Increased frequency of flash floods in Dire Dawa

Increased frequency of flash floods in Dire Dawa, Ethiopia: Change in rainfall intensity or human impact? Paolo Billi, Yonas Tadesse Alemu & Rossano Ciampalini

Natural Hazards Journal of the International Society for the Prevention and Mitigation of Natural Hazards ISSN 0921-030X Nat Hazards DOI 10.1007/s11069-014-1554-0

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Author's personal copy Nat Hazards DOI 10.1007/s11069-014-1554-0 ORIGINAL PAPER

Increased frequency of flash floods in Dire Dawa, Ethiopia: Change in rainfall intensity or human impact? Paolo Billi • Yonas Tadesse Alemu • Rossano Ciampalini

Received: 3 July 2014 / Accepted: 8 December 2014 Ó Springer Science+Business Media Dordrecht 2014

Abstract In the first decade of the twenty-first century, Ethiopia has been subjected to an increased frequency of flash floods, especially in the town of Dire Dawa. The results of international organizations studies point to no evidence of a climate-driven change in the magnitude/frequency of floods, though increases in runoff and risk of floods in East Africa are expected. Flash floods are posing constraints to the economic growth and the development process of a low-income country such as Ethiopia, and, in order to mitigate such hazard, it is crucial to understand the relative roles of two main factors: rainfall intensity and land use change. This study analyses the recent trends of rainfall intensity across Ethiopia and investigates the relative role of rainfall intensity and land use change in augmenting the frequency of flash flooding of the town of Dire Dawa by the Dechatu River. Results indicate that the increase in rainfall intensity is a more important factor than land use change in controlling the increased frequency of flash flood in Dire Dawa. Keywords

Flash flood Rainfall intensity Climate change Land use change Ethiopia

1 Introduction Ethiopia is a country with great geographic diversity. Much of its land consists of a large plateau at an elevation higher than 2,500 m a.s.l. with high mountains and deep gorges,

P. Billi (&) Dipartimento di Fisica e Scienze della Terra, Universita` di Ferrara, Via G. Saragat 1, 44122 Ferrara, Italy e-mail: [email protected] Y. T. Alemu Department of Geography, University of Dire Dawa, P.O. Box 1362, Dire Dawa, Ethiopia R. Ciampalini School of Earth and Ocean Sciences, University of Cardiff, Park Place, Cardiff CF10 3AT, UK

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Author's personal copy Nat Hazards

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500

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400

80

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60

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Aﬀecred people (x10 3)

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river valleys, and lowland plains. The rainy season in Ethiopia is concentrated in the 4 months between mid-June and mid-October when about 80 % of the annual precipitation is received as torrential downpours. Ethiopia’s topographic and climatic characteristics have made the country vulnerable to high floods that resulted in destruction, casualties and damages to economic, livelihoods, infrastructure, services, and health systems. In Ethiopia, flood disasters and the toll paid in terms of human lives and property damage show an increasing trend (Fig. 1) (Alemu 2009; Adhikari et al. 2010). Flash floods are formed from excess rain falling on upstream watersheds, flow downstream with massive concentration, high speed, and typically occur suddenly (Lin 1999). Heavy downpours in mountainous highlands can lead to surges of water that turn dry river beds or flood plains into raging torrents in minutes. Therefore, the damages of such floods become particularly pronounced and devastating when they pass across or along human settlements and infrastructures. According to IPCC (2012), the second half of the twentieth century is dominated by inter-annual to inter-decadal rainfall variations and many statistics indicate that trend estimates are spatially incoherent (Manton et al. 2001; Peterson et al. 2002; Griffiths et al. 2003; Herath and Ratnayake 2004). Moreover, Alexander et al. (2006) highlighted that, though statistically significant trends toward stronger precipitation extremes were generally found, the observed changes in precipitation extremes are much less spatially coherent and statistically significant compared with observed changes in temperature extremes. Recent studies (IPCC 2012) show rather an increase than a decrease in extreme precipitation, but there are also wide regional and seasonal variations and trends in many regions that are not statistically significant. If for East Africa, this is due to a lack of the literature on changes in heavy precipitation (IPCC 2012), the same is patent in Ethiopia for the same reason, though Easterling et al. (2000) and Seleshi and Camberlin (2006) report decreasing trends in heavy precipitation over parts of Ethiopia during the period 1965–2002. The literature on the impact of climate change on river floods (including also flash floods) is scarce, even though the changes in heavy precipitation discussed above may imply flood changes in some regions (IPCC 2012).

Fatalies

2010

2006

2003

2001

1999

1997

1995

1993

1991

1989

1987

1985

1983

1981

0

Aﬀected people

Fig. 1 Fatalities and people affected by floods in Ethiopia between 1981 and 2010 (Alemu 2009; EM-DAT 2014)

