THE

Colorado River Dam is the diverting structure for the High Line or Main Canal of the Grand Valley project. It may be briefly described as a steel roller crest surmounting an ogee concrete weir, with a sluiceway and a canal intake of a capacity of 1,425 cubic feet per second, at its west end. The most notable feature of the dam is the roller crest by

during some seasons it appeared that it would be necessary to conserve every drop of water possible, the simple wicket dam was out of the question because of the excessive leakage inherent in that design. The alternatives, then, were some form of Taintor or Stoney gates and the roller crest. The cost of the several designs was found to be not very far apart and

platform. The quantity of water required for the canal can be drawn through eight of these gates, thus permitting any one to be out of commission without decreasing the intake capacity to a point below requirements. The velocity through the gates is a little less than 4 feet per second for maximum canal draft.

In front of the intake head wall is the sluiceway 60 feet wide and 235 feet long. The sill of the sluiceway rolling gate is 8 feet 4 inches lower than the sill of the intake gates and the entrance to the sluiceway is 6 feet 4 inches lower, thus permitting water to be drawn into the canal from the upper stratum and causing the silt to be deposited in the sluiceway. It is assumed that a velocity of about 24 feet per second at the entrance of the sluiceway is required to carry silt on into the canal, and the sluiceway will take a deposit almost 211⁄2 feet deep at its entrance before this velocity is reached. When the deposit of silt reaches this depth the sluiceway roller is raised and the sluiceway channel cleared by flushing the mud down the river.

The dam proper consists of 6 bays, each 70 feet wide, or a total of 420 feet. With an assumed floodwater level of 4592.5, which is the same elevation as that of the top of the rolling crest when in place, it was computed that a discharge of 50,000 second-feet could be taken by the 6 bays and the 60-foot sluiceway and in addition about 1,600 cubic feet could be carried by the canal intake. This discharge, 50,000 second-feet was the highest flow recorded on the river during the period of 18 years previous to the construction of the dam and occurred but once during that period, and then for a very short time only. This flood was duplicated, however, the summer following the beginning of construction and again in the summer following the completion of the dam. The six bays alone carried the latter flood, with an elevation about that of the top of the closed sluiceway roller, the water occasionally just lapping over the crest.

The dam rests on gravel. There is some sand and also some cobblestones, but the principal material is gravel. Borings made in the stream a short distance above the dam site and also a short distance below indicated that rock foundation would not be reached at less than 30 feet depth, and hence the dam was designed on the basis of exclusively gravel foundation. As a matter of fact, on a portion of the dam rock was encountered before reaching the designed bottom elevation of the main cut-off.

At the time this design was made, there were but two rolling crest dams in this country, one being a short single roller controlling a logway in the Boise Dam of the Bureau of Reclamation and the other being an installation of three rollers in the Washington Water Power Co.'s dam at Long Lake near Spokane. There were, however, some 20 or 30 dams of this character being successfully operated in Europe and perhaps half that number in other parts of the

world. A dam at Kibling, Germany, had one roller of 46 feet clear span and 28 feet high. Another near Stuttgart, Germany, had two spans of 92 feet by 12 feet. The Colorado River Dam when constructed was the largest of any in the world from the standpoint of combined height and length, although there have been constructed single spans of both greater height and greater length.

ADVANTAGES OF DESIGN

Among the advantages of the dam as designed are the following: The dam is laid square across the river, and is made as long as possible in order to reduce the necessary height of the movable steel crest and also to reduce the cost of the work required to safeguard the left bank where the Orchard Mesa canal lies close to the rock cliff. The openings between piers are large enough and the crest can be raised high enough so that the lodging of floating trees at time of flood is prevented. The flow into the intakes on either side of the river is normal to the flow of the river, so that there is little interference from drift. The sill of the intake gates is sufficiently high above the floor of the sluiceway, at the right bank, to let the heavier silt pass by the gates below the level of the sill. Any silt deposit formed between the floor of the sluiceway of the Government intake and the sill of the gates can be washed downstream by opening at intervals the 60-foot wide movable crest. The scouring effect of the water with the sluiceway open is vigorous, and the sluiceway is freed from deposit in a short time and with a small expenditure of water.

PROGRESS OF CONSTRUCTION

Authority to begin clearing site and construct camp was received in August, 1913, too late to consider crossing the river that year. It was necessary to get as far along as possible, however, so it was determined to build the sluiceway and as much as practicable of the canal intake that season with a temporary plant, at the same time carrying on construction of the main plant and preparing for heavy work next season. Another reason for desiring to get the sluiceway built the first season was so that it could be used as a by-pass for the river flow during main construction.

Concrete was first placed on January 9, 1914. Freezing occurred almost every night and occasionally in the daytime. The water was thoroughly heated but not the sand or gravel. Sheds were built over the green concrete and fires kept burning in stoves and salamanders during the coldest weather. Not a yard was lost through freezing.

