New York City sinks under the weight of buildings - Experts warn of flooding threats

Recently CNN.co.jp reported the following:

ニューヨーク、ビル群の重さで沈む　洪水の脅威に専門家警鐘

2023.05.24 Wed posted at 12:45 JST

（ＣＮＮ） 米ニューヨーク市はビル群の重さで徐々に地盤沈下が進んでいるという研究結果を、米地質調査所などの研究チームが学術誌に発表した。

ニューヨーク市周辺の海面は世界の２倍を超すペースで上昇しており、２０５０年までにはさらに約２０～７６センチ上昇すると予想している。

しかも気候変動の影響で、ハリケーンなど豪雨をもたらす災害は一層頻度を増す見通しだ。

論文を執筆した米地質調査所のトム・パーソンズ氏は「海が入り込んでくるのはまだずっと先のことだ」としながらも、「ニューヨークを襲ったサンディやアイダのような大型ハリケーンでは、豪雨によって市街地が浸水し、都市化の影響で水の侵入を許した」と指摘した。

この論文は、海岸や河川、湖の沿岸部の高層ビルが将来的な洪水の原因になり得ることを示し、被害を食い止めるための対策の必要性を示す目的で発表された。

研究チームはニューヨーク市の５区にあるビル１０８万４９５４棟（当時）の重量を計算し、合計でおよそ７６２０億キロと算定した。これは満員のボーイング７４７―４００型機約１９０万機分に相当する。

このシミュレーション結果を使って、建物の重さが地面に及ぼす影響を、実際の地表の地質状況を示す衛星データと比較しながら計算した。その結果、平均で年間約１～２ミリ沈下し、場所によっては最大４．５ミリに達する場所もあったという。

２０２２年９月の別の調査では、人口の多い沿岸部の４８都市のうち４４都市で、海面上昇を上回るペースで地盤沈下が進む区域があることが判明。今回の調査ではニューヨーク市のビル群の重さに着目し、その重さが地盤沈下にどう影響しているかを調べた。

ただし地盤沈下を引き起こしているのは建物ばかりとは限らない。パーソンズ氏は「非常に柔らかい土壌や人工的な埋め立て地で建設が行われている場所では何らかの関係がみられる」と指摘する一方、「地盤沈下の説明が難しい場所もある。原因は氷河期の後に起きた氷河の融解や、地下水のくみ上げなどさまざまだ」と解説する。

マンハッタン南部やブルックリン、クイーンズなど、平均を上回るペースで沈下が進んでいる場所もあった。「一部は建設プロジェクトの進行と関係があると思われる」「しかしスタテン島の北部にも地盤沈下がみられる。これについては説明がつかず、あらゆる要因を検討したが、依然として謎は解けていない」（パーソンズ氏）

地盤沈下は洪水の原因となる恐れがあり、「これは世界的な問題だ」とパーソンズ氏は言う。ロードアイランド大学の研究チームが世界９９都市について行った調査では、大多数が地盤沈下の問題を抱えていたという。インドネシアでは首都ジャカルタで急速に地盤沈下が進んでいることから、政府が首都を移転させる計画を打ち出した。

Translation

(CNN) New York City was slowly sinking due to the weight of its buildings according to a study result of a research team, such as the U.S. Geological Survey, which was published in an academic journal.

Sea levels around New York City were rising more than twice as fast as the rest of the world and were projected to rise another 20 to 76 centimeters by 2050.

Moreover, due to the impact of climate change, disasters such as hurricanes that brought heavy rains were expected to increase in frequency.

Author of the paper Tom Parsons of the U.S. Geological Survey said "We're still a long way off from seeing the ocean coming in," and pointed out that “In major hurricanes like Sandy and Ida that hit New York, urban areas were flooded by heavy rains, allowing water to enter due to the effects of urbanization."

The paper aimed to show that for high-rise buildings along coastlines, rivers and lakes could be a potential source of future flooding, and to highlight the need for measures to limit the damage.

The research team calculated the weight of 1,084,954 (at that time) buildings in New York City's 5^th district had a total of approximately 762 billion kilograms. That was the equivalent of about 1.9 million fully loaded Boeing 747-400s.

Using the results of this simulation, the effect of the weight of the building on the ground was calculated by comparing it with satellite data that showed the geological conditions of the actual ground surface. As a result, the sinking was about 1 to 2 mm per year on average, and in some places, it reached up to 4.5 mm.

Another study in September 2022 found that 44 of the 48 most populous coastal cities had areas where land subsidence would outpace sea-level rise. The study this time focused on the weight of buildings in New York City and investigated how the weight would affect land subsidence.

However, buildings were not the only cause of land subsidence. Parsons said “There's something to do with very soft soils and where construction is being done on man-made landfills”; and explained that “Land subsidence is difficult to explain in some places. There are various issues such as the melting of glaciers and the pumping up of groundwater."

Some places, like lower Manhattan, Brooklyn, and Queens, were sinking at a faster-than-average rate. "We believe some of it has to do with construction projects going on," he said. "But we're also seeing land subsidence in the north of Staten Island. We can't explain it, and we've looked at all the factors, but it's still a mystery."  (Mr. Parsons)

Parsons said, land subsidence could cause flooding, and "it's a global problem." A study of 99 cities around the world by a research team at the University of Rhode Island found that the majority had problems with land subsidence. Indonesia's capital Jakarta was rapidly subsiding, so the government had come up with a plan to relocate the capital.

So, New York City is slowly sinking due to the weight of its buildings according to research results. It seems that many coastal cities will face relocation problems in the coming decades as sea level is rising due to global warming.