TIDAL AND CURRENT SURVEY—CANADA, 1923.
IVESTIGATIONS IN HAMILTON INLET AND IN THE NARROWS OFF RIGOLET.
TECHNICAL REPORT. By H. W. Jones.
The shores of Hamilton Inlet converge from the outer coast line of Labrador (in latitude 54° 8' to 54° 30') until about forty miles to the westward they almost meet, with Ticoralak Point, on the one hand, and Turner Head, on the other. These headlands are only about two miles apart, and from the general appearance of the map form the head of the Inlet. Beyond there is a slight expansion or basin off which a long narrow arm, Double Mer, opens to the westward while into it from the south west enters the short river which flows from Lake Melville. Here, in “ the Narrows ” off Rigolet, the behaviour of the current and the character of the water was investigated by the Tidal and Current Survey.
From Battle Harbour to Indian Harbour on the way north, and from Indian Harbour to St. Johns, Newfoundland, returning, the salinity or density of the sea water was determined in order to obtain a basis for comparison with ocean water, of all the samples taken during the season.
The density of the water was determined between Rigolet and Indian Harbour (fifty miles to the eastward at the outer extremity of the inlet) on five different runs, and on two more part way, to Ticoralak Island ; numerous samples were taken from the narrows off Rigolet at the surface and at a depth of 18 fathoms, at the times of slack water after the flood and ebb streams, during the progress of the season ; and the water of Lake Melville was tested at several points on the surface, at at different depths up to 80 fathoms near the western end. The temperature of the water was frequently taken also at the same time.
Tide gauges were erected at Rigolet, at Ticoralak Island and at Indian Harbour, to secure record that would determine the nature of the tidal fluctuation and be of use in conjunction with the rest of the work.
THE CURRENTS IN THE NARROWS OFF RIGOLET.
The width of the Narrows is one and half miles, and the depth of the water is twenty to twenty-four fathoms, excepting close to the shores. A
small scow was moored from both ends in mid-stream, as a fixed point from which to observe and meter the current. Thus the reversal of flow, or times of slack water could be determined with precision, which would be impossible on dark nights were the craft moored from one end only. The accuracy of the periods of flood and ebb flow, as found, depend on this.
Observations during a month, day and night, with some days lost because of storms, form the basis of the following data.
Strength of Current.—While the currents vary in strength under different tidal conditions, the ebb was found to be always stronger than the flood. A gale from the east may possibly reverse this order, but only while it lasts. The flood stream begins and ends more gradually than the ebb stream, which at the spring tides begins with a rush. The latter too, shows much greater fluctuations in velocity while running strongly, corresponding with the swirling of the water as in a swift river. (See Appendix “ a,” two diagrams.)
The meter for measuring velocities of the current was kept at a constant depth of 12 feet below the surface throughout the observations.
The flood stream never equalled the ebb stream in velocity while the ebb on one day was as much as five times the strength of the flood.
At spring tides the average velocities were, flood 3.37 knots and ebb 5.03 knots per hour. At neap tides the average velocities were, flood 1.89 knots and ebb 3.53 knots per hour.
(It happened that weather conditions prevented observations being taken on the exact days that the range of the spring tides was greatest, but those obtained were very close to being at the maximum of range in rise and fall.)
The velocity of the ebb was 80% greater than the flood, on the average of all the tidal streams measured.
Duration of Flow.—(1) In mid-channel. On three occasions the flood stream was noticed to run as long a time as the ebb stream preceding, or following ; but on the whole, the preponderance of the duration of the ebb over the flood on the surface was 1 hour and 7 minutes.
The under-current in mid-channel reversed its direction from ebb to flood and from flood to ebb, somewhat before the surface turned ; and it also flowed for a longer period of time outward to the sea than inwards, though not to so great an extent as did the surface current. (See Appendix “ b. ”)
On the surface the average period of the flood stream was 5 hours 37 minutes and the ebb 6 hours 44 minutes ; and for the under-current the periods were, respectively, 6 hours 00 minutes and 6 hours 21 minutes.
(2) Off the North shore. A spar buoy was anchored in ,15 fathoms of water about three cables from the shore at Square Rock Point in the narrows 40 just above Rigolet, and the turn of the current was observed by means of it, from July 5 to August 31, during daylight, and for periods during night time as well.
It will be seen from a glance at the chart that there is a curve to the south-ward in the direction of the shores of the Narrows from Double Mer Point to
Rigolet. This causes the flood stream as it moves inward to sweep along the north shore first, before extending across the whole width of the channel ; so that the current turns from ebb to flood earlier on this side. The other turn
from flood to ebb is also earlier but not as much. Thus the duration of the ebb current is necessarily less on the Rigolet shore. The observations by means of the spar show this to be the case, and give evidence that the duration of the ebb stream, though diminished relatively, is yet longer than the flood ; the average periods being 6 hours 17 minutes and 6 hours, respectively.
