Chapter 3 FLOODING
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Chapter 3 FLOODING There are 22 watersheds or drainage basins with approximately 4,000 miles of rivers and creeks and over 1,900 inland lakes in Marquette County. Over 70 miles of Lake Superior shoreline attracts seasonal or year-round residential development. Combined with heavy snow accumulations (average annual snowfall 184.7 inches) and a mean annual rainfall of 25 inches, the County of Marquette is in a prime position for flood related problems. Since 1985, Marquette County has recorded over 450 instances of flood damaged properties, with water ranging from less than an inch in some basements to 3 feet above the second floor level. Unfortunately, structures were not consistently identified and only sixteen properties have confirmed repeat damage. The vast majority of damaged structures are single-family residences, although a few rental units and commercial buildings are included. Ninety percent of federally declared disasters involve some type of flooding. Throughout the last century, flooding has been one of the most costly forms of disaster both in human casualties and in cost. Flooding can be of four types: riverine, urban, dam breach and shoreline wave action/erosion. The primary hazards associated with flooding occur due to contact with water. Secondary effects are disruption of services and the economy and impacts upon public health and the environment. 3.1 DAMAGE POTENTIAL Contact with water can cause injury and physical damage due to pressure or saturation. Upholstered furniture, drywall or insulation that becomes saturated, is generally ruined. Electrical systems are often damaged beyond repair. Damage to automobiles can extend beyond wet upholstery and shorted electrical systems. Water does not compress like the mixture of gas and air in the cylinders. Running a car that has water in a cylinder can cause the engine block to crack. With higher velocities associated with flooding, water carries a greater load of suspended particles. Velocities are frequently great enough to carry large particles such as trees and houses. Six inches of fast moving water can move a passenger car. Floating objects can cause serious injury or destruction to anything in their path. Even with the scouring by smaller particles of sand and stone, considerable damage can be done. Erosion can be extensive and undermine structures and roads, resulting in their collapse. Pressure of water soaked soil on an “empty” basement can cause the walls to cave in. When velocity finally is reduced, the suspended particles settle. The deposits of gravel, sand, mud, and ooze can be thick. They are generally difficult to remove. Debris is often tangled with vegetation. In addition, the residue is frequently contaminated with wastes and other materials causing a secondary hazard. 3-1 Water in fields can literally drown crops that are not washed away. Deposits of sediment can also be destructive. Livestock, pets, wildlife, and humans are often vulnerable to flooding. They can be carried away or drown. Secondary hazards are the pollution of drinking water sources, particularly when wells, septic tanks, and sewage treatment facilities are flooded. Gas, electric, cable, and phone service may be disrupted. Broken lines create hazards of electrocution or explosion. Roads may be closed or unstable. This could affect delivery of needed services and products or delay clean up. Tertiary effects may include permanent changing of river channels and shoreline, destruction of wildlife habitat or usefulness of land, closure of businesses and loss of jobs, and financial hardship due to repair and replacement expenses. The main causes of flooding in the Upper Peninsula are unusually heavy rainfall and rapid melting of snow or ice accumulations. By monitoring of weather and ice/snow accumulations, short-term flood predictions are possible. Flash floods can occur within a short time (usually a maximum of six hours) following excessive rainfall, a rapid snowmelt, a dam or dike failure, or sudden release of water by an ice jam. The two key elements are the intensity of water release and the duration. Topography, soil conditions, and ground cover also play an important role. Flash floods can also produce secondary hazards of mudslides and subsidence of soil. Because there is frequently no warning, the waters can be deadly. Flash flooding can occur in narrow gullies and river basins, in shoreline areas following severe storms and in urbanized areas. Impervious surfaces increase run-off two to six times over that occurring on natural land. Streets can become swift moving rivers and basements can become traps as they fill with water. The north and western portion of Marquette County are most susceptible to flash flooding. This is due to the “younger” age of the rivers, with narrower channels and steeper gradients, rock outcrops and impermeability of soils. Flash flooding incidents occurred in 1996 when unseasonably warm weather caused rapid snowmelt. At that time, a number of culverts were washed out near the Hoist Basin on the Dead River. No injuries were reported. 