Town to Be Submerged for Reservoir
On the cover of this week’s issue of FIRE AND WATER ENGINEERING is shown a view of the village of Gilboa in the heart and higher sections of the Catskill Mountains. New York State. This entire village and a portion of the Schoharie Creek Valley will be flooded to form the proposed Schoharie reservoir. To effect this the Gilboa dam will be thrown across the valley at a point slightly to the left of the church shown in the picture. The top of the steeple of the church, which is at elevation 1125, will be five feet below the reservoir flow line and on the approximate center line of dam. The Schoharie development, on which work is under way, bears equal importance with the Esopus, inasmuch as each will be called upon to yield upwards of 250,-000,000 gallons daily to supply to the full carrying capacity, 500,000,000 gallons daily, of the main aqueduct. The new Gilboa dam will be located about four miles northeast from the Grand Gorge station of the Ulster & Delaware Railroad. The Grand Gorge station is 66 miles by rail from Kingston and 156 miles from New York City. It is 120 miles in an air line north of the City Hall. New York City, and 35 miles west of the Hudson River. The southerly line of Albany County extended westerly will pass directly through the Schoharie reservoir. The tributaries have their source at elevations of nearly 2,000 feet in the localities of Hunter, Wyndham, Prattsville and Grand Gorge, in Green, Delaware and Schoharie Counties. The watershed consists chiefly of steep mountains of shale and sand stone, which are covered with wild forest growth. Recause of this steep and rocky character, the flow in the streams is a large proportion of rain fall, it yielding with only 39.5 inches rain fall 27.2 inches (69 per cent.) as stream flow. As compared with this the Croton yields 22.4 inches of stream flow and the Wachusett watershed of the Boston supply yields but 21.3 inches of stream flow on the average.
The construction of the Gilboa dam will include about 396,000 cubic yards of earth excavation, 92,500 cubic yards of rock excavation, 617,000 cubic yards of refilling and embanking, 436,000 cubic yards of masonry and 480,000 barrels of Portland cement. The dam is composed of two parts an overfall masonry portion about 1,300 feet in length and having a maximum height of about 160 feet with the crest at Elevation 1,130 feet above sea-level, and an earth section at Elevation 1,150 feet, with core-wall, approximately 1,000 feet long. Along the down-stream toe of the dam there is to be constructed a channel for collecting and conveying the overflow flood waters into the present channel of Schoharie creek below the dam.
The 1,300-foot overfall portion of the dam will be constructed of cyclopean masonry, being large blocks of stone buried in concrete; the water side will be faced with natural stone down as far as the water will be drawn: the down-stream face of the masonry section will be made in large steps from 10 to 20 feet in tread and rise, all faced with natural stone with the overfall corners composed of the largest possible stones set on edge, thoroughly anchored. Phis portion is founded on solid rock.
The 1.000-foot earth section for the left or west bank is required because of the pre-glacial gorge, which swings under the mountainside; the masonry section, where it joins the earth section is flanked at right angles both up-stream and down-stream by long, high and heavy retaining-walls, faced with natural stone; these walls will intercept the long earth slopes of the earth section; beyond them the masonry section of the dam will taper into a core-wall which will be continuous throughout the length of the earth section and extend into the mountainside; the earth section in places will extend above 100 feet in height and will be upwards of 400 feet in thickness at the base; the water-side slope of the earth embankment will be paved with heavy stone.