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Section 4: Pavement Drainage

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Design Objectives

The objective of urban storm drainage is to provide safe passage of vehicle traffic by collecting stormwater from the roadway, and to convey it safely to an adequate receiving body while preventing damage to adjacent private properties or undue risk to pedestrian traffic during the design storm event.

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 the hydraulic designer must work with the roadway geometric designer to assure efficient drainage in accordance with the geometric and pavement design.

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Ponding

The flow of water in the gutter should be restricted to a depth and corresponding width that will neither obstruct the roadway nor present a hazard to the motoring public at the design AEP. These restrictions are referred to as allowable depth and allowable ponded width. The depth and width of flow depend on the following:

Depth of flow should not exceed the curb height.

Ponded widths are limited to the following minimum acceptable standards for Department roadway design:

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  • Limit ponding to one-half the width of the outer lane for the main lanes of interstate and controlled access highways.
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  • Limit ponding to the width of the outer lane for major highways, which are highways with two or more lanes in each direction.
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  • Limit ponding to a width and depth that will allow the safe passage of one lane of traffic per direction for minor highways.

Inlets should be placed at all low points in the roadway and at suitable intervals along extended slopes as necessary to prevent excessive flow in the gutter or ponding on the roadway. An economical design uses a minimum number of inlets by allowing the ponded width and depth to approach the allowable limits. In instances such as a narrow shoulders or low grades, there may need to be a continuous removal of flow from the surface.

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Longitudinal Slopes

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

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

Figure 10-1. Rolling Gutter Profile

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Transverse (Cross) Slopes

A steep cross slope provides for proper drainage, while flat cross slopes are amenable to driver safety and comfort. Except in cases of superelevated sections, the cross slope is usually a compromise between the two requirements. The Roadway Design Manual should be consulted for guidance on pavement cross slopes.

Drainage on multi-lane roadways can be enhanced by increasing the cross slope on the outer lanes, or by increasing the cross slope on each successive lane pair. Refer to the Roadway Design Manual for guidance. Drainage on very wide multi-lane roadways such as urban interstates may require special considerations such as porous pavements or transverse trench drains. DES-HYD should be consulted when these considerations arise.

Superelevated transitions should be carefully designed to minimize the extent of flat areas. Flat slopes should not be located in the sag of a vertical curve. It is usually these transition regions where small, shallow ponds of accumulated water, or “birdbaths,” occur. Aggressive drainage techniques such as porous pavement, rough texture, or additional drains must be used to minimize ponding in these “birdbaths.”

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Hydroplaning

Hydroplaning occurs when the drainage capacity of the tire tread pattern and the pavement surface is exceeded; water builds up in front of the tire and creates a water wedge which can lift the tire off the pavement, thus reducing the tire/pavement friction to zero. Hydroplaning is a function of the water depth, roadway geometrics, vehicle speed, tread depth, tire inflation pressure, and conditions of the pavement surface, so it is difficult to calculate the exact conditions where hydroplaning will occur. The potential for hydroplaning increases as the depth of water over the roadway increases. Hydroplaning can occur at 55 mph with as little as 0.08 inches (2mm) of water.

Because the factors that influence hydroplaning are generally beyond the designer's control, hydroplaning is impossible to prevent. However, the physical characteristics that may influence hydroplaning can be minimized with the following considerations:

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  • Proper transverse slopes reduce the amount of water flowing over the pavement and prevent excessive ponding. The longitudinal slope is somewhat less influential in decreasing the potential for hydroplaning.
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  • Conscientious placement of inlets reduces or eliminates water flowing over the pavement and reduces excessive ponding. Transverse drains should not be used without serious consideration for small wheeled vehicles.
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  • Permeable surface courses and high macrotexture surface courses influence both water film thickness and the interaction of tires with the water film.
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  • Grooving may be a corrective measure for severe localized hydroplaning problems. Transverse grooving (perpendicular to the direction of traffic) produces better results that longitudinal grooving (parallel to the direction of traffic). In addition, longitudinal grooving has the potential to retard flow off the roadway.

The potential for hydroplaning can be evaluated using an empirical equation based on studies conducted for the FHWA publication “Bridge Deck Drainage Guidelines” ( HEC-21).

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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. 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.

Cross cutting (grooving) of the pavement is useful for removing small amounts of water such as in a light drizzle. The Department discourages longitudinal grooving because it usually causes problems in vehicle handling and tends to impede runoff from moving toward the curb and gutter.

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