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Flood Prevention

Ecological Enhancement Work of Yuen Long Bypass Floodway Project

Flood Prevention >Ecological Enhancement

Yuen Long Bypass Floodway affects some fishponds and agricultural land.  To compensate for the ecological loss, DSD has engineered a piece of wetland with an area of 7 ha (i.e. 70,000m2 equivalent to 10 full size soccer pitches) from 3 abandoned fishponds, providing wetland habitat to wild birds, amphibians (frogs) and dragonflies.

 One View of Wetland
                           One View of Wetland


The engineered wetland mainly comprises 3 main ponds, 1 seasonal shallow pond and 1 permanent shallow pond.  In addition, there are also crushed brick field, oyster shell field, reedbed and 1 deep pond as well.  Outside the wetland, at the downstream of the Yuen Long Bypass Floodway, DSD has also created a shallow pond within the new river channel.  Inside the engineered wetland and the channel shallow pond, various aquatic plants and different species of weeds and trees are planted extensively.  Along the south of the engineered wetland, there is a bamboo cluster to attract herons.


The construction of the engineered wetland and channel shallow pond commenced in January 2003 and completed in end 2005.  After one year of establishment, a sustainable ecological habitat has been developed.


Wetland - Sedimentation Pond, Crushed Brick Field and Oyster Shell Field


l               Sedimentation Pond

The water from the low flow channel of the Yuen Long Bypass Floodway first enters into the sedimentation pond so that solid particles in the water, e.g. sand, will settle.


l               Crushed Brick Field and Oyster Shell Field

The water then flows into the crushed brick field and the oyster shell field for natural filtration and purification so as to reduce the nutrient in the water, and thus inhabiting the development of algae boom.


Sedimentation Pond, Crushed Brick Field and Oyster Shell Field  
    Sedimentation Pond, Crushed Brick Field and Oyster Shell Field


Wetland - Reedbed


l               An area of 7,500m2

l               After treatment by the crushed brick field and oyster shell field, the water will be equally distributed into 4 reedbeds where the reeds will further absorb the nutrient from the water.

l               The reedbed is also one of the ecological habitats of the engineered wetland.  It breeds different species of invertebrates such as worms and snails etc. and, as a result, it further attracts birds to forage and inhabit.


Wetland – Main Pond and Deep Pond


l               3 Main Ponds (depth about 1.4 – 1.8 m)

The beveled bank of the main ponds provide different water depth and there are marshes and land habitat.  We have planted 20 species of a total of about 180,000 numbers of water plants to attract various wildlife such as coots, egrets, small birds, dragonflies and frogs etc to inhabit.

l               Deep Pond (depth about 2 m)

The function of the deep pond is to store the surplus water discharged into the engineered wetland for subsequent water level adjustment in the various ponds.  The outfall of the deep pond can discharge the surplus water into Kam Tin River.  The exposed mud at the bottom of the pond in the dry season is another ecological habitat.




Wetland – Shallow Pond (depth about 0.5m)


l               Seasonal shallow pond provides habitat for amphibians (frogs) to lay egg and tadpoles to grow.

l               Permanent shallow pond provides habitat to dragonflies’ larvae to grow.


Shallow pond                                   

                                      Shallow pond


Channel Shallow Pond


l               At the downstream of the Yuen Long Bypass Floodway, 350 m in length with an area of 1.8 ha (equivalent to 18 full size swimming pools).

l               The bed of the shallow pond is designed to be lower by 0.3 m and allow ponding of water to become a shallow pond.

l               Different aquatic plants at the riverbed and river bank also form a wetland ecological habitat.


Egrets in Channel Shallow Ponds
                     Egrets in Channel Shallow Ponds


Preparation Work and Procedures for Planting in Wetland


l               Remove the weeds and plants from the bottom soil.

l               Fully fill up the ponds for at least two days to ensure the bottom soil absorbed enough water.

l               Lower the water level of the pond before planting so as to facilitate transportation and planting work.

l               Planting density should be in accordance with the requirements of planting schedule; all designated planting area should have different planting composition.

l               All garbage should be cleared after planting work, plants should be adequately watered, and ponds should be filled with water with depth not less than 3 inches.


Daily Maintenance


l               Clear climbers and invasive weeds, carry out watering, cut grasses, collect garbage and adjust water level etc.

l               Although all wetland plants can survive under occasional flooding or dry condition, water should be maintained at its “optimum level” as far as possible during breeding period.  Therefore, we would use sluice gate to adjust water level.

l               The optimum water level should be determined according to operational experience after planting work is finished.


Achievements after 1-year Establishment


l               The water quality objectives and standards have been achieved through the proper functioning of the sedimentation pond, the crushed brick field, the oyster shell field as well as the reedbed.

l               All aquatic and terrestrial plants species planted are growing healthily.

l               A large quantity of other aquatic and terrestrial plants, including water fern, water shamrock and yellow velvetleaf, are colonizing naturally in the wetland.

l               5 species of the 6 bird target species are found in the first year and large amount of other bird species are found, including rare species e.g. lesser whistling duck and pheasant-tailed jacana.

l               3 species of the 5 dragonfly target species are found in the first year and large amount of other dragonfly species are found, including rare species, e.g. Acisoma panorpoides and Macrodiplax cora.


Wetland shadow
                                Wetland shadow

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