Hydrology for the Water Management of Large Riva- Basins (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 201,1991.

PREDICTION OF HYDROMETEOROLOGICAL EXTREMES IN THE SUDANESE REGION: A NEED FOR INTERNATIONAL CO-OPERATION

K. ANBAH Water Resources Research and Documentation Centre, Italian University for Foreigners, Villa La Colombella, 06080 Colombella (PG), Italy. F. SICCARDI Institute of Hydraulics, University of Genoa, Via Montallegro 1, 16145 Genoa, Italy

ABSTRACT Analysis of the 1988 floods in have shown, that a lead time of at least one month is required for mitigation interventions. The present 5 days lead time for prediction of Nile floods in Khartoum are not adequate for an efficient advance warning.The rainfall over Sudan exhibits frequent positive and negative anomalies. Two approaches are suggested in the present work. The first involves further analysis on the possible spatial coherence and teleconnections of rainfall anomalies over Africa to enhance the lead time based on prediction of extreme events in other regions. The second requires an integrated real time flood forecasting and an effective weather monitoring system covering the countries sharing the Nile Basin. These demand technical cooperation between the countries within the basin.

INTRODUCTION

Flood Hazards Prediction

For the purposes of flood prediction, the Nile basin can be divided into four main environments, namely the Equatorial Lakes, the Ethiopian, the Sudanese and the Egyptian ambient. The Equatorial Lakes and the Aswan reservoir effect long- term regulation on the flows of the White Nile and the Main Nile respectively. The Ethiopian riverine ambient with steep and confined channel does not present serious problems of inundation. On the contrary, as the floods of August 1988 have shown, a lead time of at least one month is required in the Sudanese ambient to give a timely warning for the evacuation of vulnerable population centres and agricultural installations like pumping systems and other temporary installations. However, flood prediction is presently based on the floodwave propagation of the which gives an average travel time of 5-days from Ed Deim at the Ethiopian border to Khartoum (Regulation Rules... .on the Blue Nile, 1968). These figures which presently serve as the basis for prediction of

3 K. Andah & F. Siccardi 4

Nile floods in Sudan are not adequate for an efficient advance warning, especially if the recent hydrometeorlogical events are confronted with the new socio­ economic environment ushered in by the persistent droughts of the past 15 years, characterized by unplanned settlements at the outskirts of cities and towns, especially Khartoum and also by the cultivation of farms in historical flood plains of the Nile valley. These new developments of the country are particularly vulnerable to flash floods of ephemeral natural channels and to Nile floods due to combined pressure of the drought conditions and increasing population migration. From the above, it can be seen that in order to predict, mitigate or manage natural disasters triggered by hydrometeorlogical extremes over a country like Sudan, the largest on the African continent, spanning longitudes 22 °E and 37 °E and latitudes 5°N and 22°N, it would be necessary to understand the mechanisms underlying the various climatic fluxes both in time and space across the country. In this way, it could be possible to increase the lead time from hydrometrical warning to meteorological advance warning through the prediction of causative mechanisms.

Anomalies and Trends in Rainfall Patterns

Generally, Sudan is characterized by extreme climatic zones; from the tropical south through the semi-arid Centre to the arid north. The distribution of rainfall over Sudan exhibits extreme unevenness both spatially and temporally (Hayward, 1987; Griffiths, 1972). Mean annual rainfall diminishes northwards from the south from values of about 1200 mm to about 100 mm, with coefficient of variation from 30% to 100% respectively (Sendil, 1986). The rainy season over the territory of Sudan also varies from the south to the north. While the rainy season commences from March through November in the south , the period spans from June to September in central Sudan with some scarce rainfall in August at the extreme northern fringes of the country. The general characterization of the variability of the climatic flux over the country has been based on the continental air mass circulation controlled by the Inter Tropical Convergence Zone (ITCZ), coupled with the strengths of the southwesterly monsoons and the upper easterly air currents and accentuated by local physiographic and orographic conditions (Kraus 1977; Nicholson, 1983; Zahran, 1986; El Sayem, 1986). Based on the movement of the ITCZ, rainfall is expected in the southern parts of Sudan (below ION) around March-April, while the regions up to lat. 20°N receive rain in June through September. Nicholson (1986), however, points out that variations in Sahel rainfall (including most parts of the Sudan) are generally related to changes in the intensity of the rainy season rather than to its onset or length as the ITCZ hypothesis would require. This means that extreme rainfall events cannot be solely predicted through the anoma­ lous excursions of the ITCZ as proposed by Kraus (1977) and which has been used quite extensively for analysis of rainfall variations in tropical Africa (El Sayem ,1986; Zahran, 1986). Surveys on African rainfall carried out by Klaus (1977) and Nicholson (1985, 1986) have indicated a marked coherence of rainfall variations over large portions of the continent, a characteristic found to be especially strong in the semi-arid regions south of the Sahara and hence could be 5 Prediction of hydrometeorological extremes in Sudan extended to characterize rainfall anomalies over Sudan. With this approach, Nicholson (1986) identified various patterns of rainfall anomaly fields that illustrate a strong tendency for synchronous fluctuations along the tropical and temperate margins of the Sahara. From the foregoing analysis, it becomes apparent that investigations into the behavioral patterns of rainfall must be directed toward the climatic and atmospheric mechanisms at small synoptic scales and mesoscales that could trigger this type of anomalies. With the recent high rainfalls in Sudan, which coincided with similar extreme events in the sahelian regions of Nigeria, Mali, Niger, Burkina Faso and others, it becomes pertinent to address the question of extreme climatic variations from the point of view of spatial coherence of anomalies throughout much of the tropical region (Nicholson, 1986). This approach may lead to the identification of predictors for rainfall anomalies based on extreme events both inside and outside Sudan, especially within the northern hemispheric regions of Africa.

