SNEP

Flow Duration Curves

A New Model to Envision Healthy Watersheds and Resilient Communities

Shasten Sherwell

The traditional urban design includes many impervious surfaces. As cities and towns continue to grow,
natural landscapes get replaced by buildings, houses, roads, and parking lots. These hard surfaces block
water from soaking into the ground, leaving the water with fewer places to go, which can create
problems for surrounding communities if not properly managed. In an undeveloped landscape, rain is
able to soak into the ground. Any contaminants that the rain might have come into contact with on the
surface is readily filtered out as it flows through layers of soil before finally entering the groundwater.
This natural process of infiltration and slow release of groundwater allows watersheds increased
resilience against storms and flooding. However, this resilient effect is damaged when landscapes are
paved over with impervious material that water cannot flow through. In these instances, rainfall collects
and flows, leading to a high volume of stormwater runoff that rushes down streets, pools in parking lots,
and empties into nearby rivers and streams without being afforded the filtering effect of the soil. This
can result in urban flooding, erosion, and the pollution of nearby waterways as contaminated
stormwater easily enters nearby habitats.

NATURAL ENVIRONMENT

URBAN ENVIRONMENT

10% shallow infiltration

5% deep infiltration

Impact of development on natural hydrology. Source: City of Detroit Water and Sewerage Department

Floods are one of the most common natural disasters in the United States: costing communities millions
of dollars in clean up and repairs, displacing residents, and sometimes resulting in adverse health effects
from pathogens carried in flood waters. The frequency of severe flooding events has increased in the
last three decades and is expected to continue as climate change brings wetter and more severe storms.
In response to flooding (among other issues caused by impervious surfaces) many communities have
relied on stormwater management practices to minimize the impacts of storms on their respective
watersheds. However, current stormwater management standards do not fully address the problems at
hand. Correcting the impact of development on watersheds requires a holistic watershed management
approach.

The dilemma for many communities is how to meet the demands of a growing population and the need
for future development while preserving a healthy watershed and enhancing its resilience. A new SNEP-
funded project intends to provide local decision-makers with recommendations on immediate and
future actions required to maintain natural hydrologic resiliency. The project aims to help communities

Southeast New England Program
www.epa.gov/siiecwrp


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SNEP

Flow Duration Curves

A New Model to Envision Healthy Watersheds and Resilient Communities

Shasten Sherwell

adopt conservation development practices and holistic hydrologic management approaches that will
make them more resilient to future climate conditions, while also accommodating future development.

What is a Flow Duration Curve? It describes the flow
characteristics (frequency and duration) of a stream
over a long period of time, Flow Duration Curves can
be used to quantify impacts of development on a
watershed as well as benefits of adopting alternative
development practices.

The first phase of the project involves
the development of Flow Duration
Curves (FDCs) (see box); whereas phase
two involves the creation of
recommendations for better
management practices based on the
results of compared modeled scenarios.
By comparing stream flows between pre-
development, current, and future
development scenarios, FDCs allow for powerful diagnostic evaluation of the impacts of watershed
development and the potential benefits of future management alternatives - some of which can include
the implementation of green infrastructure. FDCs will be created using existing calibrated hydrologic and
watershed management models for the Taunton River watershed. With the input of a committee of
experts, the FDC model will serve to create recommendations for municipalities and local decision-
makers to adopt sustainable development practices that conserve healthy hydrological conditions by
incorporating green infrastructure, low impact development, and landscape architecture.

This is a two-year project currently in the modeling phase.
In addition to the modeled impacts and expert
recommendations, the project will also deliver
educational materials to municipal staff, city planners,
watershed advocates, and other local decision-makers in
the SNEP region on how to implement these
recommendations. We can expect these results next year,
so stay tuned and help us envision resilient communities
and healthy watersheds.

For more information, please contact Ray Cody at

Codv.Rav(5)epa.gov

Example of green roofs in a building complex in Osaka; Japan. Source: Yuji
Kotani/Getty Images

1.	Holistic Watershed Management for Existing and Future Land Use Development Activities: Opportunities for Action for Local Decision Makers. SNEP

2.	Flinker, Peter (2010). The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality. Rhode Island Department of Environmental
Management, http://www.dem.ri.gov/programs/bpoladm/suswshed/pdfs/imperv.pdf

3.	Denchak, Melissa (2019). Flooding and Climate Change: Everything You Need to Know. NRDC. https://www.nrdc.org/stories/flooding-and-climate-change-
everything-vou-need-know#causes

Southeast New England Program
www.epa.gov/siiecwrp


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