IUCN programme on Restoring Traditional Cascading Tank Systems Technical Note # 3

Tank Ecosystem Restoration

of the Kapiriggama Small Tank Cascade System

IUCN programme on Restoring Traditional Cascading Tank Systems Technical Note # 3

Tank Ecosystem Restoration

of the Kapiriggama Small Tank Cascade System

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Natural Resources

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Citation: IUCN (2015). Tank ecosystem restoration. IUCN programme

on Restoring Traditional Cascading Tank Systems Technical Note # 3. Colombo: IUCN, International Union for Conservation of Nature, Colombo, Sri Lanka & Government of Sri Lanka. iii + 28 pp.

ISBN: 978-955-0205-33-2 Lead contributor: Dr. P. B. Dharmasena, Consultant, IUCN Cover photo: Community members engaged in a gasgommana replanting

programme S. M. M. Senavirathna © IUCN Layout by: Padmi Meegoda Produced by: IUCN, Sri Lanka Country Office Available from: IUCN, Sri Lanka Country Office 53, Horton Place

Colombo 7, Sri Lanka Phone: ++94-011-2694094, 2682418, Fax: 2682470 http:// iucn.org/srilanka

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Table of Contents

 

Table of Contents ..................................................................................................................... ii 

List of Figures ......................................................................................................................... iii 

List of Tables ........................................................................................................................... iii 

1.  Introduction and background ........................................................................................... 1 

2.  Components of tank ecosystem ................................................................................... 4 

2.1 Tank bed .................................................................................................................... 4 

2.1.1 Dead storage phase (Mada kaluwa) ................................................................... 5 

2.1.2 Deep phase (Diya gilma) ..................................................................................... 6 

2.1.3 Shallow phase (Wew thavula) ............................................................................. 7 

2.1.4 High flood phase (Wan gilma) ............................................................................. 7 

2. 2 Tank bund ................................................................................................................. 8 

2.3 Upstream reservation (gasgommana and perahana) ................................................ 9 

2.4 Upstream water hole to trap sediment (goda wala) ................................................. 11 

2.5 Upstream earth ridges to prevent sediment inflow (isweti) ...................................... 12 

2.6 Downstream reservation (kattakaduwa) — pond, marshy land, moist land, dry land ....................................................................................................................................... 13 

2.7 Command area drainage (kiul ela) .......................................................................... 19 

3.  Project interventions ................................................................................................... 24 

4.  Recommendations for future work ............................................................................. 28 

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List of Figures

Figure 1. Use of water resources in the village tank cascade system .................................... 3 Figure 2. Geometrical phases of the tank ............................................................................... 4 Figure 3. Diagram of Figure 2 .............................................................................................. 5 Figure 4. Cross section of tank showing water phases .................................................... 5 Figure 5. Cattle using the dead storage phase of water ......................................................... 6 Figure 6. The process of partial desiltation moves silt from the deep phase to the shallow phase ...................................................................................................................................... 6 Figure 7. Silt from the deep phase heaped in to the shallow phase ....................................... 7 Figure 8. The gasgommana that grows in the high flood phase ............................................. 8 Figure 9. The tank bund .......................................................................................................... 9 Figure 10. Gasgommana ...................................................................................................... 10 Figure 11. The roots of trees in the gasgommana form cages in the water where fish can breed ..................................................................................................................................... 10 Figure 12. The perahana can be seen on the upper left of the picture ................................. 11 Figure 13. Goda wala ............................................................................................................ 12 Figure 14. Iswetiya ................................................................................................................ 13 Figure 15. Kattakaduwa ........................................................................................................ 14 Figure 16.Products made from plants species of the kattakaduwa ...................................... 15 Figure 17. Kiul ela ................................................................................................................. 19 Figure 18. Cross section of the main drainage ..................................................................... 22 Figure 19. Range of interventions taken in the Kapiriggama cascade to restore the ecosystem ............................................................................................................................. 24 Figure 20. Schools program of creating awareness ............................................................. 25 Figure 21. Community and HSBC staff engaged in restoring the kiwul ela at Konakumbukwewa tank (Kumudu Herath © IUCN) .............................................................. 25 

List of Tables

Table 1. Use of water resources in the village tank system .................................................... 3 Table 2. List of plants species in the kattakaduwa and products derived from them ............ 14 Table 3. List of plant species in the kattakaduwa and their services and functions .............. 16 

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1. Introduction and background

An ecosystem functions as a unit to provide a range of ecosystem services that is extremely beneficial to humans. (See box overleaf for more detail.) A village tank cascade is a centuries-old ecosystem that has provided villagers living in its surrounds with a suite of life-sustaining ecosystem services for their daily needs — such as food, medicines and fuelwood; absorption of carbon; purification of water; control of erosion; and stabilisation of stream banks.

