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Section 4: Pavement Drainage

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Design Objectives

A chief objective in the design of a storm drain system is to move any accumulated water off the roadway as quickly and efficiently as possible. Where the flow is concentrated, the design objective should be to minimize the depth and extent of that flow.

Appropriate longitudinal and transverse slopes can serve to move water off the travel way to minimize ponding, sheet flow, and low crossovers. This means that you must work with the roadway geometric designers to assure efficient drainage in accordance with the geometric and pavement design.

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Ponding

Restrict the flow of water in the gutter to a depth and corresponding width that will not cause the water to spread out over the traveled portion of the roadway in a depth that obstructs or poses a hazard to traffic. The depth of flow should not exceed the curb height. The depth of flow depends on the following:

  • rate of flow
  • longitudinal gutter slope
  • transverse roadway slope
  • roughness characteristics of the gutter and pavement
  • inlet spacing.

Place inlets at all low points in the roadway surface and at suitable intervals along extended gutter slopes as necessary to prevent excessive ponding on the roadway. In the interest of economy, use a minimum number of inlets, allowing the ponded width to approach the limit of allowable width specified as a design criterion. In instances such as a narrow shoulder or low grades, you may need to plan a continuous removal of flow from the surface.

Longitudinal gutter slopes should usually not be less than 0.3% for curbed pavements. This minimum may be difficult to maintain in some locations. In such situations, a rolling profile (or sawtooth grade) may be necessary. You may need to warp the transverse slope to achieve a rolling gutter profile. Figure 10‑1 shows a schematic of a sawtooth grade profile. Extremely long sag-vertical curves in the curb and gutter profile are discouraged because they incorporate relatively long, flat grades at the sag. Such long, flat slopes tend to distribute runoff across the roadway surface instead of concentrating flow within a manageable area.

Sawtooth Gutter Profile (click in image to see full-size image) Anchor: #i1000308grtop

Figure 10-1. Sawtooth Gutter Profile

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Transverse Slopes

Except in cases of super-elevation for horizontal roadway curves, the pavement transverse slope is usually a compromise between the need for cross slopes adequate for proper drainage and relatively flat cross slopes that are amenable to driver safety and comfort. Generally, transverse slopes of about 2 % have little effect on driver effort or vehicle operation. If the transverse slope is too flat, more depth of water accumulation is necessary to overcome surface tension. Furthermore, once water accumulates into a concentrated flow in a flat transverse slope configuration, the spread of the flow (ponded width) may be too wide. These characteristics are the chief causes of hydroplaning situations. Therefore, an adequate transverse slope is an important countermeasure against hydroplaning.

For TxDOT projects, a recommended minimum transverse slope for tangent roadway sections is 2%. The recommended maximum transverse slopes for a tangent roadway section is 4%. Refer to the Roadway Design Manual for recommendations concerning super-elevation values for horizontal curves in roadways. Ensure that cross slope transitions, such as those required in reverse curves, are designed to avoid flat cross-slopes in sag vertical curves.

You can effectively reduce the depth of water on pavements by increasing the cross slope for each successive lane in a multi-lane facility. In very wide multi-lane facilities, the inside lanes may be sloped toward the median. However, do not drain median areas across traveled lanes. In transitions into horizontal curve super-elevation, minimize flat cross slopes and avoid them at low points of a sag profile. It is usually in these transition regions where small, shallow ponds of accumulated water, or “birdbaths,” occur.

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Use of Rough Pavement Texture

The potential for hydroplaning may be minimized to some extent if the pavement has a rough texture. Cross cutting (grooving) of the pavement is useful for removing small amounts of water such as in a light drizzle. TxDOT discourages longitudinal grooving because it usually causes problems in vehicle handling and tends to impede runoff from moving toward the curb and gutter. A very rough pavement texture benefits inlet interception. However, in a contradictory sense, very rough pavement texture is unfavorable because it causes a wider spread of water in the gutter. Rough pavement texture also inhibits runoff from the pavement.

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Gutter Flow Design Equations

Figure 10‑2 illustrates ponding spread. Ponded width is commonly designated as T.

