118

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

119

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)

120

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.

121

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.

122

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.

123

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

124

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

125

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

126

(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

127

(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

128

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).

129

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

130

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).

131

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

132

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

133

Figure 5.3 Land use map of Adayar watershed (1976)

Figure 5.4 Land use map of Adayar watershed (2005)

134

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

135

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

136

(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

137

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

138

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)

139

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 _

140

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.

141

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.

142

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

143

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

144

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.