RIVER CONTROL

Press, Volume LXIX, Issue 20978, 5 October 1933, Page 6

RIVER CONTROL

— ! PROBLEM OF MOVING SHINGLE ADDRESS BY ME H. W. HARRIS "River Control" was the subject o£ an address by Mr H. W. Harris, engineer to the Waimakariri River Trust, to the members of the Philosophical Institute last evening. Mr E. F. Stead presided. The ideal scheme of river improvement, said the speaker, aimed at obtaining a balance between the quantity of material brought down by the river into the lower reaches and the quantity discharged at the mouth, for when balance was achieved aggradation and consequent overflow were prevented. Though at first thought it would appear most logical to start a scheme by working from the mouth up, this was- not necessarily the best procedure, as the cause of flooding was generally due to aggradation by deposition in the usually flattened gradients of the lower reaches. Wnen the works in the upper reaches were intelligently designed for stabilisation of the bed, the amount of work required lower down could often be greatly modified. Of the many different methods of improvement the design of the alignment of the channel was usually the matter of greatest concern, and where cut-offs were necessary it sometimes became very difficult because of disturbance of local interests and cost of land purchases involved. Next in importance was the cross section required and possible variation of it due to changes of gradient. In rivers such as the Waimakariri contraction of the cross section was necessary in the lower reaches in order to maintain a satisfactory flood velocity over the lengths of flattened gradients. From the alignment and cross section resulted the longitudinal profile, and from this was obtained the design of the necessary works required to bring about the desired results. Tree Planting-.

Judicious tree planting was an important phase of control work, the qualities required being quick, vigorous growth, and a deep rooting system. Willows and poplars were good because they were effective almost frpm the first year. , Concernihg the controversial subject of reafforestation of water-shed areas, the speaker said "that there was no doubt that deforestation in a watershed composed of steep country such as the Waimakariri, permitted of more rapid run-off of rainfall, and that more shingle was brought down into the beds through increasing 'slips and shingle slides and by bank erosion. In rain saturated country a fiirther rainfall was not held up to any extent by the forest but in intense falls of short duration the forest had a very great regulating effect. In cases where the river entered an estuary before scharging into the sea trouble was generally experienced in the tendency that the mouth had to alter its position, thus adding length and losing gradient—in this case an endeavour was made to stabilise the mouth, as it was hoped to do with the Waimakariri, and to help to achieve this artificial sand dunes were built while the other work was in progress. Speaking of groynes Mr Harris said that these were built to form "ponds" of dead water which silted up and formed their own protection. The groynes were placed at an angle of 70 to 80 degrees to the bank. The speaker then illustrated his remarks by lantern slides of the bed of the Waimakariri and work in progress there. A vote of thanks was carried by acclamation.