The main scopes of physical model research In this paper, following main scopes of the project will be mentioned for functions of dredging channel and sand protection groin verification
Trang 1
Proceedings of the 19 th IAHR-APD Congress 2014, Hanoi, Vietnam
ISBN xxx-xxxx-xx-x
1
THE REASONS FOR SEDIMENTATION, EROSION AND SHIFTING CHANNEL IN CUA SOT
ESTUARY (HA TINH PROVINCE) ON PHYSICAL MODEL
NGUYEN NGOC NAM1, PHAM ANH TUAN1, DANG THI HONG HUE1
1 The National Key Laboratory of Coastal and River Engineering, Hanoi, VIETNAM
e-mail: 1nguyenngocnamtl@gmail.com
ABSTRACT
Cua Sot is one of the most important estuaries of Ha Tinh province, where trade, exchange, the export of agricultural products, seafood with other province in the country The estuary is also important waterways to fishing ports of Ha Tinh
in the strategic development of fishing offshore of Ha Tinh province Recently, Cua Sot channel is being filled, narrow fairways dry up, affecting large vessels from anchorage, storm shelter and annual maintenance dredging was needed To address that, localities are going intended to build a number of groin and embankments and the measures to dredging but also still being disturbed about the durability of embankments and the ability of filling sedimentation quickly Therefore, the local managers have difficulty in finding measures to overcome this risk Aiming at positive measures, finding the causes and mechanism to change the channel flow, finding a solution basis to propose appropriate measures for the sustainable regulation, the data were collected as historical data, field survey data relating to topographical and hydrological data and so on Physical model of tidal dynamics, sediment transport were calibrated and verified and used
as a tool for the study of dynamics and sediment transport of Cua Sot estuary The experiment results with different scenarios are presented in this papers and the clarification of the causes and mechanism of sediment transport in Cua Sot estuary was demonstrated
Keywords: Estuaries, erosion, deposition, sediment, waterway lane
1 INTRODUCTION
1.1 Geographical location
Cua-Sot port locates in administrative area of Loc Ha
district, Ha Tinh province with coordinations as from
18o27’10” to 18o26’58” North latitude, from 105o55’09”
to 105o54’43” East longitudinal Cua-Sot estuary is
approach 12km far form National highway No.1A and
16km far from Ha-Tinh city with advantage
transportation conditions for socio-economic
development
Figure 1: Location of project selected for research
Figure 2: Detailed location of the project being selected for
model research
Figure 3: The general model for design alternative’s - view
from upstream river to sea
Trang 21.2 Objectives and scopes of model research
1.2.1 Objectives and targets of research for project
The main objectives of the project is to build completely
technical infrastructures to minimize the adverse impact
factor due to natural disasters, storms and floods, and to
increase the efficiency of existing projects to meet
growing demand of local fishermen, fishing
productivity growth for sustainable, economic
development
To strength preventive ability on natural disaster and
rescue of victims, to maintain stability of transportation
waterway and to limit sediment in waterway into the
Cua-Sot fish port, ensuring the ship and boat ease to
access on waterway with more safety, shorter and more
convenient
To enhance drainage capacity of flood flow through
estuary and the security of sea, offshore and islands
1.2.2 The main scopes of physical model research
In this paper, following main scopes of the project will
be mentioned for functions of dredging channel and
sand protection groin (verification, revised, proposed
alternatives through hydraulic model test results):
- To maintain drainage capacity of flow through
estuary;
- To maintain water surface level meeting requirement
of normal navigation conditions in order to easy access
of boat and ship and minimum flow with Z>-0.12m;
- Hydraulic regime: to maintain hydraulic condition for
ease port-access and shelter of boat and ship;
- Trend of erosion and sediment: to reduce trend of
erosion and sediment in whole dredging channel and
waterways
1.3 The main parameters of project
1.3.1 The main parameters of navigation channel and
waterways
In Feasibility study phase, The Design Consultant (DC)
selected navigation channel and waterways for
alternative No 1 as follow:
- Section inside estuary: Based on update topographical
survey done in 2011, selected channel will be along
deeper inner stream with angle of lane is approach 40o
- Section outside estuary: Based on update topographical
