S2 Geography — Mass Movement and Landslides: Tutor Prep Guide | Slope angles, four types, slope protection

2026-11-13

Tutor lesson-prep guides · Junior Secondary Geography

Students write “it rained heavily” and stop there. This lesson turns that sentence into a six-step chain and gives them two axes, speed and water content, to classify every case.

Objectives — Students can: (1) divide vertical height by horizontal distance to get a gradient as a percentage and an angle (24 m over 30 m gives 0.8, that is 80 per cent, about 38.7 degrees, with a slope length of 38.42 m); (2) write the chain from heavy rain to collapse in six steps: infiltration, rising pore water pressure, added weight, falling shear strength, downslope force exceeding resistance, failure; (3) separate fall, slide, flow and creep using speed and water content, and explain why creep at a few millimetres a year still matters.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Gradient	Which two measurements do you divide to describe how steep a slope is?	Names vertical height and horizontal distance; “it looks steep” means rebuild this first
Weathering	How does solid rock turn into loose material?	Names physical weathering (heating and cooling, roots) and chemical weathering (rain dissolving rock)
Ratio work	What is 2.5 divided by 0.25?	Answers 10; a wrong answer here will break every ratio question later

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Case opener	Describe a rainy-season slope failure using facts only, no causes	Write down three causes of their own, right or wrong
5-13 min	Gradient maths	Draw the triangle, label 24 m and 30 m, show the division and the angle	Work out the percentage and the angle, and state the units
13-22 min	How water breaks a slope	Use 96 mm of rain in 3 hours to compute intensity and water volume	Get 32 mm per hour and 2,880 cubic metres, then say what that weighs
22-31 min	Four types	Give four description cards and ask for a ranking by speed and water	Rank them and name the deciding clue for each
31-40 min	Chain and protection	Shuffle six chain cards and ask the class to rebuild the order	Rebuild the chain and mark where engineering can intervene
40-45 min	Exit check	Collect three answers and mark correct, partly correct, wrong on the spot	Hand in three answers with one reason each

03 Teaching script (read it out)

Scene 1: Act 1: Gradient is a division, not a feeling

Say: “Steepness is not a feeling. Gradient is vertical height divided by horizontal distance. This slope rises 24 metres over 30 metres, so 24 divided by 30 is 0.8, which is 80 per cent. To get degrees you take the inverse tangent: 0.8 is about 38.7 degrees. The slope length is 38.42 metres because 24 squared plus 30 squared, then square root. If an exam asks which slope is riskier, bring these numbers, not the word steep.”

Say it back: gradient is height divided by distance, not a feeling.

Scene 2: Act 2: Water is the final push, not the only cause

Say: “Rain does three things. First it fills the pore spaces. Second, pore water pressure rises and pushes grains apart, so friction drops. Third, the water itself adds weight, so the driving force grows. Resistance falls and driving force rises; where the lines cross, the slope fails. With numbers: 96 mm in 3 hours is 32 mm per hour, and a slope face 200 by 150 metres, that is 30,000 square metres, catches 2,880 cubic metres of water, about 2,880 tonnes. Say the chain, not just the rain.”

Say it back: water raises pressure and adds weight, and together they fail the slope.

Scene 3: Act 3: Speed and water content tell the four types apart

Say: “In a fall, rock leaves the face and drops freely: from 20 metres it takes about 2.02 seconds and lands at 19.8 metres per second, roughly 71 kilometres per hour. A slide moves as one mass along a slip surface, slower than a fall but far bigger. A flow carries the most water and runs down a channel like wet concrete. Creep is the slowest: 5 millimetres a year becomes 0.25 metres in 50 years, so fences lean and poles tilt while nobody sees it move. Two axes, speed and water, sort all four.”

Say it back: falls are fast, slides move as one mass, flows are wettest, creep is slowest.

