Carbon Solutions New England

Flooding 101

Flooding 101

In recent years flooding has become commonplace in New England. Flooding occurs when the volume of water within a river or lake overflows its usual boundaries. While the amount of water in a river or lake varies with changes in precipitation, snow melt, and amount of impervious surfaces (i.e. pavement), it is commonly a signficiant flood when water endangers land areas used by humans.

To begin, lets watch a brief National Geographic video to get a background on the natural diaster of flooding.

The following page is designed to explain flooding and some of its modern causes. We also offer some suggestions and resources for how we can reduce the risks human's face from flooding and be better prepared for a future with more frequent and significant floods. To better understand flooding we pose and begin to answer the following questions. Just scroll down to see answers and resources for each question or click on a specific question to be taken directly to its explanation.

What is a 100 year flood?

A one-hundred-year flood is often thought of as the size of a flood that is expected every 100 years on average. However, it is more accurately described as the 1% annual exceedance probability flood, since it is a flood that has a 1% chance of being equaled or exceeded in any single year. The term "100-year flood" is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. Likewise, the term "100-year storm" is used to define a rainfall event that statistically has this same 1-percent chance of occurring. In other words, over the course of 1 million years, these events would be expected to occur 10,000 times. But, just because it rained 10 inches in one day last year doesn't mean it can't rain 10 inches in one day again this year (USGS).

100 year flood is an important term for a number of legal and regulatory reasons. Based on the expected flood water level, a predicted area of water cover can be mapped out. Floodplain map figures are very important for building permits, environmental regulations, and flood insurance.

Because our project addresses the Lamprey River in Newmarket, NH a daily discharge graph from that river can help to illustrate the concept of 100 year floods. The vertical axis of the graph shows the peak annual discharge (the highest amount of water moving through the river at a given point) while the horizontal axis shows the year in which it occured. The line across the top of the graph represents a discharge that indicates a 100 year flood. An official 100 year flood has occurred three times since 1987 and was very close to occuring again most recently in March of 2010. To see real time data for the river click here.

Figure Credits: Compiled by Cameron Wake & Ann Scholz based on data from USGS National Water Information System


How does land use affect flooding?

As human population expands and develops how we use our land has an increasing impact on flooding risks.

This image demonstrates how increased development (such as buildings and pavement) prevents water from being absorbed back into the ground. As impervious surface (surface that is covered by non-natural objects) increases the ground is able to absorb less and less water and more water runs off and accumulates in non-natural areas. This can cause flooding as the water does not have natural areas to infiltrate into. Click here for a larger image of the sketch below.

Image Redrawn by Kristi Donohue EOS/UNH from data provided by Urban Flood Risk Management- A Tool for Integrated Flood Management. World Meteorological Organization & Global Water Partnership. March 2008.


For more information on how land use can affect flooding check out this USGS Fact Sheet on the effects of urban development on flooding.


How does the climate affect flooding?

Background information on climate change

Indicators of Climate Change in the Northeast

In the past decade we have seen a large increase in the size and frequency of rain storms. This has contributed to increased flooding. The graph below demonstrates the increase in 4-inch percipitation events for Durham, NH (a town in the Lamprey River Watershed) and Lawrence, MA (for comparison).

This map shows trends for 4 inch precipitation events over the northeastern part of the United States. Southeastern New Hampshire and Northeastern Massachusetts have seen the greatest increase in these types of events over 5 decades.

Large flooding events often led to federally declared disasters. These disasters require significant resources for both response and clean up. The graph below shows the large increase in the cost of Presidentially declared disasters in NH over a twenty year period. This increase correlates with the increase in extreme precipitation displayed above.


What can I do in my community about flooding?

Flooding is something that can potentially affect all of us in one way or another, whether it be through damage or loss of property, road closures, higher insurance rates, the list goes on. Fortunately, there are a number of resources available to help citizens and communities prepare for and deal with flooding disasters. Listed below are a number of links to specific references citizens and homeowners may find useful.

Are you ready? An In-depth Guide to Citizen Preparedness Extensive reference manual covering many types of disasters.

New Hampshire Traffic and Travel Information Timely information for travellers on traffic accidents, current driving conditions, weather reports, road closures, etc. Information also accessible by dialing 511.

New Hampshire-specific flood safety information Safety and contact information from the N.H. Department of Environmental Services. Includes links to fact sheets on flood contamination.

Real-time hazard maps Links to hazard maps that will keep you up-to-date on hazardous conditions throughout the Northeast and the US.

Turn around, don't drown National Weather Service campaign to warn people about the hazards of walking or driving a vehicle through flood waters.

211 New Hampshire Web site and toll-free telephone number that connects callers, at no cost, to information about critical health and human services available in their community, including disaster help. Residents in New Hampshire can contact 2-1-1 NH toll-free by dialing 2-1-1 in state or 1-866-444-4211 from out of state.

There are things community decision makers can also do to try to prevent and be prepared for a flooding disasters. These can include learning about living in a floodplain and flood insurance. is a great resource and can be accessed here

What can I do in my community related to this project?

Click here to get involved!