CHAPTER 5 RESULTS AND DISCUSSION -...
Transcript of CHAPTER 5 RESULTS AND DISCUSSION -...
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CHAPTER 5
RESULTS AND DISCUSSION
5.1 GENERAL
This study is an attempt to develop a systematic methodology
integrating IFM and SSM for efficient flood management. In this research, the
available database of the Adayar watershed was analyzed for efficient
planning and management of flood related issues. Technical, social and
economic analysis was done for the clear understanding of the flooding
problem and also seeks solutions to minimize the effect of flooding. This
research work used conventional methods in addition to RS and GIS. Using
the procedure explained in Chapter 4, the flooding problems associated with
the watershed, changes in land use/ land cover, prioritization of flood source
area, the areas vulnerable to flooding in the Adayar watershed and the
monetary value of flood damages due to urban flooding in Velachery and
riverine flooding in Kotturpuram Housing Board area were all assessed. The
results obtained from the analysis are presented and discussed in this chapter.
5.2 HYDROLOGIC-HYDRAULIC ANALYSIS
5.2.1 Problem Identification Using SSM and Other Techniques
The problem identification techniques like SSM, Semi Structured
Interview etc help in diagnosing the problem with comprehensive and
coherent information. Different perceptions of the problem situation were
gathered from a number of people involved in order to identify the actors and
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issues related to flooding problem in the Adayar watershed. This
identification of actors and issues help in analyzing the facts, generation of
innovative ideas to solve the problem. The extreme range of the participants’
responses to the questions like ‘what’, ‘how’, ‘why’, ‘when’, ‘where’ etc.
even in the simple identification of problems demonstrates how ill-defined
and complicated the situation could be (Bunch 2003). Several issues, which
are relevant to flooding problems in the Adayar River is consolidated in
Table 5.1. The priorities of the participants vary. For example, State Pollution
Control Board representatives express the pollution due to tanneries in
Pallavaram area to be problematic, while participants from Institute of
Hydraulics and Hydrology, Poondi, prioritize the closing of the river mouth
due to sand bar formation.
Table 5.1 Flooding problems associated with Adayar River identifiedusing SSM and other techniques
Sl. No. Category Identified problems1 Health Hazards Mosquito breeding, threat to population in vicinity2 Hydrology and
HydraulicsInadequate storm water drainage, blockage by sand barat the estuarine mouth, clogging of storm water drains,flooding and overflowing, unpredictable flood flow, nomaintenance of hydraulic structures
3 Sensory aspects Unhygienic atmosphere, foul smell4 Pollution related
factorsSewage let into storm water drains and into the river,illegal dumping of waste, solid waste dumping, poorwater quality
5 Urbanization Change in land use pattern, migration of people,population growth, slum development, encroachmentalong river banks
6 Environmentalaspects
Degradation of river water, environmental degradation,reduction in aquatic life
7 Managementaspects
Poor allocation of funds, lack of communication andcoordination among different institutions, lack ofintegrated approach to the problem, lack of publicawareness, unwillingness of the people to cooperate,implementation of any plan not acceptable by the peopleis not feasible (political issues)
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Table 5.2 illustrates the how the situation would look if the problem
remains the same and how the situation could be if it is solved from the
stakeholders’ point of view.
Table 5.2 Responses for what would happen if the problem is solved?
and what would happen if the problem is not solved?
Problem Solved Problem not solved
Urban sustainability
Environmental sustainability
Reduction of flood intensity
Increase recharge
Increase in the economy
People will be free from mosquito menace
Disease free environment
Slum dwellers will feel healthier
Flushing activity takes place at the estuary
No bad odour
The river will be pleasant
Loss of life
Tangible and intangible damages
Environmental degradation
Disappearance of existing flora and fauna
Polluted ground water
Spreading of diseases
Health hazards
Social problems
Bad odour – Sensory problems
Higher flooding in future if sand bars are not cleared
Degradation of the river
The views were gathered and the problem is expressed in the form
of a ‘Rich picture’. This expression of the problem situation involves
identification of various actors, components and their interactions and
relationship within the system. The rich picture helps in sorting out the root
cause of the problem and gives information on the better understanding on the
present situation. Figure 5.1 presents the rich picture diagram developed by
the workshop participants and Table 5.3 illustrates the identification of the
problems from the rich picture diagram.
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Figure 5.1 Rich picture diagram
It is clear that a host of activities like effluent disposal from
treatment plants, tanneries and encroachments are taking place along the river.
The agencies that are responsible for these activities are diverse. Also due to
littoral drift, the mouth of the river gets choked due to sediment deposition.
Hence, it is seen that addressing the issue of flooding in Adayar watershed
involves different agencies and departments. This stresses the need for
integration of various agencies in addressing the problem of flooding.
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Table 5.3 Issues identified from the Rich Picture diagram
Sl.No.
Issues identifiedOrganizations
Responsible to addressthe issues
1 Quarry in Tiruneermalai (Obstruct runoff) Department of Geologyand Mining
2 Tannery in Pallavaram (Untreated effluentdischarge, pollute water)
PCB, Industries Owner
3 Illegal sewage disposal from households,industries and sewage treatment plants(Norms of treatment are not maintained)
CMWSSB, PCB, Public
4 Encroachment (Removal and resettlementof encroachers, Burial ground officiallysituated at the bank of the river)
PWD, Housing Board,Revenue Department andPublic
5 Solid waste dumping (Ground watercontamination and health hazards)
PCB, Public
6 Sand bar (Periodical dredging must be donein order to facilitate the flow of water)
PWD
7 Urbanization CMDA
From Table 5.3, it is understood that flooding problem is a inter-
departmental problem. The problem could be minimized only when all the
government agencies, NGOs and public join their hands together.
Analytic Hierachy Process (AHP) is a kind of divide-and-conquer
problem solving method. It allows one to determine the relative order
(ranking) of the criteria involved in the process using pair wise comparison
matrix (Steiguer et al 2003). In this study, the criteria listed in Table 5.4 are
the major reasons for the flooding problem. The elements of the matrix give
the relative importance between the criteria. For example, between A and B,
A is perceived to be more important, while comparison between C and E,
both C and E are perceived to be equally important. These criteria were
determined by the workshop participants.
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Table 5.4 Pair wise comparison matrix
Sl.No.
Criteria A B C D E F G H
1 Encroachment A - A AC D E AF G H
2 Uncontrolled Development B - - C D E BF G H
3 Solid waste dumping C - - - D CE CF C H
4 Waterways D - - - - D D D D
5 Sand bar formation E - - - - - E E E
6 Pollution F - - - - - - G H
7 Inadequate & impropermaintainence of microdrains
G - - - - - - - G
8 Lack of public awareness H - - - - - - - -
Based on the number of responses, the ranking is given as shown in
Table 5.5.
Table 5.5 Pair wise ranking
Sl.No.
