Modeling Ballasted Tracks for Runoff Coefficient C

In this study, the Regional Transportation District (RTD)’s light rail tracks were modeled to determine the Rational Method runoff coefficient, C, values corresponding to ballasted tracks. To accomplish this, a laboratory study utilizing a rainfall-runoff facility was conducted at Colorado State University’s Hydraulics Laboratory. The input to this model was provided by using RTD’s design criteria, data from existing installations, and a field study to sample surface materials along ballasted tracks. By subjecting the 1:1 scale model railroad segment to 1-hour rainfall events with various recurrence intervals and measuring the corresponding runoff volumes, runoff coefficients were computed for Denver hydrology. For the more frequent 2-year, 5-year, 10-year events, the average C value is approximately 0.55. For the 25-year, 50-year, and 100-year return frequency rainfall events, the C value is in excess of 0.55 and is expressed in terms of multiplication factors of this average value. The runoff coefficient for ballasted tracks is significantly larger than the previously tabulated values for railroad yards. The higher runoff coefficient reflects the design of ballasted tracks to drain rainfall as quickly as possible. As a part of the research, detention times in the ballasted tracks were also determined. The detention time is a function of antecedent soil moisture content and rainfall intensity. In general terms, for dry antecedent conditions the initial 0.3 inch-0.4 inch of rainfall is detained in the ballasted tracks. The initial 0.5 inch of rainfall produces only a small amount of runoff. For 25-year, 50-year, and 100-year events, the runoff starts 9 minutes, 7 minutes, and 6 minutes after the start of the event.


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Media Info

  • Media Type: Web
  • Edition: Final Report
  • Features: Appendices; Figures; Photos; References; Tables;
  • Pagination: 51p

Subject/Index Terms

Filing Info

  • Accession Number: 01446011
  • Record Type: Publication
  • Report/Paper Numbers: CDOT-2012-8
  • Contract Numbers: 107.00
  • Created Date: Sep 14 2012 11:14AM