Abstraction: The Red river delta is the tropical estuarine system in the West seashore of the Tonkin Gulf, Vietnam. The RRD is characterized by assorted tides with diurnal laterality and heavy fresh H2O inflow. The chief aims of the present survey were to imitate the tidal fluctuations and the longitudinal distribution of salt, and the fresh water, plumbing influences on the salt distribution. A intercrossed web numerical theoretical account was used to imitate the varying of salt under the effects of tidal circulation, fresh discharge and plumbing in the RRD estuary subdivisions. The theoretical account reproduced ascertained H2O degree and fresh discharge accurately during the field measuring in the dry season. The theoretical account could besides reproduce the longitudinal distribution of salt good and its fluctuations under the impact of regulating factors. An empirical correlativity between salt and plumbing was derived harmonizing to the categorized estuary types in the RRD delta.
The Red river is the largest river in the North of Vietnam, its H2O resources reserve many societal economic sectors in the delta field at the downstream. The Red river delta ( RRD ) is characterized by multi complex hydraulic channels and a bar-built estuary system. During the recent old ages, salt invasion is one of the most of import factors that constrict the use of H2O particularly in the dry season. The strong seasonal fluctuation that characterizes H2O exchanges and tidal commixture can play an of import function in the distribution and conveyance of salt in this river delta. Up to day of the month, a big figure of surveies have been carried out depends on two major physical factors that goaded salt, e.g. : the tidal fluctuation at the estuary oral cavity and the fresh water discharge come ining the saline country. But the salt invasion is a consequence of a complex interaction between these coercing with topographic characteristics at the estuary oral cavity. Valle-Levinson et Al. ( 2000 ) , Prandle ( 2003 ) , Savenije ( 2005 ) summarized and established the relationships among tide, river discharge, and salt with different estuary forms. However, they have been derived for individual estuary subdivision and for estuaries where the river discharge is little and the tide experiences merely modest damping or elaboration ( Nguyen, 2006 ) . Hibma et Al. ( 2003 ) , Nguyen ( 2006 ) investigated the morphological alteration and salt invasion for multi estuaries, but these estuaries are similar in form and length. Furthermore, each estuary subdivision is separated from each other by the island and it can interact with another through junctions or little connecting channels.
The RRD estuaries are the typical type of alluvial estuary with a complex hydraulic system, including several rivers and canals connected each other. Their chief topographical characteristics are shallow Waterss with big breadths to depth ratio, funnel shaped cross subdivision which is shallower and wider towards the estuary oral cavity. Each estuary has distinguishable features of tidal forcing, river discharge rate and plumbing that complicate the survey of salt invasion in RRD estuaries. We know of no survey on salt in these multi estuaries under the consequence of coincident physical factors. This paper sets out to analyse the tidal commixture, H2O exchange and salt distributions in multi subdivisions of RRD based on the field measuring conducted during 17 yearss in January, 2006 at six Stationss. In add-on, we examine some bathymetrical parametric quantities over a broad scope of estuarial conditions. The purpose of this part is to find the correlativities among denseness gradients, river flow and river plumbing and to categorise RRD estuaries in term of each impact factors.
The paper will be organized in the undermentioned mode: Section 2 briefly describes the country of survey. The inside informations of informations processing and salt measurings including uninterrupted H2O degree and coherency measuring informations are given in Section 3. The consequences of salt analysis correspond to intra-tidal government, river flows and variable forcing factors by river geometry are described in Section 4, and concluding treatment is presented in Section 5.
Red river delta
Shown in Figure 1, the RRD that is located in the West seashore of the Tonkin Gulf. The RRD is concentrated by many of import economic activities and the highest population denseness ; the entire country is about 16,644 km2, the population was 18.2 million in 2006 and wholly delta country is lying below three metre above the average sea degree ( Minh et al. , 2009 ) . The delta is restricted landwards by Son Tay territory and seawards by the coastline. RRD consists of two estuary systems located in the north-east and in the south-west. These two estuary systems are connected each other by the Duong river and the Luoc river in the upper portion and the lower portion of RRD, severally. Before making the Tonkin Gulf, the chief Red river in RRD subdivisions out into nine estuary feeders, e.g. : Da Bach, Cam, Lach Tray, Van Uc, Thai Binh rivers in the north-eastern portion and Tra Ly, Hong, Ninh Co and Day rivers in the south-western portion. This paper will concentrates on four estuary subdivisions in the south-western portion of the RRD.
