DSE Geography: River Processes and Landforms (Tutor Lesson Plan) | Discharge, sinuosity and channel roughness

2026-11-04

Tutor lesson-prep guides · Secondary Geography

Students draw V-shaped valleys and meanders, then use the wrong formula for discharge or confuse load with discharge. This lesson keeps three numbers apart: cross-section, velocity and sediment concentration.

Objectives — Students can compute discharge from cross-section and velocity with unit checks, estimate velocity from hydraulic radius and Manning, convert sediment concentration into load, and compare hard and soft catchment schemes.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Area	Area of a rectangular cross-section	Width times mean depth
Units	Litres in one cubic metre	1000 L
Processes	Order of erosion, transport and deposition	Erosion upstream, deposition downstream

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Discharge	Cross-section times velocity	14.4 cubic metres per second
10-22 min	Velocity	Hydraulic radius and Manning	About 1.02 m/s
22-34 min	Erosion and transport	Four processes each	Match landform to process
34-46 min	Sinuosity and deposition	Channel length over valley length	Sinuosity 1.5
46-56 min	Catchment management	Hard against soft engineering	Two limits for each
56-60 min	Exit check	Three questions marked at once	Independent

03 Teaching script (read it out)

Scene 1: Discharge: keep three numbers apart

Cross-section = width times mean depth = 12 times 1.5 = 18.0 square metres.
Discharge = area times velocity = 18.0 times 0.8 = 14.4 cubic metres per second, which is 14,400 litres a second.
Wetted perimeter = 12 plus 3.0 = 15.0 m, so hydraulic radius = 18.0 over 15.0 = 1.20 m.
Manning gives velocity = (1 over n) times R to the two thirds times the square root of slope, about 1.02 m/s for n = 0.035 and slope 0.001, the same order as the measured 0.8 m/s.

Tutor move: insist on units every time. Square metres times metres per second is cubic metres per second, and wrong units expose a wrong number.

Scene 2: Erosion, transport and deposition: follow the energy

Four erosion processes: abrasion, attrition, hydraulic action and solution.
Four transport processes: solution, suspension, saltation and traction, with the coarsest load moving by traction.
Deposition happens when velocity falls, so sand and silt settle downstream and deltas form at the mouth.
Sinuosity = channel length over valley length = 6.0 over 4.0 = 1.5, and above 1.5 the meandering is clear.

Ask the student: if velocity halves, by roughly what factor does the largest transportable particle fall (about sixty-four times, since the required velocity scales with the square root of grain size)?

Scene 3: Catchment management: hard against soft

Hard schemes: dams, concrete channels, flood storage basins and barrage gates. They work fast and protect urban land, but cost a lot and starve the downstream channel of sediment.
Soft schemes: meander restoration, riparian planting, wetland storage and land-use zoning. They are cheaper and better for ecology, but act slowly and need long maintenance.
With 0.8 kg per cubic metre and 14.4 cubic metres per second, the load is 11.52 kg a second, about 995 tonnes a day.

Reminder: compare schemes on timescale and on who pays, because costs and benefits without a timescale score low.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Discharge	Area times velocity	14.4 cubic metres per second
Calculation	Hydraulic radius and Manning	Area over wetted perimeter, then n and slope	R = 1.20 m, v about 1.02 m/s
Calculation	Sediment load	Discharge times concentration, then per day	About 995 tonnes a day
Comparison	Hard against soft engineering	Cost, speed and ecology	Two limits for each

Sanity check (teach this): Discharge is always in cubic metres per second, so a square metre answer means velocity was left out. The hydraulic radius must be smaller than mean depth (1.20 below 1.5), and load is a mass per time, not a volume.

05 Independent practice (3 questions)

Question	Answer	What to watch
Cross-section for width 12 m and depth 1.5 m	18.0 square metres	Width times mean depth
Discharge at 0.8 m/s	14.4 cubic metres per second	Area times velocity
Daily load at 0.8 kg per cubic metre	About 995 tonnes	Discharge times concentration times 86400

06 Common mistakes and what to say

Mistake	Student writes	Say this
Reporting the cross-section as discharge	Answering 18.0	“Multiply the area by velocity; only then are the units cubic metres per second.”
Using depth as the hydraulic radius	Answering R = 1.5 m	“The wetted perimeter includes both banks, so R is 1.20 m.”
Confusing concentration with load	Answering 0.8 kg a second	“0.8 is the mass in each cubic metre, so multiply by discharge.”
Listing only hard engineering benefits	Four advantages and no limit	“Every scheme needs a limit on cost, timescale or ecology.”

07 Homework (paper, section and question numbers)

DSE geography practice paper (rivers and landforms) — Section A data response and Section B evaluation
Secondary geography chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Cross-section for 12 m by 1.5 m	18.0 square metres	All three right: move to coasts
Discharge at 0.8 m/s	14.4 cubic metres per second	One wrong: reteach units
Which is larger, hydraulic radius or mean depth	Mean depth is larger	Two wrong: restart act one

09 Tutor FAQ

Must students memorise Manning?

No, but they should use it to explain why a rougher bed carries slower flow.

Why does discharge rise downstream?

Tributaries enlarge the catchment, so flow grows even at the same rainfall.

Why can a dam worsen downstream erosion?

It traps sediment, so clear sediment-starved water erodes the bed and banks.

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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.