Specific Hvdrological Features of the Nile Basin

The River Nile system extends over 6000 km and drains an area of about 2.9 106 km2. The river flows from the south (the Equatorial Plateaus or Lakes) and the Ethiopian Plateau (lake Tana) through the central arid regions (Sahara) to the Mediterranean sea in the north. The main course of the river valley, its tribu­ taries and lakes cover a wide range of climatic conditions in nine riparian countries, namely Zaire, Burundi, Rwanda, Tanzania, Kenya, Uganda, , Sudan and Egypt (Kashef, 1986). In order to have a fair insight into the flood- wave formation of the Nile, especially within Sudan, more attention must be paid to the Blue Nile and the Atbara rivers which together contribute more than 90% of the peak flows in the central arid regions. In fact the White Nile basically contributes about 80-90% of the low flows and around 30% of the annual flow volume. The Blue Nile which takes its source from the Lake Tana in the Ethiopi­ an Plateau produces very strong torrential flows during the rainy season and exceptionally low flows in the dry periods. The ratio of the maximum flows to the minimum can reach a very high value of 20 : 1. The runoff generation is governed by the humid climatic conditions in the Ethiopian Highlands in conjunc­ tion with the sharp topographic characteristics of the upper part of the basin. From Lake Tana to the Roseires dam, about 1000 km downstream, the channel falls steeply with a slope of about 1.3 10"3. Analysis of peak flows at Khartoum indicates that floods in and around Khartoum are generated by the Blue Nile, while at the same time causing high backwater flows into the White Nile. In the period July-August, the ratio of the flows of the Blue Nile to those of the White Nile is of the order of 97:1. It must however be noted that the flow from Roseires to Khartoum is gentler due to channel slopes of 1.2 10"4 with some minor routing effects of the Roseires and Sennar reservoirs. The most important subcatchment of the Main Nile is the Atbara river basin which has hydrological and morphological characteristics similar to the Blue Nile. The river Atbara also emerges from the Northeastern highlands of Ethiopia with an initial steep slope of 5. 10"3, carrying torrential high flows and K. Andah & F. Siccardi 6 large amounts of sediments during the rainy season (Elsheikh, 1988). The Atbara is highly seasonal and most often dries up for a period of 5 months. In exception­ ally wet years, the flows can reach about 370 106 m3.d"', measured through the Kasma Griba dam. It must be noted that the effective base width of the annual hydrograph comprises June-December with the flood season covering only August and September, (Shahin, 1985). The Atbara river is therefore essential only in flood analysis of the Main Nile, more so when its floods are superim­ posed on those from the Blue Nile. After Atbara, there is no other important inflow to the Main Nile (Nile Waters Study Report, 1979; Hassan, 1981). In the case of territorial assessment of floods, there are other catchments of relevance which from time to time create torrential and flash floods which adversely affect population centres, agricultural and other socio-economic installations. One such river of special importance is the Gash river which frequently hits the north­ eastern town of Kassala with its torrential floods, generated from Ethiopia. The Gash river is ephemeral and predominantly lies within semi-arid and arid zones characterized by progressing deforestation and erosion. On the average, the river flows from July through September in a rather braided bed with mean slope of 1.3 10"*. Torrential floods measured at Kassala since 1907 show an absolute maximum of 876 m3.s"' and a mean maximum discharge of 365 m3.s"' (Lates, 1986). An interesting fact about the river is that, the bulk of the flow is lost to evaporation and infiltration processes and does not therefore reach the Atbara, to which it was once conceived as a tributary. It must be also noted that a correct perspective on floods which sweep over the country in anomalous wet years can be achieved only when such numerous ephemeral streams scattered across the country could be properly studied, analyzed and monitored.

