CD Tidal cree
5.4 Land Elevation
Chittagong City has a complex elevation contour with a rapid variation in elevation because of hilly area elsewhere in the city. The ground elevation lies less than I m to more than 60 m above mean sea level (MSL). The contour maps collected from BWDB, have been used to produce a single map setting spot elevation on the map by axis- coordinate method. For simplicity in computation, the average land elevation is shown in Fig.5.2, and results are given in Table 5.2. The figure and the table indicate that a vast area in Chittagong, mainly along the Bay of Bengal and the Karnafuly River, is lowlying and are exposed to cyclonic surge making it vulnerable to cyclone.
Table 5.2 Area of Chittagong Under Different Land Elevation
Land elevation (m above MSL) Area (sq.km) % of total area
1-2 32 19
2-3 43 26
3-4 16 9
4-6 21 12
6-9 12 7
>9 45 27
Total 169 100
I
Fig.S.2
abov~ 15
4. 6
2-3 1-2
44
5.5 Distribution of People Habitation
Similar to land elevation, population density also differs significantly in different areas. It varies from less than 2500 persons/sq.km to over 22000 persons/sq.km (Fig.5.3). Population density in the area near the Karnafuly River is higher than that in the area near the Bay of Bengal. Thus, the expected damage to lives near the Karnafuly River is more than that near the Bay of Bengal for the same magnitude of surge height during cyclones. A population statistics with corresponding habitation area is prepared based on this map (Table 5.3). Although the population density in the area near the Karnafuly and the Bay of Bengal is relatively low in comparison to that in urban area, damage to human lives in these areas is expected to be significant because of higher surge flood depth.
Table 5.3 Population Densities in Chittagong
Land class Area (sq.km) Population density Total population
(persons/sq.km)
Urban fringe 32 <2500 <80000
Semi urban 49 2501-6400 122549-313600
Medium urban 64 6401-22000 416000-1401600
High density urban 24 >22000 >528000
Total 169
5.6 Surge Travel Distance and Flooding Depth
There remain many uncertainties in the determination of accurate surge heights and inland travel distances. The major uncertainties are as follows:
-uncertainty in the estimated wind speed of given return period or exceedence probability;
-uncertainty in the estimation of cyclonic storm land-fall probability;
-uncertainty in the estimated storm surge height at the coast;
-uncertainty in the intrusion distance of storm surge wave;
-uncertainty in the estimated height of local wind waves riding atop the storm surge wave;
-uncertainty due to local conditions of coastline geometry, near-shore topography of sea-bed and terrain characteristics of coastal land.
Due to a number of uncertainties, the results in this chapter can be dealt as indicative.
CD CD OJ OJ
Urban fringel<2S00PHsons/sq km)
S~!Jliurban
12~OO-6400 persons/sq km) Medium density urban 16500-21 900persons Isq. kml fhgh density urbol"l
~21000 Pl!rsons/SQ km'
Fig.5.3 Population Density In Chittagong Metropolitan Area
46
5.6.1 Travel Distance
The movement of storm surge over dry coastal land is a very complex process. The maximum travel distance of surge water over dry land depends on several factors, such as height of storm surge at the sea coast, water velocity, and wave velocity of the approaching surge wave, duration of storm, length of storm surge wave, tidal condition near the coast, rate of increase in water level near the coast, configuration of coast line with respect to surge direction, slope of the beach, slope of the dry land, reflection, refraction and breaking of the wave, bed material and land topography, resistance due to landuse, etc. MCSP (1992) has established a formulae for surge water intrusion distance, as follows:
4(4+1.5h)' R
x=---_
3(4+h) (Sb +f/8)
(Eqn.5.1)
Where, x is the maximum distance travelled by leading edge from the shore line, R is the factor accounting for various hydraulic and topographic uncertainties, Sb is the bed slope, h is the surge height. The suggested values of Sb , f, R for ~e coast of Bangladesh are 0.001, 0.01 and I, respectively (MCSP, 1992). The maximum travel distance over the dry land by surge heights are given in Table 5.4.