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IPCC Technical Paper VI (IPCC 2008) concluded that no evidence, based on instrumental records, has been found for a climate-driven globally widespread change in the magnitude/frequency of floods during the last decades (Rosenzweig and Tubiello 2007), though increases in runoff and increased risk of flood events in East Africa are expected. Di Baldassarre et al. (2010) found no evidence that the magnitude of African floods has increased during the twentieth century, whereas Conway et al. (2009) concluded that robust identification of hydrologic change was severely constrained by data limitations for sub-Saharan Africa. The reason of that stands in the limited number of flow gauge data in space and time and the confounding effects of land use changes and engineering interventions (Di Baldassarre et al. 2010; IPCC 2012). In fact, while the primary cause of flooding is abnormally high rainfall, there are many human-induced contributory causes such as land degradation, deforestation of catchment areas, increased population density along riverbanks, poor land use planning, zoning and control of flood plain development, inadequate drainage, particularly in cities, and inadequate management of discharges from river reservoirs (Mulugeta et al. 2007; Di Baldassarre et al. 2010). In Ethiopia, a research in Awash River basin by Terefe et al. (2006) indicates that human factor plays a crucial role in causing frequent flood disasters in upper, middle, and lower Awash. Moreover, projections of flood changes at river basin scale are scarce in the scientific literature. These same limitations and uncertainties are presently effective also in Ethiopia. In the last decade, an increasing occurrence of floods is reported for Africa in general, sub-Saharan Africa, and several arid and semiarid areas of Ethiopia. In fact, according to Conway and Schipper (2011), whereas historically floods have never been a major economic hazard in Ethiopia, recent years have seen significant socio-economic disruption due to flooding. According to Mulugeta et al. (2007), the accuracy and lead times of flood forecasts in sub-Saharan Africa are limited or questionable, thus, new research and collaborative efforts are needed to advance flood management in the future. In developing countries, studies on ephemeral streams flash floods are uncommon and very few data are available to design appropriate risk mitigation countermeasures and warning systems. In 2006, the town of Dire Dawa experienced a typical flash flood that, following the heavy rain on the upland areas of eastern Harerge highlands, within a few hours turned the dry bed of the Dechatu River into a swelling and devastating river that caused several casualties and property damage for millions of Euros. The problem of flash floods in the semiarid area of Dire Dawa is not new, but their frequency has significantly increased in the last decades (Alemu 2009; DDAEPA 2011). In response to the fact that few studies in African climate have considered temporal variations in the frequency and distribution of daily rainfall events of different magnitude (Hulme 2003) and to the lack of information about the relationship between rainfall intensity, land use change, and flash floods in Ethiopia, this study aim was threefold: (1) to analyze the recent trends of rainfall intensity across Ethiopia in order to ascertain whether the increased frequency of flash floods can be associated with an increase in daily precipitation amounts; (2) to estimate peak discharges for the Dechatu River and assess the evidence for changing flood behavior; (3) to compare the general rainfall intensity trend with the variation in the Dechatu River catchment in an attempt to investigate the causes of such flash flood hazard worsening in this area and to discern the role of climate change compared with that of land use change and human impact in general, i.e., a situation that is common in many dry lands of developing countries.

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2 Study area 2.1 The climate of Ethiopia According to the National Meteorological Agency of Ethiopia (NMA 1989), Ethiopia shows a variety of climatic conditions. This is mainly due to its rugged physiography characterized by large contrasts in altitude and vicinity to the Indian Ocean. The NMA (1989), through the analysis of the rainfall pattern of a large number of meteo-stations, demonstrated that in Ethiopia, there are different rainfall regimes. They can be summarized in the following main categories: 1. 2. 3.

Mono-modal (single maximum) Bimodal (double maxima) Diffused pattern.

The mono-modal regime is dominated by a single rainfall maximum with the wet season running from February/March or April/May to October/November and from June/ July to August/September. The wet season decreases from ten months in the SW to only two months in the NE. A good example is given by the rainfall pattern at Gore (Fig. 2). The bimodal regimes is characterized by three seasons: (1) the dry season (bega in the local language) from mid-October to January (occasional rains are recorded in places); (2) the small rainy season (belg), from mid-February to mid-June, affected by a large interannual variability; (3) the main, big rain season (kiremt), from mid-June to mid-October that, on average, accounts for about 70 % of the annual precipitation. An example of this regime is given by Alamata (Fig. 2). In some areas, the two peaks may be almost equivalent as in Dire Dawa (Fig. 2). In the diffused pattern regime, the rainfall pattern is irregular without a well-defined rainy season as it is observed in the Danakil depression. Aseb in Eritrea is a good example of that (Fig. 2).

350 300

P (mm)

250 200 150 100 50 0 J

F

M Gore

A

M Alamata

J

J

A

Dire Dawa

S

O

N

D

Aseb

Fig. 2 Typical monthly rain patterns in Ethiopia as illustrated by four representative meteo-stations: Gore, mono-modal; Alamata, bimodal with a smaller peak in March/May and a more prominent peak in July/ August; Dire Dawa, bimodal with two peaks almost equivalent; Aseb, diffused pattern

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The annual rainfall is also spatially highly variable with the deepest amounts in the southwestern plateau between the Baro and Omo rivers with about 2,400 mm year-1 to the eastern hyper-arid lowlands of Afar and Danakil where annual rainfall is commonly close to zero. According to NMA (1989) data, the largest proportion of the highlands is characterized by a low (\30 %) variation coefficient of annual precipitation, whereas the low lands and those areas with less annual rainfall such as eastern Tigray, central Ogaden, and Afar/ Danakil show a higher coefficient of variation as high as 70 % and above. In Ethiopia, mean monthly maximum temperature (Tmax) varies at both space and time scale. The highest temperatures (45 °C) occur in the Danakil depression with the secondary maxima observed in the western and southern lowlands (NMA 1989), whereas the lowest daily temperatures are recorded mostly on the highlands with values as low as 0 °C from November to January. In coincidence with the main rainfall season, Tmax decreases all over the country with the exception of the Danakil depression. The annual temperature range is rather moderate all across the country, whereas the daily temperature excursion is rather pronounced during the dry seasons reaching values as large as 30 °C on the highlands and it is very much reduced during the rainy seasons with a daily temperature range of even less than 10 °C.

