Correspondence.

Timaru Herald, Volume XXXI, Issue 1628, 9 December 1879, Page 2

Correspondence.

THE MILFOBD HARBOR SCHEME. TO THE TOITOB OB THE TIMIEn HBBALD. Sir,— Somo time ago I smuscd myself with computing .- the scouring power available for keeping open, tho channel,, should the const raotiqn of harbor, worku at . Milford •bo proceeded with, and thinking the results' may interest sorno of jour readers, I TCnlure to offer them foi; publioatioQ m .your -columns. Sir John pood,p m t)iq ispo^t", e^jrt^' Tho flrat.nnd oardioat, point to be^detctjnjnpd m this oaso is: "Whotber or. nb^ thei scouring power duo to tho influx .and tho tidal water is sufficient, under , tho jnost favorable conditions . .. .. ....'j;-;.- for securing a permanent approach . wjth a sufficient depth for navigation purposes botween. Hilford .Lagoon and , the too. Upon this; I much regret 'to e»y that lam unable to ezpren a .favorable opinion." 4lr' ; Johnston, m his '"final 'report," says— "lt 'is of the first ~impor(ahbo to ascertain , • carefully the. amount of power at command ns a scouring agent j" bat there is no hint given m that fpport that ho took any pains -to Mooitain this. HoLtafces exception ta'Bif Jolfn CoodeV opinion that tlio scouring ; power would bo insufficient for the 'work it w^nld be, required' to] do, Jmt I think he would not dq eo : if ihe.h,sd' < ascerT tamed carefully" what the amount of thot €k "^^ power might be ■ expected to be. . ...' I notice flint Sir John Coode boa taken a much, larger number to ripress the area of tho Opitii bnsin than Mr Johnston did, but it will be seen (bat tin's ia not large enough to prorido a sufficient scouring power. It will be : readilj understood : . that , the/ greatest scouringoffect ontho bed of, tup, channel wilt .be produced whonj tho tide, is 'runnirig qu.t and when tho tide is low, for ihcn'thb effebt of tho tidal , waters whi-h ,had spread: orer> the lagoon (whotlier of sea-w^tor which had entered,' or of riror-water which bad ' been [banked ,bapk),, will bo added to, the 'Water flowing down tho river, and will flow turough. a shallower channel. ■ There is considerable difficulty m forming an estioialo of tho quantity of fresh water ordinarily flowing into the lagoon, but I intend to assume, iwbat I bolic'ro to bo , yery liberal data. Mr Johnston- thiuVs Bir ; John ,Co:>de has not taken a suQloirnlly heavy rainfall m calculating tlio scouring power. I desire to avoid such an error, and prefer to err m the other direction, if at all. I therefore assume: that the ordinary stream of the Opihi.is equsl to the draining of. an annual rainfuU of 3Oin. I think that this, m addition jo tho : rainfall producing freshes and floods, is a sufficionlly cxnggora'ed estimate I shall offer a few Ggurcs r<?f pooling the power of flood' J. but m tho firit place shall conGno myself to 'a conoidcr lit ion of tho ordinary stream, as it is admitted that it would not answer lo dopend uprn fliods to keep tho olianncl clear.- Sir John Coodo reckons the aroii of Iho Opihi basin at 800 »quare ratios. For tho convenience of calcula'ion, and slilLerring on tho aids of HbornlitT, I shall take the , aroa at 000 square miles. An annual rninfoll over nn area of 900 square miles is equivalent to. a stream discharging about 265,000.. cubio jards per hour. .:'.,■ Sir John Coodo estimates Iho quantity of tidal water that would flow into and out of tho lagoon during spring tides at under 3,000,000 cubio yards. Say it is so muob,and sun'poso that tho whole of it runs out m flvo hours: the. effluent waters, fresh and' tidal, will, amount to 865,000 oabio yards, per hour. •- It will bo convenient to find, m the first place, at what rate this volume must flaw m. passing through a channel one foot' deep and of given width. In order to make tho best of the caso I will take' tbo width of tho channel at 300 ft, which is less than, tho narrowest proposed by Mr Johnston. -A. simple calculation shows the quantity of water above mentioned, m flowing through a uhanool 300 ft wido and 000 foot doep m cut

