Terrain and tidal information should be consider if the sluice gates should be opened when heavy rain causes flooding

Heavy rains in Tainan starting in the early morning of August 2, 2025, caused severe flooding. Residents of Wenshan Village in Beimen District were complaining about the government's decision not to open the sluice gates to release water into the sea. The Tainan City Water Resources Bureau stated that the sluice gates had just been completed and had not yet been inspected, and that high tides could cause seawater backflow.

Let's examine why the sluice gates shouldn't be opened.

1. This is an overlay of a contour map and orthophoto from the Ministry of the Interior's National Land Surveying and Mapping Center. Fig. 1 shows that Tainan's west coast is low-lying, with many elevations at 0 meters above sea level. Fig. 2 takes a closer look at Beimen District. Wenshan Village in Beimen District is not high. Considering the location of the township's houses, the road elevation indicated in the red box is about 1 meter, meaning that much of this area is actually between 0 and 1 meters above sea level.

2. After examining the terrain elevations, consider the day's tide information. The tide forecast provides the approximate times of high and low tides, allowing you to anticipate potential conditions. However, let's use the measured data in Fig. 3 to examine the tide. The tide height at 4:00 AM on August 2, 2025, could reach 1.05 m, and at 6:00 PM on August 2, 2025, it could reach 0.83 m.

The tidal height is comparable to the terrain here, and may even be higher. If the sluice gates were opened, seawater would inevitably flow back in. This would first submerge areas with an elevation of 0 m, then flood farmland, and finally move toward towns. Not only would buildings be submerged in seawater, but farmland would also be submerged, which would ruin all farmland. I'm not sure the critics understand what they're asking for. Do they want all villagers to be submerged in rainwater and seawater, and all farmland to be destroyed?

8/2/2025 臺南豪大雨造成大淹水，就有北門文山里的人在那裏大罵政府為什麼不開水閘門讓水排到海裡。臺南市水利局表示該閘門才剛完工還沒驗收，而且遇到漲潮會造成海水倒灌。

讓我們來看看為什麼不應該開閘門。

1. 這是內政部國土測繪中心的等高線圖和正射影像的疊圖，Fig. 1 顯示臺南西海岸地勢低窪，很多高程都是海拔高 0 m。再來用 Fig. 2 細看北門區，北門區文山里的地勢並不高，以鄉鎮房屋所在地來看，紅框裡面標示的路面高程是 1 m，也就是說這附近其實很多都是介於 0~1 m 之間的海拔高度。

2. 看完地勢高程，再來看當天的潮汐資訊，在潮汐預報上可知當天的高低潮的大約時間，事先就可以知道可能的情況。不過這裡用 Fig. 3 的實測資料來看一下，8/2/2025 凌晨 4:00 的潮高可以到 1.05 m，8/2/2025 18:00 的時候潮高維 0.83 m。

潮水高度和這裡的地勢差不多，甚至有可能更高。如果開了水閘門，海水一定會倒灌進來，海水倒灌進來，先是將那些高程為 0 m 的地方都淹沒，接著就會向農地及鄉鎮淹過來，除了建物要泡海水，農地也會泡在海水裡面，農地泡了海水就廢了。我不確定批評的人到底知不知道他在要求什麼，難道他希望所有人鄉民除了泡雨水還要泡海水，然後所有的農地全部毀掉嗎？

Fig. 1. 臺南沿海一代高程(m, AMSL) (source: 內政部國土測繪中心)

Fig. 2. 臺南北門區文山里鄉鎮區高程(m, AMSL) (source: 內政部國土測繪中心)

Fig. 3. 交通部氣象署提供的潮高資訊