Extreme Rainfall in the Northeast
Extreme Rainfall in the Northeast
It's raining a lot and our wastewater system is bursting at the seams. The resulting sewage overflows are impairing our waterways.
On 24 September 2021 a large area in the lower part of the Mohawk Watershed in NY State received more than three inches of rain overnight from a storm that was related to a moisture-rich frontal system that moved across the state. Muddy water flowed in the fast-rising creeks and rivers, and the deluge overwhelmed pipes that carry sewage and pipes that carry stormwater in the City of Schenectady. When I arrived on the Union College campus in the morning I was greeted by a fountain of sewage bursting out of a sanitary main that crosses campus. That raw sewage with unmentionable floatables ran across a campus road and drained into a stormwater line that empties into the Hans Groot Kill, a scenic creek on campus that empties into the Mohawk River. This sewage flowed for hours. It was depressing to watch.
Given the rate of rainfall, the duration of the event (about 12 hr), and the total rainfall (maximum of ~3.7 inches, but ~2 inches in Schenectady [26]), we can use the Extreme Precipitation Monitor at the Weather Prediction Center of NWS at NOAA to determine that this was about a 1 in 25 year event (in the rainfall bullseye) when it is evaluated in the context of the long-term precipitation records (about 70 years - explained below). But if rates of extreme precipitation are increasing in the Northeast, we will need to recalibrate the scale: this may now be a 1 in 10 year event. Thus in this scenario, we would expect these events to occur more frequently, and therefore we need to think carefully about our infrastructure.
What is extreme rainfall? It is safe to say that terminology is evolving but there is a technical definition, a practical definition, and a general definition. “Extremeness” depends on context, location, and past records, but we will see that it can also depend on its effect on society, including pipes and infrastructure. If pipes deteriorate over time, smaller events may be significant because the affected pipes are impaired. If extreme events and total rainfall are increasing, and simultaneously our pipes are disintegrating in place, we are really in trouble. Even small events can cause issues.
<Extreme sewage - image> Overwhelmed sanitary sewer on the Union College Campus in Schenectady NY on 24 September, 2021. Intense rainfall of more than 3” the evening before stressed and overwhelmed the sewage lines. Sewage ran into local creeks and then the Mohawk River (photo: JI Garver, 9/21).
2021: An extreme year. This has been a big year for extreme rainfall events. Across the Northern Hemisphere the news cycle seemed to be dominated by event after event of epic flooding caused by intense rainfall. These extreme events resulted in tremendous damage and fatalities. Extreme rainfall was particularly devastating in Europe in July. Between 12 and 15 July northern Europe experienced epic flooding due to a stalled frontal system that dumped between 6 and 12 inches of rain that resulted in more than 220 fatalities and widespread damage, especially in Germany [1]. In this event intense rainfall overwhelmed sewage systems in London on 12 July, resulting in significant sewage overflows and contaminated floodwaters [2].
On 21 August between 10 and 15 inches of rain fell in eastern Tennessee due to a stalled frontal boundary, and subsequent flooding resulted in 22 fatalities and extensive damage [3]. A gauge in McEwen TN recorded a rainfall total of 17.02 inches, which beat the previous state record by 3.42 inches [4].
Rainfall from extreme events has caused considerable damage in the Northeast and sewage release is a major issue in these events. Hurricane Ida (September 2021) broke the record for the highest hourly rainfall total in NYC (4.1”) and the previous record was set only two weeks before when Hurricane Henri dumped 7 inches of rain [6]. In Ida, a wide area of New York City had rainfall rates that were ~3.1”/hr, which is apparently beyond the capacity of the sewer system that can only handle rainfall rates of 1.5-2 inches per hour. In July, Sabria Shankman, who covers climate change for the Boston Globe, wrote: “If the heavy rains of this summer are emblematic of our future normal as climate change progresses, then so, too, is this: Much of that rainwater overwhelms local sewers, triggering the release of hundreds of millions of gallons of raw, untreated sewage into Massachusetts waterways” [25].
