Physical restoration of the irrigation system

IUCN Cascade Development Project Information Brief No. 5

of the Kapiriggama Village Tank Cascade System

Background

Minor tanks in the dry and intermediate zones of Sri Lanka are gradually becoming inefficient in providing an assured supply of irrigation water because of changes occurring in the size and shape of the tank bed. Water bodies become shallower as a consequence of sedimentation.

An enormous amount of foreign aid and loans has been diverted, in the last few decades, to tank rehabilitation and restoration. In these programmes, the standard actions were to strengthen the tank bund, repair or replace structures, and regain the capacity lost due to sedimentation by raising the spill and the tank bund, thereby increasing the water holding capacity of the tank. However, because the tank is still silted, the water spread also increased, resulting in the formation of larger,

shallower water bodies. Shallower, larger water bodies lose more water through evaporation and percolation. In addition, there has not been an increase in cropping intensity (the number of times a crop is grown in a given area) as a result of the interventions of the last few decades.

In addition, the following environmental issues in the tank ecosystem were also observed:.• Disappearance of the gasgommana (see Brief # 1 for

description), enhancing water loss; • Development of salinity in the upstream area around

full supply level, especially if the tank is second or third in the cascade;

• Flooding of upstream rice fields. This has caused conflict among village communities; and

• Disappearance of some of the fish species, which cannot survive in shallow waters.

Clearly, a new methodology for the restoration of small tanks is needed.

The current project set out to

• Reduce water loss through infrastructure rehabilitation;• Rehabilitate the irrigation system to provide adequate

and good quality water;• Reduce water loss in tanks by changing the shape and

size of the tank bed; and • Facilitate efficient water management through

improved irrigation facilities.

Project achievements

• Preliminary investigations were carried out on 21 tanks to identify what needed to be done. Combining the needs of the community and the results of these preliminary investigations, work was identified and carried out in 18 tanks (See Brief # 3 for a map of interventions.)

• This work included restoration of sluices, bunds, spills, irrigation structures, distribution canals, retaining walls, drainage canals, and service paths associated with irrigation systems.

• Tank bed surveys were carried out on 20 tanks to understand the shape and size of the tank bed — the existing tank geometry.

• Based on the results and in consultation with the Department of Agrarian Development and community, partial desiltation was carried out in five tanks — in Konakumbukwewa, Massalawa, Peenagama, Maha Kadiyawa, Puliyankulama. (See box on opposite page for details about partial desiltation.)

• Surveys to identify the depth of accumulated sediment were carried out in these identified tanks and partial desiltation plans were then developed.

• During partial desiltation, the following amounts of silt were removed from the tanks listed below.

Tank Total amount of silt removed (in cubic metres)

Konakumbukwewa 6,345Peenagama 3,100Massalawa 17,687 Puliyankulama 4,765Maha Kadiyawa 6,000

• In over 60% of the tanks restored, field harvest has increased by 50%.

• Shareholders (ranging 15-80 persons per tank) now engage in water management programmes for each rehabilitated tank.

What is Partial Desiltation?

Desiltation is expensive, and therefore, it is important to develop a technological concept, which generates a low-cost and effective desiltation process. The partial desiltation concept is introduced in this context, on the basis of findings from hydrological research studies conducted by the Department of Agriculture.

The main objective of the concept is to reduce tank water losses by manipulating the tank bed geometry through desiltation. Studies of sedimentation indicate that the amount of sediment deposited in minor tanks is between 20-35%, and half of this sediment is found within one-third of the tank bed area close to the bund. The same capacity can be maintained by removing sediment in this area and heaping it in the upstream tank bed. Such partial desilting increases the spread of the water and keeps more than 50% of the tank bed free of water.

Catchment area

Natural streams

Oldbed

Sedimentmounds

New bed

Tank bund

Interceptor

Methodology for Partial Desiltation

Tank bed level surveyA tank bed engineering survey is carried out to understand the tank bed geometry, storage capacity and area-capacity-elevation relationship.

Tank bed sediment surveyA sediment depth survey is then carried out to prepare an original (prior to sedimentation) contour map and area-capacity-elevation curves, which is later super-imposed to the existing tank bed survey map.

Partial desiltation design1. The design is based on above surveys along with

following criteria.2. The required live and dead storage after completing

the desiltation.3. The minimum requirement of sediment layer to

avoid excessive percolation.4. The volume of soil required to repair the upstream

side of the tank bund and ‘isweti’.

The design is initially worked out on the area-elevation diagram and transferred with relevant information to the tank bed contour map. This design map shows the locations of soil mounds and the depth of sediment to be

removed on a basis of a grid 20 m X 20 m basis. In each grid square, the volume of soil to be removed is marked, and the cost of desiltation worked out accordingly.

After completion of the above, the following is prepared. • A diagram showing the cross-sectional view of tank

bund including the sill level and the dimensions of the kattakaduwa;

• Cross-sectional view of soil mounds and isweti;• Plan of the kattakaduwa with improvements; and• Plan of gasgommana area with proposed vegetation.

