Proceedings of the 6th International Conference on the Application

of Physical Modelling in Coastal and Port Engineering and Science

(Coastlab16)

Ottawa, Canada, May 10-13, 2016

Copyright ©: Creative Commons CC BY-NC-ND 4.0

1

CLIMATIC AND CYCLONE INDUCED STORM SURGE IMPACT ON SALINITY

INTRUSION ALONG THE BANGLADESH COAST

RABEYA AKTER1, MOHIUDDIN SAKIB1, MASHREKUR RAHMAN1, SUMAIYA1, ANISUL HAQUE1, MD.

MUNSUR RAHMAN1, MD RASHEDUL ISLAM1

1Institute of Water and Flood Management, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh,

rabeya343@gmail.com

ABSTRACT

Global climate change will accelerate sea level rise that will continue to affect Bangladesh coast through increased

salinity intrusion of low-lying areas. The situation becomes more critical when a severe cyclone makes landfall in the

coastal region resulting sudden propagation of huge amount of landward saline water intrusion through estuaries. In this

study, a SIDR like cyclone (i.e. intensity same as SIDR but hydrodynamic condition different) is selected to simulate

changes of salinity intrusion. A calibrated and validated salinity model (Delft3D) and storm surge model (Delft Dashboard)

was used to simulate the surface water salinity values. Salinity isohaline lines, comparing normal dry condition and climate-

cyclone driven scenarios were drawn to generate an overview on the level of resulting landward salinity intrusion.

Upstream and downstream (including the sea level rise) boundaries are generated by using results from hydrology and

ocean models. Estuarine salinity will increase and propagate landwards through estuaries significantly due to sea level rise

and changed upstream discharge. During a cyclonic event, major changes in salinity intrusion occurs, due to sea level rise

associated with changed upstream flow, compared to a normal dry condition. Result showed that impact of cyclone on

salinity intrusion for different hydrodynamic condition was noticeable in eastern and central estuaries, especially in eastern

estuarine system (Lower Meghna and Tentulia).

KEWORDS: Cyclone, Storm surge, Climatic impact, Salinity intrusion, Salinity and storm surge model.

1 INTRODUCTION

Climate change has an influence on formation of cyclone, sea level rise and temperature that play important role for

increasing the salinity in estuaries through salt water intrusion from ocean (Bhuiyan et al., 2012).Several studies on salinity

intrusion due to sea level rise in estuarine systems of GBM delta have already been conducted (Sumaiya et al., 2015;

Susmita et al., 2014; Hussain et al., 2013).Two studies (Sumaiya et al., 2015; Susmita et al., 2014) have demonstrated the

present salinity condition of the GBM delta using contour lines. MIKE 11 Advection Dispersion Module was used to predict

location-specific river salinity using climatic scenarios from December 2049 to May 2050; the focus of which was changes

in river salinity and probable impacts (Susmita et al., 2014). Another study applied salinity 3D hydrodynamic model

(Delft3D) to predict maximum increase of salinity along the Sundarban coast of Bangladesh for the years 2020, 2050 &

2080 (Hussain et al., 2013) . According to IPCC Third Assessment report, frequencies of tropical cyclone in Bangladesh

will be intensified (Mahmuduzzaman et al., 2014). Moreover, the higher mean sea level will intensify the storm surges (Ali,

1996). To add to, impact of cyclone in different hydrodynamic condition (sea level rise and changed upstream discharge)

will influence salinity intrusion. Bangladesh experienced the deadliest cyclones in 1970, 1991, and 2007 and in 2009 (Ali,

1996). Among them SIDR in 2007 is a catastrophic cyclone which carried significant amount of saline water through the

estuaries (Mahmuduzzaman et al., 2014). Several notable numerical modeling works like Das P.K., (1972) and Dube, S.K.

(1985) have developed to model the cyclone and predict the accurate information on the height and location of peak surge

along the coastline (Das, 1972; Dube et al., 1985). However, the movement of salinity front resulting from cyclonic storm

surge in the estuaries of GBM delta has not yet conducted. In this study, salinity and storm surge model is coupled in

Delft3D modeling suite to determine salinity intrusion resulting from climate change impact and cyclone induced storm

surge.