04 Question types and worked steps

Type	Example	Steps	Answer
Gradient calculation	A slope rises 24 m over a horizontal distance of 30 m. Find the gradient as a percentage and an angle.	(1) height divided by distance (2) multiply by 100 (3) take the inverse tangent (4) check with the slope length	80 per cent, about 38.7 degrees, slope length 38.42 m.
Chain ordering	Order these: heavy rain, shear strength drops, infiltration, pore pressure rises, driving force exceeds resistance, failure.	(1) find the trigger (2) follow the water (3) track the forces (4) the result comes last	Heavy rain, infiltration, pore pressure rises, shear strength drops, driving force exceeds resistance, failure.
Type identification	A case: blocks leave the face, tumble through the air and pile at the foot, largest 2 m across. Which type?	(1) is there free fall (2) is there one slip surface (3) how wet is it (4) seconds or years	A fall, because material leaves the face and travels through the air.
Protection matching	Match cut-off drains, retaining walls, shotcrete and rock bolts to stopping infiltration, adding resistance, sealing cracks and pinning deep rock.	(1) separate water control from force control (2) water measures first (3) order force measures by depth (4) match to the case	Drains stop infiltration, walls add resistance, shotcrete seals cracks, bolts pin deep rock.

Sanity check (teach this): Reasonableness check: the percentage is always larger than the angle in degrees (80 against 38.7), so swapping them is an obvious error; the slope length must exceed the horizontal distance (38.42 against 30), so writing 30 means the wrong side was used; a 20 m drop takes about 2 seconds at 71 km per hour, so “fell for a minute” means a slide was confused with a fall; intensity is total rainfall divided by hours, 96 divided by 3 gives 32 mm per hour, and 3 divided by 96 is wrong; creep moves millimetres a year, so “metres a year” must be redone.

05 Independent practice (3 questions)

Question	Answer	What to watch
A slope rises 15 m over 20 m. Find the percentage, the angle and the slope length.	0.75, that is 75 per cent, about 36.87 degrees, slope length 25 m.	Did they divide height by distance and keep the units
120 mm of rain falls in 4 hours on a 100 m by 100 m face. Find the intensity and the water volume.	30 mm per hour; 1,200 cubic metres, about 1,200 tonnes.	Was total rain divided by hours, and was the area in square metres
A fence shifts 40 mm downslope over ten years. Which type of movement is this?	Creep, at about 4 mm a year, slow but continuous.	Was the judgement based on slow and continuous, not just movement

06 Common mistakes and what to say

Mistake	Student writes	Say this
Gradient inverted	“24 divided by 30 is 0.8 degrees.”	Say: “0.8 is the ratio, not the angle. Take the inverse tangent: 0.8 is about 38.7 degrees. Writing 0.8 degrees describes a nearly flat field.”
Rain with no chain	“It collapsed because there was heavy rain.”	Say: “Rain is the trigger, not the explanation. Name infiltration, rising pressure, added weight and falling strength. Without the middle four steps you have not answered.”
Fall called a slide	“Blocks slid down the slope, so it is a slide.”	Say: “If the blocks leave the face and travel through air, it is a fall. A slide keeps one mass on a slip surface. Check whether the material spreads out.”
Slow means harmless	“Creep is only millimetres, so it is not mass movement.”	Say: “Slow is still mass movement. Creep bends fences, cracks pipes and tilts foundations, and someone pays for it in a few years.”

07 Homework (paper, section and question numbers)

Geography index: redo practice questions 1 to 3 in this chapter, then compare with the weather elements chapter — This chapter, practice 1-3
Geography index: classify three rainy-season cases with the four-type table and write one reason each — This chapter, section 5, identification 1-3

08 Exit check (last 5 minutes)

Question	Answer	Decision
A slope rises 12 m over 16 m. What is the gradient?	0.75, 75 per cent, about 36.87 degrees.	Dividing 16 by 12 means the gradient idea is not secure
Name the third step from rain to collapse.	Pore water pressure rises, so friction falls (or weight increases).	Saying “rain loosens the soil” means the mechanism is missing
What most clearly separates a rockfall from a debris flow?	Water content and motion: a flow is far wetter and runs down a channel as a fluid.	Saying they are the same, or only naming speed, means the types are not sorted

09 Tutor FAQ

Which year is this for?

S2. It can be reused in S3 hazards work, but replace the case with a channelised debris flow.

Is 45 minutes enough for three acts?

Yes. About nine minutes each, with time left for the chain cards and the exit check; derive the gradient once, do not drill it.

Is there practice material?

Yes. The subject index links this chapter practice with answers, so the tutor can mark in the room.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.