Sediment removal in South America8 October 2014
Sediment removal is an important aspect of many South American hydropower and dam projects. Tom Jacobsen, Technical Director of Sedicon in Norway, and Gustavo Adolfo Luzuriaga from Coca Codo Sinclair hydropower project in Equador, give details about work carried out in this region.
Last autumn, the Chinese contractor Sinohydro awarded SediCon AS of Norway the contract for supplying a sediment removal system for the desanders at Coca Codo Sinclair hydropower project in Ecuador. SediCon has certainly taken on a big challenge. The 40 SediCon Sluicers will remove 6M ton of sediments annually by using gravity as the only driving force and, if they operate simultaneously, will be able to remove 25,000 ton of sediment per hour.
Coca Codo Sinclair is a unique project. With1500MW of installed capacity, capable of producing an estimated 8.631GWh/yr, with practically no reservoir and a gross head of 660m, it is one of the world's largest high head, run-of-river power plants. There will be eight desanders (150m long and 13m wide) at the intake, cleaning the water before it enters the 24.79km long headrace tunnel at design discharge of 222m3/sec. A compensation reservoir with a volume of 886.000m3, just upstream of the underground powerhouse, regulates the flow and enables a maximum discharge of 278.5m3/sec through the eight turbines.
The sediment challenge
Coca River is a tributary to the Napo River and eventually the Amazon. The catchment for the project is 3600m2, which varies in altitude from 5790masl to 1262masl. As a consequence the flora, fauna, soil, climate and rainfall are variable, and carries significant sediment loads. The power plant is able to operate with a sediment concentration of 5000ppm in the withdrawn water and 1100kg of sediments will enter the desanders every second. Annual suspended sediment load entering the desanders is estimated to be 7M ton. Such high sediment loads must be removed with a minimum of water and without interfering with power production.
In the complex intake zone, space limitations and the presence of existing infrastructure required a sediment removal system for desanders that does not modify the level of the intake and weir. The fact that the effective dam height is less than the depth of the desanders also has implications for sediment handling and conventional methods such as free surface flushing were not possible. The SediCon Sluicers do not require free surface flow; can operate at very low head, require little water and cleaning may be periodic or continuous. It therefore became the perfect solution for Coca Codo Sinclair.
The eight desanders, 150m long and 13m wide, are each designed for sediment particles greater than 0.25 mm. SediCon Sluicers consist of permanently fixed slotted pipes that suck sediment from the bottom of the desanders and into outlet pipes. The system works by gravity only and has no movable parts, except the outlet gates. Each of the eight basins has five SediCon Sluicers, and each one works independently of the others, making a flexible system with easy operation. Each SediCon Sluicer consists of five main components:
- Two slotted pipes, installed on the floor of the bottom channel of the desanders.
- A coupling unit - "black box" - that connects the two slotted pipes to the outlet pipe, and balances the flow of water and sediments. A high sediment concentration is maintained at the same time as blocking of outlet pipes is avoided.
- Up to 150m long, 500mm diameter outlet pipes pass out through the galleries underneath the desanders and discharge sediment and water in the channel used to maintain the ecological flow in the river.
- Level monitoring system of sediments in the desander.
- Outlet gates for operation of the system.
Abrasion is a challenge, therefore SediCon has selected pipe material and pipe wall thickness carefully for each of the five units in each basin. The sluicers have been optimised so that the design considers that most the most and coarsest sediment will deposit close to the intake, whereas less and finer sediments will deposit further downstream.
In mid-2012, the owner COCA SINCLAIR EP and the supervisor Comisión Federal de Electricidad de México (CFE) contacted SediCon. They wanted SediCon's solution with 40 separate and independent SediCon Sluicers to be model tested as soon as possible. After two weeks of effective preparations, Sedicon and CFE conducted a series of model tests at CFE's hydraulic laboratory in Cuernavaca. In the 1:20 model, up to 500kg of sediments were removed with an effective water head of 40cm, through 15m long outlet pipes: quite a successful achievement.
Latin American experience
This is not the first time SediCon has supplied SediCon Sluicers to Latin America. Cuyamel HPP in Honduras is a 7MW run of river plant with a head of approximately 100m. A 40m long pressurised desander immediately after the intake is equipped with two SediCon Sluicers. In order to handle debris and potentially large sediment volumes the sluicers are nearly the same size as the ones at Coca Codo Sinclair. Due to their shorter outlet pipes, the capacity is in fact higher: approximately 1000 ton of sediments per hour. The SediCon Sluicers have been working perfectly since commissioning in early 2007.
SediCon also installed four sluicers at Khimti in Nepal in September 2010, where operators measured a combined capacity of 4000 ton of sediments per hour. At the same power plant, two smaller sluicers have removed sediments from the pressurised tunnel sand trap since 2000. The sand trap has only been inspected once (in 2008) when everything was found to be in perfect order.
SediCon does not only supply equipment for desanders. The company's other main product is the SediCon Dregde, which is a gravity powered dredge based on the same principles as the sluicer. Its key component is the suction head, which due its design balances the sediment concentration so that it achieves a high output without blocking the outlet pipe. SediCon has supplied theSediCon Dredge to several projects in Central America. At El Canada in Guatemala, a 250mm product was leased to ENEL and operated by a crew of three where it has removed more than 100,000m3 of sediments per year since 2011. The highly cohesive sediments require disintegration by water jetting.
At a similar project in Costa Rica, Dona Julia, SediCon has removed 60,000m3 of sediments with a 300mm SediCon Dredge. These projects are typical and valuable to the clients as daily peaking ensures use of the reservoir volume several hundred times per year.
This technology certainly has potential in many South and Central-American countries, not least due to the large and deep reservoirs there. Colombia, Ecuador, Peru and Chile are all countries with high sediment loads due to high relief, weak geology, sparse vegetation and seasonal rainfall causing high to extreme sediment loads. In addition there is also the factoring of the El Niño weather phenomenon which can cause flooding in normally dry areas and extreme sediment loads.
The ice age deprived Norway of nearly all of its sediments tens of thousands of years ago, so one may ask why Norway is home to a company which specialises in sediment handling technology? The answer is that at the end of the Norwegian hydropower era in the 1980s, the country's engineers were forced to find work abroad. Although rich in hydropower experience, the Norwegians still lacked competence in sediment engineering.
Consequently sediment handling became an important part of hydropower courses and research at the University of Science and Technology in Trondheim, and soon PhD students, including SediCon's founder Tom Jacobsen, ended up developing a range of new sediment handling technologies. Later, in 2001, the author established SediCon AS with the SediCon Dredge and SediCon Sluicer as the main products. SediCon now has subsidiaries in Costa Rica and Mexico, and representations in Ecuador and Japan.
About the authors
Tom Jacobsen, Technical Director, SEDICON AS, Professor Brochsgate 2, 7030 Trondheim, Norway. www.sedicon.no
Gustavo Adolfo Luzuriaga, Hydraulic Specialist, Coca Codo Sinclair EP, Quito, Ecuador.