Railway and highway problems at the shingle fans north of the Clarence River (S42/P30)

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Paterson, B.R. 1980 Railway and highway problems at the shingle fans north of the Clarence River (S42/P30). Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 347. 24 p.

 Abstract: Just north of the Clarence River three streams are actively building alluvial fans across the narrow coastal plain. The large quantities of gravel deposited on the fans are a long-term problem for the highway and railway which cross near the head and toe of the fans respectively. During flood flows the highway fords become impassable and traffic is diverted over bypass bridges which are operational in all but the most severe storms. Gravel blocks the railway culverts preventing sediment from reaching the sea, and at times causing scouring of the embankment. Following major floods heavy earthmoving machinery is bought onto the site to clean out the channels and provide storage for the next event. The high cost of this work, the increasing problem of gravel disposal, and the apparent constriction to flood flows caused by the small railway culverts have all contributed to the criticism of the existing procedures. The highly erodible nature of the rocks forming the stream catchments, and the recent rejuvenation of the stream by tectonic uplift ensure that erosion will continue to be active in this area. Erosion control measures currently being carried out by the Marlborough Catchment Board will reduce the rate of erosion, but are unlikely to prevent gravel deposition during major floods. It is recommended that railway bridges be installed across the streams to replace the existing culverts. The bridges should be high enough to allow bulldozers to pass beneath them, which will therefore necessitate regrading the railway. A choice of two alternatives is proposed for the highway: (a) raising the road and installing bridges on the existing route; (b) realigning the highway along the railway