire were used to establish He current state~f-the
art methods for pavement surface drainage. Implementable techniques for improving surface
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drainage that resulted from He literature survey and from a questionnaire sent to 72 highway
agencies can be grouped into four broad categories:
· Optimization of geometric design parameters such as cross-sIope;
· Reduction of the distance that the water must flow (flow path) by installing
drainage appurtenances;
Use of internally draining (asphalt concrete) wearing course mixtures;
Use of grooving (per hard cement concrete); and
· Maximization of surface texture.
Controlling Water Film Thickness Through Pavement Geometry
Highway geometric design criteria have evolved over many years and are designed to
ensure He safe and efficient movement of vehicles. State agencies and many other
transportation agencies use the guidelines issued by the American Association of Highway and
Transportation Officials (AASHTO) (l) for geometric design. The drainage capacity of a
highway surface is determined primarily by its surface geometry, especially cross-sIope.
Geometric design criteria that enhance drainage are often in conflict with the design criteria
for safety and driver comfort. Thus, although changes in the criteria contained in current
geometric design guidelines may be desirable from the standpoint of improved drainage, there
is little possibility that such changes will be effected solely for the sake of enhanced drainage.
Geometric design criteria are presented in detail in He AASHTO design guidelines (~) but are
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reviewed briefly here to illustrate geometric criteria that control surface drainage, but that
must be satisfied during the pavement design process.
The longitudinal slope of He pavement is referred to as its "grade." Criteria for both
minimum and maximum grades are necessary for proper geometric design. Minimum
allowable grades are necessary for drainage concerns, while maximum allowable grades must
be specified for safety reasons and to control traffic flow. The longitudinal slope of the
pavement and its surrounding gutters and ditches is usually the same within each section of
highway. Therefore, this discussion covers all Free areas of the pavement system.
Maximum grades have been established based on vehicle operating characteristics,
particularly, the operating performance of larger vehicles such as tractor semitrailers. Steep
grades can be difficult to descend and vehicles often reduce speed when ascending excessively
steep grades. In general accordance with the AASHTO policy (l), maximum grades are
determined by the functional class and design speed of the roadway and He surrounding
topography. Typical maximum longitudinal grades are shown in table 1. The development of
many of He models that were reported in the literature also was performed using regression in
English units. The origins form of the models is retained throughout this report to maintain
the integrity of the original analyses. Where English units occur, conversions between English
and System International (SI) units are given in the text.
Minimum grades are required to ensure adequate drainage. This is important for
curbed roadways since water cannot drain laterally from a roadway when curbs are present.
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Table I. Maximum recommended grades All.
Design Speed mi/h (km/h)
Design Section
30 (48) 40 (64) 50 (80) 60 (96) 70 (~12)
Rural Sections,
Maximum Grade, %
Level -- 5 4 3 3
Rolling -- 6 5 4 4
Mountains -- 8 7 6 5
Urban Sections,
Maximum Grade, %
Level ~7 6 5
Rolling 9 8 7 6
Mountains ~ ~10 9 ~
The minimum grade recommended is 0.5 percent, but, if this cannot be obtained, 0.3 percent
can be used as long as no curbs are present, and the roadway is crowned properly (]J.
Vertical curves connect segments of constant grade. Since these curves often represent
a change between a positive and negative grade, a level section exists at the transitions
between positive and negative grades. AASHTO policy (]J is to design vertical curves using
a ~K-value." The K-value is defined as Me horizontal distance in feet (meters) required to
effect a I-percent change in the gradient of Me grade. To limit drainage problems, according
to AASHTO All, K-values used in design should be less than or equal to 167 ft (51 m) for both
crest and sag vertical curves. Basically, a K-value of 167 It (51 m) states that a 0.3-percent
grade is the minimum grade allowed win 50 It (15 m) of Me level pavement on a vertical
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curve. If K-values greater than 167 ft (51 m) are used, special attention should be given to the
selection of pavement geometry to ensure adequate drainage. This is critical in sag vertical
curves since water tends to collect at the bottom of these curves.
Pavement cross-slopes (transverse) are a compromise between drainage (steep slopes)
and driver comfort and safety (flat slopes). Cross-slopes may be formed in a number of ways,
as shown in figure 4. In this figure, section 2 removes the water from the roadway faster than
section 1, but more inlets are needed to collect He water at the edge of He pavement. These
sections are recommended if freeze-thaw is common. Drainage can be directed in two ways,
sloping toward the median or sloping toward the shoulder. If a highway slopes toward He
median, more inlets will be needed, but less water will be in the outer travel lane, while more
water will be in He inner, high-speed lane. Figure 4 shows a variety of drainage
configurations including drains located within the traveled way. These are discussed in more
detail in Chapter 2. This shows that cross-slopes are a compromise between many factors, and
each has to be given serious consideration.
