CLIMATE CHANGEand HYDROLOGY
New York State Dept. of Transportation
George H. Long, P.E.
Climate Change
• According to the International Panel on Climate Change (IPCC), “Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level.”
Rising Temperatures
Heat energy is what drives the hydrologic cycle.
In a warmer world, extra energy means a more vigorous hydrologic cycle:
• – Higher temperatures increase atmospheric moisture‐holding capacity
• – Higher temperatures imply globally increased evaporation
• – Worldwide, precipitation must increase – that will vary regionally – some areas will be drier
• – More intense precipitation causes flooding
• – More intense drying causes drought– Mid‐continental summertime drying
• – More rain, less snow
• – Earlier and smaller spring runoff
For transportation and public safety, we’re primarily worried about flooding due to extreme events.
Coastal and riverine flooding result chiefly from intense storm events.
• Windham, NY Hurricane Irene, Aug 28, 2011(Albany Times Union photo)
What are the expected effects of climate change on Coastal Storms?
There is no clear consensus on trends in coastal storm activity. Numerous studies have shown that the severity, if not the number, of hurricanes has increased in the last 30 years, and is expected to increase further.
Six of the 10 most active hurricane seasons have occurred since 1990.
Warmer ocean waters should feed the convection system of tropical cyclones, resulting in more intense storms.
The energy released by the average hurricane (again considering all hurricanes worldwide) seems to have increased by around 70% in the past 30 years or so, corresponding to about a 15% increase in the maximum wind speed and a 60% increase in storm lifetime
More research is needed to quantify the likely effects of rising temperatures on the intensity of coastal storms, especially of extra‐tropical cyclones.
For now, most studies of future effects of climate change on coastal flooding assume that sea level rise will dominate; they add current storm surge ranges to projected sea level.
Sea Level Rise (SLR)
• Approximately 11” of rise in NYC, 1900 – 2000
(observed)
• 12 to 23 inches by the 2080s (IPCC slow ice melt)
– 41 to 55 inches by the 2080s (IPCC rapid ice melt); 56 to 72 inches by 2100
Possible Impacts in New York
• Major NYC airports are near sea level, and would be well within coastal flood zones
• Permanent inundation of low‐lying areas and coastal wetlands
• Increasing storm‐surge flooding risk to subway, highway and rail lines, especially tunnels
• Increased risk of salinization of water supplies for communities relying on the Hudson River
Hybrid Storm Sandy14th Street, ManhattanOct 30, 2012
Hybrid Storm SandyNewburgh, NY
The NYS 2100 Commission projects 19‐29” of sea level rise by the 2050’s, 56‐72” by 2100 (Rapid Ice Melt). Considering SLR alone then:
By 2050’s Sandy‐level storm tides will
be a 10 – 15 year event.
By 2100, it will likely occur every year.
Looked at another way, by the 2050’sthe 50‐year design event for facilities in the coastal flooding zone will be 3.5’ higher than the flood tide from hybrid storm Sandy.
Inland Riverine Flooding
Riverine flooding is driven by precipitation and snowmelt. Total volume of precipitation can play a part, but much more important is the precipitation intensity. Slow, low‐intensity rainfall soaks into the ground and moves away from the storm as slow‐moving groundwater. Rain that falls faster than the infiltration rate becomes …
… Runoff
Other factors influence runoff besides rainfall intensity. Ground cover is important; impervious cover (roofs, pavement) block infiltration and promote runoff, while vegetation and forest litter that impede overland flow allow more time for water to soak into the ground.
Until the early 20th Century, logging and clearing for agriculture turned what had been forest into open ground. Much of that land in the Northeast is now reverting to forest, reducing previously increased runoff.
At the same time, large areas of former agricultural land are being developed, increasing impervious areas and lawns, increasing runoff.
Snow pack further complicates the mechanism of turning precipitation into stream flow.
Rain falling on a heavy snow pack adds the water from the rapidly melting snow, often on frozen ground. This results in greatly increased runoff.
• As winter temperatures increase from climate change, more of the precipitation will fall as rain rather than snow, reducing snowmelt, but increasing the frequency of rain‐on‐snow events.
• In New York, warmer winters mean that freezing of the Great Lakes will be delayed, allowing for longer periods of lake‐effect snows, increasing snowpack in affected areas.
Changes in Precipitation
Annual precipitation in the Great Lakes Basin has increased by avg. 4.5” since 1915
More importantly, there has been a 67% increase in the number of 2‐inch rainfall events occurring over a 48‐hour period since the 1950s.
• Chance of daily precipitation exceeding 2”– Pre‐1963 0.2 (5‐year event)
– Post‐1992 1.0 (every year)
• Chance of daily precipitation exceeding 2.5”– Pre‐1963 0.05 (20‐year event)
– Post‐1992 0.25 (4‐year event)
(US Geologic Survey Open File Rept. 2008‐1199, “Hydrologic Evidence of Climate Change in Monroe County, New York)
Most models predict a total increase of 10 percent (about four inches) in average annual
precipitation by the end of the century.
The Northeast Climate Impact Assessment (2006) projects about a 8 to 9% average increase in storm intensity (annual precipitation/number of rainy days) by mid‐century, 10 to 15% by 2090.
In addition, the number of heavy precipitation events is also expected to increase, by about the same rate.
