Download GLOBAL ENVIRONMENTAL CHANGE AND

GLOBAL ENVIRONMENTAL CHANGE AND URBAN FLOOD IN
LAGOS
ODUNUGA, Shakirudeen Ph.D
Department of Geography, Faculty of Social Sciences
University of Lagos
Email: [email protected]. [email protected]
Tel: 234-803 723 665, 234 – 01 - 7623663
Abstract
This study investigates the hydrological impact of systemic climate change (precipitation
increase) and cumulative environmental change (land use change) on urban flood
generation using Ashimowu watershed (System 6c) in Lagos as case study. This was
achieved by using Precipitation Water Inundation Model (PWIM) that integrates
projections of precipitation and urbanization drivers to simulate and analyse future flood
generations. Future climate scenarios of 25, 50 and 100 years for low, medium-low,
medium-high and high were analysed. The simulation result revealed that runoff and
peak flow for 50 years high climate change increased by 19.72% when compared with
the base year (2003) while area inundation increased by 7.46%. The 100 years high
climate change reveals that runoff and peak flow increased by 23.73% while area
inundation increased by about 10%. On the average, about 20% to 25% increase in
urban hydrological fluxes; especially flash flood will be recorded in the next 50 - 100
years due to global environmental change. Sustainable developmental projects targeted
at reducing the vulnerability and risk level of citizens to urban flooding, especially in
Sub-Sahara Africa should integrate a marginal increase of 20 to 25 percent in the
hydraulic and other design criteria.
Key Words: Climate Change; Precipitation; Urbanization; Flooding; Lagos
Introduction
Climate change is making weather less predictable, rains more uncertain and heavy storm
rainfalls with high intensity more frequent (ActionAid, 2006). The phenomenon of high
rainfall intensity in the tropics will be compounded by global environmental change. This is
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the change in the earth’s atmospheric, biological, geological and hydrological system. It
involves both systemic change and cumulative change.
Systemic changes are those changes, which occur throughout the global compartments of
the earth system such as rising CO2 in the atmosphere, global warming etc. Cumulative
changes are those which take place at discrete locations around the globe, but when
combined, have “global” importance. Examples are: acid rain, deforestation,
desertification, urbanization and others leading to change in the hydrological and
biogeochemical components of the environment. The core component of the cumulative
environmental change is the land use and land cover.
Land use and land cover are linked to climatic and weather changes in complex ways.
Key links between land cover and climate include the exchange of greenhouse gases
between the land surface and the atmosphere, the radiation balance (both solar and longwave) of the land surface, the exchange of sensible heat between the land surface and the
atmosphere, and the roughness of the land surface and its uptake of momentum from the
atmosphere. The impact of land use-change on flood events can be both positive and
negative. Generally, the removal of forest and other natural cover and the conversion of
land to agricultural uses, result in compaction of the soil and reduction of the infiltration
rate, leading to high flood peaks. Deforestation is believed to have been a significant
cause of the catastrophic flooding in the Yangtze river basin in China and in Central
America from Hurricane Mitch, both of which occurred in 1998 (UN, 2004).
Deforestation and other land use practices can also lead to greater incidences of
landslides and mud flows. As a result of these strong links between land cover and
climate, changes in land use and land cover are important contributors to climate change
and variability. The forces of global change have an impact on how well towns and cities
function and on their urban life, and also on the challenges posed by sustainable
development (Selman, 1996). The greatest impact of land use change on the hydrological
fluxes is associated with urbanization. A city will frequently have significant flooding
problems that are local in nature, but will be impacted upon by major flood events on
large streams or lakes that are not within the urban zone (UN, 2004).
