Hydroinformatics’94,

Verwey, Minns, Bobovic & Maksimovic(eds)

@ 1994 Balkema, Rotterdam, ISBN 90 54 10 51227

G.S.Reddy & V.Padmaraj

Transoft International (P) Ltd.Electronic City, Bangalore, India

The present paper concerns the development of software to process measured data with a view to assess the net sediment transport and stability of shipping channels in tidal estuaries. The model has been developed based on the measured data at regularly or irregularly spaced locations. The total sediment transportation has been estimated using the Hnsen-Englund type of empirical formulas. The error made in the prediction of sediment transport in coastal environment using the empirical type formulae is discussed.

The software developed has been tested for its efficacy in processing the measured field data at Balari reach, in Hooghly estuary. The results consists of time averaged and depth averaged values of velocity, suspended sediment concentration and salinity. The program also computes bed load transport and suspended load transport using the computed values of velocity, suspended sediment concentration and salinity. It also computes the net sediment movement at measuring stations simultaneously. The program can also be used for predicting the morphological changes of tidal riverbed.

Rapid development of instusties and population growith in the coastal areas have resulted in increased stresses on the coastal environment, affecting the flow patterns in these regions. Coastal navigation is an important activity that will be affected by these changes.

Sedimentation in shipping channel reduces the depth of the water channel and hinders the movement of vessels. To obviate these problem.

While planning for navigation, it is necessary to have a better understanding of the flow phenomena and situation and deposition processes in tidal environment. Reclamation, water conservation and tidal power projects it is important to predict the depth of water in the coastal regions. Heavy shoaling in the lower part of estuariaries acts as a navigational barrier, which may have been comparatively nominal in the port for shallow draft vessels, but with the modern deep draft.

While planning for navigation, vessels access to the port, is blocked. Thus the development of the existing ports and improvement of estuarine channels will result in changes in the regime of an estuary like shoaling patterns, circulation patterns, salinity trusion etc..., and planning of such works requires a knowledge of estuarine dynamics, sedimentation processes sources of sediment and location and amount of shoaling. Due to complexity and non-linearity of both hydrodynamic and sediment transport phenomena, the majority of the investigators primarily have been carried out by field investigations. It is fact that processing the field data manually is both laborious and time consuming. Therefore, it is necessary to use computers for processing the measured fields’ data to simulate the characteristics of flow and sedimentation process quickly.

To assess the environmental health of an estuary it is necessary to calculate time-averaged fluxes of various dissolved or suspended constituents. For this purpose net flux per unit width can be estimated form the concentration of the desolved and suspended constituents and density of water which is related to salinity and temperature. Generally density can be considered to be constant in time as the variation are small compared to velocity and salinity changes. It is necessary to measure the concentration of the constituents at several depths between the surface and bottom along with the other measurements. Depth profile of the concentration must then be considered and the value rounded off at each of the depth for each station at sampling time which is time consuming and laborious. To obviate these difficulties curve fit program can be used to

Construct the depth profiles from surface to bottom.

In the present study, the software program has been developed for processing the hydrographic data of a tidal estuary. The program can access any combination of depth, salinity, suspended sediment concentration, velocity, temperature and conductivity measurements. The program has been tested in processing the hydrographic data of River Hooghly. The computed values of depth average and time averaged values of velocity, suspended sediment concentration, salinity and variation of tidal flux have been shown in graphical form. The program can also be used to estimate bed load and suspended load transport separately at the measured stations. The usefulness of the software has been discussed regards to the prediction of morphological changes of riverbed.

The morphological changes of riverbed can be made relating to the flow characteristics. The bed elevation can be predicted using the following contribution equation of sediment.

Equation (qsc –ql) (2.1)

Where gs=dry volume od sediment; h=elevation of bed surface; qsc=settling capacity; q1=lifting capacity of bed. Qsx and Qsy are respectively unit sediment transport rate in x and y directions and its values can be written as follows :

Qsx = Qs*U/W and Qsy = Qs*V/W

Where u,v are velocities in x,y directions respectively.Qs unit sediment transport rate on the direction of a streamline, and it can be written

Qs = S.Q + Qbs (2.2)

In which Qbs unit transport rate of bed load on bed surface. S Q unit width transport rate of suspended sediment in a water mass.

All constants in the equation (2.1) can be determined from the analysis of measured field data whether the channel will be silting or scouring. Under the normal practice of operation the data collected on board the survey vessels it takes considerable time to process the data. The processing the data can be done with the suitable software.

In the hydrographic study of an estuary, velocity (direction and magnitude), temperature, suspended sediment concentration and conductivity measurements are made for several depths between the surface and bottom at different locations. The measurements are regularly spaced. To standardize the data analysis, it is desirable to interpolate the shape if each vertical profile and then read off the data values at regular spaced depth intervals. The data points used in the analysis are therefore frequently not the measured values but rather the interpolated values. The interpolation is normally done numerically and can be analyzed quickly on a computer without introducing operator errors. The most advantageous curve fit is by spline interpolation procedure.

The measured depth values of velocity, temperature, salinity, suspended sediment concentration and density can be used to compute a series of cubic spline fits. To minimize any errors due to extrapolation it is necessary to have measurements as close to the bed as possible. As such the fits of the velocity profile in the bottom region requires special consideration. This is because to determine the shear velocity and the location of datum plane several simultaneous measurements are required in the bottom 1-2 mt. depth of water column. This is because to determine the sheer velocity and the location of datum plane several simultaneous measurements are required in the bottom 1-2 mt. depth of water column. This is normally not available in hydrographic survey. For this purpose a suitable non-dimensional drag co-efficient is used involving the shear velocity and current speed measured at 1 mt. above the datum. The value of non-dimensional drag coefficient reaches a constant value when appropriate Reynolds Number exceeds a certain limit. In that case the value of bottom current speeds lies at a rang of 10-15 cm/sec. Accordingly in most of the estuaries the value of the drag coefficient has been set equal to 0.003. Adopting the above techniques the vertical profile of velocity, temperature, salinity, suspended sediment concentration and density from the surface to bottom has been build up based on discrete vertical measurements.

