Diouf et al., Geoinfor Geostat: An Overview 2013, S1 ...Optical Remote Sensing and Hydrogeological Modelling: Case Study of Dakar Flooding Area (Senegal) Diouf OC1*, Faye SC 1, Dieng

a S c i T e c h n o l j o u r n a lResearch Article

Diouf et al., Geoinfor Geostat: An Overview 2013, S1http://dx.doi.org/10.4172/2327-4581.S1-011

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Hydrological Risk Analysis with Optical Remote Sensing and Hydrogeological Modelling: Case Study of Dakar Flooding Area (Senegal)Diouf OC1*, Faye SC1, Dieng NM1, Faye S1, Faye A1, Englert A and Wohnlich S2

AbstractThe present study uses remote sensing, geographic information systems, groundwater levels and hydrogeological modelling to evaluate the influence of urbanization and modification of water pumping rates on groundwater and related inundations. Evidence of the fast urban expansion was investigated using time series aerial photos and satellite images interpretation (1966, 1978, 1989, 2000 and 2006). Results show that urban occupation increases from 36 km2 in 1966 to 150 km2 in 2006. However, areas affected by flooding were mainly located in Guediawaye and Pikine districts (municipalities). Daily groundwater monitoring levels using “Thalimede Orpheus mini” recorders and chemical data investigation support relatively high recharge values in the peri-urban areas. Groundwater recharge evolution is modelled using FEFLOW software. Predictive simulations of groundwater pumping as a solution to fight against floods at rates of 19 000 m3/d and 9 000 m3/d respectively for infrabasaltic and Thiaroye aquifers cause a decline in groundwater levels ranges between 0.6 m and 3 m in piezometer Pz7 (Guediawaye); 0.5 m and 2.9 m Pz4 (Pikine) and 0.5 m and 2.6 m Pz21 (Yeumbeul). These results show that groundwater pumping is an effective way to fold the piezometric levels. Results analyses suggest also that the flooding phenomena are due to some factors such as urbanization combined with spontaneous occupation of lowland, rainfall events accumulation, groundwater level rising caused by waste water infiltration due to the absence of drainage sanitation and a significant decrease of pumping rate.

KeywordsFloods, Remote sensing, chemistry, hydrogeological modelling

*Corresponding author: Ousmane Coly Diouf, Geology Department, Faculty of Science and Technology, University Cheikh Anta Diop (UCAD) Dakar-Fann, BP 5005, Senegal, Tel: 00221-77-561-1932; E-mail: [email protected]

Received: February 22, 2013 Accepted: June 25, 2013 Published: July 04, 2013

ranging from 15 000 m3/d to 1 300 m3/d. During the last two decades, the pumping was considerably reduced. This fact is due to nitrate pollution derived from improper sanitation system in the urbanized area.

Urban analysis is one of the most common applications of remote sensing. One of the main problems is that the spectral and spatial resolutions of sensors are too coarse to identify the desired information for urban analysis [1-3]. Implementation of this urban system requires a variety of digital data sources and temporal databases. In this study, urban and flooding area delineation was done using time series satellite images (Landsat 7 and Spot 5). The impact of this urbanization on the water quality was studied by several authors [4-7] and recently [8-10] investigated the impact of reduced pumping on groundwater regime and the consequences of the flooding phenomenon. Indeed, this phenomenon occurring these last two decades (1989, 1995, 2000, 2003, 2005, 2009 and 2010) causes damages, disasters and constitutes major preoccupation of policy makers and the government. In this context, the present study aims to analyze and propose solution to this urban flooding using satellite images, rainfall data, groundwater levels fluctuation and hydrogeological modelling.

Description of the study areaThe peninsula of Cap-Vert extends over 550 km2 between

longitudes 16°55’ and 17°30’ west and latitudes 14°55’ and 14°35’ north (Figure 1). Dakar city is a fast growing metropolitan area, its population increased from 583 000 inhabitants in 1971 to 2.5 million in 2006 with an emergency of suburban zone where the population is estimated at 1,154,316 with a density of 9335 inhabitants/km2. This region is characterized by a semi arid climate with a short rainy season, which lasts from June to October. Annual rainfall has varied between 150 mm (1983) and 664 mm (2005) over recent decades and has become slightly above the mean of 410 mm (1961-1990).

