Desalinisation of recently accreted coastal land in the eastern part of the Bay of Bengal, Bangladesh

Agricultural Water Management, 13 (1988) 1-11

1

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Desalinisation of Recently Accreted Coastal Land in the Eastern Part of the Bay of Bengal, Bangladesh L.K. SMEDEMA and A. J E N K I N S

Euroconsult, P.O. Box 441, 6800 AK Arnhem (The Netherlands) (Accepted 31 July 1987)

ABSTRACT Smedema, L.K. and Jenkins, ~,., 1988. Desalinisation of recently accreted coastal land in the eastern part of the Bay of Bengal, Bangladesh. Agric. Water Manage., 13: 1-11. The paper describes the desalinisation mechanism of recently accreted land in the eastern part of the Bay of Bengal, Bangladesh. The natural groundwater drainage of the land is such that, during the monsoon season, the upper soil layers are readily and rapidly leached naturally. However, these layers resalinise again in the dry season due to the strong capillary characteristics of the free sandy/silty soils. This resalinisation will only stop when the upper groundwater layers have become sufficiently diluted, a process which takes decades.

INTRODUCTION

The eastern part of the Bay of Bengal, constituting the main estuary of the Ganges-Brahmaputra-Meghna river system, is subjected to rapid siltation because of the heavy sediment load of the river water. New land accretes continuously along and offthe coast. This new land (locally refered to as 'charland') attains maturity when it reaches the average high-tide level and further accretion becomes negligible. Although by that time the land is partly desalinised, the soil salinity is still too high for good cropping. The studies described in this paper were aimed at learning about the salt regime and salt dynamics of the charland soils and at examining the possibilities of accelerating the desalinisation of these soils. The studies were undertaken within the framework of the land Reclamation Project, a joint Bangladesh-Netherlands development project, and conducted at research plot, a 40-ha site located on the recently accreted char Baggar Donna (Fig. 1 ). This char started accreting in the sixties and most of it reached maturity in the midseventies. The plot was embanked in 1980 and later incorporated in the pilot polder, the embankment of which was closed in 1986. 0378-3774/88/$03.50

© 1988 Elsevier Science Publishers B.V.

"----3 office

ec":,,~

j ....

Polder

\

--

j- f

J~

/

JJ

J

~.

embankment

\

1 forest reserve

km

:: 1

Fig. 1. Location and situation maps of the research plot and pilot polder.

At the start of the Land Reclamation Project, it was perceived that the desalinisation of the char soils would require the installation of a field drain systems. In the research plot, experiments were laid out with field drains of different depth (from 0.30 m to 0.90 m) and spacing ( from 20 m to 100 m ). In the course of the studies, however, it became evident that the leaching flow induced by the installed systems was far overshadowed by the leaching flow induced by the natural ground-drainage of the charlands. GENERAL DESCRIPTION OF THE STUDY SITE

The charlands are formed by marine accretion as described in the introduction. The coastal waters of the Bay carry a high sediment load year-round, and accretion occurs in both the wet and the dry seasons, although accretion of the nearly mature charlands is mostly restricted to the monsoon season when the water levels in the Bay are highest. Soils

The charland soils in the area are all still very young soils in which as yet hardly any profile development has taken place. The mature soils are usually The executing agencies of this project are the Bangladesh Water Development Board and the Delft Hydraulics Laboratory/Euroconsult.

.

\ ~, \ \~'~ \.~

0

|

0

10

.

.

.

-- ~ -

-

|

I

~

-

-

-

-

v f S L / S i L , topsoil v f S L / S i L, subsoil. SL/fSL CL/SiCL

I

20 30 40 50 moisture content ('Yo,V/V)

Fig.2.Soilmoistureretentioncurvesoffourpredominantsoiltexture. oxidized to 1-2 m depth and below this depth only along the roof channels. The most striking feature of the soilprofilesis the extreme stratification.The soillayering is especiallypronounced in the upper 2-3 m, which represents the accretion stage between M S L and average high-tide level.The different layers vary in thickness from only a few millimetres to some centimetres and in texture from (very) fine sand to siltyclay. Although most layers are essentially horizontal and of great extent, lenses and pockets of contrastingly different texture can be found at various depths, apparently marking sedimentation during storms. Hardly any homogenisation has as yet taken place, the original (micro) stratificationbeing stillvery evident in soilsof 10-20 years age. The predominant textures of the rootzone of mature char soilsare (v)fLS, (v) fSL, SiL and SiCL. The bulk density isusually around 1.30 g/ml, the drainable porosity between 3 and 5 % and the hydraulic conductivity between 0.1 and 0.5 m/day. The moisture retention curves of Fig. 2 show the strong capillary characteristicsof these soils (as to be expected given the predominant fine sandy/silty textures). Below 3-4 m depth, the predominant texture becomes (v) fSL/SL while there is less (micro) layering. This trend continuous with depth, the layers becoming thicker and more uniform and lighter-textured (Fig.

