Troubled waters settle behind Peruvian dams12 January 2000
Extreme weather conditions have accelerated soil erosion in northern Peru. Severe siltation is becoming commonplace at dams, reducing the structures’s useful service life. Gerhard Grossman, Roland Schimpf and Peter Wicke explain what remedial action is required
The northern coastal area of Peru has experienced severe weather conditions during the last two decades. The El Niño weather phenomenon has struck several times, speeding up erosion along the Pacific-Andes mountain range. Torrential rains have resulted in unexpected amounts of both sediment loads and suspension matter rushing down the river valleys. Soil degradation has increased in areas already prone to erosion, and siltation is proving to be a growing concern for dams in the area.
The problem is demonstrated by the Gallito Ciego multi purpose dam in the Jequetepeque valley. Its dead storage limit of 86M m3 has already been attained within a ten-year period, instead of a design life of 50 years.
Bathimetric investigations are under way to establish the extent of accumulated gravel, sand and clay inside the reservoir. Other dams are also suffering from the same problem and counter-measures must be taken immediately before dam operation is compromised.
The limited agricultural resources along the Peruvian coastline, which largely consists of deserts and intermittent wet valleys, has led successive governments to focus on large strategic irrigation schemes as the key to progress. Projects have been built to control the rivers belonging to the Pacific Ocean watershed.
The first attempt to retain the waters of the Jequetepeque river date back to the beginning of the 1900s but it was not until the early 1960s that positive action was taken.
The Peruvian government commissioned a German consultant to carry out a feasibility study for the construction of a dam across the Jequetepeque river. The study also looked at the construction of the main feeder channel Talambo-Zaña, related basic hydraulic infrastructure and secondary irrigation channel networks. A special administration and managing body was formed to deal with the overall aspects of the Gallito Ciego project: DEJEZA (Direccion Ejecutiva del Proyecto Jequetepeque Zaña). The feasibility study was updated in 1973 and presented to the InterAmerican Development Bank for financial support.
Although the Bank had a favourable opinion of the project, to the disappointment of the project promoters, it did not offer a soft credit framework. Eventually the feasibility study was examined by the German Government’s Co-operation Finance Institute and was accepted for bilateral German-Peruvian financing under soft loan conditions. The project went ahead and was built during 1981-8.
Prior to the start of construction, geomorphological and geotechnical aspects of the project were being discussed in detail. It became apparent that siltation could pose a serious problem for the Gallito Ciego dam. From 1969-1973 thorough investigations on riverbed loads and suspension matter had been carried out by the consultant working on the feasibility study. These were continued through sediment samples taken during dam construction by ONERN (Oficina Nacional de Estudios sobre Recursos Naturales), who carried out the delayed environmental impact assessment study in 1986-7. The results not only confirmed the earlier observations about sedimentation but heightened concerns about the problem.
Unfortunately the delay of the EIA meant that ONERN’s observations were not able to be taken into consideration as they were not anticipated. The 1969-73 studies carried out by the German consultant had led to the interpretation that in normal alluvial deposit years a combined bedload/suspension matter of a maximum of 2.3M ton (equivalent to little less than 1M m3) would appear, representing a yearly loss in storage capacity of roughly 0.2%. This would give the dam a design life of several hundred years. The progressive nature of erosion in the area was not taken into consideration.
During the rainy season of 1968/69, a ‘normal’ wet Sierra period, sudden peak sediment movements of up to 100kg/m3 were detected in the Jequetepeque river flow at the start of the flood. Such an amount was alarming and comparable with normal sediment load figures in China’s Yellow river, and was also reminiscent of sedimentation at Pakistan’s Tarbela dam. Without considering this, the 2.3M ton sedimentation data were adopted as the platform for the project. However ONERN’s late study of the 1980s did in fact show sediment movement values of between two-and-a-half and five times as much.
With the extreme events of El Niño in 1983/84 and 1998, combined bedload/suspension material masses in the order of between 20-30M m3 were discovered each time.
Unfortunately, when the project was given the green light, such catastrophic cases had not been anticipated. It was taken for granted that the surrounding landscape of the Jequetepeque catchment area, which was in a stable condition with abundant vegetation, would not change. But erosion is now common in higher slopes of the Jequetepeque valley, which starts by slices and then intensifies. Today, every confluence with Jequetepeque’s tributaries is showing increasing masses of bedload deposits. Cultivated terraces of the past are in danger of being lost.
