Table 4

Table 2.4 Considerations of Various Drainage Options

Drainage Options	Environmental Benefit	Environmental Dis-benefit	Recommendation	Reason & Considerations
Maintenance of existing river	(a) The area to be affected by the proposed drainage improvement works will be the least. (b) The existing condition of rivers will be improved. For example, the contaminated soil and waste settled in the river bed will be removed. (c) The disturbance to the ecological system and water quality in existing rivers will be of small scale.	No major environmental dis-benefits are anticipated.	Not recommended.	(a) This option can only marginally improve the capacity of the existing rivers. (b) The flood protection level provided is still very low due to the small size of existing rivers. Severe flooding in the areas will continue.
By-pass channel or box culvert	(a) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.	Extensive land resumption is required for the construction of by-pass channel or box culvert. The public will be seriously disturbed.	By-pass box culvert Pak Ngan Heung River. By-pass channel Luk Tei Tong (2) River	(a) The construction of by-pass channel or box culvert will require extensive land resumption and will impose adverse impact to other areas further away from the flooding spots. (b) Along Luk Tei Tong (2) River and Pak Ngan Heung River, most of the areas are abandoned agricultural lands and marsh. There are sufficient available space for the construction of the by-pass box culvert and channel. The diversion scheme is considered feasible and acceptable. Locals and green groups also accepted the proposal. (c) The impact to existing marsh at Luk Tei Tong would be minimum by adopted ecological friendly design details. The existing river will be untouched.
Interception and Flood Storage	(a) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.	(a) Extensive land resumption is required for construction of storage pond or interception channel. (b) Environmental impacts to other areas will result. The upstream area, which is mainly composed of dense vegetation area, agricultural land, residential area and nurseries, will be seriously affected.	Not recommended	(a) This option is also not a cost-effective solution given the large volume of runoff needed to be stored which results in resumption of large piece of land for the construction of a large stormwater storage pond. The plan area of the pond can be reduced by half if the depth of the pond is increased to 6m. However, a pumping station is then required for pumping the water away after the storms. The required land resumption is still very substantial and the objection from public cannot still be avoided. (b) This option is therefore considered not acceptable.
Floodwall and Pumping Station	(a) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.	(a) Floodwall has limited scope for improving the aesthetic. The visual impact is of concerns, in particular, if the floodwall is high.	Not recommended	(a) Floodwall can be constructed along the rivers to contain the floodwater during extreme rainstorm events. However, the capacities of the concerned rivers are very low and a high floodwall of minimum 2m is needed. (b) The floodwall protection does not provide a long-term solution because it contains floodwater at a higher water level and causes flooding of the local drainage systems behind the floodwalls. Flooding in adjacent areas still persists. (c) To eliminate the severe flooding outside the floodwalls, the floodwalls are required to be constructed further away from the existing rivers such that an additional flood plain flow area is allowed for conveyance of flow during extreme rainstorms. The formation of flood plain flow area requires substantial land resumption. (d) Consideration has been given to pumping the surface runoff in the rivers to the downstream in order to reduce the water level and hence height of the floodwalls. However, the peak discharges are very substantial and temporal. It is technically impracticable to pump the peak surface runoff away without the use of detention pond. (e) Therefore, this option is not further considered.
Deepening and Widening of Existing Rivers	(a) The existing condition of the rivers will be improved by the proposed engineering works by applying appropriate ecological friendly design. For example, Gabion lining with planting pits can enhance the ecological condition and provide shelter for the river habitats; the contaminated soil and waste settled in the riverbed will be removed, eroded river bank will be re-constructed. (b) As the improvement works will be carried out along the existing rivers, it can avoid major resumption of land and environmental impacts in other areas.	The existing rivers and ecological system will be disturbed.	River widening ü Luk Tei Tong River Improve bottlenecks ü Tai Tei Tong River	(a) The prime reason for flooding is that the existing rivers do not have adequate drainage capacity. The most effective solution is to widen and deepen the existing rivers. (b) As the river widening works will be carried out mainly along the existing rivers, there will be significant reduction of land resumption as compared to other options. (c) River widening is more reliable and low cost option to improve the drainage of rivers compared to the other options. (d) The channel design should provide an ecological and aesthetical acceptable solution, taking into account hydraulic performance and operation requirements of the channel. (e) Appropriate mitigation measures are required to be implemented such that the environmental impact is kept to acceptable limit. (f) This option is recommended and the proposed channels are lined with gabion block, and natural bed to retain the river ecology, and to improve the aesthetic, visual and environmental aspect of the river widening works. (g) For the Pak Ngan Heung River, river widening would affect the existing shrine and Fung Shui Tree. This area is culturally sensitive and any change to the shrine and Fung Shui tree at the existing bridge will be objected by the villager. Besides, one of the reasons for flooding is the sharp bend of the river near the existing bridge. Widening and deepening cannot remove the sharp bend and the problem. Therefore, this option is not recommended to Pak Ngan Heung River.
Construction of Earth Bunds at both sides of Luk Tei Tong (2) River	(a) No excavation was required for the affected area except the area covered by the foundation of the earth bunds. (b) The existing river would be untouched.	(a) Permanent loss of marsh area. (b) Visual impact due to the proposed earth bunds would be substantiated.	Not recommended	(a) The earth bunds would occupy substantial area of the marsh area and cause permanent loss of marsh area and associated ecology. (b) The earth bunds would obstruct surface runoff flowing into the existing river and aggravate the flooding situation. (c) Visual impact is of concern.