2.2 The Dechatu River catchment The Dechatu River drains the northern escarpment of the Harerge plateau, has a watershed of about 660 km2 with the shape of an elongate triangle the apex of which points to N–NW

Fig. 3 Location map of the Dechatu River catchment

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(Fig. 3). Its highest point is 2,337 m a.s.l., whereas the lowest one coincides with the confluence with the Lege Hare River at an elevation of about 1,100 m a.s.l., at the downstream end of Dire Dawa. The flow gauge is located in a narrow reach, incised in the bedrock, downstream of the confluence of the main tributaries that originate the Dechatu River, and a couple of kilometers upstream of the town (Fig. 3). Only four rain gauges are within or close to the river catchment; they are located in Dire Dawa, Kulubi, Dengego and Alemaya (Fig. 3; Table 1). The main stratigraphic units outcropping in the catchment are: (1) Precambrian metamorphic basement rocks, consisting mainly of gneiss with a variable density of thin granite injections; (2) Triassic to lower Jurassic Adigrat sandstones: reddish, medium to coarse sandstones and quartz conglomerates resting on the crystalline basement; (3) Antalo limestone (Lower Cretaceous) including neritic, fossiliferous limestones and marls, representing the most extensively outcropping formation in the catchment; (4) Cretaceous Amba Aradam sandstones, consisting of patchy outcrops of fine to medium grained, friable sandstone; (5) Quaternary alluvial and colluvial deposits (Merla et al. 1979). The mean annual precipitation ranges from 1,022 mm at Kulubi, on the plateau margin, down to 634 mm at Dire Dawa in the lowland, with both stations characterized by a

Table 1 Main characteristics of the meteo-stations considered in this study No.

Meteo-station

Elevation (m a.s.l.)

1

Alamata

1,520

750.4

96.0

2

Alemayaa

2,047

764.1

118.0

3

Dengegoa

1,650

763.4

98.8

4

Dese

2,460

1,207.1

94.0

5

Dire Dawaa

1,260

638.7

122.3

6

Fitche

2,820

1,125.2

90.9

7

Gina Ager

3,160

1,693.2

170.8

8

Gode

295

239.9

174.0

9

Gonder

1,967

1,117.2

99.1

10

Gore

2,002

2,101.3

107.7

11

Hayk

2,030

1,174.0

132.8

12

Jimma

1,725

1,563.6

105.7

13

Kebri Dehar

550

325.3

128.0

14

Kobo

1,610

752.8

101.5

15

Konso

1,053

805.6

96.9

16

Kulubia

2,410

1,022.3

100.5

17

Mekele

2,070

598.7

95.5

18

Metahara

947

549.2

96.0

19

Moyale

1,097

685.0

167.6

20

Negelle

1,544

726.6

137.0

21

Nekemte

2,080

2,037.7

137.5

22

Robe Bale

2,480

876.3

112.3

23

Zikwala

2,980

1,124.8

132.6

Ip a

Annual rain (mm)

is the highest intensity ever recorded during the working time of the station Rain gauges within or near the Dechatu River catchment

123

Ip (mm/24 h)


bimodal rainfall regime (see Sect. 2.1). In Dire Dawa, pan evaporation is about 3,140 mm and in every month, potential evapotranspiration is greater than the average monthly precipitation. Dire Dawa is the second largest town of Ethiopia with 330,000 inhabitants, whereas few, very small settlements are scattered within the Dechatu River catchment. The physiography of the catchment is rather rugged, especially in the headwaters which are located on the main escarpment of the Ethiopian Rift. Land cover consists for three quarters of scrubland, open wood, and bare soil. The Dechatu River is incised into the bedrock and no flood plain is found as far as the town of Dire Dawa. Here, the channel is about 60–70 m wide and the streambed gradient varies from 0.035 to 0.02. Upstream of Dire Dawa, the river channel is totally untouched, whereas flood retaining walls are present in the urban area. They were constructed after the 2006 flood. The Dechatu streambed is mainly sandy (70 % sand and 30 % gravel), and it is dry for the most part of the year. Very large flows occur mainly during the summer rainy season in response to short and intense downpours which sometimes cause flash flooding. 2.3 The Dechatu River flood of 2006 After a sequence of floods that occurred in 1981, 1994, 2004, and 2005 (Demessie 2007; Alemu 2009; DDAEPA 2011) that caused significant fatalities and damages to property, on August 6, 2006, the city of Dire Dawa experienced one of the largest and the most devastating flood ever. The area inundated during the 2006 flood was about 1 km2 and 86 % of this area was covered by 1–2 m of water (Alemu 2009). Since all of the dwellings in the inundated area are one story houses, this depth of inundation was big enough to cause several casualties and property damage. This flood affected more than 117,000 people (i.e., one-third of the town population) and officially resulted in the loss of 256 human lives and 244 missing. It caused also the worst property damages to housing and infrastructures in the town history with an estimated total damage of 10 million USD (Alemu 2009). Almost 90 % of the rainfall that generated the flood occurred in the day before (August 5) and the following rainfall depths in 24 h were recorded in the meteo-stations within or close to the catchment perimeter: Kulubi, 100 mm; Kersa, 159 mm; Alemaya, 118 mm; Dire Dawa, 37 mm (Fig. 3).

3 Data and methods 3.1 Rainfall intensity Though Ethiopia has presently a modern and dense network of meteo-stations, daily rain time series show commonly large gaps ranging from months to entire years and, unfortunately, no station has a long (more than 50 years) and uninterrupted data records. Moreover, given the vastness of the country and the variety of elevation, landscapes and climate, data gap filling through correlation with neighboring meteo-stations is commonly unreliable and poorly significant. The paper of Jury and Funk (2013) reports about spatial variability of rainfall over Ethiopia, but no information is given about rainfall intensity. These authors base their research on gridded data and interpolation. Given the low density of meteo-stations in Ethiopia and its rugged topography, this procedure does not seem to

