An increase in aquatic weeds in the Shire River in Malawi negatively affected operation of five hydro plants, which provide more than 98 percent of the country’s electricity supply. To remedy the problem, in 2005 owner Electricity Supply Corporation of Malawi Ltd. (ESCOM) launched a program to manage the weeds. The program includes vegetation cutting and harvesting, combined with a system to trap floating weeds. Less than three years later, the weed problem is under control.
How weeds caused problemsat five hydro projects
The major hydroelectric facilities in Malawi are on the Shire River, which is the outlet of Lake Malawi. These run-of-river plants are: 24-mw Nkula Falls A (commissioned in 1966), 40-mw Tedzani I & II (two units commissioned in 1972, two in 1977), 100-mw Nkula Falls B (two units commissioned in 1981, one in 1986, one in 1992), 52.7-mw Tedzani III (commissioned in 1995), and 64.8-mw Kapichira Falls (commissioned in 2000).
In addition, in 1965 a concrete dam was built at the town of Liwonde. The dam is 156 meters long and contains 14 radial steel gates. The dam was built to control and regulate the flow of the Shire River. A 6.6-meter-wide road runs across the crest of Liwonde Dam, which serves as a bridge across the river.
In the late 1990s, ESCOM noticed an increase in floating aquatic weeds and debris near the hydro facilities, as well as increased siltation in the intake ponds. These problems were attributed to population growth in the catchment areas of the river. As people developed gardens and used firewood and charcoal to supply heat, the forests were depleted. This removal of trees loosened the soil cover on the hills, making them prone to erosion. Because most of the farmers use artificial fertilizers, the soil that erodes into the rivers contains a great deal of nutrients. These nutrients provide the proper environment for aquatic plants to grow and multiply.
Aquatic weeds were most prevalent in the water during the rainy season (January to March or April). Rising water levels dislodged the roots of weeds that had grown on the river banks during the dry season, causing them to float into the water. The weeds include water hyacinth, red water fern, water lettuce, and elephant grass. Masses of these floating weeds were getting caught behind Liwonde Dam, where they formed a mat-like carpet covering the water surface.
To clear out the floating weeds, ESCOM simply opened the dam gates. However, a day or two later the weeds reached the downstream power stations. The weeds caused serious operational problems at the stations. ESCOM lost millions of dollars to repair the damages. For example, in December 2001 the intake structures at Tedzani Falls I & II were so blocked by accumulated weeds, water could not pass through. This situation created a vacuum in the intake tunnel. Because the machines were running, the result was a collapse of the intake screens. Repairing the screens and returning the station to service cost more than US$12 million.
In severe instances, the entire power station had to be shut down to allow cleaning away of the weeds. This resulted in lost revenue for ESCOM and affected the national economy by reducing production of the industries that use the electricity supplied by the hydro facilities. ESCOM estimated losses of about US$27,000 a day from load shedding; the losses to industries as a result of shutdowns were ten times that figure.
Developing a weed management program
The aquatic weed problem at ESCOM’s hydro facilities reached its peak in 2002. Because of the large losses being incurred, ESCOM was investigating measures to mitigate the effects from weeds. Where possible, ESCOM wanted to eliminate the weeds completely.
In January 2002, ESCOM began a program of manual harvesting of weeds, accomplished by hiring a group of local men. After a month, ESCOM determined that the amount of weeds being removed was much lower than the amount floating downstream. This program was abandoned.
Floating aquatic weeds were so prevalent in the Shire River, they damaged the intake screens at five hydro projects on the river. Occasionally, the projects had to be shut down to repair the damage.
In 2003, the government of Malawi (through ESCOM) contracted with a local company that operates ships on Lake Malawi to remove weeds near Liwonde Dam. The company used a grab crane, a tipper truck, and a lorry to dump the weeds at a deposit site. Work began in August 2003. In February 2004, the government determined that the rate of harvest was very low compared with the amount of weeds floating downstream. This contract was terminated in April 2004.
At this time, the government directed ESCOM to take over this program as an internal operation. ESCOM used the same grab crane and deployed five trucks instead of one. The program cost US$50,000, mainly to hire the crane and supply fuel for the trucks. Although ESCOM was able to reduce weeds being flushed downstream and improve operation of the hydro facilities, the crane had to be located directly on the road. In this location, it was a traffic hazard and acted as a persistent load on the dam, which was designed to carry only passing loads. Because of these concerns, ESCOM sought a better way to clear the aquatic weeds.