survey done in 2011, selected outside lane will be
parallel north-direction and along outer stream beside
estuary with angle of lane is approach 73o19’ and 3o30’
1.3.2 The main parameters of sand protection groin
Layout of groins based on design alternative will
perpendicular of contour of sea shore, focusing flow
into way for maintanance of stability of navigation lane
as well as increasing drainge flood capacity at outside
estuary
Table 1 Length and coordiantes for positioning of sand protection groin in Design alternative
3 Coordination X 2041252.535 2041794.123
4 Coordination Y 491571.777 492077.804
Groin’s length is 740m, width is 3m, top crest elevation
is +2, type of structure is reinforced concrete pipe (RC) with diameter of 1000mm, length of RC pipe is 3m
2 DATA COLLECTION AND METHODOLOGY
2.1 Data collection
In process of research, data collection was done with related references regarding to factor causing sediment, erosion and morphological change in river connecting with Cua-Sot estuary as following:
- Discharge and water level in river recorded in drought and flood seasons at typical years;
- Sedimentation of bed sand outside Cua-Sot estuary, including sand physic-mechanic features and gradation curve;
- Speed, direction and grade of wind that relate to design standard being applied in Cua-Sot estuary;
- Oceanographically design done by design Consultant that being applied for Cua-Sot estuary;
- Current topographical survey and older done in previous time; and
- Drawings of layout map for research area, boundary of groin and navigation waterways
2.2 Research physical hydraulic model Physical hydraulic model was constructed with following main parameters: a) Model style: general distorted hard bed; b) Model distorted scales: hor =180 and ver=40; c) Boundaries of prototype: total length: 5000m, maximum width: 200m, height: 9.5m (from sea bed elevation -4.5m to elevation +5m) Boundaries of model: total length (L) is 35m, maximum width (B) is 10m and height (H) is 0.5 m;
Model research was conducted with main north-east wind direction (accounting for 70% annual wind time) and two cases of flow direction which are: a) Tidal flow: from sea to river; and b) Drainage flood flow (upstream flood flow going to estuary as drainage requiring): from river to sea
Based on design alternative, physical hydraulic model experiments results will provide solutions to overcome the reversed phenomena, propose the solution for training schemes, boundary and scope of protection In whole time of model test, research team is always close contact with project owner, design consultant and relative agencies for purpose of step-by-step agreement making in order to get proposed alternative with high efficiency
2.3 Methodology
In this research, research team was followed-up the
research of design consultant (by statistic analyzing,
Trang 3morphopological process analyzing and mathematical
analyzing methods) and verifying the results by physical
hydraulic model test in order to simulating trend and
changing of dynamic axis flow through Cua-Sot estuary
Based on intended cost estimation providing for this
project, natural condition, socio-economic conditions,
and technical condition, the process of hydraulic model
test were conducted step-by-step that from beginning
design alternative, revised alternative and proposed
alternative served for construction drawings
III RESULTS AND COMMENTS
3.1 Calculation results from design consultant
Coastal near bank flow is formed by the action of waves
and wind from offshore moving into the onshore
Waves in Cua-Sot area have most of their frequency in
both directions as north (11%) and north-east (68.4%)
Two main above-mentioned wave directions are main
cause of enormous sediment transport in Cua-Sot area
To quantify the amount and determination of direction
of sediment transport, in the study of sediment
transport models were calculated for the following
conditions:
a) Two calculated sections in west (MC.Q1) and east bank (MC.Q2) going to -1.0m value of contour;
b) Monsoon velocity wind of 15m/s (99% frequency in the theoretical frequency for annual maximum wind speed);
and c) Calculation of existing conditions in both directions of north and north-west waves
Calculation results of sediment transport through Q1 and Q2 section are compiled in Table 2
Table 2 Sedimentation quantity changing between cross-section Q1 and Q2 (cubic meter per year)
Seq
Q1 711.008 71.707 660.122 66.213 1.509.051 462.61
4
Q2 491.995 54.046 451.105 49.292 1.046.437 (Remark: sign (+) shows sediment transport follow north-south direction)
1:00:00 60
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170
