Dr. Sumith Baduge, PHd.

PUBLISHED ON August 12, 2021


Aquatic weeds are plants which grow and complete their life cycle in water and cause harm to aquatic environment directly and to related eco-environment relatively. Water is one of most important natural resource and all lives on the earth depend on it. Therefore, appropriate management of water from source to its utilization is necessary to sustain the normal functions of life. The presence of excessive aquatic vegetation influences the management of water in irrigation systems including natural waterways, manmade cannels and reservoirs.

In the Southern province, for the agricultural and house hold purposes more than 843 reservoirs and tanks are utilized throughout a year. Most of the water tanks are connected with irrigation systems to supply water to faddy fields and cultivated lands. Now a day it is identified that growing of aquatic vegetations/weeds causes numerous effects not only the water tanks but also to irrigation channels connected to the water tanks.

Harmful Effects of Aquatic Weeds

 Reduces water storage capacity in reservoirs, tanks, pondsbr
 Impedes flow and amount of water in canals & drainage systems
 Reduces fish production
 Interfere with navigation and aesthetic value
 Promote habitat for mosquitoes

Aquatic weeds have been found to severely reduce the flow capacity of irrigation canals thereby reducing the availability of water to the farmers field. Aquatic weeds may also damage pumps and turbines in super thermal power stations and hydroelectric power stations influencing electric production and increasing the cost of maintenance of power stations.

In flowing water system, aquatic weeds impede the flow of water in irrigation canals and drainage channels thereby increasing evaporation damage structures in canals and dams, clog gates, siphons, valves, bridge piers, pump etc. Impediment in flow of water may result in localized floods in neighboring areas. The velocity of flowing water is reduced by about 30 - 40 percent due to the presence of aquatic weeds.

Dense weed growth slows the flow of water in rivers, canals and drainage ditches allowing silt to settle out and be deposited on the bed of the water body. This increase in silt deposition raises the bed level and finally affects the life of lakes, dams, tanks etc. and requires expenditure to be increased for frequent desilting through dredging.

Aquatic weeds (emergent, floating and submerged) interfere with the static and flow water system. They cause tremendous loss of water from water bodies like lakes and dams through evapo-transpiration.

Aquatic weeds often reduce the effectiveness of water bodies for fish production. Aquatic weeds can assimilate large quantities of nutrients from the water reducing their availability for planktonic algae. They may also cause reduction in oxygen levels and present gaseous exchange with water resulting in adverse fish production. Although excessive weed growth may provide protective cover in water for small fish growth it may also interfere with fish harvesting.

Dense growth of aquatic weeds may provide ideal habitat for the development of mosquitoes causing malaria, encephality filarasis. These weeds may also serve as vectors for disease causing organisms and can greatly reduce the aesthetic value of water bodies from a recreational point of view.

Aquatic weeds in Sri Lanka

There are many number of weed species which emerge according to to various habitat. There includes shallow and deep reservoirs, cannela with earthen and lined embankments and drainage systems with lined and unlined bunds. Some dominiant weed of the aquatic environment of Sri Lanka are Eichhornia Crassipes, Salvinia molesta, Salvinia auriculata, S. accicenlata, Potamogeton stratiotes and Panicum repens.

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Fig. 1 A free floating weed , Eichhornia crassipes (Water hyacinth , Japan Jabara)

The filed surveys on various water bodies ranging from stagnant water, ponds, pools and manmade lakes to flowing waters such as rivers, streams and channels indicates that there are two problematic weeds as shown in Fig.1 -4 , E. crassipes and S. molesta widely distributes throughout Sri Lanka.

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Fig. 2 A free floating weed, Eichhornia crassipes (Water hyacinth , Japan Jabara)

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Fig. 3 A reservoir covered by free floating weed (Water hyacinth, Japan Jabara)

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Fig. Fig. 4 A channel covered by free floating weed (Salvinia molesta)

Submerged and floating weeds such as Eichhornia crassipes propagate at a tremendous rate and it needs a special mention in this category. A pair of these plants can multiply up to about four thousand times in one season. A canal or drain surface normally gets covered and clogged in one season, from just a few germinating or introduced plants. The surface floating weeds get interwoven and form dense mats that move downstream. Often these moving mats pack up against bridges and structures creating enormous pressure that sometimes results in serious damage being caused.


