Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.1.2 Impact of widening

Along certain canal sections that will be widened by dredging, the existing bank will be removed or disturbed, which implies removal of vegetation or constructions that at the moment provide protection. Part of these canal banks will not benefit anymore from any existing protection and may need to be protected by means of natural or man made bank protection.

The alignment of the navigation channels designed as part of the MTIDP project, is as much as possible based on a design which affects only one side of the canal to reduce the negative impact of damage to the existing banks.

1.1.1.2 Tác động của sự mở rộng

Dọc theo các đoạn kênh được mở rộng bới nạo vét, các bờ cũ sẽ bị bỏ đi hoặc xáo trộn, điều này chỉ ra rằng bỏ đi thảm thực vật hay xây dựng vào lúc này là cung cấp một sự bảo vệ. Một phần của những bờ kênh này sẽ không mang lại lợi ích hơn từ sự bảo vệ cũ, và cần được bảo vệ bởi các phương tiện bảo vệ bờ tự nhiên hay con người làm.

Sự liên kết các lòng sông hàng hải được thiết kế là một phần của dự án MTIDP, được dựa theo một thiết kế có ảnh hưởng đến chỉ một bên kênh, để giảm sự tác động tiêu cực làm phá huỷ bờ cũ.


--------nhắc lại 1 số thuật ngữ---------
Slope=mái dốc
slope protection=bảo vệ mái dốc=bảo vệ bờ
erosion=xói lở
bank erosion=xói lở bờ
canal=kênh
current=dòng=dòng chảy (cơ rần)
strong current=dòng chảy mạnh
turbulent=hỗn loạn ('tơ biu lơn)
bifurcation=nhánh=rẽ đôi (bai phơ 'cây sần)
flow=dòng
bend=gập=cua gấp
susceptible=nhạy cảm=dễ thương tổn (sơ 'sép tơ bồ)
dredge=nạo vét
disturb=làm quấy rầy=làm nhiễu loạn=làm xáo trộn (đít xờ 'tớp)

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.1.3 Social function of banks and bank beautification in urban areas

Widening the waterway through urban areas will sometimes require resettlement of people living along the banks. Apart from the people who have to be resettled, this resettlement will sometimes have a large impact on the communities along the banks as, as a result of the waterway widening, suddenly part of the community has to be resettled. Also part of the infrastructure along the banks is suddenly cut, which could also have a negative impact on the communities.

The Consultants are aware of the Vietnamese appreciation for bank beautification and protection in urban areas which, apart from protecting the banks also provide residents with easy access to and facilitate use of the banks. In this respect also the social function of the infrastructure (parks, footpaths, terraces) along the banks is highlighted. It is also an issue which has been raised by the stakeholders during the various work shops conducted during the MTIDP.

To mitigate the social cost of waterway widening and acknowledging the Vietnamese appreciation for and social function of the waterway banks, the Consultants have prepared specific bank protection schemes for urban areas, which incorporate both the bank stability requirements as well as the social functions of the banks, in one design.

In the following sections the loading of the banks and the processes involved are elaborately discussed, followed by possible bank protection alternatives. Hereafter a selection is made of the preferred bank protection alternatives including feasibility designs and cost estimates for the bank protections at selected locations.


1.1.1.3 Chức năng xã hội của bờ và sự tô điểm cho bờ ở các vùng nông thôn (?)

Làm rộng các luồng lạch qua các khu vực nông thôn thường yêu cầu sự tái định cư của nhân dân dọc theo bờ sông. Một phần trong số họ phải tái định cư, sự tái định cư này có tác động lớn đến cộng đồng dân cư dọc bờ sông, đó là kết qua của sự mở rộng các luồng lạch, khi mà bỗng nhiên một phần dân cư phải tái định cư. Đương nhiên, một phần kết cấu hạ tầng ở bờ sông cũng bị cắt giảm, nên có tác động tiêu cực đến cộng đồng.
Bên tư vấn rất hiểu sự đánh giá Việt Nam về vấn đề tô điểm và bảo vệ các khu vực nông thôn, một phần của sự bảo vệ bờ sông sẽ mang lại cho dân cư sự tiếp cận dễ dàng và thuận lợi sử dụng bờ sông. Riêng về mặt này, chức năng xã hội của cơ sở hạ tầng (lối đi, nền đất cao, bãi đất) dọc theo bờ sông được nhấn mạnh. Đó cũng là một vấn đề mà được quan tâm bởi stakeholder trong suốt các công việc khác nhau trong dự án MTIDP.
Để giảm bớt chi phí xã hội của sự mở rộng đường thuỷ, cảm kích sự đánh giá cao bên Việt Nam và chức năng xã hội của bờ sông, bên Tư vấn đã chuẩn bị kế hoạch bảo vệ bờ sông đối với khu vực nông thôn, kế hoạch này kết hợp cả yêu cầu ổn định bờ sông và chức năng xã hội của bờ sông, trong cùng một thiết kế.
Trong những phần sau, tải trọng của bờ sông và các quá trình có liên quan đều được trình bày tỉ mỉ, dẫn ra qua các phương án bảo vệ bờ có thể chấp nhận được. Từ đó, một sự lựa chọn sẽ được tiến hành với các phương án bảo vệ bờ đáng giá hơn, bao gồm thiết kế khả thi và tổng dự toán cho quá trình bảo vệ bờ sông tại khu vực được chọn.