The distinguishable monsoon clime is typical characteristic in this country, the moisture season from May to October and the dry season from November to April. The one-year rainfall varies spatially and seasonally from 700 to 4,800 mm/year. Here we have 80 % of the entire rainfall in the rainy season ( MONRE, 1996-2006 ) . The difference of the H2O discharge between two seasons is rather big ( van Maren, 2007 ) . Based on the hydrological information from 1996 to 2006 at Son Tay station, this station is located at the entryway of RRD, the mean discharge in the dry season is around 1100 m3/s contrasting with the mean one in the moisture season of 14,000 m3/s. As a consequence, the river basin causes a deficit of H2O during the dry season and inundations in the moisture season.
At present, the entire sum of H2O in the Red river is diminishing significantly as a consequence of clime alteration and big addition in the population in the delta ( Lan et al. , 2007 ) . Harmonizing to Cat et al. , ( 2008 ) the lessening of river discharge in the dry season during the last 10 old ages consequences in the addition of saltwater invasion. Salinity intruded much further into the rivers. Salt concentration is observed throughout the delta for up to 30 kilometers landwards from the Hong estuary, 34 kilometer from the Tra Ly estuary, 35 kilometer from Day estuary and 44 kilometer from Ninh Co estuary ( MONRE, 2009 ) . The longitudinal distribution of salt is capable to big fluctuations during a twelvemonth due to the influence of river discharge. The highest monthly salt normally occurs in January and February, two driest months of the twelvemonth. In Tonkin Gulf, salt remains in a stable value during a tidal rhythm and ranges between 25-30 psu during the dry season ( based on the informations observed at Hon Dau station ) .
The tidal government in the Gulf of Tonkin is diurnal type with one rhythm happening every 25 H and tidal scope decreases due souths bit by bit from 4 to 2 m. The tidal amplitude at Hong estuary, the chief estuary in RRD, varies from 0.5 m during neap tides to 2.5 m during spring tides. The monsoon air currents change from northeasterly in the dry season to southeasterly in the moisture season. However, due to the topographic conditions of the delta, the fetch length for the stronger northeasterly air currents is limited ; as a consequence, wave height deaths due norths.
Shape of the Red River Delta estuaries
Most of the rivers in RRD are offshore extension. In the north-eastern delta field, the deeper characteristic of the estuaries is dominated by tidal commixture ( Tanabe et al. , 2003 ) . The estuaries are characterized by funnel-shaped type and an intricate tidal level and creek system, which accelerate the incursion of tidal H2O into the rivers. While the wave-dominated is located in the low flatter land and widening estuarine system in the south-western portion of the delta, where wave energy generated by monsoon air currents is comparatively strong. The estuaries in this portion comprise tidal flats, fens, and channels which are composed of weaving rivers. It can be seen clearly in Figure 2 the seaward spread outing tendency of the RRD estuaries. We attribute the inclination of larger gap and shallower H2O deepness of the river oral cavity to the curious characteristic of the RRD estuaries by specifying the cross-section country, storage breadth and river bed lift at the tidal averaged H2O degree. These plumbing informations was measured in the twelvemonth 2000 in the “ Red river delta inundation protection plan ” organized by the Ministry of Agriculture and Rural Development, Viet Nam ( MARD ) . The mean deepness near the oral cavity of RRD estuaries ranges from 4 to 6 m below average H2O degree, and the channel breadth at this location is about 800 m. The river channel bit by bit deepens and narrows upstream of the river oral cavity ; 10 kilometer upstream, the rivers are merely 400 m broad and 8.5 m deep.
Data sets of RRD
The informations used in this paper consist of two sets: ( I ) the first set is from field measuring from January 1 to 16 in 2006 at 6 locations for salt concentration and H2O degree, and ( two ) the 2nd set is from the aggregation by Vietnamese Institute of Meteo-hydrology ( IMH ) at fixed Stationss on the RRD.