3.2 FLOODPLAINS AND NATIONAL FLOOD INSURANCE PROGRAM The inundation area outside the normal boundary of the water body is the floodplain. Flood frequencies can be estimated by studying the yearly maximum discharge of a stream from a gauging station over a long period. By plotting and projecting the discharges over time, the chance of flooding exceeding a specified level can be estimated and maps can be generated that identify the area that would be inundated under those conditions. Using topographic data and historical information regarding river stages, maps can be constructed to show areas that are expected to be covered with water for various discharges. The severity of flooding is referred to by the probable frequency of that level of flooding (i.e. a 10- year, 50-year, and 100-year or 500-year flood). In 3-2 actuality however, it predicts the chance of receiving that level of water in any given year. A 100-year flood has a 1 in 100 chance of occurring each year. 100-year floods can (and have) been known to occur in consecutive years. On the average, one or two 200-year floods occur each year in the Upper Peninsula. Flood hazard maps are created to show areas that are susceptible to flooding when the discharge of a stream exceeds the full-bank level. As construction occurs within the drainage basin, it modifies detention areas, channels, rates of flow, run-off paths, vegetation and/or soil absorption rates. Floodplains can change dramatically. Unfortunately, maps are static. In Michigan, the average age of floodplain maps is over 16 years. The Federal Emergency Management Agency hopes to reduce that average to 5 years through upgrades of existing maps and creation of new ones for previously unmapped areas. Unfortunately, most streams in Marquette County do not have gauging stations, and many of the floodplains in Marquette County are unmapped. As part of the development of this plan, existing floodplains maps were scanned and rectified to be compatible with the County’s GIS. This allows comparison with tax roll parcels and to estimate potential property damage and threats to humans. Before 1968, most property owners were unable to purchase insurance coverage for flood related damage. The potential for enormous claims made private insurance companies reluctant to offer this type of coverage. In 1968 Congress created the National Flood Insurance Program (NFIP). Privately issued flood insurance, subsidized by the federal government, became available within communities that agreed to manage flood prone areas within their boundaries. In addition to coverage of the structure(s), coverage of contents is also available. To become eligible, the community enters an “emergency phase” by adopting preliminary actions to reduce flood threat. The coverage in the “emergency phase” is limited. After the Federal Emergency Management Authority conducts detailed flood studies, and the community enacts stringent measures to protect life and property from future flooding, the community qualifies for the regular program phase. At that time, much higher levels of insurance become available. In the late 1990’s, Increased Cost of Compliance coverage became mandatory for most new policies. Policy holders in special flood hazard areas can get up to $15,000 to help pay the costs to bring their home or business into compliance with their community’s floodplain ordinance. Flood insurance policy holders have four compliance options with the community’s floodplain management ordinance: 1. Increase elevation: This raises the home or business above the flood elevation level adopted by your community. 2. Relocation: this moves the home or business out of the floodplain. 3. Demolition: this tears down and removes flood-damaged buildings. 4. Flood proofing: This option is available only for non-residential buildings and certain residential buildings in communities certified by FEMA. It involves 3-3 making a building watertight through a combination of adjustments or additional features that reduce the potential for flood damage. Some preventive measures to reduce flood damage to an insured building are reimbursable. Policyholders may also be eligible for cost of removal and storage of insured contents when a building has been declared in imminent danger of flooding by officials. However, it is important to know that trying to obtain coverage after a declaration of imminent danger may leave property exposed to loss. There is a 5-day waiting period for policies to be come effective. Flood insurance is available only in communities that participate in the National Flood Insurance Program. Cities and Townships in Marquette County that are participating in the NFIP (Figure 3-1) are: 1. Chocolay Township 2. Ely Township 3. Ewing Township 4. City of Marquette 5. Marquette Township 6. Powell Township 7. Republic Township 8. Skandia Township 9. West Branch Township Participating communities must adopt and enforce a floodplain management ordinance that meets NFIP requirements.