2

2 STUDY AREA

The Study Area comprises south-western, south central coastal region including the estuarine system of the Ganges-

Brahmaputra-Meghna (GBM) delta which is divided into three parts named as Eastern Estuarine System (EES), Central

Estuarine System (CES) and Western Estuarine System (WES) (Haque et al., 2016; Rahman et al., 2013).EES consists of

Lower Meghna, Tetulia, Lohalia, CES consists of Bishkhali, Baleshwar, Burishwar estuaries and WES consists of the

estuaries of Sundarbans, including Rupsha, Sibsa Rivers and all other cross connecting rivers (Haque et al., 2016).

Delineation of the estuarine systems along with the track of cyclone SIDR is shown in Figure 1.

Figure 1. Study Area with Model Validation Locations

3 MODEL DESCRIPTION

Delft3D open-source hydrodynamic and salt-transport model was used to simulate the salinity values in estuaries. For

modeling setup, finer grid resolution was around 300 meter in river while for ocean the size varied up to 500 meter.

Upstream boundaries were specified as measured discharges and downstream boundaries were specified as tidal water

levels at sea. Discharge data was used from the measurements of Bangladesh Water Development Board (BWDB) for the

year of 2011 (considered as normal dry condition). Downstream tidal water level boundaries including sea level rise

scenarios were specified by using the results from GCOMS ocean model (Kay et al., 2015). Upstream discharge data for

climate extremes were generated using results from INCA model (Whitehead et al., 2015). Bathymetry of the Bay of Bengal

was generated using the open-access bathymetry data produced by General Bathymetric Chart of the Oceans (GEBCO). The

inland ground elevation data was collected from Centre for Environmental and Geographic Information Services (CEGIS),

Bangladesh (Sakib et al., 2015). Measured cross sectional data for each of the estuaries in the estuarine systems of the GBM

delta at selected location were surveyed by ESPA delta project (http://www.espa.ac.uk). For the salinity model, constant sea

salinity of 35ppt for the Bay of Bengal (Haque et al., 2016) was specified as sea boundary condition and zero for fresh

water upstream condition.For storm surge model, SIDR track was collected from Indian Meteorological Department (IMD)

(http://www.rsmcnewdelhi.imd.gov.in). The landfall of cyclone SIDR was in the maximum salinity period i.e. June 6 for

normal dry year (2011) and May 15 for climatic year (2092).The wind and pressure field of cyclone SIDR is generated by

using the Delft Dashboard which was coupled with salinity model (Delft3D interface). The simulation time step of the

coupled model was 10 minutes. Upstream and downstream boundary conditions of storm surge model was similar to that of

salinity model. Computation domain of salinity and storm surge models showing bathymetry as well as upstream discharge

and downstream ocean boundary locations are shown in Figure 2.

3

Figure 2. Model Domain and Bathymetry (in meter) including upstream discharge and downstream ocean boundary

4 MODEL CALIBRATION & VALIDATION

4.1 Salinity Model

The Delft3D salinity model was calibrated and validated using historically observed data at observed locations (Figure

1). Spatially variable parameters; roughness, eddy viscosity and diffusivity were used for calibration. Validation year which

is also considered as the normal dry condition (year 2011) was selected based on availability of measured salinity data for

the maximum number of different stations in BWDB’s data archive. Results generated from the model computation were

also validated with the findings of a World Bank report (Susmita et al., 2014) (Figure 3).

4

Figure 3. Validation of model by comparing model data with measured data, and world bank study (Susmita et al., 2014)

4.2 Storm Surge Model

The storm surge model is calibrated and validated by using measured tidal water level data during cyclone SIDR.