The field measurings were conducted in the driest month in 2006 to obtain the salt invasion curve at High Water Slack ( HWS ) and Low Water Slack ( LWS ) . The measurings were conducted at the same time at six locations including two locations on the Tra Ly river, two locations on the Ba Lat river, one location on the Ninh Co river and one location on the Day river.Salinity was measured consecutively in 17 yearss from 5 to 22 in January, 2006 at three points over the deepness: 0.5 m from the surface, 0.5 m from the underside and mid-water column. The existent recorded clip was adjusted comparatively to the clip of high H2O ( HW ) and low H2O ( LW ) . At HW period, we measured salt concentration during four hours: 1 hr before the HW, at HW and every one hr after H2O degree peaked.At LW period, salt was measured every two hours: before LW, at the lowest point, and after LW.The clip stairss for salt measuring were selected to find the fluctuations of salt during sub-tidal ( spring-neap ) corresponds to daily changing of a diurnal tidal government at four estuary subdivisions.
The 2nd information set is based on the informations collected from the IMH. The H2O degrees are collected at 6 Stationss: e.g. : Hanoi, Thuong Cat, Pha Lai, Hung Yen, Trieu Duong and Chanh Chu. The tidal degrees and salt concentrations at the oral cavity of estuaries are collected from measurings conducted by the IMH. For the intent of this survey, nevertheless, river discharge at the lower portion of the delta is non available during our field measurings. In RRD, merely three upstream Stationss ( Son Tay, and Hanoi Stationss on the Red river ; Thuong Cat station on the Duong river ) step day-to-day discharge, no other systematic records of discharge exist in RRD during the dry season except the H2O degrees. Therefore, we calculated the spacial distribution discharges over the river web in the south-western portion of RRD utilizing the MIKE 11 theoretical account. The MIKE 11 is a package bundle affecting several faculties and labs for the hydrokineticss, sediment conveyance and H2O quality in estuaries, rivers, irrigation systems and other H2O fortunes ( DHI, 2003 ) . It is being improved and going an effectual design tool for applied scientists, hydrologists, ecologists and contrivers. The hydrodynamic faculty is the karyon of the MIKE 11 and controls other faculties. In the present simulation, we use the hydrodynamic and advection-dispersion faculties. The former faculty treats the mass preservation and impulse preservation utilizing the Saint Venant equation. They are solved numerically in a infinite staggered computational grid utilizing a 6-point Abbot strategy ( Abbott, 1979 ) . The latter faculty trades with the advection-dispersion equation for dissolved stuff. It requires the discharge, H2O degree and flow speed informations as the input values that are obtained from the calculation utilizing the hydrodynamic faculty in progress. The advection-dispersion equation is solved numerically utilizing an inexplicit finite difference technique, which are devised unconditionally stable and less numerical scattering.
River web and theoretical account standardization
The computational web is established for the south-western portion of RRD. The present numerical theoretical account requires a series of 2-dimentional points depicting each cross-sectional profile along the river. It consists of 8 rivers with 280 cross subdivisions. These informations was collected from the plumbing measurings in the twelvemonth 2000. The most Vietnamese research institutes have depended on this database. The boundary conditions are given at one discharge station ( Son Tay station ) and two H2O degree Stationss ( Pha Lai and Chanh Chu Stationss ) in the upstream and at 4 tidal H2O degree locations at the estuary oral cavity in the downstream. We set the salt nothing at upstream boundary and the existent clip fluctuations at all downstream boundaries.
The undertaking of standardization is executed in two stairss. First, the theoretical account parametric quantities such as a Manning coefficient and an initial H2O degree at each cross subdivision are adjusted to obtain the best tantrum between the ascertained and computed values for both discharge and H2O degree at the prescribed station. Second, the scattering coefficient and initial salt concentration at each river section is determined for the standardization of salt.
Figure 3 shows the comparing of the ascertained and computed informations at Stationss in the upstream for the H2O degrees ( Ha Noi, Thuong Cat, Hung Yen, Trieu Duong Stationss ) and the discharges ( Ha Noi and Thuong Cat Stationss ) . The clip series observed informations for H2O degree and discharge at both Ha Noi and Thuong Cat Stationss are used for the standardization of the theoretical account while at Hung Yen and Trieu Duong Stationss merely daily ascertained H2O degree is conducted for that intent. The standardization consequence of the river-system theoretical account proves that the deliberate H2O degree at the upstream Stationss agrees good with the ascertained 1. Furthermore, the simulation besides describes accurately the discharges measured at both Thuong Cat and Hanoi Stationss.