There have been a number of intense rainfall events elsewhere in New York State, including in the Mohawk Watershed. On 8 June 2021 local heavy rainfall caused flooding in downtown Albany [7]. On 11-12 July 2021 rainfall maxima in the Schoharie and upper Susquehanna were between 2 and 4.3 inches [8]. This intense rainfall caused overland flooding due to debris in Gorge Creek in Middleburgh in the Schoharie Valley [9]. The 17-20 August storm in central to eastern NY related to Hurricane Fred resulted in significant flooding in Rome and Utica. A state of emergency was declared on 19 August in Utica and a flood warning was issued in the upper part of the watershed (Rome, Utica) [10]. Maximum rainfall totals over this period were between 4 and 8 inches, and in the upper Mohawk watershed between 3 and 5 inches [11]. A fast moving intense frontal system on 23-24 September resulted in >3 inches of rainfall in Schenectady and Saratoga counties; this is the storm with the sewage overflow on campus.
The recent intense rainfall events in NYC prompted the City to take more proactive mitigation measures in anticipation of future extreme events. In September 2021, the NYC Mayor Bill de Blasio released a task force report entitled “The new normal: Combating storm-related extreme weather in New York City” [12]. This report recognizes that the sewage and stormwater systems need to be expanded to meet the challenges of extreme rainfall: “We will re-imagine our sewage and drainage system, and rapidly increase green infrastructure and cloudburst solutions. We face a massive challenge: the City’s 7,400 miles of sewer pipes were largely designed a century ago, for a very different climate than the extreme one we now face” [12].
Recognition of the importance of extreme rainfall by the State of New York is not new, but it appears to be reaching a crisis point. Certainly the number of events in 2021 has been eye-opening, particularly after the last 10 years of weather events and recovery from them. The most extreme event in the Mohawk Watershed in recent history was Hurricane Irene in August 2011 and then Lee in September. Hurricane Irene affected most of the eastern seaboard, and it was particularly devastating in the Mohawk Watershed, especially in the Schoharie Valley, which drains part of the eastern Catskill Mountains. Rainfall totals of 3-7 inches occurred across the basin, but in the Catskills rainfall totals were between 10 and 18 inches [13]. A major issue during Irene was breach and failure of dams in the Mohawk River. Hurricane Sandy in 2012 reinforced a wider appreciation of the nature of these extreme events and how we need to prepare for them.
Following the Irene/Lee/Sandy trifecta, Eric Schneiderman, then Attorney General of NY State, released “Current and Future trends in extreme rainfall across NY State” in 2014 [14]. This publication was an attempt to establish the link between climate change, extreme precipitation, and flooding and what the State needs to do to prepare. He noted that the catastrophic flooding during Hurricane Irene resulted in estimated damage of $1.3b in NY.
The Schneiderman report focused on the observed increase in heavy rainfall events - those with 2” or more in a 24 hr period (these are pretty common), but also events with rainfall totals that have a 1% annual probability (a more technical definition, and by definition these events are less common) [14]. He suggested that the number of 2” rainfall events has nearly doubled since the mid 1990’s. But a key point for the State is that we need to accommodate this increase in the design and planning of our built environment. In other words, those planning and designing infrastructure need to “reject outdated information that does not represent current climate risks and instead begin incorporating best available data” and “If we are to overcome the challenges of climate change, we must incorporate resiliency planning and response measures into our communities and our landscape.” [14]
Let’s return to the basic question: What is extreme? Geologists tend to have a relatively good handle on extreme events because the history of the Earth is punctuated by remarkable events, some of which have changed the physical and biological trajectory of the planet. Many of the geologic events are super extreme, and not very common. In the case of precipitation we need a more practical definition. In an interesting paper entitled “Extreme Events: a research and policy framework for disasters in context,” Dan Sarewitz and Roger Pielke Jr suggest that extremeness is directly related to context, and that context in this case is societal exposure. In defining extreme events they write: “We define extreme events as occurrences that, relative to some class of related occurrences, are either notable, rare, unique, profound, or otherwise significant in terms of their impacts, effects, or outcomes” [15].
This is a pretty broad definition, but a key point here is that an event, let’s say precipitation of 3.7 inches in the Mohawk Watershed, is significant in terms of its impact. If rainfall of 3-4 inches causes flooding and overflows of sewage from underfit or compromised pipes, there is a clear impact on society, and hence one could say that it is extreme.
But if we take this one step further, what if the magnitude (size) and frequency (how often) of events stay the same over time, but the framework of society also changes? If our infrastructure is neglected and becomes increasingly impaired over time, then damage from a 3-inch rainfall event may increase over time because we have not modernized or upgraded our infrastructure as it ages. We appear to be in a scenario where extreme events are increasing and infrastructure is failing. This assessment would imply that things might well get much worse. The solution may be that we need to update and modernize, and while doing this we need to plan on bigger events. So not only do we need to replace pipes, we need to put in bigger ones.