Early in February construction of a pile bridge across the river was begun. This bridge was built principally for the purpose of constructing the cofferdam to turn the river. As soon as the ice broke up it was pushed to completion.

This high water caused great concern for the safety of the pile bridge. Ice jams, which it was feared might menace the bridge, did not occur, the principal danger being from large drift. Fortunately the larger amount of the drift passed through the sluiceway, but occasionally a tree would lodge crosswise in front of the bents and it was necessary to dynamite it in order to permit its passing between the bents. In this manner the bridge was protected during extreme flood and only one bent was damaged.

Some rock was placed from the pile bridge in the latter part of June and placing continued slowly, waiting for the water to fall. All the holes along the line of the dam were filled and the heavy rock core placed to the extent of raising the water about 2 feet by the end of July, but on account of the unusual stage of the water, the cofferdam was not sufficiently advanced to permit excavation until early in September.

Concreting was carried on through the winter with little interruption on account of weather. The piers were ready to receive the roller crests as soon as the fabricated material began to arrive in the early part of 1915.

ROLLER CRESTS

The main rollers consist each of a hollow steel cylinder 74 feet 914 inches long and 7 feet 134 inches in diameter, the ends projecting about 22 feet into recesses in the piers, and rolling on smooth tracks embedded in the side of the recesses at an angle of 20° with the vertical. Around each end of the roller is a toothed rim engaging a toothed rack fixed parallel with the track. Operation is effected by means of a chain, one end of which is fastened to and partly encircles the roller, the other being wound around a drum in the hoist house on top of the pier.

The hollow cylinder acts as an axle, and the required height of the crest is obtained by fastening to the cylinder an extension shield which rests on the sill when the roller is down, thus forming the bottom seal. The ends are sealed by means of oak strips attached to flexible diaphragms near the ends of the cylinders. The sluiceway roller crest is built on essentially the same design but with different dimensions.

The patentees submitted three different schemes for the roller system, all of which included the 15 by 60 foot sluiceway roller. The other rollers were all 10 feet 3 inches high and the schemes contemplated seven rollers of 60 feet length, six rollers of 70 feet or five rollers of 84 feet. It was found that there was practically no difference in the cost of the three schemes and the one of six rollers each 70 feet long was chosen, as that was the layout on which had been based most of the plans. The patentees were so advised and asked for proposals, first, for the fabrication of the rollers and, second, for royalty required if rollers were fabricated in this country. Upon receipt of these proposals the German company was notified of the acceptance

of its offer for fabrication of the rollers. In September, 1914, the company advised that it could not continue with the contract and the matter of working over the general drawings to conform to American practice in fabrication was immediately taken up. The drawings were completed and submitted to bidders in October. Seventeen bids were received, running from $15,000 to $50,000. The contract was awarded to the Riter Conley Co., of Pittsburgh, delivery to be made in seven shipments, beginning February 8, 1915, and running to March 22, 1915. There was from three to five days' delay in the first six shipments and 17 days in the last. This contract covered the rollers only. Contracts for the hoists were awarded to the LinkBelt Co. and the Minneapolis Steel & Machinery Co., for the 60-foot and 70-foot rollers, respectively.

OPERATING MECHANISM

The operating mechanism for each 70-foot roller consists of a hoist actuated by a 10-horsepower, 220volt, direct-current electric motor, and a similar motor of 20 horsepower for the 60-foot roller. Through a series of gears the motor rotates the 52-inch chain shaft, to which is forged an 8-tooth sprocket winding up a powerful chain which is attached to the rim at one end of the roller. The rollers are driven at one end only, the hoists for two adjacent rollers being placed on the same pier, and hoist houses are therefore provided only on alternate piers.

Each 70-foot roller weighs approximately 40 tons, and the weight of the 60-foot roller is 53 tons. The maximum chain pull required to raise the rollers is 25 tons for the 70-foot roller and 45 tons for the 60foot roller. The chains are of special design and are made up of heavy pins and links manufactured from high-test steel, with an ultimate tensile strength exceeding 80,000 pounds per square inch.

The 70-foot rollers can be raised from the lowest to the highest position, a distance of 16 feet, in 15 minutes. A power plant at the west end of the dam generates electric power for operating the rollers. This plant contains a generating set consisting of a 4cylinder automobile type gasoline engine direct connected to a 25-kilowatt, 250-volt generator. The generator is used to charge a storage battery, which has a capacity of 108 ampere hours, but can also be used to furnish power direct to the motors independent of the battery. When all the rollers must be raised, as during the high-water period or during the winter season, power is furnished direct from the gasoline engine and generator. The nine canal head gates at the west end of the dam are operated by a 3-horsepower electric motor.

The power plant and operating machinery have proved very satisfactory and no difficulty has been experienced in the operation of the rollers. As a general rule the rollers are raised to the full height only