3). Surface water The arterial drainage system is largely made up by the remainders of the creek system from which the land accreted. As the accretion reaches its final

,.°,,% • .., •

.. ,

..

, °

fine sand

•.°°••

50

....

g

¸

v 100

:l."..f.!:

>~

i..

~-.

:i:f!ii:"

E

mixed silty clay, silt and fine sand

o

o g

L

200

g

.-/:"!): fine sand silty clay

251

compact hard layer medium sand c o m p a c t e d / c e m e n t e d sand coarse sand

30(

Fig. 3. Profile characteristics a n d properties of charsoils (0-5 m, see description in text).

stages, most of the lower-order elements of this system become silted, but the larger ones remain open as they have a sufficient large tidal volume and serve as major outlet for excess rain water during the monsoon season. There are two such major creeks in the study area, the West and the East Nadi, respectively running West and East of the pilot polder ( see Fig. 1 ). Of these two, the West Bagua Nadi is by far the largest; the East Baguda Nadi was still functional during the study period although in the process of silting up (with the closure of the pilot polder embankment, this creek became an internal drain). The tidal regime of the Bay of Bengal is of the semi-diural type. The coastal waters in the project area have a low salinity level during the monsoon due to the large inflow of fresh water into the Bay from the Ganges-BrahmaputraMeghna river system• Change of wind direction also causes a rise of coastal water levels during the monsson season ( Fig. 4 ). Groundwater The groundwater in the sharland appears to be saline and unconfined to great depths. A number of geohydrological borings have been made in the area down to depths of 1000 ft ( 300 m ) and in all borings essentially the same type

(m +PD)

5EC

average high tide level

(mS/cm) 204

16-

Xx

3

X

x x

XX);

12-

X~

X

X

X X

XX

2 -x X X X X

8-

x

salinity

xx x x

x

I 4-

xx

X

X

X~ XX

O-

0

I J

I

F

M

I

A

I

I

M

x

x

J

xx I x

J

xlxX

x

A

)l

Xx

S

x x~XX, xl .

0

I

N

I

O

Figure/.. Average monthly spring tide levels and water salinity data.

Fig. 4. Average monthly high tide levels and salinity of the coastal water near the pilot polder (1984).

of textural layering was found with thicknesses varying from 1-2 m to 10-20 m. Most fine-textured layers are, however, intercalated by coarser-textured material and coarse-textured layers by fine-textured material. No thick highly permeable layers were found nor thick strata of very low permeability. There is not much information on the strata below the depth of these borings except that a number of tubewells are located in the area which pump good drinking water from a depth of some 300 m, and are semi-artesian. The phreatic groundwater levels show a pronounced seasonal pattern (Fig. 5 ). The levels are at or close to the soil surface through most of the monsoon season, especially where water is kept on bunded fields. At the end of the rains in October when the rice fields become dry, watertables fall below the soil surface, while a further steep fall of the watertable can generally be observed in November when the Aman rice crop is harvested and the soil becomes exposed to evaporation. Watertables may fall to 3-4 m depth within 1-2 months time and eventually settle at some 4-5 m depth which corresponds approximately with MSL, the regional drainage base. The rise of the watertable in April-May, when the groundwater is being recharged by the first rains, is similarly rapid. These rapid responses of the watertable are due to the earlier-mentioned strong capillary characteristics of the charland soils.

J

v

t

F

I

M

I

A

I

M

S ........

i

J

Z.J..L~k..L,~S

I

i D

0

o

~oo

I

0

_o

k

~ 300 (

400

~

300I 200

~

qC

Aman season

)

~..,.x/ j i F ' t,4

N

!

