The Red River Basin is an international river basin; its delta is completely located in Vietnam’s territory. The whole basin areas occupy 169,020 km2 of which 86,720 km2 (representing 51%) are located in Vietnam’s territory. The North Delta and Midland Region covers 14,590 km2 with a population of 18.56 millions people in the year 2000.
In recent years, big floods frequently happened in countries of the world and the region. The century flood occurred in 1998 in China is a warning of the climatic variation which caused big floods in river basins with high frequencies. Vietnam is one of the countries that suffer from impacts caused by global climate changes. Unfavorable climatic elements have increased and known sudden changes such as the August 1996 flood due to a combination of floods Nos. 2 and 4 with high tides. Hoa Binh Reservoir on Da River opened its gates during the biggest flood on Da River in last 100 years. Exceptionally the Mekong Delta has suffered in the year 2000 the unprecedented biggest and long flood in last 70 years. It is clear that flood tendency increases, floods happen with higher frequency and larger impact areas in comparison with floods occurred early of the century.
In the future, floods which are bigger then the August 1971 flood may happen in the Red River Basin as those of probabilities of 200, 300, 500 and 1000 years. The probabilities should be considered in the long-term flood control strategy for the North Delta.
Brief of previous studies and general comments
After the Red River Water Resources Development and Control in 1964, planning substances have been gradually realized and completed: Overview of cascade development on Da River, August 1970 (prepared by the Red River Committee); Overview of the cascade development on Da River in 1975 (by experts from former Soviet Union); Eco-Technical Justification and technical design of Hoa Binh Reservoir (by experts from former Soviet Union in 1973-1990); Cascade development planning of Lo River in 1976 (by the Red River Committee); Hoa Binh Reservoir Construction (1979-1990); Review of the Red River Development Planning (by Institute of Water Resources Planning in 1989-1992); Lo River cascade development planning (by Institute of Water Resources Planning in 1993-1996); Flood control planning for Red –Thai Binh Rivers Midland and Delta (by Institute of Water Resources Planning in 1994-1999); Pre-Feasibility study of Son La Hydropower Project (by Hydraulic Engineering Consultants No.1 in 1990-1996); Feasibility Study of Son La Hydropower Project: Phase 1 (by Power Engineering Consultants No.1 in 1997-1998); Pre-Feasibility Study of Dai Thi Project (by Power Engineering Consultants No.1 in 1997).
All above-mentioned studies have confirmed that cascade development on Da and Lo Rivers proposed in the 1964 planning are rational. Flood control task for the Red-Thai Binh River Midland and Delta identified as number one for reservoirs in upstream is accurate and reasonable. The six alternatives for flood prevention and control are sound and include followings:
- Strengthening dike systems
- Clearing river flows for flood discharge
- Building reservoirs to reduce floods in upstream of big rivers
- Diverting and retaining floods
- Reforesting and protecting watersheds
- Intensify dike management and protection.
Flood control solutions for the red-thai binh rivers midland and delta
The Red River Delta flood control planning has inherited previous studies. The planning process had based on proposed flood control alternatives through analyses, calculations and assessments to identify constraints and propose recommendations for implementation. The planning has especially considered and recommended flood control criteria for the Red-Thai Binh Midland and Delta in the current situation. Taking into account circumstances of big floods happened in the region and the world, the planning has given following comments on those solutions in order to work out an implementation plan for short- and long-term flood control in conformity to the Vietnamese circumstances:
* The ‘river dike system’ solution
Red River dyke system remains the most important, especially from the confluence of Da and Thao Rivers to Hung Yen and from Son Tay to Nam Dinh, which usually collapsed and eroded seriously. After the Red River dike system, Duong River dike with its both left and right sides is also a very important one because dike break occurs on the system it will cause large inundated areas with huge losses in socio-economic values and difficulties for the railway and road transportation.
Building dike is considered as a basic solution to protect 38 flood-affected areas in the Red River Delta and Midland that covers 1.44 million ha of natural areas and 18.56 million human habitants. The 45 dike systems of the Red and Thai Binh rivers have a total length of 3,500 km. The design flood water level pursuant to Circular No. 1185 on July 7, 1977 is +13.9m for Hanoi; +13.6m for dikes of levels I, II and III, and +13.3m for dikes of level IV (in accordance with the datum of 10.800 m for the Red River). However, reality have showed that when the flood water level is over +13.0m in Hanoi, incidences (such as blows, slips, push out and bubble springs) happen at many places of the dike system menacing dike security. The current dike systems were built on old dikes of hundred years with problems existent in dike geology and quality that require special attention aiming to avoid incidences during the flood season. Dike sections as well as dike crest and shoulder elevation are deficient in comparison to design. The dike systems also suffer from annual intrusion by floodwater, which requires regular maintenance. Therefore, in case of future construction of upstream reservoirs, flood water level should be designed as sound as possible in order to ensure dike operation in the rainy season.