I like the definition of extreme events provided by Sarawitz and Pielke because it is flexible, intuitive, and it allows the event to be evaluated in the context of damage and impact. In August 2021 Sarah Kaplan wrote about the Tennessee floods in the Washington Post and she really hit the nail on the head: “A warmer atmosphere that holds more water, combined with rapid development and crumbling infrastructure, is turning once-rare disasters into common occurrences.” [3].
Change is constant. Precipitation is not static and things change. There has clearly been an increase in both precipitation and extreme events in the Northeast. Huanping Huang and colleagues at Dartmouth show that between 1996 and 2016 there was a ~53% increase in intense precipitation in the Northeast [16]. In their analysis of 273 extreme events recorded in stations across the Northeast, they show that half the increase is related to tropical cyclones, and one quarter is from extratropical cyclones [16]. So we are trying to evaluate a moving target that may be changing annually.
Macy Howarth and colleagues at the Department of Atmospheric and Environmental Sciences at the University at Albany show that since 1979 there has been an increase in extreme precipitation events in the Northeast and that the number of really big extreme events (>150 mm or 6” in a 24 hr period) has increased from 6 events/yr between 1979 and 1996 to 25 events/yr between 1997 and 2014. Most of this recent change appears to be related to tropical moisture.
Extreme monitoring at the NWS. Quantitative evaluation of extreme rainfall involves understanding total rainfall amounts and rainfall rates over intervals of time (i.e., 1 hr, 6 hr, 24 hr, etc.). The Extreme Precipitation Monitor at the Weather Prediction Center of NOAA shows the Average Recurrence Intervals (ARI) regionally, and predictions provide the Annual Exceedance Probability (AEP) of an event in a given location. Like flooding, an ARI of 100 years has a 1% AEP, meaning it is the rainfall event with a 1% annual probability (the “100 yr rainfall event”). To make the calculation for an area, the NOAA Atlas-14 database is used for comparison and calculation. This database is the full record from hundreds of stations in the Northeast, and the entire record - back as far as possible - for all these stations. The mean length of record of stations with daily data is 66 years (to the early 1950s), and the mean length of record for stations with hourly records is 42 years (mid 1970s) [18]. A key point here is that the record includes all events on record, not just the last two or three decades.
The National Weather Service (NWS) has Quantitative Precipitation Forecasting (QPF) that is focussed on rainfall estimates in the context of annual exceedance probability for several different time intervals (i.e., 1h, 6 hr, 24 hr depending on the forecast lead time). This routine uses the NOAA Atlas-14 data. The annual recurrence intervals are used for making decisions about flood control, and these numbers are “...more relatable to emergency managers and key decision makers rather than absolute rain estimates alone.” [19]. We can take a look at these in graphical form using the precipitation data frequency server [20] (here). Below are the total event precipitation data for Fonda NY, which is near the center of the Mohawk Watershed.
<graph RI v rainfall total> Rainfall probability for the middle of the Mohawk Watershed (here Fonda NY). The 100 year event, also known as the event that has a 1% annual probability (black dot), is 5.1 inches of precipitation (the 1 yr recurrence for the 24 hr event is 2.09”, meaning that there almost certainty that an event of this magnitude would occur each year). The 1% AE for a 6 hour event is 3.42 inches, and the 1% AE for 30 days is 12.8”. Color lines represent other progressive increments of time. Plot modified from the NOAA Atlas-14 database (link to interactive web page here), which is based on the long-term record of records in the Northeast (mean of ~66 yr, but some stations have 100 yr of observations).
In a 2020 paper entitled “The Extreme Precipitation Forecast Table: Improving Situational Awareness When Heavy Rain is a Threat”, Diana Stovern and colleagues from the NWS note that in 2015 the NWS established the Extreme Precipitation Forecast Improvement Project, and “Feedback from NWS forecasters reveal that they still view ARIs [Annual Recurrence Intervals] as a relatively new and advanced statistical concept” [19]. They also note: “Within the last few years, addressing the societal challenges has become the forefront of the forecaster’s job duties in the NWS as the agency has started to focus on Impact-Based Decision Support Services (IDSS).” From this we can conclude that the NWS has got this, but this is a relatively new approach that has developed out of necessity because the societal impact of extreme rainfall is significant.