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provide a realistic pattern of rainfall distribution. Moreover, the procedure used by these authors includes a five-station correlation to fill data gaps and a correlation coefficient as low as 0.2 is accepted as significant. In the worst case, the data gap is filled with the mean values of the time series. This method is considered here as questionable, as it does not overtake the data limitations and introduces arbitrary conditions that are not necessarily making the data analysis stronger or more reliable than the simple use of row data. Spatial distribution of rainfall, considering 14 climate zones is investigated also by Viste et al. (2013). These authors have used raw data obtained by the Ethiopian Meteorological Agency, like in the present study, but their study focuses mainly on monthly precipitation. Finally, a consistent study by Seleshi and Camberlin (2006), dealing with spatial and seasonal (bega, belg, and kiremt) rainfall extremes, is available, but this paper is focused on 5-day consecutive rain and no data on daily rainfall is reported. Notwithstanding the awareness of the data limitations, the approach of this paper attempts to fill an existing gap of knowledge about rainfall intensity in Ethiopia and in the Dechatu River catchment and complement the analysis of seasonal and spatial variability reported by previous papers (e.g., Segele and Lamb 2005; Seleshi and Camberlin 2006; Korecha and Barnston 2007; Viste et al. 2013). For this study, 19 meteo-stations with relatively long and as much as possible continuous records were selected to investigate the variability of rainfall intensity in 24 h across Ethiopia. These 19 meteo-stations are evenly distributed across the country (Fig. 4), but their daily records encompass different time spans ranging from 1953 to 2010. For these stations, the 1964–2009 time interval was selected as it has a higher degree of uniformity. In the area of the Dechatu River, there are only four rain gauges. Three of them have a daily data record spanning from 1981 to 2009, whereas the longest time series is recorded at Dire Dawa and the 1964–2009 time interval was considered in accordance with the other 19 meteo-stations (Table 1). Though the rainfall time series selected are more continuous as possible, data gaps are still present and only in a few cases, it was possible to fill them by linear correlation with the nearest station/s. We are well aware about such a limitation, but, in the alternative of neglecting this part of the world and preventing it from any investigation on rainfall intensity variation, we believe that the data processing of these time series, though affected by the bias due to some gaps, can provide an insight into the medium- to long-term rainfall intensity trend in Ethiopia and some indications to account for the role of climate change in increasing frequency of flash flood in the Dechatu River and the town of Dire Dawa can be detected anyway.

Fig. 4 Distribution across Ethiopia of the study rainfall gauges. The numbers indicate the meteo-station name as reported in Table 1

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For each meteo-station, the following data were sorted: annual precipitation, mean monthly precipitation, the highest rainfall intensity in 24 h (I24) recorded in each year and the month in which it occurred, the mean values of I24 (Im) averaged on the entire time series, the highest value of I24 ever recorded (Ip), and the return time for an intensity I24 = 100 mm/24 h was calculated by the Gumbel method. Floods are generated by a number of factors, the most important of which are rainfall amount, catchment geomorphology (namely slope steepness), land use, and antecedent soil moisture. These conditions vary widely across Ethiopia, and, unfortunately, no information is available about any average daily rainfall intensity threshold value capable to trigger a flash flood. In the USA, the River Forecast Centers produce rainfall–runoff curves on a regular basis for each modeled basin. Changes in soil moisture due to recent rain or snowmelt are included as well in the models to produce these curves. Therefore, when soil conditions change, the rainfall–runoff relationship will also change. In their study on flash floods in Europe, Marchi et al. (2010) have studied 25 extreme events which were generated by storms with duration ranging from 7 to 22 h and rainfall amounts between 100 and 300 mm. Rainfall intensities from 100 to 250 mm are reported by Rusjan et al. (2009) to be responsible for flash floods in Slovenia and similar values were measured also by Tantanee and Prakarnrat (2006) for flash floods in northern Thailand. About 100 mm/24 h is the mean rainfall intensity recorded in the Dechatu River catchment the day before the devastating flood of Dire Dawa in 2006. Moreover, all the 23 meteo-stations considered in this study have recorded a peak intensity of almost 100 mm/ 24 h or more. Since no factual information about rainfall intensities capable to initiate flooding in other regions of Ethiopia is available, this value was taken as a possible reference threshold associated with potential conditions of flash flood risk generation, especially on steep catchment across Ethiopia. 3.2 Flow discharge of the Dechatu River Unfortunately, there is no information about the Dechatu River discharge since only flow level data are available from March 2003 to September 2010 and no rating curve has ever been constructed. In order to reconstruct the flood history and to calculate the peak discharge of the main floods occurred in such interval, field measurements of cross section, streambed gradient and bed material grain size were made in the reach with the flow gauge in order to introduce these data into a simple uniform flow equation such as the Chezy equation. The river bed at the flow gauge site is rather regular as it is straight, has a rectangular cross-sectional geometry and is confined between bedrock slope sides (only on the right banks there is a small alluvial accumulation). Given the reach geomorphology, the variability through time of the monitoring site cross-section width can be considered as negligible. As regards the variation of the streambed elevation, very scarce information is reported in the literature, especially for ephemeral streams with a sand bed and hyperconcentrated flow. Billi (2011), in his study on field measurement of bedload transport of the Gereb Oda, a sand bed ephemeral stream in Tigray, Ethiopia, found that in case of hyperconcentrated flow, streambed scouring is limited. This finding is supported also by Powell et al. (2005) in their study using scour chains in Arizona. These latter authors conclude that the streambeds experience little, if any, bed activity during an event. Following these considerations, the streambed elevation at the Dechatu flow gauge site was considered as stable.

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The cross-sectional and streambed gradient were measured by a theodolite, whereas the grain size frequency curve of bed material (Fig. 5) was obtained by the transect line, frequency by number sampling method (Leopold 1970). Since bed material includes also a non-negligible proportion of sand, the size of the sandy particles was identified by means of a visual comparator with specimens of all the sand fractions, arranged on phi scale, stuck on a wooden tablet. The modal class grain of the sand in a 1 9 1 cm area near the meter dent is considered and visually compared to the reference sieve specimens to assign it to a specific phi class (Billi, unpublished). Mean flow velocity (v) was calculated by the Chezy uniform flow equation v ¼ CðRSÞ0:5

ð1Þ

in which R is the hydraulic radius, S the streambed gradient, and C the roughness coefficient. C ¼ ð8gÞ0:5 =f 0:5

ð2Þ

0.5

in which g is gravity and 1/f is the Darcy–Weisbach friction factor. To calculate 1/f 0.5, the following equations were used: Leopold and Wolman (1957) 1=f 0:5 ¼ 1 þ 2 log (h=D84 Þ

