DSE Geography | River Management and Water Resources Lesson-Prep Guide (Tutor Edition) | Hydrographs, Urban Runoff and Supply Allocation

2026-11-02

Tutor lesson-prep guides · Secondary Geography

Students can say Hong Kong floods, yet cannot say why the same storm that was harmless twenty years ago now fills the streets. This lesson turns a hydrograph into something they can read.

Objectives — Students can: (1) compute total runoff from catchment area and rainfall and read peak discharge and lag time off a hydrograph; (2) explain how urbanisation lifts the peak from 45 to 180 cubic metres per second and cuts lag time from 9 hours to 3, pushing flow 20% above channel capacity; (3) compare reservoirs, desalination, reclaimed water and demand management, and use a 30-day reserve to explain supply vulnerability.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Catchment	Where does rain go once it lands in an area	Down tributaries into the main river and out through one outlet
Discharge units	What does 180 cubic metres per second mean	180 tonnes of water each second, about three pools an hour
Percentages	How far above 150 is 180	30 above, which is 20%

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Where rain goes	Catchment, tributaries, infiltration, evaporation	Draw one catchment arrow map
10-22 min	Water budget	Area times rainfall, then times runoff coefficient	Write 9.6 million and 3.36 million m3
22-34 min	Reading a hydrograph	Peak, lag time, rising limb, recessional limb	Read two figures off the graph
34-44 min	Urbanisation effect	Impervious surface, channelling, lost flood storage	Explain a fourfold peak
44-54 min	Supply options	Reservoirs, desalination, reclaimed water, saving	Compute a 30-day reserve
54-60 min	Exit check	Three questions marked on the spot	All three right

03 Teaching script (read it out)

Scene 1: A storm is a multiplication, not a feeling

A catchment of 120 square kilometres receives 80 mm of rain. Total water = 120 x 10^6 x 0.08 = 9.6 million cubic metres.
Not all of it becomes runoff: some infiltrates, some evaporates, some is intercepted by plants. With a runoff coefficient of 0.35, direct runoff = 9.6 million x 0.35 = 3.36 million cubic metres.
The other 6.24 million stays in soil and air. So rain does not equal river flow; the exam question is what fraction actually arrives.
A Manning estimate for a natural channel: n = 0.035, hydraulic radius 1.5 m, slope 0.001, gives velocity (1/0.035) x 1.5^(2/3) x sqrt(0.001) = 1.18 m per second. With a 30 square metre section, discharge is about 35.4 cubic metres per second.

Ask students: of 9.6 million cubic metres of rain, why only 3.36 million reaches the river (infiltration, evaporation, interception).

Scene 2: Urbanisation lifts the peak and pulls it earlier

For the same storm, after farmland becomes housing: impervious cover rises from about one tenth to seven tenths, infiltration almost disappears, and water is piped straight to the river. Peak discharge rises from 45 to 180 cubic metres per second, four times the original; lag time falls from 9 hours to 3 hours.
The channel is designed for 150 cubic metres per second, so 180 exceeds capacity by 20% and the water goes down the street.
Note what did not change: total rainfall. What changed is when the water arrives and how concentrated it is. Flood management therefore slows arrival rather than making water disappear.
Three common tools: detention ponds (hold water for hours), permeable paving (restore infiltration) and channel widening (raise capacity), each with a land cost.

Ask students: with unchanged total rainfall, how can the peak quadruple (arrival is concentrated, not increased).

Scene 3: Supply is an allocation problem, not a mining problem

Hong Kong reservoirs hold about 586 million cubic metres, yet local yield is only a few hundred million a year, with the rest from Dongjiang. Run the arithmetic: a reservoir holding 12 million cubic metres against demand of 400,000 cubic metres a day lasts 30 days; if transfer stops, the city has a month.
So options must be compared on output, cost and lead time: desalination produces 135,000 cubic metres a day but is electricity-hungry; reclaimed water is cheaper per cubic metre but needs a pipe network; demand management is cheapest but depends on behaviour.
A 1-in-50-year storm has an annual exceedance probability of 1 / 50 = 2%. Drainage design trades that probability against cost: a higher standard means a more expensive culvert.
In answers, write risk as two columns, probability and consequence, never just the word flooding.

Ask students: what does a 30-day reserve imply for planning (a second source or cross-border transfer is not optional).

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Total water and direct runoff	Area times rainfall, then times runoff coefficient	9.6 million and 3.36 million m3
Calculation	Peak against capacity	(peak – capacity) / capacity	20% above
Explanation	Hydrograph before and after urbanisation	Impervious surface, less infiltration, higher and earlier peak	Use fourfold and 9 against 3 hours
Evaluation	Choosing a supply option	Output, cost, lead time and environment, each with evidence	Balanced conclusion

Sanity check (teach this): Convert area to square metres: 1 square kilometre is 10^6 square metres; forgetting this puts the answer out by a million.
80 mm is 0.08 m, not 80 m; fix units before multiplying.
A runoff coefficient is always below 1, so runoff larger than rainfall means an arithmetic error.
Peak discharge is a rate in cubic metres per second while total water is a volume; they cannot be subtracted directly.
A 1-in-50-year event is a 2% annual chance, not a guarantee that arrives once every fifty years.

05 Independent practice (3 questions)

Question	Answer	What to watch
Total water from 120 square kilometres at 80 mm	9.6 million m3	120 x 10^6 x 0.08
Direct runoff with coefficient 0.35	3.36 million m3	9.6 million x 0.35
How far 180 exceeds a 150 capacity	20%	(180 – 150) / 150

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating rainfall as runoff	Answering that 80 mm gives 9.6 million cubic metres of river flow	Multiply by the runoff coefficient; the rest infiltrates, evaporates or is intercepted.
Assuming urbanisation adds water	Answering that building makes more rain	Rainfall is the same; timing and concentration change.
Mixing rate and volume	Answering that a peak of 180 cubic metres meets a channel of 150 cubic metres	Both are per second; the unit is cubic metres per second.
Reading a return period as a schedule	Answering that a 50-year flood happens once every 50 years	It is a 2% chance each year and can occur twice in a row.

07 Homework (paper, section and question numbers)

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

08 Exit check (last 5 minutes)

Question	Answer	Decision
Direct runoff from 9.6 million m3	3.36 million m3	All three right means move to supply
How far the peak exceeds capacity	20%	One wrong means redo percentages
What a 30-day reserve implies	A second source is needed	Two wrong means repeat act three

09 Tutor FAQ

Must students memorise reservoir capacity?

Two magnitudes are enough: about 586 million cubic metres total, with local yield under half of demand.

Is the Manning equation examinable?

DSE does not require the calculation, but students must explain discharge as velocity times cross-section.

Does urbanisation always raise the peak?

No. Detention ponds and permeable paving can lower it; judge by the individual case.

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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.