(Grid spacing 10 meter)
Figure 4a Velocity profile in Cua-Sot estuary and port in
present condition at time of tidal level going down
0 20 40 60 80 100 120 140 160 180 200
(Grid spacing 10 meter)
Figure 4b Velocity profile in Cua-Sot estuary and port in
present condition in time of tidal level going up in high tidal
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
(Grid spacing 10 meter)
Figure 5a Sediment current distribution at Cua-Sot port and
estuary affacted by wave in north direction in wind time
period having 15m/s velocity
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
(Grid spacing 10 meter)
Figure 5b Sediment current distribution at Cua-Sot port and estuary affacted by wave in north-east direction while wind
time period having velocity as 15m/s Resutls have shown that almost sand transportation from outside estuary to be setteled at navigation lane and site nearby fish port
Figure 6 Layout of physcial hydraulic scaled model for Cua-Sot estuary improvement project – various model test alternatives
3.2 Hydraulic model test results
Trang 4Physical hydraulic model tests for design, revised,
additional revised and proposed (final) alternatives are
followings:
3.2.1 Model test for design alternative
3.2.1.1 Model of design alternative:
a) Tidal flow case:
- Drainage capacity through the estuary: The design
alternative for groin line maintaining discharge flow
and ensure the water level for navigation while tidal
going down, including weak tide cases under the design
frequency (Maximum vessel dead weight is 500 DWT
and the water level ship H 99% = - 0.12 m)
- Hydraulic flow regime: When the low tidal flow with
small discharge and low water levels tidal flow lower
than the top crest of groin elevation, flow from sea to
river with main flow in outer navagation lane and sand
dune When tidal level is increasing gradually, the
near-bank flow over top crest of groin similar to flow over
weir with rapid velocity
- Flow from sand dune goes across inner waterway and
fish port Flow in outer waterway and passes through
begin groin to junction of inner lane and deep stream
The main flow focus in deep stream then discharge
from deep stream runs to river In begin of inner
waterway occurs vortex flow Flow in near fish port is
gradual and making vortex extending into inner shelter
- Trend of sediment and erosion: when tidal level rising
up and flow over top of crest of groin with high flow
velocity behind groin, the tidal water was making
erosion toe of groin and flow push sand from sand dune
behind groin to inner waterway In fish port location
inner waterway with normal flow velocity addtion by
vortex flow will make deposition of sand at inner lane,
port area and boat shelter inside
b) Drainage flow case:
- Flood drainage capacity: design alternative was
maintaining capacity through the estuary while
drainage flow requirement (from river to the sea)
- Water surface elevation: The water level along the
waterway was maintained for boat and ship easily to
access to port and shleter
- Hydraulic flow regime: the case of flood drainage
while water level in river is lowthen flow from river to
the sea occuring in deep right side stream (hill side) and
following outer lane going to sea In case of high river
water level (high flood) drainage flow with high
discharge shall run over top of groin making rapid flow
in river groin as well as outer stream
- The trend of sediment and erosion: Drainage flow
from river to the sea in case of low water level in river
was through deep stream and waterway lane bringing
sand from river to sea While high flood occurred, both
river water and sea increased, flow also forced sand and
sediment from river to sea However, due to large
discharge flood flow and high velocity of flood over
crest of groin shall cause unstable of groin’s line and
scouring of foundation in both sides Figure 7 (from case
a to case e) below indicates various flow and sediment
through Cua-Sot estuary in design alternative research
case
Figure 7a The physical hydarulic model general layout
Figure 7b Flow from sea to estuary in case of design
alternative
Figure 7c Flow in portion between groin and right hill side
Figure 7d Sediment in waterway lane in case of having groin
being constructed
Figure 7e Sediment in waterways in case of without groin
Trang 5The result of the experiment has shown that the average
annually sediment deposition in fishing port area (in
prototype) was about 220.000m3 In case of without
sand protective groin, it is verified that annual average
sediment in fisf port was aprroach 300.000m3 that higher
than 80000 m3 compared with case of having groin
3.2.1.2 Reason of sediment waterway and fish port
