Creating wetlands in reservoirs for habitat diversity, fisheries

and livelihoods

Presentation at CPWF Mekong Forum on Water, Energy and Food, Hanoi

13 November 2012

Peter-John Meynell

The issue - What happens to the aquatic ecosystem and fisheries when a dam is built ?

The free-flowing river is converted to a lake

There is an initial boom in the fishery as the flooded vegetation breaks down

The fish productivity falls back below that of the original river

The aquatic habitat and fish diversity decreases

There is often an ugly scar of barren shoreline exposed when the water level is drawn down

Changes in fisheries in Lake Kariba after impoundment – 1960s

The hypothesis

The diversity of habitats and productivity in a reservoir can be enhanced by introducing constructed wetlands within the drawdown area of the reservoir

In designing a hydropower dam, attention is also needed to design of the habitat in the reservoir,

Design objectives geared towards: increasing biodiversity, increasing productivity and contributing to the

livelihoods of the people living around the reservoir.

The idea – creating wetlands in the draw down to increase habitat diversity

• Build earth dykes with spillways across inlets and seasonal streams coming into the reservoir

• Water is trapped behind the dyke as the water in the reservoir is drawn down, creating permanent pools and wetlands

• Plant wetland plants

Has this been done before?

Not quite like this but note experience of Nam Theun 2

Created about 30 wetlands for conservation around in the forest above the reservoir

Compensated for wetlands inundated on the Nakai plateau

Wetlands in NT2 catchment

Applying this idea in the Nam Gnouang reservoir

Houay Pon

Houay Kamang

Houay Soot

Houay Kahan

Houay Mong

Nam Thong

Houay Mor

Nam Kabaa

Houay Po

Possible sites for wetlands within the drawdown of Nam Gnouang reservoir

Areas for wetlands with livelihood objectives

Areas for wetlands with conservation objectives

Keosaenkham

The feasibility study – site selection criteria

Size and shape of inlets Don’t construct on a major tributary, Relatively small catchment area Look for areas where the slope below the full supply

level is low Look for inlets with a narrow point where dyke can

be constructed. Length and height of proposed dyke – about 2 – 3 m

high and not more than 50 m across. Location within drawdown – the FSL of the Nam

Gnouang reservoir is at 455 masl with the drawdown of 35 m to a Minimum Operating Level of 420 masl. Best within the top 10 m.

Opportunities for a cascade of wetlands Source of construction materials.

Identifying possible locations in the Nam Gnouang reservoir

Site No 1 – Dyke size = 30 m long x 2.7m high

Site No 3 Dyke size 10 m long x 2.2 m high

Existing fish ponds and Site no 5Dyke size = 15 m long x 2.5 m wide

Cost effectiveness

Average cost per hectare of surface area about 1,000 $/ha Average cost per 1,000 cu.m is about 1,550 $/1,000 cu.m

Compared to building a fish pond this is cheap. An equivalent sized fish pond for Site 4 at 1,500 m2 may

cost about 5,000 $ significantly more than the cost of a fish pond at site 4

Item Dyke Length Height WideLength

Reservoir

Water surface

Area

Water Stock

Construction cost

Cost per surface area of

wetland

Cost per volume of

water

m m m m (m2) (m3) USD $/m2 $/m31 Dy-01 30 2.70 2.00 250 10,110 5,713 7,760 0.77 1.362 Dy-02 10 2.00 2.00 970 7,618 4,775 2,696 0.35 0.563 Dy-03 10 2.20 2.00 500 9,275 8,609 3,652 0.39 0.424 Dy-04 7.5 2.20 2.00 190 1,500 1,034 3,444 2.30 3.335 Dy-05 15 2.50 2.00 200 2,542 1,922 4,056 1.60 2.11

How will it perform hydrologically?

Period Hydrological functioning

Sept/Oct – Feb/Mar

• Water recharge from contact with reservoir

Feb/Mar – May/Jun

• Wetland isolated from main reservoir,

• Water loss from seepage and evapo-transpiration

May/Jun – Sept/Oct

• Water recharge from direct rainfall and run-off from catchment.

• Discharge when wetland water levels reach water flow level. E.g. after heavy rainfall events

Simulating the water levels in the reservoir and wetlands

Estimating when the wetlands and reservoir will be connected

Wetland/Dyke No

Location NGL WFL Number of days connected to main

reservoir rainfall year Masl Masl Average Low High

1 H. Gnang 441.00 443.00 124 0 206 2 H. Gnang 445.50 447.00 65 0 164 3 H. Kongko 441.98 443.50 124 0 206 4 H. Kaout 445.50 447.00 65 0 164 5 Upper fish pond 449.10 450.50 0 0 132

Existing dyke

Lower fish pond 444.50 448.00 49 0 152

Will these wetlands decrease the potential for hydropower?

Storage of water in wetlands will technically remove water from its use for generating electricity

Total water stored in wetland is 22,000 cu.m with a surface area of 3.1 ha

Water stored in these wetlands is 0.001% of the total active storage in reservoir

Water in the Nam Gnouang can be used twice to generate power at 3 cents/cu.m each time

Theoretical loss of value = 1,320 $ per year

If there were to be 50 such wetlands created, i.e. 10 times the proposed stored volume, theoretical loss would be 13,200 $ per year

Trading off fish production for hydropower

Low input aquaculture has a fish production of 1-2 tonnes/ha/yr

3.1 ha of fish ponds would yield 3.1- 6.2 tonnes per year

At a farm gate price 2 $/kg this would be worth $6,200 – 12,400 per year

Compared to theoretical loss of $1,320 per year in hydropower potential

Conclusion: Increases in fish production more than compensate for the theoretical loss in hydropower potential

Next steps

Theun Hinboun Power Company are interested to build several of these dykes during the next dry season

IWMI are developing a monitoring project to: Follow the development of the wetland ecology over

several years Monitoring hydrology and water quality, development

of wetland flora and fauna Assess use of wetlands for fish breeding and refuges Assess use of wetlands as fish ponds for livelihoods Monitor fish populations in the reservoir and

production