BRIDGES AND EMBANKMENTS.

Hawke's Bay Herald, Volume 12, Issue 912, 25 January 1868, Page 3

BRIDGES AND EMBANKMENTS.

[From the "Nelson Evening Mail," Jan. 16.] As anything pertaining to the bridging of our rivers, and the confining of them within certain limits is highly important to the public at large, situated as the New Zealand settlements are, in narrow valleys, drained by l'apid and destructive watercourses, crossing (in too many instances obstructing)our roads at every distance of five or six miles, no apology is necessary for laying before our readers the following remarks, which are founded on practical experience, and which plainly illustrate the possibility of entirely and easily controlling and directing the course of our rivers at a comparatively moderate cost. Hitherto our rivers have been looked upon as almost too formidable to cope with, but we believe this has arisen from a misapprehension of the action of the rivers, and not from any overwhelming difficulty offered by the rivers themselves ; and probably in years to come the wonder will be not how to control them, but that there should ever have been any difficulty about the matter, so simple will it appear, when once rightly understood. Piles driven at great cost deeply into the river bed is at the present time the sole means depended upon for making a foundation for a bridge pier, and pile driving and wattling are the principal means made use of for protecting river banks. In the midst of a river, as in the | case of a. bridge pier, no doubt the pile driving is effectual, the main objection being its costliness, but for protecting a bank it is useless, for the water washes underneath and behind. Besides this, the pile driving and planking or wattling has to be continued along the whole distance of the bank to be protected, at an enormous cost of time and material; whereas by concentrating the effort on one point, judiciously chosen, and placing a breakwater at such a point, at the proper angle, the current of water may be so deflected, and the force and set of the current so changed, that it will keep the new direction thus given to it for thirty or forty chains, and save this distance of river frontage. At any rate six such breakwaters to the mile may be held to be sufficient, and £10 per breakwater is an excessive estimate of the cost of such breakwaters. And now for an explanation of this contrivance that effects so much : — It is composed of three spars, about 14 feet long, bolted together with a single bolt at one end, and then stood upright on the other ends spread out like the legs of a theodolite. Outside, and about 5 feet from the bottom of these legs, three other spars are placed horizontally, and bolted together at the three corners, so that they take a bearing on the three expanded legs at the height before mentioned, and on. these horizontal spars are placed a number of other spars or branches of trees, close together, so as to form a rough flooring, on which stones are piled up to the pointed top where the three firstmentioned spars are bolted together, and the stones are kept from falling by the sides being walled up with slabs nailed to the said spars ; and in this manner five or 10 tons or more of stones are held together as one solid piece of rock, the whole weight resting on three legs, which ai'e thus tightly pressed on to the river-bed, and rendered comparatively immoveable. All that is now wanted is to close up tho side that is intended to meet and divert the stream, with planking or slabs, down to the shingle of the river-bed, and as soon as a flood comes, it will be found that the water, striking against the planked side, will be driven off and banked up considerably higher than the natural level of the rest of tho river. Tho water will be deflected from the breakwater just as a ball thrown on the ground in a slanting direction will rebound in the opposite one. But the great peculiarity and advantage of these breakwaters remains to be explained. As with pile driving and planking so with this breakwater we are describing ; the water, checked laterally, begins to expend the force thus checked perpendicularly in scooping out the shingle, and a great hole is formed, and the feet and planking are both undermined just as the planking in the case of it being nailed to piles is undermined, but the water does not get under the planking fastened to the breakwater as it does get under the planking fastened to piles, for the simple reason that the breakwater sinks as the bed of the river is removed from underneath it, and follows the bed down as stone after stone is washed away from under it, thus protecting the triangular piece of river bed left immediately under the breakwater with, a facing of planking down to the very bottom of the hole. After the water has expended its fury and got its course altered in this hole it has made for itself, it is spouted out in its new course, at the same time hurling out the debris behind the breakwater for many chains in length, thus forming a boulder bank aud shallow, and comparatively still water, where before there was deep water and a destructive current. On this fact we would base a theory of bridge building, namely, that by placing such breakwaters as we have attempted to describe immediately in front of the site of the piers of a, bridge or proposed bridge, the danger of undermining is entirely done away with; all this danger, together with the bumps of snags and floating timber, being borne by the breakwater in front, quite a distinct and. independent structure from the bridge. Thus the expensive pile driving now considered so indispensable might be superseded, and by putting such breakwaters in proper positions, not only could sites be made for bridges, just wherever the direction of the road and other considerations altogether irrespective of the river required such bridge to be placed, but such sites and bridges so protected would be rendered permanent and secure, and the cost of erecting and maintaining such bridges would be reduced, so that three or four bridges could be built for the money that is now required to build one. We have spoken of this as a theory. It is not all theory, for we could instance one breakwater which has been in operation for two years past, and which has stood the fury of the floods during that time, and saved some 25 or 30 chains of river bank. Another one has been in operation only during the past few months, but it has acted just as was intended, and during the last flood has split the water into two branches — the very purpose for which it was erected.