The maximum temperature recorded in this area between 1980 and 2010 has averaged 29.5°C. It occurs from May to June and October-November corresponding to the beginning and the end of the rainy season respectively; minimum temperatures are observed during the period from December to February (18.5 °C). Evaporation estimated by Turc Formula is ranged between 200 mm/yr and 500 mm/yr between 2000 and 2008.

From the geological standpoint, the region of Dakar belongs to the Senegalese-Mauritanian basin, the largest coastal basin of northwest Africa [11]. Most of the geological formations encountered correspond to thickness reaching several thousand meters. At the outcrop appear almost exclusively Quaternary sediments sandy and sandy clay, especially alluvial and wind deposit [12]. This Quaternary sandy formation constitutes the reservoir in the Dakar region; it overlays the impermeable marly and clay Eocene formation (Figure 2).

The aquifer extends from the Dakar peninsula to Saint Louis in the north. It is confined beneath a basaltic layer at the peninsula head

IntroductionDakar city is characterized by high population growth with an

unplanned urbanization since the drought of 1970s. The peri-urban area is under laid with an unconfined Quaternary sandy aquifer with low water table depth. This aquifer plays a major role in supplying drinking and irrigation water in rural areas. The use of this resource began in the 1950s with periods of higher and lower exploitation

Citation: Diouf OC, Faye SC, Dieng NM, Faye S, Faye A, et al. (2013) Hydrological Risk Analysis with Optical Remote Sensing and Hydrogeological Modelling: Case Study of Dakar Flooding Area (Senegal). Geoinfor Geostat: An Overview S1.

• Page 2 of 8 •Special Issue 1 • 011

doi:http://dx.doi.org/10.4172/2327-4581.S1-011

Figure 1: Urban expansion and land use (2006) in the study area.

Figure 2: Geological cross-section in the study area [13, modified].

then unconfined in the region extended from Patte d’Oie toward the East. This system consists of medium to fine sands with intercalations of sandy clay in places. Its thickness varies between 5 m (southeast) to 40 m (West) and 75 m (northwest). The permeability values of the Thiaroye aquifer are variable and range from 10-4 m/s at Thiaroye to 5.10-4 m/s around the Mbeubeuss lac. Transmissivity values vary as a function of the thickness of the sands and are between 10-2 m2/s and 10-3 m2/s.

MethodologyThe methodology used in this study follows three steps:

• Aerialsphotos(1966),SPOTandLandsatimages(1989,2000,2005 and 2006) were used to investigate urban evolution and to delineate flooding areas. Raster data used are listed in table 1.

Satellite image was performed using Arcgis 9.2 and Erdas imagine Software. Different types of land use are coded. A prospecting field was done to validate the different themes. Digital elevation model (DEM) was produced in the project entitled «rainwater drainage plan in Dakar peri-urban area» using Quickbird image (60 cm of resolution).




• DailyrainfallrecordsfromDakar-Yoffmeteorologicalstationwere analyzed in relation to groundwater levels fluctuation. These groundwater levels were obtained using “Thalimede Orpheus mini” recorders. This operation was carried out in two piezometers (PSQ1 and P3-1) located in the suburban area during the period 2010–2011; in order to determine groundwater fluctuations in response to rainfall events. Long term water table records in few piezometers, located in the urban area (P2-6, P2-2 and P2-5) and groundwater pumping were also investigated to infer the impact of the latter on the groundwater level. In addition to that, groundwater level were monitored in 37 piezometers, boreholes and wells to feature the general patterns of the groundwater flow and to calibrate the groundwater model ;

• Vectors data obtained are used formodelling groundwaterflow using Feflow 5.4. In unconfined and confined aquifer, groundwater flow equation can be respectively expressed as:

0 0( )f

ip

i

h qS B Qt x

ε ∂ ∂+ + =

∂ ∂

(1)

S f

i ijj

hq BKx

∂= −

∂

0

f

ip

i

h qS B Qt x

∂ ∂+ =

∂ ∂

(2)

0S

f

i ijj

hq Tx

∂= −

∂

B = aquifer thickness; xL =elevation; h = hydraulic head; nhq= Volumetric Darcy flux; Kij = Hydraulic conductivity; Tij = Transmissivity; ε = Porosity and S = Storage coefficient

Results and DiscussionFlooding analysis

Since the 1950s, Dakar region has been characterized by urban expansion due to population growth. Land use was made without respect to urban planning. Time series aerial photos and satellite images interpretation (1966, 1978, 1989, 2000 and 2006) show that urban occupation increased from 36 km2 in 1966 to 150 km2 in 2006. However, area occupied by marshland decreased from 80 km2 to 16 km2 between 1978 and 2008. In this order, delineation of urban flooding suggests that flooding areas ranged between 1.07 km2 and 8.65 km2 between1989 and 2005 and were mainly located in Guediawaye and Pikine districts (municipalities) (Figures 3 and 4). Urban expansion was done in areas where topography is low and water table depth shallow.