D

Fig. 5. Annual watertable regime (research plot, 1984) CROPPING

Crop seasons in Bangladesh are referred to as the 'khariff season (wet season) lasting from April/May to September/October, and the 'rabi' season (dry season) lasting from October/November to February/March. On the char soils the main crop is Aman rice, a wetland rice crop usually planted around July and harvested in November. Local salt-tolerant varieties are available which can be grown on the very young, just-mature charland soils. It is usually a lowinput/output crop sown broadcast rather than transplanted. This was the situation in the area when the research plot was established. Farmers in the research plot, however, soon switched to a higher-input/output level of Aman cropping with the crop being transplanted, fields being well bunded, higheryielding, more-sensitive varieties being used, and generally more care being applied to the land and crop. The main rabi crops are grams and pulses. Al-

though in the future irrigation may be introduced, at present rabi crops depend on residual soil moisture. The available moisture is generally sufficient but rabi cropping on young char soils often suffer from high salinity. As the desalinisation of these soils proceeds, prospects for rabi cropping become better. SOIL SALINITY STUDIES

As to be expected in marine deposits, the young char soils are initially quite saline although, upon reaching maturity the soils contain considerably less salt than would correspond with sea-water salinity. This is firstly due to the deLayer 0 - 1 5 c m

/,,

I

I

I

I

I

I

I

L a y e r 15-30cm E

4

6L

!

I

I

I

I

I

I

LW

:~6

Layer 30-60cm

-6

"8

4

U3

2 I,

6f

I

o

.

z.

I

I

I

I

I

I

Layer 60-90cm

~: Pilot Polder x Research plot

2

I

I

1979

I

1980

I

1981

I

1982

I

1983

I

1984

I

1985

Fig. 6. Pre-mid-post monsoon soil salinity in the research plot and in the pilot polder.

soil solinily

I I j --

E

//

20

'

i I

4[z

~ Lo

(EC2{ in rnS~m)

2

4

5

I ./

,/

3

,/

Reseorch Plot

I

1 i

6 m

i

6O

pre m o n s o o n 1985 mid m o n s o o n 1985 p o s t m o n s o o n 1985

80

soil

1

solinity (EC2½in mS/cm) 2

3

4 i

5

2O t~

Pilot P o l d e r

x2

6 6O

80

i(

./

Fig. 7. Soil solinityprofilesat pre-mid-postmonsoonin the researchplot and pilotpolder (1985). position of the soil material in alternating fresh and saline water environments (during the dry and wet season respectively, see Fig. 4) while, moreover, the soils have been exposed already to a considerable period of leaching upon reaching maturity. Salts in the soil and in the phreatic groundwater are mostly of the NaCL type. In the soil they occur dissolved in the soil water at seasonally varying concentrations, while the Na ions also occur absorbed on the surface of the colloidal soil particles. As most of the soil, however, are non-colloidal, this absorption is of little consequence. No significant soil structural problems due to this. Na absorption have been found. Soil salinity conditions have been monitored in the Research Plot since 1979, and in the pilot polder since 1983, by regular sampling in late April/early (premonsoon), late August/early September (mid-monsoon) and in late November-early December (post-monsoon). This sampling was done in all 60 fields of the research plot during 1979 to 1981, in half of the fields during 1982 to 1983 and in a quarter of the fields since 1984. The sampling in the Pilot Polder

was done at some 40 to 50 sites. Two important conclusions may be drawn from the results of this soil salinity monitoring ( Figs. 6 and 7):

Desalinisation An overall year-by-year decline of he salinity levels since 1979 is clearly noticeable. The 1979 data refer to the situation a few years after the char reached maturity at which time some leaching of the salts .had already occurred, although part of this was periodically negated by saline flooding. These floodings stopped with the construction of the embankment in 1980, while at the same time leaching was promoted by the initiation of intensive Aman cropping in bunded fields in the research plot. This explains the gradual fall of salinity levels in the research plot since 1980. Land development in the Pilot Polder started later and here the desalinisation has advanced less. The basic mechanism underlying this desalinisation of the char soils, the groundwater drainage flow, has been discussed earlier.