zmt's note:
bank protection la phần kè bảo vệ bờ sông, thường chỉ được làm ở thành phố/thị thôi. Và urban là thành phố/thị chứ ko phải nông thôn !!!
Sai cơ bản quá, hic


bank beautification nên dịch là cảnh quan 2 bên bờ

------------------------------------
beautification=tô điểm
terrace=nền đất cao ('te rớt xờ NOT tơ rây xờ)
issue=vấn đề
raise=đề xuất ('rây zờ NOT rai sờ)
mitigate=giảm nhẹ ('mi ti gêit)
acknowledge=chấp nhận=tỏ lòng biết ơn
incorporate=kết hợp=sát nhập (in 'co pơ rit NOT reit)
elaborately=kỹ lưỡng=tỉ mỉ (i 'le bơ rit tơ ly)
hereafter=dưới đây=sau đây

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.2 Loading of the banks in navigation channels
1.1.2.1 Introduction

The main reason for erosion in the navigation channels is scouring caused by ship induced or natural currents and waves. Indicative values for ship induced currents and waves in small rivers or restrictive navigational channels, are 1 - 2 m/s (return current) and 0.50 - 0.75 m (stern waves). Besides the ship induced hydraulic loads, also the natural wind waves and currents by tides and river discharge are important. The wind climate in the Delta is characterized by low wind speeds in general. During the rainy season, and often in combination with showers, winds can become strong ( 20 m/s). The duration, however, is usually short, while also the fetch is very small in the narrow canals. As part of the Inland Waterways and Port Modernization project dominant waves of maximum 0.30 m with wave periods of about 2 seconds have been determined. As part of the Bassac River improvement project wind waves have also been assessed but these were not considered normative for the design of the bank protection.

Furthermore the Inland Waterways and Port Modernization project reports that with respect to the tidal and discharge currents in the canals the DAC-computer was used to determine the maximum velocities in the different canals and river branches. Although the velocities usually did not exceed values of about 0.75 m/s, a number of locations were found where the extreme value of the average velocity in the cross-sections reached values of 1.00 m/s. Calculations executed with the DAC model by the Consultants as part of the MTIDP project for the studied corridors, showed more or less the same results.

At the water line the impact of waves on the slope can be very damaging, particularly when granular non-cohesive soils are present. In case of cohesive soils like the clays in the Mekong Delta the question, whether or not important damage will occur, strongly depends on the magnitude of the excess pore water pressure after the draw-down due to the passing wave, while further the magnitude of the cohesion of the soil is very important. The Inland Waterways and Port Modernisation project reports that the maximum value of the excess pore water pressure for a ship wave of 0.34 m can be calculated to be 3.3 kN/m2. From a Mohr-Coulomb stability diagram it can be derived, that for clays with a cohesion of 4 to 5 kN/m2 (which are characteristic values for the cohesion of soils in the Mekong Delta), this will not lead to loss of stability. Consequently it can be expected, that only loose clay parts can be brought into suspension due to the orbital movements in the waves.

The description of the erodability of the clay slopes by currents is a subject that relies heavily on empirical data. The following "safe permissible mean velocities" have been defined for different soils in the PIANC publication "Supplement to Bulletin No 57 (1987)", on Guidelines for the Design and Construction of Flexible Revetments incorporating Geotextiles for Inland Waterways:

sandy subsoil v = 0.50 m/s
fairly compacted clay v = 0.80 m/s
stiff clay v = 1.50 m/s
grassed clay (up to) v = 2.00 m/s

[Table 4 5 - Safe permissible mean velocities]

Practically all canal slopes in the Delta consist of the type "fairly compacted clay", while stiffer clays can also be found at selected stretches.

The rather low hydraulic loads give support to the idea to apply unprotected slopes to the extent possible. At a number of locations, however, erosion will occur, when the current becomes higher than say 0.80 m/s or the waves are higher than say 0.4-0.5m. In case this happens, this will happen during rather short periods of time, so that the erosion will only develop slowly. Furthermore the table above shows, that the presence of vegetation can have a very positive effect on the stability of the slope against scouring and that some local erosion can be stopped by bringing in vegetation. From the above assessments and considerations it can be concluded that the clay slopes in the Mekong Delta will be subject to rather limited erosion, which is in accordance with field observations. In areas with increased loading and/or unprotected/ non-vegetated banks structural bank erosion can occur.

In the following sections the loading by ship waves and currents will be discussed in more detail, as in specific locations these could be the cause of significant bank erosion, requiring bank protection.


1.1.2 Tải trọng của bờ sông tại các lòng sông hàng hải
1.1.2.1 Giới thiệu

Nguyên nhân quan trọng gây xói lở trong các lòng sông hàng hải là sự cọ xát bởi tàu hay các dòng chảy tự nhiên và sóng gây ra. Giá trị được chỉ ra để tàu có ảnh hưởng đến dòng chảy và sóng trong các dòng sông hẹp hoặc các lòng sông hàng hải bị hạn chế, là 1-2 m/s (dòng chảy dội lại) và 0.50 – 0.75m (dòng chảy cứng). Hơn nữa, tàu thuyền cũng ảnh hưởng tới các tải trọng chạy bằng sức nứơc, cả gió tự nhiên và dòng chảy bởi thuỷ triều và sự thải nứơc sông cũng là yếu tố quan trọng. Gió tại vùng Châu Thổ có đặc điểm nói chung là gió thổi chậm. Trong suốt mùa mưa, thường kết hợp cả những trận mưa rào, gió thường trở nên mạnh hơn (khoảng 20m/s). Thời gian xảy ra, thường là ngắn, trong khi lộ trình định sẵn thường là rất nhỏ trên lòng sông hẹp. Như trong dự án 2 Tuyến Đường Thuỷ và Nâng Cấp Cảng, sóng (gợn nước) được xác định lớn nhất là 0.30 m với chu kỳ khoảng 2s. Với một phần của dự án nâng cấp sông Hậu, gợn nước do gío cũng được xét đến, nhưng không được xem là quy phạm chuẩn cho sự bảo vệ bờ sông.

