Moyocoya Dam

Moyocoya Dam
Moyocoya Dam at night
Official name	Moyocoya Dam
Location	Itzatlahuitl, Xallipan
Purpose	Power, flood control, water storage, recreation
Status	Operational
Construction began	1932
Opening date	1939 (1939)
Construction cost	61 million Zacapine amatl (49 million Latin solidus) in 1932
Owner(s)	Zacapine government
Operator(s)	Roads, Bridges and Dams Administration
Dam and spillways
Type of dam	Concrete arch-gravity dam
Impounds	Yoliliz river
Height	222 m (728 ft)
Length	380 m (1,250 ft)
Elevation at crest	377 m (1,237 ft)
Width (crest)	14 m (46 ft)
Width (base)	200 m (660 ft)
Dam volume	2,480,000 m3 (3,240,000 cu yd)
Spillway type	2 controlled drum gates
Spillway capacity	11,000 m3/s (390,000 cu ft/s)
Reservoir
Creates	Lake Seti
Total capacity	40.500 km3 (32,834,000 acre⋅ft)
Active capacity	22.346 km3 (18,116,000 acre⋅ft)
Inactive capacity	18.154 km3 (14,718,000 acre⋅ft)
Catchment area	500,000 km2 (190,000 sq mi)
Surface area	544 km2 (210 sq mi)
Maximum length	175 km (109 mi)
Maximum water depth	200 m (660 ft)
Normal elevation	3,280 m (10,760 ft)
Power Station
Operator(s)	Electric Network Administration
Commission date	1939–1960
Hydraulic head	180 m (590 ft) (Max)
Turbines	16 × 150 MW 2 × 65 MW 2 × 4 MW
Installed capacity	2,538 MW
Capacity factor	29%
Annual generation	6.4 TWh (23 PJ) (2022)

The Moyocoya Dam is a hydroelectric arch-gravity dam in the Itzatlahuitl section of the Yoliliz river in the Xallipan Republic of Zacapican. Built between 1932 and 1939, the Moyocoya Dam was the last of the great Xolotecatl era megaprojects to be completed as it was finished almost a full year after the assassination of Xolotecatl Acuixoc in 1938. The Yoliliz River Project as it was originally known was the first major public works project ever proposed under the Xolotecate regime, making it both the first and last of Xolotecatl's projects. The dam was proposed by the Public Works Secretary, Achto Moyocoya, who championed the project and rallied political support to push through the many hurtles the dam would face before and during construction. The project would come to bear Secretary Moyocoya's name as a result of his vigorous advocacy. Moyocoya Dam claimed the record of the tallest dam in the world and the largest by internal volume at the time of its completion in 1939, and remains the largest and tallest dam in Zacapican. The dam is widely regarded as a great feat of engineering and an important landmark of northern Zacapican, making it a popular tourist destination as well as a functional piece of infrastructure.

The dam sits just 40 kilometers from the center of Tzopilopan, a city of 1.7 million and the capital of Xallipan. The electricity generated by the dam's 20 turbines is primarily directed towards Tzopilopan and its suburbs, and has served as the main supply of electric power in the region since the turbine halls began operating in 1939, two weeks after the completion of the dam's construction. The reservoir created by the Moyocoya Dam, known as Lake Seti, represents the largest body of water in the arid Xallipan region of the country. This water is not only piped to Tzopilopan and the other settlements of the region for the personal consumption and use of its citizens, but also represents the lifeblood of the region's agriculture and local food supply. The Yoliliz river which the Moyocoya dams is naturally temperamental, prone to heavy seasonal flooding as well as major fluctuations in flow throughout the year. The dam not only provides a means of flood control, protecting the downstream agricultural communities from the summer snowmelt flooding, but also retains water from the summer floods for use in Xallipan during the dry months from April to October when the river runs at a much lower flow level and recedes within its banks.

