Assessment of Causes and Impacts Of Flooding On Agricultural Production of Plains Surrounding Lake Tana, Ethiopia

TABLE OF CONTENTS

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

ABBREVATIONS

Abstract

CHAPTER ONE
1 INTRODUCTION
1.1 General
1.2 Statement of the Problem
1.3 Objective and Scope of the Study

CHAPTER TWO
2 LITERATURE REVIEW
2.1 Flood Disaster and Its Impacts
2.2 Plant Response to Flooding
2.3 Floodplain Management
2.4 Impacts of flood control and prevention measures
2.5 Integrated Flood Management
2.5.1 Elements of Integrated Flood Management
2.6 Waterlogging and Development of High Water Table
2.6.1 Signs of Bad Drainage
2.6.2 Harmful Effects of Excess Irrigation
2.6.3 Benefits of drainage
2.6.4 Biodrainage

CHAPTER THREE
3 DESCRIPTION AND METHDOLOGY
3.1 Description of the Study Area
3.1.1 Administrative Location
3.1.2 Geographical Location
3.2 Methodology
3.2.1 Data Collection
3.2.2 Data Processing
3.3 Basic Features of the Study Areas
3.3.1 Agro ecological Zones
3.3.2 Geology of Lake Tana Plains
3.3.3 Drainage
3.3.4 Soils of Lake Tana Surrounding Plains
3.3.5 Topography
3.3.6 Land Use
3.3.7 Socio-economy

CHAPTER FOUR
4 DATA ANALYSIS AND RESULTS
4.1 Flood Assessment Based on Variability of Rainfall and Stream Flow Data
4.1.1 Rainfall Variability
4.1.2 Stream Flow Variability
4.1.3 Lake Tana Water Level Variation and Flood Recession
4.2 Hydrologic Frequency Analysis
4.2.1 Flood Frequency Analysis
4.2.2 Frequency Analysis of Rainfall
4.3 Socio Economic Impact of Flooding on LTSPs
4.3.1 Nature of Flooding on LTSPs
4.3.2 Flood Affected Areas
4.3.3 Causes of Flooding and Waterlogging in LTSPs
4.3.4 The Impact Flooding on Agricultural Production of LTSPs
4.4 Potential Mitigation Measures
4.4.1 Flood Management Options
4.4.2 Agricultural Drainage options

5 DISCUSSION OF RESULTS
5.1 Results of the Variability Analysis of Rainfall and Runoff
5.2 Result of Hydrological Frequency Analysis
5.3 Result of Socio Economic Impact Assessment of Flooding on Agricultural Production of LTSPs
5.4 Results of the potential mitigation measures for flooding and Drainage problems

CHAPTER SIX
6 SUMMERY, CONCLUSION AND RECOMMENDATION
6.1 Summery and Conclusion
6.2 Recommendations
6.3 Limitations

REFERENCES

APPENDICES
Appendix A Figures of Mean Monthly Vs 2006 RF of LTB Stations
Figure A.10 Addis Zemen Mean Monthly Vs 2006 Rainfall
Appendix B Figures of Deviation from the mean of Annual RF of LTB Stations
Appendix C Figures of Mean of Maximum Runoff Vs 2006 Maximum Runoff of LTB Rivers
Appendix D Figures of Mean Annual Flood Deviation from Long Term Mean of LTB Rivers
Appendix E Mathematical Expressions and Standard Error of Estimate for Statistical Distributions of Annual Maximum Series
Appendix F. Questionnaire
Appendix G. Rainfall Data
Appendix H. Hydrological Data
Appendix I Aid Given by GOs and NGOs for the Victims of 2006 Flood Hazard Around LT

DEDICATION

To My Wife Biruh Tesfa & My Little Child Edomias Yirga

LIST OF TABLES

Table 1.1 Regional summery of people affected and death by flood of 2006

Table 2.1 Complementary approaches of integrated flood management

Table 2.2 Strategies and Options for Flood management

Table 3.1 Area-Slope Distribution of LTB with Percentage of Catchment Land

Table 3.2 Land use of Tana Basin

Table 4.1 Rainfall of 2006 Compared with the long term mean

Table 4.2 Coefficient of Variation of Annual RF of Stations on LTB

Table 4.3 Meteorological stations, their weighted area and mean RF of LTB

Table 4.4 Percentage Increase of 2006 maximum flood from the Long Term Mean flood

Table 4.5 Approximate duration of Over Bank Flooding of Major Rivers in LTB

Table 4.6 Specific Flood Discharge and Coefficient of Variation (CV) of Major Rivers of LTB

Table 4.7 Type of Fitted Probability Distribution and Return Period of 2006 Peak Flood of Rivers in LTB

Table 4.8 Estimated Flood Quantile of Rivers in LTB for Different Return Period

Table 4.9 Return Period of 2006 Monthly Maximum Rainfall

Table 4.10 Historical Flood Hazard on LTSPs

Table 4.11 Characteristics of 2006 Year Flood on LTSPs

Table 4.12 The Adverse Impact of 2006 Flooding on Crop Production of LTSPs

Table 4.13 The Adverse Impact of 2006 flooding on Human Being, Livestock and Infrastructures

Table 4.14 Summary of Impact Assessment of Flooding on LTSPs, its Impac Value and Duration

Table 4.15 Proposed Irrigation Demand in LTB

Table 4.16 Drainage Coefficient of Cereal Crops in LTSPs

Table 4.17 Recommended Dimensions of Collector Drains

Table 4.18 Ground Water Level Observed on LTSPs

Table 5.1 Mean Percentage Increase of RF from May to September 2006

Table 5.2 Percentage Increase of 2006 maximum flood from the Long term Mean flood

LIST OF FIGURES

Figure 1.1 Flood Vulnerable areas in Ethiopia

Figure 1.2 Flood on Fogera plain of Lake Tana Surrounding

Figure 2.1 Integrated Flood Management Model

Figure 3.1 Location Map of Lake Tana Basin

Figure 3.2 Lake Tana and its inflowing and out flowing main rivers

Figure 3.3 Sub-Agro ecological Zonation of Lake Tana Basin

Figure 3.4 Soils of Lake Tana Basin

Figure 3.5 Two dimensional elevation views of Lake Tana Basin

Figure 3.6 Land Use Map of Tana Basin

Figure 4.1 relationship of Mean Annual Rainfall and Altitude of LTB Stations

Figure 4.2 Bahirdar Mean Monthly Vs 2006 Rainfall

Figure 4.3 Deviation from the mean of Annual RF of Bahir Dar

Figure 4.4 Thiessen’s Polygon of LTB

Figure 4.5 Mean Annual Variability of rainfall on LTB

Figure 4.6 River Gauging Stations around Lake Tana

Figure 4.7 Megech Mean of Maximum Runoff Vs 2006 maximum Runoff

Figure 4.8 G/Abay Mean Annual Flood Deviation from Overall Mean

Figure 4.9 Mean Monthly Variation of Specific Flood Discharges of Rivers

Figure 4.10 Mean Monthly CV of Peak Floods of Major Rivers of LTB

Figure 4.11 Lake Tana Mean of Monthly Maximum Vs 2006 Maximum Water Surface Level