High rainfalls events characterize years where flooding phenomenon are observed. For example in August 1st-2nd, 13-19, 22-25 and 18-30 2005, rainfall recorded is respectively equal to 63.9 mm, 108.9mm, 97.3 mm and 49.21 mm. This situation causes flooding of many districts in the peri-urban area where groundwater level is highly related to rainfall events (Figures 3 and 4). However, groundwater level records in PSQ1 and P3-1 show a decrease of groundwater level after October (Figure 5).

Type Genre Resolution projection date

Raster Photo corona - UTM WGS84 1966

Raster Mosaïc of aerial photos - UTM WGS84 1978

Raster Image Landsat 5 30 m UTM WGS84 Sept.1989

Raster Panchromatic Image Spot 5 10 m UTM WGS84 October 2005

Raster multispectrale image Spot 5 10 m UTM WGS84 October 2006

Raster Panchromatic image Spot 5 2,5 m UTM WGS84 October 2006

Raster Quickbird 60cm UTM WGS84 March 2010

Table 1: Type of raster data used.

0

5000

10000

15000

20000

25000

30000

Urba

n ar

ea

Dun

es

Fore

sts, s

teppe

s, sa

vann

as

Mar

schl

and

Rain

fed

or ir

rigat

ed

crop

s Bare

soil

Are

a (ha

)

Land use

1978

1989

2000

2006

Figure 3: Land use evolution between 1978 and 2006.




Long term groundwater levels record is marked by inter-seasonal variation related to rainfall events over time. General trends show a water levels rising pattern in the urban area (P2-2, P2-5 and P2-6) between 1990 and 2008. In detail, rising groundwater levels range from 0.77 m to 4.83 m (P2-6) between October 1994 and 2008. Groundwater levels rising in urban areas evidenced reduced pumping and infiltration of domestic wastewater into the groundwater due to the poor sanitation system (Figures 6 and 7).

Groundwater modelling

The objective of this groundwater modelling was to evaluate groundwater pumping and urban development impact in the floods occurrence.

The study area was discretised in 16,333 linear triangular elements and 8,329 nodes. For assigning boundary conditions, a potential

h = *f Zf

δ δδ

−

was imposed on all meshs bordering the sea (Dirichlet condition) (Western and North-West). Z represent the sea level altitude in the unconfined part and the aquifer top altitude in the confined aquifer (δf= density of the fluid and δ=density of the water). Towards the South-east of the system where the impermeable clay and marly formation outcrops a nil flux is imposed on these meshes (Figure 8).

For groundwater model calibration a time step of 20 years was

used to study piezometric levels and recharge evolution (1969, 1989 and 2008). First, we tried to match the piezometric levels of the aquifer for the year 1969 (reference hydraulic head) by calibrating piezometric levels through changing the values of permeability and recharge. Once we obtained a fairly piezometric level, values of permeability remain unchanged and the calibration process of the piezometric levels for years 1989 and 2008 was done only using recharge values. This step enabled us to evaluate the evolution of groundwater recharge according to time.

Groundwater recharge was validated using chemical and isotopic data and results obtained suggest higher values in peri-urban area and moderate values in rural areas [14].

Groundwater modelling results show a recharge average decrease from 65 mm/yr to 50 mm/yr from 1969 to 2008 and groundwater level decreases at PS10 between 22.8 m to 21.8 m in the rural area. However, in the urban area recharge and piezometric levels rise from 78 mm/yr in 1969 to 220 mm/yr in 2008 and from 0.81 m to 5.25 m at P2-6 (Table 2).

Predictive simulations were performed considering different scenarios in relation to the groundwater pumping and sanitation drainage in peri-urban areas. These scenarios specifically target a groundwater level decline to deal with floods in the suburban area.

Thus, three scenarios were simulated, the first one aims to predict groundwater levels and understand the hydrodynamic patterns over

Figure 4: Flooding area 2005 overlapped with the DEM (Digital elevation model).