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The severity of land use change on hydrological fluxes in urban environments is more on
storm runoff because of growing pressure to develop land by converting it to compacted
surface as well as the demand for water resources. Increase in flood peak and runoff
volume in the range of 15-25% for medium – size urban watersheds has been estimated
in temperate climate (UN, 2004). This will be much higher in tropical environment due to
the nature of storm rainfall intensity, which is higher when compared with the intensity of
rainfall of similar duration and frequency in other regions of the world. These regional
tropical characteristics, coupled with the phenomena of climatic variability and global
warming on account of which precipitation has marginally increased in the tropics (Ojo et
al, 2001) and hydrological processes intensified; further exacerbate the occurrence of
flooding in Lagos. The high intensity short duration rainfalls are further enhanced in
flood generation by the tropical coastal low lying location of the city, high population
growth rate of about 10% and unregulated land use. Consequently, this research uses
PWIM to simulate future flood due to environmental change brought about by
precipitation and urbanization drivers in Lagos metropolis
Study Area:
Ashimowu watershed is located in the central parts of Lagos Metropolis. The watershed
extends from about latitude 60 27’ to 60 32’ 30’’and longitude 30 21’ to 30 24’ 42’. Fig 1
shows Ashimowu watershed in Lagos metropolis. The highest elevation within the study
watershed is about 9m above mean sea level (a.m.s.l). The terrain is flat with some
noticeable undulation. Surface lithology is entirely made up of sedimentary formation
with sandy loam composition. The climate is humid tropics and experiences a contrast
between dry and wet seasons. These two seasons are dependent on the two prevailing air
masses blowing over the country at different times of the year. The first being the warm,
dry and dusty tropical continental or the northeasterly air mass of Saharan origin, while
the second is the warm and moist tropical maritime or the humid maritime air mass
blowing from over the Atlantic.
Rainfall is highly torrential with short duration and high intensity that allows for rapid
concentration of storm runoff. Average day temperature varies between 21°C and 24°C
and falls down to about 18°C at night. The area is a region of high surface humidity of
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about 80% in January (dry season) and over 90% in June/July (Wet Season). Vegetal
cover is urban scrubs and grassland. Urbanization is accelerating due to population
increase and residential developments (Oyeleye, 2001). Housing stock and density are
high. The land use is mainly residential with some institutional land use while land cover
ranges from highly impervious built up area to permanently water log wetlands. Human
activities include urban agriculture, retailing and small scale industrial activities.
Fig 1: Ashimowu Watershed in Lagos Metropolis
Methodology
The analysis of climate change connection with intra-urban watershed flooding was
based on the analysis of change in two of the most important drivers of climate and urban
flooding. These are precipitation and urbanization (Land use change). Eleven storm
rainfall events were monitored between August 2005 and November 2005. The land use
and Land cover characteristics were derived from Ikonos 2 satellite imagery of November
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2003. Simulation of runoff, peak flow and area inundated arising from the precipitation
and land use drivers were achieved using Precipitation Water Inundation Model (PWIM).
PWIM is a model that synthesis hydrological processes on urban surface characteristics
generated in a GIS environment and simulate runoff, peak flow and area inundated from
rainfalls (Odunuga, 2008). Future runoff, peak flow and area inundated of the measured
rainfalls were generated for four climate scenario; low, medium-low, medium-high and
high for 25 years, 50 years and 100 years scenario. However, there is considerable
uncertainty in deducing changes in precipitation over short periods in small urban areas
from predictions of precipitation from models of climate change on the global and
regional scale. The uncertainty is even much higher for maximum rainfall (extremevalue) analysis (Oyebande, 1983). Based on projected increments of rainfall and intensity
due to climate change in tropical Africa (Ojo et al, 2001) the analysis, therefore, assumed
a range of multiplying factors of rainfall and intensity for each of the scenario based on
the current trend in annual precipitation (Table 1). The projections were made for 25, 50
and 100 years using 2003 as base year. Increment in urbanisation driver was based on
marginal increase in impervious area due to urbanisation as shown in Table 2.
Table 1: The multiplying factor / percentage of increments for precipitation drivers
Year
Scenario
25
Amount Intensity
50
Amount
100
Intensity
Amount
Intensity
5
10
7.5
15
10
20
Medium High
2.5
5
5
10
7.5
15
Medium Low
0
2.5
2.5
5
5
10
Low
0
0
0
2.5
2.5
5
High
5
Table 2: The multiplying factor for Urbanization drivers:
Year
Scenario
25
50
100
High
0.01
0.05
0.1
Medium High
0.002
0.01
0.05
Medium Low
0.001
0.002
0.01
Low
0.0
0.001
0.002
By the above multiplying factors, the impact for 25 years high, medium high and medium
low corresponds to the impact of 50 years medium high, medium low and low
respectively. Also, the impact of 50 years high, medium high and medium low
corresponds to the impact of 100 years medium high, medium low and low respectively.