ð3Þ

in which h is mean depth and D84 is the grain size for which 84 % of the distribution is finer, Limerinos (1970) and Knighton (1998), respectively: 1=f 0:5 ¼ 1:16 þ 2 log(R=D84 Þ

ð4Þ

1=f 0:5 ¼ 0:82 ln(4:35 R=D84 Þ

ð5Þ

Equations (3) and (4) were selected because they are among the most used in the literature and were found to be suitable for sandy gravel rivers. Equation (5) was derived by Knighton (1998) from a very large set of field data measured on fine gravel to sandy rivers and reported in the literature. In addition, the regime theory equation of Lacey (1946) was used to calculate flow velocity as:

100

% finer

80 60 40 20 0 -7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

D (Φ units)

Fig. 5 Grain size frequency distribution of bed material at the flow gauge reach

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2.0

3.0


v ¼ 10:8R2=3 S1=3

ð6Þ

and was derived for fine-grained mobile bed rivers. Finally, Grant’s equation (1997) was selected as well since it was developed on the base of critical flow condition considerations for mainly sand bed streams as: v ¼ ðghSÞ0:5 4:8ðh=D50 Þ0:11

ð7Þ

in which D50 is the grain size for which 50 % of the distribution is finer. No specific equation developed for fine-grained, ephemeral streams is available in the literature; however, Eqs. (3) to (7) that result from different approaches are based on a rather large set of field data and proved to work satisfactorily for gravelly sand rivers; hence, they represent the best alternative to be used on a dryland river such as the Dechatu. The river discharge was calculated for a wide range of flow levels recorded by the flow gauge using all the criteria described above and the results averaged. By these results, a stage/discharge rating curve was constructed (R2 = 0.95) and the discharge associated with all the flow level data recorded was calculated. This enabled to reconstruct the flood history of the Dechatu from March 2003 to September 2010 that is summarized in Fig. 6, in which only the discharges higher than 300 m3 s-1 are reported. Though August 6, 2006 flood was the most destructive, Fig. 6 shows that it was not the largest. The reasons of that will be discussed later in the paper. 3.3 Land use/cover change The following Landsat images were used to identify the soil use in the Dechatu River catchment in the years 1985 and 2006: TM-09/03/1985 and ETM? 12/04/2006. Though both images were taken during the belg (small rains) interval, they correspond to a phase of little vegetative response following 30 days of negligible or no rainfall at all. To identify the land use/cover, we adopted the NDVI index (Rouse et al. 1974), an index widely used to evaluate vegetation and soil cover changes (e.g., Sohl 1999; Kindu et al. 2013). Positive NDVI values indicate increasing amounts of green vegetation, NDVI values near zero and negative values represent non-vegetated surfaces such as bare rock and soil, water, snow, ice, and clouds. Vegetation typically ranges between 0.2 and 0.8 with higher index values associated with higher levels of healthy vegetation cover.

Fig. 6 Occurrence of floods with a discharge higher than 300 m3 s-1 observed after 2003

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The NDVI index is calculated following different phases: (1) Landsat 5 TM data are converted to the equivalent Landsat 7 ETM ? data (DN) (Vogelmann et al. 2001), (2) DN data (digital number 0–255) are converted to true radiance, then to reflectance (Chander et al. 2009), (3) the NDVI index is calculated as NDVI = ((IR - R)/(IR ? R)) in which IR and R are band 4 and 3, that is, near infrared and red band, respectively. The NDVI maps were classified by using a supervised classification (MLC—maximum likelihood classification). Four land use types were identified on the base of Google EarthÓ high-definition images of the same period and the analysis of false color of the same Landsat images: (1) bare soil, (2) scrubland, (3) open wood and (4) cultivated lands/mix scrubland. The result of the classification, after considering the overlaps in spectral signature between the four soil uses, is reported in Fig. 7. The different land uses, expressed as percentage of the Dechatu catchment area, and their variation between 1985 and 2006 are reported in Table 2.

Fig. 7 Land use/cover in the Dechatu catchment in a 1985 and b 2006

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Author's personal copy Nat Hazards Table 2 Land use/cover variation between 1985 and 2006 in the Dechatu River catchment

Land use/cover

% in 1985

% in 2006

% variation

Bare soil

9.5

11.2

1.7

Scrubland

36.3

44.0

7.7

Open wood

25.9

20.3

-5.6

Cultivated/mix scrub

28.3

24.5

-3.9

4 Results 4.1 Rainfall intensity The values of the highest rainfall intensity in 24 h ever recorded in the study meteostations (Ip) is highly variable and range between 90.9 and 174.0 mm/24 h at Fitche and Gore, respectively (Table 1). The highest annual rainfall intensity in 24 h (I24) may occur in every month, but the frequency of both I24 and Ip follow a bimodal distribution with the two modal classes coinciding with the belg and kiremt rain seasons (Fig. 8). This is not surprising since the majority of Ethiopia is subjected to the bimodal rainfall regime. The variability of both I24 and Ip is not affected by either elevation or annual rainfall as no significant correlation was found between these parameters. On the other hand, the areas with the higher values of average rainfall intensity (Im) experience also the highest peak intensities as shown by the good correlation (R2 = 0.79) between Im and Ip (Fig. 9). The return time interval for a rainfall intensity I24 = 100 mm/24 h was calculated using the Gumbel method for the meteo-stations with at least 30 years of data record. The results of this analysis show that for 50 % of the meteo-stations, the probability of a rainfall intensity of 100 mm/24 h is less than one in 20 years and only at five stations the return time is higher than 40 years (Fig. 10). In order to investigate the rainfall intensity variability across Ethiopia during the last three–five decades, 14 meteo-stations with the most uniform time series were selected and the angular coefficient, m, of the trend lines was calculated for the 1964–2009 and 1981–2009 intervals (Table 3). The latter interval was included for a comparison with the data of the Dechatu River watershed which, with the exception of Dire Dawa, cover only the last three decades (Fig. 11). The data in Table 3 show a very irregular pattern of trends, especially for the 1981–2009 interval, with m values ranging from -1.3 to 1.5. Though

