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1.1.5.4 Phong My town

Introduction
Phong My town is a busy town at the intersection of the K. Nguyen van Tiep and Song Tien. Due to the waterway widening the northern bank will have to be dredged and the inhabitants of the bank will have to be resettled. To prevent erosion of the southern bank, bank protection will also be required along the southern bank. Along the river, the banks shall also be protected against the eroding forces of the river and navigation.

Figure 4 28 – Phong My town

Proposed slope protection Phong My town
The proposed slope protection will have a length of approximately 1450m as shown in the figure on the next page. The bank protection will consist of concrete block mattresses. To limit resettlement requirements the Consultants propose a small retaining wall along the top section of the bank protection on the north bank of the K. Nguyen van Tiep. In case it fits in the spatial planning of the city and is acceptable w.r.t. the resettlement requirements, the Consultants propose a small footpath along the northern bank to make the canal accessible for the inhabitants of Phong My.

On the south bank the Consultants propose to continue the slope to the top of the bank with a small footpath along the bank. Constructing retaining walls on both sides of the banks is not advisable as retaining walls act as wave reflectors, continuously reflecting waves without dissipating wave energy significantly.

Along the Tien river, at both sides of the K. Nguyen van Tiep, the Consultants also propose a concrete block mattress bank protection with a footpath. Bathymetric data show that there is some scour close to the southern bank at the confluence of the K. Nguyen van Tiep and the Song Tien (so called confluence scour). To protect the bank against instability caused by this scour the bank protection along the southern bank is continued to a level of -10m. Along the northern bank, where there is limited scour, the bank protection will be continued to a level of -8m slowly rising to -4m before tie-ing in into the bank.

The design of the slope protection along the K. Nguyen van Tiep canal and the Song Tien has been based on different loadings as the former is a rather narrow inland waterway and the latter a large river. The preliminary designs of the bank protection have been shown in the figures on the following pages.


Figure 4 29 - Bathymetry at the confluence of the K. Nguyen Van Tiep and the Song Tien

Figure 4 30 - Plan view slope protection Phong My town

Figure 4 31 - Cross section slope protection Phong My town (A-A)

Figure 4 32 - Cross section slope protection Phong My town (B-B)

Figure 4 33 - Cross section slope protection Phong My town (C-C)

Figure 4 34 - Cross section slope protection Phong My town (D-D)

Figure 4 35 - Cross section slope protection Phong My town (E-E)

Figure 4 36 – Stairs slope protection Phong My town

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1.1.5.5 Cho Gao canal

Introduction
The Cho Gao canal is one of the busiest waterways in the Mekong Delta. At this moment the greater part of the inland waterway transport from the Mekong Delta to the greater HCMC area or vice versa, passes through this canal. The intensity of the waterway transport, the tidal difference and the absence of vegetation make these banks sensitive for erosion. Consequently a significant part of the banks which have not been protected with bank protection (including natural vegetation), suffer from erosion.

Currently two sections along the northern bank have been protected with bank protection viz.:

• Old section of bank protection from approximately km 66 to km 67.5
• New section, constructed as part of the Inland Waterways and Port Modernization project, from approximately km 64 to km 65.2
• Both consist of reno mattresses on geotextile with dimensions 6m*2m*0.23m under a slope of 1V:2H.

Figure 4 37 – Cho Gao canal

Problem description
The Consultants have visited the Cho Gao canal on several occasions, studying both navigation requirements as well as bank protection requirements. From these visits the Consultants conclude that in general the natural banks are strong enough to withstand natural as well as ship-induced wave and current attack. The problem of erosion mainly occurs at locations where the natural vegetation has disappeared through human actions. At these locations erosion is ongoing steadily threatening infrastructure along the banks.

At a number of locations man made bank protection can be found. Especially on the south bank some locally constructed bank protections are on the verge of collapse and need to be replaced. Furthermore the new bridge to be built at Cho Gao as part of this project, will require bank protection.


Erosion after removal of natural bank protection Man made bank protection on the verge of collapse

Reno mattresses along the north bank of the Cho Gao canal New bank protection required after construction of a new Cho Gao bridge

Figure 4 38 – Existing situation of slopes along the Cho Gao canal

Proposed bank protection Cho Gao canal
First of all the authorities are recommended to stimulate the growth and protection of vegetation along the banks and to make inhabitants aware of the danger which lies in removing this natural protection. Natural protection is more or less for free, does not require much maintenance and is therefore the preferred bank protection alternative.

