DIVERSION OF DEDURU OYA WATER TO RATHAMBALA OYA AND KOLAGAMU OYA BASINS

1.0 Introduction – Present Situation

During dry season about 27 tanks in the Rathambala Oya basin, downstream of the Wijaya Katupotha Tank, and about 03 tanks in the Kolagamu Oya basin, downstream of the Dewala Handiya Tank, and 07 tanks in the Sengal Oya basin were severely affected due to water scarcity. Yala cultivation was hampered during past years. More than 80% of the people living in this area fulfill their livelihood by practicing irrigated agriculture. Scarcity of water directly affects the living standards of the farmers.

2.0 Historical Background 

Ancient Kottabedda Anicut at Wilgama

Ancient Kottabedda Anicut  Layout Map in Proposed Area

A ruin of the foundations of an ancient anicut was observed at the Wilgama, across the Deduru Oya River. An ancient canal path (Yoda Ela) could also be seen on the right bank of the river.  One inch topo sheets shows that the canal travels about 10 kilo meters parallel to the river path and ends up with confluence of small tributary of the river.

Irrigable area under the Yoda ela is limited. Ground elevation / river bed elevation is not sufficient to divert water towards the Rathambala Oya basin. 

Project Proposal for Pattiyadaluwa Anicut

Co-ordinate – 118.536 E , 279.914 N

After commissioning of the Deduru Oya reservoir with the capacity of 70 MCM it regulate the river flow by accumulating water during rainy seasons and releasing the same at a constant rate after hydro electricity generation. A part of the tail race will be diverted at the Ridi-bendi-ela weir at a rate of 250 cusec. The balance part will be released to the river to fulfill environmental requirements. 

During cultivation seasons drainage water from the Magalla Scheme augments the Deduru Oya base flow downstream of the Ridi-bendi-ela anicut. After commissioning the RBMC drainage water from the paddy fields, located within 0-23km, would also drains to the Deduru Oya and augment the base flow further.   Therefore considerable amount of water flow would definitely be available at the proposed dam site for diversion.

A new concrete structure could be constructed at Pattiyadaluwa, upstream of the ancient Kottabedda anicut and similar to the existing Ridi-bendi-ela, at the 27m MSL (bed level) to impound a small reservoir. The dam crest level would be 29m MSL. 

As bed rock is visible at the river bed a concrete structure could be recommended.

3.0 Hydrological Observation

In the Field visit at March 20, 2012 we have observed the Daduru Oya in difference places at the Nikaweratiya Bridge there are no flow in river but in the proposed anicut area some amount of drain water is flowing. We can say it will be 3 cumec. In the Dry season also there is flow in lower side of the Daduru Oya.

4.0 Hydrological Studies

According to the hydrological studies the Deduru Oya discharges about 1000 MCM annually.  At present only 40% of water is used for irrigated agriculture and the balance 60% discharges to the sea without any usage. 

During dry seasons, Feb-Mar and Jul-Aug, flow rate in the Deduru Oya is considerably reduced. However after commissioning of the Deduru Oya Reservoir a regular flow could be expected in the middle and the lower reaches of the river.

After commissioning of the Deduru Oya Reservoir about 15,000 acres of paddy lands (new lands and existing lands) will be cultivated, both in the Yala and Maha seasons. Drainage water from these lands would augment the river flow further. Hence there will be a considerable hydrological potential to establish a secondary diversion from the Deduru Oya at the Pattiyadaluwa, downstream of the Ridi-bendi-ela anicut, towards the water deficit area. After commissioning of the Deduru Oya Reservoir we can expect Hydrology graph as shown below.

            Therefore water resources in the Daduru Oya Basin can be Optimized without constructing other Large Reservoirs and inundating large area.

Hydrological Analysis to be done for further studies.

5.0 Geological Studies

In the field visit we have observed the bed rock at the river bed. And we have done soil test to identify the rock level. Some places it will open the rock to ground and other places bellow 3m level in identified axis. But there is a place no rock up to 9m level therefore we will prefer to do another test also to identify rock (resistivity test or CPT test). Therefore further studies should be done. We have carried out sub surface exploration by using Iron Road and have been found that rock existing 0.75m below & near river banks Bed Rock Level not visible up to 3.0 m (Soil Report Detail Attached Herewith)

