Local Flood Hazards

Flooding in the Town of Winchester can be categorized into the following three types, as described in greater detail below:
  1. Surface water flooding associated with overtopped streams, rivers, and ponds;
  2. Flooding resulting from poor street drainage; and
  3. Flooding resulting from groundwater contributions.
Most Significant Water Sources
The most significant source of surface water flooding in the Town of Winchester is the Aberjona River and its tributary, Horn Pond Brook. Flooding may also occur along several ponds, such as Wedge Pond and, to a lesser extent, Winter Pond and Upper Mystic Lake.

Flood Insurance Rate Maps

The extent of all surface-water flooding in Winchester is mapped by FEMA on Flood Insurance Rate Maps (FIRMs) for the Town of Winchester. Theses maps identify areas that are located with the 100-year floodplain, 500-year floodplain, and areas of lower flood risk. Properties mapped into the 100-year floodplain typically experience the most devastating flooding impacts, with floodwaters potentially exceeding their first floor elevations. Rivers and ponds may take several days to recede and reach normal levels after a flooding event.

Surface-Water Flooding

This type of flooding is most prevalent in Winchester during large precipitation events, such as those experienced in October 1996, June 1998, April 2004, May 2006, and 2 events in March 2010. In some cases, the flooding may be exacerbated by contributions from snowmelt, such as the flooding event experienced in Winchester in March 2001, which resulted in the highest ever recorded flows at the USGS gaging station on the Aberjorna River in Winchester.

The table below provides a summary of the recurrence interval for the respective storms, as well as a summary of the peak streamflows, in cubic feet per second (cfs), recorded at the USGS gaging station in Winchester. For comparison purposes, the average annual streamflow at this gage is approximately 25-cfs and average streamflows during the month of August, typically the lowest flow season, are approximately 7.5-cfs.

Date Recurrence Interval Peak Streamflow (cfs)
October 21, 1996 25-year event 1150
June 14, 1998 20-year event 1070
March 22, 2001 60-year event *1590
April 1, 2004 15-year event 990
May 15, 2006 30-year event 1270
March 14, 2010 50-year event 1450
March 30, 2010 25-year event 1100
*Highest streamflow recorded at this location.

Roadway Drainage
The Town of Winchester owns and maintains an extensive drainage system consisting of catch basins, pipes, and manholes designed to convey stormwater runoff away from roadways, parking lots, and other critical infrastructure to prevent flooding.

Roadway drainage systems are typically designed to accommodate the 25-year storm event, so events exceeding this size will likely cause street flooding, since the drainage system does not have the capacity to accommodate larger quantities of precipitation. Additionally, changes in land use in the contributing watershed area may result in increased flows beyond what the system was originally designed for, causing street flooding for events less than the 25-year storm. For example, impervious surfaces from new driveways , parking lots, and building additions all contribute to increased stormwater runoff that can adversely impact the storm-drain network. Street flooding usually subsides quickly once a rainfall event has passed.

Groundwater-related Flooding
Groundwater is defined as the water beneath the ground surface that flows through soil pore spaces and fractures in bedrock. When precipitation falls or snow melts, water is absorbed into the soil and results in a rise in the groundwater table. High groundwater elevations are very common in New England during the spring months as a result of snowmelt and spring rains.
 
Basement Flooding
Any property in the Town of Winchester is susceptible to basement flooding as a result of rising groundwater or underground springs. When groundwater levels outside a basement rise above the level of the floor, hydrostatic pressure may push the water through hairline cracks in the floor or foundation. Various methods are available to help alleviate this type of flooding, including sump pumps and perimeter or “French” drains. Unfortunately, the town’s efforts to address street drainage problems and flooding along the Aberjona River and its tributaries will not impact existing groundwater flooding issues.