Dike systems play a crucial role so dike break is not allowed. It is recommended to the Government to invest in the Hanoi dike system and other important dike systems. It is indispensable to introduce and apply new technologies concerning dike security serving multifunction of flood control and transportation, etc.
In the immediate terms, it is necessary to concentrate in dike maintenance aiming to ensure design sections, in reparation and new building of sluices under dikes, which are too old or deteriorated due to collapse and break. Economic values of other solutions such as reservoirs, flood diversion and retention, flood discharge corridor improvement also involve costs for annual dike and revetment maintenance and reparation.
In view to ensure flood control for the downstream in according to the real capacities of Hoa Binh and Thac Ba Reservoirs and sustain the respective water level of 13.6m in Hanoi, an investment requirement of 4.090 billion VND is needed for dike, sluice and revetment improvement and for the first step in Day River improvement.
* The ‘channel discharge capacity improvement’ solution
Dredge and clear channels aiming to improve flood discharge capacities, reduce flood levels have been a traditional solution applied in the Red Midland and Delta since the 11th century. To Lich River was dredged in 1912, Duong River in years 1407, 1436, 1810, 1830, 1882, Ca Lo and Day Rivers were also dredged but their flood discharge capacities still decrease.
According to an analysis result, from 1930 to 1990 water level increased 1.3m and 1.06m with the respective flows of 20,000 m3/s at Son Tay and 14,000 m3/s in Hanoi respectively. Similarly, the highest water level at Pha Lai increased 1.6m in last 60 years. Below are main reasons of water level increase in recent times:
- Changes in distributary rate between tributaries
- Sedimentations in river channels and banks
- Encroachment of river banks in inferior dikes and residential areas causing narrowed channels and flood discharge corridors
- Move of river estuaries to the sea.
There are 29 and 24 flood plains on the left and right sides of the Red River respectively, of which 20 flood plains cover from 500 to 2000 ha. Distance between the two main dikes at Son Tay is 3,750 m but the distance between two levees is 935 m only while this distance at Phu Gia decreased from 2,600 m to 1,200 m, at Thanh Tri from 2,750 m to 1,000 m. Distance between dikes are very narrow at many places such as 1,600 m at Lien Mac, 1,200 m at Chem, 1,650 m at Thang Long Bridge, 1,200 m at Chuong Duong Bridge, etc…
The solution to clear channels and remove levees has been proposed at many times but could not be realized. In contrary, encroachment of riverbeds and riverbanks increasingly expands therefore levees’ elevation raises and is beyond the warning level No.3. Some levees can be used as a spillway when water level is over 10.5 m but farmers usually raise levees to prevent overflows in case of higher flood water level. Levees which protect more than 1,000 ha and some ten thousand people are namely as Vinh Tuong, Yen Lac, Yen Lang, Tu Dan, Khoai Chau, Phu Hung Cuong, Tan Lap, Duy Nhat, Trung Chau, Tu Lien, Nhat Trung, etc.
According to the 1995 data, total population who live inside 53 levees of the Red River would have to displace in case of clearing may reach 250,975 people (about 49,481 households). This number did not include those who live inside levees of Duong, Thai Binh, Luoc, and Dao Nam Dinh rivers. It is therefore indispensable to remove every hindrances on the flood discharge corridors such as levees, riverbed and riverbank encroachment aiming to ensure prompt flood discharge.
In fact, floodwater on the Red River that is discharged by the mainstream account only for 23-25% of its total floodwater volumes. Other proportions are distributed as follows: 33-40% is diverted to Thai Binh downstream (26-30% via Duong River and 7-9% via Luoc river) and 6-8% to Tra Ly River. The remaining of 23-28% is transferred to the southwest of the Delta (including 19-22% via Dao Nam Dinh River and 4-6% via Ninh Co River). This distribution to each tributary continues to vary in periods.
* The ‘flood retention’ solution
It can be cited two measures i.e. diverting floods from the Red River to Day River and retaining floods in areas in upper Red River Basin in order to protect Hanoi and other lower parts.
