Floodplains generally represent the broad, relatively flat areas directly adjacent to the rivers. These areas periodically flood during high flow events and serve to buffer against flooding problems downstream. Floodplains are an integral part of river health as they absorb flood waters, reduce peak flows, and channel scour as well as filter the water in the process. Preservation of floodplain areas has become increasingly important as the upland areas in the watershed are converted to impervious surfaces and increase the runoff volume discharged to streams and rivers during storm events. Development within the floodplain greatly compounds this problem by taking away available flood storage during storm events and increasing the runoff volumes. There have been several major flooding events in the last two decades including the most recent flood events of April 2007, May 2006, October 1996, and April 1987. These floods were relatively close in magnitude to what is statistically referred to as the 100-year flood (a flood that has a one percent chance of occurring in any given year). The recent number of major floods in this relatively short time period suggests that there may be an increasing trend of more frequent large rainfall events. The pressure to develop our floodplain areas is likely to increase as the value for other developable upland areas increases and/or becomes less available. Recent development patterns suggest floodplains along the Merrimack River are vulnerable to development pressures because they are parallel and adjacent to major transportation corridors such as I-93 and US Route 3. This makes undeveloped land space in the floodplain highly attractive for commercial development. Communities in the upper Merrimack River will need to revisit their floodplain management ordinance requirements and determine whether the development controls are sufficient to protect these areas and limit the potential for greater downstream flooding as well as in their own communities due to reduced flood storage volumes.
Goal 17: Encroachments within floodplains and floodways along the upper Merrimack River and its tributaries are avoided or minimized so that flood storage functions and ecological values are protected.
OBJECTIVE: FP-1 – By end of 2008, review floodplain zoning regulations in all UMR communities, research new floodplain control language from NHOEP or other planning resources and recommend suggested language and/or control measures for communities to adopt and improve their protection of flood plain areas.
- Compile, review, and compare floodplain ordinances for each of the upper Merrimack communities.
- Research new approaches and model ordinances developed elsewhere concerning floodplain protection.
- Identify and map key locations within watershed area that represent important floodplain areas.
- Based on a comparison of historical aerial photos, identify and quantify major changes in land use conditions within the floodplain area over the last 20 to 30 years.
- Identify areas that have been historically prone to flooding resulting in major property damage and/or disruption to vehicle travel along public roadways.