[Durham INC] DDFC Stormwater Site

[Peter Katz]

Will, I really appreciate your analysis of this issue, thank you for sharing
it with the INC List.  I think it's important for us to understand that the
question of ideal-size vs. the minimum required size for this constructed
wetland is not nearly as cut and dried as what has been presented to the
public thus far. 
I am forwarding to the Old North Durham listserve.
Best,
-Pete 

-----Original Message-----
From: inc-list-bounces at rtpnet.org [mailto:inc-list-bounces at rtpnet.org] On
Behalf Of Will Wilson
Sent: Monday, July 02, 2012 9:01 AM
To: inc listserv
Subject: [Durham INC] DDFC Stormwater Site

All,

I wanted to comment on and provide a bit of background concerning the INC
and DOST presentations on the Duke Diet and Fitness Center (DDFC) stormwater
facility, primarily concerning water volume and water quality. Sorry it's
long, but I wanted to include background information, too. I teach a
stormwater course at Duke, presently writing a book on stormwater, and have
a pretty good grasp of the science, however, the mish-mash of federal,
state, and local regulations are more confusing and less rational than
science. So, as the recent sea-level-rise fiasco shows, there's science, and
then there's regulations. I don't have a strong expertise in regulations.

The Basic Problem:
--- ----- --------
Urban areas have lots of impervious surfaces, which are covered areas where
rain doesn't directly fall on the ground: parking lots, roads, houses,
sheds, parked cars, etc. When it's not raining, pollutants settle on these
surfaces, and these pollutants include nutrients, oils, heavy metals, salt,
pesticides and herbicides, and more. Much more pollution settles out in
urban areas than rural ones, and most biologically available nitrogen (in
our area) comes from burning fossil fuels -- cars, trucks, coal, but
emissions controls help reduce that load. Regions with heavy agricultural
uses are different than ours. Even tire and brake wear add significant
amounts of various other pollutants. 
Storms wash these pollutants off of the impervious surfaces, and the highest
pollution concentrations in the stormwater are found in the "first flush",
or the initial stormwater flow off of a site.

In addition to pollutants, stormwater rushing off of impervious surfaces
leads to high peak flows downstream, like the sudden rush of water from a
toilet, and leads to a phenomenon called the "urban stream syndrome". 
High, flashy flows deeply erode streams, not only carrying the eroded
sediments downstream, but the resulting stream profile acts almost like a
drainage ditch during the dry periods. These "ditches" drain groundwater
near the stream, which dries out the stream-side vegetation (the riparian
zone), and reduces the nutrient processing abilities of these urban streams.
Leaky sewer pipes previously submerged below the water table drew water into
the pipes, but a lowered water table can instead let its contents leak into
the dry soils.

To deal with pollutants and flashy flows, "stormwater control measures
(SCMs)" try to intercept and contain the first one inch of rainfall to
promote settling, evaporation, photodegradation, and bioretention of the
contained pollutants. Slowly releasing this water also reduces peak flows
and helps preserve stream ecosystems that process nutrients, as well as
emptying out the volume of the SCM in preparation for the next storm. Of
course, these SCMs have to be built carefully to not wash away when 6 inch
storms come along.

As gardeners know, plants desperately want more nutrients, and algae in
lakes are no different. We learned long ago that nitrogen in lakes make
algae flourish, but algae-eating animals can't keep up, and when all that
algae dies, bacteria break down all that "algae compost" and use up all of
the oxygen in the water. That lack of oxygen makes fish die, the water
stinky, and leads to all sorts of other problems. We've added nutrient
regulations to deal with the problem of nutrients.

