To understand the damage that occurred at the Halligan residence and throughout the town of Pittsfield it is crucial to think of the Tweed River’s watershed as one dynamic system. This system is constantly collecting precipitation and transporting, through its rivers and streams, this water and the sediment that it picks up. The channels of rivers and streams are constantly moving and meandering over time as sediment is eroded and deposited. When a large amount of precipitation rapidly enters a watershed system, as in Irene, there are several characteristics that make it more or less susceptible to two types of flood damage, inundation—water overflowing the banks of a river, and the more damaging fluvial erosion—when a river channel erodes the riverbank and riverside structures. The characteristics that affect the severity and type of flooding that can take place are:
- The location and climate of the watershed
- The size and topography of the watershed
- The types of soil and vegetation in the watershed
- The presence of meanders and wetlands in the valley
- The geomorphology of the riverbed, specifically the presence of bedrock and erodible soils
- Historic and Current Land Use: The most important factor in flood risk is the human development both past and present in the watershed as a whole and along the river corridor.
Location and Climate: The location of the Tweed Watershed high in the Eastern spine of the Green Mountains makes it particularly susceptible to flooding. The largest and wettest storms that Vermont experiences come from Atlantic and Subtropical storms that generally move northward up the state (USGS 1989). When these saturated air masses hit the Green Mountains they rise, cool and rapidly release their moisture as was experienced in Irene.
Size and Topography: The Tweed watershed drains an area of 51 square mile into the White River. Though the watershed only makes up 7% of the White River watershed that it lies within, it is the largest tributary to the upper section of the White River. The Tweed is also a one of the steepest watersheds in the White, from its headwaters near Killington it drops from 3,500 feet to its confluence with the White River at 715 feet. This 2,785 foot drop takes place over only 9.3 miles (Tweed River Watershed Corridor Plan, 2008). The steep topography of this watershed means that when a large amount of precipitation enters this system rapid runoff and flash flooding occur frequently.
Soil and Land cover:During a large storm the types of soil and vegetation in a watershed addition to its steepness have a great impact on the amount of water that will be absorbed into the ground and the amount that will flow into rivers and streams. Soils that are high in clay, frozen, or already saturated with water can not to rapidly absorb runoff. Trees and woodland vegetation make soil more porous and absorptive and their leaves and stems help slow runoff. The Tweed watershed is 90% forested; this is a great asset to reducing the amount of runoff that enters its rivers and streams (UVM-SAL 2002). The steep topography of the area, however, means that water moves quickly down the hillsides and has less opportunity to pool and be taken into the soil.
Wetlands, Meanders, and Floodplains:
Once water runs off of the hillsides and into rivers and streams the force of its flow is determined by the meanders in the river and its access to wetlands and floodplains. Less than 1% of the Tweed watershed is composed of lakes and ponds and though there are wetlands in the higher parts of the watershed, the only way for water to slow in main stem of the river is with meanders and flood plains.
River Geomorphology: The geologic character of the river determines the ability of a stream to meander and create floodplains. The Tweed is a steep walled glacially carved valley. The superficial geology of the valley is a juxtaposition of both very solid metamorphic bedrock that is highly resistant to erosion and loose sand and gravel glacial till was deposited at the end of the last glacial period (Tweed River Watershed Corridor Plan, 2008). The steep walls and the areas of exposed bedrock constrain the rivers ability to meander putting a greater potential for erosion on the looser soils in the valley floor.
Historic and Current Land Use: These geologic characteristics of tight, steep valley walls left little room for development along Tweed. When European settlers came to the area they built buildings and roads in the only available space close to the river and logged the hillsides creating the “classic” Vermont landscape. Over the next 100 years development along the river limited its ability to meander and reduced its access to flood plains. The rivers and streams were also straightened and cleared of vegetation so that logs could be floated down them. In the lower part of the Tweed the Lumbar Railroad pushed the river against the valley wall. The confining and straightening of the river increased the speed and the power of the water. When this occurs a river erodes its riverbed and the channel sinks deeper, especially when powerful floodwaters cannot spread out over a flood plain. Modern development along the river such as Route 100 and homes built within the flood plain have maintained the straightening of the river and the limitation of its access to flood plain. Historic alterations of the river system and current land use along the river are the most important factors in the creation and mitigation of flood hazards. The historical alteration of river channels is often hard to notice because we are very used to thinking of rivers as stationary rather than dynamic features in the landscape.