Urban stormwater study finds little difference between managed,
unmanaged flows

Date:
September 16, 2021
Source:
Penn State
Summary:
At a time when many municipalities are seeking to control urban
stormwater by investing heavily in green infrastructure -- such
as water- quality ponds, infiltration basins, porous pavement and
riparian plantings -- a new study suggests that these expensive
efforts may not have much of an impact.



FULL STORY ==========================================================================
At a time when many municipalities are seeking to control urban stormwater
by investing heavily in green infrastructure -- such as water-quality
ponds, infiltration basins, porous pavement and riparian plantings --
a new study suggests that these expensive efforts may not have much of
an impact.


==========================================================================
The results of the research are especially eye-opening because the
intensity of rainstorms is increasing, and urban stormwater management
is more important than ever, noted research team member Jonathan Duncan, assistant professor of hydrology, College of Agricultural Sciences,
Penn State.

"No one wants to hear this, but we have a high level of confidence
in our data and experimental design that reduced variability across sub-watersheds we studied," he said. "A few other studies have suggested
this, but they were not conducted with the detailed watershed-scale
hydrology data we had. The bottom line is that we were not able to
detect any difference in flows created by stormwater management."
The research was unique because it was conducted in the Dead Run watershed
in Maryland's Baltimore County, "the most intensely gauged urban watershed
in the world," according to Duncan.

As a result, the researchers were able to examine two decades worth
of ecological data related to stormwater flows. "There are five
gauging stations within a 6-square-mile watershed -- other cities
are lucky if they've got a few -- and there are six just within this
one sub-watershed in Baltimore," Duncan said. "So, it's allowed for a
better mechanistic understanding of urban hydrology." To reach their conclusions, the researchers analyzed the hydrologic response - - the
change in runoff volume and timing -- in three small, highly impervious
urban sub-watersheds to "pulse" rainfall events. This allowed them to
assess how traditional stormwater management alters urban hydrographs,
which are charts showing streamflow with respect to time.



==========================================================================
The watersheds vary in stormwater management coverage from 3% to 61% and
in impervious surfaces from 45% to 67%. Those water-repelling surfaces
include building roofs, roads, highways and parking lots. For the study,
the researchers selected a set of storm events that involved a single
rainfall pulse, with more than 96% of total precipitation delivered in
60 minutes.

The research team used watershed-average rainfall data, generated
by local radars, to pinpoint local storm "hyetographs" -- graphical representations of the distribution of rainfall intensity over time --
for each event in each watershed. That adjustment, Duncan pointed out,
enhanced watershed comparability because it compensated for the extreme variability of rainfall intensity of short-duration storm events.

In findings recently published in Hydrological Processes, the researchers reported that despite dramatic differences in the fraction of watershed
area draining to stormwater management features across the three headwater tributaries studied, they did not find strong evidence that stormwater management caused significant reduction of volume or timing of peak
storm flows.

The hydrograph response for the three watersheds was remarkably uniform
despite contrasts in stormwater management, impervious cover and spatial patterns of land use, they wrote in the paper.

"Our findings contribute more evidence to the work of previous researchers suggesting that stormwater management is less effective at decreasing
urban runoff than commonly is assumed," Duncan said. "In these watersheds,
we believe that the percentage of impervious surfaces may have greater influence on runoff volume than the percent of stormwater management
coverage." Duncan explained that, historically, communities have used
gray infrastructure -- systems of detention basins to hold water back
as well as gutters, pipes and tunnels -- to move stormwater away from
where people live to treatment plants or local water bodies. But the
gray infrastructure in many municipalities across the country is aging,
and its capacity to manage large volumes of stormwater is decreasing.



==========================================================================
To meet this challenge, many communities are installing green
infrastructure systems to bolster their capacity to manage
stormwater. Green infrastructure absorbs and filters stormwater where it
falls. Although there has a been a trend toward green infrastructure in
recent years, it still comprises a small percentage of total watershed
area treated in the Baltimore study, with most stormwater management
being traditional detention ponds.

Congress enacted the Water Infrastructure Improvement Act in 2019,
which defines green infrastructure as "the range of measures that use
plant or soil systems, permeable pavement or other permeable surfaces
or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer
systems or to surface waters." Green infrastructure has many co-benefits,
such as carbon sequestration and reducing the urban heat island effect,
Duncan noted. "As the fraction of green infrastructure increases, the
sooner we understand if it is more effective in managing watershed scale
runoff than traditional stormwater practices, the better," he said.

Also involved in this research were Andrew Miller, Department of
Geography and Environmental Systems, University of Maryland; Claire
Welty, Department of Chemical, Biochemical and Environmental Engineering, University of Maryland; and Mary Lynn Baeck and James Smith, Department
of Civil and Environmental Engineering, Princeton University.

The Chesapeake Bay Trust and the National Science Foundation supported
this work.

========================================================================== Story Source: Materials provided by Penn_State. Original written by Jeff Mulhollem. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Andrew J. Miller, Claire Welty, Jonathan M. Duncan, Mary Lynn Baeck,
James A. Smith. Assessing urban rainfall‐runoff response
to stormwater management extent. Hydrological Processes, 2021; 35
(7) DOI: 10.1002/hyp.14287 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210916142843.htm

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