Raumai Bridge Erosion Subject of Special Report

Manawatu Times, Volume LV, Issue 7241, 12 June 1930, Page 2

Raumai Bridge Erosion Subject of Special Report

POSITION DANGEROUS The danger which threatens tlio eastern approach to the Raumai bridge across tho Pohangina river was the subject of a report by Mr. F. C. Hay, civil engineer, to the Pohangina County Council yesterday. Tho position, said Mr. Hay, was that about seven chains of the papa face extending up-stream from tho bridge, and on which the road was cut, wos subject to a strong attack from the flood waters of the river, which carried largo quantities of shingle and thereby caused erosion of the toe of tho papa cliff. Thcro did not seem to be any likelihood of a cessation of this attack by any natural change in tho direction of tho flood flow, and its continuation would result in undermining tho present road and necessitate its being cut still further into tho hillside, with the probability of forcing the council into heavy maintenance costs. Tho ordinary weathering of the papa faco was also a factor in the problem, but, provided the erosion at the foot of tho cliff wero prevented, tho face would tend to stabilise itself and this factor would bo almost negligible. Three ways suggested themselves to protect the cliff from the river:—*

1. To ro-align tho river from a mile above the bridgo so that tho flood waters could approach the bridge pnral lei w'ith tho cliff.

2. To build a concrete wall along the too of tho cliff above flood level. 3. To thrust tho attack of tho river away from tho cliff by groynes and a slight change in alignment up-stream. No. 1 tv as not recommended because it involved problems outside county matters and would be difficult on account of trouble with tho land owners. No. 2 would bo costly, even if foundations were obtained at a reasonable depth. No. 3 was recommended. The iul. power of the flood was directed almost at right angles to the cliff, and to divert the water two groynes were recommended. Tho body of tho fust, furthest away from tho bridge, would bo built of earth, with an end of boulder and netting construction. T.hc second grovnc, nearer the bridge, would be built wholly of netted boulders, with wings alongside the face of the cliff to prevent erosion from eddy currents. A cut would be made across the too of tho shingle bank opposite, and on the new alignment proposed for the river. Tho works would have the effect of creating a shingle bank from tho furthest groyno to near the eastern abutment of the bridge. This bank o shingle would protect the too of the cliff° from being undermined and it would form a base on which, with tho aid of anchored trees, silt would be deposited and, by the planting of willows, become a permanent support to the cliff. Tho present position was obviously dangerous and the main danger was due to river erosion. To submit to this erosion and to keep shifting the road would not only be costly, but would never be likely to result in a condition of safety being attained. Tho erosion could be stopped and the cheapest way is by the method ho proposed, coneluded Mr. Hay. The council gave consideration to the report, agreeing that tho work proposed by Mr. Hay would bo costly'. The Highways Board would help in any protection work to the extent of £2 for £l. Decision was deferred in the meantime, the plans prepared by Mr. Hay being handed to the engineer (Mr. Matson).