Based on model test results from design alternative and
verified in case of without groin, there are some reasons
that causing erosion and sediment in waterway and fish
port was to be found as below:
- Annual average flow from upstream going to sea
decreaded;
- Dynamic axis flow deviates onto right stream can
make discharge in left stream (through fish port)
decreasewith low flow velocity, causing sediment;
- Tidal flow;
- Tidal dynamic near-bank flow; and
- Wave and relative waves caused by wind, storm and
typhoon
3.2.1.3 Proposing revised alternative
With a high flow velocity behind groin will cause
erosion in the foundation and move sediment from sand
dune behind groin Based on design alternative results
test, the hydraulic parameters will be tested for
proposing the revised alternative upon lane layout and
groin as below:
- Reducing length of groin into 600m (nearly 140m
being reduced), elevation of crest of groin being
changed from +2.0m to 1.0m with inclined direction
from shore to sea;
Bottom of end groin point (sea side) being located in
-2.0m in elevation (maintaining height of groin’s
structure is of 3.0m);
- Adjusting the inside part of lane with gradual curving
in expanded location of lane; and
- End portion of lane nearby fish port being dredged
through boat lane into boat shelter
3.2.2 Model test for revised alternative
3.2.2.1 Model of revised alternative:
a) Tidal flow case:
- Drainage through the estuary: ensure discharge flow
through the estuary and water depth for boat and ship
movement when low tidal period ocurred were to be
maintained;
- Hydraulic flow regime: condition flow in inner lane
and fish port is improved comparing to design
alternative, especially in low flow from sea to river is
going in outer lane to conjunction of inner lane and
deep tream with more discharge flow into river and fish
port However, trend of moving to left of main dynamic
axis flow is still not satisfying because of flow is in deep
stream In begin of inner lane is having vortex flow,
flow is gradually going and ocuring long vortex area
extending into inner shelter The advantage of revised
alternative when applying pre-dredging extending to
river (revised alternative No 1) is that flow with more
higher discharge and smaller vortex comparing to previous design alternative
- Capacity sediment erosion: in case of tidal water rising, tidal flow from sea to river with shoreline flow is over top of groin with high velocity behind groin but falling flow is reduced comparing with design alternative due to elevation of top groin was lowered Foundation of groin is to be put deeply into sand shall increase stability of groin Sediment is from sand dune behind groin to inner lane, following tidal overtop flow Flow in inner lane with still flow and low velocity combining with vortex flow shall cause deposition in inner lane, fish port and boat shelter However,due to dredging in lane was made then sediment situation is more improved and less deposited
b) Drainage flow case:
- Drainage capacity: Maintaining capacity of flood drainage;
- Water surface level: Water surface level along lane maintains water depth for boat and ship transporting assess into port;
- Hydraulic flow regime: revised alternative has more advantage than design alternative with vortex flow is reduced When drained flood with low river water level flow from river to sea, the flow focused mainly in deeply right-side stream (hill side) and following to outer stream to sea In case of high river water level (big flood) drainage with large discharge and flow over top
of groin was causing rapid flow in both river and groin location
- Capacity of erosion and sediment: drainage flow from river to sea when river water level is low, the flow is running in deep stream and outer stream brings sediment and sand from estuary to sea When draining with high flood having high river water and tidal level, flow through estuary with high velocity and high energy slope shall push sand and sediment from river to sea However, due to high discharge flood and flow over top of groin with high velocity shall cause unstable groin and scouring in both side of groin’s foundation
Figure 8a Flow over groin in revised alternative model test
Trang 6Figure 8b Flow on portion going to port in revised alternative
model test
Figure 8c Sediment reduction behind groin in revised
alternative model test
Figure 8d Sediment reduction in Cua-Sot fish-port in revised
alternative model test
3.2.2.2 Advantages of revised alternative
Revised alternative has many advantages comparing
with design alternative: Distribution of flow discharge
increases more than 5% to the lane nearby fish port
