CREATING WETLANDS WITHIN RESERVOIRS - CGIAR · 3 DESIGN OF CONSTRUCTED ... There are two types of objectives for creating wetlands within reservoirs. ... a single embankment elevated

MEKONG CPWF| Optimising cascades of hydropower Flood control challenges for large hydro-electric reservoirs with examples from the Nam Theun – Nam Kading basins

CREATING WETLANDS WITHIN RESERVOIRS

Part I – Developing the concept

November 2013 Peter-John Meynell

MK3 Optimising cascades

of hydropower

FISHERIES & ENVIRONMENT

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Author P-J Meynell (ICEM)

Produced by Mekong Challenge Program for Water & Food Project 3 – Optimising cascades of hydropower for multiple use Lead by ICEM – International Centre for Environmental Management

Suggested citation P-J Meynell. 2013. Enhancing ecological diversity of reservoirs with constructed wetlands: Part I – Developing the Concept. Project report: Challenge Program on Water & Food Mekong project MK3 “Optimizing the management of a cascade or system of reservoirs at the catchment level”. ICEM – International Centre for Environmental Management, Hanoi Vietnam, 2013

More information www.optimisingcascades.org | www.icem.com.au

Image Cover image: Wetland plants (Salix sp) growing in created wetland in Nam Theun 2 catchment (Photo: P-J Meynell). Inside page: Reservoirs with large drawdowns can leave ugly, unproductive scars on the landscape. (Photo P-J Meynell).

Project Team Peter-John Meynell (Team Leader), Jeremy Carew-Reid, Peter Ward, Tarek Ketelsen, Matti Kummu, Timo Räsänen, Marko Keskinen, Eric Baran, Olivier Joffre, Simon Tilleard, Vikas Godara, Luke Taylor, Truong Hong, Tranh Thi Minh Hue, Paradis Someth, Chantha Sochiva, Khamfeuane Sioudom, Mai Ky Vinh, Tran Thanh Cong

Copyright 2013 ICEM - International Centre for Environmental Management 6A Lane 49, Tô Ngoc Vân| Tay Ho, HA NOI | Socialist Republic of Viet Nam

Acknowledgements Information and ideas for this project were collected during a site visit by the team in 2011. We would like to acknowledge co-operation from Theun Hinboun Power Company, Nam Theun 2 Power Company, Nam Theun Nam Kading River Basin Committee Secretariat, and Nam Theun 2 Watershed Management and Protection Authority. Long term daily flow data at TH dam and other sites were kindly provided by Theun Hinboun Power Company, as was information about the current state of flood management. Rainfall data were from Mekong River Commission Secretariat sources. Funding for this project was from the Mekong river component of the CGIAR Challenge Program on Water and Food.

http://www.optimisingcascades.org/

http://www.icem.com.au/

MEKONG CPWF| Optimising cascades of hydropower Flood control challenges for large hydro-electric reservoirs with examples from the Nam Theun – Nam Kading basins

1 IN TRODU CTION TO CON CEP T

1 . 1 H Y P O T H E S I S

The construction of a dam for hydropower, irrigation or water supply results in the creation of a reservoir converting a natural free-flowing river with a number of different habitats into a slow moving or almost stagnant lake environment with a more restricted number of habitats. Often, the newly developed lacustrine environments are less productive than the original river, especially if they are steep-sided and deep reservoirs with a large drawdown and if reservoirs are being used for storage during the wet season for slow release during the dry season. The productivity of those reservoirs is generally restricted to the top 2 m of the water surface i.e., the aerobic layer with good light penetration. During the drawdown period, the exposed banks of the reservoir are often barren and supporting little vegetation growth, and these areas do not contribute to the productivity of the reservoir when they are inundated again during the wet season.

Such reservoirs are also less biodiverse because the richer fish fauna in the river will be replaced by a more restricted fish species mix consisting of those that can breed in the reservoir and those that can survive there provided that they have access to spawning sites up the tributaries flowing into the reservoir.

The hypothesis for this research is that the diversity of habitats and productivity in a reservoir can be enhanced by introducing constructed wetlands within the drawdown area of the reservoir and by maintaining access for fish to spawn in the tributary rivers. The principle behind this hypothesis is that in designing a hydropower dam, attention should be paid to the design of the habitat in the reservoir, rather than just letting the lake environment develop by default. The design components in the reservoir may be geared towards increasing biodiversity, increasing productivity and contributing to the livelihoods of the people living around the reservoir.