A range of human-induced activities — such as deforestation, overexploitation, pollution, and the spread of invasive alien species — has degraded the traditionally rich Kapiriggama village tank cascade ecosystem. (See technical note # 2 for further details.)

There was an urgent need, therefore, to rehabilitate these tanks and restore the cascade ecosystem to ensure that the historical benefits that the community enjoyed are provided again. 1.1 Objectives The overall goal was to rehabilitate the irrigation system and restore the cascade ecosystem, with enhanced biodiversity, to provide adequate and good quality water for all needs of the community. Specific objectives were as follows:

to create awareness about the tank cascade ecosystem and its services; and to carry out ecosystem restoration, with involvement from the community.

To this end, elements of each tank (see section below) were evaluated and actions taken to rehabilitate and restore these elements.

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1.2 The tank and village – an efficient water use system Water, in the village tank cascade system, is used for many activities. Different water resources are used for one activity, and therefore, it is essential that integrated water resources management is practised.

Table 1. Use of water resources in the village tank system

Water use Water ResourceRainwater Tank water Groundwater

Domestic • • •Homestead farming • •Ran-fed farming • Lowland farming • • •Agro-well farming • Fishery • •Livestock • •

Figure 1. Use of water resources in the village tank cascade system

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2. Components of tank ecosystem A tank ecosystem comprises the following components. Each of these is discussed under separate sub-topics.

Tank bed;

Tank bund;

Upstream reservation;

Upstream water hole to trap sediment;

Upstream earth ridges to prevent sediment inflow;

Downstream reservation;

Command area drainage; and

Downstream sanitary cordon.

2.1 Tank bed The tank bed consists of four areas storing four phases of water mass: dead storage, deep phase, shallow phase and the high flood phase.

Four geometrical phases of the tank

Dead storage

Deep phase

High floodShallow phase

Figure 2. Geometrical phases of the tank

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High flood

Shallow phase

Deep phase

Dead storage

Figure 3. Diagram of Figure 2

Figure 4. Cross section of tank showing water phases

2.1.1 Dead storage phase (Mada kaluwa) This is the phase when water level depletes to sluice sill level — the volume of water remains in the tank. Water in the dead storage cannot be used to irrigate paddy fields. When the tank gains full supply level, this phase has the deepest water column. During the dry period, this small water body supports surrounding vegetation by maintaining groundwater level, survival of fish population, water for cattle and even for wildlife — such as elephants (Sinhala: Aliya/ Atha; Tamil: Yanei), mongooses (Sinhala: Mugatiya; Tamil: Keeri-pulle),

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primates (Sinhala: Wandura/Rilwa; Tamil: Kurangu) and jackals (Sinhala: Nariya; Tamil: Nari).

Figure 5. Cattle using the dead storage phase of water

2.1.2 Deep phase (Diya gilma) This is the central part of the tank bed. Water lilies and lotuses grow in this area. In the traditional system, people kept this water for yala cultivation, making maximum use of rainwater for maha cultivation. The largest amount of sediment is found in this phase. In partial desiltation, this sediment mass is removed and heaped up as mounds in the upstream area (shallow phase).

Figure 6. The process of partial desiltation moves silt from the deep phase to the shallow phase

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2.1.3 Shallow phase (Wew thavula) This is the upper part of the tank bed. Water spreads as a shallow layer. Aquatic sedges grow in this area. Birds roam in this area catching fish. In the traditional system, people practised tank bed cultivation (thavulu govithena) in low rainfall seasons by constructing a soil bund along the lower boundary of the shallow phase. When they needed to use tank water for maha season, they used this water phase, which would anyway disappear during first inter-monsoon (January – March) period. Sediment removed is heaped up in this area making habitats for many birds — such as egrets, herons (both Sinhala: Koka; Tamil: Kokku), jaçanas (Sinhala: Diyasaana; Tamil: Illaikkoli) and coots (Sinhala: Kithala; Tamil: Neelaththaazhai kohli) — as well as other aquatic species. In the water in the shallow phase are found Swamp barb (Puntius thermalis; Sinhala: Kota pethiya; Tamil: No name known), Tic tac-toe barb (Pethia melanomaculata; Sinhala: Depulliya / Tithpethiya; Tamil: No name known), and juveniles of various Rasbora and Devario species.

