How high must the Seine rise to reach your street?

#How high did the Seine rise in 2016 and 2018?

Paris measures its floods on a gauge at the Austerlitz bridge, as it has since the middle of the 19th century. A reading is the river's height above the gauge's zero, which sits 25.90 metres above sea level; it is not the depth of the water.

In early June 2016 the Seine reached 6.10 metres on that gauge. The RER C, a regional rail line that runs through Paris along the Seine's left bank, and several Métro stations closed for days. In January 2018 it rose again and peaked just under 5.9 metres (published peaks range from 5.84 to 5.88). The government's review of that flood cites simulations by the operator of the four reservoirs upstream: they suggest the reservoirs kept the river about 65 centimetres lower in Paris, and that without them it would have come close to 6.40 metres and flooded the RER C. The review treats this as a rough estimate built on unvalidated data.

Those floods were large by recent standards and small by the city's own. On 28 January 1910 the river reached 8.62 metres. It had risen for twelve days, and it did not return to its bed until 15 March. The city puts the flooded area at about a fifth of Paris, including places that had seemed safe.

#Why does the water arrive all at once?

After 1910 the state raised parts of the quays and bridges in Paris to the level of that flood, 8.60 metres on the gauge, and dug out the riverbed downstream so that flood water could leave faster. In the inner suburbs, walls built from 1936 onwards were set lower: at the level of 1924 or 1955 in some places, about 7.30 metres with 20 to 30 centimetres to spare, and at the level of 1982 or less in others. The 2018 review counts 7.40 metres on the Austerlitz gauge as the level of the inner-suburb flood walls, where the main damage begins.

The planning blueprint of the Métropole du Grand Paris, quoted in the latest flood-prevention programme for the Seine and the Marne, describes the second jump. Paris, it says, is little exposed at R0.8; its exposed area grows sharply between R1.05 and R1.15, from about 560 to 870 hectares, because the dykes and walls protecting the central arrondissements are overtopped.

The first jump, between 7.19 and 7.58 metres, falls around that 7.40-metre mark, though the maps do not say which defence gives way. At 7.58 metres the water reaches the arrondissements where the river enters and leaves Paris: the 12th and 13th upstream, the 15th and 16th downstream.

#Could the 1910 flood happen again?

Paris's flood-risk plan, which governs what may be built where, takes a flood reaching the 1910 marks as its reference and calls it centennial: a flood with a one-in-a-hundred chance of happening in any year. It is not the highest on record. In 1658 the river rose 30 to 50 centimetres above the 1910 level, according to the 2018 review; the list kept by the regional environment directorate gives 8.96 metres at Austerlitz.

Four large reservoirs now stand upstream, on the Seine, the Marne, the Aube and the Yonne, brought into service between 1949 and 1990. Together they hold about 805 million cubic metres, but they control only 17 per cent of the catchment that drains through Paris, and their effect reaches the city about eight days later. Their operator, EPTB Seine Grands Lacs, estimates that in a repeat of 1910 they would lower the river in Paris by about 70 centimetres; with the raised bridges and the deepened riverbed, by about a metre to 1.20 metres. The 2003 plan was more cautious: between 30 centimetres and a metre, depending on where and when the rain falls.

The scenario with 1910's flows sits at 8.12 metres, 50 centimetres below the 1910 mark. The regional environment directorate explains the gap: since 1910 the riverbed has been reworked to lower flood levels and ease navigation, so the same flow now rises less. The reservoirs act on something else, the flow itself, which each scenario fixes in advance.

Today's highest map, 48 centimetres above the 1910 mark, covers 8.2 per cent of Paris, well short of the fifth the city counts for 1910. The maps do not explain the gap. Two things have changed since: the quays and bridges of Paris now stand at the 1910 level, and the maps count only water spreading over the ground. A study cited in the same programme puts the return period of a 1910-type flood at roughly 90 to 130 years, depending on the stretch of river around Paris, and of the R1.15 flood at roughly 240 to 350 years.

#What do the flood maps leave out?

The maps show water spreading over the ground. Much of the damage starts below it. The government's 2018 review puts the first serious damage at 6.20 metres, when the RER C floods. The flood-prevention programme notes that the water table beside the Seine rises before the river overflows, and that its effect on basements is still poorly known outside the big network operators, such as the RATP, which runs the Métro, and the SNCF, the national railway.

In a 1997 account by the journalist Ambroise-Rendu, the water of 1910 first covered the low quays, then some of the high ones, and soon came in underground as well. On the night of 23 January the Métro stations at Châtelet, Saint-Michel and Odéon went dark as water seeped along the ballast of line 4, which passes under the river. The next day it filled the railway cuttings and tunnels along the quays. The Gare d'Orsay was the first main-line station to close. On today's maps the address of the Musée d'Orsay, as the station now is, stays outside the water at every level. By the peak, 23 of the 41 plants that kept the city running, for water, refuse, compressed air, gas and electricity, were under water.

The OECD's 2014 study of floods modelled on 1910's put the direct damage at between €3 billion and €30 billion, depending on the scenario. In the most extreme case, it found, close to five million people would be affected directly or indirectly, more than 1.5 million electricity customers could lose power and more than five million water customers around Paris could face long cuts.