Calibration and validation locations are shown in Figure 1. Roughness and wind drag coefficients are used as the calibration

parameters. Performances of the model during the calibration exercises are evaluated by computing the model reliability

(Haque et al., Unpublished). Using this indicator, the maximum model reliability for the storm surge model for the ‘most

acceptable model parameters’ is obtained approximately 60%. The calibrated model is then validated where measured tidal

water level was available during the time of landfall of the cyclone. The calibration and validation performances of the

model are shown in Figure 4.

Figure 4. Storm surge model calibration and validation

5 DESCRIPTION OF SCENARIOS

One of the ESPA scenarios (Kay et al., 2015) is used as the ‘climate extremes’ in this study. Temperature rise of

4.090C is used to generate changes of upstream flows along with 0.54m sea level rise. According to ESPA scenario, this

climate extreme is found to occur near the end of this century (in the year 2092). The conditions of climate extremes are

summarized in Table 1.

Table 1. Conditions of climate extremes

Temperature Rise (⁰C )

Sea level Rise

(m) Time & Snapshot

4.09 0.54 End Century(2092)

6 RESULTS

From historical observation of available data (1997-2011), maximum salinity had been found in maximum salinity

season (February to June) and most of the observation stations had maximum salinity during late April to early June. Hence,

May and June were selected for the purpose of analysis. Salinity situation is found through isohaline representation for a

5

particular day. 1 ppt, 5 ppt, 15 ppt and 25 ppt isohalines were drawn to depict salinity variation in estuarine system with and

without the cyclonic storm surge effect.The isohalines are drwan for surface water salinity, considering normal dry,

climatic extremes and cyclonic conditions. For this study, June 6 and May 15 were selected for drawing isohaline lines for

the normal dry condition and climate extremes respectively. Normal dry condition without cyclonic event: In normal dry condition salinity value was very low in EES. Isohaline of

1 ppt and 5 ppt passed through the Lower Meghna and Tentulia. Then the isohalines (1 & 5 ppt) was moving gradually into

the CES i.e. from Bhola towards Patuakhali District. The salinity line of 1 ppt and 5 ppt went no further than Pirojpur and

Bagerhat District. Whereas, 15 ppt salinity isohaline started from lower central estuarine system reached the Khulna district

of WES. The WES had salinity value of 25 ppt or more; as the isohaline shows (Figure 5).

Figure 5. Isohaline lines of normal dry condition (year 2011) with and without cyclonic condition (The thin line shows the normal

condition and the thick line shows the cyclonic condition)

Normal dry condition when there is a cyclone: After landfall of SIDR like cyclone at Bishkhali mouth, 1 & 5 ppt

isohalines had moved significantly upward in the EES (Figure 5). 1 ppt line moved approximately 17 km upward in

Tentulia estuary and 14-15 km upward in Lower Meghna estuary. 5 ppt salinity line intruded 17 km upward in Tentulia and

8 km in Lower Meghna estuaries (Figure 5). In CES, the change was not significant for 1 & 5 ppt salinity isohalines. On the

other hand, 15 ppt salinity moved towards upward direction by approximately 9, 8 & 6 km in Buriswar, Bishkhali and

Baleswar estuaries respectively (Figure 5) whereas 25 ppt isohaline did not change noticeably in the WES. Analyzing the

normal dry condition, It was found that 15 ppt salinity intruded more in CES due to landfall of SIDR like cyclone at

Bishkhali estuary. Whereas, 1 and 5 ppt salinity isohalines intruded significantly in EES i.e. at Lower Meghna and Tentulia

estuaries. Impact of cyclone on salinity intrusion is evident from Figure 5. The isohalines are deflected towards the cyclone

track particularly at and near the landfall locations. The magnitude of this deflection depends on the location of landfall,

track and strength of cyclone. Figure 5 shows situation just at the time of landfall. Obviously, as the cyclone propagates

towards the land after the landfall, the isohalines further inland will be deflected (not shown in this figure) showing further

intrusion of salinity inland.

