CHAPTER - II (Contd.)

1.7 Salinity

The Bay of Bengal is the source of salinity in the coastal estuaries in Bangladesh. During every high tide the saline water from the Bay enters the estuaries, mixes with the upland flow. Turbulent diffusion of this salinity travels up stream with decreasing concentration.

At every section of an estuary, salinity varies with the state of tide and also with the up land flow. Bangladesh has a monsoonic climate. The upland river flow has a distinct seasonal pattern. The upland flow increases from June to reach the peak in August/September and decreases from October. With this variation of fresh water flow salinity varies inversely. Salinity front of a particular concentration starts moving upstreams from October and starts moving towards sea during June.

The Ocean water has salinity of 35 parts per thousand whereas in the Bay of Bengal close to Bangladesh coast it is 30–32 parts per thousand. This is due to discharge of sweet water by the three big rivers.

It has been found that the relative chemical composition of Sea water is essentially constant. The greatest ionic concentrations in sea water are Chloride, Sodium, Sulfate, Magnesium, Calcium and Potassium in that order. Many other ionic constituents are present, but in smaller or trace concentration. The total dissolved solid contents of most sea water average about 34,500 PPM.

Interms of percentage they stand as follows :

Among the various units of measuring salinity the following are more popular.

Of these TDS is difficult to evaluate. Chloride can easily to determined by titrating a known quantity of water sample by AgNo₃, solution of known strength. Electrical conductivity meters with temperature compensation arrangement is easy means for field measurement of salinity.

For sea water the relation ship between the three units are as follows :

1. Salinity in PPM = 0.64×E.C.M Mhos.

2. Salinity in PPM = 30+1.805 Chlorinity (PPM)

Average monthly electrical conductivity in micro-mhos for a few stations in Satkhira area is given below:

Monthly variation of electrical conductivity at a few important stations in Bangladesh coast at High and Low water slack is shown in Tables-XIV to XX.

TABLE - XI

FREQUENCY DISTRIBUTION OF TROPICAL CYCLONES FORMING IN THE BAY OF BENGAL 1948–1970

TABLE - XII

CYCLOGENESES OVER THE BAY OF BENGAL 1948–70

TABLE - XIII

REMARKABLE CYCLONES THAT HAVE AFFECTED BANGLADESH 1795 – 1965

Table - XIV SALINITY DATA (E.C.) M/Mhos.

Table - XV SALINITY DATA (E.C.) M/Mhos.

Table - XVI SALINITY DATA (E.C.) M/Mhos.

Table - XVII SALINITY DATA (E.C.) M/Mhos.

Table - XVIII SALINITY DATA (E.C.) M/Mhos.

Table - XIX SALINITY DATA (E.C.) M/Mhos.

Table - XX SALINITY DATA (E.C.) M/Mhos.

1.8 Wind

Only a few stations in Bangladesh have wind records. Wind data may conveniently be presented as wind roses. Plate 5–7 shows wind roses in Bangladesh for the period November to February, March to May and June to October.

Monthly average wind speeds for Khulna, Barisal, Chittagong and Cox's Bazar are shown in Tables I to III.

Wind field in the November to February period

Chittagong has 25% calm. period which means very windy weather. The prevailling wind direction is north-east. A little bit inside the country at Barisal and Khulna 72% and 80% are calm period showing very much less wind movement.

Wind field in the March to May period

Khulna has 35% calm (quite windy) and Barisal has 34%. The coastal areas of Chittagong have 15% calm (very windy due to exponsure) with south east winds blowing almost parallel to the coast line.

Wind field in the June to October period

This is the monsoon period in Bangladesh. The windiest region is Chittagong with 15% calm and prevailing wind parallel to the shore line. It is a south east wind with some southerlies. The wind field appears to be the same as that of the previous period.

1.9 Waves

When wind begins to flow over a smooth surface small waves are caused by the tangential forces between the wind and water. The waves increase in size as a result of the tangential force and also due to push of the wind against the back of the waves. As the waves grow in size, their speed also increases until they move with the same speed as the wind. Since wind force acting on the waves are a function of the difference between wind speed and wave speed, further growth ceases when the speeds become equal. Duration of wind may be an important factor in the ultimate height of the waves unless the wave passes out of the region of high wind or strikes a shore line before it attains maximum growth. The growth of wind waves can be calculated by aerodynamic principles if the wave shape is assumed and viscosity is neglected.

