WAIKATO RIVER OPERATIONS.

Pukekohe & Waiuku Times, Volume 5, Issue 211, 22 September 1916, Page 1

WAIKATO RIVER OPERATIONS.

Mr Ashley Hunter's Report. Tlio fallowing is the repoit of submitted by Mr Ashley Hunter, M.1.C.E., to the Department of Lands as tlio result of an inspection of the woiks on the Waikato liver cairied out by instructions of the Miuister for Lands (ihe St. Hon. W. F Mas-ey, M P.), viz : "The matters dealt with are comprised in the following order of reference:— " 1. The Biver Board scheme as affecting reclamation of the swamp lands at Mercer and si.uth of that township. % " 2. The scheme's eff ct upon the navigation of the river. " 3. As to whether it is injuring or is likely to injure the Aka Aka and Otaua lands. " Ihe Kiver Foard's scheme as affecting the reclamation of the swamplands nearMeiccr: In considering this matter it is advisable to first draw attention to certain points in connectiowwith the primary object which the Board had in view some four or five years ago. " The Waikato Eiver Board was, I le'ieve, oiiginally constituted as the Waikato Drainage Association, and from a lepoit dated 3rd, 1911, submitted to that bodj by Mr Breakell, 0.E., it would appear that the main object aimed at was the drainage of the Whangamarino and Maramarua swamps as a direct lesult of the proposed lowering of the Waikato river level at Mercer. Mr Breakell estimated that a reduction could bo tff.cted in the river level at Mercer amounting to six feet. Subsequently, in November, 1911, Mr C. Kennedy, in a report to the Board, based upon the survey and levels made by" Mr Breakell, estimate! that the liver level might he reduced by two feet at this point. Since then the Government survey, undertaken by Mr Thompson, discloses an actual error in Mr Breakell's levels between I'ort Waikato and Mercer amounting to about .'2\ feet, and this error more than nullifys Mr Kennedy's estimated reduction of 2 feet in level at Mercer From the series of levels taken by Mr Thompson over the swamps referred to it will be noted that the swamp level is an) thing between 54 feet and 58 feet above datum, and that a fair average level may be assumed o l 5G feet above datum. (Note—This datum is 50 feet below the level of the 1907 flood on top of H.W.S.T. at the Port Waikato wharf) _ .At the time of my inspection the river gorge at Mercer stood at 1 foot 4 inches, the reduced level of which would be 53.20 feet. At the junction of the Whangamarino and Maramarua creeks the reduced lovel of the water as determined by myself from one of Mr Thompson's B Ms. was 51.06 feet, while the level of the last November Hood, as defined on what ia known -as Mail's I-Jand closo to this point, was GO.BB feet. " Since to effectively drain swamp lands it is essential that the level of the water in th 9 drains sha'l be not less than 3 feet below the general level of the swamp, it is obvious that taking the average swamp level at 56 feet, the permanent average water level in the drains must be not higher than about 53 feet. '' This is not quite a foot higher than the low summer level at the Mercer wharf. As it is, this low level is reached at odd seasons only. A record extending over the last two and a-half years of the river level at Mercer, for a copy of which I am indebted to Mr Hamerj-engineer to the Auckland Harbour Board, shows the following :—ln 11*15, during the first three or four months, the river level stcol at about 52 feet above datum, while during the rest of the year it was well over the 5-1 foot level, and went as high as 58 feet for nearly two months, reaching a maximum height for short periods of over 5'J feet in J uno, July and November. In 1914, during November the river was exceptionally low, but duiing the five months from

April to August the level fluctuated between 5 o feet and 55 feet above datum. During the present year so far the river has been ve;y low, and has averaged about 53 fee: above datum. It will be apparent that no system of swamp drainage can be regarded as successful which fails to uuwater the land dining five or six months out of the welvo. " After a careful consideration of all the facts f am of tho opinion that the summer level of the river at Motcer would require to bj lowered not less than G feet in order to drain the Whangamaiino aid Maramarua swamps by gravitation, and, turtle r, that even assuming such a lowering of tho river p ssible, these swamps would bo still subject to periodical Hoods. '• As regards the litver Board's works in sj far as they are likely to elloet any alteration in the river level at Mere r, lam definitely of the opinion that no system of groynes or dredging carried out below Mercer can possibly lower the summer level at that point beyond perhaps a few inch, a, and that it is ijuito hopeless to look to those works as likely to assist in any degree whatever tho drainage of tiro swamps referred to.

"lie the Eivor Board's sthemo as affecting the navigation of the river: In view of tho rapidly increasing trallic on tho Waikato rivor and ita tributaries, any scheme having for its object tho improvement of tho river for navigation purposes must

he looked upon as a work of national

importance. j •' lam told that last year no less I tloiii 45,000 tons were conveyed to different points on ih-3 river and creeks. This total included coil, llax, manures, cream and chefse and other products, besides the carriage of over 5000 passengers. 'The waterways up the Whangaje and Waikaro Lal.es, the itia, Otaua, Whangamarino, Awaiua, Aka Aka and other streams are all navigable, and give access to large and valuable districts, in which settlement is rapidly increasing. '■Any scheme of'iiver impvovement must therefore be a comprehensive one,' and should take into account the effecis likely to be produced on these waterways by the work done on the main liver. It is on this account that I have always looked upon the proposal to lower the river level at Mercer and elsewhere as being utterly inconsistent with any scheme of navigation improvement. In the interests of navigation the main object to bo aimed at is the formation and maintenance of a summer lo.v water channel, and this object is to bo attained ' n )t' by lowering the wator level, but through re>tiioting, by means of submergod or low level gioyues, the width of the summer channel in the wide reaches aud, as a result, inducing a scour sufficient to maintain a depth sufficient for navigation, nt the same time keeping the low summer river levels aud slopes to approximately what they are at present. Ih j Waikato river is probably without a parallel in respect of the enormous quantity of fine pumice sand which is constantly drifting down the river, causing shoals to form very rapidly and on the slightest provocation. Any: scheme of river improvement therefore requires to bo very carefully designed aud p'anned as a whole, and also from full and sifficient data. That the River Board entered upon this task on insufficient data is pretty well known, and I believe I am correct in saying that neither has tiro Board ruappe I out auy comprehensive scheme dealing with the