Figure 7. Silt from the deep phase heaped in to the shallow phase

2.1.4 High flood phase (Wan gilma) This is the area between full supply level (F.S.L) and high flood level (H.F.L). This area is upstream of the tank bed. When tank spills, various floating seeds move to this area and sink in the mud, when the water level depletes. Ultimately a strip of trees and bushes emerges in this area. This is referred to by villagers as the gasgommana — a windbreak of trees. The undergrowth becomes a meadow called perahana. During the high flood phase fish species such as Shark Catfish (Walago attu; Sinhala: Wallago attu walaya; Tamil: No

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name known), Butter catfish (Ompok bimaculatus; Sinhala: Walapoththa / Kokussa; Tamil: No name known), and eels (Anguila sp. Sinhala: Aandha; Tamil: Vilaangu) use this habitat.

Figure 8. The gasgommana that grows in the high flood phase

2. 2 Tank bund Minor irrigation tanks are constructed by placing an earth bund across a suitable location of a natural waterway. The bund is strengthened through compaction of soil. The community maintains the bund by clearing the vegetation and repairing any damaged places annually. In order to prevent the bund from breaching, villagers traditionally planted Pandanus odoratissimus (Sinhala: Wetakeyiya; Tamil: Talai) at the toe of the bund, where the breaching risk is high because of accelerated seepage from a weak point. Cattle herds are not allowed to cross the bund. Borassus flabellifer (Sinhala: Thal; Tamil: Pannai marum) is planted along the bund. With Pandanus and Borassus growing on the bund, elephants are reluctant to cross the bund. Many plant species that are used by villagers as fruits, vegetables and medicines are found on the tank bund, including Ziziphus oenoplia (Sinhala: Eraminiya; Tamil: Churai) (fruit); Aerva lanata (Sinhala: Polpala; Tamil: No name known); Acalyphaindica (Sinhala: Kuppameniya; Tamil Kuppameni) (vegetables); and Calotropis gigantea (Sinhala: Wara; Tamil: Erukkalai) (medicinal).

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Figure 9. The tank bund

2.3 Upstream reservation (gasgommana and perahana) The gasgommana is naturally grown vegetation in the upstream land strip (vaan gilma) above the tank bed, accommodating water when it spills. The gasgommana acts as a wind barrier reducing evaporation from the tank and lowering water temperature. The roots of large trees make water cages on either side of the bund, creating breeding and living places for some fish species. This strip of trees demarcates the territory between human and wild animals.

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Figure 11. The roots of trees in the gasgommana form cages in the water where fish can breed

Figure 10. Gasgommana

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Large water-tolerant trees such as Terminalia arjuna (Sinhala: Kumbuk, Tamil: Marudu), Vitex leucoxylon (Sinhala: Nabada; Tamil Kaddu-nochchi), Bauhinia racemosa (Sinhala: Maila; Tamil: Atti), Syzygium gardneri (Sinhala: Damba; Tamil: Nir nawal); and climbers such as Phyllanthus reticulatus (Sinhala: Kaila; Tamil: Mipullanti), Caesalpinia bonduc (Sinhala: Katukeliya; Tamil: Punaikkalaichchi), Derris parviflora (Sinhala: Kalawel; Tamil: no name known) and Derris scandens (Sinhala: Bokalawel; Tamil: Kalungu kodi) are found in this area. This vegetation is natural and seeds float on the water.

Birds — such as cormorants (Sinhala: Diyakava; Tamil: Neerkaham), egrets, herons use this area to nest. Barking deer (Sinhala: Oolu-muva / Vali muva; Tamil: Sembli-mann), wild boar (Sinhala: Vul oora; Tamil: Kattu pandri), sambur (Sinhala: Gona; Tamil: Marrei), wild buffalo (Sinhala: Mee haraka; Tamil: Kadu madu) are also found here, and elephants also roam in this area.

The perahana is the meadow developed under gasgommana, which filters the sediment flow from upstream chena lands.

Figure 12. The perahana can be seen on the upper left of the picture

2.4 Upstream water hole to trap sediment (goda wala) Villagers construct a water hole above the tank bed area located at the entry point of the main inflow stream. This human-made water hole aims to trap sediment and it also provides water to wild animals. This might had been a strategy to evade human-animal conflict.

