Flood storage or upstream natural measures?
The River Alden drains a 240 km² catchment. Since 1990, urban cover has risen from 12% to 38%, and the 1-in-10-year peak discharge at the gauging station has increased from 45 m³/s to 78 m³/s. The lag time between peak rainfall and peak discharge has shortened from 9 hours to 5 hours. A regional flood board is choosing between two schemes. Option A is a downstream detention basin costing $14 million: it would occupy 60 ha of grade-2 farmland and modelling suggests it would cut the 1-in-10-year peak by 32%, protecting 320 homes. Option B is upstream woodland planting of 180 ha plus permeable paving in the town on 12 ha, costing $9 million; modelling suggests an 18% peak reduction, with benefits for water quality and biodiversity. The board has a fixed budget of $15 million and must also fund a $3 million bridge repair. Residents downstream want immediate protection; farmers oppose losing land.
1. Explain the hydrological changes shown by the catchment data.
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Urbanisation replaces vegetation and soil with roofs, roads and drains. Infiltration and depression storage fall, so a larger proportion of rainfall becomes surface runoff. Drains and channels then transfer water quickly, shortening the lag time. The data show lag falling from 9 hours to 5 hours, which is consistent with faster routing. Peak discharge rises from 45 m³/s to 78 m³/s, an increase of about 73%, because runoff from many impermeable surfaces arrives at the channel at almost the same time. Compaction of remaining soil, loss of floodplain storage and possibly channel straightening would reinforce this. The result is a higher and earlier flood peak for the same rainfall event. The reasoning links land-cover change to the runoff process, then to the shape of the hydrograph; it is not enough to say that urban areas flood more.
2. Evaluate the two options and recommend one to the flood board, taking the budget into account.
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The board has $15 million but must spend $3 million on the bridge, leaving $12 million. Option A cannot be funded from the remaining budget because it costs $14 million. Option B costs $9 million, leaving $3 million, and delivers additional water-quality and biodiversity benefits. However, Option A gives a 32% peak reduction compared with 18% for Option B, and protects 320 homes, so it is technically stronger for flood risk. A defensible recommendation is therefore Option B plus a targeted package using the remaining $3 million, for example floodproofing the most vulnerable homes and improving an early-warning system. The board should also seek partnership funding for Option A only if the required standard of protection cannot be met by B. This is a judgement under constraint: affordability and multiple benefits favour B, while the protection standard favours A, so the recommendation must state the trade-off rather than ignore it.
3. Identify two limitations of the modelling used to compare the schemes and explain how they affect the conclusion.
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First, the models depend on assumed design storms. If they use historical rainfall rather than projected climate change, they may underestimate future peaks, especially for short intense storms; this would make both schemes look more effective than they will be. Second, the models must represent land-use and routing simplistically. Option B assumes that 180 ha of woodland and 12 ha of paving behave as modelled, but establishment takes years and farmer uptake is uncertain. Option A assumes the basin operates as designed, but sediment accumulation and maintenance could reduce storage. A third limitation is calibration: a model fitted to a 240 km² catchment with limited gauging data may have large uncertainty in the 32% and 18% figures. The comparison should therefore be treated as indicative, and the board should require sensitivity testing before committing funds.