Criteria No ofResponses Rank
1 Waterways (D) 7 1
2 Sand bar formation (E) 6 2
3 Solid waste dumping (C) 5 3
4 Inadequate and improper maintainenceof micro drains (G)
4 4
5 Lack of public awareness (H) 4 4
6 Pollution (F) 3 6
7 Encroachment (A) 3 6
8 Uncontrolled development (B) 1 8
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From Table 5.5 it is clear that more attention must be given to the
waterways i.e. it is necessary to maintain the hydraulic properties of the river
in order to minimize the consequences of flooding.
CATWOE analysis are helpful in framing the root definitions. They
are also employed in drawing out the important themes in the rich picture
diagram. The themes are the storm water drainage system as flood
moderation/protection measure, the storm water drainage system as
sewage/waste carriers, slum dwellers as squatters, provision of sewerage
services by CMWSSB, protection of slums by vested interests, line
agency/departmental intervention to control flooding and removal of sand bar
at the river mouth. Analyzing these themes in terms of CATWOE elements
would help to reduce complex situation to a few key relevant issues.
Typically in SSM, CATWOE analysis of a theme is used to develop a brief
description (root definition) of the core nature of the system (Bunch 2003).
These themes are similar to those identified by Bunch (2003), as the two
rivers the Cooum and the Adayar flow through Chennai almost parallely
within an average distance of not more than 5 km and share similar hydraulic
and hydrologic characteristics. In addition, the line agencies/departments
responsible are the same. Themes (that can be modelled as Human Activity
Systems) that are extracted from the Rich Picture are:
(i) Storm water drainage system as flood moderation/protection
C citizens of Chennai
A Corporation of Chennai, Tamil Nadu Public Works Department
T1 un-routed runoff –> routed runoff through waterways to the
ocean
T2 flood-prone areas –> flood protected areas
W flooding should be averted
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O Corporation of Chennai, Tamil Nadu Public Works Department
E topography of the Chennai region (flat, low-lying)
(ii) Storm water drainage system as sewage/waste carriers
C citizens of Chennai, commercial enterprises and small-scale
industries
A citizens of Chennai, commercial enterprises and small-scale
industries
T waste disposed in storm drainage –> waste to be disposed by
sewerage system
W it is convenient and less costly to dispose of wastes into the
storm water drainage system
O Corporation of Chennai, legislators, Highways Department
E tax regulatory environment relating to the disposal of waste,
insufficient and inadequate sewerage system
(iii) Slum dwellers as squatters
C slum dwellers
A slum dwellers
T unoccupied land along river banks –> occupied land
(as “objectionable land use”)
W in the absence of affordable housing, any unoccupied land
may be settled
O Tamil Nadu Housing Board, PWD, Revenue department
E cities as the location of employment, economic constraints
of the economically weaker section of society
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(iv) Provision of sewerage services by the CMWWSB
C citizens of Chennai
A Chennai Metropolitan Water Supply and Sewerage Board
T unserviced areas –> serviced areas
W sewage should be properly treated before release into the river
O Chennai Metropolitan Water Supply and Sewerage Board,legislators
E limited budget, some areas are inaccessible
(v) Protection of slums by vested interests
C slum dwellers
A political groups, Government agencies
T slums in danger of eviction/clearance –> slums protectedfrom clearance/eviction
W slums and the economically weaker section of the populationconstitute a strong potential voting constituency
O legislators, state High Court, PWD, Revenue Department
E larger political and societal systems
(vi) Line agency/departmental intervention to control flooding
C Government agencies, citizens of Chennai
A officers at Government agencies
T problem in need of action –> problem defined and addressedwithin agency’s jurisdiction
W problems falling within the jurisdiction of the agency shouldbe addressed by the agency
O legislators, Government agencies
E institutional culture, sectoral and areal jurisdictional divisions
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(vii) Removal of sand bar at the river mouth
C citizens of Chennai
A Public Works Department
T flood water obstructed by sand bars –> free flow of flood
water in to the sea
W unobstructed disposal of flood water into sea
O Public works department
E Currents, littoral drift, tide and waves
The identification of these themes helped the workshop participants
to focus on the key activities and components of the situation. Important
themes drawn from this CATWOE analysis are:
(i) The hydraulic characteristics of the river;
(ii) The topography of the region (flat) leads to low rates of flow;
(iii) There is blockage by sand bar at the estuarine mouth which
restricts the flushing action due to tides;
(iv) Inflow of waste water both treated and untreated into the river;
(v) There are numerous constrictions due to bridges and debris
dumping; and
(vi) Due to low flow and stagnation, large amount of sludge is
deposited in the river.
The above theme addresses the physical system, but the effect of
flooding transcends beyond the physical systems and affects the people
significantly. Hard Systems modeling of physical systems require extensive
and reliable data, which often are not available. Soft Systems approach is
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more appropriate than Hard Systems approach to organize, observe,
understand and generate debate which is non-prescriptive. The difficulties in
SSM approach are that it is time-consuming, all the actors do not participate
and the views are not convergent.
One technique for constructing well-formulated root definitions is
to write a statement which reflects the aspects of the CATWOE (Checkland
1979). A root definition is a kind of hypothesis about the relevant system,
which might help the problem situation. It is a precise description of the
system that should capture its essential nature (Attefalk and Langervik 2001).
The root definition for managing flood in the Adayar River was formulated in
this research as:
‘A system to support more sustainable free flow of water within the boundary
limits of the river, for a reasonable period of time (X), by means of
developing a suitable flood management strategy, which is easy to operate
and manage (Y), in order to reduce the tangible and intangible damages and
create potential benefits to the citizens within the watershed (Z)’
The above root definition incorporates the collective view of the
participants. Rich pictures and their accompanying root definitions and
CATWOE analysis provide a mechanism for
(i) mapping out a problem situation;
(ii) identifying conflicts, issues, risks and opportunities;
(iii) clarifying the central focus of the systems; and
(iv) providing some clear structured way of expressing the elements
within the system (CATWOE analysis).
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A scoping exercise was performed to qualitatively understand the
spatial and temporal extent of the components, activities and purposes
associated with the flooding problem. The results of the scoping exercises are
presented in Table 5.6.
More participants saw the system as ‘urban system’ or ‘urban waste
disposal system’ rather than a ‘river system’. It is important to determine the
actors, components and their relations in order to generate objectives and
interventions for management. The first workshop ended with these activities
(problem identification, rich picture diagram, pair wise comparison matrix,
CATWOE analysis, root definition and scoping exercises).