SOME SALIENT MOMENTS OF THE RECENT FLOODS

As the climatic analysis shows, Sudan experiences the influences of diverse continental climatic variations either directly through rainfall distribution or indirectly from the flow of the Nile which traverses across the country from the tropical South through the Central semiarid areas to the arid North. In recent history, the most devastating flood was recorded in 1946 when the Nile River overflowed its banks and inundated towns and population centres along the valleys, especially Khartoum. Another event worth mentioning is the anomalous rainfalls of 1978 which caused extensive floods in the Gezira areas, the Nile and some areas of the southern Provinces, (Zahran,1986). It must however be emphasized that the transformations and adaptations within the physical and socio-economic environ­ ments of Sudan due to the long persistent droughts played a catalytic role in aggravating the effects of the 1988 floods.

Urban and Flash Floods

Most cities and population centres in Sudan lie in flat terrains and hence are generally devoid of natural drainage systems as for example many districts of the 7 Prediction of hydrometeorological extremes in Sudan city of Khartoum are characterized by flat terrain at a mean altitude of about 380 metres above sea level. The existing urban drainage system, where existent, has been left unattended, rendering most parts malfunctional. The situation in other centres cannot be expected to be better. One other important feature of the physiographic conditions of the population centres is that they are mostly surrounded by relatively highlands accentuated by sometimes unidentifiable and ungauged ephemeral stream channels which in times of intensive rainfall conduct torrential flash flows towards living centres. From the point of view of climate, the tropical southern parts of Sudan are qualitatively different from those of the arid north. In the semi-arid areas around Khartoum and towards the North of the country, the rainfall structure is princi­ pally in accordance with the convective phenomena of high intensity and short duration events which is assumed to be triggered by the northern boundary of the Inter Tropical Convergence Zone (ITCZ). The historically observed maximum values of rainfall in Khartoum do not exceed 80 mm in 24 hours (Fig. 1) whilst in Kassala, in the rainy season from June to August, the values are not more than 100 mm in 24 hours (Fig.2). During the past month of July some isolated intensive rainfall events were observed at the stations of Atbara, Kassala, Gedaref and Ed Damazin, just to mention a few. These observed values were generally higher than the monthly historical minimum, indicating the fact that the northern boundary of the ITCZ was sufficiently more northernwards than the periods of historical minimum (dry periods). From these observations, the incoming rainy season could have been foreseen with indications that the expected rainfall events

200 -

E E 160 "55 «*— c *5 120 E E 'x as E i_ 3 O x •4- CM

~i 1 r JAN FEB MAR APR MAY JUN JUL AUG SEPT OCT NOV DEC Month

Fig. 1. Historical 24-hr. maximum rainfall at Khartoum. K, Andah & F. Siccardi 8 above the average minimum of the last few years. In the night of 4th-5th August, Khartoum and its surrounding areas received a very intensive rainfall which was estimated at about 210 mm in 12 hours and constitutes the highest amount of rainfall ever observed in 24 hours (Fig. 1). Physically, the event which has largely exceeded any observed value is this rainfall event over Khartoum and which incidentally coincided with the high levels of the Nile. However, other exceptional rainfall events have been reported in other parts of Sudan, such as Wad Medani. The devastating effects of the floods caused by direct rainfall superimposed on the torrential flows in ephemeral stream channels on population centres constituted the main direct damages incurred during the early periods of the recent events, especially in Khartoum, Wad Medani, Kassala and New Haifa, just to mention a few.

