Proceedings of the 19th IAHR-APD Congress 2014, Hanoi, Vietnam ISBN xxx-xxxx-xx-x 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 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 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 2: Detailed location of the project being selected for model research Figure 1: Location of project selected for research Figure 3: The general model for design alternative’s - view from upstream river to sea 1.2 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 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 Length and coordiantes for positioning of sand protection groin in Design alternative Seq Items End-point Begin-point Parameter Groin Length (m) 740 Coordination X 2041252.535 2041794.123 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 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, morphopological 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 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 Table Sedimentation quantity changing between crosssection Q1 and Q2 (cubic meter per year) III RESULTS AND COMMENTS 3.1 Calculation results from design consultant Tidal up 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: Seq Tidal down North Northwest North Northwest Q1 711.008 71.707 660.122 66.213 Q2 491.995 54.046 451.105 49.292 1.509 051 1.046 437 462.61 (Remark: sign (+) shows sediment transport follow north-south direction) (Grid spacing 10 meter) (Grid spacing 10 meter) 200 190 180 170 160 150 140 130 120 110 90 100 80 70 60 50 40 30 20 10 170 160 150 140 130 120 110 90 100 80 70 60 50 40 30 20 10 0 60 11/12/04 21:00:00 20 40 80 60 100 80 180 100 120 140 (Grid spacing 10 meter) 160 140 120 (Grid spacing 10 meter) 160 200 180 220 200 (Grid spacing 10 meter) 220 Figure 4a Velocity profile in Cua-Sot estuary and port in present condition at time of tidal level going down 200 180 160 140 120 100 80 60 40 20 40 11/13/04 04:00:00 20 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 60 80 Resutls have shown that almost sand transportation from outside estuary to be setteled at navigation lane and site nearby fish port 100 120 140 160 (Grid spacing 10 meter) 180 200 220 (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 200 190 180 170 160 150 140 130 120 110 90 100 80 70 60 50 40 30 20 10 0 20 40 60 80 100 120 140 (Grid spacing 10 meter) 160 180 Figure Layout of physcial hydraulic scaled model for CuaSot estuary improvement project – various model test alternatives 200 220 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 3.2 Hydraulic model test results Physical 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) Figure 7a The physical hydarulic model general layout - 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 nearbank 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 Figure 7b Flow from sea to estuary in case of design alternative - 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) Figure 7c Flow in portion between groin and right hill side - 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 low then 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 (from case a to case e) below indicates various flow and sediment through Cua-Sot estuary in design alternative research case Figure 7d Sediment in waterway lane in case of having groin being constructed Figure 7e Sediment in waterways in case of without groin The 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 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 - 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 Figure 8a Flow over groin in revised alternative model test 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 Figure 8b Flow on portion going to port in revised alternative model test 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 8c Sediment reduction behind groin in revised alternative model test Figure 9a Flow from sea to river in proposed alternative 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 Figure 9b Flow over groin and sand dune behind in proposed alternative 3.2.2.3 Disadvantages of revised alternative: Capacity of discharge separation in left stream (through fish port) increseas just to 10% compared with design alternative (reaching from 25% to 33% of total flow in both streams); Figure 9c Flow direction into section of separate discharge in proposed alternative Figure 9d: Flow direction when passing fish port 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 Figure 9e: Flow direction over groin when tide raise up in proposed alternative - 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; 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 - 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 regulations 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; - 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 c) Tidal flow; - Dredging volume decreased by: 55.000m3 d) Tidal near-shore flow; and wave and relative wave from wind, storm and typhoon - New dredging channel goes along the side of the deep stream with the alignment angle is about 45o 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; Table Parameters of new waterway lane applying for proposed (final) alternative Section Direction Length Corrdinates of Remark of lane of lane (m) begin point X (m) Y (m) inner 84o15’ 1295 2040832 492064 Bend angle 69o48’ outer 3o24’ 1840 2043155 492370 to -3.50 Table Length and coordinates of groin position for proposed (final) alternative End Seq Lenght Begin point point Lenght 600 Top crest elevation 2.00 1.00 Coordinate X 2041252.54 2041690.95 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 seastorm 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, HaTinh 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