Table 5.6 Results of scoping exercise with reference to the ‘Rich PictureDiagram’
Sl. No. System component Problem-shed Inference1 Rainfall and flooding Yearly (during monsoon
seasons)Damages (inundatedareas)
2 Storm water drains Network of drains withinthe city
Flood control
3 Sewage Throughout the citysewage is generated
Gets mixed in stormwater drain, causepollution and healthrisk
4 Encroachments Along the banks of river Obstruction to flow5 Population Mass migration Land use change6 Mosquitoes and insects During rainy seasons and
water logged areasWater borne diseases
7 Currents, littoral drift,tides and waves
Daily (spring and neaptide). Diurnal along thecoast
Choking of rivermouth due to sand barformation
8 Sand bar Slow process of accretion Stagnation of water9 Government
Organizations (PWD,Corporation, CMDA etc)
Reaches of river withinthe city
No overview ofwatershed and lack ofintegrated approach
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5.2.2 Development of Generalized IDF Equation for South Chennai
The precipitation data recorded by the rain gauge stations help to
determine the frequency and the character of the precipitation events in the
vicinity of the site. The point rainfall data were used to develop frequency
relationships among rainfall depth, intensity and duration. These relations are
known as Intensity – Duration – Frequency (IDF) curves and they are used for
designing storm water management facilities and floodway reservations
(Viessman and Lewis 2007). IDF curves were developed for Meenambakkam
rain gauge station for different return periods by Sahadevan (1980). However,
in the above study the maximum time of concentration was limited to 480
minutes. But, in the Adayar catchment area, the time of concentration is about
1552 minutes. The curves given by Sahadevan (1980) cannot be directly used
in the present study. In the absence of IDF curves for duration greater than
480 minutes, there is a need to extrapolate the IDF curves for the duration of
1552 minutes. Curve fitting was resorted in order to find the relationship
between intensity, duration and frequency. Thus, treating Sahadeven’s IDF
curve as a base, a generalized Equation was arrived at by exploiting the data.
The intensity duration curve for the Meenambakkam rain gauge station is
given in Figure 5.2 and their relationship is given in the Table 5.7.
The generalized Equation for the Meenambakkam rain gauge
station which gives relationship between intensity – duration – frequency is
found to be
i = kt (5.1)
where:
i is intensity of rainfall (mm/hour); and
tc is time of concentration (minutes).
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k = 47.74 ln(t ) + 318.02 (5.2)
n = 0.551t (5.3)
where:
tr is return period (years)
Figure 5.2 Intensity–Duration curve diagram for Meenambakkam raingauge station
Table 5.7 Relationship between intensity and duration
Sl.No. Return period (years) Relationship1 2 i = 353.09 tc
- 0.554
2 5 i = 401.93 tc– 0.509
3 10 i = 433.10 tc- 0.484
4 20 i = 452.59 tc– 0.472
5 30 i = 471.75 tc- 0.463
6 40 i = 487.95 tc– 0.457
7 50 i = 501.16 tc- 0.455
8 100 i = 537.97 tc– 0.446
9 200 i = 583.51 tc– 0.446
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5.2.3 Effect of Land Use Changes on Flood Peak
The rapid urbanization due to increase in population and the change
in land use pattern are the major reasons for occurrence of flooding. Land use
changes from 1976 to 2005 were studied for the Adayar watershed using GIS.
Land use pattern of Adayar watershed has been classified into built up area,
tanks, canal, scrub land, plantation, forest, agricultural land and barren land
The spatial distribution of various land use categories with their percentage
over total area of the watershed for the year 1976 and 2005 is presented in
Table 5.8. Figures 5.3 and 5.4 represent the land use maps for the year 1976
and 2005. The pictorial representation of the results obtained is given in
Figure 5.5 (a) and (b).
Table 5.8 Land use changes
Sl No Land useArea in sq km Percentage of total
area1976 2005 1976 2005
1 Agricultural land 273.6 195.18 38.80 27.69
2 Barren land 130.96 219.30 18.60 31.11
3 Built-up area 94.81 153.78 13.44 21.81
4 Canal 0.15 0.07 0.02 0.01
5 Forest 31.25 31.25 4.43 4.43
6 Plantation 28.9 18.75 4.10 2.66
7 River 4.17 3.64 0.59 0.52
8 Scrub land 55.59 37.29 7.88 5.29
9 Tanks 85.57 45.73 12.14 6.48
Total 705.00 705.00
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Figure 5.3 Land use map of Adayar watershed (1976)
Figure 5.4 Land use map of Adayar watershed (2005)
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Figure 5.5 (a) Spatial distributions of various land use categories (%)of the study area for the year 1976
Figure 5.5 (b) Spatial distributions of various land use categories (%) of
the study area for the year 2005
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Based on the analysis of land use map prepared, the following
observations are made
(i) From the analysis, it is revealed that there is a change in the area
of different land use categories during the period from 1976 to
2005. It is evident from the figures 5.3 and 5.4 that the Adayar
watershed has undergone rapid urbanization.
(ii) The agricultural land covering an area of about 38.80%
(273.6 km2) in 1976 has decreased to 27.69% (195.18 km2) in
2005, while the built up area has increased from 13.44%
(94.81 km2) in 1976 to 21.81% (153.78 km2) in 2005.
(iii) Tanks (water bodies) cover 12.14% (85.57 km2) of the total area
in 1976 and it has decreased to 6.48% (45.73 km2) in 2005. The
plantation area decreased from 4.1% (28.9 km2) in 1976 to
2.66% (18.75 km2) in 2005.
(iv) The scrub land has decreased from 7.88% (55.59 km2) in 1976
to 5.29% (37.29 km2) in 2005 while the barren land has
increased from 18.6% (130.96 km2) in 1976 to 31.11%
(219.30 km2) in 2005.
The results reflect that the increase in impervious area would alter
the water cycle as follows:
(i) Infiltration gets reduced thereby increasing surface runoff;
(ii) The replacement of vegetative cover reduces evapotranspiration;
(iii) Impervious surfaces absorb part of the solar energy and increase
the ambient temperature, creating islands of heat in the central
part of urban area (Tucci 2007); and
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(iv) Urban development obstructs natural runoff which in turn leads
to flood risks to inhabitants.
The change in the runoff regime is one of the significant impacts of
land use change. Developments along a river and floodplains would reduce
the carrying capacity of the river and accelerate runoff. A lumped parameter
runoff watershed model (HEC-HMS) coupled with GIS (HEC-GeoHMS) was
used to estimate the runoff from the Adayar watershed. The effect of land use
conditions on the outflow peak discharge was investigated for different return
periods from 2 to 200 years. Figure 5.6 presents a comparison between the
peak flow of the watershed corresponding to 1976 and 2005 land use
conditions. The model simulations show that, for a 100 year return period,
flood peak has increased from 1067 m3/s to 1342 m3/s as a result of change in
land use pattern. Figure 5.7 presents two runoff hydrographs for the same
watershed which corresponds to the 100 – year return period, for the years
1976 and 2005. It is clearly seen from the results that if the impervious area
increases it would increase the peak discharge.