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During periods of dry weather, any rainfall does not generate much runoff, but some water can be collected in this water hole to benefit wild animals.

The sediment filtering action of this structure is of particular importance and keeps the tank below free of sedimentation.

Figure 13. Goda wala

2.5 Upstream earth ridges to prevent sediment inflow (isweti) Tanks are generally constructed at a confluence of the stream network to reduce the bund length. For the same reason, the land slope of the both sides of the upstream area is relatively high. There is, therefore, a risk of sediment inflow from these areas into the tank. In the past, soil ridges — known as iswetiya or pota wetiya — were constructed to minimise this sediment inflow. In present tank systems, isweti are hardly found. The importance of having this structure is evident as many tanks have been heavily silted up because of the absence of perahana, godawala and iswetiya.

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Figure 14. Iswetiya

2.6 Downstream reservation (kattakaduwa) — pond, marshy land, moist land, dry land This is reserved land below the tank bund. It consists of four micro-climatic environments: 1) water hole; 2) marshy land; 3) moist land; and 4) dry upland, and therefore, diverse vegetation is developed. The kattakaduwa forest is a natural bio-filter that traps pollutants — such as salts and ferric ions — in the water, before it irrigates the low-lying paddy tracts. The thick root system of trees and shrubs act as a protective cushion for the sloping land. Well-developed kattakaduwa forests provide convenient resting sites for birds, bats, reptiles and amphibians. Common tree species in this patch of small forest include; Barringtonia acutangula (Sinhala: Midella; Tamil: Adampu); Borassus flabellifer (Sinhala: Thal; Tamil: Pannai maram); Diospyros malabarica (Sinhala: Thimbiri; Tamil: Panichchai); Ficus benghalensis (Sinhala: Nuga; Tamil: Aal); Ficus racemosa, (Sinhala: Aththikka; Tamil: Atti); Hibiscus tiliaceus (Sinhala: Belipatta; Tamil: Artia); Madhuca longifolia (Sinhala: Mee; Tamil: Illupai); Margaritaria indicus (Sinhala: Karau; Tamil: No name known), Nauclea orientalis, Milletia pinnata (Sinhala: Karanda; Tamil: Punku/Poona) and Terminalia arjuna (Sinhala: Kumbuk; Tamil: Marudu). The water hole referred to as the yathuruwala minimises bund seepage by raising the groundwater table.

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Villagers plant Pandanus odoratissimus along the toe of the bund to strengthen the bund stability.

Figure 15. Kattakaduwa

It appears to be a village garden, where people use various parts of the vegetation for purposes such as food, fruits, vegetables, fuel wood, medicine, timber, fencing materials, household and farm implements. Specifically, they harvest raw materials from this vegetation for cottage industries.

Some products derived from the vegetation of kattakaduwa are shown below.

Table 2. List of plants species in the kattakaduwa and products derived from them

Scientific name Sinhala name Tamil name Product(s) Phoenix pusilla Indi Inchu Hats, bags, basketsPandanus odoratissimus

Wetakeiya Talai Bags, baskets, mats

Bambusa vulgaris Katu-Una Mungil Wood carvings, flower vases, building materials

Calamus rotang Heen-wewel Pirambu Baskets, furniture Borassus flabellifer Thal Pannai marum Mats, bags, baskets,

sweets, toddy Cyperus pangorei Hewan -pan No name known Mats, baskets Hibiscus tiliaceus Beli-patta Artia Ropes, strings

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Figure 16.Products made from plants species of the kattakaduwa

Many plant species are found in the kattakaduwa area. The list of plant species found in the kattakaduwa is given below.

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Table 3. List of plant species in the kattakaduwa and their services and functions

Four land phases: A — dry upland; B — moist land; C — marshy land; D — Water hole