ESCOM then developed the Liwonde Aquatic Weed Management Project, which aims to minimize the effect of aquatic weeds to hydro plant generation and restore the beauty of the Shire River at Liwonde Dam. The project employs mechanical means of combating the weeds, using an aquatic weed cutter/chopper, a mechanical harvester, and a shore conveyor. In addition, a floating boom laid across the river catches any weeds that may not have been removed by the harvester or that have come downstream during the night.
In September 2004, ESCOM signed a contract with Aquarius Systems Inc. of the United States. Aquarius Systems would supply weed management equipment worth US$800,000 and provide three months of training for ESCOM staff on operation of the equipment. Equipment included: an aquatic vegetation cutter, a mechanical weed harvester, a shore conveyor, a conveyor trailer, a workboat, and a set of floating trash booms.
Vegetation cutting and harvesting
The automatic vegetation cutter arrived in April 2005 and was immediately put into operation. The cutter is a boat that contains two blades at the front, which are used for both propulsion and as cutters. It chops aquatic weeds into pieces about 15 millimeters long.
During initial use, the cutter was employed to chop weeds before they reached Liwonde Dam. The harvester was not yet available, so these weeds were left to float downstream to the power stations. Because of the smaller size of these weeds, ESCOM determined they would either easily pass through the intake screens or float off to the banks of the river.
The harvester arrived in late June 2005 and was launched in early July. In addition to collecting weeds that have been chopped by the automatic vegetation cutter, the harvester can collect weeds that have not been chopped. Once the harvester is at capacity, it off-loads the weeds using a shore conveyor system. Trucks then take the weeds to a designated depository site.
Trapping floating weeds
Due to a communication breakdown between ESCOM and the contractor, the wrong material was delivered for the floating booms. The light-weight water surface trash boom had a depth of about 30 centimeters. The boom was installed across the river about 130 meters upstream from Liwonde Dam. However, ESCOM soon noticed that aquatic weeds simply slipped beneath the boom. In addition, weeds that were trapped by the boom but not harvested within five hours absorbed enough water to sink and pass by the boom.
However, because the contractor delivered the quoted boom material, ESCOM had to make do. The decision was made to redesign and modify the existing boom. The goal was to increase the depth of the boom and increase the strength of the span length. Galvanized wire and rectangular metal plates, together with aluminum wire for a mesh, were used to extend the depth. Tension wire rope was used to increase the strength of the span length.
During the 2006 rainy season, Malawi received very high rainfall. Floods brought weeds, which were managed as they neared Liwonde Dam using the cutter and harvester. On a couple of occasions at night, there was a heavy accumulation of weeds that were successfully trapped by the boom. However, the excess loading caused the boom to snap as ESCOM began chopping and harvesting the weeds. All the weeds were released downstream, but there was little effect on the power stations. This incident prompted ESCOM to increase the size of the wire rope anchoring the boom.
During the 2007 rainy season, there were virtually no plant shutdowns due to trash. By comparison, in 2005, the Nkula Falls facility lost 349.19 hours to shutdowns.
The Liwonde Aquatic Weed Management Project has been in effect for nearly three years. Analysis of some bottlenecks has led ESCOM to make changes. First, plans are in place to procure a new boom designed to catch heavy loads at a much greater depth. Second, ESCOM plans to buy a second harvester, as the cutter chops about three times as many weeds as the harvester can remove. Third, ESCOM plans to use tipper trucks to transport weeds to the depository site, instead of conventional lorries. This should reduce time and manpower required to off-load the weeds.
The project has provided two social benefits in the nearby area. First, it has created jobs. Initially ESCOM used local unskilled laborers. Now, the project employs a marine engineer and other marine technicians, in accordance with the national water machinery regulations Second, depositing the weeds in farm fields is improving soil fertility. This allows local farmers to reduce their reliance on artificial fertilizers.
Most importantly, the aquatic weed management program has enabled ESCOM to operate its hydro facilities in an almost debris-free environment. This has allowed greater efficiency of the hydro stations and has improved the image of ESCOM to its customers and stakeholders.
— By William Liabunya, Senior Systems Engineer, ESCOM Ltd., P.O. Box 2047, Blantyre 2047 Malawi; (265) 1-870655; E-mail: [email protected]