· Potentiality for the flood retention and diversion can be discussed as follows:
+ In respective to the water level of 13.0-13.6 m in Hanoi, the maximum flood volumes to be diverted from the Red River to Day River could be 2,800-3,200 m3/s. In case of supplementary flood diversion via Luong Phu to Tich River, only 800-1,000 m3/s could be expected which may result in water level decrease of 0.1-0.12 m in Hanoi.
+ With the frequencies of 200 and 300 years, it is possible to divert floods from the Red River to Day River with a maximum flood discharge of 3,200-3,480 m3/s and a floodwater level decrease of 0.33-0.37 m.
+ With the frequencies of 500 and 1,000 years, it can be expected to divert floods from the Red River to Day River with a maximum flood discharge of 3,680-3,900 m3/s and a floodwater level decrease of 0.51-0.56 m.
+ Pursuant to the Decree No. 62/CP, flood retention in 4 areas with flood discharge not beyond 4,000 m3/s in case of on-time operation (otherwise it can not be effective like the case of August 1971 flood). On the other hand, heavy rains usually fall on-farm before floods that cause higher water levels in the four flood retention areas and resulting in decreasing flood storage capacity.
· Flood diversion from Red River to Day River would directly affect 675,000 people of Ha Tay, Ha Nam and Nam Dinh Provinces and cause inundation of 41,235 ha of agricultural lands, especially Chuong My-My Duc districts would be under 4-5m water depth with 25,000 ha of agricultural lands and inundation duration may last 35-45 days. Total fixed assets of the whole flood retention areas are estimated at 13,556 billion VND while total GDP at 1,171 billion VND (according to the 1997 data). Losses of the fixed assets and GDP are estimated at 2,711 billion VND and 585.6 billion VND (representing 20% and 50% of total fixed assets and GDP) respectively. This is excluded the worst case of human losses in the flood retention areas.
· In case of flood retention in 4 areas pursuant to the Decree No. 62/CP, it would directly affect 230,000 people (including those who live inside Vinh Tuong levee), and cause deep inundation of 15,800 ha of agricultural lands. Total fixed assets of are estimated at 9,365 billion VND while total GDP at 812 billion VND (according to the 1997 data). Losses of the fixed assets and GDP are estimated at 1,833 billion VND and 406 billion VND (representing 20% and 50% of total fixed assets and GDP) respectively.
· In a word, flood retention and diversion is a supplementary solution in emergencies when upstream reservoirs would have participated in flood storage but flood water level could still beyond 13.6 m in Hanoi. If on-time flood retention is not applied flood reduction effectiveness will be minor resulting serious losses of human lives and economic values. In both solutions, fixed asset losses and GDP losses are estimated at 4,584 billion VND and 991.6 billion VND respectively.
· It is forecasted that by the year 2015 total fixed assets and GDP of the Day flood diversion areas are about 68,305 billion VND and 4,325 billion VND respectively. Similarly, total fixed assets and GDP of the flood retention areas are about 47,900 billion VND and 2,998 billion VND. In case of flood retention and diversion, losses of fixed assets and GDP would reach 23,240 billion VND and 3,661 billion VND respectively.
* Reservoir solution for upstream area
- Recommendations on reservoirs
The construction of large reservoirs will serve as multi-purpose reservoirs, cut the floods, regulate water supply flow and generate power during dry season in the most efficient way. Therefore, Water Resource Planning and Exploitation Of Red River System, flood control for downstream should be taken as the top priority. Moreover, most of the cascade diagrams should be studied so that the flood control storage capacity will be at the allowable maximum level.
There are two alternatives for selection in Da River
Low alternative: Thac Lai + Huoi Vai + Ta Bu + Hoa Binh = Cascades
(Lai Chau will not be inundated)
- Full water supply level (m) 310 260 165 110
- Useful capacity (109 m3) 3.25 1.25 1.85 4.6 10.0
- Flood control capacity (109 m3) 1.5 0 0 4.6 6.1
High alternative: Hat Him + Ta Bu + Hoa Binh = Cascades
(Lai Chau will be inundated)
- Full water supply level (m) 310 260 110
- Useful capacity (109 m3) 0.13 21.24 0 21.37
- Flood control capacity (109 m3) 0 10.0 2.0 12.0
In August 1970, upon establishing comprehensive report of Da river as basis for engineering-economic report for Hoa Binh reservoir. Office of Red river committee suggested a flood control capacity of 12 billion m3 for the entire Da river cascades.
