Hydraulic spread calculations dictate the number and spacings of bridge deck drains and closed drainage system design. Efficient drainage is required to avoid the accumulation of runoff on bridge decks, which may result in impeding traffic and/or traffic accidents due to hydroplaning.
The main objective of this research is to investigate bridge deck drainage systems adopted by different state DOTs and update NCDOT guidelines for bridge deck drainage systems to ensure economic and efficient drainage design at the State of North Carolina.
The research team conducted a nationwide survey to investigate other DOTs drainage manuals, design guidelines, construction, and maintenance practices. Existing hydraulic spread policies in North Carolina is reviewed and compared to the research findings.
The outcomes of this research show that majority of State DOTs use the rational method for runoff quantification using an iterative process to identify the gutter flow time. Rainfall intensity is conservatively estimated using charts developed by NOAA (as recommended by the FHWA). Area contributing to rain fall includes the distance between bridge centerline and inlets for crowned bridges and the whole bridge width for super-elevated bridges. State DOTs feedback show that external closed drains are preferred to embedded drains for ease of construction, maintenance, and to avoid freeze-thaw problem.
This report includes detailed recommendation for closed drainage design equations, and a flowchart for the design process is provided. The recommendations will assist NCDOT personnel in the design, construction, and maintenance of efficient and economic closed drainage systems for bridge decks.
