The document describes the current situation of hydrogeological disasters in Rwanda.

1. State of Hydrogeological risks in
North-western Rwanda
Dibanga B. P., Dr. Gatera F., Dr. Manirakiza V. & Bavugayundi D.
Landslides and flooding
Bigogwe
MugogoGatokoto
Reference: Dibanga B. P., ManirakizaV., Gatera F. & Bavugayundi D. (2017).State of Hydrogeologial Disasters in Northwestern Rwanda,
East African Journal of Science andTechnology,Vol.7, Issue 1, 2017, pp. 1-25.

2. Presentation outline
1. Introduction
2. Methodology
3. Overview of factors of hydrogeological risks
4. Findings and discussion
5. Conclusion

3. Disasters constitute a huge
threat to people’s lives,
settlements, infrastructures,
economic activities, …
1.Introduction Examples of some reported damages cases
• Flooding of 8 ha maize, 4 houses & 2 schools
destroyed in May 2010: Muhanga;
• 60 ha of rice, 2 victims, 2 fishing dumps in
April 2010: Kamonyi;
• 118 houses: 13/03/2009; Nyakarenzo/ Rusizi;
• 57 families displaced in Janja & 186 in
Busengo sectors: 28/12/2009, Gakenke;
• 67 houses: 13/01/2010, Rutonde / Rulindo
• 215 houses, 2 churches, 855 ha: 03/10/2009,
Cyanzarwe / Rubavu;
• 258 houses, 45 ha, 2 victims, 15 bridges: May
2010, Bigogwe / Nyabihu
• 3 days road blocked at Nyabarongo : 2016
• 50 victims of landslides: 08/05/2016, Gakenke.

4. Human physical Factors:
topography, rainfall, vegetation
cover, bedrock, soils, human
settlements & activities.
Most affected zones: High
mountainous regions: Northern
& Western provinces.
Challenge: Physical factors
are associated with overuse
of soils & exploitation of
marginal lands due to high
population density & lack of
land for agriculture.
Landslides & floods: most
hydrogeological risks in Rwanda.
Various types of disasters:
landslides, floods, earthquakes,
volcanic eruptions, storms,
tornadoes, tsunamis, wildfires.
Introduction

5. Study area:
Northwestern Rwanda: Rulindo, Gakenke,
Musanze, Nyabihu & Rubavu districts.
Research objective :
To analyse the factors of hydrogeological risks
in north-western Rwanda from different
physical and human factors and propose
sustainable measures for their prevention.
Introduction

6. 2. Methodology
Three steps:
1. Inventory of recent disasters in all districts
of Rwanda : period from 2007 to 2012
2. Identification of disasters prone zone:
North-western Rwanda
3. Empirical research in selected zones & sites.

7. Observation & interviews
• Information on the state of damages, forms of floods
and landslides, land use techniques, nature of
topography & settlement.
• Identification of disasters prone area: historical records
& testimonies from local communities and population.
• Information on the period, the damages and the extent
to which the hydrogeological risk occurred.
• Photographs & maps
Research techniques : Observation, interviews & study of the risk
Methodology

8. Study of Landslide: location, environmental setting,
description of characteristics: size, soil profile, nature of
the rocks & additional factors like water springs.
Study of Flood: identification of drained & flooded
surface, origin of water flow and environmental setting.
Damages: description, quantification and social economic
impact evaluation (information from population).
Empirical study of the risk: measurement and analysis
Methodology

9. 3. Overview of the factors of hydrogeological risks
1. Nature of rocks
2. Nature of soil: type & texture
3. Topography
4. Vegetation cover
5. Rainfall
6. Human features: population & land use

10. i. Nature of rocks
- On the heights and the slopes: granites & pegmatites
- In the depressions and lowlands: volcanic rocks & quartzites
Resistant rocks and steep slopes lead to formation of
a thin layer of weathering and this causes multiple
landslides during the rainy season.
Factors of hydrogeological risks

11. ii. Nature of soil
Predominant types of soil
• Noth-west zone: andosols
• Central & southern zone:
Ferralsols, cambisols, alisols,
gleysols
Soil layers stratification
favours water stagnation in
lower horizons leading to
suffosion (water springs) &
Landslides
Factors of hydrogeological risks

12. Predominant soil texture:
Clay soils are not
permeable and cause
floods in lowlands
Sandy & coarse loams
are easly detachable and
favour landslides in high
slope zones
- Clay - Sand
- Silt - Loam
Factors of hydrogeological risks

13. • High altitude 2000 - 4,500 m
• Steep slopes : 15 – 50 %
iii. Topography
Factors of hydrogeological risks
The greater the height,
steepness and convexity of
slopes, the greater the
volumes of the landslides.
Floods occur in lower altitude
zones of Musanze and Nyabihu
and landslides in other zones.