At locations where natural bank protection is not present and cannot easily be created due to human activities like e.g. recreation, mooring, loading and unloading activities, and where valuable infrastructure or cultural heritage is threatened, the banks will be protected with man made structures. The proposed bank protection will have a length of approximately 2.5 km and will consist of reno mattresses as shown in the figures on the following pages. Slope stability calculations for Cho Gao executed as part of the Inland Waterways and Port Modernization project resulted in a stable slope of 1V:3H. This is different from the slope which has been applied for the bank protection already constructed along the Cho Gao canal. Therefore the Consultants recommend assessing the slope stability in more detail in the detailed design. For the preliminary design, a slope of 1V:3H is chosen, which corresponds with the slope of the dredged channel.

The preliminary designs of the slope protection have been shown in the figures on the following pages.


Figure 4 39 - Plan view slope protection Cho Gao canal (1)

Figure 4 40 - Plan view slope protection Cho Gao canal (2)

Figure 4 41 - Plan view slope protection Cho Gao canal (3)

Figure 4 42 - Plan view slope protection Cho Gao canal (4)

Figure 4 43 - Cross section slope protection Cho Gao canal (A-A)

Figure 4 44 - Cross section slope protection Cho Gao canal (B-B)

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1.1.6 Cost estimates

In this section a breakdown of the quantities and costs involved in providing bank protection for the selected locations is given. The cost estimate of the bank protection is depending on the dimensions (level of the bank, slope of bank, design water levels etc.) and type of structure pertaining to each individual location where slope protection is required.

Unit costs have been assessed considering different sources. Decisions for construction of infrastructure projects for Bac Lieu, An Giang and Dong Thap Province, as well as market prices in the Mekong Delta have been examined. Furthermore information from the Inland Waterways and Port Modernization Project has been considered. The unit costs acquired through these sources are presented below. These unit rates cover the costs of purchase, transport and placement in the works. Based on the unit rates and the designs of the various bank protection schemes, an estimate has been prepared for the construction costs. These costs per site and per corridor are presented in the table below.

Table 4 9 – Bank protection costs

The maintenance costs for slope protection schemes of this type are expected to be in the order of 10% of the capital costs every three years. It is important to appreciate that the present review of slope stability within the studied corridors has been on a feasibility level and therefore a detailed analysis of bank erosion and embankment stability at specific locations has not been addressed.

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1.1.7 Summary of proposed slope protection schemes along main waterways

1.1.8 Slope protection requirements along feeder canals

During several surveys of the feeder canals, the Consultants have estimated the slope protection requirements per feeder canal. The Consultants recommend further specification of type and location of the required slope protections during the detailed design phase. The lesser loading of the banks in these class IV waterways is taken into account in the unit rate applied to estimate the costs.

Table 4 10 – Slope protection requirements along feeder canals

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1.2 Dredging
1.2.1 Deepening and widening of corridors
1.2.1.1 Approach

To determine the required dredging locations and quantities to improve the selected waterways, hydrographic data and water levels on the existing waterways were required. The following information was collected to assess the dredging volumes:

• VIWA data on least available depths and widths;
• Hydro graphic surveys and maps (2003) from Sub VIWA and from other agents and authorities were collected of all the selected waterways, except for Rach Chanh canal where no maps or hydrographic survey data are available;
• Water levels (2003) from the Southern Region Hydro Meteorological Center were collected at 9 hydrological stations relevant for the project corridors.
• Low Water levels of a dry year (1998) along the waterways as a results of the Dac hydraulic model
• Surveys, to verify the selected depth and widths.

The VIWA data give least available depths and widths for all waterways of the corridors of the project and were used to make a preliminary estimate for the dredging volumes.

To determine these quantities in more detail the following assessment was carried out:

• Hydrographic maps were used to prepare cross-sections (1 cross-section for every 500 m);
• With the hydraulic model low water levels were assessed along the waterways. The outcome of the hydraulic model was verified with use of the collected water levels at hydrological stations of the Southern Region Hydro Meteorological Center;
• Low water levels and the required canal dimensions (least available depth, width at bottom level and slope) as shown in Table 4 4 were plotted into the cross-sections of the hydrographic maps. Where the required canal dimensions were larger than the existing canal dimensions the required dredging quantity was calculated.

