Paper Hydro 2007 Jacobsen-Jimenez

5/20/2018 Paper Hydro 2007 Jacobsen-Jimenez

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SPSS Sediment remover at Cuyamel pressurised sandtrap, Honduras

Tom Jacobsen Alberto Jimnez Oscar Jimnez

GTO Sediment AS GTO de Costa Rica SA Carbn IngenieraOlav Tryggvasonsgate 24b P.O. Box 1348-1200 Oficentro Alquimia,7011 Trondheim Costa Rica La Uruca, Costa RicaNorway [email protected] [email protected]

[email protected]

IntroductionIn January 2007 a Slotted Pipe Sediment Sluicer (SPSS) was installed at the pressurised sand trap at CuyamelHPP plant in Honduras. This 8 MW run of the river power plant was commissioned in July 2007. It comprises a14 m high dam which forms a small intake pond, the intake which leads directly to a pressurised tunnel. To deal

with sediment removal, a pressurised sand trap 53 m long was provided along with a specially designed Slottedpipe Sediment Sluicer (SPSS). This recently developed sediment removal system allows sediment removal fromunderground (pressurized) sand traps without dewatering and does not interfere with normal operation of thepower plant.

1 Background

Despite abundant natural resources, presently Honduras has a total installed capacity of about 1476 MW, fromwhich only 35% is of hydroelectric plants, 2% come from sugar mills, and 62% from petroleum based thermalplants. For that reason, during the last 10 years the government has tried to stimulate private investments inrenewable generation, and recently, a new promotional law which strengthen some of the benefits for this type ofgeneration has been enacted. Because of this, in the last 5 years about 37 MW on small hydro power plants and

about 60 MW of biomass generation have been commissioned, and much more are in different stages ofplanning and construction. Most of the hydropower plants are of the run of the river type, with only smalldiversion dams.

Figure 1: Proyect Location, Honduras, Central Amrica

Cuyamel HPP


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The north coast of Honduras is endowed with abundant rainfall (about 3000 to 4000 mm per year), and is alsocrossed by mountain ranges with heights up to 2500 m above sea level. These mountains are formed mainlyfrom very hard intrusive rock formations, like, granitic gneiss and schists. Quartz often occurs in drawn-outgrains of these rocks to such an extent that a particular form called quartz schist is produced. These same goodconditions for hydroelectric generation (large heads up to 400 m, large rainfall, and weathered rocks), result in a

very large river sediment transport. For that reason, all these small hydropower plants have to be very carefullyplanned regarding sediment handling.

Figure 2: Outlet tunnel, bottom gate and diversion dam during construction

Cuyamel HPP is a 8 MW run of the river power plant located about 20 km northwest from the city of San Pedro

Sula (see Figure 1). The project is owned by HECO, a private generation company. The basin of the Fro Riverup to the dam site has 108 km2, with a mean discharge of about 6 m3/s. The plant comprises a 14 m high damwhich forms a small intake pond, the intake which leads directly to a pressurised tunnel, 525 m long, and 2.5 min diameter, a buried 915 m glass reinforced plastic pipe (GRP), 1900 to 1700 mm diameter, and a powerhousewith two horizontal Francis turbines. Midway along the GRP pipe there is an underground surge tank, formed bya 47 m horizontal tunnel and a 34 m vertical pit, 4 m in diameter. The gross head is 133 m and the expected

generation is about 34 GWh per year. Fig. 2 shows a schematic of the project. Figure 3 shows a picture of thedam works under construction, at the far end there is a lateral intake (temporarily closed during construction)which leads to the underground sand trap.

GTO de Costa Rica is a subsidiary of GTO Sediment, Norway. GTO has since 1999 supplied the hydropowermarket with revolutionary technologies for sediment removal from reservoirs, sand traps and desilting basins.


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These technologies have in common that water is used for suction and transport of sediment, that they arepowered by gravity and therefore are reliable and yet very efficient.