Water surface profile, wave and hydraulic regime are
more stable Value of velociy and pressure at critical
positions are not high or unusual that no requirement to
consider Model test results show that sediment in lane
nearby fish port is decreasing lower than design
alternative Howerver, annual average sediment in fish
port area is still large with approaches of 180000 m3 per
year Therefore, it is required to find the way reducing
that sediment amount After conducting addition
revised alternative, sediment in this area continued
decreasing to 25% that equal to 140000 m3 per year
3.2.2.3 Disadvantages of revised alternative:
Capacity of discharge separation in left stream (through
fish port) increseas just 5 to 10% compared with design
alternative (reaching from 25% to 33% of total flow in
both streams);
Trend of erosion and sediment: lane flow through fish port is gradual with low velocity adding by vortex cause sediment in inner lane, fish port area and shelter inside
If dredging in inner lane being extended to river inside then sediment in navigation lane and fish port, but drainage capacity of inner lane is low compared with deep stream at connection location of two streams then dynamic axis flow behind groin is still in right side stream, causing deposited area in begin of left channel (lane nearby fish port) Hence, purpose of separating flow discharge in both lanes is not reached
Although the revised alternative provided solution with adverse impact being more reduced due to flow over top of groin, but structure of groin is still not stability in operation Therefore, the research team continues to conduct proposed (final) alternative in order to apply in construction drawings
3.2.3 Model test results for proposed (final) alternative applied in Construction Drawings stage
Figure 9a Flow from sea to river in proposed alternative
Figure 9b Flow over groin and sand dune behind in proposed
alternative
Figure 9c Flow direction into section of separate discharge in
proposed alternative
Trang 7Figure 9d: Flow direction when passing fish port in proposed
alternative
Figure 9e: Flow direction over groin when tide raise up in
proposed alternative
Figure 9f: Wave over groin when tide raises up in proposed
alternative
Figure 9g: Flow direction when coming to junction between
two streams in proposed alternative
Figure 9h Flow combining behind fish port in case of flow from river to sea in proposed alternative
a) In tidal flow case
- Drainage through the estuary: capacity is maintained
in both low tidal and peak tidal flow
- Hydraulic flow regime: main axis flow is changed to inner lane (through fish port) in case of low water flood flow and flow running along lane fluently into river In case of low tidal flow with low discharge and water in lowere than top elevation of groin, flow is distributed equally in both lanes (inner lane through fish port and deep stream in hill side) In case of peak tidal flow, discharge flow into inner lane and fish port was being reduced but remaining stability of fish and boat assess
- Capacity of erosion and deposition: when tidal level raise up the flow from sea to river is over top of groin and reaching inner lane Because of flow velocity nearby fish port in inner lane approaching 1m/s causing transportation of sand and sediment along lane to river then sediment capacity in inner lane, fish port and shelter will be reduced
b) Drainage flow case:
- Drainage through the estuary: capacity is maintained;
- Water surface level: level along lane is maintained to ensure assessement of boat and ship into port;
- Hydraulic flow regime: in case of drainage flow, hydraulic regime in inner lane (through fish port) is improved with small drained flow and low water level, but when high flood flow, the main flow is still in right lane (hill side) Due to high discharge then flow is over and expansion in both deep stream and inner lane to make discharge distribution in lane through fish port (left lane) being more better than previous, guaranteed flow for boat and ship assessment more convinent;
- Capacity of erosion and deposition: drainage flow from river to sea when river water level lowered is going in deep stream and lane to transport sand from river to sea In case of high flood flow with high river water and tidal water level, steep flow is also pushing sand from river to estuary High flood and flow over top of groin with high velocity shall cause instability of groin and erosion in both sides of groin’s foundation Begin section of groin (nearly 250m long) is still safe if reinforcing of structural strength will be conducted
IV CONCLUSIONS AND RECOMMENDATIONS