All staons 25 I24

Ip

20

Frequency (%)

Fig. 8 Histogram of the frequency of the months in which the yearly maxima and the absolute maximum of rainfall intensity were recorded at each meteostation

15 10 5 0 J

F

M

A

M

J

J

A

S

O

N

D

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All staons 250

y = 0.6014x1.2951 R² = 0.79

Ip (mm/24h)

200 150 100 50 0 0

20

40

60

80

100

Im (mm/24h) Fig. 9 Correlation between the mean yearly maximum and the absolute maximum rainfall intensity for each meteo-station

Fig. 10 Return time of a rainfall intensity of 100 mm/24 h for the study meteo-stations

I24 = 100 mm Alamata Alemaya Dengeco Dese Dire Dawa Fitche Gina Ager Gode Gonder Gore Hayk Jimma Kebri Dehar Kobo Konso Kulubi Mekele Metehara Moyale Negelle Nekemte Robe Bale Zikwala 0

10

20

30

40

50

60

Return Time (years)

more than one-third of the stations considered show a change of sign from negative to positive (i.e., from decreasing to increasing trend), the mean value of m for the two intervals remains close to zero, indicating no spatial correlation and no detectable change at country scale. By contrast, the trend lines obtained for all the meteo-stations in the Dechatu catchment show an increase in rainfall intensity in this area.

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Author's personal copy Nat Hazards Table 3 Angular coefficient, m, of the I24 trend lines

Meteo-station

1964–2009

1981–2009

Fitche

-0.081

Gina Ager

0.369

Gode

0.520

1.514

Gonder

-0.079

0.123

Gore

-0.498

0.085

Hayk

-0.037

0.371

0.026

-0.104

Jimma Kebri Dehar

a

Dechatu River catchment meteo-stations

-0.977

0.407

1.463

Mekele

-0.023

0.493

Metehara

-0.488

0.354

Moyale

0.511

0.564

Megelle

-0.614

-1.310

Nekemte

0.072

-0.241

Robe Bale

0.386

0.115

Mean

0.048

0.048

Standard deviation

0.390

0.785

Dire Dawaa

0.296

0.302

Alemayaa

0.660

Dengegoa

0.201

Kulubia

0.777

Meana

0.486

140

Fig. 11 Time variation of rainfall intensity in 24 h recorded by the Dechatu River meteostations

120

I24 (mm/24h)

100 80 60 40 20 0 1960

1970

1980

1990

2000

2010

Dire Dawa 1953-1980

Kulubi 1981-2009

Dire Dawa 1981-2009 Alemaya 1981-2009

Dengego 1982-2009

These results indicate that (1) conditions of intense rainfall are ubiquitous all across Ethiopia; (2) in the Dechatu River, rainfall intensities of the same order of magnitude of those that generated the devastating flood of August 2006 have a high probability to occur given the short return time ranging from 14 to 21 years calculated for Dire Dawa and the neighboring station; (3) no significant change in rainfall intensity occurred across Ethiopia

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during the last four decades, whereas the data of all the Dechatu stations show a marked increase becoming more prominent after the mid-1990s. As introduced in Sect. 3.1, whether or not a rainfall intensity of 100 mm in 24 h is capable to generate a flash flood in the study area may depend on many factors, with the three most important being: (1) the pattern of rainfall distribution, (2) the spatial distribution, and (3) the initial soil moisture conditions. Since of these three main parameters only the third one can be inferred from the available data for the Dechatu River, in order to point out the role of and land use/cover change in the increased frequency of flash floods in Dire Dawa during the last decades, the runoff generated by a rainfall of 100 mm in 24 h was calculated by the curve number method for the land use/cover referred to 1985 and 2006. According to this method, the change in land use/cover observed in 2006 is responsible for a runoff increase of about 4.5 %. 4.2 Flood flows The floods larger than 300 m3 s-1, calculated by the methods described in Sect. 3.2, are reported in Fig. 6. The flood of August 6, 2006 was the most devastating and killed 256 people. Its peak discharge was calculated to be around 1,508 m3 s-1 (2.28 m3 s-1 km-2). Alemu (2009), using the Soil Conservation Service Curve Number method (USDA 1986), obtained a peak discharge of 1,400 m3 s-1, i.e., a value very close to that calculated by the simple uniform flow equation. Though the SCS-CN method is empirical and developed for small catchments in the mid-western USA, it was found to be particularly suited for streams with negligible base flow, i.e., rivers for which the ratio of direct runoff to total runoff is close to one, as it is commonly observed in ephemeral streams of arid and semiarid regions (Ponce and Hawkins 1996). This is also the case of the Dechatu that is dry for most of the time and has some water flowing only in response to individual, intense rainstorms. Therefore, it is not surprising that the hydraulic and hydrologic approaches used, though conceptually different, produced a very similar result. In the last decade, the largest flood had a peak discharge of about 2,338 m3 s-1 (3.54 m3 s-1 km-2) and occurred on May 20, 2005. Other six large floods, with peak discharge (Qp) higher than 1,000 m3 s-1 occurred on April 14, 2004 (Qp = 1,095 m3 s-1), March 20, 2005 (Qp = 1,456 m3 s-1), March 25, 2006 (Qp = 1,269 m3 s-1), April 6, 2006 (Qp = 1,080 m3 s-1), April 12, 2007 (Qp = 1,508 m3 s-1), and March 8, 2010 (Qp = 1,118 m3 s-1). The August 6, 2006 flood was by far the worst in terms of human lives lost because it was a typical, very flashy flood and occurred in the night, before dawn, when most of the people were sleeping. According to eyewitness reports, in fact, the flood wave had a high velocity of propagation and peak discharge followed shortly after the flood onset. The largest flood of May 20, 2005 instead occurred during the day and people had time to move to safe places. After the August 6, 2006 flood, retaining walls were constructed throughout