Hơn thế, dự án 2 Tuyến Đường Thuỷ và Nâng Cấp Cảng được trình bày là, với sự xét đến của dòng chảy do thuỷ triều và dòng chảy ra ngoài trong lòng sông, chương trình DAC-computer sẽ được sử dụng để xác định vận tốc lớn nhất trong các lòng sông và các nhánh sông khác nhau. Dù các vận tốc thường không vượt quá giá trị khoảng 0.75 m/s, số lượng các khu vực có giá trị vận tốc rất lớn so với vận tốc trung bình ở mặt cắt ngang là 1.00 m/s. Các tính toán với mô hình DAC bởi bên Tư Vấn thuộc dự án MTIDP với nghiên cứu dải hành lang, chỉ ra rằng ít hay nhiều hơn cùng các kết quả.

Đối với đường thuỷ, sự tác động của gợn nước (sóng) tới mái dốc có thể phá hoại rất lớn, đặc biệt là với đất rời. Trong trường hợp đất dính như đất sét ở Châu thổ sông Mê kông, câu hỏi đặt ra là có xảy ra sự phá hủy nghiêm trọng hay không, có phụ thuộc nhiều vào cường độ áp lực nước lỗ rỗng quá giới hạn sau khi bị kéo xuống bởi lớp sóng đi qua, trong khi cường độ xói lở đất cũng rất quan trọng. Dự án 2 Tuyến Đường Thuỷ và Nâng Cấp Cảng trình bày rằng giá trị lớn nhất của áp lực nước lỗ rồng khi có sóng từ thuyền ảnh hưởng vào là 0.34m, có thể tính là 3.3 kN/m2. Từ biểu đồ ổn định Mohr-Coulomb, có thể suy ra với đất sét độ cố kết là 4 – 5 kN/m2 (là giá trị đặc trưng cho độ cố kết của đất ở Châu Thổ sông Mêkông), với giá trị này sẽ không dẫn đến sự mất ổn định. Hâụ quả là, chỉ có phần đất sét bị mất đi mới bị trôi theo quỹ đạo chuyển động của từng đợt sóng


Sự diễn tả xói lở của mái dốc đất sét bởi các dòng chảy là một vấn đề phụ thuộc vào các giá trị kinh nghiệm. “ Vận tốc phương tiện an tòan cho phép” được định nghĩa với các loại đất khác nhau trong ấn bản “ Phụ trương cho Bản tin số 57 ) (năm 1987), trong “Hướng dẫn thiết kế và xây dựng lớp phủ mềm kết hợp với Vải địa kỹ thuật cho cảng đường thủy nội địa”:
đất nền cát v = 0.50 m/s
đất sét nửa cứng v = 0.80 m/s
đất sét cứng v = 1.50 m/s
đất sét có cỏ mọc (gía trị lớn hơn ) v = 2.00 m/s



[Bảng 4 5 - Vận tốc phương tiện an tòan cho phép]

Trong thực tế, các mái dốc kênh, sông đào ở vùng châu thổ bao gồm các kiểu “đất sét nửa cứng”, trong khi đất sét cứng cũng có thể tìm thấy ở trên một vùng kéo dài.

Các tải trọng chạy bằng sức nước cũng góp phần hỗ trợ cho ý kiến là sẽ ứng dụng các mái dốc không được bảo vệ để mở rộng hơn có thể. Tại một vài vị trí, xói lở xảy ra khi dòng chảy trở nên mạnh hơn 0.80m/s hoặc sóng gợn cũng lên đến 0.4- 0.5 m/s. Khi trường hợp này xảy ra, sẽ xảy ra trong suốt chu kỳ ngắn ngủi, vì thế, xói lở sẽ phát triển dần dần. Hơn nữa, bảng trên cũng chỉ ra, sự xuất hiện của thảm thực vật có một hiệu ứng tích cực tới sự ổn định của mái dốc chống lại sự cọ xát, thậm chí sự xuất hiện của thảm thực vật đôi khi còn ngăn lại sự xói lở. Từ những đánh giá trên, và xem xét chúng, có thể kết luận rằng mái dốc phần đất sét ở châu thổ sông Mêkông sẽ đạt tới một sự xói lở hơn cả giớii hạn nhất định, phù hợp với sự quan sát. Ở các khu vực tải trọng tăng lên, thì kết cấu bờ sông không được bảo vệ / hay không có thảm thực vật sẽ xảy ra xói lở.

Trong phần sau, tải trọng bới gợn sóng và dòng chảy do tàu thuyền sẽ được trình bày chi tiết hơn, các vị trí đặc biệt sẽ là nguyên nhân gây xói lở cho bờ sông, yêu cầu đến sự bảo vệ bờ sông.