Gutter Flow Cross Section Definition of
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Figure 10-2. Gutter Flow Cross Section Definition of Terms

The ponded width is a geometric function of the depth of the water (y) in the curb and gutter section. For storm drain system design in TxDOT, the depth of flow in a curb and gutter section with a longitudinal slope (S) is taken as the uniform (normal) depth of flow, using Manning’s Equation for Depth of Flow as a basis. (See Chapter 6 for more information.) Ordinarily, it would not be possible to solve for uniform depth of flow directly from Manning’s Equation. For Equation 10-1, the portion of wetted perimeter represented by the vertical (or near-vertical) face of the curb is ignored. This justifiable expedient does not appreciably alter the resulting estimate of depth of flow in the curb and gutter section.

Equation 10-1.

where:

  • y = depth of water in the curb and gutter cross section (ft. or m)
  • Q = gutter flow rate (cfs or m3/s)
  • n = Manning’s roughness coefficient
  • S = longitudinal slope (ft./ft. or m/m)
  • SX = pavement cross slope (ft./ft. or m/m)
  • z = 1.24 for English measurements or 1.443 for metric.

Refer to Figure 10‑2, and translate the depth of flow to a ponded width on the basis of similar triangles.

Equation 10-2.

where:

  • T = ponded width (ft. or m).
  • Determine the ponded width in a sag configuration with Equation 10-2 using depth of standing water or head on the inlet in place of y. Combine Equation 10-1 and Equation 10-2 to compute the gutter capacity using Equation 10-3.

Equation 10-3.

where:

  • z = 0.56 for English measurements or 0.377 for metric.
  • Rearranging Equation 10-3 gives a solution for the ponded width, T.

Equation 10-4.

where:

  • z = 1.24 for English measurements or 1.443 for metric.
  • The table below presents suggested Manning’s “n” values for various pavement surfaces. The department recommends use of the rough texture values for design.

    Manning’s n-Values for Street and Pavement Gutters

    Type of gutter or pavement

    n

    Asphalt pavement:

    -

    Smooth texture

    0.013

    Rough texture

    0.016

    Concrete gutter with asphalt pavement:

    -

    Smooth texture

    0.013

    Rough texture

    0.015

    Concrete pavement:

    -

    Float finish

    0.014

    Broom finish

    0.016



    Equation 10-3 and Equation 10-4 apply to portions of roadway sections having constant cross slope and a vertical curb. Refer to the FHWA publication “Urban Drainage Design Manual” ( HEC-22, 1996) for parabolic and other shape roadway sections.

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Ponding on Continuous Grades

Avoid excessive ponding on continuous grades by placing storm drain inlets at frequent intervals. Determine the gutter ponding at a specific location (such as an inlet) on a continuous grade using the following steps:

  1. Determine the total discharge in the gutter based on the drainage area to the desired location. See Runoff for methods to determine discharge.
  2. Determine the longitudinal slope and cross-section properties of the gutter. Cross-section properties include transverse slope and Manning’s roughness coefficient.
  3. Compute the ponded depth and width. For a constant transverse slope, compute the ponded depth using Equation 10-1 and the ponded width using Equation 10-2. For parabolic gutters or sections with more than one transverse slope, refer to the FHWA publication “Urban Drainage Design Manual,” (HEC 22, 1996). For information on obtaining this publication, see References.
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Ponding at Approach to Sag Locations

At sag locations, consider sag inlet capacity, flow in the gutter approaching the left side of the sag inlet, and flow in the gutter approaching the right side of the sag inlet, and avoid exceeding allowable ponding:

  1. Estimate the apportionment of runoff to the left and right approaches. Considering the limitations of the hydrologic method employed (usually the Rational Method - see information on the Determination of Runoff), it is reasonable to compute the discharge to the sag location based on the entire drainage area and determine the approximate fraction of area contributing to each side of the sag location. Multiply each fraction by the total discharge to determine the discharge to each side.
  2. Determine the longitudinal slope of each gutter approach. For sawtooth profiles, the slopes will be the profile grades of the left and right approaches. However, if the sag is in a vertical curve, the slope at the sag is zero, which would mean that there is no gutter capacity. In reality there is a three-dimensional flow pattern resulting from the drawdown effect of the inlet. As an approximation, one reasonable approach is to assume a longitudinal slope of one half of the tangent grade.
  3. For each side of the sag, calculate the ponded depth and width. Use the appropriate flow apportionment, longitudinal slope, and Equation 10-1. Compute the ponded width using Equation 10-2.
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Hydroplaning

As rain falls on the roadway surface, the water accumulates to some depth before overcoming surface tension and running off. A vehicle encountering water on the road may hydroplane, the vehicle’s tires planing on top of the accumulated water and sliding across the water surface. Hydroplaning is a function of rainfall intensity and resulting water depth, air pressure in the tires, tread depth and siping pattern of the vehicle tires, condition and character of the pavement, and vehicle speed.

