[Durham INC] DDFC Stormwater Site

[Will Wilson]

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:
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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:
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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:
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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/