St. Mary's County Multi-Jurisdictional Hazard Mitigation Plan

Chapter 2: Hazard Identification
SMC Multi-jurisdictional Hazard Mitigation Plan
The Special Flood Hazard Area (SFHA) is a convenient tool for assessing vulnerability and risk
in flood-prone communities because many communities have maps available that show the
extent of the base flood and likely depths that will be experienced. The base flood is often
referred to as the “100-year flood” By lay persons and officially as the ” one percent annual
chance flood.” Experiencing a 100-year flood does not
Table 4.8 - Flood Probability Terms
mean a similar flood cannot happen for the next 99
years; rather, it reflects the probability that, over a long Flood
Chance of occurrence
period of time, a flood of that magnitude have a 1% Recurrence in any given year
chance of occurring in any given year. Statically
10-year 10%
smaller floods occur more often than larger and more
50-year 2%
widespread ones. Table 4.8 shows a range of flood
100-year 1%
recurrence intervals and their probabilities of
500-year 0.20%
occurrence. So every year, a 10-year flood has a
greater likelihood of occurring (10% chance) than a 100-year flood (1% chance).
Historic Activity
25 flood events were reported in St. Mary’s County, Maryland between January 1994 and
December 2010. A few of the recent major flood events are discussed below.
• On 4 February 1998, a powerful nor'easter, carrying copious moisture from the Gulf of
Mexico and Caribbean region, dumped between 2 and 4 inches of rain across much of
Maryland between the foothills of western Maryland and the Chesapeake Bay. Highest
totals of rainfall, ranging from 3 to 5 inches, fell in lower southern Maryland, causing
widespread flooding of low lying areas and small streams and creeks. The nor’easter,
coming on the heels of one just a week earlier, caused tides of 3 to 4 feet above normal
from the Calvert County/Anne Arundel County line south to Point Lookout in extreme
southeastern St Mary’s County; and along the lower tidal Potomac River along the
Charles and St Mary's Counties shoreline, including Cobb Island and St George Island.
A daily rainfall record was broken at Baltimore/Washington International Airport (BWI);
the 1.65 inches that fell on the 4th broke the 78 year-old Baltimore area mark of 1.48
inches. Flooding was most pronounced in St. Mary's County. During the peak of the
storm, 26 roads were closed due the combination of wind and rain. Nine roads were
closed due to flooding alone. State thoroughfares affected included Point Lookout Road
(route 5),and Routes 237, 238, 243, and 271. Evacuations were initiated in Great Mills
and on St. George Island due to rapid increases in tide levels. At least 200 residents
were evacuated, including one 3 year-old boy who required a water rescue. Four fire
fighters were treated for hypothermia at St. Mary's Hospital. Some cars were nearly
submerged in low-lying areas. In the Golden Beach neighborhood, Lake White spilled
over its banks, and poor drainage contributed to widespread shallow flooding . The dam
at St Mary's Lake held; overflow problems were minimal. The sewage system serving
Lexington Park failed due to the abnormally heavy flow of water which caused manholes
to flood. The combination of BG&E, PEPCO, and SMECO reported nearly 15,000
customers without power at the height of the storm. Four thousand SMECO customers
were affected in St. Mary's County. Property damage was estimated at $650,000.
• The eye of Hurricane Floyd passed east of the Chesapeake Bay between 9:00 a.m. and
midnight on 16 September 1999. Gusty winds of 30 to 50 miles per hour blew across the
area between 11 AM and midnight on the 16th, with localized wind gusts over 50 mph
near the Chesapeake Bay. Hundreds of trees and power lines were downed and over
500,000 SMECO customers lost electricity. Between 8 and 12 inches fell across St.
Mary's, Anne Arundel, and Harford Counties. The amount of damage Anne Arundel,
Calvert, Charles, Harford, and St. Mary's Counties received from the storm qualified
35