When the waterway requires widening the location of the banks was used to determine the centerline of the required waterway dimensions, considering resettlement and natural banks. Where possible it is proposed to dredge only one side of the canal to leave the vegetation untouched for stability and natural habitat.

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1.2.1.2 Dredging Volumes

Preliminary dredging locations for upgrading of the waterways to Class III standards have been determined as described above and are indicated on layout maps and cross sections in Annex J. The dredging volumes, based on cross-sectional drawings of the waterway at every 500m, are presented in Table 4 13. The additional quantities (10%) that will arise from dredging tolerances have been based on the experiences of the Inland Waterways and Port Modernization Project.

1.2.1.3 Soils to be dredged

The Mekong Delta mainly comprises very young Holocene marine sediments. Only at the border with Cambodia a narrow belt of older sediments is found consisting of old granite and limestone rock outcrops together with white clays of Pleistocene age. In most waterways the stiff clay appears to be deep enough to preclude its dredging for the purposes of creating the required navigational depth. A large part of the brackish water and marine sediments consist of potentially acid sulphate soils containing substantial amounts of pyrite (FeS2). The pyrite containing sediments have been formed under brackish water conditions in tidal swamps covered with a mangrove vegeta¬tion. At locations where acid sulphate soils need to be dredged suitable measures for containing this type of soil need to be taken. At the Inland Waterways and Port Modernization Project measurements were taken and little problems with acid sulphate soils were experienced. Therefore it is proposed to use the same approach at this project. For further details on the proposed approach the reader is referred to section 4.5.2.2. For more detailed information on the locations and intensity of the acid sulphate soils reference is made to the Environmental report submitted under separate cover.

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1.2.1.4 Choice of equipment

There are various types of dredging equipment available for executing capital dredging works. These include the following:

• trailing suction hopper dredger;
• cutter suction dredger;
• grab/clamshell dredger;
• backhoe dredger;
• bucket dredger.

Various dredging companies in Vietnam own a fleet of dredgers (cutter suction dredgers and trailing suction hopper dredgers) for executing dredging works both in the coastal zone and in inland waterways. Trailing suction hopper dredgers would not be able to operate on many sections of the inland waterways due to draught and width limitations.

At present, Vietnamese dredging companies are maintaining the water depth along various traces of the canals using cutter suction dredgers. These dredgers can operate in the dry season and the wet season (provided flow velocities remain below approx. 2.5 m/s) and can achieve a sufficiently high production to warrant their economic use.

The use of grab/clamshell dredgers, backhoe dredgers or bucket dredgers for large scale dredging in the inland waterways is impractical and costly due to their low production rate and the need to double handle the dredged spoil i.e. load into barges and then unload barges and place on the land.

Along the stretches that have to be widened, the top layers will contain remains of trees or other vegetation, while even after cleaning operations parts of foundations of houses and other debris will be present in the soil. The presence of these debris will most certainly lead to delays and break-downs of a cutter dredger in these areas and it will be more attractive to use a grab dredge or dry excavator for removal of the top layer, before employing a cutter dredger to remove the bulk quantity. Grab/clamshell dredgers can also be deployed for certain locations where dredging quantities are low and accessibility for cutter suction dredgers is restricted i.e. close to bridge foundations and existing structures or where houses are encroached along the waterways.

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1.2.2 Disposal of dredged material
1.2.2.1 Disposal options

There are various options for disposal of dredged material from inland waterways. Several are listed below, viz.:

• open and contained disposal at sea or in the waterway;
• agitation dredging;
• open and contained disposal on land.

Open and contained disposal at sea is not considered to be feasible options as the distance between the dredging area in the waterway and the open sea is very large and would imply high transportation costs. Furthermore, the material to be dredged contains pyritic soil and large scale dumping of this type of material at sea would be environmentally unacceptable.

Open and contained disposal in the waterway are not considered to be feasible options as the dimensions of the waterway are not sufficient to permit large scale dumping of the material in the waterway. Material could be dumped in the Bassac and Mekong rivers as these are sufficiently deep to place material however, large scale dumping of pyritic material in these rivers is also environmentally unacceptable.

Agitation dredging is not a feasible option for disposing of dredged material from the waterways under consideration. The flow velocities in the canals are limited and subject to tides, and therefore material agitated into suspension would most likely not easily be carried out to sea or the major rivers, without settling down beforehand. Furthermore the enormous turbidity created by agitation dredging may cause problems to aquatic life in the waterways.

