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“ The Gulf of Cambay is at the head of the great tidal wave
which flows into the Indian Ocean, and is here heaped up in an extraordinary manner, having a rise of 28 feet during spring tides, which occur at 5 hours 20 minutes. Full and change at Cambay. The corresponding node of this great tidal wave is
at the Maldive Islands. “From an examination of the co-tidal lines drawn from ascer
tained data, it is evident that the tidal wave sets into the Gulf from a South-Westerly direction, and is in fact a considerable way up it before reaching the mouth of the Taptee River on the East shores. There are five large rivers which empty themselves into the Gulf of Cambay, and it is from the detritus brought down by these rivers that the banks are formed at the entrance into the Gulf. Shallow soundings of 20 fathoms only, extend for a distance of 100 miles beyond the entrance of the Gulf. The line of 20 fathoms soundings runs from immediately opposite Bombay in a North-Westerly direction up to the entrance to the Gulf of Kutch. The waters over this expanse of sea are thick and muddy, containing matter in suspension, a portion of which is gradually deposited at the bottom of the sea, thus increasing the banks. It would be interesting to ascertain by a comparison of soundings at distant periods at what rate this deposit takes place, but owing to the great tidal rise and the scouring process going on at the bottom of the ocean, especially the latter, it is believed that the deposit is exceedingly slow, and that much of the detritus is carried down by the ebb-tide along the coast to the node of the tide near the
Maldive and Lacadive Islands. “The peculiar scouring and grinding process is very clearly shown
to a depth of 50 fathoms by the soundings (see Model Chart), though it has usually been assumed that the action of the sea does not affect the bottom at a greater depth than ten fathoms, the effect of the prevailing strong winds can however be traced to a very much greater depth. By an inspection of the Chart it is evident from the soundings that the greatest effect produced
by this scouring process is on the Eastern side of the Gulf.” Dr. Oldham in his memorandum assumes that the detritus brought down by the Taptee and other Rivers in the Gulf of Cambay is depo
sited at their mouths, and that the sea is receding along the coast. This however is only partially the case, as will be readily seen by examining a Model Chart of the Coast, for most of the silt or detritus is carried away by the scour of the receding tidal wave. This detritus finds its way gradually down the Coast—where it is deposited on “ Direction,” “ Angria,” “ Adas,” and other banks, a portion finding a resting place at the Lacadive and Maldive Islands.
The line of 15 fathoms water along the coast is :
7 miles distant from the shore at the Taptee, 16 miles do. do. at Bulsar,
6 miles at Danoo, where the coast line projects, 20 miles at Bassein,
16 miles opposite Bombay, and 10 miles opposite Rajpoor.
But the greatest accumulation near the Gulf of Cambay is on the Western shore or South-East Coast of Kattyawar, between Diu, Jaffrabad, and Goapanath, where the 15 fathoms' line extends out to upwards of 46 nautical miles in a South-Easterly direction, like a huge submarine groyne. It is this bank which checks the great tidal wave coming round from the South-East and causes it to heap up near and into the Runn of Cutch, and to scour away the adjacent coast; and as this bank extends, the effect will in a proportionate degree be increased, so that part of the waters at present heaped up in the Gulf of Cambay to a height of 28 feet (and formerly no doubt to a much greater height, judging from the appearance of the adjacent coasts on either side) will increase the elevation, and the proposed bench-marks will show the relative height of the mean sea level.
This opinion is contrary to the one usually adopted, but there is no want of substantial and reliable proofs to support this view of the subject; and as these proofs are very close at hand, it will be as well perhaps to give one or two illustrations such as will help to establish more firmly the opinion put forth.
Take the Taptee River for example. Large vessels formerly sailed up to Surat, but there is no record as to the height of the tidal rise at that time ; doubtless the river has since that time silted up considerably. The present tidal rise is about 19 feet at its mouth and 12 feet at Surat during spring tides. The mouth of the river itself is a broad
estuary, about 3 to 4 miles wide, and it is in a recess quite protected from the down scour of the Gulf of Cambay by a point running out South-West at Vaux's Tomb. The result has been a great increase of the bar at the entrance of the Taptee, and the tidal wave has to overcome this bar at its mouth before it passes on to Surat; the consequence is a much less tidal rise there than formerly ; but it would be absurd therefore to assume that the land at Surat had gradually risen because the mean sea level in the river has been lowered by a cause which is very obvious-namely, the bar at the mouth.
Again at Bombay—it is well known that formerly there were a number of separate Islands, and the tide flowed over what is noy called the flats and other parts of Bombay. Opposite Bombay to seaward there is a large and gradually growing bank, called “Direction Bank,” which checks the flow of the tidal wave, and consequently decreases the mean sea level in the Harbour, perhaps only a few inches in a hundred years, still the cause and effect are much too palpable to be gainsaid, and it would be quite a mistake to attribute this to a gradual rise of the Island of Bombay.