Resalinisation. The overall year-by-year desalinisation of the soil is anually interrupted by resalinisation during the dry season. As discussed earlier, watertables fall to great depths during this season as groundwater is drawn to the soil surface by capillary forces generated by evapotranspiration of soil moisture. As the groundwater is saline, this leads to considerable salinisation of the upper soil layers (capillary salinisation). DISCUSSION

During the monsoon season, much excess rain occurs which is discharged by one of the following two modes of drainage (schematically indicated in Fig. 8):

Surface drainage, i.e. overland flow in the general direction of the slope of the land to the nearest surface drain, usually the lowest-order element of the arterial drainage system

Groundwater drainage, i.e. groundwater flow through the substrate of the soil to the nearest groundwater drain, usually a deep creek, a river or the sea. From water-balance studies conducted at the research plot during the wet seasons of 1983 and 1984, it was concluded that, of the total rainfall, some 40-50% discharged by surface drainage and 20-30% by groundwater drainage, while the remainder was lost by evapotranspiration. This refers to well-managed Aman cropping in well-prepared bunded fields. Groundwater drainage is the main mechanism by which the young char soils desalinise. By this mechanism, a water depth of several hundreds of m m percolates through the soil profile each monsoon season, leaching away large amounts of salts in the process. Daily leaching rates were observed to be in the

10 sea or

deep river/creek embankment

#/"//A

minor c r e e k

~

a

7

b u n d e d rice fields

.r--

-

..... 7

•

~"

T

~

m

7

Fig. 8. Drainage of the eharland by surface and groundwater flow.

order of 3-5 mm during August-September. Groundwater drainage rates depend primarily on the elevation and distance of the land relative to the nearest deep creek/river or to the sea and on the transmissivity of the substrata. These conditions may not be as favorable everywhere in the project area as in the research plot but even when, under less-favorable conditions these leaching rates are only half as much, this leads to a considerable leaching in the wet season. Based on this understanding of the prevailing desalinisation mechanism of the char soils, it is considered that a deep (maintaining a drainage base at some 1.5 m below the land surface), widely spaced (1000-2000 m) system of open drains which act as the main sink lines for t h e regional groundwater drainage flow, will generally suffice. In most charlands, this system is already present in the form of natural creeks. To be effective groundwater drains, they should however be well incised. Where the intensity or depth of the creek system is insufficient, the system should be upgraded and/or intensified. Field drainage systems may be rather shallow as their function is mainly to discharge excess surface water rather than to draw groundwater, in the early part of the reclamation period, some desalinisation may be achieved by surface drainage (called flushing). Flushing is most effective when, at the end of the dry season the salts are concentrated at the soil surface. These salts dissolve and diffuse into the early rain-water retained on the bunded rice fields, a process which can be promoted by working the soil under water (puddling). In a flushing trial at the research plot in May 1981, three successive flushings removed respectively 1715, 257 and 236 kg salt per ha. it was estimated that these three flushings reduced the ECe-value of the upper 10 cm of the soil by some 1.5-2.0 mS/cm. The desalinisation effect of flushing, therefore, is limited; nevertheless it is a

11 worthwhile additional desalinisation measure during the early reclamation period. The real desalinisation of the char soils however, depends on the deep vertical flow through the soil. The classic concept that installed drains can prevent resalinisation during the dry season, obviously has little application and validity in this case. Due to the strong capillary characteristics of these fine sandy/silty soils watertables fall rapidly to great depths when the soil moisture at the surface starts to become depleted by evaporation at the onset of the dry season. Capillary salinisation continues until the watertables reach the critical depths, which appeared to be as deep as 3.0-4.0 m. (see Fig. 5). An installed groundwater drainage system would hardly increase the rate of fall of the watertable while, moreover, drains would have to be installed very deep (below the critical depth). These limitations of controlling capillary salinisation during dry periods by drain installation have also been pointed out by Boumans (1976). Another important conclusion drawn from the desalinisation studies conducted at char Bagger Donna is that under the prevailing soil and climatic conditions, groundwater salinity is the key factor in the desalinisation process. The upper soil layers can in fact readily and easily be desalinised by leaching the rainy season. However, given the strong capillary characteristics of the soils, this leaching of the upper layers is negated by capillary salinisation during the dry season as long as the groundwater remains saline. Therefore, in this case it is not sufficient to desalinise the upper soil layers; the upper groundwater layers must also be sufficiently desalinised. It is the latter desalinisation which determines when rabi crops can be successfully grown on the charlouds. ACKNOWLEDGEMENT The executing of the project are the Bangladesh Water Development Board and the Delft Hydraulics Laboratory/Euroconsult.

REFERENCES Boumans, J.H., 1976. Leaching requirements and drainage to prevent salinisationand to reclaim saline soils.Z. Bewlisserungsech., 10: 7-24.