For the existing irrigation systems it was reported that different aquatic weed control methods are practiced in the world. There are Cultural Control Methods, Mechanical Control Methods, Chemical Control Methods and Biological Control Methods.

Chemical Control Methods

EPA-approved aquatic herbicides may be used to control weeds in ponds. The first step in successful chemical control is accurate identification of the problem weed. Selective herbicides target specific weeds while allowing other plants to grow. After the weed has been identified, an herbicide that is labeled for aquatic use may be selected. Various chemicals have different product formulations; by law, only aquatic labeled pesticides may be used in aquatic applications. As with all pesticides it should be followed the label recommendations and additional chemical information exactly. Labels change frequently, and the label is the final legal document on herbicide application. Never exceed the rates recommended on the specific label of the product that is applied.

Improper handling and use without considering the label poses may cause significant threat to the aquatic environment or human health. All aquatic weed management techniques have some impact on the environment.

Biological Control Methods

Biological management of aquatic weeds is a broad term for the exploitation of living organisms or their products to reduce or prevent the growth and reproduction of weeds. The organisms that are used for biological control are diverse e.g. insects, pathogens, nematodes, parastic and competing plants Biological control involves the deliberate use of organisms such as insects or fungi to control weeds. Biological control is more complex than chemical control because it requires (a) long term planning (b) multiple tactics and (c) manipulation of cropping system to interact with the environment.


Most of the aquatic weed control methods except the mechanical control method might create some environmental problems when they are employed in a long period of time. Therefore, in this project, more attention is paid to the sustainability of the method that utilize for the purpose and the mechanical control method is chosen for this study. The advantages of mechanical methods include- utilization of available manpower resources, environmental friendly and target specific, yields immediate results, non selective (under specific situations) and provides fewer chances of permitting ecological shifts in aquatic flora. Mechanical methods often reduce massive nutrient load of eutropic water bodies, helping indirectly in diminishing the future weed population. Harvested weeds may have various utilities such as feed, manure, energy source etc. and most importantly mechanical methods can be exercised in any localized area of water bodies. The limitations include limited effectiveness as in some cases aquatic weed regrow up from their rootstocks, rhizomes and the like; physical removal especially with machines may help spreading weeds to new areas; and sometimes removal of aquatic weeds may deplete water bodies of their nutrients limiting growth of plantation.

This project is expected to complete in three steps. At the first stage, it is expected to design and fabricate a mechanical collector unit to harvest aquatic weeds. This unit is to be fitted to the front side of a small boat. With this unit two persons are needed for harvesting aquatic weeds. One person should drive the boat while other one is operating the collector unit and harvesting the aquatic weeds.

At the second stage, it is expected to motorize the collector unit. Then the requirement of one person to operate the collector unit vanishes and the operator of the boat can do all the jobs. This method is cost effective and can increase the efficiency of aquatic weed harvesting.

Finally, as an ultimate goal, it is expected to develop a multi-purpose advanced machine including a power hydraulic system and a barge which is most suitable for Sri Lankan irrigation systems. This machine can be used for different jobs by mounting respective tools on the main arm, such as cutting of aquatic weeds, removing of aquatic weeds, dredging or de-silting, rehabilitation of the river banks, etc.


A manual collector unit is design by using GI pipes including a mechanical linkage for collecting and unloading of aquatic weeds. In addition to that a conveyer is connected to the unit to send the unloaded aquatic weeds to inside of the boat. The unit is mounted on the front deck of the boat with an adjustable stand. The schematic diagram of the unit is shown in the Fig. 5. A fiber glass boat as shown in Fig. 6, with a length of (10-12 ft) is selected at this stage to store the collected weeds. The driving force of the boat is expected to be taken from 25 HP outboard engine.

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Fig. 5 (a) Schematic view of the Harvester (collecting position)

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Fig. 6 Fiber Glass Boat and its front deck used for mounting the harvester unit

Eng. Lanka Ramanayaka - CEO
Design Knigdom Lk
BSc(Eng), PG Dip.(BSE), CEng., MIE(SL), MEC(SL)
Chartered Engineer