macro and micro dredging as a tool for mitigating …...macro and micro dredging as a tool for...

MACRO AND MICRO DREDGING AS A TOOLFOR MITIGATING FLOODS AND DROUGHTSIN THE RED RIVER DELTA, VIETNAM

GIJS KOK

ABSTRACT

The Government of Vietnam is pursuing anew strategy of Integrated Water ResourcesManagement which includes economic,environmental, technical, social and politicalconsiderations. Assisted by the Governmentof the Netherlands individual studies andreports were conducted. This article coversthe initial environmental examination andthe development of mitigation measuresagainst adverse impacts of physical works. Detailed investigations of water andgroundwater, and sediment and soilssupport the assessment, backed by fieldsurveys. Though dredging was not aspecific subject of the study, the articlehighlights that integrated development ofwater resources in deltas should includesalinity, sediment and soils and it identifiesmacro and micro dredging as effective toolsfor mitigating flood and drought problems.

The author would like to thank the counter-part staff, notably Dr. Dang Quang Tinh,and the international and local experts whohelped to realise this study, especially Dr Nguyen The Truyen, EnvironmentalExpert, who emphasised the essential roleof sediment in the flood and droughtregime of the Red River.

INTRODUCTION

The Government of the Netherlands hasassisted the Government of Vietnam byfinancing the Day River Flood Diversion and Water Resources Development project.DHV Consultants BV, Amersfoort, the Netherlands (leading partner), HydraulicResearch Wallingford Ltd, Wallingford, UK and the Vietnam Institute of WaterResources Research, Hanoi, Vietnam werecontracted to do this study. The studycovered institutional aspects, hydrology,hydraulic modeling, morphology,agricultural development, engineering, dike management, sociology, environmentalimpact assessment and economic analysis.For each component, a separate report has been prepared.

The initial environmental examination andmitigation measures developed againstadverse impacts of physical works aredescribed. Detailed investigations of waterand groundwater, and sediment and soilssupport the assessment, backed by fieldsurveys and sampling campaigns in the dry

and the wet season. Dredging was not aspecific subject of the study, but macro andmicro dredging were identified as effectivetools for mitigating flood and droughtproblems.

“Macro dredging” of the Lower Day Riverto assure rapid release of Red Riverfloodwater via the Day Estuary to the Gulf of Tonkin and “Micro dredging” in the Day River floodplain (“green river”) toassure the perennial flow of Red Riverwater to combat droughts whilemaintaining soil fertility are suggested. The study highlights that integrateddevelopment of water resources in deltasshould include salinity, sediment and soils,while macro and micro dredging are toolsthat can make solutions work.

INTEGRATED WATER RESOURCESMANAGEMENT

The Government of Vietnam is pursuing a new strategy for water resourcesmanagement, in which the water sectorand the (water) environment is consideredin an integrated manner in relation toeconomic, environmental, technical, social and political considerations:

Above, a drought-prone area, Chuong My, Vietnam.

Dredging should be considered as a tool in regulating

flood and drought zones in the Red River Delta.

Macro and Micro Dredging as a Tool for Mitigating Floods and Droughts in the Red River Delta, Vietnam 3

4 Terra et Aqua | Number 103 | June 2006

in short Integrated Water ResourcesManagement. The passing of the WaterLaw in 1998 was an importantdevelopment in this process and theinitiation of the Day River project was oneof the concrete actions taken in thisrespect. Integrated Water Resources

Management should take place throughinteraction with users (population),resources and institutions. Integrated WaterResources Management thus applied takesaccount of the use of water resources inrelation to social and economic activitiesand functions, and the required water

infrastructure, considering the soil fertilityand salinity aspects.