Figure 5.6 Comparison of peak outflow for different rainfall returnperiods corresponding to 1976 and 2005 land use conditions
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From Figure 5.6, the trend line for 1976 land use pattern is
= 2.3528 + 819.42 (5.4)
The trend line for 2005 land use pattern is
= 3.0959 + 1007.7 (5.5)
Figure 5.7 Runoff hydrograph for 100 year return period for 1976 and 2005
5.2.4 Validation of HEC-HMS
In order to assess the accuracy of UFR predictions using
HEC-HMS model, the results from HEC-HMS model are compared with the
field data. The discharges measured for 1976 and 2005 rainfall conditions at
Thiru Vi Ka bridge are 1411 and 1331 m3/s (Source: PWD Chennai)
respectively. The simulated values obtained from the model are found to be
1412.9 and 1342.2 m3/s respectively. The model predictions and the measured
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field discharges are in fair agreement. The actual stage values obtained for the
year 2005 flood in HEC-RAS (100 year rainfall return period) in
Nandambakkam-Porur brigde (which is 12.2 km from the sea mouth) and
Maraimalai Adigal bridge (which is 7 km from the sea mouth) are 9.4 m and 5.6 m
respectively. The simulated values from the model are 9.54 m and 5.82 m
respectively. This may be attributed to the changes in the cross sections of the
river that have been progressively taking place.
5.2.5 Flood Index Results Using UFR Approach
As stated in the previous chapter, HEC-HMS model was used to
determine the flood index values corresponding to 2005 land use conditions
and a 100 year return period rainfall. The contribution of each sub-watershed
to the flood peak at the outlet was quantified using Unit Flood Response
(UFR) approach (Saghafian and Khosroshahi 2005) under two conditions
(i) with no spatial distribution of rainfall; and (ii) with spatial distribution of
rainfall. The rainfall values of the five rain gauge stations were spatially
distributed over the watershed using inverse square distance method. This
forms a basis to rank the sub-watershed in terms of their flood potential.
Tables 5.9 and 5.10 show the ranking of subwatersheds in terms of their flood
characteristics.
A dimensionless flood index fi* was introduced and is given in
Equation (5.6) as
i = (5.6)
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Table 5.9 Flood index without spatial variability
Sl.No.
Category
Das
arik
uppa
msu
bwat
ersh
ed
Man
iman
gala
msu
bwat
ersh
ed
Ora
thur
subw
ater
shed
Thi
ruso
olam
subw
ater
shed
Ada
yar
wat
ersh
ed
1 Area (km2) 146.99 112.82 145.44 299.75 705
2Subwatershed peakdischarge (m3/s)
256.4 148.7 302.9 443.5 1151.5
3Priority based onsubwatershed peakdischarge
3 4 2 1 _
4Priority based onsubwatershed specificpeak discharge
2 4 1 3 _
5Outlet peak dischargewithout subwatershed(m3/s)
866.9 429.2 489.2 823.9 _
6 Flood index FI (%) 25 63 58 28 _
7 Priority based on FI 4 1 2 3 _
8 Flood index fi (m3/s/km2) 1.94 6.4 4.55 1.09 _
9 Priority based on fi 3 1 2 4 _
10 Dimensionless fi* 0.32 0.75 0.73 0.49 _
11 Priority based on fi* 4 1 2 3 _
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Table 5.10 Flood index with spatial variability
Sl.No. Category
Das
arik
uppa
msu
bwat
ersh
ed
Man
iman
gala
msu
bwat
ersh
ed
Ora
thur
subw
ater
shed
Thi
ruso
olam
subw
ater
shed
Ada
yar
wat
ersh
ed
1 Area (km2) 146.99 112.82 145.44 299.75 705
2Subwatershed peakdischarge (m3/s)
269.9 168.5 318.4 483.8 1342.2
3Priority based onsubwatershed peakdischarge
3 4 2 1 _
4Priority based onsubwatershed specificpeak discharge
2 4 1 3 _
5Outlet peak dischargewithout subwatershed(m3/s)
1036.9 483.8 552.8 911.4 _
6 Flood index FI (%) 23 64 59 32 _
7 Priority based on FI 4 1 2 3 _
8Flood index fi(m3/s/km2)
2.08 7.61 5.43 1.44 _
9 Priority based on fi 3 1 2 4 _
10 Dimensionless fi* 0.29 0.81 0.74 0.56 _
11 Priority based on fi* 4 1 2 3 _
From the Tables 5.9 and 5.10, it is obvious that by accounting for
the spatial variability of rainfall, better results can be obtained.
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As seen in Table 5.10, Thirusoolam subwatershed generates thelargest peak discharge of 483.8 m3/s and Manimangalam subwatershedproduces the smallest peak flow of 168.5 m3/s. Based on the UFR approach,the flood source areas are identified and ranked as in row 7 (peak discharge
contributions, FI) and row 9 (per unit subwatershed area peak-dischargecontribution, fi) in the above table. Although Thirusoolam andManimangalam subwatersheds hold first and last ranks, respectively in botharea and peak discharge, they are in difference with respect to FI, fi and
fi*ranks. For example, the rank of Dasarikuppam subwatershed, which hassecond largest area, has the third highest peak, and ranks fourth for FI floodindex and third for fi flood index. This illustrates that integrated effect ofdifferent factors such as spatial distribution of rainfall, physical characteristics
of watershed, topology of river network, river routing play an important rolein determining the contribution to flood peak at the outlet. The effects of thesefactors were simulated using hydrologic models.
Figure 5.8 compares the flood index FI and the percentage area for
each subwatersheds. The area of Manimangalam subwatershed (112.82 km2)is 1.28 times less than that of Orathur subwatershed (145.44 km2) and the
ratio of their contributions to the outlet peak discharge stands at 1 to 1.89.These two subwatersheds ranked first and second, respectively in the flood
index, FI and flood contribution per unit area, fi. The flood contribution perunit area, fi is considered an important criterion in determining the location of
head water flood – abatement measures (Saghafian and Khosroshahi 2005).Figure 5.9 depicts values of row 9 in Table 5.10 in a histogram form and
shows the relative rankings of subwatersheds with respect to unit areacontribution. According to Table 5.10 and Figures 5.8 and 5.9, it is
understood that rankings of flood index, FI and flood contribution per unitarea, fi and dimensionless fi* may differ i.e. Thirusoolam watershed ranks 3 in
FI and fi* but holds rank 4 in fi. Manimangalam watershed which ranks firstin FI, fi and fi* is identified as a flood source area and therefore it should be
prioritized for planning flood – abatement measures.
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Figure 5.8 Comparison of area and flood index (FI) for the respectivesubwatersheds
Figure 5.9 Area (in percent of total area) and outlet peak dischargecontributions per unit area (fi) of subwatersheds
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From Figure 5.10, it is clear that for higher return periods, rural
watershed tends to show a small decrease, urban watershed tends to show a
sharp increase and in a mix of rural and urban watershed, it tends to show a
small increase in dimensionless flood index. This dimensionless flood index
can also be used to prioritize the sub watersheds. Figure 5.11 presents the
effect of return period on coefficient of variation and mean of subwatershed
flood index, fi. It is seen that the mean of the sub-watershed flood index
increases with the return period, whereas, there is no consistent trend in
coefficient of variation with respect to the return period. This trend exists due
to the flat topography of the watershed. The results indicate that the rainfall
with higher return period would tend to increase the sub-watershed flood
responses in urban areas.
