5.1 INTRODUCTION

When rain falls on a sloped pavement surface, it forms a thin layer of water that increases in thickness as it flows to the sides of the roadway. This accumulation of water on travel lanes can defeat the purpose of a highway by disrupting traffic flow, reducing vehicle skid resistance, increasing potential for motorist hydroplaning and visibility problems, and accelerating roadway deterioration. The water may also freeze, making vehicle maneuvering even more difficult. The objective in highway drainage design is to minimize such problems by collecting runoff in gutters and intercepting it using stormwater inlets that subsequently direct flow to subsurface conveyance systems, culverts, or ditches. Proper design of these facilities is thus essential to maintaining safe vehicular and pedestrian travel conditions and ensures that highway service levels will avoid disruption.

This chapter provides guidance on the selection of runoff frequency and spread for design computations, design flow estimation, flow in gutters, inlet design, and bridge deck drainage. For additional information, the reader is referred to Federal Highway Administration guidance documents, including Hydraulic Engineering Circular (HEC) No. 12 (Johnson and Chang, 1984), HEC-21 (Young et al., 1993), and HEC No. 22 (Brown et al., 1996). In addition, Anderson et al. (1995) provides further information on the mechanics of surface drainage.

5.2 FACTORS AFFECTING INLET DESIGN

The spread of water onto pavement, or the top width of gutter flow measured perpendicular to the edge of the roadway, is of...