RIVER PROTECTIVE WORKS.

Marlborough Express, Volume XXXVI, Issue 77, 3 April 1902, Page 4

RIVER PROTECTIVE WORKS.

r> IMPORTANT REPORT. Some months ago the Lower Wairau River Board asked the Government for an engineer's report on the overflow of the Waihopai River into Gibson's Creek, recognising the diversion of the waters of the Waihopai and Wairau Rivers into the Opawa, via Gibson's Creek, as a serious danger to the town and district. The Government 1 acceded to this request and the application for a report on the proposed diversion of the Fairhall and Mill Creek into the Opawa. The work was carried out by Mr C. H. Williams, Government Road Engineer, Blenheim ; and at last night's meeting the Boxrd received from the Chief Engineer of Roads a copy of Mr Williams's report, which is as follows:— " To enable a detailed report, to be made, it was necessary to traverse the line of Gibson's Creek, also portions of the Wairau and Waihopai Rivers. The total length of Gibson's Creek from the intake to the Opawa River, to the breach at the groins at WaihoDai River, marked X on the plan No. 1, "is 7 miles 72| chains. A great quantity of gravel and silt has been deposited in Gibson's Creek from the overflow of the Waihopai. I understand it is about two years since the breach in the groins at Gibson's Creek took place, and no effect of any note has been made to remedy the evil, with the exception of two temporary groins erected a few months ago, as shown by the black line marked A and B on plan No. 1. You will see that the line of the work referred to by the letters A and B will not have the desired effect of diverting the flood waters below the Waihopai bridge. By inspecting the flood line of the river on the eastern side of the Waihopai bridge, it will be seen that the flood waters will flow at the back of the present facine work, and from thence into Gibson's Creek. The work referred to, I understand, was carried out by the settlers in the Renwick district. The class of work is faulty in construction, and its position for future benefits to be derived will be of little value. Protective works of a permanent character, consisting of wire netting and stones, will require to be constructed in the form of a training wall, starting from the end of the Waihopai bridge and extending to the Wairau River. This wall will require to be protected by a series of current baffles, the position of these will be fixed on completion of training wall. If the whole of the work cannot be undertaken at once I should recommend that the sections marked on plan No. 1, 14 to 21 and 23 to 24 ba undertaken, but if possible the whole work should be carried out. After the protective works are completed, current baffles consisting of wire netting and stones will require to be built 14 to 18 feet in length, and not less than \\ to 2 chains from the protective works where practicable. These baffles will assist to divert the stream, and will.al9O be the cause of the accumulation of silt and gravel between the baffles and the protective works. In all probability these baffles will, after a heavy flood, be forced slightly out of position, but they will be the cause while in position of forcing the river to do its own banking. If after a heavy flood it is found that the force of the water has scoured at the toe of the works and caused them to settle, new baffles will require to be built on top of the old work. Between the protective works and the river, where practicable, willow trees should be planted a few feet apart, and also the back of the protactive works, towards the maintenance. Present Protective Works:—As previously stated, I am of opinion that the work completed by the settlers is not of a permanent character, and I would suggest that the class of work that should be adopted for training wall and current baffles should consist of wire netting and large boulders; foundation of training wall to be of two rows of sacks laid longitudinally and transversely, filled with gravel and stones; the top row to be level with the surface of the ground and the bottom row to be excavated below the surface. The training wall to be not less than lft Gin higher than the current baffles. Current baffles may be built with stones only secured with wire netting. If the work is carried out I would strongly recommend that great attention be paid to it, and if possible the whole length of training wall should be undertaken. If this is carried out and properly maintained the Waihopai River will be confined to almost a straight course. Slight improvements may require to fca made from time to time; and damage to works caused by floods immediately repaired. Attention must also be paid to the encroachment of the Wairau river, at the point of main terrace on the western side of the Wairopai river. If a strong groin is built at the place referred to, it will probably be the mean* of saving a considerable area of ground from being washed away, as the main body of the Wairau river is row on the southern side, and if allowed to further encroach at the point of terrace referred to, and the protactive works at the Waihopai river Jnot carried out, it will only be a matter of time when the Wairau and Waihopai rivers will flow down Gibson's Creek into the Opawa river. With reference to the proposed diversion of the Fairhall river and Mill Creek into the Opawa \—\ wish to point out lit would be very unwise to divert the rivers referred to at the present time. The works at Waiho* pai will require to be carried out first, and their benefits ascertained before attempting to divert the present course of the above rivers. ;It may not be out of place to give the local bodies interested a few facts relating to the cause of the present congested state of the Opawa river. At the present time and for 'he last two years the greater portion of the Waihopai and Wairau river waters has been diverted from their original chann/ejß ? viz., Spring Creek district, into the Opaxv^ river. This latter river flows through the town of Blenheim, and is unable at the present time to discharge the accumulated R/fti&ers of the Waihopai and Wairau rivers. is now required to be done is to lessen jth/3 ypliime of water in the 'Opawa, and the jjcey tp the position to assist to carry this into effect is situated at the Waihopai river at the base foj? operations. Evidently the River Boards dp not think that the ay cumulated waters of the Waihopai and Wairau rivers flowing into the Opawa river is a serious matter, but from my point of view it is, for the town of Blenheim and the surrounding district. The local bodies have evidently overlooked the important fact that in former years the now called ,Qp#wO> fiver was only a small creek, the wfc#je of jbhp waters from the previously mentioned riyejjs b**ing discharged by way of Sprjng Creak district. This beipg the case, it necessarily follows that the cause of the present congested stata of the Opawa river, and the consequent flooding of the town of {Blenheim» must be sought for at the junction of the Waihopai and Wairau rivers. The drainage area of both these rivers is situated on the same watershed, although flowing from different (directions (as shown on plan). The ! jd#ain#gje avea of the Wairau river be • ing greater ih.an .tint pf ]fchp Waihopai, consequently it discharges mpr,6 watep; ' but the velocity of the Waihopai, which" has a fall of Ss'soft per mile, more than equals the increased volume of water in the Wairau, (its fall being only 17.08 per mile), and if confined will assist in diverting the Wairau water to

the northern and original channel, viz, Spring Creek district, and will then lessen the volume ol water in the Opawa. I find at the present time the greater portion of the flood water of the Waihopai is discharged below the town of Renwick into the Opawa, by way of Gibson's Creek, thus allowing the full force of the Wairau to be discharged to the southern side in the direction of ' the Opawa. Before the rivers referred to are diverted, the River Board must find some means to equalise the discharge of the Waihopai and Wairau Rivers, and this can only be accomplished by carrying out the proposed scheme. If sufficient funds are not available to carry out the proposed works, then it will be necessary to repair the temporary groin marked X on plan No 1, but the main works will require to bo undertaken at a future date, and if delayed for an indefinite period the Wairau and Waihopai will flow down Gibson's Creek into the Opawa. I would therefore advise the local bodies interested to endeavour to carry out the protective works. Estimated cost of proposed works :— 110 chains of training wall, including wire netting, £1000 ; repairing breach in old groins and strengthening pre- j sent work, £50 ; total, £1050." £ After discussion, it was resolved { that the Department and Mr Wil- j liams be thanked for the report, and £ that the request that run 111 be j vested in the Board to enable it to { carry out protective work in that t locality be re-urged on the Govern- j ment. ] The consideration of further action j was deferred. f