Figure 4.12 Annual Variation of Maximum Flood Level of Lake Tana

Figure 4.13 Daily Water Levels of LT above 1787 masl

Figure 4.14 Fogera Flood Plain around Lake Tana

Figure 4.15 Megech river and Dembia Flood Plain around Lake Tana

Figure 4.16 Proposed Irrigation Projects on LTB

Figure 4.17 Sandbag Levee

Figure 4.18 Typical Design of Levee for Inflow Rivers and LT

Figure 5.1 Deviation of Annual Areal RF of LTB from Long Term Mean

Figure 5.2 Deviation Mean Annual Inflow to LT from Long Term Mean Annual Flood

Figure 5.3 Deviation of Annual flood level of LT from the mean flood level

Figure 5.4 Causes of Flood and Flood-intensifying factors in LTSPs

ABBREVATIONS

APFM Associated Programme on Flood Management

CSA Central Statistical Authority

CV Coefficient of Variation

DC Drainage Coefficient

GOs Governmental Organizations

GWP Global Water Partnership

GWT Ground Water Table

ha hectare

IFM Integrated Flood Management

IWRM Integrated Water Resource Management

LT Lake Tana

LTB Lake Tana Basin

LTSPs Lake Tana Surrounding Plains

m meter

masl meter above sea level

M2-1 Tepid to cool moist plains

M2-5 Tepid to cool moist mountains and plateau

M3-7 Cold to very cold moist mountains

MAR Mean Annual Rainfall

MoARD Ministry of Agriculture and Rural Development

MoWR Ministry of Water Resources

NGOs Non Governmental Organizations

PAs Peasant Associations

SH2-5 Tepid to cool sub-moist plains and mountains and plateau

SH2-7 Tepid to cool sub-humid mountains

SM3-7 Cold to very cold sub-moist mountains

SSD Subsurface Drainage

WMO World Meteorological Organization

WT Water Table

Abstract

The main objective of this study is to identify the main causes and effects of flooding and drainage problems on agricultural production of Lake Tana surrounding plains, mainly on Fogera, Dembia and Kunzila floodplains. The paper intends to answer 1) variability of runoff and rainfall data in identify the cause of 2006 flooding on LT shores 2) the best-fit flood analysis distribution function for major rivers 3) The socio economic impact of flooding on agricultural production and 4) potential mitigation measures that reduces adverse effect of flood and drainage problems.

The rainfall variability analysis of LT basin in 2006 was shows that on average 43% increase in wet season rainfall than the normal (mean). All rainfall gauging stations shows an increase in rainfall in 2006. Similarly the variability analysis of major rivers was also shows that on average 35% increase in flood season stream flow of G/Abay, Gumara, Rib, Megech and Koga. The trend of these rivers shows that maximum runoff for the year 2006 was higher than the mean of the long term maximum flood. Whereas Lake Tana maximum flood level of 2006 (1787.155masl) show an increase by 16 cm only from the mean flood levels of pervious records (1787 masl). Therefore LT backwater effect in 2006 was less and it doesn’t have much contribution for 2006 flooding.

Flood frequency analysis of major rivers were done and it is found that Extreme Value Type One maximum likelihood probability distribution is best fit for G/Abay and Koga rivers. Pearson III method of moment probability distribution is a best fit for Megech and Rib rivers. For Gumara river Pearson III probability weighted moment distribution better estimate flood quantiles with less standard error. It is also found that Gamma two probability weighted moment is best fit for Lake Tana water surface level. In general from rainfall and flood frequency analysis 2006 year flooding may have a chance to occur once in six years in LTB.

The 2006 year flood damage indicates that there is high impact on agricultural production of Lake Tana surrounding plains. 107,647 peoples were actually affected by flood. At least 448, 910 quintal of food grain, 1230 domestic animals, 9634 chickens and 1088 bee-hives were damaged by 2006 flood. The impacts of flooding on socio- economic and environmental resource indicators were qualitatively assessed. Totally twenty seven indicators were assessed. Based on the assessment ten indicators gave beneficial impact and thirteen indicators gave adverse impact the rest indicators gave that no impact on flooding or the impact is not identified. In general 2006 flooding has a negative socioeconomic impact, which over shadow the beneficial impact with respect to the environment and socio-economy.

CHAPTER ONE

1 INTRODUCTION

1.1 General

Natural hazards and flood events are part of nature. They have always existed and will continue to exist. With the exception of some floods generated by dam failure or landslides, floods are climatological phenomena influenced by the geology, geo-morphology, relief, soil, and vegetation conditions. Meteorological and hydrological processes can be fast or slow and can produce flash floods or more predictable slow-developing floods, also called riverine floods.

Excessive flooding in rainy season of Ethiopia is becoming problem which in many parts results in loss of life and extensive damage to the infrastructure and agricultural production. Much of the international emergency assistance is directed to alleviate the immediate short-term problems arising as a result of excessive floods. However, more lasting solutions are required to overcome and reduce the negative effects of flooding in a sustainable way.

Flooding on the lower plains of LT due to the overflowing of Rib and Gumara rivers is a recurrent event and each year causes some hazard, in varying degrees and area coverage, to the agricultural production, rural infrastructure and human settlements. Floods reached to disastrous proportions in 2006 with serious damages in food production and human settlement. The floods occur during the monsoon period from July till November and are caused by heavy tropical storms in its drainage basin.

Fogera, Dembia and Kunzila are the areas around LT which are flooded severely. Even if the farmers know that the areas are vulnerable to flood, they are not willing to flee to other areas. Rather the population increases from time to time, because these plains are fertile and high market access. The farmers cultivate at least two times in a year one by the long rainy season and the other by the moisture stored after the heavy rains. The main problems which affect agricultural production on these plains are the recurrent flooding and waterlogging.