Open disposal of dredged material on land is not recommended as surplus dredge water used to hydraulically transport the dredge material through the pipeline would drain to low lying areas. These low lying areas are often used for agriculture and the acidity of the surplus dredge water could negatively affect this. In addition, the material is very fine grained which means it will spread over very large areas during the reclamation process.

Considering the above, there remains only one option for material disposal and that is contained disposal on land. When the transportation distance is kept short by disposal on the adjacent canal bank, costs can be kept low. The material will be contained between bunds, meaning the process of dewatering the disposal area and the distribution of material over the land can be controlled. This is essential when disposing this material in an environmentally friendly way.

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1.2.2.2 Method of disposal

A contained clay disposal area comprises a perimeter dyke which can be previously constructed by draglines from material excavated from the disposal site. The ditch created by the excavation of material will form the drainage ditch for the disposal area which is located on the outside of the perimeter dyke.

The disposal area is normally divided into a number of sub-containment areas by a suitable arrangement of internal bunds. These sub-containment areas are filled in sequence during the dredging operation. This is required as once a sub-containment area is filled the surplus dredge water can not be discharged directly to the waterway as suspended sediments have not had sufficient time to settle. One day should be sufficient for suspended sediments to settle and then the surplus dredge water can be drained from the area.

Drainage structures are placed within the perimeter dyke to facilitate drainage of surplus dredge water. A drainage ditch located around the periphery of the perimeter dyke will collect water from the disposal area and lead it to the main discharge channel back to the waterway.

The thickness of dredged material deposited in a continuous operation is usually limited to about 1.5 m as dewatering of this fine grained clay becomes increasingly difficult and time consuming as the thickness increases.

Following the experiences at the Inland Waterways and Port Modernization Project for acid sulphate soils the dykes will be covered with PVC membrane to ensure drainage does not occur through the dyke walls and a lime layer will be used to cover the material.

These disposal areas can prove to be very dangerous places for people (especially children) before consolidation has taken place. It is therefore important that sufficient measures are taken to prevent the local population from entering this area.

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1.2.2.3 Volumetric changes and land requirements for disposal

Changes in bulk density of the soil will vary at different stages of the dredging, transport and placement processes. The alteration in density is caused by the formation of additional voids in the soils, which fill with water when it is disturbed. This usually means that the volume of the disposed material on the disposal site is higher than the in-situ dredged volume. This increase can be expressed as a percentage of the in-situ volume or as a ratio of the two volumes. The latter is known as the bulking factor . General values for the bulking factor are given in Table 4 11.

Soil type Bulking factor for mechanical excavation (B)
Sand, compact 1.25-1.35
Sand, medium to compact 1.15-1.25
Sand, loose 1.05-1.15
Silts, freshly deposited 1.00-1.10
Silts, consolidated 1.10-1.40
Clay, very hard 1.15-1.25
Clay, medium soft to hard 1.10-1.15
Clay, soft 1.00-1.10

Table 4 11 – Bulking factor for different soil types using mechanical excavation

The abovementioned values of bulking are related to mechanical excavation. When hydraulic excavation and placement techniques are adopted, bulking can be much higher (1.30 to 1.50). In order to determine the dimensions of the disposal sites an average bulking factor of 1.40 will be applied. The total required area of the disposal sites will be determined using the calculated dredging quantities plus tolerances, multiplied by the bulking factor, whilst using a fill height of 1.5m. For e.g. a dredging volume of 1 million m3 and following the abovementioned approach, a disposal area of 93ha will be required.

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1.2.2.4 Suitable disposal areas

The suitability of a discharge area is determined by environmental, social and economic factors. In respect of the techno-economic aspects of a reclamation area it can be concluded, that in general waste land and agricultural areas with yearly crops are most suitable as (temporary) disposal areas. The costs for the use of this land can usually be limited to crop compensation for the land users but experiences at the Inland Waterways and Port Modernization Project show that this can cause delay and problems with obtaining the land when the people consider the compensation too low. At this project the compensation for roads was approximately four times higher than for waterways, which was difficult to explain to the people. Therefore the assumed costs for compensation are chosen in line with the road compensations.

On the other hand disposal areas can have socio-economic benefits. In low-lying areas with frequent flooding the dredged soils may be used as landfill material which will raise the land of the people by approximately 1.5 m. Further people may sell the dredged material as building or landfill material. In this way disposal of dredging materials creates better living conditions for the poor and may induce urban economic development.

