FLOOD PLAIN ROAD DESIGN ANALYSIS IN LAKSAR AREA USING GEOMATICS TECH-NIQUES

Abstract

Flood plains may be regarded place nearby riverside which are prone to frequent floods when the river is flowing at high stage. In these conditions the areas along the riverside get submerged under the influence of floods, consequently these areas get waterlogged for a considerable period of time an if any mode of transportation such as a roadway in the flood plains, the same also get affected and become dysfunctional during flood. Therefore special design considerations are required to be taken in account while designing the roads passing through flood plains. For this, the region under consideration was divided into various sub catchments and peak discharge (maximum surface run-off) for each sub catchment was estimated for maximum rainfall of return period 50 years and corre-sponding to the peak discharge, high flood level for each sub catchment was estimated. The peak discharge was calculated from different methodologies such as synthetic unit hydrograph method, rational method, empirical relationships and maximum of the peak discharge obtained from the dif-ferent methodologies was in the calculation of high flood level. The peak discharge estimated in sub catchments through which the Laksar Purquaji major district road was passes were 179.7 m3/sec, 357.5 m3/sec, 427.3 m3/sec, 174.3 m3/sec and corresponding high flood levels of sub catchments were 236.03 m, 232.35 m, 230.8 m, 230.94 m. A total stretch of road length of about 17 km was found to be below the high flood level that is road under submergence during floods and was de-signed as per IRC SP 34:2011 specifications to be properly functional during floods of 50 year return period. At the stream and road crossing, in various sub catchment a cross-drainage of suitable size was designed for surface runoff to recede from the catchment without affecting the road. The de-sign bridge opening after providing appropriate vertical clearance as per IRC SP 13:2011 for various sub catchments was estimated as 123.54 m2, 229.0 m2, 294.6 m2, 115.0 m2. Another region taken into consideration was the proposed NH58 bypass from Mangalore towards Rehmadpur which has a crossing at Solani river. The peak discharge estimated for the sub-catchment in the region corre-sponding to maximum rainfall of return period 50 years was 255.33 m3/sec and the inlet discharge in sub catchment was added to the peak discharge to obtain the total discharge at the outlet of the sub catchment .The total discharge hence obtained was 2522 m3/sec. The high flood level corresponding to the observed discharge in the stream was estimated as 252.9 m and a length of about 3 km of proposed path for the bypass was observed below high flood level that is the region under submerg-ence during floods. The elevation of road subgrade in this region under submergence was designed as per IRC SP 34:2011 recommendations and the size of design bridge opening beneath the road for surface run off to recede from the sub catchment during floods after providing appropriate vertical clearance(between water level and bottom of bridge deck) as per IRC SP 13:2011 estimated was 1078.3 m2.