This paper aims to provide a conceptual framework for the development of constructed wetlands within the Nam Gnouang reservoir in Lao PDR, a hydropower project operated by the Theun Hinboun Power Company (THPC), in order to demonstrate how such wetlands may be designed and installed.

MEKONG CPWF| Optimising cascades of hydropower (MK3) Flood control challenges for large hydro-electric reservoirs: the Nam Theun – Nam Kading basins

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TA BLE OF CON TEN TS 1 INTRODUCTION TO CONCEPT .................................................................................................III

2 OBJECTIVES FOR CREATING WETLANDS WITHIN RESERVOIRS ................................................. 2 2.1 Conservat ...................................................................................................................................... 2 2.2 Livelihoo ........................................................................................................................................ 2

3 DESIGN OF CONSTRUCTED WETLANDS WITHIN THE RESERVOIR AREA .................................... 2 3.1 Constructed wetlands in Nam Theun 2 catchment ....................................................................... 2 3.2 Cambodian ring dykes ................................................................................................................... 4 3.3 Design principles for wetlands constructed within the reservoir ................................................. 5

4 CRITERIA FOR SITE SELECTION ................................................................................................. 6

5 MANAGING THE CONSTRUCTED WETLANDS ........................................................................... 8

6 APPLICATION TO NAM GNOUANG RESERVOIR ........................................................................ 9

7 PROPOSED RESEARCH AND DEVELOPMENT........................................................................... 11 7.1 Feasibility study ........................................................................................................................... 11 7.2 Detailed design and construction................................................................................................ 12 7.3 Community consultations ........................................................................................................... 12 7.4 Monitoring .................................................................................................................................. 12

ADB| TA7779 – Support for the NTP with a Focus on Transport & Energy |ICEM ThanhHoa Field Mission Report – Adaptation Team Secondary Mission

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2 OBJ ECTIV ES F OR CREA TING W ETLAN DS W ITHIN RES ERV OIRS There are two types of objectives for creating wetlands within reservoirs. These may have a focus on conservation, i.e., the wetland is created for maintaining or enhancing the biodiversity of the reservoir, or alternatively the focus may be upon livelihoods of local communities, enhancing the potential for using the natural resources around the reservoir. These are not incompatible, in that a constructed wetland aiming at conservation of the fish diversity, for example, would have a beneficial effect upon fish populations and productivity in the reservoir which may then enhance the fishery potential in other parts of the reservoir. However, a constructed wetland that has a conservation objective should not itself be used for fish capture or culture.

2 . 1 C O N S E R V A T I O N

The conservation objectives may be framed in terms of constructing wetland areas in the reservoir in order to:

• Increase the diversity and productivity of the reservoir habitats, especially in drawdown areas • Enhance and maintain access to a diversity of spawning grounds for reservoir fish species • Enhance fish species diversity and fish productivity in the reservoir • Provide wetland areas in the catchment as habitats for endangered species (see example

from Nam Theun 2)

2 . 2 L I V E L I H O O D S

The livelihood objectives may be framed in terms of constructing wetland areas in the reservoir in order to:

• Enhance the reservoir fishery for the livelihoods of local communities • Create fish ponds in the drawdown area to be managed by local communities • Retain water in the drawdown area that may be used for recession agriculture by local

communities

3 D ESIGN OF CON S TRU CTED W ETLAND S W I THI N TH E RES ERV OIR AREA The ideas for the design of constructed wetlands within reservoirs stem from two examples within the Mekong: (i) the wetlands constructed in the catchment above the Nam Theun 2 reservoir and b) Cambodian ring dykes. These are outlined below and followed by conceptual design principles for wetlands constructed within the drawdown of the reservoir.

3 . 1 C O N S T R U C T E D W E T L A N D S I N N A M T H E U N 2 C A T C H M E N T

About 30 wetland areas have been constructed in the Nam Theun 2 catchment, close to the reservoir. The original objective for constructing these wetlands was as compensation for the loss of small ponds and wetlands in the plateau area caused by inundation. These small ponds were considered important habitats for the endangered White-winged Duck and, when created, for the relocation of a number of turtle species captured before inundation.1

1 Taken from Nam Theun 2, Wetland Construction and Vegetation Project. Monitoring Report. December 2008

MEKONG CPWF| Optimising cascades of hydropower (MK3) Enhancing ecological diversity of reservoirs with constructed wetland

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These constructed wetlands are located just above the full supply level (FSL) of the reservoir and in the forest conservation area on the northeast bank of the reservoir as indicated in Figure 3-1. They consist of a small dyke, not more than 100 m long, across the numerous streams running into the reservoir from the catchment. The dykes are protected from flash flooding by a spillway as shown in Figure 3-2.