Table 4 Discharge in m3 s-1 of the Dechatu River at Dire Dawa for floods with different return time Method

Q5

Q10

Q20

Q50

Q100

Gumbel EV

1,611

1,981

2,336

2,795

3,139

Semilog

1,227

1,636

2,045

2,586

2,995

Q5–100 indicate floods with return time from 5 to 100 years

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the river reach within the town of Dire Dawa and the subsequent floods, though a couple of them were of the same order of magnitude as the previous ones, had limited impact. In order to quantify the magnitude of the August 6, 2006 flood and the largest flood of May 20, 2005 (Demessie 2007; Alemu 2009), a flood frequency curve was constructed with the discharge data calculated on the basis of the water level measured by the flow gauge. The discharges with the return times of 5, 10, 20, 50, and 100 years were calculated using the Gumbel EV method and by a simple semilog interpolation (Table 4). The results in Table 4 indicate that the return interval of the largest flood of May 20, 2005 is 20 years, whereas that of August 6, 2006 flood is about 4.5 years. The data used to calculate the flood frequency curve, though from one side may reflect the most recent flood trend, from the other cover a very short time interval (March 2003 to September 2010) that can make the results obtained questionable. Given the limitation of the data available, in an attempt to, at least, indirectly confirm the results obtained, the most renown empirical equations to calculate the maximum flood (i.e., Q100) reported in the literature were applied to the Dechatu. They are the equations of Pagliaro (1936, in Maione 1977); Rodier and Roche (1984); and Griffiths and McKerchar (2008) which returned the following discharges for Q100, 2,552, 8,154 and 4,536 m3 s-1, respectively. None of these criteria were specifically developed for ephemeral streams of arid and semiarid regions. In fact, the data of Pagliaro refers to Italian rivers data of the early twentieth century, that is, with less human impact and less forest cover than the present day; Rodier and Roche’s equation is very general since it was derived from data of the world catalog of very large floods including rivers from all over the world; and Griffiths and McKerchar data refer to rivers in the Westland of New Zealand South Island, where a temperate climate with high rainfall prevails. A subset of data of maximum flood recorded in rivers of arid and semiarid environments, with catchment area less than 2,500 km2, was extracted from Rodier and Roche (1984) and data of catchment area (A) were plotted versus maximum recorded discharge (Qp). The best fitting curve (R2 = 0.70) is the following: Qp ¼ 92:625A0:5902 3

-1

ð8Þ

2

with Qp in m s and A in km , and explains 70 % of the maximum discharge variability. According to Rodier and Roche, Qp is very close to Q100. By applying Eq. (8) to the Dechatu, we obtain Q100 = 4,257 m3 s-1. This value is about 25 % larger than that calculated by Gumbel EV method with the flow data recordings available but, given the brevity of the time series, which may result in an approximation for defect, a discharge between 3,100 and 4,200 m3 s-1 can be considered as a possible actual value for the Dechatu’s Q100. Following this result, the return time intervals of the largest flood of May 20, 2005 and of the devastating flood of August 6, 2006 are expected to be larger. The return intervals of I24 = 100 mm/24 h calculated for the four meteo-stations within or close to the Dechatu catchment (Dengego, Dire Dawa, Alemaya and Kulubi—Fig. 3) range from 14 to 21 years and seem to confirm a larger return time for these high floods.

5 Discussion A large number of studies have examined potential trends of river discharge during the twentieth century. Many of them, however, have found no trends or have been unable to separate the effects of variations in temperature and precipitation from the effects of human interventions in the catchment (Di Baldassarre et al. 2010). In the case of the Dechatu

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River, though flow data have been recorded only during the last decade, local authorities (DDAEPA 2011) and local witnesses have confirmed the increased frequency of high flash floods. In the recent past, in fact, only four, minor floods affected Dire Dawa in 1945, 1977, 1981, and 1994 (Alemu 2009). No flow data are available for these floods and are classified as ‘‘minor’’ because they are not reported to have caused fatalities and property damage. The comparison between the flood frequency in the 1945–1994 and the 2003–2010 (Fig. 6) intervals confirms that the flood hazard in Dire Dawa has markedly increased in the second interval. The analysis of I24 time series produced contrasting results for the 14 meteo-stations selected to represent the variability of extreme rainfall at country scale. Eight out of 14 stations are characterized by an increasing trend over the 1964–2009 interval, whereas the proportion of decreasing trend lines is 8 out of 15 stations for the 1981–2009 period. Though the number of data with negative values of m is a little higher in the last three decades, it is worth noticing that only two stations maintain the same sign and the change from positive to negative is observed only in three stations. At country scale, it can be concluded that there is a high variability of extreme rainfalls in time and space, but no clear overall trend can be recognized (mean m = 0.048—Table 3). By contrast, in the Dechatu stations, the trend lines are all positive (mean m = 0.486—Table 3) and indicate an average rainfall intensity increase of 32 % from 1981 to 2009 (Fig. 11). In his 2009 paper, Morin investigated the minimal detectable absolute trends in annual precipitation and concluded that similar analyses may be applied to other related variables. The daily rainfall intensity trends calculated in this study show rates of change that are within the limits reported by this author for the study areas to be considered as minimal detectable absolute trend. The reasons why different areas of Ethiopia are experiencing different patterns of extreme rainfall variability are complex and beyond the scope of this paper; however, it is rather evident that the Dechatu River has been subjected to a marked increase in extreme rains throughout the last three decades (Fig. 11). Furthermore, if the data of Dire Dawa are considered, it appears that rainfall intensities start to increase in 1970 and proceed at an almost constant rate as far as 2009 (Fig. 11); therefore, this factor alone cannot be invoked to account for the concentration of high floods observed in the last decade (Fig. 6) and other factors are to be considered. Land use/cover changed significantly from 1985 to 2006 and a nick point can be identified in the first years following the change of Government in Ethiopia that formally occurred in 1992. Some problems were encountered in defining with accuracy the boundaries between some cultivated areas and the neighboring scrubland. In the study area, traditionally, people were not used to cultivate perennial crops and that resulted in very serious soil erosion problems. From the late period of the DERG government up to the present, construction of terraces and planting eucalyptus trees have been the physical and biological measures implemented in the Dechatu catchment, particularly in the downstream portion, to contrast erosion. Late land management plans of the DERG forced the farmers to set up cultivations also on the steep slopes of the Dechatu headwater and, though soil countermeasures such as terracing and eucalyptus tree plantations were implemented through the food for work program, the local farmers considered the natural constraints and limitations too hard to cope with and making such cultivations scarcely remunerative and rewarding. After the fall of the DERG regime in 1991, these cultivations were abandoned and grown trees were cut and sold for daily consumption. The abandoned cultivations, though partly obliterated by the spontaneous re-growth of scrub vegetation, still maintain traces of their original structure, and for these reasons, they are not easily