Implementation

• The Department of Agrarian Services planned the above activities in collaboration with Project staff;

• Although excavation of soil needs the support of machinery, other activities — such as stabilisation of bunds and mounds with vegetative cover, construction of upstream drains, establishment of the gasgommana and kattakaduwa were carried out with support from the community;

• Data collection and required technical supervision were the responsibility of the Department of Agrarian Services staff.

Impact assessment

The monitoring and evaluation programme commenced immediately after site selection. The following assessments evaluated the success and performance of partial desiltation:

Before partial desiltation:• Collection of basic data related to each tank such as

capacity, tank bed area, full supply level; • Water balance study.• Cropping intensity of the command area.

At partial desiltation:• Cost of planning;• Cost of desiltation, repairing tank bund and

construction of eye-brow bunds (‘U’ or ‘V’ shaped or eye-brow shaped bunds built to divert water towards the crop);

• Cost of restoring the kattakaduwa and gasgommana; and

• Farmer participation in implementation.

After partial desiltation:• Collection of the same data as before partial

desiltation; and • A cost-benefit analysis of the process.

Indicators of progress:• Increase in the annual income of farmers from paddy

fields;• Opportunities available for cottage industries;• Tank water storing efficiency;• Cropping intensity;• Other agricultural opportunities generated; and• Plant diversity index.

January 2016

For more information contact

Shamen Vidanage, Programme Coordinator, IUCN Sri Lanka Country Office53, Horton Place, Colombo 7, Sri Lankatel: +94-112682418, fax: +94-112682470email: shamen.vidanage@iucn.org; https://www.iucn.org

Access technical note from here.

What are the benefits of Partial Desiltation?

Although the main objective of partial desiltation is to decrease water loss from small tanks, there are other benefits associated with this process, as listed below:

Increased extent for cultivation

Even though the asweddumized lands are available for cultivation in most of the command areas, water availability in tanks limits the cultivable extent. Reduction of tank water losses from partial desiltation leads to improved water availability, in turn, increasing opportunities for cultivating a larger extent. The project has already noted a 50% increase in field harvest in more than 60% of the restored tanks.

More agriculturally productive lands

Partial desiltation reduces the water spread area. More than half the land inundated with tank water will be free of surface water after a successful partial desiltation and the water body is confined close to tank bund. The land area freed from such water spread can be used for agricultural purposes, if measures for preventing erosion are followed. This soil is fertile with nutrients and high level of organic matter (5-8%). There is also easy access to groundwater. This area can be transformed to paddy land or any perennial crop, which is tolerant to shallow ground water.

Increased cropping intensity

Because the water storage efficiency of the tank is increased by partial desiltation, any water remaining in the tank after `maha’ cultivation can be kept without much loss for a yala cultivation.

Further, this tank storage can raise the groundwater in the command area and yala cultivation can be supplemented by well water. This, in turn, leads to an increase in the cropping intensity of the command area.

Opportunities for cottage industries

The new area emerged at the upper portion of the tank bed and the soil mounds formed from partial desiltation are more suitable for perennial crops because soil erosion and sedimentation must be prevented. For a cottage industry type programme, this land could be used to grow plants — such as bamboo (Bambusa spp.), rattan (Calamus spp.), mat grass (Cyperus pangorei), screwpine (Pandanus spp.), Hibiscus tiliaceus, palmyrah (Borassus flabellifer), fish tail palm (Caryota urens) — all of which provide various raw materials for cottage industries.

Freshwater fishery

Minor tanks are mostly seasonal reservoirs. They can be used for raising fish species for a short duration or harvesting half-matured fish stock. An adequate dry season storage of a tank with favourable geometry can improve this situation for rearing even fish species that need a longer time for maturation. However, the choice

of species used for such a fishery must be ecologically sound, ensuring that only non-invasive species are selected.

Recommendations for the Future

• Strengthen community participation to obtain labour and thereby reduce the cost of rehabilitation work;

• Refine the partial desiltation technology to reduce costs;

• Ensure ecological stabilisation, with replanting, after hard improvement — such as soil movement;

• Conduct technical studies to determine the tank bed area which can be freed for ecological restoration, and obtain the consent of relevant authorities and the community;

• Establish a legal provision to reduce the spill and free the tank bed for such ecological restoration;

• Link the above improvement to a study on water budgets and provide technical support to irrigation water management in order to increase the cropping intensity; and

• Adopt soil conservation measures to minimise siltation.

Photographs: Cover: Soil mound formed with the soil dredged during partial desiltation at Maha Kadiyawa tank; page 2 top: Tank bund and sluice repair at Palugonamariyawa tank; middle: Group of farmers engaged in lining a distribution canal at Mailagammana tank; bottom: partial desiltation of Pulitankulama tankThis page above: sealing the toe of Puliyankulama tank. All photographs” Kumudu Herath © IUCN