14. • Challenge: deforestation & overuse of uncovered soils
• Large part is located in deforestation area which should be
intense forest zone (de Martonne aridity index (IdM)).
Aridity index (I):
- 30,3 in 2013
- 38,8 in 2014
! when I > 30 : dense forest
iv.Vegetation cover
Poor vegetation cover is not a consequence of droughts,
but a consequence of intensive human activities.
Factors of hydrogeological risks

15. • Highest rainfall > 1200 mm ( eg. 1487 mm volcanic area).
• 8 months of rain season : from September to May;
• Atmospheric drought lasts from July to August;
• The soil drought covers only July;
v. Rainfall
Factors of hydrogeological risks
There is no hydrological drought in the region

16. • Fertile highlands (Rukiga,
Buberuka)
• Densely populated area
District Density
Rulindo 509
Gakenke 481
Musanze 695
Nyabihu 556
Rubavu 1041
vi. Human features
Population density
Factors of hydrogeological risks
 Population

17. • Dispersed settlements
• Scattered homesteads,
• Subsistence farming
• Traditional land use
practices & techniques
Land degradation and
soil erosion.
 Land use
Factors of hydrogeological risks

18. 4. Findings & discussion
Nr Site Nature of risk District Sector
1 Bushoki Landslide Rulindo Bushoki
2 Gashenyi Landslide Gakenke Gashenyi
3 Busengo* Landslide Gakenke Busengo
4 Mugogo* Flood Musanze Busogo
5 Gatokoro* Landslide Nyabihu Rambura
6 Bigogwe* Landslide & Flood Nyabihu Bigogwe
7 Rusongati Landslide Rubavu Kanama
8 Nyundo & Mahoko Flooding control Rubavu Nyundo
Location of studied sites
* Discussed sites in this presentation

19. i. Busengo rotational landslide (slump)
Environmental setting
• Loose materials on the steep slopes (30 %)
• Near Gaseke river bed (100 m)
• Size: 36 m in width and more than 58 m in length
• Soil profile: 3 layers & no identifiable bedrock
- upper layer: fine colluvial materials (40 cm)
- medium layer: coarser materials (30 cm)
- lower layer : sandy materials
• Water-springs at the bottom of the main scarp
• A settlement at the top of the hill
• No forest but grass vegetation cover

20. Preventive measures
- Re-forestation programme
- Integrated management of river banks
- Resettlement of the population
Causes of Busengo landslide
- High gradient of the slope
- River banks undermining
- Unstable colluvial soils materials
- Water springs and undrained rain water
from the settlement on the top of the hill

21. ii. Gatokoro landslide (Rambura)
Environmental setting
• Located on a pegmatitic granite
• Steep terrain, slope of 27 %.
• Soil profile: sandy soils with 4 layers:
- upper layer of colluvium (60 cm)
- stone layer (40 cm)
- weathered rock
- pegmatitic parental rock.
• Weathering of parental rock results
into sand (white micas, quartz. )
• The movement of land is about 7m.

22. Causes of Gatokoro landslide
• Sandy soils (highly permeable)
• Many water springs that cause sliding
• Rainwater and undrained rain water
from the top of hill that speed up sliding
Preventive measures
• Re-forestation,
• Resettlement of people
• Engineering works (gabions, stone walls)
to avoid falling rocks.

23. • Floods of Mizingo & Kinoni
rivers in Mizingo & Mugogo
marshlands
• Frequent floods mainly in
November and April
• Site bordered by the hills and
mountain with steep slopes,
densily populated due to fertile
volcanic soil
• High rainfall causes excessive
runoff which leads to flooding.
iii. Bigogwe & Mugogo floods

24. Combined factors :
• Escarped topography that does not facilitate
water infiltration in soil,
• Limited soil permeability (clay and silt soils)
• Intensive rainfall & Deforestation
• Densely populated watershed
• Accelerated runoff: overgrazing & cultivation
Measures:
• Afforestation and re-afforestation,
• Terracing of slopes,
Nyundo, protection against flooding
Nyundo
abandon
ned
health
center
due to
Sebeya
flooding
Construction along the banks of rivers
and widening river channel through
dredging. Eg: Sebeya - Nyundo

25. 5. Conclusion
• All factors: nature of rocks, slopes, soil granulometry, rainfall intensity,
vegetation cover and human activities act together to contribute to
hydrogeological risks in the north-western region.
• Rainfall is a triggering factor because landslides and floods recurrently
happen during the rainy season.
• Even if all natural factors may relatively be the same, disasters do not
happen in all slopes to the same extent.
• Land use is therefore a main factor of disasters in Rwanda
1. Research contribution

26. 2. Preventives measures for sustainable land use
• Resettlement, afforestation and appropriate land use techniques.
• Careful analysis of where to locate a settlement because on each
top of the slope where occurred a landslide there is a settlement.
• Slopes of over 20° are not suitable for settlement and agriculture, as
well as depressions surrounded by steep slopes and colluvial soils.
• Control of population growth and its adequate distribution.
• Sensitization of the population to bad effects of hydrological risks,
their prevention and appropriate and sustainable land use.

27. The research team thanks you
Rusongati bridge