Although Fogera and Dembia plains form from repeated deposition of sediments, they are ideal habitats for human settlement and cultivation; these are areas at risk from catastrophic flood events and suffering from water borne diseases (Malaria and Bilharzias). There should be a mechanism of living with flood, adapting to floods, developing local flood warning systems and devising flood mitigation systems are practical alternatives to improve flood use. Furthermore, there is a need to protect against the excess flood water in wetland and swamp areas. Land drainage is required to meet the demand for agricultural land and eradicate breeding ground for waterborne disease causing agents.

This study is therefore an attempt to identify the main causes and effect of flooding on agricultural production and the peoples living in the Lake Tana surroundings.

1.2 Statement of the Problem

Although floods are relatively common during the June to September rainy season in Ethiopia, the magnitude of the current flooding in 2006 is unprecedented. In year 2006 the country has experienced some of the heaviest and most intense rains on record; resulting in flash floods and/or the overflow of rivers, lakes and dams, where local residents have been advised to leave. The impact of the disaster in terms of lives, infrastructure, livelihoods, and basic coping mechanisms has yet to be assessed.

By the end of August 2006, large areas in as many as eight regions had been affected. The map of 2006 year flood vulnerable areas in Ethiopia is depicted on Figure 1.1. According to the latest information issued by the Ethiopian Government Disaster Prevention and Preparedness Agency (DPPA) more than 500,000 people are vulnerable, and about 200,000 people have been affected. The details of the affected and death toll are furnished in the Table 1.1.

Table 1.1 Regional summery of people affected and death by flood as of August 24, 2006 (DPPA, 2006).

Abbildung in dieser Leseprobe nicht enthalten

Figure 1.1 Flood vulnerable areas in Ethiopia in August 24, 2006 flood (DPPA, 2006)

There are about sixty one (BCEOM, 1998) large and small streams that draining into the Lake; with only one outlet (Abay). The outlet is already too elevated (by lava flow) to allow the water to gush out as much as it enter the Lake. Moreover, as part of the Tis-Issat hydropower scheme, the outflow is being regulated. These have resulted in a more or less settled water body allowing most of the sediment in suspension to settle onto the bottom of the Lake. As the sedimentation increases, the water holding capacity of the Lake decreases.

The four major rivers (Gumara, Megech, Rib and Gilgel Abay) contribute 93% of water to the Lake (Kebede S. et.al, 1998). The amounts of annual flow from the streams in recent years have high fluctuations. For e.g. in 2003, the level of LT became so low that rocks were exposed making navigation difficult but on the other hand, this year 2006 excess inflow that caused large areas to flood. 2006 flood is one of the severe floods which inundated large area of cultivated and grazing land in surrounding of LT (the flood situation in Fogera plain of LT are shown in Figure 1.2 ). The drainage situation of the area has worsened the problem; cultivated crops were damaged and livestock grazing become impossible which had significant impact on food security. Due to increasing population in the area, the number of people affected by flood will rise in the foreseeable future. The causes, problems and the effect of this flood should be assessed to avoid the damages being caused by future flooding. The occasional advantage of the area should be weighed together with the damage caused by flooding.

Causes of flooding, flood control, drainage investigation of LTSPs in the basin as a whole is not studied separately and in detail except the regional study of the Abay River Basin Master Plan for different development activities as a scale of 1:250,000 (BCEOM, 1998). The project area lacks a detail hydrological and socio-economic study on floods and flood management in order to launch developmental activities in the basin. It is believed that this paper will play an important role towards sustainable development in the basin.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1.2 Flood on Fogera plain of Lake Tana Surrounding (September 2006)

1.3 Objective and Scope of the Study

General Objective

To identify the main causes and effects of flooding and drainage problems on agricultural production of Lake Tana surrounding plains, mainly on Fogera, Dembia and Kunzila floodplains.

Specific Objective

- To assess the variability of runoff and rainfall data in identify the cause of 2006 flooding on Lake Tana shores.
- To determine the best-fit flood analysis distribution function for major rivers.
- To assess the socio economic impact of flooding relative to the livelihood of the rural people and production of the land.
- To suggest the potential mitigation measures that reduces adverse effect of flood and drainage problems.

CHAPTER TWO

2 LITERATURE REVIEW

2.1 Flood Disaster and Its Impacts

Natural disasters cause much misery, especially in developing countries where low-income economies are greatly stressed by their recurrence. Statistics show that around 70 per cent of all global disasters are linked to hydro-meteorological events. Flooding is one of the greatest natural disasters known to humankind. Flood losses reduce the asset base of households, communities and societies by destroying standing crops, dwellings, infrastructure, machinery and buildings. In some cases, the effect of flooding is dramatic, not only at the individual household level but on the nation as a whole. The 1982 floods in Bolivia are reported to have resulted in a loss equivalent to 19.8 per cent of the country’s GDP. It may, however, be argued that looking at the impact of floods on a piecemeal basis, rather than making holistic appraisals, has too narrowly assessed their impact (APFM,2003).

As it is also indicated by APFM (2003) technical document, Absolute protection from flooding is neither technically feasible nor economically or environmentally viable. Thinking in terms of setting a design standard of protection is both a trap and a delusion: such a standard conflicts with the principle of managing all floods and not just some. It is also a delusion because estimates of the magnitude of extreme floods are very inaccurate and, due to climate change, likely to get modified over time.

Although living on a flood plain exposes its occupants to one set of disturbance – i.e. flooding – it also offers enormous advantages. The deep, fertile alluvial soil of flood plains – the result of aeons of flooding – is ideal for higher crop yields and helps reduce vulnerability of the flood plain occupant to a wide range of other disturbances.

In turn, flood plains typically support very high densities of human settlement. It is not entirely coincidental that the population densities of the Netherlands and Bangladesh are so high and that the gross domestic product (GDP) per square kilometer is high in countries whose territories are comprised mostly of flood plains, such as in the Netherlands – which has the highest GDP per square kilometer in Europe.

Recurrent flood events are natural phenomena in many areas in the world. Regular flooding by itself does not form a problem. Traditionally, farming systems were based on rice production and incorporated floods. Damage to crops under these conditions is restricted to abnormal conditions, i.e. when floods occur much earlier or later, or when water level rise higher or quicker than normal. However, with the introduction of improved varieties, crop intensification, diversification, and farm mechanization better water control conditions are required and all floods started to be considered as problematic. Also under influence of the ever-increasing world population there is often enormous pressure to develop tidal, deltaic and other flood prone areas due to their strategic location. These factors have led to increased necessity for flood control and protection measures (APFM, 2003).