Figure 3: Schematic of Cuyamel HPP

2 Sediment Transport

In Honduras, long term suspended sediment measurements are very limited. The National Power Company(ENEE) took samples during the eighties in several sites of major rivers. The more representative and reliabledata gives values from 500 to 1200 t/km2/year. It is very likely, that small mountains rivers like Fro River mayhave much larger suspended sediment loads, as well as a large quantity of bed load, as seen upstream of the damsite (see Figure 4).

For this particular project, based on experiences in other small mountain basins in Costa Rica, a figure of total

sediment transport of about 2,500 ton/km2/year was estimated. This gives a total sediment load of about 270,000

ton/year, from which about 25% corresponds to bead load. This means that about 25,000 m3of bead load will bedeposited in the small reservoir, which only has a total volume of about 70,000 m

3. Therefore, this reservoir will

probably be filled up by sediments in a few years.

For handling the bed load, a bottom outlet gate (2.5x2.5 m) was provided immediately close to the intake with a

discharge capacity of about 57 m3/s, which the normal operating water level. Under free flow condition, the gate

discharges 28 m3/s, which is about four times the yearly average discharge. The height difference between the

bed and intake sill, of about 4 m, allows for some sediment deposition and avoids coarse sediment entering thewaterways. The gate will guarantee that at least close to the intake it will be possible to remove sediments.


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Figure 4: Heavy bead load, Fro River

3 Sediment handling at Cuyamel

A pressurized sand trap was provided immediately downstream of the intake. This structure is underground, as

show in Figure 6. The structure was designed to catch particles of 0.5 mm and above with an efficiency of 95%.The typical section is 4.5 m wide with a horse-shoe shape. Initially the sand trap was planed to function as a

Bchi type, which would require draining the whole hydraulic circuit to flush sediments. For that purpose, a

short flushing tunnel with a valve was provided. A Bieri type sand trap was also considered, but it wasconsidered very complicated because the required mechanical and electrical equipment, in this case locatedunderground. Finally, GTO sediment AS offered a very novel sediment handling system, which was adopted forthe project, the so called Slotted Pipe Sediment System (SPSS), explained in the next section.

3.1 Measurement of sediment volume inside sand trap.

Several devices to measure the sediment level have been installed inside the sand trap of the Cuyamel HPP,including, pressure transmitters, higher level, test flushing system and two echo sound sensors that are attachedto the roof of the sand trap. The reading of these systems will assist the operator to take the decision on flushing

the sediments through the SPSS.

3.2 Slotted Pipe Sediment Sluicer, SPSS

A new technology, the Slotted Pipe sediment Sluicer (SPSS,) which have been developed since 1993 enablesefficient removal of sediment from sand traps and desilting basins. Sediment removal is done with a minimum

use of water. Not least, sediment removal is performed without interrupting the supply of clean water.The SPSS can be described as a pipe with a continuous, longitudinal slot or row of slots along its lower surface.It is fixed close to the original bed and connected to a pipe whose outlet is at a lower level. The SPSS is operatedin two phases:

1. Sediment is allowed to deposit on top of the slotted pipe until the thickness of the sediment deposit issufficient for flushing. Because the slots are on the bottom side sediment will not accumulate inside thepipe. Water can thus flow freely through the slotted pipe and out of the outlet pipe.

2. The valve on the outlet pipe is opened, and flushing of sediment starts. Water is drawn through the slots

and picks up sediment, and as the sediment is sluiced the suction point moves downstream until allsediment that cover the slotted pipe has been removed.


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Due its special design the SPSS ensures that an absolute minimum of water is used for transport of sediment.The design also ensures that no pumps are required, and that a very low head height difference between watersurface and outlet is required.

Figure 5: Use of SPSS in open sand trap

In the case of Cuyamel HPP, two SPSSs have been installed; covering both of themnearly the first and secondhalf of the sand traps longitude respectively. Each of the SPSS has a valve located in the outlet tunnel; the

valve is manually opened and closed to proceed with the flushing procedure. Figure 6 shows a plan view of thesystem Figure 7 shows a typical cross section of the sand trap.