Bathimetric measurements carried out by Pacific SA in 1998, on behalf of the dam operating body Opema-Jequetepeque, showed that the predicted silt deposits (0.2% of reservoir volume annually) were underestimated. After several El Niño events and with only ten years of dam operation, much larger amounts of settled river bedload/suspension matter have been detected — around 90M m3. This already corresponds to the maximum dead storage capacity of the reservoir, reducing the dam’s expected lifespan considerably.
Such investigations must be continued in order to obtain further detailed information about the surface of settled sediment loads and suspension material.
One solution to the siltation problem could be to remove the suspension matter as a slurry. This can be poured onto agricultural fields, where such fertile material was available before dam construction, and is badly needed. Heavy ‘flygt’ pumping could also be used through the use of long conduits or a special slurry tunnel. But more practically, an integrated Jequetepeque river management plan could consist of:
•Building sediment retention ponds up in the higher river reaches of Jequetepeque and its tributaries.
•Terracing river beds and increasing the siltation capacities of rivers upstream.
•Constructing retention walls in erosion- prone areas — a costly measure.
•Other approaches could consist of the establishment of communities in higher catchment areas. Cattle farming and agricultural activities could be combined with afforestation, aided by joint technical consultancies and NGOs. Similar pilot experiences in the Cajamarca area have only required the revitalisation of former afforestation programmes with a multi-criteria approach.
Both the water consumers of the Jequetepeque-Zaña valleys and the hydro power station owner have been invited to take part in efforts to restore the damaged, partly eroded landscape. To help finance the schemes, negotiations are under way to establish an irrigation water price of at least US0.9 cents/m3 (the current tariff is much lower), along with a 10% surcharge.
Agricultural landowners need to understand that their future welfare depends on the Gallito Ciego dam, which in turn relies upon an effective overall river management programme. In the past they have been reluctant to commit themselves financially but they must be convinced of the benefits of cost sharing in order to achieve efficient ecological impact mitigation. Financial support has also been requested from the hydro power station owner.
The discussions held in the late 1970s between the German Ministry of International Co-operation, the Peruvian Ministry of Agriculture and Nutrition and Junta de Usuarios Jequetepeque-Zaña focused on integrated land as well as river catchment management. Special project approaches (afforestation in particular) progressed well in the region at the time but soon slowed down. Such renewed efforts are imperative.
Ecologically oriented consultants and NGOs are known to be keen to help control the erosion and subsequent sedimentation problems.
In view of the river management deficiencies of the past, the question arises whether Opema, the dam operating body, should be privatised in the future. In the context of the overall Jequetepeque river repair approaches the constitution and structure of Opema should not be excluded from general discussion.
Poechos is another Peruvian dam showing similar sedimentation difficulties to Gallito Ciego — although a lack of comprehensive bathimetric surveys means there is little data to demonstrate this. It is thought that since the dam’s inauguration in 1976, repeated El Niño events (severe floods in 1983, 1987, 1992 and 1998) have led to the accumulation of large amounts of silt inside the reservoir. Thorough sediment monitoring is now required and mitigation measures along the Chira river and its tributaries are needed as well. A joint cross-border plan will be necessary because the neighbouring country Ecuador is involved in the project.
Sediment flushing could prove to be the most advantageous mitigation approach. Three accessible radial gates capable of handling a maximum spill/outlet flow of up to 1650m3/sec each, could carry almost 5000m3/sec of slurry mix away from Poechos.
|Gallito Ciego dam|
|Gallito Ciego is a multi-purpose dam. Part of an intended two-phase development, the first phase's objectives were to provide: Year-round irrigation water distribution, enabling additional agriculture campaigns and second harvests. Flood control. Electricity generation. (The second phase was to include water transfer through the Andes but has been postponed and may even be cancelled). Characteristics of the Gallito Ciego dam project: Jequetepeque river catchment area (4230km2). Earth dam - 102m high with a volume of 13.9M m3 . Free flow surface spillway; capacity of 1600m3/sec, combined with irrigation water intake tunnel/bottom outlet with bifurcation for water power pressure conduit. Useful operation storage capacity (400M m3). Dead storage capacity 86M m3. Ratio of annual inflow volume/reservoir capacity 0.6. Large stilling basin allowing for certain downstream compensation storage (daily balanced irrigation water distribution). 34MW hydro power station (built 1995/96) feeding the interconnected grid for electricity sales, mainly in peak demand hours. Privately run by a cement factory. Improved and extended irrigated land (42700 ha). Total project cost was DM 500M (approximately US$200M at the time). A 100% budget increase occurred due to technical modifications, the inclusion of socio-cultural requirements and inflation. German credit lines corresponded to approximately 50% of the investment.|