Rain and Snowmelt Augmented Design Flood for Highways Bridges in Snowy Mountains

There is lack of codal and engineering practices to consider the rain mixed snowmelt induced flood flows for the design discharge in deciding hydrologic/hydraulic features of road/ highway bridges, resulting in varying degree of damages to structures and traffic movement. Synthetic unit hydrograph and watershed models are not suited to get reliable peak flood of numerous un-gauged catchments for concerned structures. The paper presents the methodology to consider the probable effective snowmelt and intensity collectively at desired return periods. Instead of neglecting the impact of meltflow or just enhancing the design discharge by 10%, five options of peak runoff generating catchment criteria due to probable rain and/or snowmelt have been envisaged having rainfall intensity on time of concentration and melt intensity on degree-day melt index. Consequently, results of two small bridges, Wagund and Pernigaon located in Doda and Anantnag districts of J&K on NH1A using empirical, rational and slope-area methods have been presented. The peak design discharge computed was 395 and 71 cumecs at the Wagund and Pernigaon sites, respectively at 50 years return period during 2005-2009. Slope-Area method has been found useful to compute HFL for combined design discharge of snowmelt and rainfall, while Weir/Orifice method provided afflux of 0.81 and 1.03 m for 20 and 12 m clear span, respectively.