Scientific name Sinhala name Tamil name Services/ functions Land phase Terminalia arjuna Kumbuk Marudu Salt absorption, lime preparation, timber C,D Milletia pinnata Karanda Punku/Poona Salt absorption, medicinal C, Nauclea orientails Bakmee Atuvangi Medicinal, timber, fuelwood C,D Hibiscus tiliaceus Belipatta Artia Rope making C Memecylon umbellatum Kayan Kaya/Kora kaya Timber A Cassia alata Aththora No name known Medicinal B,C Cyperus pangorei Thunhiriya No name known Weaving of mats, open baskets C Agave vera-cruz Hana No name known Bags, strings A, B Pandanus kaida Vetakeya No name known Weaving of mats, open baskets B,C Calamus rotang We-wel Pirambu Furniture, open baskets B,C Typha angustifolia Hambunpan No name known Weaving of mats, open baskets B,C Colocasia esculenta Kiri Ala Shamakkilangu Tuber as food B Barringtonia acutangula Midella Adampu Timber C, D Azadirachta indica Kohomba Vembu Oil, insecticide, timber A Madhuca longifolia Mee Illupai Honey, oil B,C Bauhinia racemosa Maila Atti Rope-making A Limonia acidissima Divul Vila maram Jam, soft drinks A,B Syzygium cumini Damba Naval Mortar and pestle, fruits, timber B,C Cassia fistula Ehela Thirukondrai Medicinal, timber, fuelwood A Tamarindus indica Siyambala Pulli Sweet and soft drinks A Mitragyna parvifolia Helamba Chelampai Timber A Acacia leucophloea Keeriya Velvalayam Timber A Schleichera oleosa Koon Kula, Puvu Fruits, chutney A Bridelia retusa Ketakala Mul-venkai Timber A Holoptelea integrifolia Godakirala Ayil, Kauchia, Velaylii Timber A Diospyros malabarica Thimbiri Panichchai Timber C Diospyros ebenum Kaluwara Panichchai Wood carvings, timber A

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Scientific name Sinhala name Tamil name Services/ functions Land phase Chukrasia tabularis Hik Aglai, Kaloti Medicinal, timber, fuelwood A Cordia dichotoma Lolu Naruvilli, Vidi Fruits, medicinal, timber, fuelwood A Albizia odoratissima Kabal mara Ponnaimurunkai Timber A Glenniea unijuga Daambu (gal-mora) Kuma Fuelwood A Morinda coreia Ahu Manchavanna Medicinal, timber, fuelwood A Bombax ceiba Imbul Kaddu-olaga Fruits, medicinal, timber, fuelwood A

Borassus flabellifer Thal Pannai mara Honey, fruits, toddy, weaving of mats,

open baskets A, B, Thespesia populnea Gan sooriya Thandrimaram Timber, green manure A Gliricidia sepium Vet mara Kona Green manure A, B, Albizia lebbeck Mara Vahai Timber A Mangifera indica Amba Mangai Fruits, timber A Caryota urens Kithul Tippillipanai Honey, toddy, timber A, B Ceiba pentandra Kotta No name known Kapok cotton A Berrya cordifolia Halmilla Chavandalai Timber A Datura metel Aththana Venumattai Medicinal A Muntingia calabura Jam gaha Jam maram Fruits A Solanum indicum Thibbatu No name known Vegetable A, B Alternanthera sessilis Mukunuwenna Ponnankani Leafy vegetable B, C Centella asiatica Gotukola Vallarai Leafy vegetable B, C Cissus quadrangularis Heerassa Arugni Medicine A Dregea volubdis Thiththa aguna No name known Leafy vegetable A Abrus precatorius Olinda Kundu-Mani Medicinal A Bambusa vulgaris Katu-Una Mungil Handicraft, building material B Erythrina variegata Erabadu Mulmurukku Medicinal, green manure A, B Strebles asper Nithul Patpirai Animal feed A, B Nerium oleander Kaneru Alari Medicinal, A Ficus microcarpa Ehetu No name known Fencing material A Alstonia scholaris Rukaththana Elilaippalai Medicinal A, B Ficus benghalensis Nuga Aal Fuelwood A Phoenix pusilla Indi Inchu Handicraft B, C Ficus racemosa Aththikka Atti Fuelwood A

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Scientific name Sinhala name Tamil name Services/ functions Land phase Ficus hispida Kotadimbula No name known Fuelwood A Atalantia ceylanica WaInaran Pey kuruntu Medicine A Oroxylum indicum Thotila No name known Fuelwood A Ficus religiosa Bo Aresu Religious purposes A Trema orientalis Geduma No name known Fuelwood A Senna auriculata Ranawara Avarai Medicinal brew B, C Grewia helicterifolia Boradamana Taviddai Fuelwood A Dichrostachys cinerea Andara Vindattai Fuelwood A Cardiospermum helicacabum Penela No name known Fuelwood A Cassia roxburghii Vaa Vaahai Fuelwood A Vitex negundo Nika Nochchi/Vennochchii Medicine A Crataeva adansonii Lunuwarana Nawala Fuelwood A Manilkara hexandra Palu Palai Fuelwood A Haldina cordifolia Kolon Manchal kandampa Timber A

