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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 1 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). 2 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 3 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 4 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%. 6 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 7 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 8 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 9 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. 10 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 Watershed, Lagos, Nigeria. PhD Thesis (unpl) University of Lagos, Akoka, Lagos, 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. UN, (2004): Guidelines for reducing flood losses, UN inter-agency secretariat for the international strategy for disaster reduction, Geneva. 83p 11