whole of the river between Hamilton and the Heads. The method of putting in a groyne here aud another one there, more or less as fancy dictates is a wrong one, because, although such work may have the desired effect in tin immediato locality, yet the ultimate effects produced from the work may be a bad one in other parts of the river, resulting in further works being required to combat the harm done To quote a well known authority on the subject: 'The regulation, improvement and control of rivers constitutes one of the mott important and at the same time one of the most difficult branches of civil engineering :

<; The works carried out so far by the Liver Board on the lower reaches of the river consists of four separate sets of groynes. The object aimed at by the construction of these groynes ii the cutting off of the numerouschanuels below Kaitangata and the confinement of the river to a single channel along the south shore. The immediate effect of these groynes is to bottle up the water to a' ceitain extent above Devi's Elbow, and by slightly increasing the height there to give a greater fall and velocity down to where the groynes teimiuate. These longitudinal groynes are of more or less oj;.on construction and as a consequence silting will gradually take place at the back of them. This shoaling at the back will of course be of diiect benolit in further assisting to conlino channels. The shoaling and silting as a result of these works is most noticeable at tho back of the Kaitangata groynes, which wire the first ones put in. Hero a very considerable si ting up has tAcu place and is still going on. At low water it is possible actually to see the growth of this shoaliug, the sand rolling down in a constant stream. There is no doubt in my mind, but that as time goes on, this shoaling will gradually but surely extend down the northern channels and will ultimately block them for navigation purposes

" Having iu view the fact that tho Aka Aku and Otaua districts contain about 10,000 acres of most valuable land and that the settlers are voiy largely dependent on the northern channels as a means of access, it is of the first importance that nothing should be done which would in any way interfere with the present approaches to tho northern shore. That such an intoifercnco with waterways will ultimately take place unless precautionary measiuos are adopted, I feel convinced. It is perhaps premature to oiler any suggestion in regard to this point, hut it appears to mo that the Kiver Board would iiave been better advised had they split the river at Kaitangata into two main channels, ono on tho north eide and one on the south, thua insuring adequate access to the valuable lauds on tho north bank of tho river, " In order to determine tho increase in Bho.iling at tho back of tho Kaitangata groynes, I took a cross section from tho mouth of the eastern drain to a point noar thß foot of tho Kaitangata groynes. This section is about intermediate between crosseectious Nos. lo and 11 on Mr Thompson's plans, and is sullicieut to show pluii.ly what is taking place. " With respect to the Board's scheme as affecting the navigation above Mercer: It is impossible at present to oiler an opinion seeing that so far the Board has not formulated any comprehensive scheme

i deding wiih thi rivtr as a whole. I ' can oa y say that any lowering of : | the river *ioi>e will have a detiiil ment il offtct on the lakes und |en eke, a:;d at the fame tin e is pretty ceitiin to had to ii created ! shoaling in the liv.r j " Summarising the foregoing 10j marks I m-y say that in my ! opi ion—i "(a) Tha works so far carried out j will, in time, improvo tho navigable i channel between I'evil's Elbow and j Port Waikato. j " (b) Tho presold, or any other ! works, are rmt likely to produce any appreciable lowering of the river level nt M*:cer. "(c) The ultimate effect on the chancels giving access to the Aka Aka aui Otaua lands will Ih highly detrimental so far as navigation ia concerned. " Dealing cow with tho question as to whether the scheme is injuring or likely to injure tho Aka Aka jftitl Otaua lands, I take it that this refers more especially to the effects of the scheme on the drain igo of the lands referre 1 to. " As what is known as the eastern drain opposite Kaitangata is tiro one mostly alfticted by *tho works, a special examination was made of it wi'h the following results: "At low water 1-vcl neap tides ! the depth of water in the drain be tween a point one chain from the mouth to about three chains up averaged from 2fc 3in t'j 2ft Gin. From a point one chain up the drain the depth gradually shadowed t»wards the mouth until at the latter point it was only 18in. "This shoaling of 12in near the mouth consisted of p'imice sand, evidently brought in on ll >o 1 tides from the shoal banks outside. These shoal banks at the back of the Kaitangata gioynes are gradually extending down stream iu front of the entrance to this eastern drain, and it is merely a question of time when the drain will become more or less blocked aud its efficiency con-idir-ably impaired, as it is oxtremely doubtful if the velocity of flow in the drain is sufficient in itself to scour and keep open a channel to the depth required in face of the constantly increasing shoaling that is taking place. The depth of water in tho gut-.er at outlet of drain, observed at tha tame time, was from 3ft to 2ft Gin on the down stream sido, but shallowed to 2ft above the drain.

"As regards the diain a little to the east of the Mawhitiwhiti btream, 1 found a good depth of water and channel at tho outlet, aud do not see any likelihood of this draiu beinjr immediately affected by the Board's works, although ultimately it may be. The other main drainage is that into the Aka Aka, and so far I can see no evidence injsupport of the assumption that" tie Hoard's works so far as at present constructed are having any prejudi.al effect on the drainage, tho only result being as already pointed out that the navigation up the Aka Aka is likoly to be intervened with "