Aldermere: More Floods, Less Rain
Aldermere is a town of 48,000 people built on a river delta. Between 1990 and 2020, annual rainfall in the region fell about 8%, yet the average number of flood days per year rose from 3 to 11. Town records show the following:
- Wells and farm pumps have lowered the water table by about 1.2 m since 1995.
- The delta surface is sinking about 12 mm per year.
- 60% of the tidal marsh that once fringed the shore has been filled for housing.
- The port channel was dredged and straightened in 2004.
- The nearest tide gauge shows open-ocean sea level rising about 3 mm per year.
The council blames climate change alone and proposes a $40 million seawall along 2 km of waterfront. A residents' group says halting groundwater pumping is enough. The council can fund only one measure this year and meets in two weeks.
1. Flood days rose even though rainfall fell. Using the numbers, rank the causes and explain why the rainfall trend does not contradict the flood trend.
Read the worked answer
Convert the numbers to a common time scale. Over 30 years, sinking adds 12 mm x 30 = 36 cm, and ocean rise adds 3 mm x 30 = 9 cm, for roughly 45 cm of relative sea-level rise at the shore. That is the largest measured change and it is the direct driver of tidal and storm-surge flooding.
Second is the loss of marsh: 60% of the buffer that absorbed wave energy is gone. Third is the dredged, straightened channel, which lets surge travel farther inland.
The pumping figure is not a separate cause from subsidence - pumping is probably a major cause of the sinking, so the two sides of the argument are partly describing one mechanism.
The rainfall trend is consistent because it changes which floods happen: less rain means fewer river-peak floods, while higher relative sea level means more tide- and storm-driven floods. The council's phrase 'climate change alone' misses that most of the relative rise here is local sinking, which local policy can affect.
2. Which of the two proposals is better supported by the evidence, and what would you want measured in the two weeks before the vote?
Read the worked answer
Neither proposal as stated fits the evidence. The seawall addresses surge, but a wall built on a sinking delta also sinks, so it loses height each year; it protects only 2 km, and no one has calculated its cost per protected household. Halting pumping targets a real driver, but compaction already caused by draining the aquifer is partly permanent, and the water table would recover slowly, so it cannot deliver results this year.
Measurable questions for the two weeks: (a) a parcel map of which properties actually flood - is the proposed 2 km the right 2 km? (b) pump records matched to subsidence rates, to test whether cutting pumping slows sinking, as has happened in other deltas; (c) cost per household for each option; (d) marsh restoration cost per kilometre, usually far below a wall.
A defensible decision would be a package - phased pumping cuts, marsh restoration, and building rules - with monitoring, rather than committing $40 million on one structure.
3. A council member says: Sea level rose only 9 cm, so it cannot explain eight extra flood days. Respond with reasoning about thresholds and about what the tide gauge measures.
Read the worked answer
The threshold argument: flooding begins when water crosses a fixed height - a road lip, a storm-drain outfall, a low wall. If the baseline rises, many events that used to stop just short now cross the line. So a modest rise in the baseline can multiply flood days without any change in the number or strength of storms. Picture raising the floor of a bathtub a few centimetres: many near-misses become overflows.
The comparison is also wrong. Nine centimetres is only the ocean part; adding 36 cm of local sinking gives about 45 cm at Aldermere's shore, five times the figure the member cites. On top of that, the marsh loss and channel dredging change how far a given surge reaches.
Finally, the tide gauge measures open water offshore, not the town. A gauge fixed to the land would show the relative rise that residents actually experience, which is the relevant number for flood days.