Formation of Relief Features in Rwanda
Senior 4 · Geography
Formation of Relief Features in Rwanda
Lesson 4 of 5
25 min
On 2 and 3 May 2023 heavy rain fell on the Northern, Western and Southern provinces. Floods and landslides followed. The Government of Rwanda reported at least 127 deaths by 4 May; by 16 May media reports gave up to 131 deaths, more than 5,800 displaced families and 6,391 damaged houses, with 2 health centres, 29 bridges and many roads also damaged (ReliefWeb, "Rwanda: Floods and Landslides, May 2023"). The map shows how unusual the rain was.
Rain alone does not cause a landslide. It triggers one on slopes that are already steep, covered with deep, weathered soil, and often cleared of their natural forest. To understand why the west and north of Rwanda are so exposed, we need two processes: weathering, which prepares the material, and mass wasting, which moves it downhill.
Definition
Weathering: the breakdown of rocks in place, at or near the surface, by physical, chemical and biological processes. Nothing is carried away.
Regolith: the layer of loose, weathered material above solid rock. Mass wasting (mass movement): the downslope movement of rock and regolith under the pull of gravity, without being carried by a river, ice or wind.
Example: hydrolysis turns the feldspar in granite into clay (weathering); the clay-rich regolith later slides downhill in a storm (mass wasting).
| Type | Process | Where it matters in Rwanda |
|---|---|---|
| Chemical: hydrolysis | Water reacts with feldspar and mica, turning them into clay minerals | Granites and schists everywhere; produces thick clay regolith |
| Chemical: oxidation | Iron in minerals combines with oxygen to form red and yellow iron oxides | The red and brown soils of the hills; iron-rich laterite crusts |
| Chemical: solution and carbonation | Rain water, a weak carbonic acid, dissolves minerals, especially limestone | Limestone and other carbonate rocks, which cover only small areas of Rwanda |
| Physical: frost action | Water freezes in cracks, expands and splits the rock | Only on the highest volcano summits, where REMA notes temperatures can fall below 0°C |
| Physical: unloading and exfoliation | Rock expands as overlying rock is eroded, and sheets peel off | Exposed granite domes and quartzite outcrops |
| Biological | Roots widen cracks; termites, worms and burrowing animals mix and loosen regolith; plants release acids | Everywhere, especially under forest and grassland |
Rwanda is warm and wet: REMA gives average annual temperatures of about 16–20°C and rainfall from about 900 mm in the east to 1,500 mm in the north-west (REMA, 2009). Chemical reactions run faster when it is warm and need water, so chemical weathering dominates, and the old rocks of the hills are covered by a regolith many metres thick.
Tip
Five factors control weathering: climate (temperature and rainfall), rock type and structure (joints, minerals), relief (steep slopes lose their regolith, gentle ones keep it), vegetation and organisms, and time. In an exam, explain each factor with a Rwandan example.
Figure 6: two kinds of mass wasting. Soil creep is too slow to see but leaves clues; a rotational landslide moves a block of saturated soil along a curved surface in seconds or minutes.
| Type | Speed | What happens | Typical setting in Rwanda |
|---|---|---|---|
| Soil creep | Very slow (millimetres a year) | Soil particles shuffle downhill as the soil wets and dries, heats and cools | Most cultivated hillsides |
| Slump (rotational landslide) | Fast | A block of saturated regolith rotates down a curved slip plane | Steep, deep-soiled slopes of the west and north after heavy rain |
| Translational landslide | Fast | A sheet of regolith slides over a flat surface such as the top of solid rock | Thin soils on very steep slopes; road cuttings |
| Mudflow (earthflow) | Fast to very fast | Saturated soil flows like a thick liquid down valleys | Gullies and stream channels during storms |
| Rockfall | Very fast | Blocks fall freely from cliffs and steep cuttings | Road cuttings, quarry faces, volcanic slopes |
Definition
Shear stress: the part of a slope material's weight that pulls it downslope. Shear strength: the resistance of the material to moving (friction between grains, cohesion, roots). A slope fails when shear stress exceeds shear strength.
A simple measure is the factor of safety: . The slope is stable if and fails if .
Worked example 1: why steep, wet slopes fail
Model. For dry, loose regolith with an angle of internal friction on a slope of angle , the factor of safety is
(a) Dry regolith with on a slope of : . Stable.
(b) The same regolith on a slope of : . It fails.
(c) After days of heavy rain, water pressure between the grains reduces friction; suppose the effective falls to . On the slope, : the slope is on the point of failing.
This simple model explains the pattern of May 2023: slopes that stand safely for years can fail when soaked, and the steeper the slope, the less rain it takes. (Real slopes also have cohesion and roots, which add strength.)
Worked example 2: how steep is the slope on the map?
Problem. On a 1:50 000 map two contours 20 m apart are 1.5 mm apart on a hillside above a road. Find the slope angle.
Solution. HE .
Steeper slopes appear where the contours crowd together; planners use such measurements to map landslide risk.
| Causes | Control measures |
|---|---|
| Natural: steep slopes; deep clay-rich regolith from chemical weathering; heavy and intense rain; earthquakes in the west | Map high-risk zones and keep houses off them; move families out of the most dangerous places |
| Human: deforestation of steep slopes; cultivation and grazing on steep land; road cuttings that steepen and undercut slopes; poor drainage that lets water soak the slope | Reforest and keep tree cover on steep land; build terraces and contour ditches that slow and drain run-off; retaining walls and drains along roads; weather warnings and early evacuation |
REMA has long identified the flood and landslide risk zones, defined by how often daily rainfall exceeds 50 mm, in the Southern, Northern and Western provinces, and noted that these zones were heavily deforested (REMA, 2009).
Warning
Weathering, erosion, mass wasting: keep them apart. Weathering breaks rock down in place; erosion wears land away and carries the material by an agent (water, wind, ice, waves); mass wasting moves material downhill under gravity alone, with water only helping by adding weight and reducing friction.
Pimlico Geography: "What is the difference between sliding and slumping?". The second half uses a model to show both. Notice the curved slip surface of the slump, as in Figure 6.
Practice
Key points
Sources and credits
Sources: Rwanda: Floods and Landslides, May 2023, ReliefWeb (UN OCHA), quoting the Government of Rwanda, ECHO and media reports; Rwanda State of Environment and Outlook Report, REMA, 2009 (climate, risk zones).
Map: ECDM 20230510 FL DRC Rwanda (percent of normal rainfall), Emergency Response Coordination Centre (ERCC), European Commission, public domain, via Wikimedia Commons.
Figure 6, the worked examples and the exercises were drawn and written by Ganzaa; the factor-of-safety values in Worked example 1 are illustrative.
Video: "What is the difference between sliding and slumping?", Pimlico Geography, embedded from YouTube.
Quick check
Why is chemical weathering dominant in most of Rwanda?