Figure 5.10 Comparison of fi* for different return periods
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Figure 5.11 Effect of return period on coefficient of variation and meanof subwatershed flood index, fi
5.2.6 Flood Hazard Mapping Using GIS
Flood hazard mapping forms the basic tool for flood preparedness
and mitigation activities, including flood insurance programs. This map
would help the authorities to develop, design and operate the flood control
infrastructure and plan for the relief operations for high risk areas during
floods. The annual rainfall for the Padappai rain gauge is 1818 mm and for
the remaining rain gauges it is greater than 2000 mm for the return period of
100 years. The rainfall map is shown in Figure 5.12. The average basin slope
at the downstream of the Adayar watershed is less than 15% and the
remaining part varies from 15 – 100%. Figure 5.13 presents the slope map.
The gradient map is shown in Figure 5.14. In the land use map shown in
Figure 5.15, the urban area is given the highest weightage and the rural area is
given the lowest weightage. From Figure 5.16, it is found that the hydrologic
soil group A occupies 42%, B occupies 38%, C occupies 16% and D occupies
4% of the total area of the Adayar watershed.
145
Figure 5.12 Rainfall map (100 year rainfall return period)
Figure 5.13 Slope map
146
Figure 5.14 Gradient map
Figure 5.15 Land use map
147
Figure 5.16 Hydrologic soil map
Thus, the thematic maps were prepared and the rank of each was
assigned on the basis of its estimated significance in causing flooding. Flood
hazard map was prepared by overlaying all the thematic maps and also by
considering the following figures for identifying the degree of probability of
occurrence of flooding in identifying the flood hazard zone. Figure 5.17
presents the flood hazard map of the Adayar watershed. The hazard areas are
calculated as shown in Table 5.11.
148
Figure 5.17 Flood Hazard Map (FHM) for 100 year return period rainfall
Table 5.11 Flood hazard area
Sl. No. Degree of probability offlooding
Area(sq km)
Area (%)
1 High hazard 282.02 44
2 Moderate hazard 338.11 52
3 Low hazard 24.61 4
From the Figure 5.17, it can be seen that the entire portion of
Adayar watershed is within moderate to high hazard zones. Urban peripheral
of Adayar watershed and areas adjacent to river banks are characterized as
high hazard zone. Except 4% of the remaining portion, the other areas are
designated as moderate hazard zone. Table 5.12 presents the areas vulnerable
to flooding along the Adayar River.
149
Table 5.12 Areas vulnerable to flooding along Adayar River
Sl. No Sub-watershed Areas vulnerable to flooding
1 Manimangalam Manimangalam, Karunakaracheri, Somangalam
2 Orathur Mudichur, Veradarajapuram
3 Thirusoolam Kandaipalayam, Tirunirmalai, Kamaraj Nagar,Valuthalembedu, Saidapet, Jafferkhanpet,Kotturpuram
5.2.7 Flood Zone Mapping using HEC-RAS
Delineation of flood extent and depth within the floodplain in
Adayar River was carried out for different return periods based on the
integration of hydraulic simulation results and GIS analysis using the
HEC-GeoRAS extension of Arcview. Application of hydraulic modeling in
GIS environment provides the capability to simulate flood depth in different
parts of the floodplain (Salimi et al 2008). HEC- GeoRAS performs both pre
and post processing processes and produces the floodplain of the selected
flood profile. HEC- RAS generates water surface profiles of the river for
different return periods. Flood zone maps show the extent and depth of
flooding for different return periods. Inundation depth grid and floodplain
polygon for the selected water surface profile were created using
HEC- GeoRAS. The floodplain polygon denotes the boundary region whereas
the inundation depth grid denotes the real flood risk. The results indicate that
the flooding pattern varies for different return periods.
150
In this study, the flood zone categories are classified as very low,
low, moderate, high and very high. Flood plain maps for different return
period rainfall (2, 5, 10, 20, 30, 40, 50, 100, 200 and 350 years) were prepared
and presented in Figures 5.18(a) to 5.18(j). Figures 5.19(a) and (b) depicts the
overlaid flood zone map for 2- and 350- year return period rainfall and overall
flood zone map of Adayar River respectively. Table 5.13 presents the
summary of flooded area and the maximum flood depth. The results indicate
that the flooded area along the river banks increased by 18.85 sq km for 350
year return period and the maximum flood depth varies up to 2.11 m. Figure
5.20 shows the changes in flooded areas for different return periods.
Table 5.13 Summary of the flooded area and the maximum flood depth
Sl.No Return period
(years)Flooded
area (km2)
Areadifference
(km2)
Max flooddepth (m)
Depthdifference
(m)1 2 26.94 - 12.287 -
2 5 27.28 0.34 12.330 0.043
3 10 27.67 0.73 12.375 0.088
4 20 28.08 1.14 12.440 0.153
5 30 28.91 1.97 12.535 0.248
6 40 30.07 3.13 12.635 0.348
7 50 32.05 5.11 12.757 0.470
8 100 36.61 9.67 13.063 0.776
9 200 40.30 13.36 13.591 1.304
10 350 45.79 18.85 14.395 2.108
151
Figure 5.18(a) Flood zone map for 2 year return period rainfall
Figure 5.18(b) Flood zone map for 5 year return period rainfall
152
Figure 5.18(c) Flood zone map for 10 year return period rainfall
Figure 5.18(d) Flood zone map for 20 year return period rainfall
153
Figure 5.18(e) Flood zone map for 30 year return period rainfall
Figure 5.18(f) Flood zone map for 40 year return period rainfall
154
Figure 5.18(g) Flood zone map for 50 year return period rainfall
Figure 5.18(h) Flood zone map for 100 year return period rainfall
155
Figure 5.18(i) Flood zone map for 200 year return period rainfall
Figure 5.18(j) Flood zone map for 350 year return period rainfall
156
Figure 5.19 (a) Overlaid flood zone map of Adayar River for 2 and
350 year return period rainfall
Figure 5.19 (b) Flood zone map of Adayar River
157
Figure 5.20 Inundated area versus return period
5.2.8 Results of Second Workshop on SSM
The second workshop is conducted to complete the stages 5, 6 and
7 (Figure 4.5) of SSM. The results of hydrologic – hydraulic modeling were
discussed with the stakeholders in the second workshop. The root definition
expresses what the system is but the conceptual model explains what it does
(Theppitak 2006). On the basis of root definition, the conceptual model was
constructed. The methodology flow chart presented in Figure 4.1 depicts the
activities to be implemented in order to achieve the root definition. This
conceptual model contains a set of activities for the system to work in a
sequential manner. The comparison of the results with the real world situation
was done in order to generate desirable and feasible changes to resolve the
key issues. Regardless of how the comparison with the real world was
undertaken, the aim was not to improve the models but to ‘find an
accommodation between different interests in the situation, an
158
accommodation which can be argued to constitute an improvement of the
initial problem situation’ (Checkland and Scholes 1990). Checkland (1979)
suggests four ways to compare the model with reality. They are:
(i) Unstructured discussions;
(ii) Structured questioning of the model;
(iii) Scenario or dynamic modeling; and
(iv) Trying to model the real world using the same structure as the
conceptual model.