As with longitudinal pavement slopes, a maximum and minimum superelevation is
suggested by AASHTO (IJ. Research by Gallaway et al. (4) has shown that superelevations of
two percent have lime effect on driver comfort or vehicle stability. The maximum transverse
slope recommended by AASHTO is two percent per successive lane. The maximum slope
permissible, as recommended by AASHTO (1), is four percent. Typical cross-slopes for
various pavement types are presented in table 2.
17
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J
o
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o
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co
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Figure 4. Different lateral drainage configurations with and without lateral drains.
18
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Table 2. Typical cross-sIopes for different
pavement surfaces Ail.
Pavement Type
Cross-slope
-
High
Intermediate
Low
1.5-2
1.5-3
2-6
On longitudinal curved sections of highways, the pavement is typically superelevated.
A limit is placed on the rate of superelevation for driver comfort and safety. If the
superelevation is too high and speeds are too low, drivers will need to steer up the slope.
Also, vehicles can slide toward the inside of Me curve if ice is present on pavements with high
superelevations.
For reasons given above, the absolute maximum superelevation is 12 percent. Other
maximum cross-slopes exist and are applied depending on the situation (]J. If ice anti snow
are common, a maximum superelevation of eight percent is used. When heavy traffic volumes
and low speeds prevail, a maximum slope of six percent is used. In urban areas, when speeds
are low, curves can be designed without superelevation. A summary of maximum allowable
values for superelevation rates is presented in table 3.
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Table 3. Maximum allowable superelevation (1J.
Situation
Slope (%)
Absolute Maximum
Ice and Snow Uncommon
Ice and Snow Common
Urban, Low Speed
12
10
8
6
In order to obtain full superelevation on the curve from a tangent section of roadway,
the n 1/3 rule" is commonly applied. This rule states that 2/3 of the superelevation should be
obtained before the beginning of the horizontal curve. This transition distance is called the
length of ~runoff." This runoff length can be obtained from most highway design manuals and
is a function of design speed, highway curvature, and lane width. Transition is an important
element in drainage design: When moving from a normally crowned pavement to a
superelevated pavement, the pavement surface is usually rotated about the center line of the
highway. This causes a section of Me pavement to be level. Consequently, when considering
pavement surface drainage, special attention should be paid to these transition areas.
After Me water has drained from the traveled lanes, the shoulders or parking lanes
must either convey the water to an inlet or drain the water to ditches. Shoulders are typically
used on rural roadways, while parking lanes and gutters are used in urban areas.
Consideration of curbs, gutters, and other drainage appurtenances is beyond Me scope of this
project; Hey are examined elsewhere (5,69.
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Based on this research project, the authors recommend that AASHTO review its current
policy on the geometric design of highways arm streets to consider establishing minimum
cross-slope recor'?mendatior~s for highway pavements (1). The results of this study show that
as the longitudinal slope or grade increases, the cross-slope of a pavement section should also
be increased in order to remove water more rapidly from the pavement. This effectively
shortens the distance a droplet of water must travel to reach He nearest appurtenance of a
pavement edge (maximum flow path length; see figure 5), a critical design parameter for
pavement drainage.
In summary, the use of geometry to reduce water film thickness on pavements is
constrained by the need to ensure driver comfort and vehicle stability. This effectively limits
the maximum cross-slopes that can be used to remove water from the pavement, and thus other
methods are required to enhance drainage and reduce He depth of water on the pavement.
Most importantly, even though pavement geometry is an important factor in determining water
film thicknesses, it alone may not correct drainage situations that lead to the potential for
hydroplaning. Consequently, other means of drainage and water film Sickness control are
needed as described in the following.
Controlling Water Film Thickness Through Use of Appurtenances
Drainage appurtenances are a very effective means for removing water and shortening
the distance that water must flow in order to be removed from the pavement surface.
Shortened flow paws imply reduced water film thickness. Traditionally, flow from He
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The breakout point was computed by considering the equilibrium flow rate and the
capacity of the grooves, bow of which were a function of rainfall rate and down-sIope
distance. The equation for the breakout point is (311:
L, (1.50) / (s i ng)
(1)
where
Breakout distance measured from top edge of pavement (ft)(1 It - 3.05 m)
s = Groove spacing (in)(1 in = 25.4 mm)
i - Rainfall rate (in/h)(1 in/in = 25.4 mm/in)
n. Manning roughness coefficient for grooves
. g
The coefficient 1.50 is a function of groove geometry, and as given in equation 1 is for 6-mm-
by-6-mm (0.25in-by~.25-in) rectangular grooves with a pavement surface slope equal to 1.5
percent.