Another instance may be furnished where a contrary result has taken place by artificial works, namely the Clyde in Scotland. This River had formerly a tidal rise at Glasgow of only 3} feet, whereas now by removing the sand banks it has a rise of 7 feet at the Broomielair Quay; would it not then be contrary to fact to attribute this to a depression of the adjacent quay walls and of the surrounding country?
This is just exactly what is occurring at the Runn of Kutch ; the land is not being depressed, but the tidal wave is being kept or driven back westward, becoming more and more heaped up by the gradually increasing size of the large bank between Goapanath, Jaffrabad, and Diu—as most clearly shown by a Model Chart of this part of the Kattyawar Coast.
Were a chart available of the coast near to Kutch, the action of the sea could be as distinctly shown, and the probable changes readily deduced, without waiting an indefinite period to solve what is really a very simple question ; for there is nothing whatever either unaccountable or mysterious exhibited in the phenomena observable at the Gulf of Cambay, the Runn of Kutch, or the intervening Coast line.
In the memoir on the Western Coast Harbours which has been referred to, it has been pointed out that there is an oscillation
of the great tidal wave commencing on the Coast of South Australia ; the distance between the node and head of each tide is 30° or 1,800 nautical miles. This curious fact was discovered when laying down the cotidal lines on a chart of the Indian ocean. It is a subject worthy of further investigation, and it is intended to follow it up as opportunities are afforded. The rate at which these great tidal waves travel or are propagated is from 75 to 100 miles per hour. Should a strong wind prevail during a length of time from the direction in which these waves flow, then such disasters follow as those which occurred in Masulipatum and Calcutta in 1864, where the tidal wave swept over the whole country, devastating it for miles. The elevation then attained being doubtless at one period the mean sea level ; a few feet of rise would lay an immense extent of country under water both in Bengal, Madras, and Guzerat.
SUPPLEMENTARY MEMORANDUM ON THE GEOLOGI
CAL ACTION ON THE SOUTH COAST OF KATTYAWAR, AND IN THE RUNN OF KUTCH ; also on the ADJACENT COAST LINE, AS FAR AS KURRACHEE. By WILLIAM SOWERBY, Esq., C.E., F.G.S. Contributed by the Author. 3rd February 1868.
In the former Memorandum it was stated that the increase of the tidal rise in the Runn of Kutch, and the apparent subsidence of the land, was because of the great tidal wave being checked from flowing into the Bay of Cambay by the growing bank opposite to Diu, Jaffrabad, and Goapanath.
There are two Model Charts accompanying this Memoraudum ; these Charts show the various soundings by colours, each different colour showing a difference of five fathoms in depth ; also the co-tidal lines are indicated by red lines with Roman numerals.
By reference to Model Chart No. I., the action of the scour of the tide will be seen by the formation of a Submarine Bay to the south of Diu Head. This Bay is 35 nautical miles at its broadest point, and it runs in to landward about 12 nautical miles. This Bay is shown where the purple colour indents into the shallower water-(coloured red.)
The comparative velocity with which the tidal wave flows before it is checked by the bank between Diu, Jaffrabad, and Goapanath, and after it has to pass over the bank, is shown by the co-tidal lines, for it flows from opposite Mool-Dwarka to Jaffrabad, a distance of about 50 nautical miles, in 1 hour and 5 minutes, as per Admiralty Tide Tables, namely, from 10 hrs. 30 m. to 11 hrs. 35 m., while from Jaffrabad to Goapanath, a distance also of about 50 miles, it takes from 11 hrs. 35 m. to 2 hrs. 15 m., or two hours and forty minutes, or at little more than one-third of the velocity it travelled over the former fifty miles. The retarding effect of this bank is therefore very conspicuous and decided.
On an inspection of Model Chart No. 2 (which was not in the possession of the Author when the former Memorandum was written), it will be readily seen that there is another very formidable bank which helps to force up the tidal waters into the Runn of Kutch, namely, the bank to the Westward, formed by the sediment and detritus brought down by the Indus. The area of this bank is between 4,000 and 5,000 square miles ; and its retarding influence is clearly indicated by its taking two hours and a quarter for the tidal wave to travel 35 miles, namely from Dwarka to Assar point.
The great tidal wave is thus influenced by the submarine groyne to the Eastward, at the entrance of the Gulf of Cambay, between Diu, Jaffrabad, and Goapanath, and by the enormous bank formed by the detritus from the Indus, lying between the mouths of that river and the Gulf of Kutch, and thus the waters are forced up the Runn of Kutch in greater quantity, and to a greater height, gradually increasing from time to time, as the two Banks on either side increase in extent, and the consequent increased effect in retarding the flow of the tidal wave.
There are two submarine bays near the entrance to the Gulf of Kutch, in 20 fathoms water; the one is opposite Veerwarrah, and is 50 nautical miles wide at its outer entrance, and runs 12 miles inwards towards the land. The other is to the Westward of Lushington Shoal, and is about 60 nautical miles wide, and runs inwards for about 20 miles—these are shown by the purple colour-bordering the shallow water (coloured red).