Flood hazard map of the Adayar watershed and flood zone map of
Adayar River were presented in the workshop. The participants of the
workshop concurred with the results obtained from the hydrologic and
hydraulic models and opined that results compared with ‘real world’, and
accepted the model after an unstructured discussion by the participants of the
workshop. They also suggested that these maps can be used for flood
mitigation purposes. A Force Field Analysis was done in order to trace the
forces, which can halt or encourage the change. It carefully examined the
probability of reaching agreed upon or disagreed upon the changes.
Figure 5.21 portrays the Force Field Analysis diagram developed
by the workshop participants. The objective is to ‘develop the flood
management policy’. In this analysis, the participants identified political,
financial and institutional support as the restraining forces. Non-
responsiveness by the departments, lack of data and co-ordination are
identified as the external forces, which act as barriers and play an important
role in the restraining forces list. Good governance and public recognition are
found to be the driving forces. Loss of human life, livestock and agricultural
productivity would pave the way for the formation of a flood management
159
policy. The workshop participants identified more and stronger driving forces
than restraining forces. The forces are given weightages (5 – extremely
strong, 4 – strong, 3 – moderate, 2 – weak and 1 – extremely weak). The total
score of the driving forces and restraining forces is found to be 56 and 43
respectively. Thus, the participants perceive that the driving forces are
favorable towards an integrated flood management policy. The relationship
between the institutions and organizations in the context of flood management
is drawn as an influence diagram by the workshop participants. The influence
diagram is given in Figure 5.22.
Based on the outcome of the two workshops, key issues have been
identified. They are:
(i) waterways must be properly maintained;
(ii) encroachments to be removed;
(iii) sand bar formation at the riverine mouth to be cleaned to
facilitate tidal flushing;
(iv) creation of public awareness; and
(v) stakeholders’ should participate in flood management.
It is indicated that there seems to be a lack of coordination and
cooperation among Government agencies/departments. The workshop also
addressed tools and techniques used within an integrated approach. It should
be noted that there is some limitation to this workshop, although a wide range
of interests are represented. Some stakeholders did not participate. Hence,
their perspective and experience are not incorporated in the products of this
workshop.
160
Score Driving Forces Restraining Forces Score
Equlibrium
Figure 5.21 Force Field Analysis for flood management policy
5 Loss of human life
5 Loss of livestock
5 Loss of agricultural
productivity
4 Economic loss
5 Damage to property and
infrastructure
4 Epidemic prevention
4 Good Governance
3 Demand by people
3 Participation of
stakeholders
3 Public recognition
3 Economist
5 Ecologist
2 Social Activist
5 Environmental Activist
56 Total for change
Huge Investment 5
Urbanization 4
Encroachment 4
Non co-operation
of public 4
Lack of
co-ordination 3
Negligence by
the department 4
Lack of data 4
River morphology 4
Livelihood
dependency 3
Profit making
attribute of
private firms 3
Obstruction to
waterways 3
Political
Interference 2
Total for no change 43
Change No Change
161
162
5.3 SENSITIVITY ANALYSIS
Franklin et al (2002) define sensitivity analysis as that ‘in which the
model system is subjected to various changes in the starting database and the
outputs are compared with what changes in the outputs would be expected’.
In this study, the sensitivity of runoff with effect of urbanization is examined
under two scenarios.
(i) The first scenario was based on the trends in land use changes
(1976 to 2005); and
(ii) The second scenario was based on the incremental increase in
urbanization.
5.3.1 Scenario I
The land use maps of 1976 and 2005 were analyzed. It is found
from the maps that decrease in agricultural land is accompanied by an
increase in the area of barren land and built up area. The increase in the ratio
of built up area to barren land is based on the changes between the 1976 and
the 2005 land use maps (a decrease of 22.44 km2 in agricultural land has
increased the barren land by 15.24 km2 and built up area by 7.2 km2; 2.58 km2
decrease in tank area has increased the barren land by 1.56 km2 and built up
area by 1.02 km2). This ratio was applied linearly for consecutive 10 years, 20
years and 30 years and the hydrologic impact is simulated by varying the CN
numbers of the micro watershed. From the Figure 5.23, it is seen that a
decrease in agricultural area tends to increase the peak discharge, while
decrease in the tank areas tend to decrease the discharge. This is attributed to
the changes in the curve number. Table 5.14 depicts the projected
agricultural, built up, barren land and tank areas.
163
Figure 5.23 Effect of decrease in agricultural and tank areas on discharge
Table 5.14 Projected agricultural, built up, barren land and tank areas
YearProjected
agricultural area(km2)
Projectedbuilt up area
(km2)
Projectedbarren land
(km2)
Projectedtank area
(km2)2015 187.72 155.76 224.87 44.90
2025 179.85 158.37 230.88 44.14
2035 171.94 161.14 236.99 43.17
5.3.2 Scenario II
This analysis was carried out in order to assess the hypothetical
increase in runoff due to urbanization. Land use maps with different
percentages (10, 20, 30, 40 and 50) of urbanization were prepared using GIS.
HEC – HMS model was run by giving different CN value as the input for
each and every micro watershed based on the land use change. The effect of
flood peak at the outlet on the variation of micro watershed CN value is
164
investigated. Figure 5.24 shows the flood hydrograph at the outlet for various
percentages of urbanization. The increase in the peak discharge for different
percentage of urbanization is shown in Figure 5.25.
Figure 5.24 Flood hydrograph for various percentage of urbanization
Figure 5.25 Effect of urbanization on runoff
165
5.4 SOCIO-ECONOMIC ANALYSIS
Socio economic analysis was carried out in order to assess the flood
damage. The socio economic characteristics of the community of both
riverine and urban flooding are listed in the Table 5.15, the respondents’
estimates of flood problems are given in Table 5.16.
Table 5.15 Socio-economic details
Sl.No. Category Riverine flooding Urban flooding
1 No of respondents 54 54
2 Sampling technique Strategic sampling(block wise)
Strategic sampling(street wise)
3 Education levels in %(a) Primary(b) Secondary(c) Higher Secondary(d) Bachelor degree(e) Master degree(f) Illiterate
114333
724
415243522
04 Occupation in %
(a) Government(b) Private(c) Business(d) Others
0582220
4442032
5 Annual income (Rs) in %(a) < 50000(b) 50001 to 100000(c) 100001 to 500000(d) 500001 to 1000000(e) > 1000000
4054
600
691
220
6 Type of dwelling in %(a) Individual(b) Apartment(c) Others
00
100
9442
7 Ownership details in %(a) Owner(b) Tenant(c) Lease
6931
0
5446
0
166
Table 5.16 Respondents’ estimates of flood problems
Sl.No.