(3) No observations of the current were taken near the south shore ; but the behaviour of the flood stream, going in along the north shore first, crowds the last of the ebb stream over to the south side so that it must persist there later than in mid-stream. From the direction of the channel above, and the suddenness with which the ebb stream begins, the reversal from mid-stream over, must be about simultaneous. Therefore, with the ebb beginning as in mid-stream, and ending later than it does there, the duration of the ebb stream on the south shore must be even longer than in other places across
From the foregoing facts regarding the current, its strength and duration of flow in the two directions, it is evident that the mean level of the water in Lake Melville must be appreciably above mean sea level. Also from the same observations, the effect of the rivers flowing into the lake is apparent at the outlet through the narrows, and this is also shown later by a comparison of the water densities after flood and after ebb currents.
DENSITY OF THE WATER.
The density of the water as determined, is its actual specific gravity.
The numerous tests taken on the voyage from Indian Harbour to St. Johns (September 8 to 15) show that well off the coast of Newfoundland the density of the water is 1.0240, while in the vicinity of the coast from Hamilton Inlet almost to Belle Isle Strait, a lower density than of the open sea is found due to the effect of the numerous adjacent rivers. This density of 1.0240 is taken as standard sea water for all comparisons made.
Samples taken in the same locality in June and early July with the drifting ice about, and again in September when all ice with the exception of occasional ice-bergs had disappeared, indicated a less density for the early part of the summer, the difference being nearly 0.0010. Thus Indian Harbour water in the early part of the season may be as low as 94% of the density taken as standard sea water.
The samples were all brought to the standard temperature of 60° F., within half a degree, before being tested. This method avoided the need of applying any correction for temperature.
Between Rigolet and Indian Harbour.—The course taken by small open boats making this run is to the north of the middle of the estuary in its outer reaches, because precautions have to be taken with regard to the weather. Prevailing North-east winds doubtless drift the fresh water from the river
above to the south shore ; but it was impossible to go for samples along this coast in the time at our diposal. The results of the tests taken on our trips to Indian Harbour are best shown on the accompanying map. It is to be seen that evidence of river water is noticeable all the way down the inlet. (See appendix “ c.”)
The last of the series of tests on September 2–3 show a distinctly fresher state of the water for twenty miles below Rigolet. This may be due to heavy rains which my have fallen a short time previously in the back cournty, or to the general freshening of the water in Lake Melville with the progress of the season, and the last sample taken opposite Rigolet on August 27, compared with the earlier ones, seem to bear this out.
In the Narrows off Rigolet.—Samples were taken during the slack at the end of the ebb and flood, from July 13 to August 27, at the surface and at a depth of 18 fathoms, with the following results.
Average density of surface water after flood strea, 1.0182 or 76% of the standard sea water. Average density of surface water after ebb stream 1.0139 or 58% of standard sea water.
The water as it comes form the lake above is thus 18% less in density than when the current is inwards.
Average density at depth of 18 fathoms, after flood current is 1.0241 or the same as sea water. Average density at depth of 18 fathoms after ebb current, is 1.0208 or 87% of sea water.
It is thus apparent that the sea water flows in along the bottom towards the end of the flood tide, while at the end of the ebb tide it is 13% less salt.
On August 11 tests were made at the end of the flood, at 5 fathoms, 10 fathoms and 18 fathoms showing densities of 1.0200, 1.0219 and 1.0241, respectively.
Immediately below Henrietta Island.
—Samples taken at 29, 30 and 38 fathoms after the ebb stream gave densities of 1.0221, 1.0226 and 1.0228 or 92%, 94% and 95% of standard sea water. After the flood stream the full saltness of the sea must prevail here, at the lower-depths, as already it is found to do opposite Rigolet, near the bottom.
In Lake Melville.—Tests were made on August 21 and 22 as follows :
Two miles of St. Johns Island on course to westward ; surface density 1.0050 or 21% of sea density.
One and a half miles off Mulligans Head to the south ; surface density, 1.0031 or 13% of sea density.
At about three miles northward off Epinette Point after crossing the 8 fathom shoal, on which we sounded, on course from Mulligans Head, samples were taken at depths to 80 fathoms approximately. The restults of the densities graduated from 1.00184 or 7% to 1.0211 or 88% of sea salinity at 34 fathoms, and from that down they were practically the same. The water in this region, when taken from the surface of course is considered fresh enough for drinking. (See detail in appendix “ c.”)