Wave height data gathered at major reseruoir confirm the theoritical and experimental data for Ocean waves if a modified value of fetch is used. The derived equation is Zw = 0.034 Vw 1.06F^0.47 (i) Where Zw is the average height (inft.) of the highest one-third of the waves and is called the significant wave hight. Vw is the wind valocity (in m/h) about 25 ft. above the water surface, and F is the fetch in miles.

The design of embankment will have to take into account the wind generated waves. If the embankment has to be designed against cyclone, the probable cyclonic wind has to be considered in the above formula.

The prevailing wind and its direction may be considered for estimating wind waves within the pond.

1.10 Tide

The tide along Bangladesh coast originates from the Indian Ocean. It travels through the deap Bay of Bengal and arrives at Hiron point and Cox's Bazar at about the same time. These two points being practically at the heads of submarine canyons i.e. swatch of no ground and Burma trench.

The tide in the Bangladesh coast is semi-diurnal with slight diurnal inequality. In the extensive shallow areas in the north-eastern corner of the bay, the tidal range increases due to partial reflection. The rising limb of the tide curve become steeper and under some circumstances moving hydraulic jumps locally known as tidal bore may occur. As the tidal wave travels along inland rivers, the frictional forces causes gradual decay in the tide. This process is further accelerated by the upland flow. The horizontal tide i.e. reversal of flow direction due to tide is felt in the low water season as far up as Bardia on the Gorai, the confluence of lower Kumar with the Arial Khan, Lohajang on the Padma etc. the back water effect of this tide is felt further up stream.

The tide from the deeper part of the Bay of Bengal enter into the continental shelf through the two submarine canyons. The ranges of the tides at the tip of these two canyons i.e. Hiron point and Cox's Bazar being about, 10 feet during the spring tides of the equinoxes.

In the Passur, Bhairab and the Baleswar-Kocha rivers funelling effect upstream of Khulna and Kaukhali respectively causes the strong tides in the vicinity of those places. In the Passur it even results in the greatest range in the upper reach inspite of attenuation due to friction. The back water effect of tide on the upland flow is felt even at the maximum flood upto Chandpur on the Meghna and Bardia on the Bhairab-Nabaganga.

Quick discharges of flood water in the Bay of Bengal is retarded by this tidal back water effect.

Plate 8 & 9 shows variation of mean annual high and low tide in the Bangladesh Coast.

Water Level Hydrographs

In order to asses the fluctuation of tide level round the year, the daily high and low tide has been plotted for the following stations.

1. Cox's Bazar, Patenga and Khulna for the period from April 1981 - March 1982.

2. Chiringa, Dhumghat, Kaikhali, Satkhira and Mongla for the period from April 1982 - March 1983.

3. Khepupara for the period from April 1980 - March 1981.

It has to be understood that the actual course of water level changes are within this two envelopes. The effect of spring tide (higher high water and lower low water) and effect of neap tide (Lower high water and higher low water are also visible in the presented graphs.

Examination of hydrograph of Cox's Bazar and Khepupara will show a general rise of sea level by about 2 to 3 ft. during the monsoon.

High and Low tides of different frequencies

High and Low tides of 50% and 80% probabilities one shown in the tables XXI - XXIV, tabulated. These levels when considered with topography within the ponded areas will show the possibilities of water exchange with tides.

Seasonal Variation of Tide

Seasonal variation in high tide and Tidal range (Table-XXIVA) accounts for available heads for drainage or for flushing certain area by tide. High tide increases from January attains highest value during July to September and then Starts falling. Tidal range however is high during lean season i.e. January to June and is comparatively low during July to September.

Comparison of Salinity data however shows the salinity starts increasing from January attains higher value during April-May and then starts receding.

