Flooding

This section addresses non-coastal floods, which occur when rivers or streams overflow their channel, flooding the adjacent land or flood plain. In flood-prone areas, "damage" is contingent on the presence of humans and infrastructure. However, in New York State many original settlements were concentrated within the most viable transportation corridors, typically along rivers and their valleys, and much of the State's infrastructure reflects these early patterns of development.

Assessing Changes in Flood Risk

As detailed earlier, climate change may result in more total precipitation as well as more total rainfall in larger storm events in New York. This likelihood of increased total annual rainfall as well as higher rainfall intensities is often used as justification for predictions of an increased likelihood of flooding. To appropriately assess whether flooding in New York State may increase, it is important to focus on the dominant processes leading to flows of different sizes on streams and rivers in NY and to realize studies carried out in other regions may not be applicable.

To better understand flood processes in NYS, linkages between stream discharge and precipitation and snowmelt were examined for three moderately sized watersheds in three different regions of the state: Ten Mile River in the lower Hudson Valley, Fall Creek in the Finger Lakes Region, and the Poultney River in the Lake Champlain Valley. The three water bodies were selected because they have at least 50 years of stream gage and precipitation records and do not have any major impoundments or diversions.

For each waterbody, the following indicators were examined: the annual maximum daily mean flow, the daily mean flow associated with the annual maximum 2-day precipitation event, and the annual maximum daily flow associated with the maximum 3-day snow melt event. The basic intent was to investigate the relationship between maximum stream discharges and two of the most obvious causes of flooding, large precipitation events and melt of a sizable snowpack. The results are summarized in Table 1.

  % Annual Max Discharge Events Occurring When:
Watershed 2-Day Annual Max Rainfall 3-Day Annual Max Snowmelt May to October
Ten Mile River 15 10 14
Fall Creek 20 20 5
Poultney River 17 9 10

Table 1. Summary of causative conditions of annual maximum discharges on three watersheds representative of conditions in NYS.

The three watersheds generally reveal the same patterns in flooding. Most notably, less than 20% of annual maximum daily discharges correspond to the 2-day maximum rainfall. most 2-day maximum rainfall events occurred between May and October. During this time of year, moisture laden southern air reaches New York State and causes 2-day rainfall amounts that can exceed 5 inches. However, counteracting these larger rainfall amounts is an increase in available soil water storage capacity due to the drier soil conditions and lowered water tables that are common in the same timeframe. Using data from the Fall Creek watershed in central New York, Figure 3 shows the correlation between the 2-day storm precipitation amount and the resulting mean daily discharge in the stream. Although there is a general upward trend (larger stream discharges occur with larger precipitation amounts), rainfall is clearly not the only factor related to peak discharge. For instance, there are three days with discharges around 6000 cubic feet per second (cfs), but these correspond to medium-sized storm rainfall amounts ranging anywhere from 3-5 inches.

storm rainfall amount/peak discharge

Figure 3. Storm rainfall amount and resulting instantaneous peak discharge on Fall Creek, Tompkins County, NYS for the annual peak discharges from 1974 to 2007.

In brief, given the number of interacting factors that affect flooding (snowmelt, precipitation, growing season length, evaporation rates, etc.), it remains uncertain whether the magnitude of annual maximum flows will increase with climate change. If it does, it would seem probable that wetter spring conditions would likely increase the number of moderate floods (10-25 year return floods).

Additionally, more frequent flooding of larger magnitude are possible and more certain in areas in which flooding is directly linked to the intensity and amount of rainfall. This is the case in urbanized areas because impervious surfaces, reduced vegetative cover, and compacted soils minimize the buffering effects of soil water storage and make stream flows closely correlated to rainfall. Similarly, small, steep basins with highly convergent topography, such as those found in the Southern Tier, rapidly aggregate water and have a limited capacity to attenuate rainfall inputs, making increases in flash floods likely to follow from increases in rainfall intensity.

Suggested Adaptations

In response to reducing flood risks in urban areas, there are federal stormwater management regulations (National Pollutant Discharge Elimination System (NPDES) stormwater program) applicable to both large and small communities that are in the process of being implemented. Both when retrofitting existing development and designing new developments, a continued emphasis should be placed on encouraging cost-effective stormwater management infrastructure that enhances natural hydrologic processes (infiltration, recharge, evaporation) instead of rapidly conveying stormwater to receiving waterbodies.

Although there remains uncertainty as to whether climate change will increase flooding in more rural systems, consideration should be given to moving development out of flood plains as buildings, infrastructure, and flood protection structures age and it becomes time to rebuild. This strategy of phased withdrawal from the highest-risk, flood-prone areas is currently recommended by the National Association of Floodplain and Stormwater Managers and was publicly endorsed by the NYSDEC commissioner at the 2008 Flood Summit.

Waste water treatment plants within flood plains are one facility that should receive special consideration for being moved, even with a limited degree of change in flood risk. A brief interruption of operations during infrequent floods may be acceptable (high flood water would dilute and rapidly transport discharge), but floods that routinely interrupt operations for an extended time period pose a risk to public health as well as the waterbody health. Especially since many aging waste water treatment plants are in need of replacement, the possibility of moving the waste water treatment plants out of the flood plain should be considered when new facilities are designed.

Learn More

Climate Change Links

Intergovernmental Panel on Climate Change (IPCC) (link)

Northeast Climate Choices (UCS Reports) (link)

Climate Change and Northeast Agriculture (link)

Climate Change and Water Resources (NCAR) (link)

USDA Global Change Program Office (GCPO) (link)