The results of data generated by Reed et al. t30) for grooved Portland cement concrete
pavements are summarized in figure 7. The figure was developed for a rainfall intensity of 75
mm/in (3 in/h). The breakout points are shown on the graph as the intersection of the curves
with the abscissa. The smaller the groove spacing, the greater is the distance to the breakout
point L. The Marlning roughness coefficient for the grooves, ns, was taken as 0.01 (31).
Grooving can reduce the water film Sickness on pavements by acting as drainage
channels and thereby carrying water from the pavement surface. However, unless grooves are
parallel to the slope of the pavement, their ability to conduct flow is reduced and their
effectiveness minimized. In summary, grooving Portland cement concrete pavements can
reduce water film thickness and thus increase the speed at which hydroplaning will occur.
This has been demonstrated for grooves whose principal orientation is in Me direction of the
flow paw of the water (309. The PAVDRN model, documented in Appendix A, uses
34
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at at
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Figure 7. Predicted water film thickness, WFr, for grooved portland cement concrete
pavement, rainfall iritensity 75 mm/in (309.
3s
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information about groove spacing, width, and depth to effectively increase the mean texture
depth of Me pavement and thus increase the speed at which hydroplaning occurs.
ControNing Water Film Thickness with Surface Texture
Another method for controlling water film thickness is by maximizing the texture of the
pavement surface. The water film thickness is the total thickness of the film of water on the
pavement minus the water trapped in the macrotexture of the pavement surface. Water film
thickness is reduced in direct proportion to the increase in macrotexture (total macrotexture
volume, not MID). The importance of macrotexture for asphalt surfaces is discussed in a
previous section on the use of porous asphalt to control water film thickness. Since porous
asphalt surfaces are typically prepared from relatively coarse aggregates or gradations with a
minimal quantity of sand-sized material, they generally yield large levels of macrotexture.
The macrotexture of other asphalt surfaces is also controlled by the gradation of the aggregate,
ranging from very low levels of macrotexture for sand asphalt to relatively large levels of
macrotexture for coarse-graded mixture and surface treatments. The importance of
macrotexture is recognized in French practice where microsurfacing techniques are now
widely used and have replaced porous asphalt in areas where the performance of porous
asphalt has been suspect (see also the section on porous asphalt as a method for controlling
water film thickness). Micros urfaces are Tin lifts of hot-mix asphalt concrete graded to
maximize surface texture.
Macrotexture is also important for Portland cement concrete surfaces. New Portland
cement concrete pavement surfaces in the United States are typically constructed with fined
surfaces to enhance macrotexture. Macrotexture produced by fining or brooming is to be
distinguished from grooving. The texture of Portland cement concrete pavement can be
enhanced by etching away the mortar exposing the coarse aggregate (new construction) or by
grinding (to restore texture in old pavements) although these techniques are not used often in
practice and often result in high levels of tire noise.
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The importance of texture is recognized in the reproposed Design Guidelines for
Improving Pavement Surface Drainage" (2) where the pavement texture is one of the design
options. The importance of macrotexture may not always be demonstrated with the standard
ASTM E 274, "Standard Test for Skid Resistance of Paved Surfaces Using a Full-Scale Tired
locked wheel tests because the test is not conducted on a flooded pavement. Instead, the water
introduced in front of the tire of the moving skid tester. For example, in full-scale tests
conducted at the Turner-Fairbanks Highway Research Center, no correlation was shown
between macrotexture and hydroplaning speed (32J.
PROPOSED DESIGN GUIDELINES FOR IMPROVING PAVEMENT
DRAINAGE- IMPLEMENTATION OF FIN1)INGS
The design guidelines were developed as a ~stand-alone" document for use by design
engineers in the design of new roadway systems or the rehabilitation of existing pavements (2).
The guidelines can be used by highway design engineers to evaluate the effect of different
pavement parameters on the water film thickness and the potential for hydroplaning. The
guidelines are complemented by an interactive computer program, PAVDRN, which allows
the pavement engineer to predict water film thickness and the propensity for hydroplaning
(Appendix A). The treatment of the different design parameters is reviewed briefly in this
section. The reader is referred to the Proposed Design Guidelines for Improving Pavement
Surface Drainage" (2) for details.