--------------------------------------------
scour=cọ rửa=chùi=tẩy (xờ cau ơ)
indicative=ngụ ý=biểu lộ
stern=sau tàu=mông (slang)
discharge=chảy ra=bốc dỡ
fetch=lộ trình định sẵn
Inland Waterways and Port Modernization project=Dự án 2 tuyến đường thuỷ và nâng cấp cảng (tên 1 dự án VN - ko nhớ rõ)
dominant=trội=có ưu thế lớn
normative=mang tính quy phạm=quy chuẩn
Bassac River=sông Hậu
DAC-computer=có khả năng đây chỉ là một chương trình vi tính tên là DAC nhiều hơn là 1 cái máy vi tính
granular=hình hột=có hột
cohesive=dính=cố kết
granular non-cohesive soil=đất rời ?
clay=đất sét
magnitude=tầm quan trọng=tính trọng yếu
pore=mải mê=lỗ chân lông (có lẽ từ này bị viết sai chính tả -> pure water pressure = ? p re shơ)
Mohr-Coulomb stability diagram=?
derive=nhận thấy=xuất phát
orbital=quỹ đạo
erodability=? (không tra thấy trong từ điển nào erode=xói mòn (i 'rao đờ), LV có erodibility là noun của erode)
empirical=theo kinh nghiệm
stiff=cứng=đặc
conclude=kết luận
assessment=đánh giá
consideration=cân nhắc

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.2.2 Ship induced waves and currents

Design vessel

To assess the normative loading on the banks a design vessel has to be selected. During several site visits and in meetings with River Management Stations it became clear that oversized vessels often make use of the waterways, even though they are not allowed. The proposed design vessel is therefore a 600t self propelled vessel with the following dimensions:

Lengthpp = 42 m
Beam = 8.8 m
Draught = 2.75m

This vessel is one of the largest vessels in the fleet making use of the navigation canals and will give normative loading on the banks under bankfull conditions.

Ship waves and currents

Ships cause a complicated pattern of waves and currents which has been visualised in Figure 4 8. The water level depression with a length of approximately the ship’s length is called the primary wave. From the point of view of an observer on the bank, the primary wave starts with the front wave, followed by the depression and ending with the stern wave. Within the primary wave, this stern wave usually gives the most severe attack on the banks. The much smaller waves that come from the hull are the so-called secondary waves.

Figure 4 8 – Pattern of ship waves in a canal

Limit speed

Regardless of engine power a ship can not sail faster than its own primary wave. This limit speed is analogous to the sound barrier for planes. Self propelled ships can pass this barrier when they are able to plane on the water (e.g. speed boats). The limit speed can be calculated with the following formula:

where:
vl=limit speed
c=wave celerity
L=ship length
h=water depth

For deep water, vl is proportional only with the square root of the length of the ship. In shallow water vl is proportional only with the water depth and the length of the ship does not play a role. When the cross-sectional area of the ship is not negligible to that of the waterway, vl can be calculated with the following formula:

in which:
As= Bs*d=cross sectional area of the ship in which:
Bs=Beam of the ship
d=Draught of the ship
Ac=h*b=cross sectional area of the canal in which:
h=average depth of the canal as defined in Figure 4 9
b=width of the canal as defined in Figure 4 9
Fr=Froude number= with:
vl=limit speed of the ship and
h=average water depth in canal

Figure 4 9 - Parameter Definition Diagram

The advised vessel speed in traffic regulations is normally set at 90% of the limit speed. A higher velocity will lead to:
• Higher velocities of the return currents
• More squat (sinking of the ship as a result of its movement)
• Unfavourable conditions to the canal banks (whether protected or not)
• Higher energy consumption by the ships.

For the purpose of bank design, a speed of 90% of Vl is recommended.

Secondary waves

Secondary waves are caused by the pressure pattern due to discontinuities in the hull-profile. These discontinuities are found at the bow and the stern which both emit waves. The bow is usually dominant. The wave emission from the bow can be thought of a succession of travelling disturbances. Each disturbance creates a circular wave. The envelop of these circles forms the so called diverging wave. Behind the ship the remnants of the circles can be seen as transverse waves. The transverse waves travel in the same direction and with the same speed as the ship, while the diverging waves travel slower. Where transverse and diverging waves meet, there is interference and cusps are formed. From the secondary waves these cusps are dominant for the stability of the bank protection.

Figure 4 10 - Origin of diverging and transverse waves

Figure 4 11 – Pattern of secondary waves

The secondary wave height and period can be calculated with the following formulae:

in which:
Hc=height of the interference peak, the cusps
h=average water depth in canal (see Figure 4 9.)
ς=coefficient of proportionality=1.2 (a reasonable upper limit)
s=distance from ship’s sailing line (corresponding with a ship sailing at the foot of the slope)
T=wave period
Fr=Froude number= with vs=sailing speed of the ship
The normative loading by secondary waves on the canal bank occurs in a wide and shallow canal (limited blockage, small average water depth) when the ship sails close to the riverbank at maximum speed.

Primary waves: return current and stern wave

The return current (ur) can be derived from the following formula:


with z=water level depression

The water level depression can be calculated with the following formula


To take an eccentric position of the ship into account, the values for z and ur can be corrected with the following formulae:

where y and b are defined according to Figure 4 9.

An approximation for the stern wave is:
zmax=1.5zecc

On inland waterways the dominant loading on the banks is generally caused by stern waves.

