Abstract:
The rational formula has been used for the peak flow estimation over 150 years and still
remains as the most widely used flood estimation technique. The method is applied
assuming that rainfall intensity and storm duration is uniform over the study area; storm
duration is equal to the time of concentration of the catchment; and the runoff coefficient is
constant during a storm. The assignment of precise upper limit on the catchment area for
reliable application of rational method varies from country to country and among literatures.
As many literatures indicated, it has been specified that rational method is appropriate for
small catchments. However, the definition of small catchment is not consistent across
practitioners.
The peak flow estimation of drainage structures would be a vital design consideration in
evaluating the capacity adequacy of existing drainage system and to propose extension/new
addition to the present drainage system, if it failed to satisfy the anticipated peak flow of a
structure.
The hydrologic model determines the runoff that occurs following a particular rainfall event.
The primary output from the hydrologic model is quantity, rate and timing of stream flow that
results from rainfall events. The Hydrologic Modeling System (HEC-HMS) originally
developed by the U.S. Army Corps of Engineers is used to simulate precipitation-runoff
processes of dendritic watershed systems. It includes many of the well-known and wellapplicable
hydrologic methods to simulate rainfall-runoff processes in river basins.
The study focuses on the hydrologic design of cross drainage structures of road sections
located in different hydrological and geographical zones in Sri Lanka. The topographic maps
of 1:10,000 and 1:50,000 together with google terrain maps were used to identify the
respective catchment areas and catchment characteristics. The peak flow corresponds to
return periods of 25,50 and 100 years determined by the rational formula and comparison of these values with the outputs derived through HEC – HMS model were then used for the
determination of upper limit of the catchment area where the rational formula can be applied.
Citation:
Dissanayake, G.A., & Pasindu, H.R. (2015). Applicability of rational formula for larger size catchments in highway drainage design [Abstract]. In H.R. Pasindu (Ed.), Proceedings of the Transportation Research Forum 2015 (pp. 18-19). Department of Civil Engineering, University of Moratuwa. https://uom.lk/sites/default/files/civil/files/TRF%202015_0.pdf