Pavement Geometry
Five different types of design sections are considered in the guidelines and in the
PAVDRN computer program. They include (~) tangent sections, (2) superelevated horizontal
curves, (3) transition sections, (4) vertical crest curves, and (5) vertical sag curves. Each of
these sections can be analyzed using the PAVDRN model. In the analysis, the pavement is
divided into successive sections or planes according to one of the five types of design section
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(see Appendix A, especially figure A-2). The flow from one type of section to another can be
linked in the analysis. Geometric information is required for each section in the analysis.
For tangent sections, the guidelines recommend that, as grade increases, pavement
cross-slope should also be increased up to maximum recommended values. The guidelines
also recommend other control methods such as slotted drains between traveled lanes. For
superelevated sections the guidelines suggest the use of a maximum recommended
superelevation to minimize water film thickness on horizontal curves and the use of other
methods, such as increased mean texture depth or grooving, if superelevation does not reduce
the potential for hydroplaning to desired levels.
In transition sections, the effects of changes in the pavement geometry on the flow path
length are fairly complex. The location of the maximum flow path length changes depending
upon the difference between the cross-slope at the curve end of the transition and the cross-
slope at the tangent end. Runout length also affects the location of the flow path and its
length. The runout length is the distance, measured from the start of the plane and in the
direction of the traveled way, to the point where the Towpath exits He plane. In general, the
guidelines recommend that the runout length be shortened as cross-slopes increase. However,
in transition sections concern for safety and driver comfort must be balanced. Other measures
to control water film thickness might need to be applied if the shortest recommended runout
length is used, and He potential for hydroplaning still exists.
Pavement Properties
There are two pavement-related factors that can be controlled by the designer to control
the water film thickness: (1) pavement type and (2) mean texture Kept. Four pavement types
are considered in the guidelines and the PAVDRN software. The four pavement types are: (1)
Portland cement concrete (PCC), (2) grooved PCC (GPCC), (3) dense-graded asphalt concrete
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(DGAC), and (4) OGAC, which includes both porous asphalt and open-graded asphalt friction
course (OGAFC).
The design information required to specify the design section varies with He pavement
type. In PAVDRN, the mean texture depth (MTD) is a function of several parameters that are
determined by the designer. For PCC surfaces, the water-to-cement ratio and the surface
finish (e.g. degree of lining) affect the MTD. Maximum aggregate size, gradation, and air
void content affect the texture of asphalt concrete mixes. OGAFC and porous asphalt surfaces
have larger macrotexture than dense-graded surfaces. Porous mixtures and high air-void
content mixtures both contribute to the mean texture depth and, in An, to a reduction in the
water film Sickness.
Grooving PCC pavements reduces water film thickness if the grooves are oriented so
that they conduct flow off the pavement. Otherwise, the effect of grooving is localized and
can lead to increased water film thickness on other parts of the pavement. The guidelines
provide specific recommendations win respect to groove size and spacing based upon an
analysis using PAVDRN and survey responses from highway engineers.
Drainage Appurtenances
The designers ability to reduce water film thickness on a highway pavement using
geometry and pavement properties is limited. Drainage appurtenances are typically necessary
to control water film thickness, especially on large, multilane facilities where the flow path
length spans more Man two travel lanes. The most promising technology for multilane
highways is Be use of slotted drains placed between the Gavel lanes. At least four state
transportation departments reported using slotted drains in this manner. Slotted drains can
also be placed transversely or across the traffic lane to capture flow. Drains used in either
manner reduce the water film thickness on a pavement by removing or reducing flow over the
pavement.
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PAVDRN SOFIA
PAVDRN is intended for use by highway design engineers to determine the likelihood
of hydroplaning on various highway pavement sections. It does this by computing the longest
flow path length over the design pavement section and determining the water film thickness
(depth of water above the asperities of the pavement surface) at points along the path. The
water film thickness is used to estimate the speed at which hydroplaning will occur (if at all)
along the longest flow path, the critical path in the section. The predicted hydroplaning speed
along this path is then compared to the design speed of the facility, a parameter selected by the
designer.
PAVDRN nuns under Windows 3. ~ and above. The user interface was programmed
in Visual Basic. The computational algorithms were programmed in FORTRAN 77. The user
interface uses point-and~lick technology with pull-down menus and context-sensitive help.
The user's guide is available on-line and is installed as part of the help files with the PAVDRN
program.
