Applicability of rational formula in hydrological analysis of highway cross drainage structures

Abstract

The peak flow estimation at Highway -Waterway intersects has substantial effects on Hydraulic design aspects of culverts and bridges. Fair and accurate estimation of peak flow would be the basis in deciding the size of cross drainage structures to ensure proper transfer ofrunoff collected at inlet towards the downstream. The history of hydrology modelling varies from well-known Rational Formula to sophisticated computer aided models. Rational Formula has been used for over 150 years and still remains as the most widely used method due to its simplicity in approach. The present study considered five numbers of flood estimation techniques; Rational Formula, Fuller Formula, Snyder’s Method, Flood transportation and HEC-HMS for catchments located in four road segments where area varies from 9 - 6663 hectares (ha). All catchments are ungauged and prediction of flood flows for ungauged basins is extensively discussed in past decades and still remains a question due to accuracy and validity of assumptions made in the analysis. The study focused on different methods of time of concentration, runoff coefficient and design discharge estimations. Three empirical relationships along with Irrigation Department guideline (1984) were considered for the computation of Time of Concentration. Time of concentration results of Kirpich equation was substantially lower than other methods while UK Flood Studies Report equation was identified as the highest. Only the Bransby-William equation showed fair agreement with Irrigation Department Guideline. Peak flow estimations of flood transportation technique is highly sensitive for the drainage area ratio and relatively low drainage area ratios correspond to culvert and bridge sites caused significant deviation from other methods. Fuller formula estimations were also considerably deviated from other methods. The difference in peak flow estimation methods was not significant up to catchment area of 100 ha. Peak flow estimations of cross drainage structures in studied road segments revealed that Rational formula can be applied up to 770 ha. As a comparative assessment catchments larger than 770 ha to be modeled using Rational formula and alternative flood estimation methods.