EROSION MENACE

Manawatu Standard, Volume XLIX, Issue 230, 28 August 1929, Page 8

EROSION MENACE

FROM MANAWATU RIVER. [WORK OF PROTECTION. REVIEW BY ENGINEER. For many decades the Manawatu River has swept along its course on the outskirts,of Palmerston North, anc destructive in flood, difficulty to control and swift in its velocity, it has encroached steadily on the banks along its tortuous length. Valuable land has crumbled before the pressure of swirling waters which have bitten their ivay in, despite efforts to control the riverHowever, in the last twelve months the gargantuan appetite of the current appears to have been reasonably checked, and the degree of success achieved seems to promise that in the_ seven mile length in the Palmerston North River Board district at least excellent results may be anticipated in the near future, according to statements made yesterday by the engineer to a “Standard reporter. Fed by a watershed estimated to cover an area of one million and a half acres, and reaching a velocity of ten miles an hour in time of maximum flood, the Manawatu River is a potent force, and at the height of its turbulence can pour along at the rate of 100,000 cubic feet per second at the Fitzherbert Bridge. Nevertheless, some thirty groynes on the Palmerston North frontage have offered stout insistence this winter, and the erosion menace appeared, from an inspection made yesterday by the River Board engineer (Mr E. Fisher) to have been lessened to a very appreciable extent. Moreover, it was pointed out that a scheme of works was in hand with which it was hoped eventually to remove practically all danger of senous encroachment along the banks at Palmerston North. Contrary to the customary nature of events, two maximum floods had been experienced within twelve months, stated Mr Fisher, the last preceding flood of any extent having occurred three years previously, in July of 1926. The floods which took place in November and May last were exceptionally sever on new works or those of recent construction, as most of the protective work undertaken was really unsettled, but it withstood well the strain to which it was subjected. At indicated, however, that under certain conditions there would have to be a modification of the scheme, particularly at Mcßae’s bend, which constituted the worst problem with which the Palmerston North River Board had to deal in its area. , . Extending from a point at Te Matai to another some 25 chains below Botanical Road, the Palmerston North River Board had about seven miles of tortuous river front on which various remedial measures had been adopted to check the erosion, but it was a difficult stretch to control on account of the great velocity of the water, there being a fall of about twenty feet in the river level over the distance mentioned, and the greater the bends the more intensified was the erosion. Unless the flow of water around them was controlled and directed, bank works would be rendered inefficient or practically useless. It was estimated that during the last decade about thirty acres of first-class land, the value of which might be computed at £IOO an acre at least, had been swallowed up by the river, while seven acres disappeared from Mcßae’s bend in three years prior to the undertaking of protection work there. Very little land had been lost from the board’s district lately owing to the precautions taken. CONFIDENT OF SUCCESS. “I am confident of eventual success in permanently enecking the erosion,” staied Mr I’isner, who added that, although a considerable amount of money had been expended at Mcßae’s bend, it had made the Fitzroy bend aosoiutely secure from the menace of erosion. Actually not more than £IUUU had been spent on the Fitzroy bend. Serious erosion had in the past taken place at the old racecourse reserve, but it was intended to put in a system of diversion groynes which would have the ultimate effect of retrieving or reclaiming lost laud. “I think that the erosion menace will be overcome in time,” he continued. “It is hoped to see the completion, within two years, of the scheme now in hand to make the works fully effective. Much has been learned from the effects of the two maximum floods so far as the efficiency of the work is concerned. There is always great trouble encountered with the bends, where not only do large floods deposit debris on the convex side, accentuating the trouble of erosion opposite, but sweep away material along their course.”