Velocity temperature, suspended sediment concentration and conductivity measurements are made at several depths between surface and bottom at a number of different stations. Water depth at each station is measured with Echo sounder. Two velocity measurements are made one, which is positive in the main ebb direction and another lateral component positive 90 degrees clockwise relative to the positive longitudinal direction. It is important to select and specify the ebb orientation of the estuarine location relative to the north. It is necessary to measure both current speed and direction when the estuary is wide and has complex bathymetry and shape. The input of the program consists of three components. The first component specifies the number of stations, which will be analyzed in a single computer run. The program has been written to allow as many as 999 stations to be treated simultaneously. Each station has an identification data followed by "N" data values, where "N" is the number of measurements along there depth of parameter.

The objectives of hydrographic surveys varies from situation to situation. One of the goals is usually to describe the circulation of the particular estuary and to show the net moment of a dissolved and suspended constituents. Apart from the informations such as tidal flux, variations of bed shear stress during a tidal cycle, variation of salinity, suspended sediment concentration, temperature, velocity are required for engineering analysis. For each parameter it is necessary to compute net or time averaged values for non dimensional depth Z=0,0.1,…,1.0 rather than for fixed distances below the surface or above the bottom. Measurement should be at constant sampling distances, over at least node tidal cycle and if possible it should be continued for several cycles as net values may vary from one to another. It is suggested that measurements begin at either low or high tide to minimize possible averaging errors due to erroneous choice of the duration, of a particular tidal cycle. In a semi-diurnal or mixed tide regime, if the ‘n’ is selected to be 12, the sampling rate equals to 1.035 hour or one lunar hour, which in most cases is longest acceptable sampling rate. In such cases it is necessary to plot curves of each variable at all depths as function of time and divide each time series into ‘n’ equal increments. The interpolated values at each depth at n+1 time would then be the data to be used in computing time average.

The velocity time averaging has been done vectorially. For this purpose it is necessary to decompose into the orthogonal velocity components and then the time-average of each component have been carried out separately and the resultant velocity vector is obtained by combining two sets of velocity components. The velocity measurements at a station can be used to calculate the net discharge per unit cross sectional width. With the number of stations in a section knows it is possible to obtain the value of net cross section discharge.

In coastal environment the bed is composed of very fine sand, so most of the sediment will be transported in the form of suspension. The total transport load during the tidal cycle can be assessed from the difference of total load between flood and ebb cycle. Total load consists of both bed-load and suspended load. Normally, the suspended load can be estimated based on measured suspended concentration and velocity profile over a tidal cycle at the measured station. The bed load can not be measured but it can be computed based on certain assumptions. In the present case the bed load is calculated using the Hansen-Englund type of bed formulation.

The software program has been developed followed by Dyer, Ghosh and Reddy approaches as shown in flow diagram fig.1. The program requires various combinations of input values, such as velocity, temperature, salinity and suspended concentration. The program is developed in such a fashion that it computes the profiles of velocity, suspended sediment concentration, temperature and salinity according to the desired options.

The program then gives the – final output of the values of two orthogonal horizontal velocity values, salinity, temperature and density at eleven equally spaced distances beginning at the surface and ending at the bottom. It also computes frictional velocity bottom shear stress, dynamic roughness from the velocity profile. It provides also the time-averaged values of velocity, salinity and net tidal flux and net salinity and net tidal flux and net salinity flux per unit width. The program also computes suspended load transport and bed load transport separately from the computed profiles of velocity and suspended sediment concentration. Finally, Hansen-Englund empirical relation is used to transport. The program can also be used for predicting the morphological changes of tidal riverbed.

The software program has been applied to process the measured data at Balari, Hooghly estuary, India. From the available measured input data (Biswas and Ghosh) of velocity, suspended sediment concentration and salinity (1), the computed profiles has been building up. These are shown has been build up. These are shown in fig.2 to fig.4. The computed values have been used to obtain the time averaged and depth-averaged values of salinity, suspended sediment concentration and velocity profiles. Fig.5 shows the time averaged net velocity. The time averaged tidal flux both flood and ebb over a tidal cycle is shown in fig.6. From the figure it can be said that the net tidal flux is in ebb direction at the measured station. Fig.7 and Fig. 8 shown the variation of velocity and bed shear stress over a tidal cycle at the measured station respectively. The bed load transport has been estimated based on the net bed shear stress. Total sediment transport load has also been estimated using the Hansen-Englund formula. The values of net salt flux, net sediment transport are shown in table I.

The software tested for its efficacy in hydrographic data processing in navigational channel has been found to be performing satisfactorily. However for its acceptability in fields many more trial computations need to be carried out and for this extensive field data are required. It is believed the software will help faster data processing on board the survey vessel thereby facilitation better monitoring of the shipping channel and evolving better management practice.

Grateful acknowledgements are made to Mr. Arun Murthy, Dr. Sharad Tripathi and Mr. Ram Tripathi of M/s Transoft Internation for their encouragement and suggestions during preparation of the manuscript. The authors also express their sincere appreciation to Ms. Waheeda Abdul Rehman for her valuable help during the preparation of the menuscript.