Stormwater & Flood Management
Stormwater in the community is the result of runoff from developed areas. What is not immediately absorbed into the ground or collected in the stormwater system often pools in low-lying areas before infiltrating into the groundwater table. One of Keene’s goals for the future is to establish stormwater systems that work with natural systems to manage stormwater as close to its source as possible. The planning process made it clear that a multipronged strategy will be required to manage stormwater adequately, prevent large-scale flooding and remain financially responsible through the establishment of this infrastructure. One way that Keene is moving toward this goal is through use of innovative stormwater management, a low-impact, low-cost design that is effective while allowing the existing system to remain at its current capacity. It should be noted, however, that the city will replace an individual component of the system, such as a culvert, on a case-by-case basis if it is found to be below the current capacity standard.
Conventional stormwater management methods usually capture water flowing off impervious surfaces, channelize or pipe it, and discharge it into undeveloped areas or streams. Often called “hard” engineering solutions, these methods aim to alleviate problems in individual areas but can lead to larger environmental problems such as increased soil and stream-bank erosion, increased flooding potential throughout the community and downstream, and interruption of the natural infiltration process. In most cases, rather than being intended to manage stormwater at its source, these conventional methods were reactions to development patterns that ignored ecological impacts. The key to responsibly managing stormwater is integrating development into the natural landscape – or in areas where infill development is occurring, integrating natural systems back into the built environment. A citywide stormwater strategy will ensure that development considers its effect on the entire system, not just at the local site.
Given that most of Keene is situated within the 100-year flood plain, and given Keene’s history of floods and the expected future increase in the frequency and amount of precipitation because of climate change, the city itself is the ideal built environment in which to incorporate innovative stormwater management solutions. Combined with a system of integrated flood-control practices, Keene’s ability to manage stormwater can be greatly improved and overall community resiliency can be increased.
The 2000 Flood Hazard Mitigation Plan and 2005 All-Hazard Mitigation Plan provide detailed review and analysis of Keene’s flooding risk, and recommendations in these plans are still appropriate in 2010. While many of the actions in the plan have been implemented, the following remain relevant:
- Apply for additional flood mitigation funds to finish flood proofing or other feasible mitigation strategies for the remaining Krif Road and Krif Court commercial and industrial properties.
- Develop and implement a mitigation plan to protect the Kingsbury Corporation facility, which is partially located in the floodway and entirely in the 100-year floodplain.
- Develop a program to mitigate risks and secondary hazards associated with flooding at the Tanglewood Estates manufactured home park.
- Conduct site visits to properties on the Hazardous Material Inventory to determine risk of release during flooding or other hazard events.
- Strive to create programs to retrofit existing flood channels and detention basins with trails and other recreational amenities.
- Evaluate systemwide solutions for the Beaver Brook watershed, including assessing the feasibility of modifying or replacing bridges that create obstructions and backwater flooding.
- Restore flood-storage capacity of filled areas within Woodland Cemetery wetland complex.
- Assess the feasibility of expanding the Three-Mile Swamp structure to create greater storage capacity.
It is recommended that the city pursue specific stormwater management strategies and incorporate them into land-use regulations and code. The goal in highly developed areas is to minimize imperviousness, harvest stormwater and maximize water infiltration. Where feasible, components of the stormwater system should be incorporated into the community’s architecture. The city should explore the use of greenroofs, cisterns and rain barrels; if feasible, a program should be created to encourage their use in residential and commercial development throughout the community.
A program should also be created to educate citizens and developers on best management practices to manage stormwater, such as rain gardens, bio-swales, and topographic depressions. Planning Board regulations should be revised to incorporate a zero-runoff policy for new development as well as to require innovative stormwater techniques in proposed development. Many developers are incorporating porous concrete and asphalt, porous pavers, bio-swales and other innovative methods into their site plans, as the ability to tie into the city’s existing system becomes less feasible.