123


distinguished by the NDVI analysis. Presently, only little cultivation is taking place due to the limitations of soil degradation severity. This important land management transformation, consisting substantially in crop and conservation practices abandonment, which occurred after 1991, has left the slopes, especially in the river headwaters, devoid of any efficient soil maintenance and water conservations practices, de facto giving rise to conditions that favor higher than before overland flow volumes In order to assess the relative influence of land use/cover change, the runoff generated by a rainfall of 100 mm in 24 h was calculated by the curve number method and an increase of about 4.5 % was found for the land use/cover conditions of 2006 with respect to 1985. The average rainfall intensity calculated from the data of the Dechatu meteo-stations is 52.8 mm/24 h for the 1981–1991 interval and becomes 60.1 mm/24 h for the period 1992–2009, i.e., after agriculture abandonment, with an increment of 14 %. These results suggest that in the Dechatu catchment, notwithstanding the marked change in land use/cover and conservation practices that occurred in the last three decades, the recent increase in rainfall intensity seems to be a key concurrent factor, presumably more important than poor land management in contributing to rise the risk level of flash floods in the town of Dire Dawa. In the very recent years, however, the local administrations have constructed high flood retaining walls that have contained the last large floods of 2010 with peak discharges around 1,000 m3 s-1 (Fig. 6), preventing the town from further inundation, and started a new program of flash flood risk reduction by expanding and renovating the old soil and water conservation measures on the slopes of the Dechatu catchment headwater.

6 Conclusions The analysis of daily rainfall data over a time span ranging from 50 to 30 years from 19 meteo-stations scattered across Ethiopia has shown that almost all the country is subjected to high rainfall intensities ranging from 90 to 170 mm/24 h. Though the highest values of daily rainfall may be recorded in every month, i.e., also in the dry interval, their distributions follow the typical monthly rainfall pattern characterized by two almost equivalent modal classes in April and August. On the base of the rainfall data associated with the devastating flood of Dire Dawa on August 6, 2006, a rainfall intensity of 100 mm/24 h was taken as a reference value capable to set conditions for a flash flood. In about 60 % of the meteo-stations considered, a rainfall intensity of 100 mm/24 h has a probability to occur as once in less than 20 years, and in all but five meteo-stations such intensity has a return time \40 years. The Dechatu River is an ephemeral stream which is dry for most of the time. During the last decade (2003–2010), however, 23 flash floods were recorded. These floods range from 335 to 2,338 m3 s-1, equivalent to 0.51 and 3.54 m3 s-1 km-2, respectively, and seven of them had a discharge higher than 1,000 m3 s-1 (1.52 m3 s-1 km-1). The return time of the largest flood recorded on May 20, 2005 and of the most devastating flood of August 6, 2006 is calculated as 20 and 4.5 years, respectively. These values were obtained from a very short time series and a comparison with the maximum possible flood (Q100), derived by regression analysis between measured maximum flood and catchment area of rivers in arid and semiarid areas (subset of data from Rodier and Roche 1984), and the maximum rainfall intensity in 24 h, recorded at the Dechatu catchment meteo-stations, indicates that

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the higher floods return interval is likely longer than that calculated by the Gumbel flood frequency analysis. The maximum rainfall intensity (I24) recorded in the study meteo-stations shows a contrasting pattern of decrease and increase trends for different stations across Ethiopia, but the long-term mean of the trend lines angular coefficient does not show any evidence of change. By contrast, a marked increase is observed in the Dechatu catchment. Here, in the last decades after 1991 (i.e., after the general abandonment of agriculture and land management practices, following the fall of the DERG Government, especially in the Dechatu River headwaters), extreme rainfall intensity has increased by 14 %. The land use/cover change in the Dechatu catchment was analyzed by means of 1985 and 2006 satellite images interpretation. This analysis indicates that the land transformations occurred between 1985 and 2006 resulted in an increase in all the factors that favor larger volumes of overland flow. The use of the curve number method, refereed to the 1985 and 2006 land use/cover situations shows a runoff increase of 4.5 %. This value, however, is modest if compared with the three times larger increase in rainfall intensity observed after 1991. The increase in extreme rains, paired by a marked change in land use/cover and management practices, is considered the main factor responsible for the increased frequency of high flash floods in the town of Dire Dawa during the last decade, though the increase in rainfall intensity is likely playing a more relevant role. Acknowledgments The authors are indebted to two anonymous referees who posed key points, stimulated discussion, and greatly improved the general quality of the manuscript. This paper was funded by the National Geography Grant 8400-08, University of Ferrara and University of Dire Dawa funds. The authors are indebted to Girma Moges and to the technical staff of the Geography Department of Dire Dawa University for their assistance during the field measurements on the Dechatu River. The Department of Hydrology of the Ethiopian Ministry of Water Resources is acknowledged for providing the flow level data of the Dechatu River.

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