Table 5.4 Maximum Travel Distance by Surge Water of Different Exceedence Probabilities
Exceedence probabilities 0.1 0.05 0.02 0.001
Travelled distance (km) 6.40 7.80 9.75 11.30
5.6.2 Flooding Depth
The flooding depth due to surge at a location is mainly dependent on surge height and wind generated local waves ridding atop the surge wave. The surge wave is a gradually varying flow while the wind generated local wave is a rapidly varying flow. The rapidly moving local waves cause most harm to people desperately trying to survive in the flowing water of the storm surge wave.
The following formula may be applied for the determination of flooding depth due to surge at a location in the coastal area.
h = hT- (x-I)k + hw, x=1 ifx<1 hw= h-(x-I)K/4, hw =1 ifhw< I D =h-y
(Eqn.5.2) (Eqn.5.3) (Eqn.5.4)
Where, D is the surge flooding depth in m due to surge height of magnitude, hT corresponding to return period T, hw is the amplitude of local waves (in m) from mean water level, and y is the ground elevation with respect to mean sea level. x is the distance of the location, where surge height is to be predicted from a reference line, the coast line is considered as the reference line (the line up to which the water reaches during spring). K is the retardation factor which accounts for the reduction in surge height due to various resistance or obstruction during its propagation. MCSP (1992) study recommends a value of k 112 per km for Chittagong-Cox's Bazar-Teknaf area and 1/3 for other coastal areas. There is no data on wind generated local wave. MCSP (1992 ) study suggests
a
value of 1/4 of the surge height for local waves, but should not be less than I m.Using the values of surge heights, hT for Chittagong coastal regions from Table 4.8, in Eq.5.1, the maximum possible water levels at different locations at various distances from the coastline have been predicted and presented in Table 5.5. It is assumed that surge water is coming from various directions and moving toward inland perpendicularly. This assumption is somewhat conservative for planning mitigative measures against surge flooding. With the help of the water levels in Table 5.5, and using the land elevation value, y from Fig.5.2, surge flooding depth at various locations have been computed and shown in Fig.5.4. The results in the figure are shown in terms of different flood depth contour due to surge at different exceedence probabilities.
Table 5.5 Maximum Possible Water Levels Over Dry Inland at different Exceedence Probabilities
Distance, x (km) Exceedence Probabilities
0.1 0.05 0.02 0.01
1 4.20 5.40 7.30 8.80
2 3.70 4.80 6.60 8.00
3 3.20 4.30 6.00 7.50
4 2.70 3.80 5.40 6.90
5 2.20 3.30 4.80 6.30
6 1.70 2.80 4.30 5.60
7 0.5 2.30 3.80 5.00
8 0 1.00 3.50 4.50
9 0 0 3.00 4.00
10 0 0 0 3.50
5.7 Delineation Of Risk Zone and High Risk Area in Context to Damage to Human Lives
Risk is the possibility of loss or injury to people and property. During a cyclonic storm loss and injury to people and property may OCcur from two sources. One is the strong wind and the other is the storm surge flooding. Despite the wind velocity, over 90% death during cyclones are caused by surge flooding. The present study is mainly concerned with the risk due to storm surge flooding. As followed in MCSP (1992) study, the following criteria have been adopted in delineating the Risk Zone
-11- Floodd'plh1m' contour lin e
'1
'\
o.BAY OF BENGAL
N
48
\\
\
o
!
S( ALE
LEGENO
1 km
!
l'ig.S.4(a)
I'lood Depth Contour Cauoed by Surge !Ieight With Exceedence Probability O. I
Flood Depth Contour Caused by Surge Height With Exceedence Probability 0.05 2 k
,
mo
I
~ LEGEND
-2-3~ Flood del'fh(mI
confour line 2.9
BAY OF BENGAL
Fig.5.4(b)
50
1 km
o
SCALE
illQ!.Q
-1-6- Flood depth I mI contour line
Flood Depth Contour Caused by Surge Height With Exceedence Probability 0.02 BAY OF BENGAL
Fig.5.4(c)
Flood depth 1m' contour line
N
BAY OF BEN GAL
LEG ENO
o
, 1t
jkm
f'ig.5.4(d)
f'lood Depth Contour CRused by Surge Height With Exceedence ProbRbility 0.0 I
52
(RZ) and the High Risk Area (HRA). Delineation of HRA herein is on the basis of that damage to human lives and cattleheads is extreme in this area.
