DSE Geography | River Landforms and Flood Management Lesson-Prep Guide (Tutor Edition) | Basin Arithmetic, Landform Reading and Flood Works

2026-11-02

Tutor lesson-prep guides · Secondary Geography

Students can sketch a meander yet cannot say why the river downstream rises so fast after the city is paved. This lesson links landforms and floods through volume and time.

Objectives — Students can: (1) compute annual runoff from basin area and rainfall, and explain how urbanisation changes the runoff coefficient; (2) read the upper, middle and lower course landform sequence from slope and velocity, naming the process behind each landform; (3) compare hard and soft flood works and state how much of a flood peak a storage tank can shave off.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Water cycle	Where does rain go after it falls	Evaporation, infiltration, surface runoff
Area and volume	How many square metres in one square kilometre	1,000,000
Discharge	How do width, depth and velocity relate to discharge	Discharge = width x depth x velocity

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Basin budget	Area x rainfall gives input; multiply by the coefficient	Write 264 million m3
10-24 min	Urbanisation	Less infiltration, shorter lag time	Coefficient 0.45 to 0.75
24-38 min	Landform sequence	V-valley upstream, meanders mid, floodplain and delta below	Four landforms, four processes
38-48 min	Hydrograph	Peak, lag time, rising limb	Point out both differences
48-56 min	Flood works	Hard acts fast, soft is cheaper but slower	Compute a 38.6% cut
56-60 min	Exit check	Three questions marked on the spot	All three right

03 Teaching script (read it out)

Scene 1: A basin is a budget: everything starts with area times rainfall

The basin is 120 square kilometres with 2,200 mm of rain a year, so input = 120 x 1,000,000 x 2.2 = 264 million cubic metres.
Only part becomes river water: a rural runoff coefficient of 0.45 gives 118.8 million cubic metres; after paving the coefficient rises to 0.75, giving 198 million, an extra 79.2 million cubic metres (+66.7%). The lag time also falls from 6 hours to 2, so the same rain arrives faster and more concentrated.
The downstream peak rises not because more rain fell but because the water travels faster.

Ask students: where does rain go once the soil is covered by tarmac (less infiltration, more surface runoff).

Scene 2: Landforms form a sequence set by slope

Steep upper slopes give fast flow and vertical erosion, producing V-shaped valleys and waterfalls; gentler middle slopes alternate lateral erosion and deposition, producing meanders and oxbow lakes; nearly flat lower courses are depositional, producing floodplains and deltas.
The reading routine is a three-part pairing: slope, process, landform. A student who names the landform without the process is reciting, not explaining.
On a meander the outer bank is fast and eroded while the inner bank is slow and depositional; both happen at once along one bend, which is where marks are lost.

Ask students: what two features appear on the outer and inner banks of one bend (steep bank outside, bar inside).

Scene 3: Flood works are trade-offs, not answers

Hard works include embankments, channel straightening and dredging. Widening a channel from 20 m to 28 m and raising velocity from 1.5 to 2.0 m/s lifts discharge from 90 to 168 cubic metres per second, 86.7% more: quick results, but maintenance is endless and the peak downstream grows.
Soft works include storage tanks, wetland restoration and zoning. A peak of 120 cubic metres per second lasting 3 hours is 1.296 million cubic metres, so a 500,000 cubic metre tank shaves only 38.6%; cheap and ecologically useful, but it needs land and time.
A 50-year return period is a 2% chance each year; the chance of at least one event in 30 years is 45.5%, so design cannot lean on rarity alone.

Ask students: why embankments alone fail (the peak downstream grows and risk concentrates behind higher walls).

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Annual runoff	Area x rainfall x coefficient, watch the units	118.8 million m3
Calculation	Peak shaved by a tank	Peak x hours x 3600 for volume, then divide by tank size	38.6%
Explanation	Urbanisation on a hydrograph	Impermeable surface, higher coefficient, higher peak, shorter lag	Describe both lines
Essay	Choosing flood works	Effect, cost, limit, then a combination	Hard plus soft

Sanity check (teach this): Area is in square kilometres, so multiply by 1,000,000 first; rainfall is in millimetres, so divide by 1,000 first.
The runoff coefficient is always below 1; runoff larger than rainfall means the coefficient was misused.
A storage tank holds water rather than removing it, so the cut is never 100%; a return period is a probability, not a calendar.
A deeper channel does not automatically mean faster flow: discharge needs all three variables.

05 Independent practice (3 questions)

Question	Answer	What to watch
Input for 120 km2 at 2,200 mm	264 million m3	Convert area, then multiply by 2.2 m
Runoff at a coefficient of 0.45	118.8 million m3	Input times 0.45
Chance of a 50-year event in 30 years	45.5%	One minus 0.98 to the power 30

06 Common mistakes and what to say

Mistake	Student writes	Say this
Blaming the peak on more rain	Answering that it rained more	Rainfall is unchanged; the coefficient moved from 0.45 to 0.75.
Skipping unit conversion	Runoff given as 26.4 billion m3	A square kilometre is one million square metres.
Naming landforms without processes	Answering that this is a meander	Add outer-bank erosion and inner-bank deposition to finish the answer.
Treating a return period as a timetable	Answering that it repeats in 50 years	It is a 2% chance each year, not a date.

07 Homework (paper, section and question numbers)

Secondary geography chapter index — All chapters and practice
All lesson-prep guides — Compare other subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
Annual runoff at 0.45	118.8 million m3	All three right: move to coasts
Percentage rise after urbanisation	66.7%	One wrong: reteach the coefficient
Peak shaved by the tank	38.6%	Two wrong: return to act three

09 Tutor FAQ

Must students memorise the world rivers?

No. Exams supply data; the skill is inferring processes from slope and velocity.

Are hard works always better?

They act fast but cost more and shift risk downstream, so answers must weigh trade-offs.

Can a runoff coefficient exceed 1?

No. It is the share of rainfall becoming runoff, so its maximum is 1.

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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.