Drainage Options

Environmental Benefit

Environmental Dis-benefit

Recommendation

Reason & Considerations

Maintenance of existing river

(a) The area to be affected by the proposed drainage improvement works will be the least.

(b) The existing condition of rivers will be improved. For example, the contaminated soil and waste settled in the river bed will be removed.

No major environmental dis-benefits are anticipated.

Not recommended.

(a) This option can only marginally improve the capacity of the existing rivers.

(b) The flood protection level provided is still very low due to the small size of existing rivers. Severe flooding in the areas will continue.

By-pass channel or box culvert

(a) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.

Extensive land resumption is required for the construction of by-pass channel or box culvert. The public will be seriously disturbed.

By-pass box culvert

Pak Ngan Heung River.

By-pass channel

Luk Tei Tong (2) River

(a) The construction of by-pass channel or box culvert will require extensive land resumption and will impose adverse impact to other areas further away from the flooding spots.

(b) Along Luk Tei Tong (2) River and Pak Ngan Heung River, most of the areas are abandoned agricultural lands and marsh. There are sufficient available space for the construction of the by-pass box culvert and channel. The diversion scheme is considered feasible and acceptable. Locals and green groups also accepted the proposal.

(c) The impact to existing marsh at Luk Tei Tong would be minimum by adopted ecological friendly design details. The existing river will be untouched.

Interception and Flood Storage

(a) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.

(a) Extensive land resumption is required for construction of storage pond or interception channel.

(b) Environmental impacts to other areas will result. The upstream area, which is mainly composed of dense vegetation area, agricultural land, residential area and nurseries, will be seriously affected.

Not recommended

(a) This option is also not a cost-effective solution given the large volume of runoff needed to be stored which results in resumption of large piece of land for the construction of a large stormwater storage pond. The plan area of the pond can be reduced by half if the depth of the pond is increased to 6m. However, a pumping station is then required for pumping the water away after the storms. The required land resumption is still very substantial and the objection from public cannot still be avoided.

(b) This option is therefore considered not acceptable.

Floodwall and Pumping Station

(a) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.

(a) Floodwall has limited scope for improving the aesthetic. The visual impact is of concerns, in particular, if the floodwall is high.

Not recommended

(a) Floodwall can be constructed along the rivers to contain the floodwater during extreme rainstorm events. However, the capacities of the concerned rivers are very low and a high floodwall of minimum 2m is needed.

(b) The floodwall protection does not provide a long-term solution because it contains floodwater at a higher water level and causes flooding of the local drainage systems behind the floodwalls. Flooding in adjacent areas still persists.

(c) To eliminate the severe flooding outside the floodwalls, the floodwalls are required to be constructed further away from the existing rivers such that an additional flood plain flow area is allowed for conveyance of flow during extreme rainstorms. The formation of flood plain flow area requires substantial land resumption.

(d) Consideration has been given to pumping the surface runoff in the rivers to the downstream in order to reduce the water level and hence height of the floodwalls. However, the peak discharges are very substantial and temporal. It is technically impracticable to pump the peak surface runoff away without the use of detention pond.

(e) Therefore, this option is not further considered.

Deepening and Widening of Existing Rivers

(a) The existing condition of the rivers will be improved by the proposed engineering works by applying appropriate ecological friendly design. For example, Gabion lining with planting pits can enhance the ecological condition and provide shelter for the river habitats; the contaminated soil and waste settled in the riverbed will be removed, eroded river bank will be re-constructed.

(b) As the improvement works will be carried out along the existing rivers, it can avoid major resumption of land and environmental impacts in other areas.

The existing rivers and ecological system will be disturbed.

River widening

ü Luk Tei Tong River

Improve bottlenecks

ü Tai Tei Tong River

(a) The prime reason for flooding is that the existing rivers do not have adequate drainage capacity. The most effective solution is to widen and deepen the existing rivers.

(b) As the river widening works will be carried out mainly along the existing rivers, there will be significant reduction of land resumption as compared to other options.

(d) The channel design should provide an ecological and aesthetical acceptable solution, taking into account hydraulic performance and operation requirements of the channel.

(e) Appropriate mitigation measures are required to be implemented such that the environmental impact is kept to acceptable limit.

(f) This option is recommended and the proposed channels are lined with gabion block, and natural bed to retain the river ecology, and to improve the aesthetic, visual and environmental aspect of the river widening works.

(g) For the Pak Ngan Heung River, river widening would affect the existing shrine and Fung Shui Tree. This area is culturally sensitive and any change to the shrine and Fung Shui tree at the existing bridge will be objected by the villager. Besides, one of the reasons for flooding is the sharp bend of the river near the existing bridge. Widening and deepening cannot remove the sharp bend and the problem. Therefore, this option is not recommended to Pak Ngan Heung River.

Construction of Earth Bunds at both sides of Luk Tei Tong (2) River

(a) No excavation was required for the affected area except the area covered by the foundation of the earth bunds.

(b) The existing river would be untouched.

(a) Permanent loss of marsh area.

(b) Visual impact due to the proposed earth bunds would be substantiated.

Not recommended

(a) The earth bunds would occupy substantial area of the marsh area and cause permanent loss of marsh area and associated ecology.

(b) The earth bunds would obstruct surface runoff flowing into the existing river and aggravate the flooding situation.