As the saline invasion along a river is really sensitive to the initial conditions, boundary conditions and scattering coefficients, the standardization of salt is by and large more complicated when compared with that of discharge or H2O degree. Figure 4 shows computed and the mean ascertained salt concentrations at four Stationss at the downstream, e.g. : the Ba Lat, Dong Quy, Phu Le and Nhu Tan Stationss. The H2O degrees measured during field study at these Stationss are combined to compare with the anticipation of the theoretical account. Although there are several disagreements between lower salt concentrations, the theoretical account predicts the salt fluctuation moderately good. Additionally, the unmoved H2O degrees agree strongly with the tidal fluctuation from the theoretical account computations. Therefore, the parametric quantities obtained from the standardization are acceptable. These findings validate the attack we used to obtain the discharge from the limited information available at the IMH.
Discussion( Salt in RRD estuaries )
Effectss of tides
Salt in RRD varies following the tidal government. The lower salts near the river caput and higher salts seaward are related to the tidal fluctuations. At the indicated Stationss nearby the river oral cavities ( 6-9 kilometer from the sea ) , the salt measured did non look to be in equilibrium. Salt differences are high during a tidal rhythm due to ebb-flood flows at all seaward Stationss where the maximal salt measured at the HW and the minimal 1 obtained at the LW. The salt construction was stratified at high H2O column of a tidal rhythm but it was to the full assorted at the terminal. The fluctuation of salt besides good corresponds to the spring-neap fluctuation. The highest salt occurs during spring tides and lessenings towards neap tides due to the commixture. Harmonizing to Simpson et Al. 1990, blending rates are highest and smallest during spring and neap tides, severally. Salt cuneuss can therefore irrupt farther into an estuary during the neap tide. Our observation of salt fluctuations reflects this tendency at Phu Le and Nhu Tan Stationss ( see Figure 4 ) . The stratification happening during the neap tide was related to blending in combination with tidal straining or advection. However at the Ba Lat and Dong Quy Stationss, a contrast of better assorted type was observed during our measuring in the dry season. The mensural perpendicular salt construction at two Stationss suggests that baroclinic circulation is non of import in the dry season, go forthing tidal straining as the chief stratifying procedure. Tidal advection besides leaves sea H2O intruded further into rivers, nevertheless, salt reduces rapidly. In up-estuaries, as distance upstream additions, salt dropped quickly to values between 0.5 and 3.5 psu ; the highest value fell about 3 hours after HW at Phuc Khe station and Duong Lieu station. Longitudinal denseness advection is supposed much influenced by river discharge and topographic features of each river.
It is found that in RRD both partial assorted and well-mixed estuary types are existed during the dry season ( Figure 5 ) . The changing salt responds quickly to the alteration of tidal fluctuations in the latter type ( Hong and Tra Ly estuary subdivisions ) while it reacts more easy in the former one ( Ninh Co and Day subdivisions ) . Decreased tidal commixture in well-mixed estuaries decreases the exchange flow, and therefore lower salt was obtained at these subdivisions during our measurings. During low river discharge in January, HWS salt at the Ba Lat and Dong Quy Stationss was around 24 psu compared to 29 psu at Phu Le station. Lower salt was observed at Nhu Tan station, nevertheless, we expect fresh discharge and river plumbing are significantly diluted salt H2O. After LWS, salt increased quickly to sea value throughout the estuary. High salt was obtained at all four seaward Stationss during HWS. At two Stationss Phu Le and Nhu Tan, salt concentration was comparatively high at LW even through the tidal scope during the study was about the same. We can non govern out that a alteration in the sub-tidal salt government over that period could account for the local plumbing alteration or the dismissing of fresh discharge. Both likely play a function in higher salt. However, without any available informations it is hard to measure the salt fluctuation at these Stationss.