PROJECT LOCATION

The Day River Project is situated in the Red River Delta of Vietnam, with an area of16,500 km2 and a population of 18.5 million.Next to the Mekong Delta, the Red RiverDelta is the second largest granary,producing 15 to 20 percent of Vietnam’srice. The catchment of the Red River is169,000 km2, half of which lies in China(Figure 1). The Day River project area,situated East of Hanoi, covers parts of theprovinces Ha Tay and Ha Nam (Figure 2).Around 600,000 people live in the DayRiver region, 90 percent of whom work inagriculture. The Red River upstream ofHanoi comprises three major tributarysystems: the Da, Thao and Lo Rivers. The flood season in the Red River lasts fromJune to October, with particularly widespreadflooding in August. For the rest of the year,the area is subject to occasional flooding.When large floods occur, as they did in 1971,the flooded area is some 500 km2 in Ha Tayand 300 km2 in Ha Nam.

HYDROLOGY OF THE RED RIVER

The Red River at Hanoi comprises threemajor tributary river systems, the Da, Thaoand Lo Rivers. A number of reservoirs havean impact on the discharge of the rivers:the Thac Ba reservoir on the Chay Rivercompleted in 1972 and the Hoa Binhreservoir on the Da River completed in1989. The potential storage in the Hoa Binhreservoir is approximately 10 times more thanthe Thac Ba reservoir, and therefore has asignificantly higher impact on reducing highflows in the Red River at Son Tay station.Figure 3 shows the seasonal averageddischarge of the Red River.

Low FlowsThe minimum flows at Son Tay after theconstruction of Hoa Binh reservoir are higherthan before, but still below 1000 m3/s inthe period March to April. The rainy seasonin the Red River basin ends in October.November-December is the transitional

Figure 1. Protected and flooded zones of the Red River Delta in 1925 (ENPC; Normandin).

period between the rainy season and theseason of light rain. The dry season beginsin December and lasts until May. In the dryseason, rainfall is around 20-30% of theannual rainfall; 60% of this is concentratedin April-May; there is a little rain in January,February and March. The dry continentalmonsoon blows particularly strongly inDecember and January. Large precipitation isonly 60-80 mm/ month; normal precipitationis 20-40 mm/month and small precipitationis less than 20 mm/month. This is notenough to maintain the humidity of the soil cover. The total rainfall added to flowsin the river system is negligible because ofsmall precipitation. Dry season floworiginates mainly from groundwater.Agricultural production in January requiresthe maximum water volume for hoeing soilfor the spring rice crop. Acid cleansing andsalt removal in coastal areas needs plenty of fresh water. In addition, tillage in theDay River needs water, which supply thepumping station at Ham Mon and transferfrom the Red River through the Nhue andNam Dinh rivers.

Average DischargeThe average discharge as recorded at Son Tayover the period 1957-1988 is 3,545 m3/s,just 2.6% higher than the combinedaverage of the three tributaries, which was3,455 m3/s. Thus relatively little flow isadded downstream of the three gaugingstations in the tributaries. The impact of theHoa Binh reservoir on the discharges is


significant. The percentage of time that a discharge is less than 1000 m3/s hasincreased after 1989. The percentage of timethat discharge is between 1000 m3/s and4000 m3/s has decreased. The percentageof time that discharges exceed 4000 m3/shas increased. This phenomenon mayreflect a strategy of water release for flood

GIJS KOK

Gijs Kok graduated as a land and water

engineer from Wageningen University,

the Netherlands (1972). He has been

working with NEDECO members, a group

of independent consultancies and

institutes, since then. In 1977 he joined

Royal Haskoning as a water specialist

and team leader for environmental

projects. In 1994 he joined DHV BV as a

senior project manager and advisor for

water and environment. He has worked

extensively in China and Vietnam

dealing with soil fertility, water excesses,

shortages and pollution, including flood

and drought preparedness.

Figure 2. The Day River project area.

control, in which release of a certain highdischarge creates storage to prevent evenhigher discharges when a real flood comesdown the Da River.