Propeller wash

For canal sections where ships lie still or manoeuvre near the bank, the slope protection should be designed based on loading by propeller wash. The flow velocities due to propeller wash can be calculated with the following formulae:

with:
P=power of the engine [W]
d=0.7 times the diameter for a normal propeller (when the diameter of the propeller is not known, it can be estimated at 65% of the ship’s draught for a conventional motorship)
w=mass density of water
x=horizontal distance from the propeller in the direction of the flow [m]
r=vertical distance between the propeller axis and the bottom [m]

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.3 Slope protection schemes

1.1.3.1 Introduction

The main objective of slope protection schemes is the protection of slopes susceptible for erosion. However site specifically other objectives could be:

• Beautification of the banks
• Easy access to the water and use of the banks for all kinds of household activities
• Landing stage for boats

aterials and structures used for the protection of inland waterway banks generally include:

• rip-rap;
• gabion/reno mattresses;
• concrete blocks;
• concrete block mattresses;
• open stone asphalt;
• geotextile mattresses;
• combinations of the aforementioned.

In navigation channels in Viet Nam in general the following bank protection schemes can be found:

• rip-rap
• concrete blocks
• concrete mattresses (not much used)
• gabion/reno mattresses
• combinations of the aforementioned

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.3.2 Permanent vs temporary slope protection schemes

As has been mentioned in one of the previous sections, most of the slopes are naturally quite stable, especially when they are vegetated, and will not require any bank protection. However one should realize that after widening the canals at least one side of the widened banks is no longer protected by natural or man made slope protection. In cases in which:

• some erosion, before growing back of the natural vegetation, is not acceptable
• the circumstances are such that the natural vegetation can not easily grow back

nature could be given a hand by constructing a temporary and low cost bank protection which stimulates vegetation of the banks. Most of the available systems are based on strong, natural and biodegradable products to establish the natural vegetation. There are several systems available of which a few examples are given below:


Fascine mattress with one layer of rock and natural vegetation

Natural filter mattress consisting of coconut fibers

Figure 4 12 – Low-cost, environmental friendly, temporary slope protection elements

The Consultants recommend experimenting with the applicability of this type of slope protection during the implementation phase of the project. The performance should be carefully monitored to apply the lessons learned in the cause of this project or future projects.


In the following sections the Consultants will elaborate on the design of permanent slope protection schemes.

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.3.3 Main dimensions and design considerations

Upper level bank protection
It is recommended to construct bank protections to a level that is determined by the maximum wave run-up (Ru) above the highest expected water level in the canal or the natural bank level, which ever is lowest. Constructing the bank protection above the natural bank level would make it an obstruction to the natural flooding and receding waters which is not desirable. As a first estimate Ru= zmax should be taken. At certain locations a path/road has been constructed along the bank. In those cases it is recommended to connect the bank protection to this path/road to construct a continuous protection.

Lower level bank protection
The bank protection offering protection against waves, should be continued to a level that lies 1.5m below the lowest expected water level in the canal to protect the bank protection against damage by smaller ships sailing through the canal and people and animals walking on the banks. Below this level the bank protection should be based on the loading by the return currents and canal flow.

To asses the stability of the natural river bank under different flow conditions the critical flow velocity for various types of clay without vegetation has been summarised in the following table. In addition the reader is referred to Table 4 6.

Porosity 20% 40% 60%
Critical flow velocity 1.5-1.8 m/s 1.0-1.2 m/s 0.1-0.4 m/s
Table 4 6 - Critical flow velocity for various types of clay

Considering the flow velocities in the tables and the flow velocities occurring in the waterways, it is recommended to extend the bank protection to the toe of slope. This is in agreement with general practice in which the bank protection is extended to the toe of slope for canals having a depth less than twice the draught of the loaded ship

Filter layer
A filter layer will be required to avoid loss of soil through the protection layer. The bank slope will have to be compacted and trimmed before laying the geotextile in order to avoid further settlement and assure an even surface. Consecutive layers of geotextile should have a minimum overlap of approximately 0.5m.

Toe
Toe protection is supplemental armouring of the surface in front of a structure which supports the slope and prevents waves from scouring and undercutting it. Factors that affect the severity of toe scour include wave breaking (when near the toe), wave run-up and backwash, wave reflection, and grain distribution of the bottom materials. Toe stability is essential because failure of the toe will generally lead to failure throughout the entire structure. Toe scour is a complex process for which specific guidance has not been developed yet, although some general indicative guidelines are available:

• Scouring caused by waves:

To protect the stability of the face, the toe soil must be kept in place beneath a surface defined by an extension of the face surface into the bottom to the maximum depth of scour. This can be accomplished by burying the toe, where construction conditions permit, thereby extending the face into an excavated trench with a depth of the expected scour. Where an apron must be placed on the existing bottom or can only be buried partly, its width should not be less than twice the local wave height. With slopes milder than 1V:3H and/or a water depth higher than twice the wave height, much of the wave force will be dissipated on the structure face and a smaller apron width may be adequate, but shall be at least equal to the wave height (minimum requirement).

• Scouring caused by currents:

Toe protection against scouring caused by currents may require a less heavy armour, but wider aprons. Special attention must be given to sections of the structure where scour is intensified like at the head of structures protruding in the waterway.

In the canals the expected scour will be limited due to the large depth compared to the expected wave height and the limited currents, justifying a flat toe with a width between 1 and 3 times the maximum wave height. The chosen width will depend on the location and the expected morphological changes. Along river banks the width of the flat toe shall be assessed on the basis of the expected scour. In the detailed design phase the expected scour and consequently the design of the toe structures should be assessed in more detail.

Slope
On average stable slopes in the Mekong Delta can be constructed with a steepness of 1V:3H. At specific locations this can be steeper, sometimes the slopes should be shallower. A slope of 1V:3H is proposed for the feasibility design.