On Gam river (Dai Thi reservoir at Na Hang site is recommended in the feasibility study):
- Full water supply level (m) 120
- Useful capacity (109 m3) 1.6
- Flood control capacity (109 m3) 1.0
In Hoa Binh reservoir design, the agreed full water supply level is 115 m for larger flood control capacity is required to absorb the floods of similar magnitude as one in August 1971 with downstream flood control capacity of 4.9 - 5.6 billion m3.
Thac Ba reservoir was designed in 1960 when the Water Resource Planning and Exploitation of Red River System were not developed. As a result, the reservoir has not been assigned with the task of downstream flood control. As the reservoir was completed and put into operation, downstream flood control was added as one of its tasks with spare capacity of 450 million m3, which enable the reservoir to absorb a discharge of 300 to 1,900 m3/s (the pre-flood water level of the reservoir at elevation of 56.5 m must be kept by the end of August). In fact, during its operation period from 1990 to 1998, although spare downstream flood control capacity was not available, Thac Ba Reservoir failed to store water to a full level as per design by the end of flood season for consecutive 5 years (i.e. 50% of the time).
- Hoa Binh reservoir in Da river and flood control task
In feasibility study and engineering design of Hoa Binh reservoir, the top priority task is downstream flood control. Hoa Binh reservoir is expected to absorb floods to maintain the water level in Hanoi at a level below 13.3m in the event of the floods equivalent to one in August 1971 with different flood types. In order to fulfill the above-mentioned task, Hoa Binh reservoir is designed with following parameters:
Installation capacity 1960 MW
Dam crest elevation 123 m
Elevation of highest flood water level 120 m
Full water supply level 115 m
Face spill gate elevation 117 m
Compulsory capacity (117-120 m) 0.7 billion m3
Flood control capacity (85 -115 m) 4.9 billion m3
Pre-flood water level 85 m
Dead water level 80 m
Hoa Binh reservoir engages in flood control and operates in accordance with Downstream Flood Control Procedure in 3 months i.e. from 15 June to 15 September annually. The duration includes 3 phases, namely early flood from 15 June to 15 July; main flood 16 July to 25 August and late flood 26 August to 15 September.
Since its first-time operation in 1989, together with Thac Ba, Hoa Binh reservoir in Da river have helped absorb floods for downstream. In the event of reoccurrence of floods in 1945, 1969, 1971, 1986 and 1996 or adverse flood types on Da river, the highest water levels in Hanoi will be well kept under the elevation of 13m and 7.2 at Pha Lai without resorting on retention basins.
Given both Thac Ba and Hoa Binh reservoirs carry out the task of flood control in accordance with procedures approved by the Government, the water levels in Hanoi can be kept at elevation of 11.5 - 12.0 m in the flood water level of below 12.5m. In the event of 1969 and 1996 floods with flood water level of 13.0 - 13.3 m, the water levels in Hanoi can be kept at elevation of 12.0 - 12 m and 12.5 - 13.0m in a big flood with flood water level exceeding 14.0m as ones in August 1945 and August 1971. In the event of extremely large floods having return period of 200, 300, 500 and 1000 years, together with flood retention into Day river, the water level in Hanoi can be kept at 13.45 m, 14.29 m and 14.69 m respectively (given the current condition of retention basins of day river). At the water level above elevation of 13.0m, the dyke system will be in the danger of being broken, resulting in numerous significant economic and urban area flooded in deep water.
The above mentioned flood control procedures will increasingly ensure flood absorption for relevant water levels in small and medium size floods in Hanoi. In cases of extremely large floods having return period of 200, 300, 500 and 1000 years, the dyke system will be unable to protect the city from being inundated.
- Cascade reservoirs in Da and Lo rivers and flood control task
For convenient comparison, in all calculations and analyses, the total flood control capacity of cascade reservoirs in Da river are taken as 5, 7, 10 and 12 billion m3. Flood control capacity of Lo river cascades is 1 to 1.5 billion m3. Either of the two situations is considered in details as follows (according to calculation result basing on river channel topography in 1996):
· Flood control capacity of cascades in Da river (Hoa Binh + Son La): 5, 7, 10 and 12 billion m3.
· Flood control capacity of cascades in Da river of 5, 7, 10 billion m3 incorporated with flood control capacity of cascades in Lo river of 1-1.5 billion m3.
Calculation results showed:
- Floods with frequency P = 0.5% (return period of 200 years) and a total flood control capacity of cascades in Da river of 7 to 10 billion m3 can well keep the water levels in Hanoi at the elevation under 13.29 - 12.74 m and 7.44 - 7.21 m at Pha Lai.