The board's district extends to a mile above the protective scheme commenced at Anderson’s bend, and Mr Fisher pointed out that formerly the waters from big floods used to sweep over the embankment there, cutting across valuable farm land, and joining the river again opposite Ruahine Street. When that occurred, about forty acres were inundated and the worst feature was that the swiftly flowing water degraded back from the lowest point to within 70 chains of where it overflowed. An embankment was constructed ten years ago, but erosion subsequently took away the end, again exposing the area until flooding occurred. Another stop-bank was made, and by the scheme at present in hand the erosion had been stopped to such an extent that the protective works were already well established and the embankment rendered immune, fiom further erosion. In about forty years, some fifty acres of land had disappeared from the title of Mr Anderson’s property, but for the past four years no land had been lost, and the protective scheme embarked upon over 2i years ago had proved to be an unqualified success. A certain amount of relief was, however, afforded at that point by the flood channel across tHe sand accretion opposite. Both that bend and Mcßae’s "were vciy acute, but while the former bad only a radius of 25 chains the latter had a radius of 80 chains. With the view to straightening the course of the river and bringing it nearly parallel in its course to the existing groynes, which would greatly simplify the problem, it was intended to make a start in cutting off the low-lying point of land opposite Mr Gosling’s property. That would be undertaken by means of open groynes after the flood season was over, and it was anticipated that it would be put through in the coming summer. It had been expected that the scheme already in existence would have been sufficient, but while it would have achieved the desired effect eventually by reclamation, the process was too slow to afford the immediate relief sought at Mcßae’s bend. For the effective protection of McRae’s bend, the system of groynes would have to be continued right round, and it was proposed to construct another six. As far as the bend itself was concerned, the object was to dislodge the shingle beach opposite and with groynes to take the flood pressure off the bank. Starting opposite Mr Gosling’s land, the radius of*

the bend would be enlarged to give < a more gradual course, as at present the formation was more like a loop than a bend. The river would then have a quicker discharge, preventing the flood waters from contracting at that point, as it was the retardation caused at the bend which accounted for the excessivo erosion. What was once the swiftly-flowing main course at Fitzroy bend was now dead water, added Mr Fisher, and the diversion cut was sufficiently developed in area and capacity to be controlled. The diversion channel would be so regulated as to eliminate the possibility of the water returning to its old and destructive course. RESISTANCE BY WORKS. “The works have stood up remarkably well tins winter,” commented Mr Eisner, “and 76 per cent, have resisted tne strain in an exceptional manner. Under the conditions it was found at Mcßae’s bend that the bank scheme of work was insufficient, owing to the abnormal circumstances. It is really the only place which has suffered any damage, and it must be admitted that so far as Mcßae’s bend itself was concerned, the work was not effective. That is an argument in support of the facilitation of the scheme in hand. The principal activities have been the construction of works of a tentative nature at Mcßae’s bend pending the undertaking of the revised scheme. Eventual success is anticipated, but the potency of a voluminous and swift river cannot be foretold with any degree of certainty. Anderson’s bend* is regarded as a secure base from which to proceed with corrective measures in regard to Mcßae’s bend.” With the new work in contemplation another punt 36 feet by 20 feet was in course of construction, as progress was too slow with only one plant to cope with the activities. The new punt would be equipped with up-to-date pile driving machinery and derrick, and would probably be in commission in about two months. Material for the groynes was already being hauled to the sites. Various control works had been employed for many years past, stated Mr Fisher, and latterly much had been accomplished in minimising the waste of land due to erosion. That applied particularly to the upper reaches of the Manawatu River from the Gorge to the town of Palmerston North, where lands under maximum flood conditions of the river were not subject to inundations, while the lands on the lower reaches of the river towards Jb'oxton were, with maximum floods, extensively flooded, and consequently the settlers in that area were seriously affected: Much had been done in eliminating erosion in the upper part of the riyer, where the scheme of works specially designed to prevent erosion was being comprehensively applied, and, generally speaking, had proved successful, although much still remained to be done to bring about their completion. Extraordinary conditions had been encountered but valuable knowledge had been acquired in observing the effect obtained by the various works constituting the scheme under maximum flood conditions. The scheme of works consisted of a. combination of open groynes and willows, tho latter being the chief factor ultimately providing efficient and permanent protection of the river banks. The principle of the groynes being pervious they served well under all conditions and especially so in voluminous rivers of high flood velocity like the Manawatu. It was obvious that each year all fast flowing rivers assumed a more tortuous and distorted course, destroying an increasing area of rich alluvial land by erosion. That action was intensified by the congestion of the river channels with the accumulation of large shingle deposits dislodged by erosion and deposited on the convex sides of the bends, which restricted the flow and directed the pressure on to the vertical concave banks. The regulation of the channels was of vital importance in formulating a scheme for the prevention of erosion. It had to be pointed out, moreover, that land drainage operations had reached such a state of efficiency that the water found its way into the nver quicker than in former years, aggravating the danger of erosion and flood.