seems the bridge is near the mouth of the Tamsui River, the river that flows to Taipei ?
according to Gemini Ai :
"This is the most critical reason. The Tamsui River is the direct, aquatic highway straight into the heart of Taipei (Taiwan's capital).
In a conflict scenario, the river mouth is a prime target for an amphibious invasion. By keeping the Danjiang Bridge low to the water, Taiwan's military ensures that large enemy warships, landing craft, or captured commercial vessels cannot sail up the river to drop troops off in the middle of the capital. It acts as a permanent, physical barrier."