SECTION 3: HAZARD IDENTIFICATION AND ANALYSIS This section provides a description of the known flood hazards in Savannah according to best available data. It includes information on the source, frequency, extent and magnitude of various types of known flood hazards as well as a discussion of historically recorded flood events. It also includes the current statistics for insured at-risk properties as made available through the National Flood Insurance Program, as well as for those properties that have filed repetitive flood claims. It concludes with a general discussion on the probability of future flood hazard occurrences for Savannah. Scope of Study Area The flood risk assessment was completed for those areas in Chatham County that are within the incorporated City limits of Savannah. Figure 3.1 provides a base map that illustrates the boundaries of the study area in addition to major transportation routes and aerial photography. The City of Savannah is located within portions of two major river basins: the Ogeechee River Basin and the Savannah River Basin. There is an expansive series of manmade canals and tributaries that help drain the City to the rivers, marshes, sounds and ultimately to the Atlantic Ocean. Figure 3.2 illustrates the various drainage basins for Chatham County. While all of Savannah’s incorporated areas were included in the flood risk assessment (including those recently annexed by the City), the core study area for this plan includes fourteen (14) drainage basins as listed below. Most of Savannah’s reported flood hazard problems are located within these basins, all of which are contiguous to Savannah’s most populated and developed areas. Bilbo Basin Casey North Casey South Chippewa Coffee Bluff Dundee Canal Fell Street Harmon Canal Horseshoe Placentia Springfield North Springfield South Wilshire Windsor Description of Study Area The City of Savannah has a total land area of approximately 97 square miles located in southeastern Georgia, nestled in close proximity to the Savannah River and the Atlantic Ocean. In 2007 the City had an estimated total population of 130,331, with another 118,138 residents living in other areas of Chatham County. The City also hosts millions of visitors every year, with tourism being an active and rapidly growing segment of the economy. The City's attractiveness as a visitor destination is enhanced by its internationally renowned historic district, abundant accommodations and easy accessibility. It is served by several primary highways including Interstates 95 and 16, as well as the Savannah / Hilton Head International Airport. SECTION 3: HAZARD IDENTIFICATION AND ANALYSIS Figure 3.1: Base Map Source: Savannah Area Geographic Information System City of Savannah Flood Hazard Mitigation Plan 3-2 SECTION 3: HAZARD IDENTIFICATION AND ANALYSIS Figure 3.2: Chatham County Drainage Basins Source: Savannah Area Geographic Information System City of Savannah Flood Hazard Mitigation Plan 3-3 SECTION 3: HAZARD IDENTIFICATION AND ANALYSIS The climate in southeastern Georgia is warm and temperate to subtropical. The average temperature in January is 63 degrees Fahrenheit (°F), and is 92°F in July, and the average annual precipitation is approximately 50 inches, with most precipitation occurring in August. While the Atlantic hurricane season lasts from June through November, most of the historic storms that have impacted Savannah occurred during the months of August and September. The City is situated on a low coastal plain with much of its surrounding area consisting of tidal marshes. Elevations range from sea level along the coast to approximately 40 feet in downtown Savannah. The Savannah River (north of City) and the Ogeechee River (south of City) have drainage areas extending far beyond the limits of Savannah and Chatham County. Other streams and creeks have chiefly tidal estuaries within the area and include the Little Ogeechee River, Vernon River, Bear River, Wilmington River, Bull River, and numerous tributaries to these. Main openings to the Atlantic Ocean are Ossabaw Sound and Wassaw Sound, both of which are wide and deep. Description of Known Flood Hazards Flooding is the most frequent and costly natural hazard in the United States, a hazard that has caused more than 10,000 deaths since 1900. Nearly 90 percent of presidential disaster declarations result from natural events where flooding was a major component – including the two historical presidential disaster declarations for the City of Savannah and Chatham County, Georgia (1994 and 1998). As disastrous as it can be, flooding is part of the earth’s natural hydrologic cycle. This cycle constantly circulates water throughout the environment and helps maintain an overall balance between water in the air, on the surface and in the ground. Sometimes however this system has temporary imbalances that send more water to an area than it can normally handle. This situation results in flooding. Throughout its long history, the City of Savannah has always been prone to flood hazards. Historical data show that structural flooding has been a regular and costly occurrence in Savannah for more than a century. In addition, the City has seen the frequent flooding of streets and intersections which in turn impedes traffic flow and causes significant interruptions to mobility and commerce. Either way, when flooding does occur in Savannah, the community experiences significant social, economic and environmental impacts that are deemed unacceptable to the City. The cause of flooding in Savannah can be attributed to a number of factors, including its low elevation, relatively flat terrain, close proximity to the coast, 6 to 9 foot tides, and an abundance of rivers, creeks and streams that surround and traverse the City. Another contributing factor is the large amount of developed and impervious land throughout the City which limits ground absorption and increases surface water runoff. Early in its history, the City constructed a series of canals to convey stormwater. Without these canals, development in many parts of the City would not have been possible, and over the years, these canals have been supplemented by additional drainage improvements of varying capacities to help alleviate potential flooding. As a result, flood hazard mitigation has long remained a top priority for the City of Savannah as is evident by numerous engineering studies, task force reports and drainage improvements throughout the last 20 years. This includes the preparation of the 1987 Drainage Improvement Task Force Report and the 1994 Flood Mitigation Capital Improvements Plan. While many City of Savannah Flood Hazard Mitigation Plan 3-4 SECTION 3: HAZARD IDENTIFICATION AND ANALYSIS recurring flood problems have been solved, there remains a variety of known flood hazards for the City to continue addressing in its sustained mitigation planning efforts. Sources and Types of Flooding In Savannah, flood hazards are generally the result of excessive precipitation, but for planning purposes they have been classified under three unique categories: (1) riverine flood; (2) flash flood; and (3) coastal flood. Each of these different types of flood hazards is further described below. Riverine Flood Riverine flood hazards are characterized by the slow rise of water levels as the result of heavy precipitation over a given watershed for a long period of time. When the actual amount of river or stream flow is larger than the amount that the channel or basin can hold, the water will overflow its banks and flood the adjacent low-lying areas called floodplains. Riverine floods typically develop slowly and may last for several days or even weeks. Riverine floods are described in terms of their extent (including the horizontal area affected and the vertical depth of floodwater) and the related probability of occurrence. Flood studies use detailed hydrological and hydraulic analyses to determine the extent and historical records to determine the probability of occurrence for different extents of flooding. The probability of occurrence is expressed as the percentage chance that a flood of a specific extent will occur in any given year. The severity of riverine flooding is typically determined by a combination of several major factors, including: duration and intensity of precipitation; stream and river basin topography and physiography; recent soil moisture conditions; the degree of vegetative clearing and impervious surface; and the capacity of existing stormwater control facilities. The City of Savannah lies immediately along the banks of the tidally-influenced Savannah River in addition to Wassaw and Ossabaw sounds, with numerous tributaries and manmade canals running throughout its jurisdiction that are very susceptible to overflowing their banks during and following excessive precipitation and coastal storm events. While flash flooding caused by surface water runoff is certainly more common in Savannah, the less frequent but large-scale riverine flood event (such as the “100-year flood”) will cause significantly more damage and economic disruption for the area. Savannah’s floodplains, the areas at most risk to such events, have been studied and mapped by the Federal Emergency Management Agency (FEMA) as special flood hazard areas. Flash Flood Flash floods, as the name suggests, occur suddenly after a brief but intense and concentrated downpour. The most destructive and dangerous of flash floods occur in areas with steep slopes and narrow stream valleys. However, flash flooding