Climate extremes without cyclonic event: In climatic extreme (year 2092), the salinity values showed a drastic change

for all isohalines. Fresh areas of EES as well as CES were not fresh anymore, as the 1 ppt and 5 ppt isohalines shifted

spatially upward. 1ppt salinity zone intruded 10 to 25 km from Lower Meghna estuary and more than 80 km upward from

6

Tentulia estuary (Figure 6). This movement continued in upward direction until it reached the previously saline free central

estuarine system (Jhalokathi to Barisal District). Similarly, 5 ppt isohaline shifted upwardly in the estuary system of EES,

CES and intruded into Khulna District. The estuaries which had salinity values of 1 ppt in normal flow year had salinity

values of 5 ppt in a climate extreme year (Figure 6). This change indicated that both CES and EES were significantly

affected by higher levels of salinity intrusion. However, 15 ppt line shifted upwards, indicating more salinity intrusion in

CES and Lower Meghna of EES. The shifting of 15 ppt isohaline was 25-27 km landwards through Baleswar, Burishwar,

Bishkhali (CES) and 11 to 28 km in WES of the study area. The model result showed change of salinity levels for 25 ppt

isohaline line in WES was near the downstream of the Rupsha-Sibsa-Passur system that likely to arrive near estuary mouth

of EES (Figure 6).

When Figure 6 is compared with Figure5, it is seen that impact of climatic extremes (sea level rise accompanied by

changed upstream discharge) on the pattern of isohaline movement is different from what was observed for isohaline

movement due to cyclonic event. Instead of ‘deflection’ (as observed during cyclonic event), saline fronts are ‘moved’

systematically upstream. The amount of movement depends upon ‘strength’ of variables representing the climatic extremes.

Figure 6. Isohaline lines of normal dry condition (year 2011) and climatic extremes (year 2092) without cyclonic condition

(The thin line shows the normal dry condition and the thick line shows the climatic extremes)

Combination of climate extremes and cyclonic event: In EES, 1 ppt line had intruded approximately 23 km through

Lower Meghna estuary towards inland (Figure 7). Whereas, 5 ppt invaded 11km and 17 km in Lower Meghna and Tentulia

estuaries respectively. The intrusion continued through CES but was not much drastic compared to EES. 5 ppt salinity

isohaline moved upwards by 13 km in Buriswar estuary of CES. Moreover, 15 ppt isohaline followed same pattern of

movement as well. It intruded approximately 13 km, 22 km, 10 km and 6 km respectively in Lower Meghna, Tentulia,

Buriswar and Bishkhali estuaries respectively. Again 25 ppt isohaline showed that salinity intrusion occurred in CES and

EES systems compared to WES (Figure 7).

Analyzing the combination of climatic extremes and cyclonic event, it was found that salinity intrusion occurred

considerably in EES and CES compared to WES due to the landfall of SIDR like cyclone (Figure 7). When there is a

cyclone on a climate extreme situation, the movement of isohaline is almost similar to what was observed when there was

cyclone in normal dry condition (Figure 5). The isohalines are ‘deflected’ along the cyclone track (Figure 7) similar to what

7

was observed during a normal condition. From model simulation it was found that after the landfall of cyclone SIDR in both

normal dry and climatic condition, the recovery time of isohalines to the ‘undisturbed’ flow condition was around 4-5 days.

Figure 7. Isohaline lines of climate extremes (year 2092) with and without cyclonic condition (The thin line shows the climate

extremes and the thick line shows the cyclonic condition imposed on climate extreme)

7 CONCLUSION

When there is a cyclone, isohalines are deflected along the cyclone track indicating increased salinity intrusion inland. On

the other hand, isohalines are systematically moved inland due to climatic extreme. In both cases, the deflection and

movement of isohalines depend on the strengths of cyclone and climate extremes.

In this study, salinity intrusion was noticeable in the eastern and central region of Bangladesh coast due to landfall of

SIDR like cyclone both during normal dry condition and climatic extremes. Compared to the eastern and central region, the

western region did not face any abrupt change.

