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CHAPTER 1
INTRODUCTION AND RESEARCH APPROACH
INTRODUCTION
The primary objective of this research project was to identify improved methods for
draining rainwater from the surface of multi-lane pavements and to develop guidelines for their
implementation. Improved methods for drawing water from the surface of multi-lane
pavements are needed because of the import role that drainage plays ~ the mitigation of
hydroplaning and splash and spray. The tendency for hydroplaning and splash and spray
depends on the thickness of Me film of water on the pavement. Therefore, ~ order to
quantitatively evaluate the effectiveness of the different methods, a mode! for predicting the
depth of flow, or water film thickness, resulting from rainfall on multi-lane pavements was
developed and Incorporated Into computer-based design guidelines. In the process of
completing this study, a number of specific tasks were addressed. These included:
.
.
A literature review to establish the state of practice regarding analytical models
for predicting rainfall water depths and to establish current design practices for
removing rainfall runoff from multilane pavements.
A review of current design methods and analytical procedures for estimating
sheet flow across highway pavements.
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.
.
.
.
The development of improved models that describe the water film thickness
resulting from sheet flow on Impervious and pervious pavement surfaces.
Laboratory rainfall runoff data for determining the roughness coefficient
(Mann~ng's n) for pavement surfaces for which data was not available ~ ache
literature.
Skid resistance measurements to supplement current hydroplaning data in the
literature and to better quantify the onset of hydroplaning as a function of the
depth of the water film flowing over the pavement surface.
Desian procedures and criteria that can be used bv user agencies In the selection
of the most cost-effective means for controlling surface drainage and incorporate
the procedures and criteria into a computer program.
RESEARCH APPROACH
The need for improved drainage is prompted by the hydroplaning and excessive splash
and spray that can result when thick films of water develop on the pavement surface. The
tendency for hydroplaning and splash and spray are micronized if the thickness of the water
film on the pavement surface is minimized. Therefore, the mam focus of this study
concentrated on methods for predicting and controlling the flow and flow path length of rain
water flowing across the pavement surface.
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Water FiEn Thickness
Figure ~ provides a definition of Me water film thickness as it flows across the
pavement surface. The thickness of the water film that contributes to hydroplaning is the mean
texture depth (MTD) plus the thickness of the water film above the tops of the surface
asperities. The MTD depends on the macrotexture of the pavement surface. The macrotexture
is the texture or roughness of the pavement surface that is caused primarily by the coarse
aggregate. Techniques for measuring the macrotexture are described later in this report. The
water below the MTD is trapped in the surface and does not contribute to the drainage of the
pavement. Drainage or flow occurs in the total flow layer, y, which is the water film thickness
(WFT) plus the mean texture depth. Increasing the macrotexture or depth is important because
it allows a reservoir for water (depth below the MTD) and enhances drainage (depth above the
MTD).
The flow path for a particle of water falling on a pavement surface is simply defined as
the line determined by the slope along the pavement surface. Thus, the maximum flow path
for a pavement section is the longest flow path for the section-the maximum distance that a
rainfall droplet can flow between the point of contact with the water film and its point of exit
from the pavement, as presented in figure 2. For a given quantity of rainfall per unit area of
pavement, reducing the flow path will result in a more shallow depth of flow and a
concomitant reduction in the propensity for hydroplaning or excessive splash and spray.
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Figure 1. Definition of water film thickness, mean texture depth, and total flow.
4
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Note: The figures above show a plan view of a design plane (nght) and a
profile of the water film thickness (WFT) as water flows along flow path
(left). Water falling on the pavement first fills the macrotexture flower left,
here 1.27 mm deep) at which point it reaches tops of the asperities of the
coarse aggregate particles. At this point, the depth of the water film
increases (from zero) until the water exits either an edge of the pavement
plane (case here) or a drainage appurtenance.
A plane is defined as a section of pavement that has the same geometric
charactenstics. In the drainage mode! used in this study, the drainage
across the pavement is modeled by linking adjacent design planes.
Figure 2. Definition of flow path and design plane.
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In order to develop quantitative guidelines for increasing pavement surface drainage, it
was necessary to develop models that can predict the thickness of the water film flowing over
the pavement surface. This type of flow is called sheet flow. The aforementioned models,
which are an essential part of the guidelines, depend on values of Manning's n (hydraulic
roughness coefficient) for the pavement surface. This fact necessitated the measurements of
Manning's n for some selected surfaces for which data was not available In the literature.