Consequence of fresh discharge
The river discharge, together with relevant parametric quantities specifying estuary form and tidal forcing, is the cardinal parametric quantity finding salt invasion in alluvial estuaries ( Nguyen, 2006 ) . In alluvial estuaries during periods of low flow, when salt is highest, the river discharge is by and large little compared to the tidal flow. This makes the finding of the fresh water discharge a ambitious undertaking. Even if discharge observations are available during a full tidal rhythm, the fresh water discharge is rarely much larger than the tidal discharge. Observations farther upstream, outside the tidal part, do non ever reflect the existent flow in the saline country due to backdowns or extra drainage. Discharge calculation is even more hard in a complex system such as the RRD, which is a multi-channel estuary consisting of many subdivisions, over which the fresh water discharge distribution can non be measured straight.
Before making the Tonkin Gulf, the chief Red river distributes its flow through six subdivisions with 25 % of the flow of the Red River dispatching into the sea via the Hong estuary, 10 % via the Tra Ly estuary, 6 % via the Ninh Co estuary, 22 % via the Day estuary and the other H2O volume is discharged into the Duong and the Luoc rivers ( Pruszak el al. , 2005 ) . The one-year discharge of the Red river is about 137 A- 109 M3 nevertheless less than 20 % of that H2O is supplied for the RRD in the dry season. During our measuring, the mean discharge at the Son Tay hydrological station is 1500 m3/s. However, the distribution of the discharge over the subdivisions downstream depends on a complex interaction of topography, tide, hydraulic channel web. Therefore, it is hard to obtain a sensible estimation of the discharge distribution over the subdivisions of the system.
The new attack to utilize the salt measurings to gauge the discharge distribution is tested against the consequences of a hydraulic theoretical account that uses the ascertained upstream discharge as the upstream input and the ascertained tidal fluctuation at the downstream boundary. For this intent, we have used the MIKE1 theoretical account. The information for the schematisation of the hydraulic theoretical account, including topographical information, has been obtained in the twelvemonth 2000. Based on the theoretical account consequences, we are able to deduce the fresh water discharge values in the subdivisions of the RRD. The computed consequences are presented in Table 2, as a per centum of the ascertained discharge at Son Tay station in the upstream.
Highest discharge distributed via Hong estuary, and so followed by Tra Ly estuary.
Well-mixed salt observed at two Stationss Ba Lat and Dong Quy. Discharge flush out salt H2O seawards, salt lessening quickly
Smallest rate obtained in Ninh Co estuary.
Highest salt value observed at Phu Le station on this estuary subdivision. Partial assorted salt obtained during neap tide period.
Apparently the discharge rate at Day estuary is comparatively little
It is noted that the amount of the discharge values of all estuaries is non necessary equal to the entire discharge ( see Figure 4 ) . This is apprehensible since a certain sum of H2O can come in or go forth the inland channel system and a certain sum of H2O can travel into the estuary subdivision.
At Ba Lat station, big sum of sea H2O fluxing into the Hong estuary, fresh H2O is comparatively big here, because of being the largest estuary in the RRD system.But merely outflow observed during the neap tide. The similar tendency besides obtained at Dong Quy station in Tra Ly estuary. This may explicate why in two estuaries salt is comparatively lower.During the neap tide period, the escape is much higher than inflow, most disruptive energy is supposed for the commixture that leads to well assorted type of salt.
The contrast characteristic can be seen in the Day and Ninh Co estuaries. The partial assorted obtain at Nhu Tan and Phu Le Stationss. This may be due to the influence of bathymertry.
We have seen that the discharge distributed into RRD subdivisions performs good with the salt observed in our measurings, salt measured at the same time over every subdivision within the same period. A good overview of how salt and discharge distribute over the subdivisions of the delta, this implies that a simple theoretical account can supply a good penetration into a complicated system like the RRD.
Consequence of plumbing
Topographical characteristics can hold an consequence on the commixture, based upon the geometry and coercing influences of the estuary. A peculiar instance for a characteristic impacting commixture is the form of the estuary entryway.
At the oral cavity of RRD estuaries, river channel is about 1000 m broad and the bed lift is about 5 m below average H2O degree. The river channel in the Hong river and the Tra Ly river bit by bit deepens and narrows landwards within the distance of 15 kilometer from the river oral cavity. While it keeps a level characteristic in the Ninh Co river and the Day river, the bed degree is about 6 m and 8 m below the average H2O degree, severally.
