Discussion
Flooding Source: Storm Events
- Can include hurricanes and tropical storms/depressions, which may flood coastal areas due to a combination of tidal surge, wave action associated with high winds, and rainfall of significant intensity and/or duration.
- Can flood non-tidal areas due to rainfall of significant intensity and/or duration swelling rivers, streams and creeks, overwhelming drainage systems and culvert/bridge openings, and sweeping debris into waterways which can block flow paths.
- Non-hurricanes or tropical storms/depressions that deliver significant rainfall that can overwhelm waterways and drainage systems. Rapid snow or ice buildup from severe snow storms can also result in flooding.
Risk and Vulnerability
- Transit facility infrastructure including rail lines, stations, and other facilities should be evaluated for risk and vulnerability to possible flood events that could occur. This analysis should:
- Identify any past experiences with flooding events including the characteristics of the storm, root cause of the flooding, resulting damage caused by the flood with the cost to repair, and impact to service, if any.
- Include a hydrologic/hydraulic engineering study of the range of possible storm events and the elevation of resulting flood waters.
- Develop a cost/benefit analysis complying with all applicable federal, State, and local regulations to determine the appropriate level of flood mitigation for a location.
- Underground facilities, which will retain flood waters until they can be pumped to a safe outfall.
Flood Mitigation
- May consist of permanent structural measures to prevent flood waters from entering or otherwise impacting a transit facility, temporary measures to prevent flood waters from entering a facility, and/or operational procedures to minimize the impact of flood events. Flood mitigation also includes measures to ensure the ability of a facility to withstand a flood event without damage. Where designing permanent or temporary flood mitigation measures are not feasible, operational procedures should be established to close facilities in advance of expected flooding conditions.
- In all cases, proper drainage system maintenance must be provided to ensure all flood mitigation measures operate as designed when the flood event occurs.
- Key flooding mitigation measures:
- Establish track profile, where feasible, such that bottom of rail is safely above flood elevation developed in risk and vulnerability analysis (i.e. design flood elevation) and ensure embankments inundated by these flood waters are sufficiently armored.
- Incorporate materials able to withstand inundation up to the design flood elevation. These include mold resistant materials and coatings/additives to resist salt-water intrusion, where applicable.
- Seal electrical service entries
- At tunnel portals, establish track profile to create a physical barrier against inundation of floodwaters into the tunnel.
- Regrade or provide ramps and steps so that entrances and exterior doors are at or above the design flood elevation.
- Design ventilation grates for underground structures to be above design flood elevation.
- Provide:
- Structural configuration or protection elements that are a part of the overall station/facility construction that serves as a dam or barrier against flooding.
- Waterproofing non-essential spaces at ground level that allows them to take on floodwaters without that water migrating to other spaces within the station or otherwise concentrating and/or redirecting flows inconsistent with pre-construction drainage paths.
- Configure exit stairs so that the final run of each stair is from an elevated landing down to the street to allow the final run to be flooded without floodwater migrating to other spaces within the station or otherwise concentrating and/or redirecting flows inconsistent with pre-construction drainage paths
- Flood gates (for vertical openings), hatches (for horizontal openings), and elevator doors that can withstand the pressure of flooding and to limit water infiltration to a rate that can be removed by the station or tunnel drainage systems.
- Check valves at ejector pump discharges that connect to public storm drain systems to prevent flood waters from back-flowing into underground spaces.
- Backwater valves for house drains connecting to plumbing fixtures that are above street sewer level, but below the design flood elevation.
- Mobile pumping capacity before and during extreme events;
- Temporary measures such as sandbags.
- Adequately size tunnel/station ground water and storm water pumping systems to handle flood conditions where feasible.
- Locate Critical Transit Facilities (such as Police or OCC) in areas not subject to flooding;
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