Areas that are characterized by high temperatures, high rainfall and recurrent flood events have resulted in farming systems traditionally based on rice production. Inundation depth, during and timing determined cropping patterns, crop selection and yields. With the introduction of improved varieties, crop intensification, diversification, and farm mechanization better water control conditions are required. Better water control can be achieved through the implementation of drainage alone or in combination with flood control measures. Especially in lowlands, delta and tidal areas excess rainfall and floods are the main limitations for crop intensification and diversification.

Although, lowlands, delta and tidal zones are in general unsuitable for development due to soil conditions, waterlogging and inundation risks, and their environmental value there is often enormous pressure to develop these areas due to their strategic location. Development of these areas involves flood control schemes, in combination with the provision of drainage and in several cases polder development. The type and extend of flood protection and drainage development in an area depends on rainfall patterns, the type, intensity and depths of flooding, and the advance in rural development.

Besides the natural floods caused by run-off during exceptionally heavy rainfall, there are also floods caused by human intervention in nature these include: sudden breaching of an embankments; sustained failure of polder drainage pumps during the monsoon season, thus allowing ponding of rainwater within a polder; abnormally high rates of release of water from dams; ponding of water behind road, railway and flood embankments following heavy rainfall; drainage congestion; and river siltation.

Human interventions in the drainage catchment area have often resulted in increased flooding risks. Examples are deforestation, canalization of river streams and disappearance of wetlands. Potential solutions are introduction of flood control and protection measures often in combination with drainage, integrated river basin management to reduce the risks of flooding and introduction of precaution measures to reduce damage to crops.

2.2 Plant Response to Flooding

Flooding of soil rapidly depletes soil oxygen and alters plant metabolism, thereby inhibits growth. Reduced growth is preceded by stomatal closure: reduced photosynthesis, carbohydrate translocation, and mineral absorption; as well as altered hormone balance. Flood tolerance varies widely among plant species, cultivars, and ecotypes and is associated with both morphological and physiological adaptations (Kozlowski, 1983).

Also, stagnant water or waterlogging cause excessive soil reduction which alters the chemistry of wetland soils and usually causes nutrient deficiency or toxicity or both to rice crops. Adequate water movement through rice fields is recognized by most rice farmers as essential. Incoming water is normally aerated and its oxygen prevents the soil redox potential from falling too far. Rice yields in millions of hectares with prolonged stagnant flood with depth of 50 cm or more are generally low. In these areas, the depth, duration and frequency of flooding are important factors affecting yields of wetland rice (Kozlowski, 1983) .

Certain plants overcome the bad effects of excess water in soil and waterlogging by developing certain adaptations. They are as follows:

- Plants like sugarcane and maize grow aerial adventitious roots for respiration owing to absence or inadequate supply of oxygen in the soil having excess water or waterlogged condition. Greater concentration of roots occurs in upper layers of soil profile when the water table is high and occupies a part of the crop root zone.
- Plants develop large intercellular space, thin cell wall, low or no cell wall suberization, hollow pith and large cell to contract the excess soil water condition. Aquatic plants have all these adaptations.
- Plants develop specialized air conducting tissues in the stem and roots in order to help transport of atmospheric air through leaves and stems to roots for root respiration. Rice crop under waterlogged condition develops such tissue called aerenchyma for root respiration.
- Plants increase gradually there tolerance to submergence or excess water conditions.

Crops like rice, sugarcane, cotton and maize are tolerant to excess water in the root zone, while tobacco, beans, tomatoes and fruit trees are susceptible to it. Plants are usually little tolerant to waterlogging during germination process, but become susceptible immediately after. Flowering stage is the most susceptible stage of plants to waterlogging. However, plants become slightly tolerant to this condition during the grand gross period and in the maturation stage (Dilip, 2000).

2.3 Floodplain Management

Floodplain management refers to all the actions society can take to responsibly, sustainably, and equitably manage the areas where floods occur and which serve to meet many different social, economic, natural resource and ecological needs. Since this includes reducing the hazard and suffering caused by floods, floodplain, and flood management consist of many common activities. However, floodplain management recognizes explicitly that other factors of a social, economic, natural resource management and ecological nature also have to be taken into account in “managing” floods.

Effective floodplain management, like watershed management, is an iterative process of identifying and assessing alternative ways of reducing the impact of floods (particularly of catastrophic events) in flood-prone areas. Decision-making in floodplain management involves compromises between the costs and benefits of alternative actions. It also requires that upper catchment areas be considered part of the solution and not as the ‘source’ of the problem.

In the past, structural responses (e.g., dams, levees, dikes, etc.) were emphasized and indeed, in the early- to mid-20th century engineers prevailed in debates over the best means to tame the awesome power of floodwaters. With ‘flood control’ as their explicit objective, engineers around the world spent decades (and billions of dollars) building dams, embankments and levees to prevent floodwaters from inundating floodplains. These structures were often combined with dredging to straighten and deepen stream channels. According to the World Commission on Dams (WCD 2000), some 13 % of all large dams, or over 3,000 worldwide, were built with a specific flood-mitigation function.

Most flood defenses were built as individual local schemes, with little consideration of their impacts across the wider river catchment, their impacts on the aquatic and coastal environments or, indeed, even their broad economic impacts. The fact that embankments and other engineering structures were most effective only for small- to medium-sized flood events was often not recognized.

Although attention is usually focused on the negative effects of floods, there are highly important positive effects that warrant recognition and consideration. Flooding in many low-lying areas of Asia is a vital element of the culture and economy of the people. Annual floods along many rivers carry fine sediments and nutrients that renew the fertility of the land and aquatic habitats, and the continuous flow of silt-bearing irrigation water helps control diseases in many areas. In a region where agriculture and fishing remain vitally important, the loss of these beneficial effects could potentially lead to unacceptable economic and social disruption. However, what is beneficial to some may inflict heavy economic costs upon others. The challenge is to balance costs and benefits.

New flood management approaches are steadily introducing or expanding the role of non-structural measures within integrated floodplain management programs. Key measures include the identification of natural storage areas, such as swamps and wetlands, where excess water can be diverted and temporarily stored during periods of flooding. The World Commission on Dams (WCD 2000) categorizes the components of an integrated approach to floodplain management according to those which reduce the scale of floods, those which isolate the threat of floods and those which increase people’s capacity to cope with floods.