The SPSS could be installed without making big changes to the original design. One change though, was that theoutlet tunnel through which SPSS has outlet could be moved upstream closer to the dam and made considerablyshorter.

Figure 6: Plan view of intake and sediment works


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In this particular case, a conventional flushing system was kept in the design, also for draining the sand trap andcarry out inspections of the tunnel and the SPSS once or twice a year. A by pass pipe of 500 mm is included tobring fresh water to the downstream end of the sand trap and perform a conventional flushing operation withwater flowing backwards.

Advantages of removing sediments through the SPSS (compared with conventional system):

Continuous production: It is not required to dewater the sand trap and the tunnel to remove thesediments. Sediment removal does not interfere with production and vice versa.

Saves water: A minimum amount of water is used to remove the sediments, as discharged sedimentconcentration is very high. Use of SPSS can be combined with small and medium size floods, meaningthat spill water is used to remove the sediments, thus, there is virtually no production loss.

Reduces construction costs, as outlet tunnel(s) can be made shorter or omitted altogether.

Reduces turbine wear: Sediment removal can be performed whenever required and does not have to bescheduled to periods with no production.

Requires small resources: SPSS has no movable parts except valves, and low maintenance cost and isoperated by local personnel.

Figure 7: Typical cross section of sand trap

3.3 Types of sediment to be sluiced

The SPSS is designed according to the geometry of the actual sand trap and is capable of sluicing a wide rangeof sediments. For Cuyamel the sediment is expected to be medium sand with varying grain size according totime of year and size of flood. Occasional large items like rocks and pieces of wood must be expected, even

though the trash rack is relatively fine. The SPSS at Cuyamel are therefore capable of sluicing rock up 200 mmdiameter and debris up to substantial length without blocking.


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3.4 Capacities

The capacity of the SPSS will depend on a number of factors such as geometry and sediment properties.

Assuming average grain size of trapped sediment is 1,0 mm, the combined (both SPSSs) theoretical capacity isin the order of 1000 ton sediment / hour. The capacity is therefore more than sufficient and each flushingoperation can be performed in less than one hour.

4 Conclusions

The sediment removal system installed at Cuyamel in Honduras represents a breakthrough in efficient andreliable sediment removal from underground sand traps. Sediments are removed without interfering power

production and with a minimum use of water. The system has no movable parts, uses no external energy andonly a minimum of water and therefore gives substantial benefits both for owner and operator.

The Authors

T. Jacobsen, graduated in Hydraulic engineering from the University of Science and Technology in Trondheim, Norway in

1990, and then worked for a construction company Skanska for two years. In 1997 he defended his doctoral thesis SedimentProblems in reservoirs control of sediment deposits. From 1998 he worked for consultant engineering Sweco. He hasinvented a number of sediment handling technologies. From 1999 has been working for the Norwegian company GTO

Sediment with a number of sediment handling projects both offshore (Subsea) as well as for hydropower projects.

A. Jimnez graduated in Civil Engineering from the University of Costa Rica (UCR) in 1995, and obtained a Master Degreefrom the Norwegian University of Science and Technology (NTNU) in 1999. He worked two years teaching hydraulics, laterfor the Costarrican National Power Company (ICE) for about 2 years, and then with Ghella Group studying hydropower

projects. During the last years, he has been involved in feasibility and design studies for a number of small and mediumhydropower projects in Central America. He is currently working with GTO Sediment AS as representative for LatinAmerican countries.

O. F. Jimnez graduated in Civil Engineering from the University of Costa Rica (UCR) in 1981, and obtained a MasterDegree from Washington State University. He worked for the Costarrican National Power Company (ICE) for about 25

years, before joining Carbon Ingenieria, a regional consulting firm. He has been involved in feasibility and design studies fora number of small and medium hydro projects in Central America. He is currently member of the board of the LatinamericanChapter of IAHR.

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Paper Hydro 2007 Jacobsen-Jimenez