Since it is a one-dimension model' PAVDRN first analyzes the section geometry to
determine the maximum or longest flow path length over the pavement section. The program
determines water depth, time to equilibrium, and velocity at points along the longest flow path
length; equations for determining these values are presented in Chapter 3. The mean texture
depth is subtracted from He depth to determine the water film Sickness. The water film
thickness, computed in this manner, is used to determine the speed at which hydroplaning will
occur. Results are printed in a summary report format. They are also available as a text file
that can be imported to a third-party graphics program and plotted. A sample of the summary
output table is provided in table 5 based on the analysis of a tangent section with zero grade
and standard I.S-percent cross-slope.
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Table 5. PAVDRN summary output table.
X Y Di stance OFT
Flow/width Manning' I; n Reynold' s No. Hydr. Speed
(ft,m) (ft,m)(ft,m) (in,mm) (cfs/ft,cms/m)(mi/h)
150.0.0 .00 -.SOE+OO .00E+00 .000 O. 999999
150.01.0 1.00 .64E+00 .14E-04 .310 11. 109
150.02.0 2.00 .88E+00 .28E-04 .076 21. 100
150.03.0 3.00 .lOE+01 .42E-04 .061 32. 96*
150.04.0 4.00 .12E+01 .56E-04 .0S2 42. 93*
150.05.0 5.00 .13~+01 .69E-04 .046 53. 91*
* Indicates hydroplaning speed is; ~ es`; than design "peed .
Note: PAVDRN ha" the option of producing output in SI or English unit=. The data shown in the
table abjure are in U.S. unit".
Design Ex~nple Using SIoHed Drains
' This analysis considers a tangent section consisting of three lanes of Gavel in Me same
direction. The geometric input for the analysis of ache tangent section in this example is listed
in table 6.
Table 6. Tangent section properties.
Property
Value
No. of Planes
Length of Each Plane
Longitudinal Slope
Width of Each Plane
Pavement Type
Mean Texture Depth
Cross-Slope of Plane ~
Cross-SIope of Plane 2
Cross-Slope of Plane 3
3
300 m
0.02 m/m
4m
PCC
0.50 mm
0.015 m/m
0.025 m/m
0.035 m/m
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Additionally, a rainfall intensity of 80 mm/in is assumed, and a kinematic viscosity of the
water of 1.306 x 10 ~6m2/s (water temperature = 10 °C) was chosen. These values of intensity
and water temperature are conservative but might be observed in some locations in the United
States. A summary of the output of the model is shown In table 7.
Table 7. PAVDRN output for tangent section.
Water Film
End of Drainage Length Thickness Flow/Widd ~Hydroplaning
Plane (m) (mm) (m3/s/m) Speed (krn/h)
1 6.66 1.3 0.00013 90
2 11.79 1.5 0.00023 88
3 16.39 1.6 0.00032 86
The results In table 7 present the final value of the water fihn thickness at the end of the
longest drainage path length across each section of the pavement. In this example, since each
lane has a different cross-slope a plane consists of one lame of travel. At the end of the first
plane, the model has predicted that the flow length of water across the
innermost lane will be 6.66 mm, and the hydroplaning speed at that point is 90 km/in. The
lane is only 4 m wide, but the flow length will be along a distance that is the resultant of the
cross-slope and the longitudinal slope. Therefore, the drainage path length will be greater than
the 4-m width.
For a design speed of 90 kn'/h, the computed hydroplaning speed just meets the criteria
to prevent hydroplaning. However, as the drainage length increases across the second and
third lanes of travel, the water film thickness increases to a point where the hydroplaning speed
for the third, outermost lane of travel is significantly below the design speed of 90 1an/h. One
solution for increasing the hydroplaning speed could be to install a longitudinal slotted drain
between the second and third lanes of travel In the direction of travel (see figure 5~. This drain
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would intercept the flow from Me second lane, reduce the water film thickness at the end of the
second lane, and additionally reduce We water film thickness across the entire third lane of
travel. This would reduce the hydroplaning potential of the entire roadway system to be in
accordance with a design speed of 90 km/in.
From the analysis, ache flow at the end of the second plane needs to be reduced to a value
that will eliminate the hydroplaning potential for the system. A slotted vane grate is selected
and placed between the second and third lanes. Using a design chart provided by the
manufacturer to obtain a grate inlet coefficient, K = 39, the grate will capture (0.000516 m3/s
per meter of length of the slotted drain inlet) as determined by equation 2:
Q = K D5/3
where
Q
D
Flow rate (cfs/ft)
Depth of flow (ft)
( lcfs - 0.028 m3/s, 1 It = 305 mm)
At this location in the pavement, the flow is only 0.00023 m31sim, and therefore the
total flow will be captured.
43
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Representative terms from entire chapter:
film thickness