Murraya koengii Karapincha Karivempu/Karuvetpilai Spice A

Ixora coccinea Rathmal Vedchi Medicine B, C Vitex altissima Milla Kaaddamanakku Timber A

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2.7 Command area drainage (kiul ela) This is the old natural stream used as common drainage. The kiul ela removes pollutants — such as salts and iron — and improves drainage of the paddy tract. Tree species such as Milletia pinnata; Madhuca longifolia, Cyperus pangorei, Hygrophila schulli (Sinhala: Ikiri/Katuikiri; Tamil: Neeramulli), Pandanus and a few species of small fish — such as the Malabar danio (Devario malabaricus; Sinhala1: Rath kailaya), Carverii Rasbora (Rasbora microcephalus; Sinhala: Caveri Randiya), Rasbora dandia (Sinhala: Dandiya / Kehel Dandiya) and the Common spiny loach (Lepidocephalichthys thermalis; Sinhala: Thith Ahirawa) are also found in water holes along the kiul ela.

Figure 17. Kiul ela

In water management, drainage is as important as irrigation. Absence of proper drainage in many irrigation schemes in Sri Lanka has led to the development of salinity in paddy fields.

At present, there is hardly any drainage in minor irrigation schemes to dispose of the excess runoff from the command area. During the period of cultivation, any excess water in the field

1 There are no Tamil names for these fish.

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should be removed at a safe velocity but rapidly. Thus, the presence of a common drainage is essential for irrigable paddy fields.

The step by step procedure to design a drainage system is given below.

1. Find out the designed discharge for drainage. Under the prevailing conditions in the Project area, a design discharge of 0.3 cusec/ acre is acceptable.

2. Prepare the existing longitudinal section of the main drain. The cross sections may be taken at suitable intervals.

3. The existing slope may be adopted if the velocity is within the maximum permissible values, otherwise the drop structures will have to be provided.

The drain can be designed using Manning’s equation.

Keeping in view the permissible velocities and the existing topography, a designed bed slope of 0.3 and 0.25% is adopted for upper and lower portions respectively. If required, drop structures need to be designed.

Design calculations:

Example:

Command area 60 acresMaximum unit discharge 0.3 cusec/acreMaximum design discharge 60 x 0.3 = 18 cusecSide slope 1.5: 1Manning’s Roughness Coefficient 0.04 (assumed) V = R2/3. S1/2

-------------------------

n where V is the average velocity of flow in m/s. R is the hydraulic radius in m. S is the gradient in m/m and n is the roughness coefficient

Q = S1/2 (bY + my2)5/3

n [b+2y •(m2 + 1)]2/3 = S1/2 y1/3 (b/y + m)5/3

ny3[b/y + 2 •(m2 + 1)]2/3

In which: m = Slope of channel sides defined as ratio of horizontal to vertical projection (m/m) b = Width of the channel (m), y = Depth of the channel (m)

Q = S1/2 . Y8/3 . A , where N Where A = (b/y + m)5/3 [b/y + 2 •(m2 + 1)]2/3

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For an assumed value of b/y and m and given values of S and n the depth of channel can be determined using the above equation for Q. The width may be determined from the calculated value of depth.

The step by step procedure for the design calculation is as given below.

1. Based on the reservation width available, assume certain value of b/y; 2. Calculate the value of Y using the equation for Q. It is assumed that the value of S, n,

m and Q are known; 3. Determine the value of b from the calculated value of Y; and 4. Calculate the velocity of flow. It should be within the maximum safe velocity. If not

increase the cross section suitably, keeping the b/y ratio constant.

Let b/y = 4, slope = 0.25 % = 0.0025 in fraction

m = 1.5

Therefore, A = (b/y + m)5/3 [b/y + 2 •(m2 + 1)]2/3 = (4 + 1.5)5/3 [4 + 2 •(2.25 + 1)2/3 = 4.43

Q = S1/2 . Y-8/3 . A ,

n 1 cumec = 1 cusec x123 x 2.543

1003

= 1 cusec/ 35.3 18 = (0.0025)1/2. Y8/3 x 4.43 35.3 0.04

0.51 = 0.050 y8/3 x 4.43

0.04 Or y8/3 = 0.51 x 0.04

0.05 x 4.43 = 0.092

Or y = 0.4 m = 40 cm or say 1.5 ft Depth = 1.5 ft and width = 1.5 x 4 = 6 ft.

