Draft Study Plan Vol 1 (PDF) - Alaska Power and Telephone Company

DRAFT DOCUMENT
Dam
Dam Type: Because there would not be a road to the Connelly Lake dam site,
GLH believes the most economical type of dam would be a rockfill dam, with the rockfill
derived from a quarry near the dam site.
GLH believes that the most suitable dam types would be either a concrete-faced
rockfill dam or a rockfill dam with an upstream geomembrane protected by riprap; both
options will be evaluated. Rockfill volumes would not be significantly different between
the two types, and GLH does not expect any significantly differing environmental
impacts. Both types can be developed with acceptable levels of safety for the seismic
environment of the site.
Dam Height: Compared to a low dam, a higher dam would provide more storage
and greater generation, but at greater cost. GLH will evaluate the costs and benefits of
four dam heights to provide a basis for the final selection, as shown in the following
Table I. The highest dam (75 feet high with the crest at El 2350) is about the maximum
that can be constructed using a rockfill type of dam with appropriate face slopes. GLH
recommends that the highest dam option be the basis for study requests.
Table I: Dam Height Options
Dam height, feet 0 (1) 25 50 75
Dam crest elevation, feet N.A. 2300 2325 2350
Normal maximum water
surface elevation, feet
2278 (2) 2290 2315 2340
Active storage, acre-feet (3) 1,250 2,640 6,380 10,630
(1) Use existing lake only (no dam, i.e. Alternative 1)
(2) Approximate existing lake elevation
(3) Between El 2258 and the normal maximum water surface elevation
Transportation: Construction materials, equipment, and personnel would need
to be transported to the dam site from the powerhouse area. Initially, this would need to
be accomplished by helicopter; a heavy-lift helicopter would be necessary for large
pieces of equipment on a one-time basis, but personnel, materials, and tools would
require only a smaller machine on a daily basis.
GLH will also evaluate the feasibility of using the highline system for
transportation to the dam site. Note that with the proposed rockfill type of dam,
construction materials to be brought up to the dam site would be mostly cement,
reinforcing steel, form plywood, diesel fuel, and explosives; rock, sand, and gravel would
be derived from excavations up at the dam site.
Spillway
For reliability and ease of maintenance, GLH would use an ungated spillway with
a concrete control weir. GLH believes that it would be appropriate to size the spillway to
pass the Probable Maximum Flood (PMF) with the water level at or below the dam crest.
Draft Study Plan – June 2012 p. 6 Connelly Lake Hydroelectric Project