7.0 Proposed Canal Trace

A new right bank canal path ( Refer Fig 01. ) needs to be started from the new anicut at Pattiyadaluwa (water level at the canal intake would be 29m MSL), travels Westerly above the Nammuwawa and Nelumwewa tracks, enters to the Kadigawa tank at 27.5m MSL (water level) and leave the tank at the same level, travels Westerly through a high cutting area (ridge) up to Allankulama area (27m MSL), turn the path to the North-Western direction and enters in to the Rathambala Oya basin. The canal confluence with the Rathambala Oya main stream at Allaperumandaluwa and impounds a small reservoir with a full supply level of 25.5m MSL. The canal leaves the reservoir at the same level, travels North-Westerly, crosses the Thamarakkulama basin (a right bank tributary of the Rathambala Oya) and travels on the same direction until ends up with the Kolagamu Oya basin at 20m MSL, just upstream of the Dewala Handiya tank. Full Supply Level of the Wijaya Katupotha tank and the Dewala Handiya tanks are located below the 20m MSL contour   line and augmenting of water resources in those tanks could be possible.  

Proposed canal trace goes through the jungle and paddy lands therefore no harm to People live in that area. We try different canal traces within Proposed Anicut at pattiyadaluwa to Wijaya Katupotha tank and Dewala Handiya tank that was shown in Fig 01. Also LS of those conceded canal trace attached herewith.

Lands could be cultivated

Pallama / Kolaeliya / Adippala tanks                              - 400 acres

Sengal Oya tank complex                                                     - 1000 acres

Wijaya Katupotha tank                                                         - 600 acres

27 small tanks (downstream of Wijaya Katupotha)         - 2500 acres

Dewala Handiya Tank                                                          - 774 acres + 300 acres (extra)

Pullichankulama Tank                                                          - 1000 acres

Along the canal path                                                              - 1500 (gravity) + 500 (lift)

Total                                                                                         - 8574 acres

8.0 Problems Associated Because of Scarcity of water

·           Depletion of drinking water resources causes serious health problems. Water born diseases have been spreaded.

·         Serious environmental problems have been emerged. Endemic aquatic lives, 156 species of birds and breading grounds of them have been threatened. During dry seasons, pumping out dead storage of the tanks for agricultural purposes aggravates the situation further.

·         Salinity condition in the Dutch canal and the Mundalama lagoon has been increased due to reduction of fresh water intake. Hence prawn and fish cultivation has been severely affected and reduces foreign earnings as well as personal income. 

·         After collapsing the Sengal Oya anicut the Sengal Oya feeder canal is not functioning and creates water scarcity at the Sengal Oya tank complex (07 tanks) and the Anawilundawa wetland.

·         In order to address the above mentioned problems a new proposal has been studied to divert water by constructing a concrete structure across the Deduru Oya at Pattiyadaluwa area.  

All the above problems can be solved by this proposal and other benefits as well.

9.0 Benefit of the new Proposal

• ·         Several numbers of small tanks (about 20 tanks), which are located along the canal path, could also be fed. The canal could be traveled further along the Northern direction up to certain extend.  Canal extension would depend on the availability of excess water resources. 
• ·         A branch canal needs to be started from the main canal at the Allankulama area (27m MSL) and travels Westerly to feed water to small tributaries of the Deduru Oya, which are flowing on the South-Western direction, in order to augment Pallama, Kolaeliya, Vihara wewa and Pottukulama tanks ( 400 acres).
• ·         Drainage water from the Pallama and Kolaeliya tanks enters in to the Sengal Oya feeder canal naturally. In addition to that an extra quantity of water could be released to the Sengal Oya feeder canal through the same path in order to reactivate the Sengal Oya complex (07 small tanks) and there-by re-establish the ecosystem under the Anawilundawa (Ramsar) wetland. 
• ·         Augmenting water resources in the other two basins, the Rathambala Oya and the Kolagamu Oya, would increases crop intensity in paddy and cash crop cultivation, supports cattle farming, develop prawn cultivation and fishery, regulate salinity level in the Dutch canal and the lagoon, improve livelihood practices, escalate personal income and ultimately grows gross domestic production of the country.
• ·         Augment irrigation water supplies through a network of canal system to 8574 acres of paddy lands under major and minor irrigation systems. Improve crop intensity up to 2.0
• ·         Supply drinking water to 20,000 families in Pallama, Arachchikattuwa and Mahakumbukkadawala D.S. Divisions. Improve drinking water quality. Recharge Ground Water Aquifers - Improve water table.Enhance Micro Climate conditions. 

                    

• ·         Increases Gross Domestic Production.
• ·         Improve livelihood practices/ living standards/ prevent youth unrest.
• ·         Rural development towards reduction of demographic transition.
• ·         New / additional rural road network will be provided for linking villages. 

• ·         5,000 farmer families will directly be benefited by the project while over 20,000 families will indirectly be benefited by getting drinking water and gaining new employment opportunities etc.