Tie-in
The locations where the bank protections stop and the natural bank line continues, need specific care. Ending the bank protection at those locations without a proper tie-in will leave the bank protection vulnerable for damage caused by bank erosion of the unprotected bank. The design of the tie-in should create a smooth transition between the bank protection and the unprotected bank. The depth of the tie-in, is among others based on the expected bank erosion of the unprotected banks.

Transitions
Transitions in bank protections are sometimes inevitable since different locations on the slope often require different solutions. But at the same time transitions are vulnerable elements, damage often starts in these places. A transition can decrease the strength and/or be subjected to an increased load. The ideal transition is just as strong and flexible as the adjoining layers. As this is very hard to realize transitions should preferably be prevented. In case this is not possible the following aspects should be considered:

• Care: with extra attention during construction, inspection and maintenance many problems can be avoided
• Permeability: when there is a difference in permeability of the bank protection on both sides of the transition, the transition should be dimensioned with the transition in mind
• Overlap: a split down to the bottom material should be avoided, there should always be some overlap between layers

Given the possible negative impact of transitions due to the difficulty in constructing it properly, the Consultants propose to construct the bank protections without any transition, to ensure a high quality and durable bank protection. Having assessed the loading of the banks, the physical characteristics of the banks and the bank protections applied in the Mekong Delta, the Consultants propose to study the following bank protection schemes in more detail:

• Bank protection consisting of riprap
• Bank protection consisting of concrete block mattresses
• Bank protection consisting of reno mattresses

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.3.4 Design approach rip-rap slope protection

Loading by secondary waves
For secondary waves the following formula can be applied:


where:

= system specific stability upgrading factor =1 for rip rap
= stability factor for wave load = 2.25
= wave stability parameter =

Loading by return current
To determine the nominal rock diameter to withstand return flow forces the following formula can be applied:

in which
φ=stability parameter = 0.75
KT=turbulence factor = 1
Ψc=critical dimensionless shear stress = 0.35
u= ur-ecc+ maximum flow velocity in waterway in m/s (maximum flow velocity in canal acquired from hydraulic model)

Kh=depth and velocity distribution factor=
Ksl=slope factor=
= slope angle
β= angle of repose

r=density of rock = 2650 kg/m3
w=density of water = 1025 kg/m3


Loading by stern waves
For the stability calculations to withstand stern waves the following formula can be applied:


Loading by propeller wash
To determine the nominal rock diameter to withstand propeller wash forces the same formula as for the return current can be used. However due to turbulent nature of propeller wash, the turbulence factor should be increased to 4. Propeller wash should be considered for the dimensioning of bank protection in areas where vessels need to manoeuvre near the banks (e.g. in ports). For the design of the regular bank protection this loading will not be considered.

On inland waterways the dominant loading on the bank is generally caused by stern waves, while in wide and shallow waterways the secondary waves can become dominant.


Standard rock gradings
Some common rock gradings which are often used in riprap bank protections, are presented in the table below. It should be realized that especially the 10-40kg rock grading can be easily damaged as a result of walking on the bank protection, without additional protective measures. To reduce the maintenance requirements, it sometimes proves to be worthwhile to select a heavier grading, and consequently higher capital construction costs.

Table 4 7 - Rock gradings and nominal mean rock diameter (based on a rock density of 2650 kg/m3)
Grading W50/Wem Wem, min Wem,max Wem,av Dn50=(W50, av/ρr)1/3
10-40 kg 1.3 10 20 15 0.18 m
10-60 kg 1.3 20 kg 35 kg 27.5 kg 0.24 m


Construction
Based on the calculated Dn50 the thickness of the armour layer can be calculated. The armour layer is generally 1.5-2*Dn50. When the armour layer consists of rock grading 10-60kg or lighter, the armour can be placed directly on the geotextile, and does not require an additional filter layer (assuming a good quality composite geotextile is used).

In the figures below an example is shown of the construction of a rip-rap slope protection.

Figure 4 13 – Example of the construction of a rip-rap slope protection

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1.1.3.5 Design approach reno mattress slope protection

As mentioned in the previous section the normative loading is most likely caused by stern waves on the inland waterways. On wide and shallow waterways secondary waves could become normative. Loading by return currents and propeller wash will not be normative and are therefore not considered in the following sections.

Loading by stern waves
The minimum dimensions of the reno mattresses loaded by stern waves can be determined by using the following formula.


where:
= system specific stability upgrading factor =2.25 for mattress filling by stones
= 1 for stern waves
D = thickness of the reno mattress

Loading by secondary waves
For loading by secondary waves the abovementioned formula can be applied. The wave height to be used in the calculation should be determined with the following formula:

and
= wave stability parameter =

Rock grading
The recommended grading range of stone or rock material for rip-rap bank protections and mattress filling, related to the W50-value (defined by W50 = (D50)3/ρs) is shown in Figure 4 14 (adapted from PIANC, 1987). More specifically:

• The upper limit of the W100 stone should equal the maximum size that can be economically obtained from the quarry, but not exceed four times W50min
• The lower limit of the W100 stone should not be less than twice W50min
• The upper limit of the W50 stone should be about 1.5 times W50min
• The lower limit of the W15 stone should be about 0.4 times W50min
• The upper limit of the W15 stone should be slightly greater than W50min

Figure 4 14 - Recommended grading range of stone filling for rip-rap and mattress systems (adapted from PIANC, 1987)

Construction
For practical reasons, the thickness of reno mattresses shall not be less than 0.15m. The minimum thickness is also depending on the stone filling and shall not be chosen thinner than 1.8D50 (with D50 = Dn/0.85).