Long or short record? How has the record of precipitation and extreme precipitation varied over the last 100 years? Has the record been constant, or is it changing over time? Because we need to use past records to plan our response for the future, this question is really important. Stationarity means that the mean and the variance of a process over time does not change significantly - instead it is static, and thus easier to model [21]. It means that the variation in a time series, like precipitation at Albany, is constant and differences over time are statistically insignificant (read about the details here) [21]. But if the rate changes over time, things get more complicated because the recent record (last 10-20 yr) may be more relevant than the historic record (say the entire 70-100 yr record).
If recent findings of an increase in extreme events in the Northeast are correct, then an assumption of stationarity in the rainfall record is incorrect. If the tempo and pace of extreme rainfall is increasing, the record of rainfall events 50 or 100 years ago is not relevant to understanding where we are today. This reasoning means that the Atlas 14 data and the RI of extreme rainfall will underestimate the actual return period or recurrence interval. So if we return to the 3.7 inch event on 24 September in Upstate NY, we would need to recast and recalculate the probability for the last few decades rather than for the last 70 to 100 years. In this case we may find that this event was a 10 year event rather than a 25 year event. This means that our stormwater infrastructure may be underfit for the current job - or even more underfit than we already think it is.
<image pipes/precipitation and time> Cartoon showing a hypothetical relationship between sewage pipes - many built and installed a century ago - and a changing rainfall-runoff regime. Pipes buried in the ground age and slowly become impaired due to cracks, breaks, root growth, etc., so that their effective capacity may diminish over time. Meanwhile, a precipitation regime with more extreme events may swamp the system, and thus the pipes may be underfit for the current situation.
A path forward. A decade ago the Intergovernmental Panel on Climate Change report warned: “It is likely that the frequency of heavy precipitation or the proportion of total rainfall from heavy rainfalls will increase in the 21st century over many areas of the globe” (IPCC report - Seneviratne et al., 2012) [22]. A recent news feature in Nature explored the link between an increase in atmospheric moisture and the increase in extreme rainfall, but also the unevenness of precipitation. Angeline Pendergrass at the National Center for Atmospheric Research (NCAR), who recently published on extreme precipitation, notes that: “The next 20 years will be worse than the last 20 years — all indications point to that.... And things will be completely nuts by the end of the century ....” [23]. In her paper she notes that: “Rather than assuming more rain in general, society needs to take measures to deal with little change most of the time and a handful of events with much more rain” [24].
For many, 2021 appears to have been a turning point in the recognition that we are seeing a change in precipitation intensity in the Northeast generally, in New York overall, and in the Mohawk Watershed.
In July 2021 Dan Shapley, Interim VP for Programs and Interim Hudson Riverkeeper, wrote in a blog post: “Extreme rains will overwhelm sewers that can’t handle even routine storms today. If we don’t upgrade our sewers, we’ll find the water unfit for recreation just when we need it most….New York State has the greatest documented need for sewer upgrades of any state.” [27]
We need to directly explore the implications of these extreme rainfall events on our infrastructure. It would be prudent to consider how probable maximum floods may threaten intermediate and high hazard dams, and we also need to invest in our aging pipes to reduce sewage overflows and impairment of our surface streams and rivers. Some of our communities - like Colonie and Cohoes - use raw water from these rivers for drinking water, and thus overflows driven by extreme precipitation can be considered an emerging threat to municipal water supplies.
This and other Notes from a Watershed are available at: Mohawk.substack.com
Further reading
[1] 2021 European floods, Wikipedia - downloaded 8 October 2021 (here)
[2] BBC, 2021. Flash floods: Parts of London receive a month of rain in one day. 13 July 2021. (here).
[3] Kaplan, S., 2021. Tennessee floods show a pressing climate danger across America: ‘Walls of water’ ‘Climate change has come barging through the front doors of America’ Washington Post, 23 August 2021 (here)
[4] Iowa Environmental Mesonet. National Weather Service raw text product. 21-08-23. (here).
[5] 2021 Tennessee Floods. Wikipedia. (here).
[6] University at Albany. Ida Dumps Record-Breaking Rain at New York State Mesonet Network Sites. 7 Sept 2021. (here).
