Challenge at Glen Canyon | 1983-1984 Lake Powell Flood and Spillway Damage скачать в хорошем качестве

Challenge at Glen Canyon | 1983-1984 Lake Powell Flood and Spillway Damage

Challenge at Glen Canyon, 1983-1984 is a film produced by The United States Bureau of Reclamation. The film depicts the effects of the 1983 flooding in the Colorado River System on Lake Powell and Glen Canyon Dam, with an in-depth look at the redesign and reconstruction of the spillways in the dam to prepare for future flooding. Powell reservoir first filled in June 1980, seventeen years after the diversion tunnels around the new dam near the lower end of Glen Canyon were closed. Brim full, the reservoir holds almost exactly two average annual flows of the Colorado River: twenty-seven million acre feet (An acre foot will cover one acre with one foot of water. A football field, minus the end zones, is about an acre). During the fill period most of the water had to be bypassed to meet downstream users needs, human as well as wildlife. Glen Canyon Dam and its reservoir are managed to maximize electrical power production—a “cash register” that was intended not only to pay its own construction cost but also the costs of other Bureau of Reclamation structures in the Colorado River Basin. Accordingly, once the reservoir topped off, the Bureau has consistently interpreted the Congressional authorization for the “Glen Canyon Unit” as requiring Powell to be kept as full as Nature will permit. Under the government’s ideal scenario, in a non-drought situation, every year the spring runoff will just, exactly, fill the tank again. The snowpack in the mountains ringing the Colorado River Upper Basin is monitored and educated guesses are made about late spring precipitation events. In early May 1983 the Bureau found itself with too much water in Powell Reservoir. Winter had lingered unusually long and cold in much of the high country, and the government’s runoff modeling was, to be kind, inaccurate. Then it suddenly turned warm and rain began to fall over much of the 108,000 square mile basin above the reservoir. For the first time other than for brief tests, the dam’s spillways had to be placed in service. Many dams have simple over-the-crest spillways, but large dams, particularly ones with their powerplants located at the toe of the dam, must route overflows around the structures. The Bureau’s first big dam, Hoover, built in the 1930s, uses tunnels. Glen Canyon Dam was designed in much the same way, incorporating portions of the river diversion tunnels that had to be constructed around the dam site to manage the river during construction. It was an efficient arrangement, therefore appealing to the engineers, because the diversion tunnels could be partially utilized for the lower ends of the spillways. The downside, however, is that tunnels have a finite capacity. Boring two or three thousand feet of tunnel through rock is time consuming and expensive, so the tunnels were sized in a tradeoff between anticipated flood flows and cost. The tunnel spillways at both Hoover and Glen Canyon are not designed to run full, for then they would be under pressure, like water pipes. In fact, the tunnels are intended to operate like covered flumes, with a minimum 30% air gap, at atmospheric pressure, throughout their lengths. The design capacity of the combined spillways at Glen Canyon is 276,000 cubic feet per second when the reservoir is full. Historic flows in the Colorado at Lees Ferry (fifteen miles downstream from the dam) have often exceeded 200,000 cfs, and there is strong evidence, from debris found at the time Hoover was surveyed, that flows through the Grand Canyon have exceeded 400,000 cfs. When the Upper Basin decided to relieve itself purposefully in late May 1983, the Bureau began to bypass a few thousand cfs. To do so, the spillways’ radial gates were raised, permitting the water to flow under their lower edges and down the tunnels. Each tunnel carried the water down at a 55 degree slope through a smooth curve into a horizontal section that was part of the original river diversion tunnel in both abutments. Each diversion tunnel was plugged at the point where the descending spillway curves into the horizontal section. (Behind each plug is the water pressure at the bottom of the reservoir: 250 pounds per square inch with a full reservoir.) At the end of each tunnel, the 120-mph jet of water was directed away from the canyon wall and deflected upward by a 40 degree ramp to dissipate much of the water’s energy before it hit the river. Steve Hannon