Floods The floods in the Red River are thecombination of floodwaters from the Da,


Thao and Lo rivers. The waters of thesethree tributaries come together at Viet Tri,and thereafter form one flow channel toHanoi. The floods may rise quickly, takingjust 2 or 3 days to attain the peak level, andsubside more slowly, taking a 3- to 4-foldlonger period. However, the opposite hasalso happened. Furthermore, one floodmay not have fully subsided when anotherflood already begins. The flood season inthe Red River lasts from June to the end ofOctober, with particularly big flood eventsin August. Statistically distributed, the peakflows have occurred as follows: July (32%),August (55%), September (7.3%), andOctober (4.4%). In some years, the peakflood level occurred in June (l.6%). There are three alarm levels for floodwarning in Hanoi. The records of themeasurements show that during the last 90 years in only two years (1917 and 1931)was the water level of the Red River in Hanoiabove the alarm level.

Dangerous Floods Dangerous floods in the Red River canoccur when large floods occur in one, two or all three of these rivers. Such specialcases of coincidence did take place: 1. in l945 resulting from the historically

large floods in Da River and the relativelylarge floods in the other two rivers;

2. in l97l resulting from historically largefloods in Lo River combined with verylarge floods in both Thao and Da rivers;

3. in l968 resulting from historically largefloods in Thao River combined withsmall floods in Da and Lo rivers;

4. in l969 resulting from large floods in Da River combined with large floods in Lo River and medium floods in Thao River.

In 1989, the Hoa Binh reservoir on the Da River was completed. This reservoir hashad a significant impact on reducing largeflood flows in the Red River at Son Tayupstream of the Day River Flood Diversion.

THE DAY RIVER PROJECT

The Hanoi area has grown to be of primary importance to Vietnam as adevelopment centre for trade and industryand as the national administrative centre.Reducing the risk of flooding in the areabecause of high discharges in the Red Riveris a major concern of national importance.The recent floods in China have furtheremphasised this concern. In line with theWater Law, the Government of Vietnamhas formulated two main objectives for theDay River Flood Diversion and Water

Resources Development Project:1. To reduce the risks of inundation caused

by flood flows in the Red River andresulting damage to the city of Hanoiand other important areas downstream;and

2. Rural development through integratedwater resources management in the DayRiver leading to increased agriculturalproduction.

In Ha Nam Province the industrial activitiesinclude cement production and quarryingof rocks and aggregates for concrete androad construction. Ships transport them toHanoi to support the booming buildingsector. The navigation route leads firstsouth before taking the Red River channelnorthwards and has a length of about 200 kilometres. This long route in effecthampers the planned development of Ha Nam’s industrial development. For thisreason “Improving navigation facilitiesbetween Ha Nam province and the maineconomic development areas near Hanoi”is formulated as a third objective, though of secondary importance.

DAY RIVER PROJECT HISTORY

The Day River is a tributary of the Red River,which branches off from the Red Riversome 30-km upstream east of Hanoi,flowing to Ninh Bihn, where the Day Rivermeets the Nam Dihn Branch of the Red River, jointly flowing to the sea through the Day Estuary. Before 1937, the Day River acted as a natural flooddiversion route, an event that happenedyearly (see Figure 1, Normandin, M.A).

Each year, large parts of the Day RiverRegion were flooded. This happened inthose parts not protected by dikes, south of the present Highway No 6, between Ha Dong and Hao Binh and in the low areadownstream of Phu Ly. The inundationlasted for months, creating the largestwaterlogged area in the Red River Delta,apparent after every flood. These two areaswere almost the only areas being floodedyearly, since from 1925 dikes protected thewhole delta, except some patches of landin the northeast of the Red River Delta.

Figure 3. Chart of the seasonal averaged discharge of the Red River.


This situation prompted the FrenchGovernment to construct the Day Damwith movable gates in the upstream part ofthe Day River, some 8 km downstream ofHat Mon (Jean Rigal, 1939) (Figure 4). This was a logical location for the Dam,since the dikes of the Day River and theRed River used to converge here, creating a natural funnel. The Day Dam wascommissioned in 1937 and flood diversionfollowed by several times since then.Unfortunately, the intervention in the Day River resulted in the closing-off of the Day River for dry season flows, because of the silting up of the entrance.The floodplain of the Red River at Hat Monbecame higher and higher, preventing theRed River water at low flows from enteringthe Day River. This prompted constructionof the Van Coc sluices commissioned in theyear 1966 to facilitate passage of largefloods (Figure 5). The regular water supplyfrom the Red River to the Day River ceasedcompletely after the construction of a lowdike of 8-km length in 1974.