0

0.5

1

1.5

2

2.5

3

0

10

20

30

40

50

60

6 1524 5 1423 1 101928 7 1827 5 1423 2 112029 7 1625 3 122130 9 1827 5 1423 2 112029 7 1625 3 1221 2 112029 7 16

Rai

nfal

l (m

m)

Daily rainfall (mm) Piezometric levels P3-1 (mm)

Piezometric levels (m

)

Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Jan. Feb Mar.

2011 2012

Time (days;months and years)

5.4

5.6

5.8

6

6.2

6.4

6.6

6.8

7

7.2

7.4

0

10

20

30

40

50

60

27 6 13 20 27 3 10 17 24 1 8 17 24 31 7 14 21 28 5 12 19 26 2 9 16 23 30 6 13 20 27 4 11 18 25 1 8 15 22 29 6 13 20 27 3 10 17

Rai

nfal

l (m

m)

Time (days;months and years)

Daily rainfall (mm) Piezometric levels (PSQ1) (m)

Piezometric levels (m

)

Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Jan.

2011 2012

Figure 5: Water level fluctuation in two piezometers (P3-1 and PSQ1).




0

50

100

150

200

250

300

350

400

0

1

1

2

2

3

3

4

4

5

5

1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9 1 5 9

Piez

omet

ric

leve

ls (

m)

Years

Rainfall P2-6 P2-2 P2-5

Rainfall (m

m)

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Figure 6: Long term groundwater levels records in the urban area.

0

500000

1000000

1500000

2000000

2500000

3000000

3500000

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Pum

ping

rat

e (m

3)

Years

Figure 7: Pumping rate evolution in the Thiaroye field.

a period of 20 years if the current operating conditions (groundwater pumping) and sanitation drainage are maintained. However, the impacts of pumping and sanitation drainage in peri-urban areas were assessed in the second and third scenario.

Prediction of groundwater levels to 2030 (scenario 1) was simulated in the context of a recharge increased in the peri-urban area. This recharge increased was induced by the unplanned urbanization and lack of sanitation drainage. If the current trend continues, the amount of groundwater infiltration can reach 383 mm/yr if the sanitation drainage in the peri-urban area is not done and if the trend return of the rainfall continues. Results obtained in this scenario indicate a groundwater level rising ranged between 1.28 m and 0.28 m in piezometers Pz2 and P2-6 (Table 2). This situation can

cause flooding expansion in Guinaw Rail North and South, Diacksao, Thiaroye and Thiaroye sur Mer districts.

Groundwater levels rising due to domestic wastewater infiltration and reduced pumping cause recurring floods in Dakar city. The second scenario aims to produce groundwater polluted water (9000 m3/d) for farmers in the conditions where sanitation drainage is not achieved. The results show a groundwater levels decline in all piezometers located in the Thiaroye basin. This water level decrease was greater in piezometers located near the pumping wells (Pz7 and Pz4) and ranged between 0.02 m and 0.6 m in 2030 (Figure 9 and Table 2). This decrease may be related to increase of wastewater infiltration in peri-urban areas over time.

Resumption of groundwater pumping in Thiaroye Basin can play

Scenario 1 Scenario 2 Scenario 3

Piezo. 1969 1989 2008 2030 2030 2030

PL Rech. PL Rech. PL Rech. PL Rech. PL Rech. PL Rech.

Inhabitedarea

PS10 22.860-70

22.4-

2250

21.830-40

21.830-40

21.830-40

PS11 20.2 20.07 19.5 18.5 18.5 18.5

Urbanarea

Pz7

80-90 190

3.53

220

4.70

383

2.85

383

0.42Pz4 3.58 4.71 3 0.64

Pz21 3.65 4.53 3.08 0.98Pz2 4.57 5.85 4.56 1.98P2-6 0.81 3.33 5.25 5.72 4.68 2.70

Table 2: Recharge (mm/yr) and Groundwater levels (m) evolution (Rech = recharge, PL= piezometric level, PS10, PS11, Pz7, Pz4, Pz21, Pz2 and P2-6= piezometers).

Figure 9




Figure 8: Discretisation of the study area.

Figure 9: Predictive groundwater levels (scenario 2).




an important role in the development of agriculture and irrigation especially in the «Niayes» where is produced mainly vegetables from Senegal. However, governmental programs to fight against floods will certainly eliminate or reduce long-term domestic wastewater infiltration. Groundwater level evolution in response to combined increasing groundwater pumping (9000 m3/d) and reducing waste water infiltration about 50% (sanitation drainage) was simulated in the third scenario. The results indicate that groundwater level decrease varies over time and space between 2.55 m and 3.13 m in 2030 (Table 2).

