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Reducing Physical DamageMitigating Flood and Storm Water Runoff Protecting a facility from the negative effects of flooding is not always as simple as merely locating it outside known flood zones; inappropriate site layout and building design can create a storm water runoff flood exposure anywhere.Flooding can be caused by bodies of water (rivers, streams, oceans, bays, lakes, canals, etc.) overflowing their normal boundaries, or as the result of storm water runoff accumulating in normally dry areas. Off-site flooding can block access routes to and from the site, as well as interrupt vital utilities. If a facility is located within a known flood zone, the challenge of managing the flood risk is greatly increased. The goals, according to FM Global’s Property Loss Prevention Data Sheet on Storm Water Runoff and Terrain Management, then become to ensure that: (1) Operations can continue without interruption, and (2) The facility suffers the least possible amount of physical damage. These goals can be achieved by developing a flood-mitigation strategy throughout the facility that address overall layout, electrical and mechanical systems, and vital utilities, and applying it during all phases of the site’s lifespan, including design and construction. While instituting the flood-mitigation strategy is most effective during the design and construction phases, practical mitigation solutions included in FM’s data sheet can be very effective for existing locations. Site Selection Proper site selection is the best solution for avoiding the effects of flooding. Selecting the correct site is far less difficult than designing a facility located in a flood zone to resist the effects of flooding. Select a location where the entire site and all access routes (highway, marine, railroad, etc.) are outside 500-year flood zones (by both elevation and footprint). Verify flood studies for the selected site are up-to-date by having a qualified hydrologist review the study and recent flood data.Select a building site that is at least 2 ft (0.6 m) higher than the predicted 500-year flood elevation and at least 500 ft (152 m) from direct wave impacts and or high flood-flow velocities. Select a site that is not in an area protected by a levee or other man-made flood control works. Ensure electrical and communication services, drinking and process water, wastewater treatment, steam supplies, etc. obtained from off-site locations will remain unaffected during flooding in their area. If this cannot be ensured, establish alternative sources for backup. Proper design of the facility’s storm water management system is needed for all locations to ensure a flood exposure isn’t created or flooding made worse by the layout, grading, storm-water management system, etc. Ensure drainage systems are sized to handle a 100-year rainfall event without causing property damage. The duration of the 100-year rainfall should be the storm that causes the most intense runoff across the site. Both sub-surface routing (drainage piping) and overland flow can be used to direct the runoff. Ensure the facility’s storm-water management system uses grading sufficient to route predicted rain-water and snow-melt runoff away from buildings, outside storage, and equipment. Ensure water runoff originating from offsite areas is included in the facility’s site water management plan. If on-site ponding or storm water routing is essential, ensure it is arranged so water will not enter or come into contact with buildings, outside storage, or equipment. Use customary drainage design features to limit soil erosion and avoid excessively high flow velocities. Use grates, trash racks, curbs, etc. to protect the inlet to all drains and storm-water drainage systems against debris blockage. Do not use landscaping materials, such as wood chips, pine needles, etc, that can be easily dislodged by rain water. They may obstruct or clog drainage systems, catch basins, culverts, or overland flow patterns. Do not locate buildings, outside storage, or fire protection equipment within natural storm water drainage flow paths such as small streams or swales. Ensure walls, fences, and landscaping do not direct water on to buildings, outside storage, or fire protection equipment. Install backflow preventers equipped with manual shut-off valves on each side of the backflow preventers on effluent-discharge lines that connect to combined sewer systems (wastewater and storm-water runoff) and any other areas that have a history of backups. Additionally, provide a backflow preventer by-pass line and normally closed shut-off valve to allow for maintenance. If it is not possible to comply with these recommendations, the risk of flooding may be greatly reduced by building up land levels so they are 2 ft (0.6 m) higher than the predicted 500-year flood elevation. If this can-not be avoided, provide erosion protection designed by a qualified engineer. Design fill material to be stable when exposed to flood action, including rapid rise and drawdown, prolonged inundation, scour, and erosion. Ensure the facility and grounds are designed by a qualified registered civil or structural engineer with previous experience in flood-related loading and geotechnical conditions. Ensure the geotechnical properties used for the foundation design (e.g., bearing and frictional resistance, active and passive pressure, and settlement) are based on diminished structural capacities that are associated with flood level and floodwater action. Elevating Individual Buildings and Key Equipment If it is not possible to comply with the above, the risk of flooding to specific buildings and equipment may be greatly reduced by building them 2 ft (0.6 m) higher than the predicted 500-year flood elevation. Design buildings, outside storage areas and equipment (whether owned by the facility or a utility company) to be at least 2 ft (0.6 m) above the 500-year flood level, by using raised foundations or elevated structures. Design foundations, buildings, and outdoor structures to withstand the predicted 500-year flood elevation plus 2 ft (0.6 m), and to resist erosion from high water-velocity. Do not build foundations in areas subject to high or moderate velocity flows. If this cannot be avoided, provide erosion protection designed by a qualified engineer. Do not narrow, re-route, or change the on-site watercourse. Design and build structures to adequately resist all flood-related loads and conditions, including hydrostatic loads, hydrodynamic loads, breaking wave action, debris impact, ice floes, ice and debris jams, rapid rise and drawdown of floodwaters, prolonged inundation, soil liquefaction, soil consolidation and subsistence, sediment deposition, mudslides, and wave-induced and flood-related erosion and scour. Consider long-term erosion over the design life of the structure when determining the effects of flooding on building and foundation design. Ensure design considerations also account for other applicable loads (e.g., gravity and wind) that will act on the structure concurrently with the flood. Consider all appropriate load combinations when analyzing flood loads for actions, including overturning, sliding, undermining (erosion and scour), and uplift (buoyant forces). Use load combinations, load factors, and resistance factors as specified in the governing model codes and standards. Where local codes do not specify load combinations with flood loads, use load combinations from the most recent editions of ASCE 7 or the International Building Code (IBC). However, in no case use flood load factors of less than 1.3 in strength design or 1.0 in allowable stress design. Retain a qualified registered civil/structural engineer with previous experience in flood-related loading and flood-related geotechnical conditions to design buildings, structures, and protective works (e.g., flood walls, retaining walls, bulkheads, levees, dams, channels, and diversions). FSM
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