The changes for the 25 years medium low and low are highly insignificant when
compared with the 2003 flood parameters generated for the observed storms.
Results and Discussions
The hydrological parameters are simulated based on the projected systemic climate
change (precipitation driver) and cumulative environmental change (urbanization driver)
for similar storms comparable to the base year. Table 3 shows the observed rainfall
characteristics and the PWIM simulation of the observed storms on the 2003 land use.
Tables 4 and Table 5 show the simulated flood parameters for 25 years medium-high
climate change and high climate change respectively. Runoff and peak flow for the
medium-high climate change increased by an average of 13.10% when compared with
the 2003 urbanization base year while area inundated increased by 1.80 %. The high
climate change scenario shows average increment of 16.1% for runoff and peak flow
when compared with the base year while PWIM area inundation increased by 2.46%.
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Table 3: PWIM simulated Runoff, Peak flow and Area inundation for 2003 base year
Average
Date
Amount (mm) Duration (Min)
Intensity/hr
22/08/05
28/08/05
1/9/2005
7/9/2005
8/9/2005
20/09/05
24/09/05
27/09/05
4/10/2005
11/10/2005
3/11/2005
Maximum
Minimum
Mean
S/D
CV(%)
4.75
9.75
9.25
1.25
2.25
6.5
4.75
7.5
2.25
1.8
16.48
16.48
1.25
6.05
4.35
71.9
0.61
2.6
4.63
0.45
9
13
1.9
2.5
3
1.8
32.96
32.96
0.45
6.59
9.1
138.09
465
120
225
165
15
30
150
180
45
75
30
465
15
136.36
129.81
96.20
PWIM Runoff
(mm)
PWIM Peak
Flow
(m3s-1)
63.317
236.79
34.658
129.58
123.30
461.01
16.662
62.299
29.992
112.14
86.645
323.96
63.317
236.74
99.975
373.80
29.992
112.14
23.994
89.711
219.678
821.35
PWIM
Area
inundation (ha)
178.2426
185.526
192.9558
177.657
208.95
223.59
182.964
185.16
186.99
182.598
296.6436
Table 4: PWIM simulated flood parameters for 25 years Medium High Climate Change
PWIM Area inundation
Date
PWIM RUNOFF mm
PWIM PEAK m3/s
(ha)
22/08/05
71.606
267.728
182.541
28/08/05
39.195
146.546
189.589
01/09/05
139.444
521.366
196.780
07/09/05
18.844
70.455
181.974
08/09/05
33.919
126.818
212.258
20/09/05
97.988
366.365
226.426
24/09/05
71.606
267.728
187.110
27/09/05
113.063
422.729
189.235
04/10/05
33.919
126.818
191.006
11/10/05
27.135
101.455
186.755
03/11/05
248.436
928.876
297.124
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Table 5: PWIM simulated flood parameters for 25 years High Climate Change
PWIM Area inundation
Date
PWIM RUNOFF mm
PWIM PEAK m3/s
(ha)
22/08/05
73.490
274.772
182.608
28/08/05
40.226
150.401
190.078
01/09/05
143.113
535.083
197.698
07/09/05
19.339
72.308
182.007
08/09/05
34.811
130.155
214.102
20/09/05
100.566
376.004
229.117
24/09/05
73.490
274.772
187.450
27/09/05
116.037
433.851
189.702
04/10/05
34.811
130.155
191.579
11/10/05
27.849
104.124
187.075
03/11/05
254.973
953.315
304.042
Table 6 shows the simulated flood parameters for 50 years high climate change. Runoff
and peak flow increased by an average of 19.72% when compared with the base year
(2003). This reveals an additional marginal increment of 3.6% when compared with 25
years high climate change. PWIM area inundation increased by 7.46% and this also
shows an additional increment of 4% in area inundation.