PHYSICAL WORKS PROPOSED BY THEDAY RIVER STUDY

The present Day River study recommendedphysical works for flood diversion and flood control works, dike improvement,

intake of irrigation water and shortening of the navigation lane to Hanoi. These were based on study of hydrology,hydraulic modelling, morphology,agricultural development, engineering and dike management, taking into account sociological, economic andenvironmental conditions.

Flood Diversion and Flood ControlThe study classified flood mitigation andflood control measures into four broadcategories: – flood storage, – river diversion,– increased river capacity; and – flood defense.

Figure 4. The Day Barrier built in 1937, with six moveable weirs with a maximum capacity of 5,000 m3/s.

Figure 5. Van Coc’s Flood Diversion Structure with 25 gates; and a spillway of 8.6 km length carrying together a

maximum diversion of 5,000 m3/s from the Red River into the Day River.


Though these were described as separateoptions, it was recognised that theoptimum strategy might involve acombination of measures, for example,flood defense measures implemented inconjunction with improved upstream floodstorage. The hydraulic model confirmedthat the current level of flood protection toHanoi is approximately once in 450 years.

It demonstrated that flood diversionthrough the Van Coc-Day Dam system ismost effective when carried out based onthe time of the flood peak in the Red River,rather than based on a fixed water level atHanoi, as is the current practice.

Though costly, it also showed that a new structure replacing the Day Dam andthe Van Coc sluices would reduce thefrequency of flood diversion to once every250 years and increase the level of floodprotection to Hanoi to once every 550 years.This structure would have a discharge

capacity of 8,000 m3/s, and it confirmedthat use of upstream flood retention areascould have a significant impact on reducingwater levels in Hanoi. For the Day River, thesolution would imply that the likelihood ofa flood would be much less, but the impactwould be much greater than under currentcircumstances. Routing a larger floodthrough the same profile would implylarger flow velocities and correspondinglyhigher flood induced damages.

For Hanoi and the Red River Delta, theimpact is limited since the recurrence periodwould change from once in 450 to once in 550 years. Current operating proceduresfor Hoa Binh Reservoir could increase floodattenuation downstream and so reduce the flood risk to Hanoi (Figure 6). Furtherupstream storage, such as the proposedSon La reservoir, would also reduce theflood risk to Hanoi. In light of the highsediment loads in the Red River, the optionof trying to increase the flow capacity of

the Red River by carrying out dredgingwould not be economical and waseliminated early on in the study.

Dike Improvement Consultants assessed the safety of the dikesby considering the hydraulic and geotechnicalfailure mechanisms. These mechanisms are:overflow, wave overtopping, erosion of theouter and inner slope, macro stability of theouter and inner slope, piping or sand carryingwells and micro-stability of surface layers onthe dike slopes. The safety assessmentrevealed that a number of dike subsectionsare not safe, meaning that additional areasmay flood. The breaching of a dike by failuremechanisms other than overtopping causesthis flooding. Consultants developedscenarios for dike improvement along thearea affected by the flood diversion.

Intake of Irrigation WaterConsultants proposed a structure for theintake of irrigation water into the Day River

Figure 6. Da River downstream from the Hoa Binh Dam at 500 m3/s, with one gate of eight opened.


generally can serve as sources for drinkingwater supply. However, in some areas,microorganisms and NH4

+ pollute some wells.Heavy metals incidentally occur at levels nottolerated in OECD countries. Partly theyreflect the chemical footprint of the Red River, originating from copper mines inChina. Otherwise, heavy metals originatefrom industrial parks along the Red Riverand village industry in the Day River,notably lead. High copper concentrationspossibly affect biodiversity in the projectarea, acting as inhibitor for aquatic life.