Nile Floods

As explained above, flood waves of the Nile through Khartoum up to the confluence with Atbara river is generated exclusively by the Blue Nile. Further downstream, the magnitude of flood events is defined by the effect of superim­ position of flood waves from both the Blue Nile and the Atbara rivers. The sequence of events leading to the extensive flooding of the country could be described as follows: As a result of intensive rainfalls in the Ethiopian Highlands, the Atbara river developed in the first days of August a flood, the maximum of which was

280

Historical 220 - 2 E E —1 =" 1988 •2 160 - c E2 3 X£ 100 - (0

40 -

f——f—"""—[•"" I— JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Month

Fig. 2. Historical maximum rainfall at Kassala in the Atbara basin. 9 Prediction of hydrometeorological extremes in Sudan attained on 7th August 1988, the highest value registered in the last 20 years. Two days later, the Blue Nile also presented a peak at Roseires, a little over the recorded maximum for the last twenty years (Fig.3). In the successive days, the hydrograph of the Blue Nile presented an increased peak at Roseires, and in spite of the bottom sluices all opened (an annual standard operation during the rising of the seasonal flood), the water level of the reservoir rose two metres above the crest of the spillways. However, the floodwave of the Blue Nile reaching Khartoum in the second week of August, creating a relatively high level, did not equal the peak of 1946 (Fig.4). The second peak observed at Roseires on 18th-19th August arrived at Khartoum on August 21-22, but still below the observed peak of 1946. From the analysis of the available data up to 22nd August, the trend of the floodwave of the Blue Nile at Khartoum from the 19th August onwards seemed to coincide with that of 1946 (Fig.4). It was therefore expected that, if the rains in the Ethiopian Highlands persisted, the peak of the Blue Nile at Khartoum, which is normally observed from the last week of August up to the end of the first week of September, might even exceed the 1946 peak value. However, the combined effects of the steadily rising levels of the Blue Nile and the erratic high flows of the Atbara river produced unprecedented flood levels downstream of the Main Nile. The Nile levels at Dongola exceeded the absolute maximum of 15.12 m on 19th August, reaching 15.53 m on 22nd August.

Socio-economic Impacts

From the analysis of the data collected during the 1988 floods (up to 22nd August), in comparison with historical observed data, there is an indication that the level of the Blue Nile was a little over the 50 years average maximum, while that of the Atbara exceeds the maximum recorded in 20 years (Andah, 1988). A certain amount of discrepancy may be seen between the dramatic consequences and the physical entity of the phenomena as regards the effects of major river networks, at least as the outlined available observed data show. However, a correct assessment of the impacts of the Nile floods must be based on the new physical environment created by the socio-economic transformations due to the long persistent droughts. It must also be emphasized that due to the erratic climatic variability between anomalously wet and dry periods, characteristic of semi-arid and arid ambient, the new socio-economic reality of the country could be considered as permanent. As a consequence of the above, there were serious widespread damages to agricultural farms and pumping installations along the Nile valley, especially the northern areas of Atbara and Dongola. Most of the islands situated in the Main Nile were inundated or cut away from the rest of the country with grave consequences to life and property. The most serious socio­ economic effects of the flood events was the complete loss of the year's farming season. The Northern provincial towns of Atbara, Dongola, New Haifa and many others were heavily hit by both the Nile floods and the causative effects of the high intensive rainfalls on urban inundation and flash floods through temporary natural channels. A vivid example was the town of Dongola, which apart from the damages caused by the high Nile water levels was also surrounded by flash K. Andah & F. Siccardi 10

750

a> 650 - E> to o •=0D) 550 - _>. 'to •a E 450 3 E "x 350 to

250 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Days in august

Fig. 3. Blue Nile discharges at Roseires dam.

floods emerging through ephemeral streams. The nature of damages in Khartoum reveals a potential weakness of the infrastructural system of the city, if confronted further with the vulnerability of the city to Nile floods; the consequences of the latter cannot be predicted due to unavailability of flood risk maps. In the present circumstances, it can be inferred that the Sudan was not only psychologically but also physically unprepared for managing a flood situation as has been outlined. The physical quantities involved in the extensive damages incurred to life, property and economic installations across the country due to direct rainfall, flash floods and Nile floods were quite disastrous (Andah, 1988). From the foregoing analysis, it is clear that the damages were caused by two sources, namely rainfall and Nile flood, and consequently calls for two possible approaches to mitigation actions.

CONCLUSION

In such an ambient, where the existing infrastructure is fragile and the Nile Basin monitoring system is not effective for managing such emergencies, it is important to propose two levels of measures. The first level involves the design, redesign or reorganization of urban infrastructure of major population centres and econo­ mic installations, especially in Khartoum, in accordance with the hydrometeorological information being currently acquired. The second requires an integrated monitoring system within the countries sharing the Nile Basin, 11 Prediction of hydrometeorological extremes in Sudan especially in those countries where the bulk of the storm runoffs emerge, through technical cooperation both at the national and international level (Andah et al,

17 -Ô~=-=fl=-=-«~

0) 1988 > 1987 >• 15 - 1946 "c5

E 3 .§ 14 X /*' CO .S ^ 13

i i i i i i r i i -i 1 r 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Days in august

Fig. 4. Blue Nile water levels at Khartoum.