[7] Staff, Albany Times Union. 8 June 2021. Storm causes flooding in Albany, more rain to come. (here).
[8] NWS Binghamton (NY), Rainfall total for July 11-12, 2021. (here).
[9] Levy, Dan. 12 July 2021. WNYT. Middleburgh deals with serious flooding after heavy rain. (here).
[10] Schneider, H.R., 20 August 2021. Utica Observer-Dispatch. 'It was a mess': Rome hit hard by flooding; officials monitor flooding in North Utica. (here).
[11] National Weather Service, Binghamton. Heavy Rainfall August 17 to 20, 2021 Fred. (here).
[12] Office of the Deputy Mayor (NYC), 2021. The New Normal: Combating storm-related extreme weather in New York City. PDF available here.
[13] Lumia, R., Firda, G.D., and Smith, T.L., 2014, Floods of 2011 in New York: U.S. Geological Survey Scientific Investigations Report 2014–5058, 236 p., http://dx.doi.org/10.3133/sir20145058. http://pubs.usgs.gov/sir/2014/5058/
[14] Schneiderman, E.T., 2014. Current and Future trends in extreme rainfall across NY State Across New York State. NY State office of the Attorney General. (here).
[15] Sarawitz, D, and Pielke, R.. Jr, 2001. Extreme Events: a research and policy framework for disasters in context, International Geology Review, Vol. 43, 2001, p. 406-418.
[16] Huang, H., Winter, J. M., & Osterberg, E. C. (2018). Mechanisms of abrupt extreme precipitation change over the northeastern United States. Journal of Geophysical Research: Atmospheres, 123, 7179–7192. https://doi.org/10.1029/ 2017JD028136
[17] Howarth, M.E., Thorncroft, C.D. and Bosart, L.F., 2019. Changes in extreme precipitation in the northeast United States: 1979–2014. Journal of Hydrometeorology, 20(4), pp.673-689.
[18] Perica, S, Pavovic, S., St Laurent, M., Trypaluk, C, Unruh, D., Martin, D., Wilhite, O., 2015 (revised 2019). NOAA Atlas 14 - Precipitation-Frequency atlas of the United States. Vol 10, version 3: Northeastern States. Connecticut, Maine, Massachusetts, New Hampshire, New York, Vermont. US Department of Commerce, Silver Spring MD. https://www.weather.gov/media/owp/oh/hdsc/docs/Atlas14_Volume10.pdf
[19] Stovern, D. R., J. A. Nelson, S. Czyzyk. M. Klein, K. Landry-Guyton, K. Mattarochia, E. Nipper, and J. W. Zeitler, 2020: The Extreme Precipitation Forecast Table: improving situational awareness when heavy rain is a threat. J. Operational Meteor., 8 (7), 93-104, doi: https://doi.org/10.15191/nwajom.2020.0807 (here).
[20] Analyzing Alpha. April 2021. What Is Stationarity in Time Series Analysis? A Visual Guide (web site here).
[21] NWS Weather Prediction center. Extreme precipitation monitor. https://www.wpc.ncep.noaa.gov/qpf/epm/extreme_precip_monitor.php
[22] Seneviratne, S.I., N. Nicholls, D. Easterling, C.M. Goodess, S. Kanae, J. Kossin, Y. Luo, J. Marengo, K. McInnes, M. Rahimi, M. Reichstein, A. Sorteberg, C. Vera, and X. Zhang, 2012: Changes in climate extremes and their impacts on the natural physical environment. In: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)]. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press, Cambridge, UK, and New York, NY, USA, pp. 109-230.
[23] Witze, A., 2018. Why extreme rains are gaining strength as the climate warms; from Atlantic hurricanes to the Indian Monsoon, storms are getting worse and becoming more erratic. Nature 563, 458-460.
[24] Pendergrass, AG, and Knutti, R. 2018. The Uneven Nature of Daily Precipitation and Its Change. Geophysical Research Letters., v. 45, issue 21. P. 11980-11988.
[25] Shankman, S., 23 July 2021. Climate change will bring heavier storms and our sewers aren’t ready. Boston Globe. (here).
[26] Rosenbaum, M., 7 October 2021. Union’s delphic waters polluted by sewage overflow. Concordiensis, v. 144, issue 20, p. 1.
[27] Shapley, D. 29 July 2021. Climate extreme and beach closures on the Hudson. Riverkeeper. Link here.