Soils in Day RiverSoils in the Day River strongly reflectpresent or antecedent deposition of RedRiver sediment, or the absence of suchdepositions. Red River sediment is rich inlime that maintains the buffer capacity ofsoils under hot climate. Soils receiving Red River sediments usually are more fertile,less acid and better drained. They have thehighest crop yields and allow cropdiversification. Soils receiving less Red Riversediment are less fertile. This notablyapplies to gleyic soils in the lower areas,remote from the Red River, showing yellowand gray mottling produced by partialoxidation and reduction of iron, caused byintermittent water logging. The specificproblem here is acidification caused by highurea gifts, in a waterlogged environment.

It results in loss of fertility and landdegradation. In addition, the Vietnamesesoil scientists reported other soildeficiencies, connected with the drainageregime, the application of fertiliser and thetropical climate. Their observationsconfirmed that regular flooding anddeposition of Red River sediment isparamount for maintaining soil fertility andguaranteeing sustainable use of the land.

Socio-EconomyThe population in the Day River FloodDiversion area is developing fast, from150,000 in the year 1937 to 600,000 in theyear 2001. Still 90% of Day population isworking in agriculture. They are subject toconstraints in development, imposed by its“green river function”. They face costs ofemergency preparedness, guaranteeing thesafety of the other 17.9 million people

west of the Van Coc sluice. The designcapacity of the structure is 50 m3/s. This includes the irrigation demand and anenvironmental duty flow of 10 m3/s.

There are two alternatives for the structure.Alternative 1 is an intake structure withgates that connect to a canal plannedimmediately east of the Ngoc Thao dikewest of the retention area. The otheralternative (2) is a pumping station. This will also feed a canal with the samealignment. Alternative 1 will have a veryflat slope, requiring all year-roundmaintenance dredging. The canal ofAlternative 2 can have a steeper slope that does not require dredging.

Shortening Navigation Distance toHanoiThis component envisages reopening the Chau Giang shipping route between Ha Nam and Hanoi by constructing newsluices (Phu Ly Navigation Sluice and PhucNavigation Sluice) and navigation locks (Tac Giang Ship-Lock). Maintenancedredging of the Chau Giang fairway will be required every 5 years.

INITIAL ENVIRONMENTALEXAMINATION

Extended consultations were held with thecompetent authority at the different levelsof Government: National (MARD), Provincial(DARD) and Regional Water ManagementAuthorities (Day Dam Operational Unit) and the Flood Preparedness Committee at Commune level. Interviews with selected beneficiaries and affected people helped identify stakeholders’positions and opinions. The consultantscreened the Day River Project followingthe recommendations of the EnvironmentalLegislation of Vietnam (Part IV, GuidelineDocuments, 1998). The consultant scopedthe project following a Driver, Pressure,State, Impact, and Response (DPSIR)approach, to recognise the environmentalissues and to identify to which extent thenature and extent of the project affects them.

The consultant collected baseline data inGovernment and Research Institutes, and

organised a physical-chemical field survey,covering the dry and the wet season in theyear 2001 to typify the initial conditionsbefore the project intervention.

The survey included:1. The water quality and sediment

boundary conditions set by the Red River;2. The primary flood zones in the Day River

bed (quality of surface water, sediments,groundwater and soils);

3. The secondary flood zones outside theDay Riverbed (quality of surface water,sediments, groundwater and soils); and

4. The other areas possibly impacted bycollateral water logging, downstream ofHa Nam (quality of surface water,sediments, groundwater and soils).

The aim was to identify the fingerprint ofRed River deposits and effects of pollutioncaused by human interventions. The in-situobserved parameters were pH, DO,turbidity, EC. The Laboratory of the Centerof Water Resources Development andEnvironment (CWE) of the Vietnam Instituteof Water Resources Research analysedphysical, chemical and biologicalparameters for water, groundwater, soil and sediment, including heavy metals.The Department of Vegetation Protection ofthe Northern Pesticide Control Centertested soil and sediment on pesticides’residues.