hour, would have a moan velocity of 14.714 («ay 14i) miles, per hour.. . .] „, . Tha aulhorilios at my command (fire .Y36 miloa per hour as tho rate of current jre-' quired lo move grovel the Biw of brans; . • f 30<7 n« tho ' rate : for ono-inch pebbles ; ' 223 ' f r shingle thetixo of lions' eggs ; and 2.736 ns the rate for larger and rough stones liko r , brokon metal. Dividing thn rate of current m '» channel ona foot deep by tlieso numbers -, will give tho maximum depth of ohannol m .'. which the stream can moro tho different kind* , of material. 14.744 divided by .730 gives 20ft ], . 'as .tho greatest depth along which gavel tha , sizp of brans could be moved ; divided by ,1.396 "it gives about lOJft us the great out , depth along which pebblos one inch' m diameter could be moved ; divided by 2.28 it -, ( £>v<)s nboiit 6£ft as the greatest depth for j the tj transport of ahingls tho ai«e of • hens' eggs ) and divided by 2.736 it gives about sJft as . Iho limit of depth at which somewhat lafgor stones could bo moTod. • These figures show.. ' that; assuming a narrower channel than im. proposed, osnggernting all favorable data, and entirely 'neglecting the opposing action of tho sea, the depth to which the ordinary, river nnd tidal wattir*, during Spring tides, could srour such a channel, if not insigniQcant or altogether ,• - insufficient for navigation pnrposw, is very, emali, nod nothing like the depth? Mr Johnston snid he ox pec ted to obtain. And when ,/- Hie degree to which all favorable data have been exaggerated, and the oppoiing actiop of tho ten, are taken into accoant, and when it - is remembered that the depths arrived ot ■ oonld only be obtained by the current running; ' ronlinuously at tho minimum depth — which" it. could not do— it is plain that the channel . t . , which would.be really available would boa very pwr one from a maritime point of .view. A word, or two now about the scouring „ : pgwcif.^qf . floods : Mr Jphneton ,e«ljm»ted . ' tliAt on one occasion as much na 2,000,000 cubic feet- of water per ioioute .was disj»..chargrd through the lagoon. .This was a large quiutity, and one very seldom reaobed, I a.kould ray. It represented the total of a rainfall of about V3B inches m a day over, the whole of the assamed area of 900 square miles— a very heavy drainage indeed. It will be of iffiportaooe to know to what . depth even such an rxceptionol flood could fcour tho bottom of the> lagoon,. for although tho ontrance channel need not aecersarily be deep enough to float a vessel of a certain size nt low tide, the basin inside mast bo so. The . lugoon is said to about eight chains m width throughcat the portion proposed .to be used as a harbor, 2,000,000 cabio feet of witter per minute running through a ohannol of this width id a current one foot deep, would have a mean velocity of about 43. miles per hour. Divide this number by 2.736, the rate of current required to move stones ' like broken metal,, and we get 15.7 foot as the maximum ' depth at which even encli an exceptional flood cnuld move such materials along tho bottom. Sir John Coode states that tho depth of the lngoon at prcicnt is from three to four feet at> low water, so it is no wonder that the bottom is swept ulear of shinglo by floods now. If. tho Ingoou wero deepened,: by dredging or otherwise, a limit would very anon . bo. reached beyond which it would bo useless to deepen it, unless it .were also intended to dredge onb as it came all tho shingle brought down by the river. . Much smaller floods than the one calculated would be strong enough to bring shingle into tho logeun if it were deepened to 15ft at low water, but would be unable to carry itout to aea. - : ■ I am, 40, .. . . . ' Nota:-,Bbkb.