Choosing a Feeder-Road Alignment in the Bamenda Highlands
A rural council in the Bamenda Highlands must choose between two alignments for a 22 km feeder road linking three villages to the regional market. Alignment A follows the existing track along a ridge crest between 1,700 and 1,850 m, crossing nine gully heads, with slopes averaging 18 degrees and reaching 32 degrees where the track bends. Alignment B drops into the valley and runs along the footslope at 1,350-1,450 m, adding 6 km; its slopes are 4-8 degrees, but the valley floor is black clay that stays waterlogged from July to October. Rainfall averages 2,300 mm a year, concentrated in June-September. The underlying basalt weathers to thick clay-rich saprolite. Since 2015 farmers have cleared about 40% of the ridge forest for eucalyptus and potato plots, and several gully heads have widened. The council has a fixed construction budget and a maintenance allowance of less than 2% of capital cost per year. Traffic today is about 60 motorcycles and 8 lorries a day and would grow if the road were paved.
1. Which alignment should the council build? Justify your choice using the evidence given, and state two conditions under which your recommendation would change.
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Framework. Compare capital cost, recurrent cost, hazard to users and construction risk. With maintenance capped below 2% of capital, a road that fails every rainy season is worse than a longer one that survives.
Case for A (ridge crest). It is 22 km against 28 km, so fill, culverts and clearing are spread over fewer kilometres. Crest sites drain naturally and need fewer side drains except at the nine gully heads. But mean slopes of 18 degrees, and 32 degrees at bends, force cut slopes into weathered basalt saprolite, which loses shear strength when saturated. With 2,300 mm in four months and 40% of the ridge forest cleared since 2015, runoff is concentrated and the gully heads are already retreating. Nine stream crossings on a road with almost no maintenance budget invites washouts and closed sections.
Case for B (footslope). Gradients of 4-8 degrees suit the 8 daily lorries and future sealing, and drainage design is straightforward. Penalties: 6 km extra length (about 27% more road), black clay with a seasonally high water table demanding a raised embankment, imported fill and deep side drains, plus possible dry-season-only construction.
Recommendation. Build B, conditional on a geotechnical check of clay depth. The reasoning is that the council controls construction once but maintenance forever; choosing the alignment where natural processes do least work reduces the risk of an unfunded repair bill.
My answer would change if: (1) the cost premium for B exceeds roughly 30% of A's tender price, or suitable fill cannot be found within a short haul, making B unfundable; or (2) the clay is deeper than about 3 m and the water table stands near the surface, so B needs piles or extensive replacement and its cost advantage disappears. In that case A becomes defensible only with nine box culverts and an enforced ban on further clearing above the gully heads.
2. Explain how the land-use change on the ridge since 2015 has altered runoff generation and slope stability along Alignment A.
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Interception and rainfall energy. Closed forest intercepts a significant share of the 2,300 mm and its multilayered canopy breaks raindrop energy. Eucalyptus and potato plots intercept less and, crucially, leave bare soil between harvests, so raindrops compact and seal the surface. Infiltration falls and overland flow rises.
Root reinforcement. Forest root networks add apparent cohesion to the upper saprolite. Shallow-rooted annual crops and short-rotation eucalyptus provide much less reinforcement at 0.5-1.5 m depth, which is where shallow translational slides usually start on these slopes. The effect is not that trees use more water; it is that root strength and litter cover are lost, especially during harvest and replanting.
Concentration into gullies. Compacted farm tracks and plot boundaries funnel water into the nine gully heads. Once a head-cut forms, it migrates upslope during each storm, widening the crossing that the road must span. This is why the report notes the gully heads widening rather than general sheet erosion.
Pore pressure and timing. During June-September the saprolite approaches saturation. A storm of, say, 60 mm in a few hours raises pore pressure, reduces effective stress and can trigger failure on the 32-degree bends where the road cut has removed the toe support. Cut slopes therefore fail in the same weeks that traffic most needs the road.
Conclusion. The land-use change has shifted the slope from a slow, forest-moderated system to one where runoff is delivered quickly to lines of weakness. Any engineering on A must address water concentration, not just the road surface.
3. Design a monitoring and maintenance scheme the council could realistically run on less than 2% of capital cost per year.
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Why a scheme is needed. At under 2% of capital per year, the council cannot rebuild washouts. It can only detect problems early, when they are cheap. The scheme should therefore be cheap, repeatable and tied to the rainfall calendar.
Elements.
- Gully-head watch list. Photograph and mark nine fixed points with painted stakes each April. A named farmer or village committee reports head-cut retreat of more than about 1 m after any storm above roughly 40 mm. Cost: paint and a notebook.
- Drainage clearing before June. Clear side drains, culvert inlets and the crossings of cut branches and sediment in May, before the first heavy rains. This is the single highest-return task and can use local labour paid by the day.
- Vegetative treatment. Plant vetiver or napier grass strips across gully floors and along cut-slope toes. Cheap, self-repairing, and it reduces flow velocity where a masonry structure would be unaffordable.
- Simple records. A one-page log per road section: date, rainfall estimate, damage, action, cost. This lets the council argue for funds and shows which sections are failing repeatedly.
- Rules. Enforce a ban on cultivation within about 5 m of gully heads, agreed with the village council rather than imposed.
Judgement. The scheme deliberately favours low-cost, high-frequency actions over engineering. The trade-off is that vegetation and clearing will not stop a major landslide on a 32-degree cut slope; for that the council must budget separately or realign the section. Stating this limit matters, because a 2% budget should not be presented as sufficient for a hazardous alignment.