BASELINE DATA

Surface and GroundwaterSurface water quality in Day River and Red River generally is good and suitable as sources for domestic water supply withproper treatment. Nitrite and ammonia are the most common pollutants in thestagnant parts of the Day River, causingabundant growth of water plants. They reflect the disposal of untreatedwastewater and the absence of flow.

Wastewater discharge from resident areas,industrial areas and agriculture productionareas locally pollute Day and adjacentrivers. Nevertheless, mixing with Red Riverwater can generally improve water qualityof Day River and Nhue River. Groundwater


living in the Red River Delta. They also facedelay in agricultural development, causedby water shortages, water logging and lackof fertilisation by Red River sediments afterthe siltation of the Day River entrance. In parallel, flood diversion functions retardedother village infrastructure developments,like tapped water supply, sanitation, andelectrification, while primitive villageindustries substantially contribute to thepollution of water and soils with heavymetals and other pollutants.

The flood diversion function of the DayRiver otherwise prompted a restrictedspatial planning. It imposed constraints tourbanisation and industrialisation in theflood zone, demanding careful control ofbuilding licences in the area. It alsoprompted the establishment of emergencypreparedness. The National Flood ControlOffice takes the decision to open the Van Coc sluices; the Day Dam Officeimplements it by delegation to Village Flood Committees, who are directly incharge of emergency preparedness,evacuation or sheltering. Hence, all peoplein the Day River observe a yearly period ofemergency preparedness, extending fromJuly 1 to September 30.

IMPACT OF THE PROJECT AND ITSMITIGATION

Day River Flood Diversion By interviewing authorities and stakeholders,consultants extensively studied the impactof the 1971 event (Figure 7). The impact offlood diversion in the Day River stronglydepends on the following factors:– Area flooded– Depth of inundation– Destructive power of the currents– Scour and deposition of sediment– Displacement and accumulation of debris– Duration of water logging– Accumulation of pollutants, their decay

and release of nutrients– Aggravate effect of coincident or

possibly prolonged rainfall– Rate of denudation as related to micro

and macro drainage system– Evaporation and infiltration of water

from the fields Figure 7. The flooded area in 1971.


Flooding potentially results in loss of life,flood-induced injury, economic damageowing to loss of property, crops andinfrastructure, to outbreak of waterborneand water related diseases and socialinfringements. In the perception of theauthorities, there are three zones of impact:front zones, tail zones and danger zones:

1. Front zones: People at the front endinundate fast and deep, prompting ahigh state of emergency preparedness,but the lands denude quickly as well.Upon return, the people will findsediment (“chocolate”) everywhere.Though crops will be lost, the fertilisingeffect of sediment deposition hasprofound positive effects on the cropyields, notably in the areas inundateddeepest. Rapid clean up, repair andrehabilitation of properties, utilities andinfrastructure is possible here. If the dry

season sets on quickly, business cansoon be as usual.

2. Tail zones: Land at the tail end inundatesmore slowly and less deeply, promptinga lesser state of emergency preparedness(Figure 8). People have to make thedifficult decision, whether to evacuate orto stay in their isolated hamlets, cut offfrom transport, communication, schools,and so on. Sediment is not coveringtheir property, but debris and scumcover their premises. The watersurrounding them will be stagnant formonths. It will accumulate decayingorganic matter, prompting pathogensand varmints to flourish. People aresusceptible to the outbreak of waterrelated diseases and the situation mightnot improve for months.

3. Danger zones: The power of the flood

directly affects some places upstreamand downstream of man-made andnatural structures. Obstacles will inducestreamlines to converge and velocity toaccelerate. Downstream of obstacles,velocity streamlines will diverge andvelocity dissipates energy. The flood willlevel such obstacles, unless they arereinforced and resistant to thedestructive power of the flood. Forexample it concerns zones behindspillways where overtopping starts,around bridges, in narrow zonesbetween the dikes and such.