Predictive simulations suggest that the groundwater pumping and sanitation drainage impact varies across the study area. However, increasing groundwater pumping in order to satisfy water supply for vegetable production and reducing the infiltration of wastewater through the implementation of an adequate sanitation drainage system can help to effectively fight against the floods causing water level decline about 3 m.

ConclusionThis study focused on Dakar peri-urban area and showed that

Thiaroye aquifer is strongly influenced by rainwater and wastewater infiltration. It follows that flooding phenomena are observed in the rainy season. Results analysis showed that the flooding is mainly due to high rainfall events, unplanned urbanization and rising groundwater level due to reduced pumping and domestic wastewater infiltration. Thus, modelling groundwater flow showed an increase in groundwater levels in the densely urban area and a decrease in the rural area. The predictive simulations suggest that groundwater levels will decrease about 1.5 m at PS11 in 2030. Increased groundwater level in the urban area is about 4 m at piezometer P2-6 and can be related to reducing groundwater pumping from 15 000 m3/d in 1969 to 1300 m3/d in 2009 but also to domestic wastewater infiltration. References

1. Todd WJ (1977) Urban and Regional Land use Change Detected by Using

Landsat Data. Journal of research by the US Geological Survey. 5: 527-534.

2. Barrett EC, Curtis LF (1992) Demography and Social Change, Introduction to Environmental Remote Sensing, Chapman & Hall, London, UK

3. Stefanov WL, Ramsey MS, Christensen PR, (2001) Monitoring urban land cover change: An expert system approach to land cover classification of semiarid to arid urban centers. Remote Sensing of Environment 77: 173-185.

4. Martin A (1970) The layers of the peninsula of Cape Verde: their use for water supply Dakar. BRGM report, 56 p.

5. WHO (1972) Water Supply and Sanitation and around Dakar. Groundwater Study, Volume II: Tablecloths sands.

6. Sonees (1986) Strengthening of water supply in the region of Dakar (1986-1991) interim phase. Final Report 1: 305.

7. Tandia AA (2000) Origin, evolution and migration of mineral forms of nitrogen in aquifers beneath suburban environment not cleaned: the case of water Quaternary sands in the region of Dakar (Senegal) 208 p. State doctoral thesis. Dept. of Geology, University Cheikh Anta Dakar, Senegal.

8. Cissé Faye S (2001) Unconfined Quaternary sands Thiaroye / Beer Thialane. Study of nitrate contamination on the basis of a geographic information system (PC ARC / INFO).

9. Antea/senagrosol (2004) Modeling of groundwater: Scenario modeling stop-Evaluation of hydro geological impacts, phase B and C. Final Report A33129/A.

10. Dieng NM (2009) Analysis of hydrological risks with optical satellite imagery: the case of floods in the region of Dakar.

11. Castalain J (1965) Stratigraphic and micro paleontological overview of the Senegal River Basin. History of the paleontological discovery. In Coll. Int. micropaleontology, Dakar and memory BRGM 32: 357-365.

12. Bellion YJC (1987) Post-Paleozoic geodynamic history of West Africa from the study of some sedimentary basins (Senegal, Taoudenni Iullemmeden, Chad). PhD Thesis of Science, University of Avignon (Paris, France).

13. Chaoui S (1996) Hydrogeology and hydrochemistry of the peninsula of Cape Verde (infra basaltique water and ground water Thiaroye) Senegal. Internship Report Office of Scientific and Technical Research Overseas (ORSTOM).

14. Diouf OC, Faye SC, Diedhiou M, Kaba M, Faye S, et al. (2012) Combined uses of WTF, CMB and Environmental Isotopes methods to investigate groundwater recharge in the Thiaroye sandy aquifer (DAKAR, SENEGAL). African Journal of Environmental Science and Technology 6: 425-437.

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Author Affiliations Top1Geology Department, Faculty of Science and Technology, University Cheikh Anta Diop (UCAD) Dakar-Fann, BP 5005, Senegal2Ruhr University of Bochum, Department of Applied Geology, Universitaets str. 150, D-44801 Bochum, Germany

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Diouf et al., Geoinfor Geostat: An Overview 2013, S1 ...Optical Remote Sensing and Hydrogeological Modelling: Case Study of Dakar Flooding Area (Senegal) Diouf OC1*, Faye SC 1, Dieng