Reading a Contour Map for a Reservoir Siting Decision
A municipal team is choosing between two sites for a small irrigation reservoir in a hilly catchment. Site A lies in a broad, gently sloping valley where the 1:25 000 map shows contour lines spaced about 8 mm apart; the valley floor is at 240 m and the nearest contour above the dam line is 280 m. Site B lies in a narrow, steep-sided gorge where contours are 1.5 mm apart; the floor is at 300 m and the contour above the dam line is 380 m. The dam at each site would be 200 m long. The map scale is 1:25 000. The team also notes that Site B sits on limestone and Site A on clay-rich marl. Average annual rainfall is 620 mm, concentrated in spring and early summer.
1. Using contour spacing and the map scale, compare the likely storage volume and dam height of the two sites. Which site would you shortlist on topographic grounds and why?
Read the worked answer
First convert contour spacing into real ground distance. At 1:25 000, 1 mm on the map equals 25 m on the ground. Site A: 8 mm spacing means the ground rises 40 m (240 to 280 m) over roughly 200 m horizontally, a gradient near 20 percent - a gentle slope. Site B: 1.5 mm spacing means the ground rises 80 m (300 to 380 m) over roughly 37 m horizontally, a gradient above 200 percent - a near-vertical wall.
Storage is controlled by the area flooded behind the dam times average depth. A broad gentle valley (A) floods a wide area for a modest rise in water level, so a low dam gives a large surface area and a workable volume. A narrow gorge (B) gives a small surface area but a deep, steep-sided pool; a high dam is needed to store much water. On pure topography, A is easier and cheaper to build (lower dam, gentler abutments) but loses more land and evaporates more per unit stored; B stores more per hectare flooded but demands a much taller, more expensive dam and has steeper, less stable abutments.
The defensible shortlist answer is A on cost and constructability, with the caveat that the final choice must weigh geology (limestone at B risks leakage through karst) and the land-take and evaporation penalty at A. A good answer states the topographic trade-off explicitly rather than picking a site on one number.
2. The rainfall is 620 mm concentrated in spring and early summer. How does this seasonal pattern affect the reliability of a reservoir at either site, and what map evidence would you look for to judge it?
Read the worked answer
A single annual total hides the seasonal problem. If most of the 620 mm falls in a few spring and early-summer months, the reservoir must fill in a short window and then carry the dry season on stored water. Reliability therefore depends on catchment area above the dam, not just local rainfall, and on how much of the inflow can be captured before it spills.
Map evidence to check: the size of the contributing catchment upstream of each dam line (drawn from the watershed divide along ridge crests); the presence of tributaries shown as blue lines feeding the site; whether the valley narrows just below a large upstream basin, which favours storage; and the position of the dam relative to the highest contour so spillway capacity can be judged. A steep gorge like B may have a small catchment and flashy inflow, filling fast and spilling fast; a broad valley like A may collect from a wider area but lose water to evaporation over its large surface.
A sound conclusion: neither site is reliable on rainfall alone; the answer must combine catchment size, seasonal concentration, and storage shape, and recommend a gauging record or a conservative yield estimate before committing.