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CHANNEL CIIANGES—BOLIVAR CHANNEL.
Between headlands this channel has widened slightly and deepened materially; passing outside of the headlands, it branches out to the right and left, thus forming several channels across the bar.
The central channel (known as the Cylinder Channel) crosses the bar directly, and, while affording little less depth than the side channels, apparently has its ebb and Hood currents less deflected from the line of its axis by the winds.
Along the shore of Bolivar Peninsula is a smash channel, south of the central channel; and in direction following the line of Galveston Island shore is a wide channel, nised by the deepest-draught vessels admitted to the harbor, and, by reason of the slightly greater depth, properly considered the main channel.
Galveston Island was a swasli chapnel similar to that along the face of Bolivar Peninsula.
The changes in these are interesting. For convenience I will refer to them as Nos. 1, 2, 3, and 4, in the order above named.
The axis of No. 1 in 1867 was found north (near Bolivar Peninsula) of its position of 1851; in 1872 it had moved yet farther north.
This movement bas been followed by the disappearance of a small islet (Bird Key) on that side, and by the shoaling of No. 2.
No. 3 in the mean time has been shifting its axis in the contrary direction, or nearer Galveston Island, and at the same time it has advanced its 12-foot curve very greatly to the southward. The effect of this move has been the obliteration of No. 4.
The charts of 1851, 1867, and 1872 show these movements to have been continuous between their dates, and yet in progress.
Passing to the head of Bolivar Channel, we find that a similar deflection of the deep water axis has taken place in an opposite direction, or toward Pelican Island, thus throwing the main channel into Galveston Bay, farther to the west, and extending the branch that heads toward East Bay.
Between 1851 and 1867 Galveston Harbor shoaled and shortened noticeably at its upper extreinity, besides being shortened at its lower by the format of the inn bar. It also narrowed perceptibly, which was a matter of more importance, considering the little width of the deep water portion. In 1872 the width was found restored to nearly that of 1851, and the length of deep water about the same.
In Galveston Bay, west and south of Pelican Island, comparison of charts shows appreciable shoaling along the western margin of the bay and at the entrance to West Bay.
That these changes have greatly injured Galveston Harbor is patent, as also is the inference that if they are allowed to continue in the same direction they will ultimately destroy the harbor.
AGENCIES EFFECTING CIIANGES.
The Gulf tides and the currents (into Galveston Bay) caused by them having formed Bolivar Channel and Galveston Harbor, if afterward unintluenced by other causes, would probably not have been the means of effecting changes noted.
The winds have alone been the cause of changes-by the changes
they effect in direction and relocity of currents, and by the littoral cur. rent they cause.
The relations between the tides, winds, and their resulting currents are so variable that they are best studied from the chart. Tidal diagram and wind tables submitted, and these are arranged for the purpose of making unnecessary a long written statement.
The following may be useful in directing study and indicating effects:
The bighest tide observed during the time covered by our gange record was 3% feet above mean low-tide, the lowest 20 feet below this plane, giving what may be considered as a fair annual range of 6,'. feet.
The daily range of the tides is much less, and so variable because of winds that it cannot be stated; the greatest observed was 27%. feet, the least 10 feet.
The prevailing winds are from south around to northeast. Those next in frequency of occurrence are from the north and north west; those most infrequent are from the west and southwest. (See tables.)
These various winds apparently produce the following effects:
Continued from the southeast and east, they raise the tides iu Galveston Bay, making strong floods and weak ebbs.
From north and northwest they deplete the bay, reversing the above.
From northeast they retard the ebb from West Bay after the tide has commenced to flood through Bolivar Channel, thus causing the eddy forming the middle ground on the inner bar.
All winds from the eastward drive the tide currents toward the head of Galveston Island and Pelican Spit, thus moving them continually westward.
Easterly and northeast winds cause a littoral currentin the Gulf, carry: ing drifting sand to the west.
Westerly, south, and southwesterly winds reverse this current and the direction of drift.
The outer bar has been formed by this drift, and its shape given by the ebb and flood currents through Bolivar Channel; all, or nearly all, of the alluvial matter brought down by the streams emptying into the bay being deposited before nearing the bar.
The prevailing winds favor the flood current in filling West Bay around Pelican Island, rather than through Galveston Harbor; the difference in distance by the two routes being only about one mile. The effect of this is to shorten and narrow the harbor.
Winds from the south and southwest drive a large portion of the ebb from West Bay around Pelican Island, decreasing the volume of ebb through Galveston Harbor to the detriment of the channel across the inner bar. Northers, besides being dangerous to shipping in the harbor, bring in deposit from the west part of Galveston Bay.
PLANS FOR IMPROVEMENT.
It is evident from the character of the bar formation, and the exposed position of both bars, that improvement by dredging is not worthy of consideration.
Improvement can only be made by employment of jetties and training-walls.