Keep 0.5 ft as free board, then total depth = 2 ft.

For the upstream section a width of 4 ft and a depth of 2 ft will be sufficient as the contributing discharge will be much less than the maximum design discharge. Check for maximum permissible velocity:

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Area of cross-section = (6 + 2.25 + 2.25 + 6) x 1.5 2 = 12.375 ft2 Wetted perimeter = 6 + 2 •(2.25 + 6.2) = 6 + (2 x 2.9) = 11.8 ft R = 12.375/ 11.8 = 1.05 V = 1.486 x (1.05)2/3 x (0.0025)1/2 0.04 = 1.486 x 1.03 x 0.05 0.04 = 1.91 ft./sec The actual velocity is less than the maximum permissible velocity of about 3ft./sec. Therefore, the design is in order.

3”

2” 1.5”

3” 3”

Cross section of the main drainage

Figure 18. Cross section of the main drainage

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All field excess water should be drained freely. The side bank should be kept at a minimum height by spreading soil to form passages either side of the canal providing outlets (wakkada) at the end of each paddy strip (issara). Farmers can participate in this work. One passage can be kept for farmers to use as a service path and the other can be used to generate a line of green manure plants and also any economically valuable trees such as Areca catechu (Sinhala: Puwak; Tamil: Paaku); Cocos nucifera (Sinhala: Pol; Tamil: Thengai); Tectona grandis (Sinhala: Teak; Tamil: Tekku) and Berrya cordifolia (Sinhala:Hal milla; Tamil: Chavandalai).

Sustainable traditional villages with the above environmental components have been in existence up to the recent past, but these have deteriorated and collapsed during the last few decades.

Although we may not be able to achieve the prosperity we had in the past, we should be able to learn lessons and best practices of sustainability from our historical use, such as the following:

Groundwater was preserved without using it for cultivation, to assure the water security;

Communities were accustomed to a risk-evading farming system;

They used the environment for their needs without destroying it;

They lived with minimum needs and in the simplest way;

They harvested rainwater and used it in the most efficient manner;

They functioned as a group to ward off wild animals, epidemics and droughts.

They stored food for future use.

Because they used the ecosystem sustainably, they were able cope with external events such as natural disasters.

They protected the knowledge they gathered and attempted to find methods and strategies to solve problems.

They considered social well-being and a healthy environment fundamental needs of the community.

These ten rules are called daha ana.

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3. Project interventions A range of actions were taken to meet the overall goal and objectives. These are listed below in detail, and illustrated in the map below.

Figure 19. Range of interventions taken in the Kapiriggama cascade to restore the ecosystem

3.1 Creation of awareness

Four awareness raising programmes were conducted in three schools (Konakumbukwewa Maha Vidyalaya, Konakumbukwewa; Kapiriggama Prathamika Vidyalaya, Kapiriggama; and Kallanchiya Maha Vidyalaya, Kallanchiya) in the project area on the importance of the cascade tank ecosystem and its conservation.

About 350 students participated in these programmes.

About 600 books related to the history, ecology, biodiversity and culture of ancient cascade systems were donated to the three schools of the project area.

A herbal garden was established in one of the schools in the project area.

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Figure 20. Schools program of creating awareness

3.2 Community and partner participation

Eighteen shramadana programmes were carried out. About 700 villagers from 10 villages participated in these activities.

About 110 HSBC staff participated in cascade ecosystem restoration activities.

Figure 21. Community and HSBC staff engaged in restoring the kiwul ela at Konakumbukwewa tank (Kumudu Herath © IUCN)

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3.3 Physical rehabilitation of the tanks

Partial desilting was carried out in five tanks — Konakumbukwewa, Massalawa,

Peenagama, Maha Kadiyawa, Puliyankulama. (More details about partial desiltation can be found in technical note # 5.)

Upstream earthen ridges (isweti) were formed using the dredged soil in four the five desilted tanks

The toe of one of the tanks, which had been damaged during a former desiltation attempt, was sealed.

The tank bund was repaired, sloped, and then turfed to prevent erosion (planting associated with the bund is included in section on kattakaduwa) in seven tanks —in Kuda Kadiyawa, Maha Kadiyawa, Puliyankulama, Palugonamariyawa, Galkadawala, Kayan wewa, Wanniyankulama.