Methods for Reducing Water Film Thickness
There are five techniques that can be used to reduce water film thickness: alteration of
surface geometry, installation of drainage appurtenances, use of permeable or porous asphalt
paving mixtures, grooving (portland cement concrete), and enhancement of surface texture
through mixture selection and design. Surface geometry factors, such as cross-slope and
superelevation, have traditionally been employed to remove water from the pavement surface.
However, pavement geometry must be designed in accordance with American Association of
Highway and Transportation Officials (AASHTO) design guidelines (1), limiting the degree to
which surface geometry can be used to ~ninimize water film thickness. Therefore, other
approaches, in addition to the modification of surface geometry, are needed.
Appurtenances, such as grate inlets and slotted drains, are a means for removing
surface water from the pavement. Permeable asphalt concrete pavements, such as open-graded
friction courses (OGAFC) used in the United States and porous asphalt as used in many parts
of Europe, are another means for reducing the flow of surface water across the pavement.
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These surfaces also provide a means for draining water from beneath the tire, thereby reducing
hydroplaning potential. Finally, texture modification, as typified by the recent developments
in the texturing of concrete pavements, and the grooving of asphalt and Portland cement
concrete (PCC) pavements also provide a means for reducing water film thickness.
Research Program
The research program that was followed during this study was designed to provide the
additional data needed to implement the methods that were identified for reducing water film
thickness, to provide models that predict the depth of sheet flow when these techniques are
used, and to provide guidelines so that the design engineer can facilitate their implementation.
An overview of the research program is illustrated in figure 3. The primary focus of the
research was placed on identifying the most promising techniques for predicting and
controlling water film thickness as a means for minimizing the potential for hydroplaning.
Limited attention was given during the research project to the mitigation of splash and spray.
Research Products
The two major products of this research were (~) a set of guidelines (2) Mat can be used
by highway design engineers to consider alternate methods for improved surface drainage and
(2) an interactive computer program (PAVDRN) for predicting the depth of sheet flow on
pavement surfaces. In order to develop the guidelines, it was necessary to develop a hydraulic
model for predicting the depth of the water flow on the pavement surface. This model is the
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Task 1.
Literature Review
1
Task 2.
State of Practice
1 ' ~
Task 4.
Interim Report
Task 5.
Evaluation and
Model Refinement
I I
Task 6.
Development
of Guidelines
~ 1
Environmental Factors
Pavement Geometry
Appurtenances
Material Properties
Models:
- Sheet flow
- Skid resistance
- Hydroplaning
Task 3.
Model Analysis
} 1 1
Figure 3. Research tasks.
8
Task 7.
Final Report
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basis of an interactive computer program that can be used by a design engineer in the process
of designing a highway pavement section. The model, which predicts the depth of sheet flow
resulting from rainfall, is based on pavement geometry, pavement surface type and texture, the
presence of appurtenances, and rainfall rate. By selecting maximum allowable water film
thicknesses that can be allowed without the onset of hydroplaning, the design engineer can use
the proposed design guidelines to select and specify the pavement geometry, surface
characteristics and mixture type, and appurtenance design required to satisfy the hydroplaning
criteria. The design guidelines and associated computer program allow the design engineer to
select pavement geometries that minimize sheet flow; to select and locate drainage
appurtenances; and to select various mixture types and surface textures that will also minimize
water film thickness and the potential for hydroplaning.
In order to develop the computer model and the design guidelines, it was necessary to
conduct permeability studies on various open-graded or porous asphalt mixtures, to establish
Manning's n for selected PCC and asphalt concrete surfaces, and to conduct full-scale skid
testing on open-graded pavement surfaces.
Findings from the literature review and based on assessment of the current state-of-the-
art based are summarized in Chapter 2 along with an overview of the proposal methods for
controlling surface drainage. The rationale for choosing the models that were used in
developing the guidelines is provided in Chapter 3. The results of testing performed during this
study are presented in Chapter 4, and a set of recommendations and conclusions are given in
Chapter 5. An overview of the interactive computer or program, PAVDRN, which was
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developed as part of this study, is given in Appendix A. Other supporting documentation is
given in Appendices B through D. The "Proposal Design Guidelines for Improving Pavement
Surface Drainage" and the PAVDRN program are available from NCHRP via the ~nternet and
present detailed descriptions of several of the experiments to determine Man~g's n.
10
Representative terms from entire chapter:
pavement surface