Mitigation measures for Day River FloodDiversion include: – Structural measures (civil works)– Emergency preparedness (contingency

planning)– Flood forecasting (rainfall and runoff

monitoring)

Figure 8. Flood-prone area in Kim Bang.


– Flood warning (communicating thelikelihood of a flood diversion)

– Attenuation and delay the flood byproper operation of structures

– Sheltering or evacuation (people cattle,properties)

– Denudation (making the land dry afterflood)

– Return, clean-up and rehabilitation ofproperties

– Acting upon lessons learnt (applies to allmitigative measures)

– Improved spatial planning observing thevulnerability zones

Day River Water ResourcesDevelopment The construction of the flood diversionstructures in the entrance of the Red Riverto the Day River blocked the regular flow of water and sediments. Currently some25,000 ha irrigated land in the project areaderives its irrigation water from pumpingstations along the Day River. Most of thepumps have an irrigation function, some of them have a dual irrigation / drainagefunction, others are for drainage purposesonly. Most of them need overhaul orreplacement. Water shortages are prevalentin the upper and middle reach of the Day River and its tributary, the Tich River.

On the other hand, intensive rainfall at thestart and end of the monsoon often createsdrainage problems, incidentally in all placesor, more commonly and persistently, in thelower reaches of the Day River. The shortageof water for paddy is most serious duringspring when land preparation requires largeamounts of water. Water logging occursmostly during late spring (close to the timeof harvesting) and late summer, with theonset and end of the monsoon. High use ofurea and lacking supply of Red Riversediment, in combination with waterlogging, cause acidification of soils anddegradation of low lying lands.

For mitigating droughts and infertilitylosses, the water taken from the Red Rivershould bring the optimum amounts ofwater and sediment to Day River’s irrigationcommand area, notably to fields at greatdistance from the intake, requiringsediment most. During the irrigationseason, sediment level in Red River isrelatively low (200 mg/l). This is the periodthat water brought into the Day River willbe largest (50 m3/s), so that sediment is insuspension. Potentially a maximum amountof 10 kg sediment / second will be takenfrom Red River. The amount of 40 m3/ssupports irrigation and 10 m3/s is a

minimum base flow needed for flushing the Day River down to the confluence with the Tich River. If there would be nosedimentation losses, 8 kg TSS/s wouldenter the fields and 2 kg TSS/s would godownstream. Otherwise, the irrigation anddrainage infrastructure should be robust atthe front, but more flexible at the tail toabsorb the impact of flood diversion.

SCOPE FOR DREDGING

Amongst alternative solutions developedduring the environmental impactassessment, dredging had a prominentplace. Though traditional dredging ispracticed in the Red River Delta, forexample for the maintenance dredging ofthe intakes of the irrigation canals, thepresent technical opportunities are not usedto the full extent.

Macro DredgingIn addition to the scope of the presentproject, consultants identified the optionfor “macro-dredging” of the Lower DayRiver between Phu Ly and Ninh Bin. TheLower Day River here has to pass limestonebarriers at the surface and in theunderground, preventing natural scour anddeepening of the river. As demonstrated bythe 1971 floods, the bottom slope and theperimeter of the Lower Day River do notallow rapid runoff of the wateraccumulated in the area inundated by the Day River Flood Diversion (Figure 9).

Widening of the Lower Day River and / orpossible deepening would speed-up thedenudation of the waterlogged areas.

Figure 9. Shallow narrowness of lower

Day River between PhyULi and Ninh Binh.


It would require robust interventions, basedon blasting and cutting the limestone overa length of several kilometres. One optionis to widen the perimeter, requiringresettlement of several hundreds of peopleand industrial facilities from the denselypopulated embankment. The socialeconomic costs of expropriation andresettlement will be substantial.

Another option is to deepen the perimeter,creating risks for intrusion of salt waterfrom the Day Estuary in episodes with lowriver discharge. Prevention of seawaterintrusion might require a larger intake ofirrigation water from the Red River to theDay River than foreseen in the 50 m3/sscenario. There even are prospects for alarger dry weather flow through the Day River. One scenario consideredreconstruction of the Day River into aperennial river with a nominal capacity of1,000 m3/s, which would have certainphysical and economic benefits.