The common drainage canal (kiwul ela) of the Konakumbukwewa tank was reconstructed to the design given in the previous section.

The spill canals of two tanks — Aluthgama and Massalawa — were restored.

3.4 Ecosystem restoration of tanks elements

3.4.1 Kattakaduwa (interceptor): The kattakaduwa area was demarcated in six tanks — facilitating restoration

activities and also for the conservation, in future, of this element;

In 12 tanks — Puliyankulama, Maha Kadiyawa, Kuda Kadiyawa, Peenagama, Kayangollawa, Kikilige wewa, Aluthgama, Massalawa, Konakumbukwewa, Galkadawala, Kayan wewa, Wanniyankulama — the kattakaduwa was replanted and gaps filled;

The vegetation in encroached areas of the kattakaduwa was enriched with economically important crops such as Areca catechu (Sinhala: Puwak; Tamil: Paaku);

Plants used for ecological restoration of the kattakaduawa included Terminalia arjuna; Madhuca longifolia; Milletia pinnata; Limonia acidissima (Sinhala: Divul; Tamil: Vila maram); Syzygium gardneri (Sinhala: Damba; Tamil: Nir nawal); Azadirachta indica (Sinhala: Kohomba; Tamil: Arulundi); Diospyros malabarica; Ficus spp. (Sinhala: Nuga) and Areca catechu (Sinhala: Puwak; Tamil: Paaku).

3.4.2 Yathuru wala: In four tanks — Aluthgama, Massalawa, Konakumbukwewa, Galkadawala — the

vegetation of the yathuru wala was improved by augmenting it with Lasia spinosa (Sinhala: Kohila; Tamil: No name known) and Ipomoea aquatica (Sinhala: Kang kung; Tamil: Kang kung kirai), as well as reeds.

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3.4.3 Gasgommana: In eight tanks — Massalawa, Maha Kadiyawa, Konakumbukwewa, Peenagama,

Mailagammana, Kuda Kadiyawa, Galkadawala, Kayangollawas — the gasgommana was replanted with the same species, as for the kattakaduwa area, except for Areca catechu.

3.4.4 Plant nurseries: The land for two plant nurseries was donated by two villagers and maintained for the

project duration by them.

About 7,500 plants from these nurseries were used in the process of restoration.

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4. Recommendations for future work Use the newly developed institutional framework to spearhead restoration and

maintenance;

Continue demarcation of tank elements and the cascade ecosystem;

Maintain existing plant nurseries, a source of plants for continuing restoration;

Continue tank ecosystem restoration and ensure maintenance of restored areas;

Set up a long-term study, using schoolchildren, to assess the biodiversity of the restored areas;

Improve, and if needed, establish legal provisions and a mechanism to resolve legal issues related to restoration;

Develop a study to value, in hard currency, what has been done through ecological restoration in terms of ecosystem services to the community;

The above will allow for linking ecological restoration to economic activities;

Link restoration to sustainable use; and Improve female participation by sustainable use to cottage industries.

Explore possibilities of establishing a financing mechanism for restoration, from contributions by the beneficiaries of tank ecosystems.

5. Conclusions There is much to be re-learned from our heritage of a hydraulic civilisation. Historically, our people lived sustainably, respecting the environment, dealing well with wild animals and coping with natural hazards. A village tank cascade ecosystem exemplifies such a sustainable system. It was essential, therefore, to rehabilitate the physical infrastructure of various tanks and restore the ecosystem so that the community may reap the benefits from such as restored ecosystem.

About IUCN

IUCN, International Union for Conservation of Nature, helps the world find pragmatic solutions to our most pressing environment and development challenges.

IUCN’s work focuses on valuing and conserving nature, ensuring effective and equitable governance of its use, and deploying nature-based solutions to global challenges in climate, food and development. IUCN supports scientific research, manages field projects all over the world, and brings governments, NGOs, the UN and companies together to develop policy, laws and best practice.

IUCN is the world’s oldest and largest global environmental organisation, with almost 1,300 government and NGO Members and more than 15,000 volunteer experts in 185 countries. Its work is supported by almost 1,000 staff in 45 offices and hundreds of partners in public, NGO and private sectors around the world. IUCN has been working in Sri Lanka since 1986.

www.iucn.org

December 2015

INTERNATIONAL UNION FOR CONSERVATION OF NATURE

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