Further the Inland Waterways and Port Modernization Project experienced problems with finding large disposal areas because the land is divided and it is needed to negotiate with all the different land owners. While it was estimated that 20 disposal areas would be required, at the end of the project approximately 500 disposal areas were used, covering the same total area. Therefore at this moment the suitable areas for disposal are not identified in detail.

Most stretches of the waterways are characterized by the presence of a road at one side of the canal, while along the roads and canals most houses are located. Behind the houses usually the agricultural fields are located, which can be, (and in fact are already often used during maintenance dredging campaigns), as reclama¬tion area.

Densely populated areas and high investment agricultural areas should be avoided as much as possible. Dredging in large villages or towns, or at locations near extensive coconut plantations will require consideration of alternative disposal techniques. At these locations the use of grab dredgers and barges for transport will be a more feasible option.

It should be realized, that dredged clay is being transformed by the cutter dredger into a slurry with a low density. Depending on the type of clay and layer thickness of the reclamation the soft mud will have to dry out and may require some years before it regains its bearing capacity and can be utilized again for agricultural or for resettlement purposes. Also the effects of increased ground levels as a result of the deposition of dredged material on drainage and irrigation systems should be given due attention

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1.2.3 Impact of widened and deepened waterways on hydraulics

Model simulations have been carried out in order to assess the effects on the water levels and the current velocities of upgrading the selected corridors to class III. These model simulations have been done for the base case (no improvement works included) as well as for a schematization of the improvement works like deepening and widening of the relevant stretches. The model schematization was adapted such that the channel dimensions depth and width meet at least the requirements for the class III waterways.

The model simulations showed a decrease in water level and in general an increase in current velocities for the improvement scenario as can be seen in the figures below.

Figure 4 45 – Water levels for base case and improvement scenario for corridor 2

Figure 4 46 – Water levels for base case and improvement scenario for corridor 3

Figure 4 47 – Current velocities exceeded 5% of the time from hydraulic model along corridor 2 for base case and improvement scenario

Figure 4 48 – Current velocities exceeded 5% of the time from hydraulic model along corridor 3 for base case and improvement scenario

Annex A reports more details on the hydraulic model and the outputs.

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1.2.4 Cost estimates
1.2.4.1 Unit rates

Unit rates have been assessed considering different sources. Decisions for construction of infrastructure projects for Bac Lieu, An Giang and Dong Thap Province, as well as market prices in the Mekong Delta have been examined. Furthermore information from the Inland Waterways and Port Modernization Project and the Bassac River Improvement project has been considered. The unit costs acquired through these sources are presented below and also cover the costs of mobilization and demobilisation.

Activity Unit Unit rates
Dredging US$ / m3 2
Construction of disposal areas (including areas for acid sulphate soils) US$ / ha 1000
Land compensation US$ / ha 500
Dredging tolerances % 10
Table 4 12 – Unit rates of dredging

1.2.4.2 Cost estimate dredging main waterways

Based on the unit rates and the waterway designs, an estimate has been prepared for the dredging and disposal costs. These costs per site and per corridor are presented in Table 4 13.

Table 4 13 - Preliminary dredging costs for main waterways (in M US$)


1.2.4.3 Cost estimate dredging feeder canals

Based on the unit rates and the feeder waterway designs, an estimate has been prepared for the dredging and disposal costs. These costs per province are presented in Table 4 14.

Table 4 14 - Preliminary dredging and disposal costs for feeder waterways


1.2.4.4 Cost estimate maintenance dredging
For maintenance dredging (once every three years) an average allowance of 10% of the investment cost has been assumed.

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turbulent flow/current = dòng chảy rối

laminar " = dòng chảy phiến (song song, lớp... )

granular soil = đất hạt, có hạt, đá cát có hạt

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Few words:

1. Pore pressure = Áp lực nược lỗ rỗng
(u = tỷ trọng nước x chieu sau ngap nuoc)

2. Igneous rocks = Đá magma phun trào

3. Granular materials = đất rời. Cái này phân biệt với cohesive soils = đất dính.

4. Regulating reservoir = hồ điều tiết (không phải "Dieu tiet ho chua").

5. Breakwater = de pha' so'ng/ lu*o*n pha' so'ng --> S-slope breakwater = lu*o*n pha song chu S.

6. Spillway = ma'ng tra`n