Thursday, April 1, 2010
Rivers in a flood
Unprecedented amounts of water are flowing under the bridge at Pawtuxet Falls and through rivers statewide. What does this volume of water do to the rivers around the state? Generally, rivers reach what we call "bank full" conditions every one to two years. These are conditions that form the river's channel. The 5-10 year storm will reach the floodplain and deposit sediment as waters recede. What we are seeing now are two 100 year floods within one month. The current conditions might be considered a 500 year flood. Rivers naturally respond to floods like this by creating new channels, scouring out sediment and debris, and making new areas of wildlife habitat. While there may be a temporary disruption for fish and other stream wildlife, floods create new healthy habitat.
Problems with floods like this are mainly for people. Many of the houses and businesses being flooded today are outside of what we would consider the river's floodplain, and don't have flood insurance. Our floodplain maps are out of date, and will all have to be rewritten. We are also seeing the consequence of 100 year old infrastructure reaching its breaking point. Dams were built too close to bridges, mills encroach on the river.
Even bridges built within the last decade are undersized. This is because they are often built within the "footprint" of the old bridge without any thought. State and City highway departments save considerable amounts of time and money on bridge design when they create a "footprint" bridge because they have fewer environmental permits to file. Widening a bridge span when replacing a bridge requires excavating stream banks, designing new abutments and filing wetland permits. Infrastructure in the Northeast is undersized by 30%, including bridges and culverts. We need to take this seriously and not repeat past mistakes. Click on the video below to see the Pawtuxet in action!