ACKNOWLEDGEMENT

This work was carried out under the Collaborative Adaptation Research Initiative in Africa and Asia (CARIAA), with

financial support from the UK Government’s Department for International Development (DFID) and the International

Development Research Centre (IDRC), Canada. The views expressed in this work are those of the creators and do not

necessarily represent those of DFiD and IDRC or its Board of Governors. Website: www.deccma.com.

REFERENCES

Ali, A., 1996. Vulnerability of Bangladesh to climate change and sea level rise through tropical cyclones and storm surges. In Climate

Change Vulnerability and Adaptation in Asia and the Pacific, pp. 171-179

Bhuiyan M and Dutta D, 2012. Assessing Impacts of Sea Level Rise on River Salinity in the Gorai River Network, Bangladesh, Estuarine

Coastal and Shelf Science, VOL. 96, PP. 219-227.

Das,P.K., 1972.Aprediction model for storm surges in the Bay of Bengal. Nature,239,211-213.

Dube, S.K., P.C. Sinha and G.D. Roy., 1985. The numerical simulation of storm surges along the Bangladesh coast, Dynamic of

8

Atmosphere and Oceans, 9, 121-133.

Haque A, Sumaiya and Rahman M, 2016. Flow Distribution and Sediment Transport Mechanism in the Estuarine Systems of the Ganges-

Brahmaputra-Meghna Delta, International Journal of Environmental Science and Development, VOL. 7, NO. 1.

Haque, A., Sumaiya, S., and Rahman, M., Reliability analysis of dynamic models, “unpublished” (Journal)

Hussain MA, Islam A.K.MS, Hasan MA, and Bhaskaran B, 2013. Changes of the seasonal salinity distribution at the Sundarbans coast

due to impact of climate change,4th International Conference on Water & Flood Management.

Kay S, Caesar J, Wolf J, Bricheno L, Nicholls RJ, Islam AKMS, Haque A, Pardaens A, and Lowe JA, 2015. Modelling the increased

frequency of extreme sea levels in the Ganges–Brahmaputra–Meghna delta due to sea level rise and other effects of climate change,

Environmental Science: Processes & Impacts, VOL. 17, PP. 1311-1322

Md. Mahmuduzzaman, Zahir Uddin Ahmed, A. K. M. Nuruzzaman, Fazle Rabbi Sadeque Ahmed,2014. Causes of Salinity Intrusion in

Coastal Belt of Bangladesh ,International Journal of Plant Research,4(4A): 8-13

Whitehead P. G., Barbour E., FutterM. N, SarkarS.,RoddaH., CaesarJ., ButterfieldD., JinL., SinhaR., NichollsR. andSalehinM., 2015.

Impacts of climate change and socio-economic scenarios on flow and water quality of the Ganges, Brahmaputra and Meghna

(GBM) river systems: low flow and flood statistics, Environ. Sci.: Processes Impacts, VOL. 17, PP. 1057-1069

Rahman M, Haque A, Siddique MKB, et al., 2013. A preliminary assessment of the impact of fluvio-tidal regime on Ganges-

Brahmaputra-Meghna delta and its impact on the ecosystem resources, International Conference on Climate Change Impact and

Adaptation(I3CIA-2013), Center for Climate Change and Sustainability Research (3CSR), Department of Civil Engineering, DUET,

Gazipur, Bangladesh

Sakib M, Nihal F, Haque A, Rahman M, and Ali M, 2015. Sundarban as a Buffer against Storm Surge Flooding, World Journal of

Engineering & Technology, VOL. 3, PP. 59-64

Sumaiya, Haque A, Rahman M, Elahi WE, Ahmed I, Rimi RA and Alam S, 2015. Modelling Salinity Extremes in Bangladesh Coast, 5th

International Conference on Water & Flood Management (ICWFM-2015), Dhaka, Bangladesh

Susmita D, Akhter KF, Huque KZ, Sharifuzzaman C, Ainun N, 2014, “River salinity and climate change : evidence from coastal

Bangladesh”, Policy Research working paper ; no. WPS 6817. Washington, DC: World Bank Group