Map Extract and Fieldwork Plan for a Valley Village
On a 1:25,000 map extract (1 km grid squares) a village of 900 people lies on a valley floor at 452 786, where the ground is 60 m above sea level. The contour interval is 10 m and the 140 m contour closes around the catchment upstream. The stream flows north-east, entering the sea at 468 808; it falls from 74 m at 455 792 to 14 m at 457 794. A narrow two-arch bridge, 4.2 m wide, carries the only road into the village across the stream at 459 797. The map shows terraced fields, a disused quarry at 461 781, a patch of woodland at 447 790 and a band of blue clay marked at 449 785. Mean annual rainfall is 560 mm and the catchment measured on the map is 3.6 km². A student plans to sample discharge and channel characteristics at five sites at 500 m intervals, beginning 300 m downstream of the bridge, on a day in July.
1. Calculate the straight-line distance and the channel gradient between 455 792 and 457 794, and express the gradient both as a ratio and as a percentage.
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Distance. On a 1:25,000 map, one grid square is 4 cm on the map and represents 1 km on the ground.
Easting difference: 457 − 455 = 2 grid units = 2 km. Northing difference: 794 − 792 = 2 grid units = 2 km.
Straight-line distance = √(2² + 2²) = √8 = 2.83 km = 2,830 m.
Fall. 74 m − 14 m = 60 m.
Gradient = fall ÷ horizontal distance = 60 ÷ 2,830 = 0.0212.
As a ratio: 1 ÷ 0.0212 = 1 in 47. As a percentage: 0.0212 × 100 = 2.1%.
So the stream falls about 21 m per kilometre over this reach.
Interpretation. A gradient of 1 in 47 is gentle-to-moderate for a Maltese valley and suggests a lower-reach channel with deposition rather than incision. The measurement is a straight-line approximation, so the true channel gradient is slightly gentler because the stream meanders and its actual length exceeds 2.83 km. A student should say this explicitly rather than presenting the figure as exact. Note also that grid references give four-figure precision, so the distance carries an uncertainty of roughly ±0.1 km.
2. Evaluate the fieldwork plan for measuring discharge and channel characteristics.
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Fatal issue - season. July on a Maltese valley stream means low or zero flow. Many such channels are intermittent and dry by midsummer, so discharge may be unmeasurable and the comparison between five sites impossible. If the aim is discharge, the fieldwork should follow substantial rain, or measure a reference flow and record the days since rainfall.
Design issue - extent. Five sites at 500 m intervals covers 2.5 km starting 300 m below the bridge at 459 797. From there to the sea at 468 808 is roughly 12 grid units, well beyond the sampled reach, so the plan is workable in extent but ignores the estuarine outlet where tidal and saline effects begin - an important site for a downstream comparison.
Systematic sampling at equal intervals is appropriate for a longitudinal study and avoids the bias of choosing 'interesting' spots, but pools and riffles alternate at a scale shorter than 500 m, so a single reading per site may miss the pattern. Three readings per site, or sites stratified by pool and riffle, would be better.
Method gaps: discharge needs cross-sectional area × velocity, with depth measured at several points across the channel and velocity by current meter or float with a correction factor; record width, wetted perimeter, bedload size and bank height. Safety at the outlet and on 4.2 m bridge is a real concern; access and permission must be secured. Repeat measurements and record the weather, because a single day gives a snapshot, not a regime.
3. Recommend a site for a small winter storage reservoir to supply the village, justifying it with map evidence.
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Requirements: upstream of the village for gravity supply, a narrow valley for a short dam, impermeable ground, adequate catchment, above the flood level and away from woodland and buildings.
Preferred site: the valley near 449 785, just below the 90 m contour.
- Upstream of 452 786, so water reaches the 900 residents by gravity - no pumping costs.
- A band of blue clay is mapped at 449 785. Clay is impermeable; a reservoir on clay loses far less water than one on jointed limestone, which would leak along fissures.
- The valley is narrow here, judged by the spacing of the 90 m and 100 m contours, so a short dam wall impounds a large volume.
- The 140 m contour closes upstream, confirming a defined catchment; 3.6 km² with 560 mm rainfall gives a usable yield, though the catchment is small and a dry winter is a real risk.
- Avoids the woodland at 447 790, which should be retained for bank stability and interception, and avoids the terraced fields downstream.
Rejected alternatives: the disused quarry at 461 781 is downstream of the village and would need pumping, though it has potential as a lined secondary store. The site near the bridge is unsuitable - a reservoir there would flood the only road access and raise flood risk at a 4.2 m pinch point.
Caveats: the yield must be checked against a demand estimate and against drought years; an environmental assessment is needed, since impounding interrupts sediment supply to the downstream reach studied in the fieldwork.