New Data for Extreme Rainfall Events Denise Savageau, Conservation Director Town of Greenwich

Impacts of global warming: Sea level rise – 1 ft in 50 years, 2-4 ft in 100 yrs Extreme changes to hydrologic cycle drought in some areas increased precipitation and flooding in others Changes in vegetation/agricultural production Impact on biodiversity Changes in severity and frequency of storms hurricanes -In past 35 years there has been an 80% increase in the number of category 4 & 5 hurricanes tornadoes

What is flooding? Flooding is a naturally occurring hydrologic event where water inundates land not normally submerged. Storm events, such as hurricanes, nor’easters and heavy rain events, are the most common causes of flooding in our area. Flooding becomes a problem when human development takes place in flood prone areas and when development alters natural drainage patterns.

What is a 100 year storm event? 100 year storm is a storm that has a 1% chance of occurring annually. It does not mean that a storm will occur only once every 100 years. 100 year storm = 1% chance every year 50 year storm = 2% chance every year 25 year storm = 4% chance every year 10 year storm = 10% chance every year 2 year storm = 50% chance every year

What does no net increase in runoff mean? No net increase in runoff refers to no net increase in peak discharge – not in total runoff. The challenge for municipalities is to reduce both total runoff (maintain or improve existing infiltration) and peak discharge. Town’s need to update their drainage manuals to address both total run-off and peak discharge. Town’s also need updated rainfall data based on changing climate conditions.

In May 2012, the Town of Greenwich adopted new drainage manual. This new manual requires low impact development and best management practices for water quality and quantity. The manual also adopted the new extreme rainfall data developed by USDA-NRCS and the Northeast Regional Climate Center at Cornell.

We know that rainfall data needs to be updated but this is not happening fast enough. Town’s need to be pro-active by using and requesting updated data.

Extreme Rainfall for NY/New England website CT DEEP Climate Change Adaptation Town of Greenwich Drainage Manual orks/Engineering_Division/Stormwater_Information/drainage_m anual/