Where floods fastest

Everything below is statistics of a simulation: the WRA third-generation flood potential maps (2022 edition) after gridding. It says "if this much rain fell, where does the model flood first and how widely", not what has happened anywhere. Every township name links to its full ten-scenario table.

Nationwide

Land 37,014.8 km². Simulated area flooded above 0.3 m under each scenario:

ScenarioFlooded km²Share
6 h / 150 mm163.70.4%
6 h / 250 mm566.41.5%
6 h / 350 mm1,177.63.2%
12 h / 200 mm264.60.7%
12 h / 300 mm686.21.9%
12 h / 400 mm1,224.23.3%
24 h / 200 mm192.70.5%
24 h / 350 mm674.51.8%
24 h / 500 mm1,362.93.7%
24 h / 650 mm2,060.45.6%

Highest share (24 h / 350 mm)

Small urban districts (Lingya, Qianjin and Xinxing in Kaohsiung cover only 2–8 km²) show inflated shares. The places that are both large and high are the Lanyang Plain and the Chiayi–Tainan coast.

#TownshipArea km²Flooded km²Shareof which >1 m
1Lingya District, Kaohsiung City Map8.43.238.0%0.2
2Qianjin District, Kaohsiung City Map2.00.632.4%0.0
3Xinxing District, Kaohsiung City Map2.50.829.9%0.0
4Zhuangwei Township, Yilan County Map39.611.629.3%3.2
5Wujie Township, Yilan County Map40.49.623.6%2.1
6Qiaotou District, Kaohsiung City Map24.55.522.4%1.4
7Dongshi Township, Chiayi County Map82.417.621.4%2.4
8Luodong Township, Yilan County Map11.32.320.6%0.1
9Liujiao Township, Chiayi County Map62.612.520.0%2.2
10Yuanchang Township, Yunlin County Map72.513.819.0%1.0
11Qieding District, Kaohsiung City Map17.13.218.6%1.1
12Budai Township, Chiayi County Map79.414.618.4%2.6
13Donggang Township, Pingtung County Map31.05.517.9%0.1
14Xiaying District, Tainan City Map33.76.017.8%1.1
15Sanmin District, Kaohsiung City Map17.13.017.5%0.2

Highest share (24 h / 650 mm)

In the extreme scenario 58 townships have more than a quarter of their area simulated flooded, 8 more than half.

#TownshipArea km²Flooded km²Share
1Wujie Township, Yilan County Map40.426.966.6%
2Zhuangwei Township, Yilan County Map39.625.965.4%
3Lingya District, Kaohsiung City Map8.45.059.5%
4Yilan City, Yilan County Map30.617.757.7%
5Qieding District, Kaohsiung City Map17.19.756.6%
6Qianjin District, Kaohsiung City Map2.01.153.9%
7Liujiao Township, Chiayi County Map62.633.152.9%
8Luodong Township, Yilan County Map11.35.850.8%
9Hunei District, Kaohsiung City Map21.910.849.3%
10Dongshi Township, Chiayi County Map82.440.148.6%
11North District, Hsinchu City Map23.711.347.5%
12Kanding Township, Pingtung County Map25.411.846.3%
13Yanshui District, Tainan City Map51.823.745.7%
14Xinxing District, Kaohsiung City Map2.51.245.3%
15Qiaotou District, Kaohsiung City Map24.511.145.3%

Biggest jump from 200 to 650 mm

These places look fine in an ordinary downpour and flood almost entirely once rainfall climbs. Several Lanyang Plain townships appear together because the plain is a bowl. The table shows the change in share between the 24-hour 200 mm and 650 mm scenarios.

#Township24h 200 mm24h 650 mm200→650 mm
1Wujie Township, Yilan County Map7.1%66.6%+59.5 pp
2Yilan City, Yilan County Map2.5%57.7%+55.2 pp
3Zhuangwei Township, Yilan County Map10.3%65.4%+55.1 pp
4Qieding District, Kaohsiung City Map5.4%56.6%+51.2 pp
5Liujiao Township, Chiayi County Map3.7%52.9%+49.3 pp
6Hunei District, Kaohsiung City Map1.0%49.3%+48.3 pp
7North District, Hsinchu City Map0.6%47.5%+46.9 pp
8Luodong Township, Yilan County Map5.2%50.8%+45.6 pp
9Kanding Township, Pingtung County Map2.5%46.3%+43.8 pp
10Baozhong Township, Yunlin County Map1.8%44.6%+42.8 pp
11Nanzhou Township, Pingtung County Map2.4%45.2%+42.8 pp
12Yanshui District, Tainan City Map3.2%45.7%+42.4 pp
13Xiushui Township, Changhua County Map0.5%42.1%+41.6 pp
14Qiaotou District, Kaohsiung City Map4.4%45.3%+40.9 pp
15Shuilin Township, Yunlin County Map3.2%43.9%+40.6 pp

Where short intense rain is worse than a long event

The same 350 mm packed into 6 hours floods far more than 350 mm spread over 24. In 166 townships the 6-hour scenario floods at least 1.5 times the area (and more than 1 km²); the table lists the biggest gaps. Most of them drain by gravity and simply cannot keep up.