Because the factors that influence hydroplaning are generally beyond the designer’s control, it is impossible to prevent the phenomenon. However, minimize the physical characteristics that may influence hydroplaning:

  • The greater the transverse slope on the pavement, the less the potential for water depth buildup and potential for hydroplaning. A minimum cross slope of 2% is recommended. The longitudinal slope is somewhat less influential in decreasing the potential for hydroplaning. You must establish coordinate establishment of these slopes with the geometric design to ensure adequate provisions against hydroplaning.
  • Studies have indicated that a permeable surface course or a high macrotexture surface course has the highest potential for reducing hydroplaning problems.
  • As a guideline, a wheel path depression in excess of about 0.2 in. (5 mm) has potential for causing conditions that may lead to hydroplaning.
  • Grooving may be a corrective measure for severe localized hydroplaning problems. However, grooving that is parallel to the roadway traffic direction may be more harmful than useful because of the potential for retarding sheet flow movement.
  • Do not use transverse surface drains located on the pavement surface.

Rainfall intensities can be so high in Texas that the designer cannot eliminate the potential for hydroplaning. Because rainfall intensities and vehicle speed are primary factors in hydroplaning, the driver must be aware of the dangers of hydroplaning. In areas especially prone to hydroplaning where you have employed reasonable measures to minimize the potential for hydroplaning, the department should use wet weather warning signs to warn the driver of the danger.

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Vehicle Speed in Relation to Hydroplaning

You can evaluate the potential for hydroplaning using an empirical equation based on studies conducted for the USDOT (FHWA-RD-79-30 and 31-1979, Bridge Deck Drainage Guidelines, RD-87-014).

Equation 10-5 and Equation 10-6 provide in English and metric units a means of estimating the vehicle speed at which hydroplaning occurs.

English:

Equation 10-5.

Metric:

Equation 10-6.

where:

  • v = vehicle speed at which hydroplaning occurs (mph or km/h)
  • SD = [Wd-Ww/Wd]*(100) = spindown percent (10 % spindown is used as an indicator of hydroplaning)
  • Wd = rotational velocity of a rolling wheel on a dry surface
  • Ww = rotational velocity of a wheel after spinning down due to contact with a flooded pavement
  • P = tire pressure (psi or kPa), use 24 psi or 165 kPa for design
  • TD = tire tread depth (in. or mm), use 2/32-in. or 0.5 mm for design)
  • WD = water depth, in. or mm (see Equation 10-7).
  • A = For English measurement, the greater of:
  • For metric, the greater of
  • [12.639/WD0.06] + 3.50 or {[22.351/WD0.06] - 4.97} * TXD0.14
  • NOTE: This equation is limited to vehicle speeds of less than 55 mph (90 km/h).

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Water Depth in Relation to Hydroplaning

Equation 10-7 provides for evaluating the depth of storm water on pavement.�

Equation 10-7.

where:

  • z = 0.00338 for English measurement or 0.01485 for metric
  • WD = water depth (in. or mm)
  • TXD = pavement texture depth (in. or mm) (use 0.02 in. or 0.5 mm for design)
  • L = pavement width (ft. or m)
  • I = rainfall intensity (in./hr or mm/hr)
  • S = pavement cross slope (ft./ft. or m/m).
  • After calculating water depth, check design speed. If hydroplaning is a concern, several possibilities exist:

  • The cross-slope could be increased. Pavement cross-slope is the dominant factor in removing water from the pavement surface. A minimum cross-slope of 2% is recommended.
  • Pavement texture could be increased. However, no technical guidance appears to be available on the relationship between texture depth and pavement surface type.
  • Reduce the drainage area. If possible, reduce width of drained pavement by providing crowned section or by intercepting some sheet flow with inlets such as slotted drains.
  • The speed limit could be reduced for wet conditions.

If physical adjustments to the roadway conditions are not practicable, consider providing appropriate warning of the potential hazard during wet conditions.

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