Investigating the feasibility of thisintervention was outside the scope of the present study. An additional study isrequired for investigating macro dredgingas a feasible tool for river training, coveringthe whole Red River Delta and the jointimpact on water, salinity and sediment inan integrated manner

Micro DredgingConsultants identified the need for smallerdredging operations for severalcomponents of the project. These includedmaintenance dredging of the Hoa BinhReservoir to remove silt, capital andmaintenance dredging of the fairway of theChau Giang shipping route and ecologicaldredging in the Day River floodplain todeliver the Red River water to the irrigationcommand areas.

Though consultants recommend a linedtrapezoidal channel (Q=50 m3/s) for routingRed River water from the intake from theVan Coc Sluice to the Day Dam, supportedby the required structures for passing thestructures, they consider a less robustsupply solution downstream from the DayDam. Rather than continuing the linedtrapezoidal channel, they suggest recurrent

micro dredging, harvesting the Red Riversludge for deposition on the command areafor maintaining and improving fertility. Easytransportable small dredgers would buildup and maintain natural waterways in theold Day River bed and floodplain. The smalldredger would guarantee a sustainablesupply of irrigation water, reception ofdrainage water and help create a naturalbuffer for water and sediment in abandonedoxbow lakes. They would enable a flexibleresponse after a major flood diversion,affecting the floodplain.

The antecedent Day River bed would guidethe dredgers in finding the best canal’salignment, as topography is the footprint ofearlier flood and sedimentation patterns.

Abandoned meanders, steep levees andswampy depressions mark the old DayRiver. Planting wicker, bamboo and canewould create levees in the wider parts ofthe dredged channel. Wetlands would bestimulated beyond these levees. Infiltrationand overflows from the main artery wouldfeed these impoundments and strengthentheir traditional use as village ponds,enabling production of fish, waterfowl,vegetables, water supply or treatment andmaturation of wastewater. Dredgers shouldremove and isolate sediment from “hotspots”, contaminated by heavy metals fromvillage industry. Dredgers also shouldcarefully remove, dispose and containsulfurous clay, apparent near the surface inthe lower delta. The primary task of thedredgers however would be to keep theirrigation and drainage canals open, whiledepositing the fertile Red River sludge onthe lands in the command area.

CONCLUSIONS

Proper management of the deposition ofsediment is of paramount importanceunder tropical climate. Sustainabledevelopment of land and water in deltasnot only depends on large flood diversionstructures, dikes and pumping stations.

The present study scope found scope for“micro dredging” in the Day River flooddiversion zone by mobilising movable

dredgers for maintaining irrigation anddrainage canals, while depositing fertileRed River sediment on agricultural lands.

The example suggests that dredging canhelp develop flexible responses to flood-and drought-prone deltas under tropicalclimate, while maintaining soil fertility andtheir granary function. To avoid thinking in mega structures and giant dikes,integrated development of water resourcesin deltas should consider macro and microdredging as flexible options for keeping up soil fertility and managing floods anddroughts.

REFERENCES

Normandin, M.A (1925). “Les crues duFleuve Rouge et la Défense du Delta duTonkin contre les inondations”. Monographof the “Ecole Nationale des Pont etChaussées (ENPC)”. Paris, France.Rigal, Jean (1939). “Le barrage du Day(Tonkin)”. Traveaux, Fevrier, No 74(Courtesy of ENPC).

Part IV of the Guideline Documents toImplement Environmental Protection inUrban and Rural Management andDevelopment and Construction Investment,issued as a Circular Letter of the Ministry of Science, Technology and EnvironmentNo. 490/1998/TTBKHCNMT 29 April 1998,acting as a guide for EIA in Vietnam. For the water sector, reference is made toSectoral Standard, 14 TCN – 1997,“Guideline for Environmental ImpactAssessment of Water ResourcesDevelopment Project”.