(1) The RZ extends from the coast line (sea coast or river bank) to an inland limit upto which surge water can reach.
(2) The HRA includes a strip of land within the RZ. It extends from the coast line upto a limit where the depth of storm surge inundation may reach upto one meter. MCSP (1992) study has selected the one meter depth criterion for HRA delineation with the view that an adult could force his way through water as long as the depth of water remained below his waist height. So a depth of one meter, which is near the height of the waist of an average adult, has been selected as the criterion for delineation of HRA.
Based on the above assumptions RZ and the HRA have been delineated, as shown in Fig.5.5 and Fig.5.6, respectively.
5.7. I Population under Risk Zone and High Risk Area in Chittagong
The areas under the RZ and HRA at surge heights with different exceedence probabilities and associated affected people are given in Table 5.6.
Table 5.6 Risk Zone and High Risk Area at Surge Height with Different Exceedence Probabilities
Exceedence Risk Zone High Risk Area
Probabilities
Area (sq.km) Population Area (sq.km) Population
0.10 59 218500-482900 48 177762-392868
0.05 82 424800-917100 65 336732-726969
0.02 90 527300-1026000 72 421840-820800
0.01 108 665200-1240000 86 529696-987407
5.7.2 Submerged Land in Risk Zones
Different land types which would be submerged under flood water due to storm surge are given in Table 5.7. A significant portion of Chittagong district is hilly and thus free of surge flooding. It may be summarized from this table that agricultural area constitutes the major part of affected area;
damage to crops is likely to be severe. Because the agricultural lands are Iowlying and near the Bay of Bengal and the Karnafuly River, where surge heights and flooding depths are expected to be more;
and are lowlying. Since the industrial area is near the Bay of Bengal and the Karnafuly river as can be seen from Fig.5.I, it is expected that these areas are highly surge vulnerable. The commercial
N
BAY OF BENGAL
o
LEGEND
1 km
Boundary line of RlskZD('lf of l!:xcl!l!dl!nef p"obobW.,
0.'
Fig.5.5 Risk Zones Boundary Under Surge Flooding With Different Exceedence Probability
54
N
BAY OF SUIGAL
o
!Boundary lin!! of High ~isk Ar!Q ot
exceldenc! probabilily 001
Fig.5.G High Risk Area Boundary Under Surge Flooding With Different Exceedence Probabilities
areas are less susceptible to surge flooding compared to other areas as they are far inland from the sea and the river. Table 5.7 indicates that a great care should be taken to protect the industrial area from cyclonic damage caused by surge flooding.
Table 5.7 Different Types of Lands Under Flooding Due to Storm Surge With Different Exceedence Probabilities
Exceedence Agricultural area Industrial area Housing & commercial area Total area
probabilities in sq.km(%) in sq.km(%) sq.km(%) in sq.km
(%)
0.10 35 (76) 12 (75) 12 (22) 59 (35%)
0.05 46 (100) 12 (75) 20 (37) 82 (49%)
0.02 46 (100) 12 (75) 28 (51) 90 (53%)
0.01 46 (100) 12 (75) 46 (83) 108 (64%)
5.8 Delineation of High Risk Area in Context to Industrial Damage
The probability of financial and economic loss incurred due to cyclonic surge flooding in the Chittagong industrial area is enomorous. Thus, it seems practical to identify the high risk zones in the Chittagong industrial area separately from other areas of the city.
Assumptions of high risk area in context to industrial damage caused by surge are quite different from those for delineation of HRA in context to damages to human lives. In this study, it has been assumed that damage to industry, mechanical or electrical, may occur only when the water level exceeds the plinth level of the facility. However, the type and extent of damage will depend on the type of the industry itself and the duration of the cyclonic surge sustained. For simplicity these factors are assumed to be uniform for all purposes.