FOR IMPROVEMENT OF OUTER BAR. It is proposed to construct parallel jetties along the lines marked C D and E F on the chart. The jetties are to be made of cement-covered
gabions, the details of which are shown by a drawing herewith. The gabions are to be six feet in diameter and six feet high, two rows in each jetty; all to be fastened to. gether by copper wire at the tops, and filled with sand by a dredge-boat alongside as they are placed in posi. tion. They may be called submerged jetties since they will not, except on a short portion of their lines, be built up to the plane of mean low-tide, while for the greater part their tops will be five or six feet below that plane.
The present area of discharge over the bar is 411.181260 square feet. The contracted area will be 274.112 square feet, the former measured along the crest of the bar, and the latter along the jetties and that portion of the crest between them.
The jetties are expected to act as training walls for the lower ebb current, while the upper will pass over them. They are calculated to give a depth on the outer bar of between 18 and 19 feet, and at the same time only confine and direct so much of the ebb and flood currents as may be useful, thus preventing as great advance of the bar gulrward as might be expected, were the jetties built up to the plane of mean low tide; further making them less exposed to the destructive action of storms, giving them less weight to sink them into the unstable material offered for a foundation, and making them less expensive.
It is thought these jetties will serve to catch the drifting sand brought by the littoral currents until they are covered by it, and ibat afterward the sand will be car. ried over them rather than around their ends, giving a cross-section something like this:
Mean low water.
Jetty C 1 20,200 feet long.
$369, 990 Saud filling 77,480 cubic yards, at 50 cents.
33, 744 Total cost..
408, 734 The cost of the gabions has been determined by making experimental ones; the cost of the filling was determined from experience in dredging on Red Fish Bar, where the work was as much exposed as it will be on the outer bar.
The two jetties should be built out so that their heads may be kept nearly opposite, for the reason that during the course of construction it may be found that shorter jetties than those laid down may answer all our purposes.
This reason suggests what is true, that this project, and all others for the improvement of the entrance to Galveston Harbor, must involve, while being carried out, modifications of the original plan, either to economize or render the work more effective. For example, it may be found not necessary to carry the jetties to the crest of the bar. In this case they will cost less than the estimate. On the other hand it may be found necessary to raise them. In this case I present the following estimate.
For one row of gabions placed 100 feet from the two rows already placed, and for two rows on top: (See sketch.)
18,199 gabions, at $30
$554, 970 00
58, 114 50
613, 084 50 405, 734 00
1,021, 828 50 It is possible that the addition for which the last estimate is made may be found necessary; I do not think it will.
After construction these jetties will, from time to time, require extension to keep pace with extension of the bar gulfward.
The times and amounts of such extensions cannot be stated, but it is my opinion that the advance of the bar will not be rapid.
This is based on the fact that the bar does not receive additions from the bay, but only from the littoral current which drifts the sand along the coast,
IMPROVEMENT OF INNER BAR. The jetty CD, by extending the head of Galveston Island to the position it had in 1841, when there was 30 feet of water over what is now the bar, will, I think, have the effect of giving at least a 20.foot channel across the bar.
The effect of the short jetty already built to C in. dicates this.
IMPROVEMENT OF THE HARBOR. The dams M N and KI, with Pelican Island and Pelican Spit, cut off West Bay from Galveston Bay, and are designed to have the effect of throwing all the water to fill West Bay as far as Caronkaway Point, through Galveston Harbor, for the purpose of widening the harbor. They will also return all of the ebb through the harbor and over the inner bar, resulting in increase of depth of channel across the latter. They will also protect the harbor from northers.
The jetty D H is designed to cut off all the flood-tide that would find its way over the shoal the jetty covers, and to carry the full volume of the tide into the harbor to widen and extend it opposite the city front.
G H, 5,000 feet long; number of gabions, 4 feet by 4 feet, 1,250; cubic yards of sandfilling, 2,282. 1,250 gabions, at $1,580
$19, 750 00 2,282 cubic yards filling, at 40 cents, (by band).
912 80 Total cost.....
20, 662 80
After the construction of the jetty G H, it may be found necessary to move the city wharf-front, which is now reached by long causeways, nearer the city. This will be the duty of the wharf company.
It will probably be advisable to connect the jetty C D with the wharves by a cheap levee, closing the break across Galveston Island, between the city and Fort Point, to prevent the water during northers from escaping from the harbor in that direction. Plans and estimates for this work can only be made after ascertaining the effects of the other harbor structures recommended.
1. Jetties ( D and EF. Two rows of gabions.
For year ending June 30, 1875, $500,000.
Much of the work can be done by contract, under the close supervision of an officer of engineers.
The commercial importance of the work has been stated in previous reports from this office. Now that St. Louis is in direct railroad communication with Galveston Harbor, the importance of improving the latter has been greatly increased.
Commercial statistics will be found in my last annual report on improvement of inner bar, Galveston Harbor; also location of work and nearest light-house.
The plan of improvement here submitted has been carefully studied. The details of construction are without precedent, but commend