At these two rivers, big tidal discharge flows back and Forth through an indistinguishable cross sectional country and hence speed difference between the surface and bottom beds is comparatively little and mixing rates are low.
In the Hong and the Tra Ly rivers, the plumbing has a complex alteration in bed lift ; it is shallow at the entryway of the estuary and deepens landwards. Additionally, the plumbing at the Hong river is characterized by barrier-spit system close to the estuary oral cavity. The river flow decelerates as it enters the Tonkin Gullf, lodging flaxen deposits that form the river oral cavity saloon ( Maren, 2007 ) . Based on the old surveies on RRD, the period of the lifting phase of the tide ( inflow ) is shorter than that of the falling phase of the tide ( out-flow ) : 42 % and 58 % of clip, severally. The same tidal prism flows through an indistinguishable cross-sectional country in a shorter clip span, and hence there it can be expected that inflow speeds exceed outflow speeds, ensuing in a flood-tide dissymmetry. However, this flood-tide dissymmetry might be compensated by river escape which is in the way of the wane currents. In the dry season, this fresh H2O flow balanced the tidal dissymmetry in the chief channel ensuing in equal influx and outflow speeds, although the continuance of escape was longer.
Scenario 1: Changing river discharge in the upstream
Based on the hydrological database at the Son Tay station from 1996 to 2006, the averaged discharge varies from 900 to 1500 m3/s. In order to analyze the sensitiveness of longitudinal salt distributions correspond to the ratio of H2O discharge in the RRD estuarin system, we divided the discharge at Son Tay station into three degrees: 800 m3/s, 1200 m3/s, and 1600 m3/s. Numeric simulations are carried out throughout the estuarine system. Harmonizing to this indicated ratio, the rate of discharge flows into RRD estuary subdivisions in our calculations proved the similar ratios that are highest in the Hong river, lowest in the Ninh Co river, and chair in the Day and Tra Ly rivers, severally.
When the fresh H2O additions ( Figure 6 ) , as predictable, the invasion length by seawater will be decreased. There is an upstream displacement in the brackish seawater due to the inflow of river flow during the dry season. For illustration, if we define a discharge of 800 m3/s at Son Tay station as a threshold saline invasion, the intruded length reduces 4 % and 7 % at the Tra Ly river, 3 % and 5 % in the Ninh Co river, 8 % and 11 % in the Day river, 21 % and 34 % in the Hong river when discharge increased by 50 % and 100 % . However the diminutions in saline intruded length are comparatively little compared to the increased discharge rate except the Red river. The salt profile for changing river discharges does non demo a noticeable alteration in the Tra Ly, Ninh Co, and Day rivers. A closer expression at these profiles reveals that the deliberate consequence is non sensitive, the escape discharges in the dry season are excessively little that can ease the saline invasion.
In malice of little decrease of seawater invasion length, the accommodation of salt profiles are comparatively big in the distance of 10-25 kilometer from the river oral cavities. One can happen that at the high H2O in the spring tide, the H2O keeps the high saline denseness from the river oral cavity to the distance of 10 kilometers, and decreases the denseness quickly to the following 15 kilometer and easy to the terminal of computation section for the Hong river. In contrast, the denseness profile decreases bit by bit from the river oral cavity to the upper watercourse in the same status for the Ninh Co river. The other tendencies of salt invasion look like although these forms are non curious.
The consequence of the analysis demonstrates that there are two types of estuary based on the tendency of saline invasion curve: bell form and dome form ( Savenije, 2005 ) . The former type can be seen in the Ninh Co river, the narrow breadth in the upstream cross-section but strongly funnel shaped near the river oral cavity. The latter type is typically obtained in the Hong river characterized by broad channel with a marked funnel form and sand bars at the oral cavity. Both rivers have wider and shallower characteristics at the river oral cavity, nevertheless the salt differences are comparatively high. The partly assorted and well-mixed estuaries in these rivers of the RRD estuary system likely due to the differences in discharge rate and form of river plumbing. Other factors as air current, wind-wave forcing are besides of import to disruptive commixture, but they will non be treated in this paper.