Figure 3-1: Wetland sites located in the catchment of the Nam Theun 2 Reservoir

Source: NTPC report


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Figure 3-2: Wetland vegetation zones in the Nam Theun 2 wetland sites

Upstream of the dyke, a pool of permanent water remains trapped, and during the wet season water from the catchment will flow down to replenish it creating a larger area of seasonally inundated land that will dry out as the dry season progresses. These areas of different wetness create a diversity of wetland habitat which can be planted with emergent and floating plants.

A detailed survey report2 from the Nam Theun 2 catchment area can be used to illustrate both survey process and the design of the dyke, spillway and open water areas. This is illustrated in Annex 1.

3 . 2 C A M B O D I A N R I N G D Y K E S The Cambodian ring dyke system is described in papers by Someth et al. (2007) and Someth et al. (2009)3. These papers assess the water balance in constructed ring dykes for the storage of water and use for irrigation in the dry season in the floodplain of the Tonle Sap Lake. The dyke system consists of a single embankment elevated by excavated soil at the upstream part and submerged in the rainy season due to the expansion of the Tonle Sap lake. The dyke is tangential to the natural slope, so that when the floodwater recedes some water is retained upstream of the dyke.

The dykes studied include the irrigation system of the West Baray established in the Angkorian period near the town of Siem Reap and the Batheay irrigation system initiated during the Pol Pot regime

2 Nam Theun 2 (2007) C1038 & C1039 Wetland creation – survey, construction and vegetation. Wetland survey and Design for Zone B. 3 Paradis Someth, Naritaka Kubo and Hajime Tanji. 2007. A combined technique of floodplain storage and reservoir irrigation for paddy rice cultivation. Paddy Water Environ 5:101-112 Someth, P., Kubo, N., Tanji, H. and Ly, S. 2009. Ring dyke system to harness floodwater from the Mekong River for paddy rice cultivation in the Tonle Sap Lake floodplain in Cambodia.

Source: NTPC report


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(1975 – 1979) at the southern end of the Tonle Sap Lake. The dykes in the Tonle Sap floodplain around the West Baray Reservoir consists of a single embankment elevated by excavated soil with a total length of about 10 km, standing about 2 m above the natural land surface and covering an area of 6 sq. km, with a water volume of 6 million m3. When all the floodwater stored in the ring dyke has been used for dry season rice cultivation, water from the West Baray Reservoir is used.

The Batheay system consists of an earth ring dyke with a total perimeter of 9,500 m connecting four hills. It has a top width of 8 m and a bottom width of 20 m and a height of 5 m. It can store 18.5 million m3 in an area of 700 ha. It allows irrigation of an area of 2200 ha of which 800 ha is cultivated in the wet season (700 ha inside the ring dyke) and 1,400 ha cultivated in the dry season (outside the ring dyke). The soil for the dyke was excavated from the ground on each side, so that the ditch on the outside serves as the irrigation canal.

On the ring dyke there are two main sluice gate structures (4 gates of 2.0 m and 2 gates of 2.5m) which act as intake structures - during the flooding season, and eight circular culverts (0.5 – 0.8 m in diameter) acting as supplying structures in the dry season.

These two dyke examples are on a much larger scale than that proposed for constructed wetlands and have been designed principally for irrigation, but they illustrate the possible earth bank construction for the storage of flood waters, either by overbank filling or through culverts and sluices.

3 . 3 D E S I G N P R I N C I P L E S F O R W E T L A N D S C O N S T R U C T E D W I T H I N T H E R E S E R V O I R

The basic operational concept for wetlands constructed within the drawdown area of a reservoir is that a simple earth bund encloses an area of water when the reservoir is at FSL. The most suitable locations may be across inlets in the perimeter of the reservoir, so that bund length is limited.