Since rainfall intensity is a major factor in generating runoff, this means that flood flows will, on average, get larger.
And floods will come more frequently.
How much larger, and how much more frequent?
Flow Prediction
• Deterministic models– Rational Method
– SCS Curve methods
• Statistical models– Regression equations
Rational Method
Q = CIA
Q = discharge, in ft3/sec (cfs)
C = runoff coefficient
I = rainfall intensity, inches/hour
A = watershed area, in acres
NRCS (SCS) Curve Methods – TR‐55 and TR‐20
Runoff Q is defined as
Q = (P – Ia)2 / (P – Ia + S)
P is the “design depth” of a 24‐hr rainfall with a selected design frequency of 2, 5, 10, 25, 50 or 100 years, i.e. a rainfall intensity
Statistical Method – Regression Eqn
Prepared for NYSDOT by the USGS and periodically updated (1990, 2006) and incorporated into StreamStats
Based on observed flows in gaged streams, they arenot dependent on a modeled relationship
to rainfall, but deal with a range of variables.
So how do we address Climate Change in predicting floods?
Clearly, more research is needed.
Climatologists and engineers need to communicate, so that climate models can be analyzed for those variables that engineers can use.
In some cases, scaling up the rainfall intensity or annual precipitation in a model like the Rational Method or in Curve Number methods may be sufficient ‐‐ in both these methods, streamflow is directly proportional to rainfall intensity.
For the Regression equations, the easiest adjustment is by analogy; if the deterministic models suggest an increase, increase the regression values by the same proportion.
So, if rainfall intensities are expected to increase 10 to 15% by the end of the century, you could simply increase your calculated flows by 10 to 15%.
BUT…Rainfall intensities have already increased by about 20 to 25% compared to the long‐term historical record, so you really need to increase them by 30 to 40%
Remember, hydrologic analysis of flood discharge is imprecise at best.
Statistical uncertainties in discharges from Regression Equations is typically on the order of ± 30%.
Uncertainties from deterministic techniques are inherent in the models and hence unquantified, but probably at least as large.
The complexity of weather systems within the overall climate framework makes quantitative prediction of storm and rainfall intensities very difficult.
The additional level of complexity involved in deriving streamflow from storm data makes its prediction even more daunting.
What is New York Doing to Prepare?
Legislature created NYS Sea Level Rise Task Force (2007). Final Report (Dec. 2010) recommended adaptation strategies.
Executive Order #24 (Aug 2009)– Established a statewide goal for greenhouse gas
reduction
– Established NYS Climate Action Council,
composed of heads of various State agencies. (Report issued Nov. 2010)
NYS2100 Commission formed Nov. 15, 2012 by Gov. Cuomo to evaluate vulnerabilities and recommend actions to increase resiliency of infrastructure.
Report issued Jan 2013
2100 Commission Recommendations:1. Develop a Risk Assessment of the State’s
transportation infrastructure, prioritize a Transportation Lifeline network
2. Strengthen existing transportation networks, including protecting transit systems and tunnels, and upgrading scour vulnerable bridges
3. Strategically expand transportation networks to provide redundancy, including two new tunnels under the Hudson River into Manhattan
At NYSDOT:
• Energy Efficiency & Climate Change task force, including Climate Adaptation Work Group
• Sustainability and Climate Change Section in Statewide Policy Group formed in 2010
• NYSDOT member of NYS interagency Climate Action Council. Report Nov 2010
NYSDOT Programs:
• Pooled‐fund NOAA Atlas 14 project to update rainfall intensity curves.– Will still be averaged over period of record, with no projection of trends
• Funding research by Northeast Regional Climate Center at Cornell University on downscaling projections of extreme rainfall in NYS
• Funding work by USGS to incorporate projected climate changes into StreamStats
NYSDOT is currently engaged in a statewide Vulnerability Assessment, and awaiting adoption of the specific policy guidance to develop programs to mitigate and adapt to the effects of what is turning out to be a rapidly changing climate.
But given that we are now designing facilities that we hope will last through the end of the century, we need to move as quickly as possible.
Selected References
Climate Risk Information – New York City Panel on Climate Change, 2009 www.nyc.gov/html/om/pdf/2009/NPCC_CRI.pdf
NECIA – Northeast Climate Impact Assessment, Union of Concerned Scientists 2006
www.climatechoices.org/assets/documents/climatechoices/NECIA_climate_report_final.pdf
Responding to Climate Change in New York State.
ClimAID Synthesis Report , NYSERDA 2011
www.nyserda.ny.gov , search on “ClimAID”
NYS 2100 Commission Report, Jan 2013
http://www.governor.ny.gov/assets/documents/NYS2100.pdf
Evaluation Questions
Coastal and riverine flooding results chiefly from:1. Long‐term climate factors
2. Intense storm events
3. Land‐use factors within the watershed
Answer: 2
Under the Rapid Ice Melt scenario, by the end of the century sea level is expected to:
1. Rise 8 to 9 %
2. Rise 12 to 23 inches
3. Rise 56 to 72 inches
Answer: 3
The most significant effect of climate change on inland flooding will be from:1. Increasing rainfall intensity
2. An increasing percentage of snow vs. rain
events
3. Increasing storm durations
Answer: 1