TABLE - XXI

STATION: COX'S BAZAR

WATER LEVEL AT DIFFERENT PROBABILITY
( IN METRE)

Probability of excedence

TABLE - XXII

WATER LEVEL AT DIFFERENT PROBABILITY (IN METRE)
(Probability of Excedence)

TABLE - XXIII

WATER LEVEL AT DIFFERENT PROBABILITY (IN METRE)
Probability of excedence

TABLE - XXIV

WATER LEVEL AT DIFFERENT PROBABILITY ( IN METRE)
Probability of excedence

TABLE - XXIVA

SEASONAL MEAN TIDE AND RANGE ( IN METRE)

1.11 Co-relation of Pond and outside salinity with the rainfall during the salinity observation period

Fig : 1–2 shows plotting of pond and outside salinity with time. The salinity values are averages during spring and neap tide over periods of 3 to 7 days. On the same fig 1–2 rainfall of the nearest station for the same number of days for which salinity data has been collected (Table-XXV) have also been plotted. As is observed in fig 1–2 although salinity was continuously falling there had been no rainfall during corresponding period of June and July. There was however rainfall during these two months on days other then salinity observation days. Rainfall during June and July was 139 mm & 124 mm at Satkhira. Temperature and salinity data for Kaliganj, Chiringa and Khepupara are also presented in fig 3 to fig 5.

It is therefore concluded that salinity does not respond immediately with the rainfall of the same day/days. However cumulative rainfall has a definite effect on salinity. The same conclusion holds good both for Satkhira and Chiringa sites. The variation of Pond and outside salinity is not very significant and difference possibly decreases with the higher rate of water exchange.

Co-relation of Salinity with flow

In khulna area there is no significant upland fresh water flow. This is because only Goria receives fresh water flow from the Ganges and distributes the same through passur and Madhumati system. During monsoon there is a general exchange of fresh water from Madhumati system and streams to the east towards the net-work of rivers to the west of Gorai-Madhumati system. This along with monsoon rainfall causes seasonal decreases of salinity in Satkhira area.

Chakaria forest area in Chittagong is situated on the outfall of Matamuhuri river. It is therefore expected that salinity in the experimental pond in these area will strongly respond with the flow of Matamuhuri river. Table-XXVI shows the mean daily discharge of the Matamuhuri river at Lama.

Average flow during the salinity observation period has been computed and tabulated in Table-XXVII-XXX. The data has been plotted in fig. 6–9 for pond salinity and as well as outside salinity against these average flow. Salinity seems to be fairly strongly Co-related with the flow of Lama. This means, for a given flow of Matamuhuri river average and over a period of 3 to 7 days, expected salinity can be read from the Co-relation graphs. As has been mentioned earlier salinity within the pond and outside is not significantly different due to high rate of water exchange. In Chiringa area it might be possible to maintain a comparatively higher salinity in the pond if water exchange is decreased during the passage of flood flow. Annexure-A presents the tabulated data and charts on hydrology.

TABLE - XXV

SALINITY(%) AND RAINFALL (mm) DATA AT DIFFERENT STATION

TABLE - XXVI

Mean Daily Discharge of Lama

TABLE - XXVII

TABLE - XXVIII

TABLE - XXIX

TABLE - XXX

FIG - 1

FIG.   SHOWING MONTHLY FULL MOON (F) NEW MOON (N) FIUCTUATION OF SALINITY (‰) AND WATER TEMPERATURE (°c) IN STATION S-1 AND POND SP-1 AT SATKHIRA.

FIG - 2

FIG.   SHOWING MONTHLY FULL MOON (F)-NEW MOON (N) FLUCTUATIONS OF WATER TEMPERATURE (°C) AND SALINITY (‰) IN STATION C-1 AND POND CP-1 AT CHITTAGONG.

FIG - 3

FIG.   SHOWING MONTHLY FULLMOON (F) - NEWMOON (N) FLUCTUATIONS OF SALINITY (‰) AND WATER TEMPERATURE (°c) IN STATION S-2 AND POND SP-2 AT SATKHIRA.

FIG - 4

FIG.   SHOWING MONTHLY FULL MOON (F)- NEW MOON (N) FLUCTUATIONS OF WATER TEMPERATURE (°C) AND SALINITY (‰)IN STATION C-2 AND POND CP-2 AT CHITTAGONG.

FIG - 5

FIG.   SHOWING MONTHLY FLUCTUATIONS OF WATER TEMPERATURE (°C) AND SALINITY (‰) IN STATIONS K-1 & K-2 AT KHEPUPARA.

FIG - 6

FIG - 7

FIG - 8

FIG - 9