Fish gain access to this area of water at FSL over the top of the bund (which would be located maybe 1 - 2 m below FSL) or through a sluice gate that is kept open as the water level rises in the reservoir in the wet season (see Fig 3-3). As the water level is drawn down during the following dry season, the water behind the bund is retained, or the sluice gate is closed, so that a standing body of relatively shallow water (between 2 - 5m deep) is retained, thus creating a pond. The land around this pond remains wetted during the dry season and could be planted with emergent and floating aquatic plants to create a diverse wetland habitat.

Each wet season, the water in the pond will be replenished as the reservoir approaches full supply level. Some additional water may flow in from the small catchment immediately around the pond.

It is important that the soil lining of the wetland area be suitable for retaining the water and for the growth of aquatic plants. An impervious clay soil would be suitable as a base to retain the water, covered by a layer of fine silt and mud.

The area of wetland behind the bund would have to be sufficiently large so that enough water is retained without too great evaporation losses so that the pond lasts throughout the dry season.

The simplest type of bund might be of earth-fill construction that is easy to repair and maintain. The bund should not be constructed over a perennial stream coming into the reservoir, for two reasons: (i) the flows in the stream during the wet season may be too strong with a risk of washing out the bund and (ii) the stream may be an important route for fish moving up out of the reservoir for spawning. The bund should be located so that it does not prevent access to such streams for migrating fish. The bund should have a reinforced overflow to protect the structure in the event of a flash flood. If fitted with a sluice gate this should be kept open while the reservoir is filling, and be closed at the onset of drawdown.

The issue of siltation of the wetland areas has to be considered if sediment from upper catchment is high. If there is a high risk of collection of sediment in the wetland which would reduce its life, measures for sediment management in the catchment above the wetland may have to be put in place.


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It might also be possible to create a series of ponds further down the drawdown, which could act as refuges for fish in the dry season. These might be excavated rather than constructed with earth bunds. These would be unlikely to develop any aquatic plants and would rely upon algal productivity as a source of primary production.

Figure 3-3: Creating constructed wetlands with earth-fill dyke (a) simple bund below FSL and (b) with culvert and slice gate to control water entry and exit to constructed wetland.

4 CRITERIA F OR S ITE S E LECTION

It will not be possible to construct wetlands in every reservoir. The characteristics of the reservoir and its topography will determine the suitability of this approach. Some of the parameters that will have to be considered in scoping the reservoirs and site selection include:

• Drawdown: Storage reservoirs with a large annual drawdown will be more suitable for created wetlands than reservoirs where there is smaller drawdown and shorter storage period. Run of river schemes would be less suitable.

• Perimeter and area exposed with drawdown: A quick metric for identifying which reservoirs have a high indentation of the perimeter which might be suitable is the difference between the perimeter at FSL and the perimeter at Minimum Operation Level (MOL). Similarly, the area exposed with drawdown could also be used. For example, the perimeter drawdown indices for the reservoirs in the wider Nam Theun/Nam Kading Basin are shown in Table 4-1, with a map of the Nam Theun-Nam Kading Basin and the existing and proposed reservoirs.

Table 4-1: Calculations of perimeter/drawdown indices for the reservoirs in the Nam Theun/Nam Kading Basin

UnitNam

Theun 2Theun

HinbounNam

GnouangNam

Theun 1Nam

Theun 4Nam

MouanXe Bang

Fai

Area of reservoir at FSL – sq km sq km 483.1 47.9 133.5 102.4 11.8 59.9 153.6Perimeter of reservoir at FSL – km km 974 437.1 479.1 353.4 55.8 261.6 325.2Length of reservoir at FSL (dam to top of reservoir) km 99.6 32.92 65.1 61.7 20.4 49.8 28.7Drawdown m 12.5 5.0 35.0 32.0 40.0 20.0 5.0

Area of reservoir at MOL – sq km sq km 325.6 10.6 47.5 60.6 5.5 40.5 87.1Perimeter of reservoir at MOL – km km 834.9 71.8 193.9 338.8 38.7 250.4 243Length of reservoir at MOL (dam to top of reservoir) km 82.1 32.7 37.2 36.4 8.0 49.6 28.6IndicesChange in perimeter with drawdown (PFSL-PMOL) km 139.1 365.3 285.2 14.6 17.1 11.2 82.2Change in area with drawdown sq km 157.5 37.3 86 41.8 6.3 19.4 66.5

Determinant factor

Existing and proposed reservoirs in Nam Theun-Nam Kading Basin, Lao PDR

Full supply level

Minimum operating level


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Figure 4-1: Map of Nam Theun Nam Kading River Basin, showing existing and proposed reservoirs considered for constructing wetlands