Besides the stability of the whole mattress, the weight of the individual stones should be sufficient to prevent excessive movement and thus loads on the wire mesh material. The minimum size of the stones must be larger as compared to the width of the wire mesh. Steel wire used in the mattresses should preferably be both galvanized as well as PVC coated to prevent corrosion. The PVC must be able to withstand UVL degradation. Additional resistance to mechanical damage must be considered - if the risk of mechanical damage, e.g. in urban areas or at moorings, is high then gabions may not be the best solution.

In the PIANC publication "Supplement to Bulletin No 57 (1987)", on Guidelines for the Design and Construction of Flexible Revetments incorporating Geotextiles for Inland Waterways it is stated that when the current exceeds 3m/s or the wave height exceeds 1m then a granular sublayer (about 0.2m thick) should be incorporated. In other cases it is satisfactory to place the gabion directly onto the geotextile. It is important that both the subsoil as well as the stone filling inside the mattress are adequately compacted.

The figures below show reno mattresses applied in the Cho Gao Canal.

Figure 4 15 – Reno mattresses in the Cho Gao canal

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.3.6 Design approach concrete block mats slope protection

Introduction
A wide range of concrete block mats have been developed all over the world. The practical experience obtained from subsequent use has enabled further optimization of the design and use. Block mats are usually employed for protection of embankments and bed in small water courses. In principle there are two types of block mats: mats with and mats without a fabric filter. In the case of a mat without a fabric filter, the geotextile is applied separately in the structure. Examples are cable mats, comprising of concrete blocks which are usually arranged on steel or synthetic cables forming a rectangular mat with the blocks arranged in a staggered pattern. Block mats with a filter fabric are made by casting the concrete blocks directly onto the fabric or by joining afterwards, as prefabricated blocks. The jointing members comprise of e.g. flanged pegs of synthetic material which are pierced through the fabric cloth.

Most concrete block mattresses are patented by the firms who have developed these systems. The consultants have contacted both firms abroad as well as in Viet Nam to survey the market of concrete block mattresses, to complement their knowledge of the applicability of these systems and the costs involved. There are numerous systems available but no selection has been made of the preferred system as part of this feasibility study. In general all systems can be produced under license in Viet Nam. The Consultants recommend studying these systems in more detail during the detailed design, and contact potential suppliers abroad and in Viet Nam to provide quotations. In this respect ‘economies of scale’ apply and it is recommended to apply one system for the whole project.

The Consultants is aware that concrete block mats are not widely used yet in Viet Nam although there are a few examples where these have been applied successfully. The ease of construction, the acceptable cost, and the quality which can be achieved, make the Consultant believe that this alternative, although not widely used yet, will be applied a lot in the near future. Some examples of the use and construction of concrete block mattresses are presented in Figure 4 16.

Figure 4 16 - Examples of the use and placing of concrete block mattresses

Loading by stern waves
The minimum dimensions of the block mattress can be determined by using the following formula:

where:
= 2.0
1 for stern waves

c=2300 kg/m3
w=1025 kg/m3

Loading by secondary waves
For loading by secondary waves the abovementioned formula can be applied. The wave height to be used in the calculation should be determined with the following formula:

and
= wave stability parameter =

Construction
The concrete block mattresses should be placed on a geotextile or connected to the geotextile. These mattresses should be placed on the bank (after compaction and trimming) without additional filter layer. The voids between the blocks should remain open and not be plastered, to allow for drainage of water, to prevent pressure build-up under the slope.

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.3.7 Design approach geotextile filter layer

In general a woven type geotextile is used for strength, the unwoven type for sandtightness. Depending on the subsoil, sometimes a woven geotextile can also guarantee sandtightness. As the soil investigations show predominantly clay, it is most likely that a composite geotextile consisting of a woven and a non-woven type should be used. Below a design approach has been presented to be able to specify the required geotextile. In absence of sufficient soil data only a limited number of properties can be determined as part of this feasibility study.

• Weight of geotextile (for rip rap)

The required weight of a geotextile, expressed as a function of the drop height of the rock material (to be applied in case of a riprap bank protection) and the dimensions of the rock itself, can be determined with the following formula:

where :
Ma=mass of the geotextile per unit of surface area [kg/m2]
H=drop height of the rock material [m]
D85=characteristic grain size of rock material [m]
cs=damage factor by mass dumping [-]:
cs = 1.20 : no damage to geotextile
cs = 0.75 : 10% of the surface is damaged
(the values of cs are based on practical observations)

• Filter stability

The stability rule for stationary flow through geometrically closed geotextiles is given by:

O90<2 D90b

For cyclically loaded geotextiles 2 to 4 times lower values for the apertures should be used, depending on the permissibility of some sediment loss through the geotextile.

• Permeability

Usually a geotextile is more permeable than the subsoil. However two phenomena can decrease the permeability: blocking and clogging. Blocking occurs when large particles seal the openings in the textile. Clogging is the trapping of (very) fine particles in the openings of the geotextile. This can happen when water is contaminated with chemicals, e.g. iron. In contrast to blocking, clogging is a time-dependent process. The idea is that it stabilises at a certain level after a certain time, but not much experimental evidence is available to support this thesis. When the permeability of the geotextile is 10 times greater than that of the subsoil, there is usually no significant pressure build-up, not even when clogging takes place. However when used in very contaminated (ground)water, one should be very careful. In this respect the influence of acid-sulphate soils on the performance and durability of the geotextiles should be taken into account. When the danger of blocking or clogging can be ruled out, a permeability ratio of 2 or 3 is sufficient.