Table 2.1 Complementary approaches of integrated flood management (WCD 2000)

Abbildung in dieser Leseprobe nicht enthalten

2.4 Impacts of flood control and prevention measures

In general terms the role of flood control and protection measures is to protect life and property against damage caused by floods. In agricultural terms the role of flood control and protection measures is to reduce the damage to crops and to increase agricultural potential.

Implementation of flood control and protection measures in flood plains and deltaic zones will regulate the depth and timing of flooding. Where large areas are fully protected from floods by embankments, drainage of excess rainfall is normally obtained by gravity through sluices. Under this concept, excess rainfall is drained slowly through the monsoon season. If supplementary pumping is added, so changing to a polder-type of management, the area will be separated from the surrounding hydrological regime and independent water table control can be accomplished.

Positive impacts

- Crops are protected against damage from floods;
- Infrastructure, houses, farm equipment, stocks and livestock are protected against damage from floods;
- Opportunities for crop intensification and diversification;
- Improved farm management;
- Possibilities for farm mechanization.
- Prevention of damage to farm equipment, stocks, livestock, infrastructure and houses. Negative impacts
- In general the flood level rises in areas unprotected by dikes or embankments and thus increasing the damage in these areas;
- Beds of rivers confined by embankments might experience a rise due to siltation causing drainage problems;
- If inadequate drainage measures are incorporated in flood protection works the area behind the embankments and dikes might suffer from drainage problems as rainwater will accumulate behind the flood protection work;
- Damage from floods caused by breaches in dikes and embankments are normally very destructive;
- Deltaic and tidal areas are often fragile ecosystems that are easily disturbed by development works.

2.5 Integrated Flood Management

Integrated Flood Management is a process promoting an integrated – rather than fragmented – approach to flood management. It integrates land and water resources development in a river basin, within the context of IWRM, and aims at maximizing the net benefits from flood plains and minimizing loss to life from flooding.

Globally, land, particularly arable land, and water resources are scarce. Most productive arable land is located on flood plains. When implementing policies to maximize the efficient use of the resources of the river basin as a whole, efforts should be made to maintain or augment the productivity of flood plains. On the other hand, economic and human life losses due to flooding cannot be ignored. Treating floods as problems in isolation almost necessarily results in a piecemeal, localized approach. Integrated Flood Management calls for a paradigm shift from the traditional fragmented approach of flood management.

Integrated Flood Management (IFM) recognizes the river basin as a dynamic system in which there are many interactions and fluxes between land and water bodies. In IFM the starting point is a vision of what the river basin should be. Incorporating a sustainable livelihood perspective means looking for ways of working towards identifying opportunities to enhance the performance of the system as a whole. The flows of water, sediment and pollutants from the river into the coastal zone – often taken to extend dozens of kilometers inland and to cover much of the river basin – can have significant consequences. As estuaries overlap the river basin and coastal zone it is important to integrate coastal zone management into IFM. Figure 2.1 depicts an IFM model (APFM, 2003).

Abbildung in dieser Leseprobe nicht enthalten

Figure 2.1 Integrated Flood Management Model (APFM, 2003).

The attempt is, therefore, to try to improve the functioning of the river basin as a whole while recognizing that gains and losses arise from changes in interactions between the water and land environment and that there is a need to balance development requirements and flood losses. It has to be recognized that the objective in IFM is not only to reduce the losses from floods but also to maximize the efficient use of flood plains – particularly where land resources are limited.

However, while reducing loss of life should remain the top priority, the objective of flood loss reduction should be secondary to the overall goal of optimum use of flood plains. In turn, increases in flood losses can be consistent with an increase in the efficient use of flood plains in particular and the basin in general.

2.5.1 Elements of Integrated Flood Management

According to APFM (2003) technical document, the defining characteristic of IFM is integration, expressed simultaneously in different forms: an appropriate mix of strategies, points of interventions, types of interventions (i.e. structural or non-structural), short or long-term, and a participatory and transparent approach to decision making – particularly in terms of institutional integration and how decisions are made and implemented within the given institutional structure.

Therefore, an integrated flood management plan should address the following five key elements that would seem to follow logically for managing floods in the context of an IWRM approach (APFM, 2003).

- Manage the water cycle as a whole;
- Integrate land and water management;
- Adopt a best mix of strategies;
- Ensure a participatory approach;
- Adopt integrated hazard management approaches.

Manage the Water Cycle as a Whole

Recognizing that water is a finite and vulnerable resource, differentiation between water resources management, flood management and drought management needs to be circumvented. Flood management plans need to be intertwined with drought management through the effective use of floodwater and/or by maximizing the “positive” aspects of floods. In arid and semi-arid climates in particular, floods are essentially the water resource. Whilst for most of the time runoff is essentially the water resource, it is only at the times of extremes that runoff is a problem. The positive effects of floodwater should be recognized in national/local water management plans. Groundwater and floodwater should be treated as linked resources and the role of flood plain retention capacities for groundwater recharge should be considered. Alluvial flood plains particularly provide opportunities for groundwater storage of the floodwaters. Possibilities of accelerated artificial recharge, under given geological conditions need to be explored and utilized. The possibility of retaining part of the flood flows, as green water should be explored. However, in considering interventions that will change the runoff regime, one needs to consider the effects holistically. For example, taking measures to reduce runoff during the rainy season could be counter-productive if it also reduces runoff at other times of the year.

Integrate Land and Water Management

Land use planning and water management have to be combined in one synthesized plan through co-ordination between land management and water management authorities to achieve consistency in planning. The rationale for this integration is that the use of land has impacts upon both water quantity and quality. The three main elements of river basin management – water quantity, water quality, and the processes of erosion and deposition – are inherently linked and are the primary reasons for adopting a river basin-based approach to IFM.

Upstream changes in land use can drastically change the characteristics of a flood and associated water quality and sediment transport characteristics. Upstream urbanization can cause an accentuation of flood peaks and their early occurrence in downstream reaches. Using low lying depressions that play an important role in flood attenuation for dumping solid waste may worsen hygienic conditions and increase flood peaks in downstream reaches during floods. Ignoring these linkages in the past has lead to failures. These linkages need to be recognized, understood and accounted for to lead to synergies in improving river basin performance in several different ways simultaneously. Taking advantage of these potential synergies will, however, require a wider perspective of the issues of development of the river basin in its entirety, rather than attempting to resolve local problems in an isolated manner.

By adopting a functional approach to flood management, a problem orientation is an almost inevitable consequence. Taking a wider perspective can allow the situation to be viewed as one of opportunities, of looking for ways in which the performance of the basin as a whole can be enhanced.