The Theun Hinboun Reservoir has the highest change in perimeter with drawdown, but is not considered further because it is not a storage reservoir. Nam Theun 2 has both a high change in


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perimeter with drawdown and high change in area exposed with drawdown. This also indicates that the area exposed has a relatively gentle slope. The NG dam has a high perimeter change but a lower area exposed, indicating higher exposed slopes than the NT2. Nam Theun 1, 4 and Nam Mouan would appear to be less suitable because they have relatively low changes in perimeter and area with drawdown. Xe Bang Fai would appear to have moderate suitability with mid-range changes in perimeter and area.

• Shape of reservoir at full supply level and minimum operating levels – the visual inspection of the shape of the reservoir will provide a rapid short list of possible inlets where such wetlands could be constructed.

• Avoid inlets with major streams flowing into the reservoir. • Topography and slope of the inlets – the shallower the slope, the larger the wetland area to

be created behind the earth bund. • Width across the mouth of the inlets – choose inlets which have a narrower mouth, since

this will limit construction effort and costs. • Sub-catchment area - ensure that the sub-catchment above the inlet is relatively small so

that earthworks will not get washed away by high flows. • Soil type and permeability in the wetland area – clay soils will be best so that water will be

retained in the wetland. • Source of materials for earth-fill bund – source of suitable construction materials should be

nearby. • Area and volume of water stored – estimate the area of the wetland and volume of water

stored and use evapotranspiration and infiltration data to estimate water losses during dry season.

• Impact upon hydropower potential – use these stored water volumes to calculate the impact upon hydropower potential. This will be small for a few constructed wetlands, but may be more significant if a number are created.

• Fishery management – the identification of fishery management zones in the reservoir, i.e., non- fishing zones, will help to separate wetland areas suitable for conservation objectives from those with livelihood objectives.

• Proximity to communities – constructed wetland areas closer to communities may be more suitable for livelihood objectives.

5 MA NA GING TH E CON S TRUCTED W ETLAN DS

Once constructed, the wetlands will need to be maintained and managed. It is obviously worthwhile constructing the dykes well and with good quality materials, so that maintenance costs are manageable.

Management of the constructed wetland should be seen as the part of the overall reservoir management, which would be the responsibility of the hydropower operator, but with strong community involvement. This applies equally to wetlands which have conservation or livelihood objectives.

If the wetlands are constructed with conservation objectives, the wetland should be used for neither fish capture or fish culture. However, there should be a link with the fishery management on the reservoir, since the benefits of having these constructed wetlands will result in a better fishery within


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the wider reservoir. Fishermen should be aware of this link and be involved in the management and maintenance of the wetland as part of community fishery initiatives.

If the constructed wetlands have a greater livelihood objective, e.g., as community fish ponds, or have retaining water in the drawdown area that could be used for recession agriculture by local communities, then attention must be paid at the outset to developing a management mechanism. This should include definition of access rights, benefit sharing and responsibilities. The physical maintenance of the structures and the enhancement of the fish stocks will be important responsibilities, but with this will come security risks from poaching of the fish. These too will need to be managed. Some ideas for community management of fish ponds in Lao PDR are described in recent papers.4 5

Managing the security risks may call for a regulatory framework with the collaboration of the power company, government and local users. This will mean looking at the interplay between current access rights to the reservoir, the investment agreements between government and the company, the company’s own policies and what is currently allowed/disallowed by national law both in terms of access but also the various community management models possible. The management of the balance between conservation and livelihood objectives of the different wetland areas would have to be considered.

6 A P P LICA TION TO NA M GNOU A NG RES ERV OIR

An initial screening of the Nam Gnouang Reservoir shows that it has a high index of change in perimeter with drawdown (though not as high as Nam Theun 2) indicating that there is high potential for the development of such constructed wetland areas.

The MK1 project has developed a series of maps of the reservoir at different levels of drawdown, shown in Figure 6-1.6 These also indicate the proposed no fishing zone and the location of the main resettlement area at Keosaenkham.

Initially, the inlets of the reservoir coloured in green, which are exposed when the water level of the reservoir is drawn down from the FSL at 455 masl to 450 or 445 masl, would appear to be the most suitable locations to investigate. The inlets with the major streams should be avoided as indicated on the larger map on Figure 6-2. The inlets in the no fishing zone should be considered for conservation objectives. The inlets near Keosaenkham may be considered for livelihood objectives.