Floodcharacteristics
Riverine flooding Urban flooding
Standarddeviation(N = 54)
Minimum MaximumStandarddeviation(N = 54)
Minimum Maximum
1 Frequency offlooding in ayear (no of
times)
0 1 1 1.374 1 4
2 Inundatedperiod in days 0 4 4 14.946 4 90
3 Depth offlooding in m 0.55 0.1 3.7 0.32 0.3 1.5
5.4.1 Riverine Flooding
The width of the Adayar River in the stretch between Saidapet
Bridge and Thiru-vi-ka Bridge has reduced because of the encroachments on
the river course. After analyzing the responses to the questionnaire survey in
SPSS, it is found that the annual average income of the people varied from
Rs. 50,000 to 100,000 (1 USD = Rs.47 approx.) in the year 2008. 13 % of the
people are graduates. 87 % have completed schooling. Government has
provided the houses along the right bank of the Adayar River to the
economically weaker section of the society (Figure 4.15).
An average of about 5 people every year gets trapped in the Adayar
River bed near Kotturpuram Bridge (situated at the chainage of 2.5 km from
the river mouth). This is due to excessive sediments getting deposited, which
in turn increases the depth of flooding. Total number of blocks in
Kotturpuram Housing Board area is 84 (24 blocks of 8 houses each, 2 houses
in the ground floor plus 6 houses in three storeys above and 60 blocks of
6 houses each, 2 houses in the ground floor plus 4 houses in two storeys
above. Number of blocks surveyed is 27 (7 in three storeyed buildings and 20
167
in two storeyed building). From the analysis it is found that all the ground
floor in 84 blocks has undergone property damages. The people in the ground
floor move to the first floor with their belongings and stay with the first floor
occupants for about 5 days. They cook and live together in a 237 sq ft flat.
The flooding depth varies from 0.3 m to 3.66 m. The suggestions given by the
people to minimize the effect of flooding are to remove the debris from the
river, increase the embankment height and provide proper drainage facility.
Figure 5.26 shows the TIN of Kotturpuram Housing Board area. As
mentioned in the previous chapter, out of 54 households, 27 households fall
under Group I category and remaining fall under Group II category.
Table 5.17 shows the replacement cost incurred by Group I.
Figure 5.26 TIN of Kotturpuram Housing Board area
168
Table 5.17 Replacement cost incurred by Group I in KotturpuramHousing Board Area
Sl.No. Property damage No of
householdsReplacement cost
(Rs)1 House 24 1,14,0002 Furniture 17 44,0003 Tv and Fridge 22 80,0004 Vehicle 22 1,73,0005 Miscellaneous 22 43,0006 Total replacement cost 4,54,000
The stage damage curve, shown in the Figure 5.27, was constructed
to derive the relationship between flood damage and flood depth, which was
collected from the field survey. It can be used for the estimation of
damage/household under different flood depth in the study area. The
information about damages to the assets under different flood depths was
collected from the survey. It is observed from the graph that the ground floor
of the apartments is fully submerged if the flooding is greater than 3.5 m.
Figure 5.27 Stage damage curve for Kotturpuram Housing Board area
0
0.5
1
1.5
2
2.5
3
3.5
4
Inside thehouse
Furniture TV andFridge
TV andFridge
Vehicle Vehicle Completehouse wassubmerged
Property damaged
169
The details of the amount spent by the people during floods for
transportation and health are tabulated in the Table 5.18. Flood cost (without
considering the expenses during normal period) for 27 households in Group I
is Rs 17,145, while the flood cost (health, water and milk) for 27 households
in Group II is Rs 11,645.
Table 5.18 Comparative table for the amount spent during normal andflood times in Kotturpuram Housing Board area
Sl. No. Content Amount spent (Rs)Group I Group II
1 Water Normal 2,376 2,376Flood 2,700 2,700
2 Health Normal 1,500 1,500Flood 13,500 8,000
3 Milk Normal 675 675Flood 945 945
4 Total Normal 4,551 4,551Flood 17,145 11,645
The summary of Tables 5.17 and 5.18 is provided in Table 5.19,
which is the aggregated flood (damage) cost.
Table 5.19 Total damage assessment of Kotturpuram Housing Board area
Sl. No. Flood cost/problemNo of
householdssurveyed
% ofhouseholds
Total loss(Rs)
1 Property damage (only theground floor) 24 44.4 4,54,000
2 Water 54 100 6483 Health 54 100 18,5004 Milk 54 100 5405 Loss of daily wages 54 100 12,7506 Total flood cost 4,86,438
170
Total money spent during flood by Group I = Rs 4, 72, 969
[4, 54,000 + (17, 145 – 4,551) + (12, 750 / 2)]
Total money spent during flood by Group II = Rs 13, 649
[(11,645 – 4,551) + (12, 750 / 2)]
No of households which come under group I = 168
No of households which come under group II = 384
Total cost spent during flood by 168 households = Rs 29, 42, 918
Total cost spent during flood by 1 household (Group I) = Rs 17, 517
Total cost spent during flood by 384 households = Rs 1, 94, 119
Total cost spent during flood by 1 household (Group II) = Rs 330
The total cost spent by 552 households = Rs 31, 37, 037
The inundated area is found to be 79866 m2. Therefore, the total
cost of the flood damage is Rs 39/ m2.
5.4.2 Urban Flooding
Urban flooding is mainly due to inadequate drainage facilities.
Even a low intensity rainfall can cause flooding. The flooding causes great
economic loss to the city, individuals and to the society. The inundation
spread area in Velachery was calculated using the raster calculator in
ArcGIS 9.3. The total area and volume of flood patches are found to be
422.48 hectares and 348.4 ha m respectively. The depth of inundation varies
from o.1 m to 1.5 m. Figure 5.28 shows the TIN and flood spread area in
Velachery.
171
Figure 5.28 TIN and flood spread area in Velachery
The study area has been experiencing growth of anthropogenic
activities in the past three decades. The intensity of land use and development
has increased rapidly. After analyzing the responses to the questionnaire
survey using SPSS, it is found that the majority of the people live in
individual dwellings and the annual average income of the people varied from
Rs. 50,000 to 100,000 (1 USD = Rs 47 approx.). From the survey, it is found
that 31 households fall under Group I and the remaining 23 households fall
under Group II. The property losses for Group I are in the form of furniture,
TV, house, garage and vehicle. Of the 54 house surveyed, 31 houses are
flooded above the floor level. All the people living in this area faced some
intangible damages like difficulty to access to schools, colleges and
workplaces, hardship in getting essential things, mental agony and physical
stress etc., which cannot be expressed in terms of monetary value.