1988; Siccardi et al., 1988). As verified in Sudan with the current events, it is necessary to have a lead time in the order of 10 days for any eventual forecasting of the Nile flood to allow for the evacuation of people and property from vulnerable areas. For the first part, a risk map for floods is to be developed to serve as a guide for siting of economic installations and agricultural farms. This requires a hydraulic model of the river basin (WMO, 1986). The necessity of risk maps has become the more urgent due to die new socio-economic environment created in the past 15 years. The production of such maps could be used in planning plant cultivation such that seasonal and long term crops could be cultivated at areas of different levels of flood risk. An integrated real time flood forecasting monitoring and an effective weather forecasting system coupled with risk maps, capable of giving ample lead time (one month) could be used to advise farmers on the types of crops and where to cultivate if the wetness or otherwise of a particular year could be predicted. The hypothesis of spatial coherence of rainfall anomalies over Africa, especially in the Sahel region should be further studied as it could enhance the lead time of prediction of anomalous years based on similar occur­ rences in other parts with earlier inception of the rainy season. This would K Andah & F. Siccardi 12

demand a high level of technical cooperation between not only the riparian coun­ tries, where the storm runoff formation occurs but also with the larger internati­ onal arena.

REFERENCES

Andah, K., Borsetto, M. & Siccardi F. (1988) Report on the recent floods in Sudan. ISMES, Bergamo. El Sayem, F. E. (1986) The Effect of Deforestation in Equatorial Africa on Rainfall in Central Sudan. Proc. of the International conference on Water resources needs and planning in drought prone areas, Khartoum, 155-171. ElSheikh, S. E., Kaikai, A. & Sharif, I. (1988) Effects of siltation on the Roseires reservoir. Case study report, 5th Intern. Advanced Course on Water Resources Management, WARREDOC, Perugia. Griffiths, J. E. (ed.) (1972) Climates of Africa - World Survey of Climatology (editor in chief, LandsbergH. E.). Vol. 10, Elsevier Publ., Amsterdam. Hassan, F. A. (1981) Historical Nile Floods and their Implications for Climatic Change. Science, Vol.212, No.4499, June 5. Hayward, D. F. & Oguntoyinbo, J. S. (1987): The climatology of West Africa; Hutchinson, London. Kashef, A. I. (1981) The Nile - One river and nine countries. Journal of Hydrology, 53, 53-71. Kraus, E. B. (1977) Subtropical droughts and cross-equatorial energy transports. Monthly weather review, 105, 1009-1018. Lates, E. M., ElMonshid, B. E. F. & Abbas, A. I. (1986) Comments on the results of extrapolation of series of flood peak values for the River Gash at Kassala. Proc., Intern, confer, on Water resources needs and planning in drought prone areas, Khartoum, 759-764. Nicholson, S.E. (1983): Sub-Saharan rainfall in the years 1976-1980: Evidence of continued drought. Monthly weather review, 111, pp. 1646-1654. Nicholson, S.E. (1985) Sub-Saharan rainfall 1982-84. Journal of climate and applied meteorology, vol. 24, No. 12. Nicholson, S.E. (1986) The spatial coherence of African rainfall anomalies: Interhemispheric teleconnections. Journal of climate and applied meteorology, vol. 25, No. 10. Nile waters study report (1979) Sir Alexander Gibb and Partners (UK), London. Regulation rules for the working of the reservoirs at Roseires and Sennar on the Blue Nile. (1968) Min. of Irrigation and Hydro-electric power, Khartoum. Sendil, U. & Salih, A. M. A. (1986) Frequency of rainfall in the Central Sudan. Proc., Intern. confer, on Water resources needs and planning in drought prone areas, Khartoum, 819-826. Shahin, M. (1985) Hydrology of the Mie Basin, Developments in Water Sciences, 21. Elsevier, Amsterdam. Siccardi, E&Rossi, F. (1988) Coping with floods: the policy of the Italian Group for the Prevention of Hydro-Geological Hazards. Proc., Intern, workshop on Natural disasters in European- Mediterranean countries, Perugia. W.M.O. (1986) Hydrological modelling and real time forecasting in the Nile Basin. Proc., Intern. confer, on Water resources needs and planning in drought prone areas, Khartoum, 77-84. Zahran, A. B. (1986) Sudan rainfall variability-Towards a drought assessment model. Proc., Intern. conf. on Water resources needs and planning in drought prone areas, Khartoum, 85-106.