This report describes the effects of the downspout piping on the capacity of a bridge deck drain. Full-scale hydraulic modeling of the drain was performed using two separate physical models to observe the hydraulic behavior of the drain and the influence of various piping configurations on the drain capacity. It was found that the drain behaved under one of two different control regimes for any given set of modeling conditions. Weir control occurred when the drain pan was not completely full and the approach flow was in free-fall over at least a portion of the drain grate. Orifice flow occurred when the drain pan was full so that the flow control was the capacity of the entrance into the drain piping. The capacity of the drain is greater for weir flow than for orifice flow. Thus, the downspout piping above the vent affected the capacity of the drain by influencing the flow at which the transition from weir to orifice control occurred. All vertical piping which followed a piping elbow never flowed full, so the location of the first elbow was found to have significant impact on the drain capacity because this elbow affected the total head on the drain due to the drain piping system. This report presents details of these results with conclusions and design recommendations. In addition, the results in this report and two previous reports were used to determine the accuracy of calculating the drain flow as the minimum of weir flow using HEC-12 and orifice flow using a constant orifice coefficient and the head across the orifice at the entrance to the drain piping. For most of the conditions, the calculations overestimated the capacity of the drain. The error was as much as 120% just for weir flows or 65% when taking the minimum of weir and orifice controlled flows.

  • Supplemental Notes:
    • Research study title: Hydraulic Characteristics of Recessed Curb Inlets and Bridge Drains: Phase 2.
  • Corporate Authors:

    University of Texas, Austin

    Center for Transportation Research, 3208 Red River Street
    Austin, TX  United States  78705

    Texas Department of Transportation

    Research and Technology Transfer Office, P.O. Box 5051
    Austin, TX  United States  78763-5051

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Authors:
    • SMITH, S
    • Holley, E
  • Publication Date: 1995-12


  • English

Media Info

  • Features: Appendices; Figures; Photos; References; Tables;
  • Pagination: 139 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00724960
  • Record Type: Publication
  • Report/Paper Numbers: FHWA/TX-96/1409-2F, Res Rept 1409-2F, CTR 1409-2F
  • Contract Numbers: Study 0-1409
  • Created Date: Aug 26 1996 12:00AM