#Township6h 350 mm km²24h 350 mm km²×
1Yuli Township, Hualien County Map32.55.65.8
2Wufeng District, Taichung City Map16.91.115.4
3Xingang Township, Chiayi County Map20.810.42.0
4Minxiong Township, Chiayi County Map16.47.22.3
5Zhubei City, Hsinchu County Map13.25.62.3
6Houbi District, Tainan City Map14.37.51.9
7Fenglin Township, Hualien County Map12.05.22.3
8Liujiao Township, Chiayi County Map19.112.51.5
9Wuri District, Taichung City Map7.00.416.5
10North District, Hsinchu City Map8.82.33.8
11Shoufeng Township, Hualien County Map9.73.52.8
12Taitung City, Taitung County Map13.88.21.7
13Neipu Township, Pingtung County Map11.56.01.9
14Yizhu Township, Chiayi County Map13.17.81.7
15Ji'an Township, Hualien County Map7.52.23.4

Counties: 6 h 350 mm versus 24 h 350 mm

The same 350 mm over a short and a long duration, by county. Where the ratio is large, drainage is especially sensitive to rainfall intensity.

County6h 350 mm km²24h 350 mm km²6h 350 mm24h 350 mm×
Taipei City3.91.11.4%0.4%3.7
Taichung City61.317.82.7%0.8%3.4
Chiayi City5.01.58.4%2.6%3.3
Hsinchu City14.14.411.2%3.5%3.2
Hualien County87.527.51.9%0.6%3.2
New Taipei City25.411.01.2%0.5%2.3
Nantou County17.78.10.4%0.2%2.2
Hsinchu County36.717.72.6%1.2%2.1
Miaoli County18.08.71.0%0.5%2.1
Pingtung County106.452.23.8%1.9%2.0
Taitung County24.512.00.7%0.3%2.0
Kinmen County3.71.92.0%1.0%2.0
Penghu County1.40.71.0%0.5%2.0
Keelung City1.10.60.8%0.4%2.0
Taoyuan City44.924.73.7%2.0%1.8
Kaohsiung City109.164.13.6%2.1%1.7
Chiayi County164.399.58.4%5.1%1.7
Tainan City180.4109.38.0%4.8%1.7
Changhua County74.946.96.0%3.7%1.6
Yunlin County135.7111.79.7%7.9%1.2
Yilan County61.453.12.8%2.4%1.2
Lienchiang County0.10.10.4%0.2%0.0

Townships with no flooded cell even in the largest scenario

32 townships, all in the mountains. That does not make them safe: these maps simulate flooding, while the mountain hazards are debris flows and landslides, which belong to a different map.

Xinyi Township, Nantou County Map · Lugu Township, Nantou County Map · Dapu Township, Chiayi County Map · Alishan Township, Chiayi County Map · Datong Township, Yilan County Map · Sandimen Township, Pingtung County Map · Laiyi Township, Pingtung County Map · Taiwu Township, Pingtung County Map · Mudan Township, Pingtung County Map · Shizi Township, Pingtung County Map · Liuqiu Township, Pingtung County Map · Majia Township, Pingtung County Map · Wutai Township, Pingtung County Map · Pinglin District, New Taipei City Map · Pingxi District, New Taipei City Map · Wulai District, New Taipei City Map · Shiding District, New Taipei City Map · Wufeng Township, Hsinchu County Map · Jianshi Township, Hsinchu County Map · Central District, Taichung City Map · Heping District, Taichung City Map · Dadu District, Taichung City Map · Nanxi District, Tainan City Map · Tai'an Township, Miaoli County Map · Wuqiu Township, Kinmen County Map · Neimen District, Kaohsiung City Map · Liugui District, Kaohsiung City Map · Shanlin District, Kaohsiung City Map · Taoyuan District, Kaohsiung City Map · Jiaxian District, Kaohsiung City Map · Maolin District, Kaohsiung City Map · Namaxia District, Kaohsiung City Map

Full figures on each township page; method and limits under Source and limitations.

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