Inland or riverine flooding often occurs after heavy rain. Inland flooding accounts for more than 50 percent of hurricane-related deaths each year.(1) Flash flooding commonly occurs during the spring snowmelt, during which water levels are higher in freshwater bodies and soils are saturated. Other potential flooding events can occur in the spring, when frozen ground conditions contribute to low rainfall infiltration and high runoff events. Loss of natural coastal buffers and higher groundwater levels can also contribute to inland flooding. Existing infrastructure can also compound flood concerns. Flooding can be exacerbated in urban areas, where impervious surface covers (e.g. roads) and highly compacted soils prevent proper drainage and increase runoff into water bodies. Furthermore, heavy precipitation can overburden storm management systems, further intensifying flooding. Tropical storms and hurricanes can generate high water runoff when soil infiltration rates are exceeded, leading to river flooding.
The term "100-year flood" is commonly used to describe a flood with great magnitude. According to the United States Geological Survey (USGS), the term is used to "simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year".(2) In other words, there is a 1-in-100 chance that a flood of large magnitude will be equal or exceeded within a given year. Similarly, the term "500-year flood" refers to much larger magnitude flooding that has a 1-in-500 chance of occurring in any given year. The Federal Emergency Management Agency (FEMA) recently provided designations of where 1-in-100 and 1-in-500 chance floods may occur. A flood hazard map for Massachusetts depicting FEMA designated 1-in-100 and 1-in-500 chance of flooding annually can be found here.
Increases in sea level will impact inland flooding. As discussed in the Sea Level Rise Climate Profile, the frequency of the 100-year flood events is predicted to increase significantly in response to sea level rise. Evidence also suggests that Boston could experience the current 100-year flood every two to three years on average by 2050 and every one to two years by 2100. Massachusetts is also predicted to experience an increase in frequency of high precipitation events as well as more intense tropical storm and hurricanes (see Severe Weather Events Climate Profile); both these conditions can result in increased flooding potential. Areas of Western Massachusetts characterized by high slopes and minimal soil cover are particularly susceptible to flash flooding caused by rapid runoff that occurs in heavy precipitation events. Flooding can impact public health and safety in a number of ways. Prior to and during a flood event, individuals may have difficulty relocating from an impacted area. Immediately following flood events, residents may be displaced and lack access to necessary services, critical services may be disrupted or damaged. Spread of infectious diseases may also be of concern. Longer-term health impacts may include stress, complications to chronic health conditions, and infrastructure needs.