Creating Wetlands in Reservoirs for Habitat Diversity, Fisheries and Livelihoods
Creating wetlands in reservoirs for habitat diversity, fisheries and livelihoods 13 November 2012 Peter-John MeynellPresentation at CPWF Mekong Forum onWater, Energy and Food, Hanoi
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 Karibaafter 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 thedraw 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
Possible sites for wetlands within thedrawdown of Nam Gnouang reservoir Nam Thong Houay Mor Keosaenkham Areas for wetlands with conservation objectives Nam Kabaa Areas for wetlands with livelihood objectives
The feasibility study – site selectioncriteria 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 theNam Gnouang reservoir
Existing fish ponds and Site no 5Dyke size = 15 m long x 2.5 mwide
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 Cost per Water Cost per Length Water Construction surfaceItem Dyke Length Height Wide surface volume of Reservoir Stock cost area of Area water wetland m m m m (m2) (m3) USD $/m2 $/m3 1 Dy-01 30 2.70 2.00 250 10,110 5,713 7,760 0.77 1.36 2 Dy-02 10 2.00 2.00 970 7,618 4,775 2,696 0.35 0.56 3 Dy-03 10 2.20 2.00 500 9,275 8,609 3,652 0.39 0.42 4 Dy-04 7.5 2.20 2.00 190 1,500 1,034 3,444 2.30 3.33 5 Dy-05 15 2.50 2.00 200 2,542 1,922 4,056 1.60 2.11 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
How will it performhydrologically?Period Hydrological functioningSept/Oct – • Water recharge from contact withFeb/Mar reservoirFeb/Mar – • Wetland isolated from mainMay/Jun reservoir, • Water loss from seepage and evapo-transpirationMay/Jun – • Water recharge from direct rainfallSept/Oct and run-off from catchment. • Discharge when wetland water levels reach water flow level. E.g.
Simulating the water levels in thereservoir and wetlands
Estimating when the wetlands andreservoir will be connectedWetland/ Location NGL WFL Number of daysDyke No 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 132Existing Lower fish pond 444.50 448.00 49 0 152 dyke
Will these wetlands decrease thepotential 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 forhydropower 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 potentialConclusion: Increases in fish production more thancompensate for the theoretical loss in hydropowerpotential
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