Adopt a Best Mix of Strategies

Strategies and options generally used in any flood management approach are given in Table 2.2. Adoption of a strategy depends critically on the hydrological and hydraulic characteristics of the river system and the region.

Three linked factors determining which strategy or combination of strategies is likely to be appropriate in a particular river basin are the climate, the basin characteristics and the socio-economic conditions in the region. Taken together, they determine the nature of the floods that are experienced and their consequential effects. Quite different strategies are likely to be appropriate in different situations and in different countries. However, the strategies often involve a combination of complementary options – a layered approach that includes intervention at several points in the process of flooding. The differences in the performance of the different options also suggest that adopting a layered flood management strategy will often be the best strategy.

Table 2.2 Strategies and Options for Flood management (APFM, 2003).

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It is important to avoid isolated perspectives and the trap of assuming that some forms of intervention are necessarily always appropriate and others are always necessarily bad. Instead, it is necessary to look at the situation as a whole, compare the available options and select a strategy or a combination of strategies that is most appropriate to a particular situation. While recognizing the merits and demerits of various structural and non-structural measures, a good combination of both kinds of measures needs to be evaluated, adopted and implemented. Measures that create new hazards or shift the problem in time and space, sometimes merely temporarily, need to be guarded against.

Ensure a Participatory Approach

IFM, like IWRM, should be based on a participatory approach, involving users, planners and policy makers at all levels. For the approach to be participatory it needs to be open, transparent, inclusive and communicative and requires decentralization of decision making with full public consultation and involvement of stakeholders in planning and implementation. All the upstream and downstream stakeholders representing different parts of the river basin need to be involved. The core of the debate in the stakeholder consultation process is frequently not what the objectives are but what they ought to be. Two aspects of this argument are: who has standing in the decision, what is the legitimacy of their standing, and by what right are they entitled to be heard; and, secondly, how to ensure that the powerful do not dominate the debate.

An extreme “bottom-up” approach risks fragmentation rather than integration. On the other hand, the lessons from past attempts at “top-down” approaches clearly indicate that local institutions and groups tend to spend a great deal of effort subverting the intentions of the institution supposedly responsible for overall management of the basin. It is important to make use of the strengths of both the approaches using an appropriate mix.

All institutions necessarily have geographical and functional boundaries. It is necessary to bring all the sectoral views and interests to the decision making process. All the activities of local, regional and national development agencies, departments and ministries working in the field of agriculture, urban development, watershed development, industries and mines, transport, drinking water and sanitation, poverty alleviation, health, environment, forestry, fisheries and all other related fields should be coordinated at the highest level.

Adopt Integrated Hazard Management Approaches

Communities are exposed to various natural and man-made hazards and risks. A wide range of activities and agencies are involved in the successful implementation of disaster management strategies. They involve individuals, families and communities along with a cross-section of civil society such as research institutions, governments and voluntary organizations. All these institutions play vital roles in transforming warnings into preventive action. Members from all sectors, involving different disciplines must be involved in the process and carry out activities to ensure the implementation of disaster management plans.

The success of disaster mitigation will be measured from the public understanding of the adoption of appropriate strategies and their implementation and preparedness. Integrated natural hazard impact mitigation to address all hazards holistically (“all hazard” emergency planning and management) is preferable to hazard specific approaches and hence IFM should be integrated into a wider risk management system. This helps in structured information exchange and the formation of effective organizational relationships. The approach has the benefit of improved treatment of common risks to life, efficient use of resources and personnel and includes development concerns along with emergency planning, prevention, recovery and mitigation schemes. It consequently ensures consistency in approaches to natural hazard management in all relevant national or local plans.

Early warnings and forecasts are key links to the series of steps required to reduce the social and economic impact of all natural hazards including floods. However, to be effective, early warnings of all forms of natural hazards must emanate from a single officially designated authority with a legally assigned responsibility.

2.6 Waterlogging and Development of High Water Table

High water table and waterlogging of land pose a serious problem in humid areas. Mostly, excess and high intensity rainfall and absence of proper drainage have been the primary causes of waterlogging and development of high water table in croplands. The sustainability of crop production and soil health are under great threat in irrigated areas owing to improper irrigation practices, particularly over-irrigation. Lack of proper drainage system has made the situation worse.

Waterlogging is caused in a location when the inflow of water into it exceeds the outflow resulting in progressive rise of water table. The inflow may be due to excessive and high intensity rainfall, seepage from canals, reservoirs, flood and over-irrigation. The outflow declines with impaired drainage, lack of adequate drainage, rise of water table owing to construction of reservoirs, rise in water level in rivers (Dilip, 2000).

2.6.1 Signs of Bad Drainage

There may be a number of indications by which a land can be identified as badly drained land (Dilip, 2000). They are:

- Soil is very soft and wet. It sticks to farm implements and tools and feet of animals and shoes of farm laborers.
- Occurrence of spots or pools of free water. These may be few or many, big or small.
- Presence of good growth of bright green grasses or weeds in some places.
- Animals avoid resting on the ground where they are grazing, because of land being cold, particularly in winter. Free water may be flowing out of the field from sides of ditches or over the soil surface.
- Aquatic and water loving plants are seen growing.
- Plants look usually yellowish or pale colour and unhealthy and are stunted in growth.
- When crops are sown, seedlings grow slowly. Many seeds may not germinate as there is excess water in the soil.
- Wild plants like pulicaria dysenterica (Fleabane) are found growing on the land.

2.6.2 Harmful Effects of Excess Irrigation

Irrigation is beneficial only when it is properly managed and controlled. Faulty and careless irrigation does harm to crops and damages lands, besides causing waste of valuable water. Rice is the exception and it is grown under soil submergence. Wide knowledge and experience are required for efficient water management. When plenty of water is available, farmers are tempted to over irrigate their lands without being conscious of harmful effects.