4.1 Conclusions The research team of National Key Laboratory of Coastal and River engineering has carried out various model tests to satisfy with requirements of present
Trang 8regulations and standards with high quality to serve for
detail design of the project Model test results have
found the cause of erosion and deposition in waterway
assess to Cua-Sot fish port and shelter in Loc Ha district,
Ha Tinh province The main causes are following:
a)Annual average flow from upstream to sea
decreasing;
b) Dynamic axis flow comes to right side stream shall
make discharge in left side stream lowering (passing
through fish port) and water velocity decreasing
causing deposition of sediment;
c) Tidal flow;
d) Tidal near-shore flow; and wave and relative wave
from wind, storm and typhoon
Based on finding causes that got from step-by-step
careful study in model test results, the research team
has proposed final alternative with advantages as
follow:
- Adverse impact due to deposition of waterways, fish
port and shelter caused by tidal flow in Cua-Sot area is
being reduced;
- Partial advese impact of near-shore dynamic flow is to
be deducted;
- Dynamic axis flow will be moved to left side of main
stream (fish port side) that increasing portion of flow
separation into 10-15% comparing design and revised
alternatives;
- Sediment was reduced in fisf port and shelter If final
alternative will be applied then the annual sediment
quantity is to be dredged approach 100000m3;
- To train the waterway more suitable; and
- To reduce dredging volume equal to 55000m3
4.2 Recommendations
Based on model test results of final alternative, research
team recommended project owner and design
consultant upon research results and revising design of
lane of waterway and groin depending on cost
investment, geographical condition, structural,
geotechnical and constructional conditions as follows:
- Layout of groin should be maintained as mentioned in
alternative No 1, length of groin should be 600m
Elevation of crest of groin should be lowered gradually
from 2m to 1m Typical standard height of structural
groin should be 3m (from bottom to top) Reinforcing
steadly groin body and foundation in range of 50-70m
along cross section and 250m from begin of groin to the
sea along longitudinal section;
- Dredging inner waterway lane nearby fish port as
recommended by research team: changing of angle of
lane is 84015’ and continuing dredging into river side;
- To plant the sea-side tree and sand-flying protective
vegetation in sand dune of both sides of groin;
- Constructing additional groin in order to reduce wave impact and dynamic near-shore flow; and
- Applying the extension suitable training method with target as driving dynamic axle flow coming to left side stream (pass through fish port) etc
Parameters of new boat transport lane and groin for final alternative is as following:
- Length of the old dredging channel: 3200m
- Length of the new dredging channel: 3135m
- Dredging volume decreased by: 55.000m3
- New dredging channel goes along the side of the deep stream with the alignment angle is about 45o
Table 3 Parameters of new waterway lane applying for proposed (final) alternative
Section
of lane Direction of lane Length (m) Corrdinates of begin point Remark
X (m) Y (m) inner 84o15’ 1295 2040832 492064 Bend angle
69o48’ outer 3o24’ 1840 2043155 492370 to -3.50
Table 4 Length and coordinates of groin position for proposed (final) alternative
2 Top crest elevation 2.00 1.00
3 Coordinate X 2041252.54 2041690.95
4 Coordinate Y 491571.77 491981.40
ACKNOWLEDGMENT
The group of authors expresses their thanks to Key National Laboratory of Coastal and River Engineering and Hydraulic Research Center as well as colleages whom participated the model research in support and helpful assistance while testing physical hydraulic model for Investment project of dreging and waterway lane training into Cua-Sot boat and sea-storm shelter, Loc-Ha district, Ha-Tinh province
REFERENCES
Nguyễn Ngọc Nam, Phạm Anh Tuấn, Đặng Thị Hồng
Huệ et all (2014), Report on hydraulic and oceannographic model test results under investment project for dreging and navigation waterway training into Cua-Sot boat and sea-storm shelter, Loc-Ha district, Ha-Tinh province, Hanoi Project Investment Report (2014), “Investment project for construction of dreging and navigation waterway training into Cua-Sot boat and sea-storm shelter, Loc-Ha district, Ha-Tinh province”, Register No HT-TK4/04-2011,
Haiphong
Structure of boat shelter – port (1992), Sector Standard
No 22 TCN 207:1992, Hanoi Guidelines for sea channel design (1973), Institute of Transportation Design, Hanoi.