• Stability between riprap and geotextile

To prevent the rock on the geotextile from sliding down the slope, friction is needed between the filter layer and the geotextile. The amount of friction follows from:
fu W cosα ≥ W sinα
where:
W= weight of rock [kg]
α=slope angle [deg]
fu=friction factor between rock and geotextile=0.70 +/- 0.05 for riprap on geotextile
When fu > tanα the rock will not slide down the slope, a requirement which is usually met.

 Overall stability
The equilibrium demand for the whole protection, including geotextile, on a slope reads:
Ft>ρmEFF1 g d Δx1 (sinα – f cosα) + ρmEFF2 g d Δx2 (sinα – f cosα)

where:
Ft=tension force in the geotextile
fb=friction factor between geotextile and subsoil which is usually in the range of (0.6 -0.9)tanφ
ρmEFF1 = effective density of the layer as a whole above water
ρmEFF2 = effective density of the layer as a whole under water

Δx1= length of the slope above water [m]
Δx2= length of the slope under water [m]
d=thickness of the layer [m]

Low water determines the maximum tension force, since the uplift effect from the water is then minimal. On the basis of the maximum tensile force the required tensile strength of the geotextile (warp and weft) can be determined.

• Geotextile properties dependent on construction method

Depending on the construction method the geotextile should have certain properties. For example, when applying concrete block mattresses which are connected to the geotextile, the geotextile should have enough tensile strength to allow lifting of the mattress.

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1.1.4 Basic evaluation of studied permanent slope protection alternatives

Based on interviews with various stakeholders and based on the experience of the Consultants with the design of bank protection in general, and in the Mekong Delta in particular, the following table has been put together, listing the advantages and disadvantages of the considered alternatives.

Riprap
Advantage

• Rip rap bank protection is the cheapest of the considered alternatives
• The flexibility of a riprap bank protection is unmatched by other bank protection types
• The self healing effect of a riprap bank protection allows for limited damage before maintenance is required

Disadvantage

• Sensitive for vandalism. There are examples of riprap bank protections being stripped of their armour layers in Viet Nam. Alternatives with asphalt penetration will reduce this risk, but will increase the price.
• Requiring regular maintenance
• Not considered beautiful

Concrete block mattress
Advantage

• When constructed well a durable effective bank protection
• Considered beautiful
• When constructed well, requiring minimum maintenance

Disadvantage

• Expensive compared to other bank protection alternatives
• Not much experience in Viet Nam, most likely requiring foreign expertise
• Sensitive for construction errors

Reno mattresses
Advantage

• Applied with success in Viet Nam, constructions function satisfactory
• Not sensitive for vandalism, people tend to respect the structure

Disadvantage

• Expensive compared to riprap bank protection alternatives
• Sensitive for damage
• Requiring regular maintenance
• Not considered beautiful

Table 4 8 - Basic evaluation of studied permanent slope protection alternatives

One of the key criteria for evaluating bank protection alternatives is the durability of the bank protections. Sensitivity for vandalism, make the riprap alternative not suitable without additional protection like asphalt penetration. Asphalt penetration will increase the price of the riprap bank protection to such a level that it will not be competitive to the other bank protection alternatives.

Of the two remaining alternatives only the concrete block mattresses are considered beautiful and will be acceptable in urban areas. However this will involve higher capital costs compared to the reno mattresses. Nevertheless it is envisaged that, in case the structures are constructed according to (international) standards, the structures will require less maintenance than reno mattresses. Outside the urban areas reno mattresses will be applied as in these areas the structure will mainly be evaluated on the basis of their functionality.

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.5 Areas considered for slope protection along main waterways

1.1.5.1 Introduction

After numerous site visits and discussions with stakeholders the Consultants propose bank protection at the following locations:

Corridor 2:

• Thu Thua town, over a length of approximately 1.3 km, along the north bank of the waterway
• My An town, over a length of approximately 0.4 km, along the south bank of the waterway
• Phong My town, over a length of 1.45 km (total), along both banks of the waterway and along the river

Corridor 3:

• Cho Gao canal, over a length of approximately 2.5 km (total), mainly along the south bank of the waterway

The proposed bank protections will be discussed in more detail in the following sections.

Ðề: Thử thách dịch thuật về Đường Thuỷ

1.1.5.2 Thu Thua town

Introduction
Thu Thua town is a busy town along the K. Thu Thua. 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 bank erosion the northern bank will have to be protected by means of bank protection. In case it fits in the spatial planning plans of the city, the Consultants propose to make the channel bank accessible to the public by means of a footpath and/or road.

Figure 4 17 – Thu Thua town

Proposed slope protection Thu Thua town
The proposed slope protection will have a length of approximately 1300m as shown in the figure on the next page. The slope protection will consist of concrete block mattresses. To limit the resettlement requirements a small retaining wall is proposed along the top section. The reader is referred to the figures on the following pages.


Figure 4 18 - Plan view slope protection Thu Thua town (1)

Figure 4 19 - Plan view slope protection Thu Thua town (2)

Figure 4 20 - Cross section slope protection Thu Thua town (A-A)

Figure 4 21 - Cross section slope protection Thu Thua town (B-B)

Figure 4 22 – Stairs slope protection Thu Thua town