As indicated by Dilip, 2000, the following are some harmful effects of faulty and excess irrigation:

1. Impaired soil aeration . Excess irrigation fills all soil pores expelling soil air completely. This leads to deficiency of oxygen in the soil and disturbs seriously the root respiration and root growth. However, in rice, the supply of oxygen to roots is made from leaves through aerenchyma cells which are continuous from leaves to roots.
2. Imbalance in nutrient uptake . Plants derive energy by root respiration and the energy is needed for nutrient uptake. Reduced or lack of root respiration owing to improper soil aeration under excess soil water condition greatly disturbs the nutrient uptake. The decline in uptake occurs in the order of K > N > Ca > Mg > P. the potassium uptake is affected the most and the phosphorus uptake, the least. Some nutrients such as manganese and iron become more soluble and their increased availability may reach the toxic level to plants, whereas boron and molybdenum become less available.
3. Physiological imbalance in plants . Physiological activities of plants get seriously disturbed due to lack of adequate oxygen in poorly aerated soils under excess soil water condition. An imbalance in nutrient uptake that may occur due to reduced or excess availability of nutrients under impaired respiration disturbs the physiological activities of plants greatly.
4. Restricted root system . Excess water and lack of adequate oxygen in soil restricts the root development and feeding zone of plants. Roots do not grow in wet soils and usually remain shallow particularly where water table rises and encroaches the normal root zone of crops. With restricted feeding area the availability and uptake of nutrient decline. Consequently, crop growth and yields are affected. Shallow root system exposes the crop more to the risk of low yields from drought.
5. Toxicity of nutrients . Under excess soil water condition and in waterlogged soils, some nutrients like manganese and iron get reduced in the soil and their solubility increases. Their increased availability leads to their toxic uptake by plants.
6. Loss of soil fertility . Uneven and excess irrigation leads to leaching of nutrients beyond plant root zone. Often, careless and heavy irrigation causes erosion of fertile surface soil runoff that washes out plant nutrients into drains.
7. Soil erosion. An uncontrolled heavy irrigation in sloping and undulated lands may cause erosion of surface soil. The stream size and amount of irrigation applied should be decided based on water intake rate, hydraulic conductivity, textural class, and water retentive capacity of soil, land slope and soil water depletion status.
8. Destruction of beneficial soil structure and soil aggregates . Waterlogging and excess soil water conditions for a long period destroy the crumb structure and soil aggregates and encourage the development of platy structure. Crumb structure and soil aggregates favour crop growth and yield.
9. Production of harmful gases. Under excess soil water and waterlogging conditions harmful gases such as ethane, methane, hydrogen sulphide, carbon monoxide and hydrogen gas are produced due to anaerobic decomposition of organic matter present in the soil. These gases are toxic to crop plant. Under waterlogged condition it is often observed that plants turn yellowish and become stunted and diseased.
10. Rise of water table . Faulty and over-irrigation in a farm if continued over a long period leads to rise of water table. This occurs particularly in lands where the root zone is underlaid by an impervious soil or rocky layer. Rise of water table restricts the root and feeding zones of crops. Growing of fruit trees and deep rooted crops is very much restricted in areas where the water table rises high up and gets near to the soil surface.
11. Waterlogging . When irrigation is done with a large stream and if that is not turned off in proper time, water accumulates in the lower part of the field and causes waterlogging. Waterlogging is harmful to crops and lands in many ways that have been stated in this section.
12. Activities of micro-organisms. Excess soil water due to excess irrigation causes deficiency of oxygen in the soil. Useful aerobic bacteria such as ammonifying, nitrifying and nitrogen fixing bacteria cannot function well or at all under oxygen deficiency. Decomposition and mineralization of organic matter, atmospheric nitrogen fixation and availability of nutrients to plants are hampered. On the other hand, anaerobic bacteria are activated causing loss of nitrogen as gas, evolution of harmful gases and appearance of plant diseases.

2.6.3 Benefits of drainage

The main direct benefit of installing a drainage system, to remove excess water for crop development and growths, is that the soil is better aerated. This leads to a higher productivity of cropland or grassland because:

- The crops have better root development and root more deeply.
- There will be better nutrient uptake and therefore fertilizers will be used more efficiently.
- Activity of micro-organisms will be increased and therefore the decomposition of organic matter will be enhanced.
- In the absence of oxygen certain soil bacteria will transform nitrate, which is a plant nutrient, into nitrogen gas. When the soil is better aerated less nitrate is lost.
- Salinity can be controlled better.

Other agricultural benefits of well-drained soils are:

- The land is easier accessible, with a better bearing capacity and workability.
- The period in which tillage operations can take place is longer.
- The choice of crops is greater.
- The growing seasons will be extended, as early planting will be possible.
- The soil structure is better, which also improves permeability.
- Soil temperatures are higher, so that crops (particularly horticultural crops) and grasses can be grown earlier.

Besides the agricultural benefits there are a number of social benefits that direct contribute to rural development and improvement of the environment. These benefits include:

- Improved public health through reduced risks of vector born and water-born diseases.
- Better sanitation.
- Improved animal health.
- Reduced maintenance cost to infrastructure, buildings etc

2.6.4 Biodrainage

The limitations and shortcomings of the conventional drainage techniques call for alternative approaches to help keep agriculture sustainable over the long term. Alternative techniques must be effective, affordable, socially acceptable and environmentally friendly and not cause degradation of natural land and water resources. Biodrainage is one of these alternative options. The absence of effluent makes the system attractive. However, for biodrainage systems to be long-term sustainable, careful consideration is required of the salt-balance under the biodrainage crops (FAO, 2002).

Biodrainage relies on vegetation, rather than engineering mechanisms to remove excess soil water through evapotranspiration. It is often considered attractive because it requires only an initial investment in site development (planting of a “biodrainage crop”) and (potentially) returns a benefit when the biocrop is harvested for fodder, wood or fiber. In addition, under some management scenarios, viz. certain cropping systems and slightly saline conditions, it might offer limited scope to achieve nutrient and/or salt-balance through removal of biomass, thus alleviating the problem of the disposal of polluted drainage effluent from the biodrainage crop area by reducing volumes and improving the quality of the effluent.

Applicability of biodrainage systems is not restricted to the simple substitution of a pipe drain or tubewell. Canal leakage always occurs in irrigation projects. Tree plantations have effectively drained the ponds formed alongside canal embankments, as compared with large areas that have been inundated and became saline. A further application of tree plantations is that they can effectively drain natural inundated depressions or areas where effluent is produced by conventional drainage techniques where water is disposed of in evaporation ponds.

