解题
**(a) (i)** To measure the depth, the students should stretch the tape measure tightly across the river from wet bank to wet bank, ensuring it is perpendicular to the flow. They should divide the total width into equal intervals (e.g., every 50 cm). At each interval, they must lower a graduated meter rule vertically into the water until it rests flat on the riverbed, ensuring the narrow edge faces the current to minimize water piling up on the rule. They then read and record the water level at each interval.
**(a) (ii)** Safety precautions: 1. Wear appropriate waterproof footwear with good grip (e.g., waders/wellingtons) to prevent slipping on wet rocks. 2. Work in teams and stay out of the water if it is too deep or fast-flowing.
**(b) (i)**
Mean depth calculation:
\(\text{Mean Depth} = \frac{0.15 + 0.22 + 0.28 + 0.35 + 0.25 + 0.19}{6} = \frac{1.44}{6} = 0.24\text{ m}\)
Cross-sectional area calculation:
\(\text{Cross-Sectional Area} = 4.20\text{ m} \times 0.24\text{ m} = 1.008\text{ m}^2\) (accept 1.01 \(m^2\) or 1.0 \(m^2\)).
**(b) (ii)** The hypothesis is completely true. The cross-sectional area increases consistently downstream from Site 1 to Site 4. For instance, at Site 1 (1 km downstream), the cross-sectional area is 0.18 \(m^2\), which rises to 1.01 \(m^2\) at Site 2, 2.15 \(m^2\) at Site 3, and reaches 4.01 \(m^2\) at Site 4 (13 km downstream).
**(c) (i)** Lay a tape measure across the river bed as a transect line. Select a pebble at regular intervals (e.g., every 30 cm) along this line. To avoid selective bias, the student should close their eyes or look away, reach down, and pick up the exact pebble that their finger touches first on the riverbed. Repeat this across the full width of the channel to collect 20 pebbles.
**(c) (ii)** Opportunistic bank sampling leads to size and shape bias, as only easily accessible pebbles are chosen, which are often smaller/lighter depositional materials. Systematic sampling covers the entire cross-section (including pools, riffles, and the faster-flowing center channel), representing the true bedload population.
**(c) (iii)** Pattern: Pebble roundness increases downstream, moving from a score of 1.7 (very angular) at Site 1 to 5.1 (rounded) at Site 4.
Explanation: This is caused by the transport process of **attrition**, where pebbles collide and rub against each other, chipping away sharp corners. It is also caused by **abrasion**, where pebbles scrape against the rocky channel bed. The longer the distance traveled downstream, the longer these processes act on the bedload.
**(d) (i)**
1. Take depth measurements at closer, more frequent intervals (e.g., every 10 cm instead of 50 cm) to capture minor bed irregularities more accurately.
2. Measure cross-sections at three different locations within the same site area and average the results to eliminate local anomalies.
**(d) (ii)** Any three of:
1. Discharge increases.
2. Average velocity increases.
3. Bedload size decreases.
4. Gradient decreases.
评分标准
**(a) (i)** [Max 4 marks]
- Lay tape measure tightly from wet bank to wet bank / at right angles to current (1)
- Measure total width (1)
- Take depth readings at equal/regular intervals (1)
- Place rule vertically / flat on the bed (1)
- Read water surface level with eyes at water height / ensure rule is not facing current to prevent splash height errors (1)
**(a) (ii)** [Max 2 marks]
- Wear protective footwear/grippy shoes/waders (1)
- Do not enter river if it is flooded / too deep / too rapid (1)
- Work in groups/pairs / have a lookout on the bank (1)
- Check weather forecast / avoid rain events upstream (1)
- Wear buoyancy aid/life jacket (1)
**(b) (i)** [Max 2 marks]
- Correct calculation of mean depth: 0.24 m (1)
- Correct calculation of cross-sectional area: 1.01 \(m^2\) (allow 1.008 or 1.0) (1)
**(b) (ii)** [Max 4 marks]
- Correct decision: Hypothesis is true/completely correct (1) [Reserve 1 mark]
- Cross-sectional area increases from Site 1 to Site 4 / increases with distance downstream (1)
- Use of paired area data: e.g. Site 1 is 0.18 \(m^2\) and Site 4 is 4.01 \(m^2\) (1)
- Use of intermediate site data to show trend: e.g. Site 2 is 1.01 \(m^2\) and Site 3 is 2.15 \(m^2\) (1)
- Mention that width also increases downstream (1.80 m to 8.90 m) (1)
**(c) (i)** [Max 4 marks]
- Lay tape measure/transect line across the river bed (1)
- Measure at set/equal/regular distance intervals (1)
- Pick up the pebble touching index finger / pick up without looking/bias (1)
- Sample across the entire wet width of the river (1)
- Repeat to get a sample size of 20 (1)
**(c) (ii)** [Max 2 marks]
- Opportunistic/bank sampling causes bias (1)
- Pebbles near the bank are not representative of the center/bedload (1)
- Systematic sampling covers different flow zones (pools/riffles/thalweg) (1)
**(c) (iii)** [Max 5 marks]
- Roundness score increases downstream / from 1.7 at Site 1 to 5.1 at Site 4 (1)
- Attrition process (1) [Reserve 1 mark]
- Attrition explained: pebbles collide / rub against each other during transport and wear down (1)
- Abrasion/Corrasion process (1)
- Abrasion explained: pebbles grind against the channel bed/sides (1)
- Distance/time factor: transport over longer distances means processes occur for longer (1)
**(d) (i)** [Max 4 marks; 2 marks for suggestions, 2 marks for explanation]
- More frequent depth measurements (1) to better capture uneven river bed (1)
- Take multiple transects at each site and average (1) to reduce anomalies (1)
- Use a graduated rod with a flat base plate (1) so it doesn't sink into soft mud/sediment (1)
- Do measurements on the same day / under stable discharge (1) to prevent stage height fluctuations from skewing comparisons (1)
**(d) (ii)** [Max 3 marks]
- Discharge increases (1)
- Channel width increases (1)
- Channel depth increases (1)
- Average velocity increases (1)
- Channel gradient decreases (1)
- Bedload size decreases (1)
- Channel roughness decreases (1)