With the he'lp of land use pattern (Fig.5.l) and flood depths depicted earlier (Fig. 5.4), an estimation of industrial belt under HRA is generated in Table 5.8. As the most industrial areas, as shown in Fig.5. I, are near the Bay of Bengal where surge flood depths are always higher than any other areas, they are under HRA for a surge flooding of even a low magnitude. Fig.5.7 shows the industrial areas under HRA for a surge flooding of low magnitude (0.1 exceedence probability). It is evident from this figure that the most important industrial area the Chittagong Export Processing Zone near the Bay of Bengal is always at a high in risk of surge flooding from the Bay of Bengal. The other industrial areas lie on elevated lands near the hilly areas far away from the Bay of Bengal and the Karnafuly River, where there is no surge flooding even for high intensity cyclones, so these industrial areas are free from damage by surge flooding. The only damage to these industries is expected by high wind velocities during cyclone.
56
N
InduSlries under High Risk due to Surge Flooding with Exceedcnce Probability O. I
1 km
,
\
o
I
SC ALE
LEGE HO
-11_ Floodd.pfh1m!
contour lin!
\\
17
BAY
or
BEHGALFig.5.?
Table 5.8 Risk Zone and High Risk Area in the context of Industrial Damage by Cyclonic Surge
Exceedence Probability Industrial Area under Surge Flooding
Risk Area in sq.km(%) High Risk Area in sq.km(%)
0.10 12 (75) 12 (75)
0.05. 12 (75) 12 (75)
0.02 12 (75) 12 (75)
0.01 12 (75) 12 (75)
Inland flooding due to surge during cyclones is a great disaster to Chittagong City. Approximately 35%, 49%, 53% and 64% area may be subjected to surge flooding due to surge height with exceedence probabilities 0.1, 0.05, 0.02 and 0.01, respectively. A considerable number of people may be threatened due to surge flooding in High Risk Areas. On an average, 28500, 531800, 6213320, 758551 persons, respectively with exceedence probabilities mentioned earlier may be affected. Most agricultural areas may be subjected to complete inundation at these exceedence probabilities. The damaging effect of inundation to agricultural lands and industrial areas are expected to be severe, The commercial areas are less susceptible to surge compared to other areas, because most of them are far inland from the coastline. Beside the construction of embankment, non-structural measures such as, landzoning interms of surge vulnerability, may prove to be effective in reducing the damaging effect of surge inundation. Since, various uncertainties are involved in determining surge heights, the results reported here are indicative.
•
58
CHAPTER 6
PERFORMANCE OF COASTAL EMBANKMENT AGAINST SURGE FLOODING
History of coastal embankments of Bangladesh, its design characteristics and construction procedures have been discussed earlier. Primary focus of the following sections will be on the coastal embankment in the study area, Chittagong City, and its performance evaluation against surge flooding.
6.1 Coastal Embankment in the Study Area
The embankments in the Chittagong Metropolitan City area are constructed along a portion of the Bay of Bengal as a part of CEP by BWDB (Fig.6.1). These embankments are under POLDER-62 of CEP.
A typical section of embankment has already been shown in Fig. 3. I.
A survey was conducted on the embankment of the study area by the consultants of Cyclone Protection Project II FAP-7 under Flood Action Plan (CPP, 1992). Their description on this embankment was verified during field visits as a part of the present study, and are described here.
The polder belongs to Patenga Thana (Chittagong port area) under Chittagong circle. This embankment starts from the coastal sites of Chittagong airport area and extends upto the end of Uttar Halisahar. About 19.0 kilometers of embankment were surveyed as shown in Fig.6.2. Just from the .beginning of the polder up to the Naval Office a good plantation of Babla (Acacica Arab/ca) trees were found to be growing on both sides of the slope which were planted during 1968-70. The trees are in good growth and the average height and girth were measured and found to be approximately 12.0 m and 35.0 cm, respectively. Under the trees, the embankment was well covered with durba grass (Cynodon Dactylon). Beyond the Naval Office, the embankment bent sharply and continues for a few kilometers. This area is the most seriously affected area from the direct sea wave action and cyclone surges. On the embankment, few scattered date palm trees, of height about 8-10 m, and local bushes are found on the slopes of the countryside. The sea side has been protected by concrete and brick blocks as well as bolders against damage by high tidal waves and cyclonic surges.
