Roadway drainage includes the entire system from pavement drainage through storm drains. Drainage features that make up the system include curbs, gutters, drop inlets, median drains, overside drains, roadside ditches, and storm drains. The basic design procedure for roadway drainage includes hydrology, surface water removal, and disposal. A properly designed system must adequately accommodate the design runoff by removing it from the roadway surface and conveying it to the outfall, avoiding damage to adjacent property and roadway hazards from overflowing and ponding.
Pavement may be drained in one of two ways. The runoff may be allowed to sheet — flow across the roadway surface and into roadside ditches. This may not always be possible or cost-effective, because of right-of-way constrictions. Alternatively, a curb and gutter section is used to channel the flow.
DESIGN WATER SPREAD
h ■- ■* -_ 4
~1 LCURB and SHOULDER^ GUTTER
FIGURE 5.6 Illustration of design water spread. (From Highway Drainage Guidelines, Vol. IX, American Association of State Highway and Transportation Officials, Washington, D. C., 1999, with permission)
An appropriate design storm must be selected so that the drainage facilities may be properly designed. This design storm must relate to an acceptable level of flooding of the roadway with regard to both area and frequency. The acceptable level of flooding is termed the design water spread (Fig. 5.6) and is defined by the acceptable amount of encroachment on the roadway surface that is assumed to have a certain probability of occurrence. It may not be economically feasible to completely prevent encroachment on the roadway. Alternatively, it is unwise to allow spread that results in unsafe driving conditions. Greater water spread produces hydroplaning, greater splash and spray effect, and an accompanying decrease in visibility and vehicle control by the users of the facility. The amount and frequency of encroachment should vary with the type of roadway being designed, because roads with higher volumes and speeds can tolerate less loss of visibility than local and collector roads.
AASHTO has developed general guidelines on highway drainage that may be used to formulate roadway surface drainage criteria. Table 5.7 shows the suggested AASHTO procedure for relating the road classification, the frequency of the design storm, and
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TABLE 5.7 Minimum Design Frequency Design spread
*Average daily traffic. Source: From Highway Drainage Guidelines, Vol. IX, American Association of State Highway and Transportation Officials, Washington, D. C., 1999, with permission. |
|
Design storm |
Design water spread |
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|
Highway type/category/feature |
4% 10% (25 yr) (10 yr) |
or parking lane |
outer lane |
Local standard |
|
Freeways |
||||
|
Through traffic lanes, branch connections, and other major ramp connections |
x — |
x |
— |
— |
|
Minor ramps |
— x |
x |
— |
— |
|
Frontage roads Conventional highways |
—x |
— |
— |
x |
|
High-volume, multilane, speeds over 45 mi/h (70 km/h) |
x— |
x |
— |
— |
|
High-volume, multilane, speeds 45 mi/h (70 km/h) and under |
—x |
— |
x |
— |
|
Low-volume, rural, speeds over 45 mi/h (70 km/h) |
x— |
x |
— |
— |
|
Urban, speeds 45 mi/h (70 km/h) and under All state highways |
—x |
— |
— |
x |
|
Depressed sections that require pumping: Use a 2% (50-year) design storm for freeways and conventional state highways. Design water spread at depressed sections should not exceed that of adjacent roadway sections. A 4% (25-year) design storm may be used on local streets or road undercrossings that require pumping. |
Source: From Highway Design Manual, California Department of Transportation, with permission.
the design spread. However, more specific local or regional guidelines are usually developed and should be referenced for highway drainage design. An example of a regional guideline developed by the California Department of Transportation (Caltrans) is shown in Table 5.8. It is apparent that a more severe storm (25-year versus 10-year mean recurrence interval) is used for roadways with higher volumes and speeds, as well as a more limited design water spread.