Background

Dams had been proposed to be built at various locations along the upper course of the Yoliliz river in the upper Meco valley and Xallipan plateau due to the region's aridity and the severity of the river's seasonal floods, with proposals going as far back as 1880 under pre-revolutionary Aztapamatlan. This region of the country had been a sparsely populated and largely uninhabited border zone for centuries, in large part due to the lack of an significant water resources to sustain large scale agriculture. What agriculture there was usually established itself on the banks of the Yoliliz river which in turn rendered it vulnerable to the unpredictable floods which would hit between October and April every year. As late as the 17th century, large sections of the population in the region were semi-nomadic tribesmen of the highland desert. Xallipan was the last region of modern day Zacapican to have urbanized and remains one of the least developed constituent republics, thanks both to its high elevation, desert climate, and its status as the only landlocked republic in Zacapican. Because of this, Xallipan has long been the target of government projects attempting to foster development and a transformation of Xallipan into a more productive region of the country. The Yoliliz River Project would be the centerpiece of the Xolotecate era initiative to establish the infrastructural basis for Xallipan to join the rest of the country in the ongoing modernization and industrialization process of that era.

The Itzatlahuitl (White Canyon) site had been singled out as a potential site for a future dam as early as 1914, the same year Xolotecatl Acuixoc came to power in Zacapican. The canyon as seen as a key restriction in the river as it wound its way down the Xallipan plateau from its headwaters in the northern Mixtepemec mountain chain, and promised to be the most feasible site for a future dam. Itzatlahuitl has the logical site for such a dam to be built, but many of the possible locations along the length of the canyon were found to be lacking when surveyed. Criteria for a dam site required that the canyon be within a certain width, narrow enough for the dam to effectively span the width yet also wide enough that a work site could be established within the canyon to build the dam. The geological characteristics of the canyon walls would also need to be taken into account, as the rock would not only need to bear the weight of the water transmitted through the arch-dam design but also have several tunnels excavated through it to allow the river to be diverted around the dam site during construction. Several possible sites were also disqualified from consideration when significant faults were found in the canyons walls which would present serious vulnerabilities in the structural integrity of the project. The future site of the dam was surveyed and found to be suitable in 1921. Construction of the dam would be delayed by a further eleven years, however, thanks a lack of funding and a rise in political resistance from within the Xolotecatl regime during the 1920s.

Construction

Diversion tunnels

In order for any construction of the dam itself to proceed, the section of canyon would have to be drained for the full vertical height of the canyon rock faces to be cleared and prepared and for the dam to begin construction from the riverbed upward. This would require the flow of the river to be blocked using two temporary cofferdams made of blasted rock and earth fill to block the river from taking its natural course through the canyon, while diversion tunnels would have to be dug through the canyon walls themselves to bypass this blocked off section and allow the waters of Yoliliz to flow past the site unimpeded. The tunnels, which would have to be cut before the cofferdams could be put in place, represented the first major hurtle in the construction of the dam. The plans called for one tunnel to be cut into each side of the canyon, providing redundancy in case either tunnel would be clogged, collapsed or otherwise fail during the construction of the dam. Each tunnel would be roughly 1.3 kilometers long and some 25 meters in diameter to carry the river's waters through the canyon walls all the way around the dammed off section of the canyon and dump it back into the canyon downriver of the work site. They would also have to be wide enough and durable enough to withstand the duress of the summer floods for several years in a row while construction proceeded.

Tunneling began using the well proven method of drilling bore holes which would be filled with dynamite and then blasting out the layers of rock to expand the tunnel. In order to speed this process up, the construction crews would acquire specialized vehicles which were nicknamed totolontic trucks for their large size. This custom vehicles were lowered by crane from the top of the canyon and reversed into the tunnels, and provided a mobile platform for a dozen bore holes to be drilled at once from a three-decked superstructure built onto the back of the vehicle. The Totolontics are regarded as one of the earliest examples of a mechanized tunnel boring machine. One is preserved on the site of the dam's visitor center as a museum piece. Once the tunnels were blasted out using this method, a concrete lining a meter thick in places would be laid onto the walls via mobile crane set up inside the tunnel on rails. Construction of the cofferdams began early 1933, shortly before the tunnels were finished and nearly a year since construction had begun, finally draining the dam work site.

Rock clearance

Before the main construction project could begin, the unstable walls of the canyon would need to be cleared of loose rocks and stabilized in preparation for the dam to be build directly abutting the canyon walls. Clearance would need to take place along a significant length of the canyon in order to facilitate the construction of the turbine halls downriver of the dam structure itself.