172
Figure 5.29 Stage damage curve for Velachery area
It is observed from the Figure 5.29 that flooding extends inside the
house only if the depth of flooding is greater than 0.6 m. The total
replacement costs for the flood damages in 2005 are found to be Rs 1, 62,000.
The details are provided in the Table 5.20.
Table 5.20 Replacement cost for Group I in Velachery area
Sl.No. Property No of households Replacement cost (Rs)
1 House 7 61,000
2 Furniture 18 32,000
3 TV 2 6,000
4 Garage/shed 10 18,500
5 Vehicle 4 20,500
6 Miscellaneous 7 24,000
7 Total replacement cost 1,62,000
173
The details of the amount spent by the people during floods for
transportation and health are tabulated in the Table 5.21. Flood costs for
Group I (health and transportation without considering the expenses during
normal period) is Rs 92,160 and for Group II (health and transportation) is
Rs 67,340.
Table 5.21 Comparative table for the amount spent during normal andflood times in Velachery area
Sl.No.
ContentNo of households Amount spent (Rs)
Group I Group II Group I Group II
1 TransportationNormal 26 18 10,520 8,350Flood 26 18 38,530 27,550
2 HealthNormal 31 23 16,070 11,900Flood 31 23 53,630 39,790
3 TotalNormal 26,590 20,250Flood 92,160 67,340
From the survey it is found that certain mitigation measures were
adapted to minimize the effect of flooding. The details are given in Table 5.22.
Flood mitigation cost spent by Group I is Rs 1, 35,500 and by Group II is Rs
58,500.
Table 5.22 Comparative table for the amount spent for mitigationmeasures in Velachery area
Sl. No. Item (MitigationOptions)
No of households Amount spent (Rs)Group I Group II Group I Group II
1 Elevated electric box 18 16 28,500 22,0002 Elevated appliances 3 2 9,500 8,0003 Elevated structure 19 9 61,500 25,5004 Improved drain around
home10 1 36,000 3,000
5 Total 1,35,500 58,500
174
The summary of Tables 5.20, 5.21 and 5.22 is provided in
Table 5.23, which is the aggregated flood (damage) cost.
Table 5.23 Total damage assessment of Velachery area
Sl.No.
Flood cost/problemActual no ofhouseholds
% ofhouseholds
Total loss(Rs)
1 Property damage 31 57 1,62,000
2 Transportation 44 81 47,210
3 Health 54 100 65,450
4 Elevated electric box 34 63 50,500
5 Elevated appliances 5 9 17,500
6 Elevated structure 28 52 87,000
7 Improved drain around home 11 20 39,000
8 Total flood cost 4,68,660
Total cost spent during flood by Group I = Rs 3, 63,070
[1, 62, 000 + (92, 160 – 26, 590) + 1, 35, 500]
Total cost spent during flood by Group II = Rs 1, 05,590
[(67, 340 – 20, 250) + 58, 500]
On an average, there are about 20 households in a street. From theanalysis, it is found that 31 streets have undergone property damage (Group I)and the remaining 24 streets fall under Group II category.
No of households which come under group I = 620
No of households which come under group II = 460
Total cost spent during flood by 620 households = Rs 72, 61,400
Total cost spent during flood by 460 households = Rs 21, 11, 800
The total cost spent by 1080 households =Rs 93, 73,200
175
The inundated area in patch B (Figure 4.15) is found to be 140.83
ha. The total flood cost is Rs.66, 557/ha.
5.5 FLOOD MITIGATION MEASURES
Flood mitigation is defined ‘as measures aimed at decreasing or
eliminating the impact of floods on society and the environment’
(BTRE Report 2002). Andjelkovic (2001) states that ‘mitigation means
planning, programming, setting policies, co-coordinating, facilitating, raising
awareness, assisting and strengthening and it does not include insuring,
assessing, financing, relieving and rehabilitating’. Mitigation is split into two
types: Structural mitigation, which refers to the engineering measures such as
construction of levee, check dams, flood gates, diversions and channel
improvements etc,. Non-structural mitigation, which refers to knowledge
development, public awareness including participatory mechanisms and the
provision of information can reduce risk with related impacts.
Structural mitigation measures should be adapted in urban
watershed, while in rural watersheds, non-structural mitigation measures
should be adapted. Since, Manimangalam subwatershed (non-urban
watershed) has been prioritized as flood source area, it would be better to
make interventions in that subwatershed. Hence, potential recharge zones
were identified in Manimangalam subwatershed. The concept behind the
identification of potential recharge zone in the upstream is to minimize the
effect of flooding in the downstream tributaries. In this study, the recharge
zones were identified by overlaying the soil map, geology map,
geomorphology map and lithology map. The overlaid map was once again
overlaid with modeled stream lines (blue lines) and surveyed streamlines
(light green lines) as found in Figure 5.30. There is a mismatch between
surveyed and simulated streamlines. Most of the mismatches are found in the
Manimangam subwatershed, which is identified as flood source area and is
176
located in the upstream end of the Adayar watershed. It is also found that the
modeled streamlines extend beyond the surveyed streamlines. Saraf et al 2004
showed that where the ground water recharge is high, the degree of mismatch
between the surveyed and the simulated lines are also high and this mismatch
can be used to delineate the ground water recharge zones. Combining with
soil, lithology, geology and geomorphology information of the study area, the
high recharge zones are identified, where there is misfit between the surveyed
and simulated stream lines. From the Figure 5.31, it is seen that some portions
of microwatersheds 0, 3, 4, 8 and 11 are the locations suitable to be recharge
zones in Manimangalam subwatershed.
Figure 5.30 Identification of potential recharge zones
Modeled line
Surveyed line
177
Figure 5.31 Sites suitable for recharge zones
178
5.6 SUMMARY
The results of this study indicate that for the same amount of
rainfall and different land use conditions between 1976 and 2005, the flooded
area and the water depth has increased for the 2005 land use conditions. Flood
prone areas are identified using the flood hazard map and the flood details are
got from the flood zone map and these maps would assist in the appropriate
planning of developmental works. Manimangalam subwatershed is identified
as flood source area using UFR technique and some portions of
microwatersheds 0, 3, 4, 8 and 11 in Manimangalam watershed are found to
be the sites suitable to be recharge zones. Results presented in this study have
shown the potential increase in flood risk as a result of urbanization in
Velachery. Flood volume of the area was calculated using GIS. In addition, the
total flood cost spent by affected people in East Velachery was derived for the
flood in 2005. The cost comes to be around Rs. 93, 72,000 (9.3 million INR) and
the flooding extends inside the house only if the depth of flooding is greater than
0.6 m. Stage damage curve is built for the residential sector of both Velachery
and Kotturpuram Housing Board area and can be used extensively for flood
damage assessment as well as damage mitigation. The stage damage curves
were derived from the data for the study area.