Table 6: PWIM simulated flood parameters for 50 years High Climate Change
PWIM Area
Date
PWIM RUNOFF mm
PWIM PEAK m3/s
inundation (ha)
22/08/05
75.803
283.419
191.519
28/08/05
41.492
155.135
199.355
01/09/05
147.616
551.922
207.348
07/09/05
19.948
74.584
190.889
08/09/05
35.907
134.251
224.555
20/09/05
103.730
387.837
240.306
24/09/05
75.803
283.419
196.598
27/09/05
119.689
447.504
198.961
04/10/05
35.907
134.251
200.930
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11/10/05
28.725
107.401
196.205
03/11/05
262.997
983.316
318.900
Table 7 shows the simulated flood parameters for 100 years high climate change. Runoff
and peak flow increased by an average of 23.73% when compared with the base year
while PWIM area inundation increased by 9.96%. This reveals an additional marginal
increment of 4% each in runoff and peak flow when compared with 50 years high climate
change and 2.50% for area PWIM inundation. Fig 2 shows the projected area inundated
for 100 year high climate change of three of the observed storms (3/11/05, 1/09/05 and
24/09/05) for Ashimowu watershed in Lagos.
Table 7: PWIM simulated flood parameters for 100 years High Climate Change
PWIM Area
Date
PWIM RUNOFF mm
3
PWIM PEAK m /s
inundation (ha)
22/08/05
78.344
292.918
195.972
28/08/05
42.883
160.334
203.989
01/09/05
152.564
570.419
212.168
07/09/05
20.617
77.083
195.327
08/09/05
37.110
138.750
229.773
20/09/05
107.207
400.835
245.888
24/09/05
78.344
292.918
201.169
27/09/05
123.700
462.502
203.586
04/10/05
37.110
138.750
205.601
11/10/05
29.688
111.000
200.766
03/11/05
271.811
1016.272
326.302
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Fig 2: PWIM projected area inundation of Ashimowu watershed for (3/11/05, 1/09/05
and 24/09/05) storms
The PWIM projections reveal that the impact of climate change on urban flooding
phenomenon will be more severe in the first 20 to 30 years. However, with respect to
sustainable livelihood in a changing Earth System especially in tropical Africa, with the
world’s fasted urbanising region and cities, global environmental change is already
threatening the right to adequate housing and good living conditions. The already strained
megacity of Lagos with over 15 million inhabitants is being put under additional stress
with continuous increase in frequency and extent of flooding phenomenon.
Environmental refugees due to flooding are becoming annual rituals especially amongst
the urban poor who live in the blighted environments of Lagos such as Bariga, Makoko
and Idi-Araba to mention but few. Increasing urban floods in most parts of Lagos has
resulted in damage to houses, assets and income. A questionnaire survey of six flood
prone areas in Lagos (Ilaje-Bariga, Idi-Araba, Surulere / Itire / Lawanson, Iwaya /
Makoko, Orile and Victoria Island) reveals that flooding has caused damage to goods
and property of about 71.8% respondents.
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Conclusion and Recommendations
With respect to environmental change, the projections reveal that the impact of global
environmental change on urban flooding will be more severe in the first 20 to 30 years,
using year 2003 urbanization driver as the base year, and that peak flow and other related
velocity for the 25years high climate change will increase by an average of 16.1% while
inundation area will increase by an average of 2.46%. Policy implementations that
embrace the use of the PWIM for design and urban renewal purposes, incorporation of
estimated future flood due to climate change phenomenon in current intervention, public
campaign / awareness, community participation as well as improving the socio-economic
status of the vulnerable individual to urban flood are hereby recommended.
References:
Actionaid, (2006): Climate change, urban flooding and the rights of the urban poor in
Africa; Key findings from six African cities, A report by ActionAid , actionaid
international
Odunuga, S. (2008) Urban Land use change and the flooding patterns in Ashimowu
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Nigeria.
Ojo, O., Ojo, K and Oni, F. (2001): Fundamental of Physical and Dynamic Climatology.
Sedec publishers. Lagos. pp 477.
Oyebande, L. (1983): Rainfall intensity – duration frequency curves for selected station and
maps for Nigeria. Occasional paper Number 2, Department of Geography, University
of Lagos. Lagos.
Oyeleye, D. A. (2001) Settlement Geography. University of Lagos Press, Lagos, Nigeria.
Selman, P. (1996) Local Sustainability Managing and Planning Ecologically Sound
Places. St. Martin’s Press, New York, USA.
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