Flood Design Criteria
1. Flood Design Considerations
Design criteria of flood prevention schemes vary in different cities in respect to their unique characteristics. For example, flood prevention measures were set up in Osaka, Japan to prevent flooding during high tides in Osaka Harbor; in Washington D.C., against overflow from the Potomac River and high tides in Chesapeake Bay; and in Zurich, Switzerland, against the overflow of lakes and rivers. Taipei MRT enhanced systemwide flood protection on its exclusive right-of-way and underground routes by protecting against the flooding caused by rainstorms and typhoons. After conducting a comprehensive evaluation – covering the functions and lifespan of MRT facilities, the risk to safety, engineering economics, etc. – Taipei MRT adopted an estimated flood level for a return period of 200 years plus an additional 50 cm. All entrances to stations, structural openings and depots were designed to prevent water intrusion to this level.
2. Flood Prevention Facilities
The most economical and efficient way of meeting the above criteria is to elevate the level of entrances to prevent surface water from entering underground facilities. However, passengers will be inconvenienced if the level is too high. In places where entrances are of insufficient height to prevent flooding due to convenience concerns, the addition of floodgates or full-dimension waterproof gates may be necessary. Early flood prevention facilities for entrances on the Taipei MRT could be summarized as follows:
(1) All entrances shall be a minimum of 15 cm above the flood level with a return period of 100 years and shall be a minimum of 60 to 120 cm above the adjacent ground level.
(2) All entrances shall have floodgates that reach 50 cm above the flood level for a return period of 200 years.
3. Upgrading Flood Protection Criteria
After flooding caused by Typhoon Nari in 2001, an overall review of flood prevention systems for the Taipei MRT was carried out and enhancement measures were adopted as follows:
(1) The flood prevention standard of the lines built after the initial network—Xinzhuang line, Luzhou line, Tucheng line, Neihu line (part of the Wenshan-Neihu line), Nangang eastern extension, Banqiao line BL1&BL2, etc.—was set at a return period of 200 years plus an additional 110 cm, or above the level of flooding recorded during Typhoon Nari.
(2) Watertight doors shall be considered for installation on sidewalks connecting to the Taipei MRT, in joint development buildings and at underground connections with other transportation facilities.
(3) Gaps between vital electrical & mechanical facility rooms and conduits & pipes shall be filled with watertight materials.
(4) Assessment of the installation of full-dimension floodgates guarding daylighting facilities, transfer stations with tunnel interfaces and on both sides where lines cross rivers or faults. These included two floodgates at Station O8, two on the crossover of the Xinzhuang and Luzhou lines, two at Station O6, two on the daylighting section of Luzhou Depot, one on the daylighting section of Tucheng Depot, two at Station G17, two at Station G22, two at Station BL17, and eight at Station O14/R10.
(5) Existing flood prevention facilities shall be improved and floodgates heightened at stations on the initial network, in accordance with the flooding height recorded during Typhoon Nari.