As the planet warms, science has predicted flood warnings will increase. Two of the most recent papers published on the subject as it relates to the Northeastern United States, do not tell the apocalyptic tale shared by Noah director Darren Aronofsky. Rather, these studies are straightforward reminders -- or subdued warnings -- of the expected impacts of climate change in one region.
Matt Collins, a hydrologist and earth scientist with the National Oceanic and Atmospheric Administration Fisheries Restoration Center in Gloucester, Massachusetts, is one of the scientists behind the papers. He oversees a restoration monitoring program and also provides technical support for fish passage restoration projects throughout the United States. The projects he is involved with can involve the addition or removal of structures such as dams or fish ladders. In those cases flood estimates, which come from data from nearby stream gauges, have to be analyzed.
Collins was motivated to be involved in the research after noticing the statistical properties of many flood records, even in comparatively undeveloped areas, were not constant over time in the Northeast. "This is important because most statistical methods that use past flood records for estimating future floods assume that the record being used is stationary," Collins said.
Estimating flooding is a common step in the design process for roads, bridges, and general urban infrastructure. Collins explained that until now it has been assumed that looking at the history of floods or rainfall events has provided data that represents a reasonable outlook for the future.
"The problem is what if past is changing," said Collins.
In our research we identified upward trends in the Northeast starting around 1970. Our question then is, the past still a good guide to the future given projections for human induced climate warming and if not is the post 1970 period a better guide than the full record or is it even an adequate guide.
Results of the paper "Hydroclimatic flood trends in the Northeastern United States and linkages with large-scale atmospheric circulation patterns"
A combination of natural variability and climate warming may be affecting both conditions that predispose the land surface to flooding and flood-producing storm events."In general, most of the Northeast except Eastern and Central Maine has been seeing these upward trends in magnitude and in frequency," Collins said.
The biggest change is the small floods are getting bigger, though they are still relatively small, and they are getting more frequent. These smaller floods are important for stream and floodplain processes and habitat.
He believes the results would be stronger yet if rural watersheds in Northeast had not been reforesting for the past 100 years. Scientists can see an increase in flood flows if there is more water (rainfall) or by a change to the land surface. Urbanization -- paving over large areas -- creates water run off, because there is nothing to soak it up (note all the puddles on the side of a road after it has rained). In an unpaved area, natural vegetation (trees, grass) will help impede runoff.
As big as Irene was, the flood of 1927 in Vermont was quite a bit bigger and more destructive. The big difference, Collins explained, is they were both hurricane floods and tracks up there were similar, but one hit in August when leaves were on trees and one in November after the leaves fell.
"Leaves play a big part in run off," Collins said. "They not only slow the precipitation from hitting the ground, but the trees are also pulling up water when they are all leafed out. That's how they live, they are sucking water out of the ground."
Collins explained soil moisture conditions can also create very different impacts. Hurricanes Floyd and Irene were almost identical storms meteorologically, but Floyd did not produce much for flooding, because for weeks preceding the storm there had been no rain so the land surface was fairly dry and could absorb more of the rainfall. Before Irene there had been a couple weeks of decent rain and so a lot of the soils were saturated, and saturated soil works a lot like pavement where there is more runoff.
Results of the paper "Annual floods in New England (USA) and Atlantic Canada: synoptic climatology and generating mechanisms"
There has been more precipitation and generally more intense precipitation in the Northeast as a result of climate change.
Over half the annual peak floods in the Northeastern United States and Atlantic Canada occur in the late winter or early spring. Depending on the location, early winter or fall are secondarily important flood seasons.
"The dominant precipitation mechanism for producing annual floods is rainfall, and we find evidence that rainfall events are associated with the largest annual floods," said Collins.
Taken together, our analyses of annual flood seasonality and precipitation mechanisms suggest that rain falling during leaf-off conditions, and particularly when the ground is saturated or frozen, frequently cause annual floods -- and sometimes the largest annual floods.