The use of biodrainage systems in recharge areas seems to be widely accepted as a sustainable management option. In discharge areas with shallow groundwater tables, deep-rooted plant-based biodrainage systems are often associated with salt accumulation. Under these situations, combining conventional drainage with biodrainage would be the most appropriate design option.The aim of biodrainage is to remove excess groundwater through the process of transpiration by vegetation. This is achieved by enhancing the transpiration capacity of the landscape by introducing high-water use vegetation types in large enough areas to balance recharge/discharge processes to maintain groundwater balances below the root zone of the agriculture crops. The following issues should be considered in the development of biodrainage systems (FAO, 2002):

i . Water balance: Biodrainage plantations should be able to extract groundwater volumes equal to the net recharge. The water balance is to be maintained such that the water table is kept below the root zone.
ii . Plantation area: The biodrainage plantation area should be kept as small as possible. Agriculture (particularly irrigated agriculture) is practiced primarily to produce high-value crops. Conversion of high-value cropping land to relatively low-return forestry may be difficult. Often good quality water is in short supply while land is not a limited resource. Particularly in arid and semi-arid regions, dryland areas surrounded by irrigated land could be earmarked for tree plantations without loss of productive resources.
iii. Salt tolerance: Biodrainage crops need to be salt tolerant. Groundwater qualities can vary greatly spatially, normally they have a higher salinity than irrigation supplies. The water use capacity of trees and other crops decreases with increase in water salinity. In the case of Eucalypt species, it reduces to about one-half of potential when the water salinity increases to about 8 ds/m according Oster cited by FAO (2002).
iv. Drawdown of water table: Crops, including trees, act as biopumps; they depress the water table directly underneath plantation areas and consequently lower the water table in the surrounding area. The drawdown effect under trees/crops depends on the tree/crop water use, the rate of recharge in the surrounding area, the hydraulic conductivity of substrata and the depth to deeper barrier layers. Biodrainage plantings should be established in blocks or strips and spaced to keep water table levels in the irrigated farmland in between the plantings below the root zone. The harvesting of the biodrainage plantations would need to be planned in such a manner that the “drainage” function is not lost (thinning regimes).
v. Salt balance: The introduction of irrigation always upsets the salt balance. Although irrigation supplies often have relatively low salinities, the large volumes of water that are introduced in the landscape increase salt imports significantly. Drainage of effluent to export these salts is therefore generally considered a necessity. To achieve salt balance without conventional drainage, the irrigated crops, along with interspersed biodrainage plantings, would have to accumulate the salts introduced by irrigation, and would subsequently have to be harvested and removed from the region. This is only (potentially) achievable in situations where very low-salinity water is available to the plants.
vi. Economic aspects: The growing of biodrainage trees and crops requires a different operational management approach than the growing of agricultural crops. Up-front costs associated with planting and maintenance precedes the income from harvesting by many years. Some form of contract growing, based on annual payments might have to be considered to make the system acceptable to landholders.
vii. Social acceptance: The introduction of new crops such as tree plantations affects rural social societies. New markets might have to be developed, security arrangements differ from those for normal crops (illegal pruning or cutting for fuel wood) and fires could destroy the results of many years of Labour in a single day. Active participation of local communities in the development of tree plantation-based biodrainage systems is extremely important to overcome problems and ascertain that the benefits of the biodrainage systems are reaped to the maximum extent.

The need for drainage is not restricted to irrigation areas. In rainfed areas without irrigation, water (and salt) balances, disturbed by land use changes, often need to be managed to minimize negative environmental impacts. As the land use in these areas is often less intensive than in those using irrigation, economic considerations prevent the adoption of expensive engineering inputs. This fact makes the biodrainage approach especially attractive for the management of drainage problems.

CHAPTER THREE

3 DESCRIPTION AND METHDOLOGY

3.1 Description of the Study Area

3.1.1 Administrative Location

The Lake Tana basin is located in three administrative zones of the Amhara Region. North Gondar in the north and northwest, South Gondar in the east & southeast and West Gojjam in the south and southwest are the three administrative zones. It covers five woredas (Wegera (20%), Gondar Zuria (fully), Dembia (fully), Chilga (10%) & Alefa (30%)) in north Gondar, five woredas (Kemkem & Dera 60% each, Estie-20%, and Farta and Fogera fully each) in South Gondar and six weredas (Achefer & Merawi fully each, Bahir Dar Zuria and Sekela 80% each, Fagta Laka -50% and Banja-10%) in west Gojjam. Large portion of the Tana water surface (the reservoir) is located in north Gondar, and some portion in south Gondar and west Gojjam administrative zones (MoWR, 2005).

3.1.2 Geographical Location

The geographical location of the Tana basin extends from 10.95oN to 12.78oN latitude and from 36.89oE to 38.25oE longitude. The reservoir area is located from 11.62oN to 12.31oN latitude and from 37.01oE to 37.64oE longitude. The Tana basin location in relation to the Abay Basin and country is shown in Figure 3.1.

The basin has a water divide from Tekeze basin. Its top elevation (4100masl) starts from mount Guna. Debre tabor and Gondar are zonal towns and Bahir Dar Regional capital and is 570km from Addis Ababa. Other small towns on the study area are Woreta, Addis Zemen, Koladiba, Durebete, Kunzila and others. The study area is north of Addis Ababa and East, Southwest and North of Lake Tana. The study area is situated within Fogera, Dera, Lib-Kemkem, Dembia, and Achefer woredas.

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Figure 3.1 Location Map of Lake Tana Basin

Lake Tana sub-basin is located at the headwaters of the Blue Nile (Abay) basin. The total area draining into the lake, including the lake water surface, is about 15319 km2. The lake is the source of Abay River, which is known internationally as the Blue Nile. It includes the highland escarpments of Gondar (Guna and Armacheho) and Gojjam (Sekela) highlands and the lower plains surrounding the lake, which are often flooded during the rainy season and forming extensive wetland such as the Fogera plain to the east, Dembia plain to the north, the Kunzila plain to the southwest and the Bahir-Dar city to the south.

Lake Tana with an area of about 3100 km2 located at an altitude of about 1830m above sea level, is a crater lake formed two million years ago due to volcanic blocking of the Blue Nile River (Howell & Allan, 1994). It is the largest fresh water body in the country bordering Dembia to the north, Bahir Dar to the south, Fogera to the east and Kunzila to the west. Its bottom is volcanic basalt mostly covered with a muddy substratum with only little organic matter.

The main tributaries to the lake are Gilgel Abay, Gumara, Rib, Megech, Arno-Garno, Dirma and Gelda. There are wetlands on all sides of the lake resulting from hydrological and land use changes. The Dembia plain to the north, the Fogera plain to the east and the Kunzila plain to the south-west are low areas bordering the lake which are often flooded during the rainy season. Rocks border the west and north-east of the lake. Main tributaries to Lake Tana and surrounding flood plains are shown on Figure 3.2.

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Figure 3.2 Lake Tana and its inflowing and out flowing main rivers

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