The forest department planted mangrove species in the foreland several times earlier during 1975 to 1980 but the afforestation could not be established due to heavy wave action and repeated erosion.
From the chainage 3.56 km to 5.50 km, the whole embankment was completely breached out during the devastating cyclone in 1991. A new embankment was found to be under construction in this area.
From chainage 7.0 km to 9.9 km the whole embankment is covered with good planation of Babla (Acacica Arab/ca), shill koroi (Alb/u/a Procora;, Ipil-Ipil (Leuceana Leococephylla), khoir tree
SCALE
m 1000, !leo! lJ'[ 1000, 1000 'COO: •••
Proposed ortorlSrolron [xis ling embankment Proposed embankment E"dslinq offortstolion
a
•
4 I '.
...L. ..L...J...1
LEGEND
Coaslal Embankment in the Study Area Fig.5.1
60
(Acacia Catechu), Rain tree (Samanea Saman), date palm (Phoenix dactylifera) and are growing very well in both the slopes of the embankment. On the foreland, the forest department established a good mangrove afforestation with the species of Baen (Avicinea ojficinalis), Garan (Cereops decandra), Gulpata (Nypa fruitcans), Gewa (Exoecaria agallocha), Keora (Sonneratia apatala), Kankra (Bruguiera Gymnorhiza), etc during 1980-1985. But the Gulpata species was not established in this . area. The mangrove species are growing very well. A portion of the mangrove afforestation was destroyed and felled out by the shrimp cultivators. From the chainage 9.91 km to 10.67 km, just opposite to the Chittagong Export Processing zone (CEPZ), the embankment was relocated few years back and the plantation on the embankment was not yet established. In the foreland, there is no mangrove afforestation although mangrove afforestation could be established here. After this, the embankment is continued upto the beginning of Dakhin Kattali. The whole area is covered with good plantations of Babla, Shilkoroi and Raintree, etc. and some scattered mangrove afforestation on the foreland. The existing and the proposed afforestation on the slope of the embankment as well as on the foreland of the seaside are shown in Fig.6.1.
6.2 Damage to Embanlonent
Once an embankment is constructed, it can sustain damage in several ways. Failure may be either from its internal construction fault or from damages caused by outside forces, such as wave attack, erosion due to surge wave overtopping during cyclone, etc.
6.2.1 Damage by Cyclonic Surge Wave
Erosion from surge wave overtopping:
If the design water level for embankment is exceeded, overflow can take place. The height of embankment in the study area is 6.04 m. Table 6.1 represents the maximum water level for different return periods under monsoon and cyclonic conditions. This table indicates that the embankment height corresponds only with the surge height of return periods between 10 and 20 year.
Table 6.1 Monsoon Water Levels (CPP, 1992) and Displacement of Water Surface During Cyclones
[tern
Return period (years)
10 20 50 100 200
Monsoon water levels (m) 3.75 4.04 4.4 4.68 4.93
Cyclonic water levels (including local 5.90 7.08 8.95 10.45 12.08 waves)(m)
Cyclonic still water levels 4.90 6.00 7.50 8.70 10.00
Formulae stated below has been used to estimate the overflow rate per unit length of embankment for different water levels and given in Fig.6.2.
The discharge per meter width of embankment can be roughly estimated as
q =O.38h(2gh)05
Where,
q=discharge per meter width (mJ/s/m) g = acceleration due to gravity (m/s')
h = height of water over embankment crest level (m)
(Eqn.6.1)
8 7
~6 .c
"-
E 5
~"-
E
'0 4
x
~3 l:'
"
.cv
.~
0Hei ght of overflow above crest (m)
. Fig.6.2 Discharge During Overflow under Cyclone
Using Fig.6.2, and formula given below, the maximum overflow velocities have been computed as shown in Fig.6.3.
The maximum velocity (not considering waves) on the inner slope can be roughly estimated by the following formula (Engelund,1965):
v
rna);= V =q°.4Mo.0 6(Sina)o.JWhere
Vm"= overflow velocity (m/s)
q = Discharge per meter of embankment length (mJ/s/m) M=Manning Number
a= Inner slope angle
(Eqn.6.2)