- Floods with frequency P = 0.3% (return period of 300 years) and a total flood control capacity of cascades in Da river of 7 to 10 billion m3 can well keep the water levels in Hanoi at the elevation under 13.79 - 13.32 m and 7.64 - 7.41 m at Pha Lai.
- Floods with frequency P = 0.2% (return period of 500 years) and a total flood control capacity of cascades in Da river of 7 to 10 billion m3 can well keep the water levels in Hanoi at the elevation under 14.28 - 13.82 m and 7.85 - 7.61 m at Pha Lai.
- Floods with frequency P = 0.1% (return period of 1000 years) and a total flood control capacity of cascades in Da river of 7 billion m3 and another capacity of 1 billion m3 of Dai Thi reservoir can well keep the water levels in Hanoi at the elevation of 13.96 m and 7.76 m at Pha Lai. The water level in Hanoi cannot be kept under 13.60m unless the flood will be absorbed into retention basins of day river.
- Floods with frequency P = 0.2% (return period of 500 years) and a total flood control capacity of cascades in Da river of 7 billion m3, another capacity of 1 billion m3 of Dai Thi reservoir; Bac Me 0.5 billion m3 and Thac Ba 0.5 billion m3 can keep the water levels in Hanoi at the elevation of 13.50 m and 7.6 m at Pha Lai.
- Floods with frequency P = 0.1% (return period of 1000 years) and a total flood control capacity of cascades in Da river of 7 billion m3, another capacity of 1 billion m3 of Dai Thi reservoir; Bac Me 0.5 billion m3 and Thac Ba 0.5 billion m3 can keep the water levels in Hanoi above the elevation of 14.00 m and at Pha Lai above 7.70 m.
- Floods with frequency P = 0.1% (return period of 1000 years) and a total flood control capacity of cascades in Da river of 10 billion m3, another capacity of 1 billion m3 of Dai Thi reservoir can keep the water levels in Hanoi under the elevation of 13.48 m and under 7.52 m at Pha Lai without absorbing flood in retention basins of day river.
In order to control against 500-year probable flood, it is recommended that the total flood control capacity of Da river be 7 billion m3and larger in the long-term period. The construction of Dai Thi reservoir on Gam river with flood control capacity of 1 billion m3 and Bac Me reservoir 0.5 billion m3 are also proposed.
Reforestation and forest protection
Reforestation and forest protection are two of traditional solutions to the reserve land and water source, which will help, avoid land erosion and exhaustion, flash flood, especially in the areas where reservoir has not been constructed, midland and Thai Binh river basin.
According to recent statistics of forest investigation and planning institute, the natural area of North East part is 6.748 million ha in which forest area is 2.368 million ha (including both natural and artificial forest). In the South West part, the natural land area is 3.572 million ha of which nearly 1 million ha is the area of forest. The current forest coverage of North East part is estimated at 35.11% and South West part 26.97 % and 32.29 % for the entire region (the data of forest land on the territory of china are not available).
The planned forestation land in the 5 million ha program is expected to cover the North East part with an additional area of 1.755 million ha and South West part another 0.645 million ha. The vegetable coverage of the entire region can be increased to 55.55% by the year 2010. After 2010, the forest coverage can be better maintained through the effective and proper exploitation, planting and protection.
Management arrangement and effective exploitation of flood control measures for red river delta and midland
- Develop the observation station network for meteorological and hydrological elements, which will serve as basis for more reliable flood early warning and forecasting. Flood warnings should be issued 3-5 days in advance about possible floods on tributaries. Moreover, the warning about water level in Hanoi should be made 36 - 48 hours in advance with an error of ± 10 cm and a reliability of 90%.
- Management arrangement of watershed, channel system downstream and flood control infrastructure in accordance with Acts on Flood and Storm Control, Law on Forest Planting and Protection, Law on Water Resource, law on Environmental Protection and other related laws and acts.
- Establish the operation procedures for reservoirs in order to better absorb flood and prevent the potential problems through appropriate coordination and management of existing flood control measures.
- Public awareness enhancing programs, policy development and financial mechanism are necessary for the effective implementation of structural and non-structural measures.
- Organize protection staff and human forces for relief, rescue and recovery in the flood basing on four in-place guideline: in-place human force, material, direction and logistics.
- Promotion of international cooperation, especially one with china so as to meet the requirements of weather information concerning floods, rains or other events on the basin which potentially lead to the formation of flood or structure problems.
By Dr. To Trung Nghia - Director of Institute of Water Resources Planning
