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St. Mary's County HIRA

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Click on the document above to review the HIRA completed for the St. Mary's County.


Hazard Identification & Risk Assessment 

As part of the plan update process for St. Mary’s County, Maryland, a Hazard Identification Risk Assessment (HIRA) has been completed for the County.


Twelve (12) natural hazards have been identified and a hazard risk has been assigned to each. Only natural hazards are included in this assessment as they lend themselves better to data collection related to geographic extent than technological and man-made hazards. 

Hurricane, Tropical Storms & Storm Surge



Although St. Mary’s County has not been directly hit by a hurricane, it is very vulnerable to one, by virtue of being a peninsula. The county is subject to the wind and flooding effects from hurricanes that hit the east coast and travel inland.

Sea Level Rise



Maryland has 3,100 miles of tidal shoreline and low-lying rural and urban lands that will be impacted.  The experts’ best estimate for the amount of sea level rise in 2050 is 1.4 feet. It is unlikely to be less than 0.9 feet or greater than 2.1 feet.

Shoreline Erosion



St Mary’s County has 534 miles of shoreline, which less than 7% of the has any significant risk for erosion. Some of the impacts from shoreline erosion include the direct loss of land and its economic, cultural, and ecological values as well as the offsite impacts caused by increased sediment.

Winter Storm



All areas of St. Mary’s County are subject to the effects of winter storms. These storms may include snow, freezing rain, sleet, and extreme cold. Major winter storms and occasional blizzard conditions bring bursts of heavy snow accumulating 3-6 inches in short periods or 1-2 feet in 12-24 hours.




FEMA designated floodplains results in a high level of vulnerability to flood hazards. Given the large number of people that can be affected by flooding, high economic costs and moderate response costs, the vulnerability to flooding is high in St. Mary’s County.




The primary hazard caused by wind is the transport of debris, which can cause casualties and property loss or even the dislodging of manufactured homes from their foundations or vehicles.




Tornadoes have occurred in St. Mary’s County in the past and are expected to occur in the future. Tornadoes often result in buildings with missing roofs, uprooted road signs, fallen powerlines and trees, destroyed homes and water towers, and damaged cars.

Drought & Extreme Heat



Problems of drought can affect St. Mary’s County with implications for the availability of water for agricultural, industrial, and household uses, as well as, recreational purposes such as boating and fishing.

Emerging Infectious Disease


Emerging Infectious Diseases can be considered as part of a broad hazard category that could be termed “public health emergencies.”   In addition to disease epidemics, such events can take the form of large scale incidents of food or water contamination, infestations of disease bearing insects or rodents, or extended periods without adequate water or sewer service.




According to the DNR, the urban- wildland interface fire threat potential to the St. Mary’s County forestlands is considered very high, due to the pressure to develop large tracts of open land. The probability of wildfires in St. Mary’s County would also be tied to periods of prolonged drought when forests are more vulnerable to ignite from lightning strikes or human carelessness or arson.




The thunderstorm hazard includes lightning and hail events. Impacts from severe storms have been moderate, with localized flooding occurring from severe thunderstorms, minor damages from high wind events, and power and transportation disruptions from winter storms. The impact from hail and lightning has been limited to minor damages at specific locations. Severe storms could have a major economic impact on St. Mary’s County when utility systems, including electricity, are disrupted for an extended period.

Dam Failure



A dam failure is simply an uncontrolled release of water from a reservoir through a dam as a result of structural failures or deficiencies in the dam.  There is one dam (St. Mary’s River Watershed Dam, Site 1, on the western branch of the St. Mary’s River) located just west of Great Mills with a high downstream hazard rating.  A dam with a high downstream hazard rating means there is a potential loss of life or property damage downstream due to flood waters being released or structure failure.

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