Solutions:
----------

The Falls Lake nutrient reductions are part of those regulations, and the
exact numbers for the various implementation stages aren't so important, but
the concept is clear: we must reduce the nutrients flowing into the lake. We
are "graded" on the nutrients in the streamwater right where it flows into
the lake, so as far as nutrients go at that spot, a reduction of 100 pounds
anywhere upstream is a reduction of 100 pounds. We should all support our
stormwater services department in finding the places upstream where the most
reductions can be done for the least amount of taxpayer money. Downtown is
likely a disproportionately high source of the nutrients, but it's also an
extremely expensive place to reduce them, in part because land prices are
high and undeveloped land is limited. So, just from the perspective of
nutrients, there is a reasonable argument that we should make the reductions
where they're the cheapest.

Stormwater volume is a different matter. In undeveloped watersheds, rain
falls across the entire watershed, evaporates or helps plants and trees
grow, or soaks into the ground. That's a process that naturally holds back
water throughout the watershed until it seeps from groundwater to
streamwater. In urban areas, all that water quickly flushes downstream
because impervious surfaces prevent it from soaking into the ground. We need
to hold that stormwater back or sacrifice all of the natural, downstream
nutrient processing capabilities. We could hold back that volume with one
big facility way downstream like the DDFC site, or lots of smaller
facilities farther upstream using a variety of control measures.

Let's think about the Duke Diet and Fitness proposal. Above that spot is
about 500 acres of watershed with a high fraction of imperviousness, and, if
you have a one inch rain over that area, you could think of the volume as a
temporary 500 acre lake that's just one inch deep, or a one acre lake that's
500 inches deep, or anything in between. The DDFC site is 9 acres, meaning
it would need to be about 56 inches deep (about 5
feet) to hold that volume, assuming no wetlands media in the pond. Using
less land for the pond at that site means a deeper pond, or not holding all
of the water volume at that location.

There are many ways to deal with this volume, including "caverns" built
under parking lots, which provide irrigation water for green roofs and urban
trees that help cool our city that reduce our energy demands and improve air
quality. These large cisterns also just collect stormwater that can slowly
be released like a detention pond, without any water use. Here is a link to
one such parking lot approach: 
http://stormtrap.com/. Do we have a few acres of parking lot downtown that
could serve this purpose? Let me also provide a link to a free magazine,
imaginatively called "Stormwater": http://www.stormh2o.com/. 
Many more approaches to stormwater treatment are discussed here: 
http://www.epa.gov/oaintrnt/stormwater/best_practices.htm.


The DDFC Presentation:
--- ---- -------------

During the presentations I've heard, it has been stated that using less than
the full site simply won't work. I have seen much smaller constructed
wetlands, and it is not at all clear why a 3-, 5-, or 7-acre pond simply
can't work at the DDFC site. Small constructed wetlands exist and function
properly, so it can't be a matter of science. I don't understand that
assertion and direct questions on that issued are deflected.

The DDFC stormwater presentation describes and promotes a policy choice, and
lacks a full explanation of the range of policy choices available to
citizens. Various folks at INC described the broader issue nicely: In the
urban area, we need to think holistically about the solution to the
stormwater problem, and break down any silos that prevent considering other
benefits like water parks, education, urban cooling, and urban trees. There
will be a lot of money devoted to the issue no matter the choices, and if we
can solve problems beyond stormwater at the same time from the same dollars,
or even if alternative approaches might well be more expensive, citizens
might prefer these additional environmental amenities and services for a
little additional cost. This situation is a wonderful opportunity to discuss
downtown green infrastructure desires and needs, and the DDFC presentation
sidesteps that opportunity.

Durhamites may well choose the cheapest option of using the DDFC site to
handle all the water volume and incentivize optional green roofs downtown.
However, we should also consider the option of a partial solution at the
DDFC site and requiring subsidized green roofs downtown, along with a more
comprehensive set of green infrastructure. That is a policy choice for
citizens to make, and we need complete information from the stormwater
department. The stormwater department faces a challenging problems and
continuously changing regulations, but I would encourage them to present the
full array of policy choices available to the city.

Will Wilson

--
http://www.biology.duke.edu/wilson/
New Book: http://www.constructedclimates.org/

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