Example of a wetland site: Figure 6-3 illustrates the constructed wetland concept at the road crossing to the resettlement village of Keosaenkham inundated at FSL. The installation of a sluice to control the water level across the culverts would enable water to be retained in the wetland area. The soil suitability to retain the water level in the draw down period, the flows in the creek during heavy

4 Saphakdy, B., Phomsouvanh, A., Davy, B., Nguyen, T.T.T. and De Silva, S.S. 2009. Contrasting community management and revenue sharing practices of culture-based fisheries in Lao PDR. Aquaculture Asia Magazine, Volume XIV No. 3, July - September 2009. 5 Brad Collis. Fish Ponds Yield New Food Enterprises. Aquaculture November 2010 – February 2011. 6 Given the limits of resolution of the DEM available to us for the GIS analysis, this is as far as the mapped outputs can be used. This first-cut identification of the locations needs to be followed up with more detailed field observation and survey to actually locate where the bunds should be constructed. More detailed topographic data (with much smaller contour intervals, say of at least 1 m) is needed if it is necessary to map out the actual extent and configuration of the constructed wetlands.


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storms and access up the creek by migrating fish at different times of year would have to be checked. The constructed wetland could be used as a community fish pond.

Figure 6-1: Maps of the Nam Gnouang Reservoir at different drawdown levels.

Figure 6-2: Map of the Nam Gnouang Reservoir indicating possible locations to be investigated for constructed wetlands.


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Figure 6-3: Road to Keosaenkham, inundated at FSL of Nam Gnouang Reservoir

7 P ROP OS ED RES EA RCH A ND D EV ELOP MEN T

There are four stages envisaged to take this concept further:

7 . 1 F E A S I B I L I T Y S T U D Y

This will involve a site visit for partner representatives to investigate the potential for constructed wetlands using the initial map as a first guide for possible locations. The field visit should consider:

a. The best locations for five constructed wetlands, with both conservation and livelihood objectives, considering the criteria listed above

b. The size and location of earth banks required c. The design of the earth banks and ancillary structures (spillway, culverts, sluice gates) d. The availability of construction materials e. The suitability of soils in the area of the constructed wetlands f. The total area of the wetland to be constructed at FSL g. The suitability of wetland vegetation that may be planted h. Water balance, evaporation and infiltration and impact upon water availability for electricity

generation i. Overall cost and labour required.

As part of the feasibility study, an assessment of current policy, legal and institutional frameworks around hydropower management, access and community-based fisheries management is done so options for the management framework can be developed along with the structural plans. Knowing these options would also be a necessary primer for the community consultations and the agreements on access as well as on shared responsibilities with local communities.


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7 . 2 D E T A I L E D D E S I G N A N D C O N S T R U C T I O N

If the feasibility study proves positive and a decision is taken by THPC to experiment with at least two constructed wetlands in the areas selected, then local labour would be mobilised by THPC to undertake this. No schedule is set for this, but it should be before the FSL is reached.

7 . 3 C O M M U N I T Y C O N S U L T A T I O N S

It will be necessary to develop these wetland areas, those with both conservation and livelihood objectives, in consultation with communities living around and using the reservoir. Community involvement in the wetlands will be required for construction, maintenance and management. Agreements will have to be developed on access and use of the wetland areas, sharing of the benefits and how issues such as security risks may be addressed.

7 . 4 M O N I T O R I N G

The performance of the wetland should be monitored in terms of the following aspects: a. Retention of water during the drawdown period

b. Establishment of aquatic plants

c. Presence and productivity of fish species

d. Movements and spawning activities of fish

e. Opportunities for livelihood development and how these are distributed amongst the communities

f. Contribution to the overall productivity and ecological diversity of the NG Reservoir

g. Functioning of local/community organisations as both the beneficiaries and the guardians of the wetlands resources.

The feasibility study was carried out in 2012 and indicated the locations of at least five possible sites within the Nam Gnouang, on which some designs and costings were developed. This study is described in Creating Wetlands in Reservoirs: Part 2.

Documents

CREATING WETLANDS WITHIN RESERVOIRS - CGIAR · 3 DESIGN OF CONSTRUCTED ... There are two types of objectives for creating wetlands within reservoirs. ... a single embankment elevated