Urban Drainage Design: Pavement, Gutters, and Roadside Channels

BasePDH  |  Course No. 017  |  2 PDH

Source: HEC-22, 4th Edition

Title: Urban Stormwater Management in the United States — Hydraulic Engineering Circular No. 22 (HEC-22), 4th Edition
Publisher: Federal Highway Administration (FHWA)
Sections: Chapter 5 — Roadway Pavement Drainage; Chapter 6 — Roadside and Median Channels
Supporting references: HEC-12 (FHWA 1984), HEC-15 (FHWA 2005), HEC-21 (FHWA 1993), AASHTO Drainage Manual (2014), AASHTO Policy on Geometric Design (2018)

Roadway Pavement Drainage

The paved surface of the driving lanes is the critical area of interaction between vehicles and the roadway; effective drainage of the pavement facilitates safe roadway use. Water on the pavement adversely affects safe use of the road, interrupts traffic, reduces skid resistance, increases potential for hydroplaning, and limits visibility by splash and spray from traffic (AASHTO 2014). Pavement drainage design involves knowledge and consideration of surface drainage, gutter flow, and inlet capacity. The design of these elements depends on the design rainfall frequency and the allowable spread of stormwater on the pavement surface. This chapter presents information for the design of roadway features to meet the desired safety and serviceability levels. The chapter draws on HEC-12, Drainage of Highway Pavements (FHWA 1984), and AASHTO's Drainage Manual (2014) for most of the information presented.

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5.1 Spread

The width of pavement (outward from the face of curb) covered by water during rainfall is called "spread." Spread at any point on the roadway relates to the intensity and duration of rainfall. Allowable spread represents the maximum width of pavement covered during the design storm. Designers also evaluate spread associated with a larger (less frequent) storm.

5.1.1 Design Frequency and Spread

Highway storm drainage provides safe passage for vehicles under a set of conditions called a design storm. The drainage system, including the curb and gutter and additional width such as parking lanes, conveys stormwater to pavement inlets to provide reasonable safety for traffic and pedestrians at a reasonable cost. As spread from the curb increases, the risk of traffic accidents increases because of hydroplaning and spray, along with nuisance to pedestrian, bicycle, and scooter traffic. When specifying the AEP or return period for the design storm and the allowable spread during such a storm, designers make decisions regarding risk of accidents and traffic delays, and acceptable costs for the drainage system. Risk associated with water on traffic lanes increases with increasing traffic volume and traffic speed not only for safety reasons but also because of increased potential for delay.

The safety of the public represents the primary consideration for designers in specifying the design frequency and design spread. State and local Departments of Transportation (DOTs) establish design storm and spread criteria based on consideration of:

• Functional classification of the roadway, and possibly traffic volume within the functional classification.
• Traffic speed, which is a primary factor in hydroplaning when water is on the pavement.
• Existing and projected traffic volume, which may be an indicator of the economic importance of keeping the highway open to traffic. The costs of traffic delays and accidents increase with increasing traffic volumes.
• Cost. Balance between desirable and practicable criteria is sometimes necessary because of cost, and because of external factors such as constrained right-of-way (ROW) or utilities. The costs and feasibility of providing for a given design frequency and spread may vary significantly between projects. In some cases, it may be practicable to significantly upgrade the drainage design and reduce risks at moderate costs. In other instances, costs may be very sensitive to the criteria selected for design.

Designers consider inconvenience, hazards, and nuisances to pedestrian, bicycle, and other personal transport traffic. In some places, such as in commercial areas, this consideration may assume major importance. Local design practice may also be a major consideration since it can affect the feasibility of designing to higher standards, and it influences public perception of acceptable practice. Designers also consider the relative elevation of the highway and surrounding terrain where water can be drained only through a storm drainage system, as in underpasses and depressed roadway sections. They consider the potential for ponding to undesirable depths when selecting the frequency and spread criteria and in checking the design against storm runoff events of lesser frequency than the design event.

Selection of design criteria for intermediate types of facilities may be the most difficult. For example, some arterials with relatively high traffic volumes and speeds may not have shoulders which will convey the design runoff without encroaching on the traffic lanes. In these instances, practitioners typically assess the relative risks and costs of various design spreads to select appropriate design criteria. Table 5.1 provides example minimum design frequencies and spread based on the type of highway and traffic speed. Along with the situations covered in Table 5.1, for depressed sections and underpasses where ponded water can be removed only through the storm drainage system, designers frequently consider additional criteria including a 0.02 AEP event. The use of a more severe event, such as a 0.01 AEP event to assess hazards at critical locations where water can pond to appreciable depths is commonly referred to as a "check storm."

Table 5.1. Example minimum design frequency and spread.