Number range CHAPTER© 2004 by Leo Poppoff

February 17, 2004

Basin Watch

by

Leo Poppoff

A HOLISTIC VIEW OF BMPs - PART II

Think of green roofs as sponges. Then imagine roofs at Tahoe covered
with plants and organic soil sopping up precipitation. It would be
scenic -- and greatly reduce runoff. That's just one of the ideas in
Eric Strecker's portfolio of Best Management Practices (BMPs). Strecker,
with the Portland Office of GeoSyntec Consultants, has been studying
BMPs in the Tahoe Basin for several years. He is also one of the
principal investigators developing an International BMP data base for
the EPA. We covered some of his work in last week's column.

Strecker strives to increase the "sponge" effect of urban watersheds.
Vegetation and organic soils soak up water like a sponge. The water is
then evaporated (or transpired by plants while they're growing) during
dry periods and doesn't run off into streams, gutters or lakes.
Vegetated swales, infiltration of water into shallow soil layers, tree
canopies (lots of snow held in trees evaporates for example), and green
roofs help this natural process.

"When I first heard about green roofs, I thought it was crazy," Strecker
said. "But, the more you look into it, the more you realize it isn't so
crazy. For example, what breaks down a roof? It's sunlight. So, if sun
is beating down on plants and soils on a roof, rather than on the
roofing materials, it actually lasts longer. The roofs have to be
designed right, and it might cost a little more to begin with, and it
means that you'll need to do a better job of waterproofing. However,
because these roofs last longer, overall costs are not so different. So,
I've turned into much more of an advocate of green roofs as a way to cut
down the effect of impervious areas.

"In Portland, they've used succulents for planting these roofs. There
are some drought-tolerant and cold-tolerant succulents. You don't have
to irrigate them very much -- perhaps a couple times a summer. Some
folks tried grasses, but you don't want a lot of combustible materials
on roofs. It's a fire issue.

"Here in Portland," Strecker added, "they're using two-inch and
four-inch soil systems. That's not a lot of extra load when the soil is
soaked. Even with a two-inch soil roof, they're showing a 40 to 50%
volume reduction in runoff, even during the winter time. But, you have
to be careful about the kind of soil you put on the roof, because some
soils naturally leach phosphorus."

Are green roofs practical in snow country? "I was up at Snowbird Ski
Resort recently," Strecker answered, "and they had vegetated roofs on
parking covers. It was probably for aesthetic reasons (e.g., the areas
looked like part of the landscape from above), but it helped water
quality."

Strecker likes to find many ways to cut down on runoff -- a holistic
approach. For example, at Tahoe, he notes that snow removed from roads
is stored in large piles. If it were to rain on those piles, rapid
melting would cause rapid runoff. But, Strecker points out, if the snow
was spread on vegetated swales, some snowmelt would soak into the ground
and evaporate later. Or snow could be spread out and melted slowly
during sunny winter days, avoiding large flows later in spring.

If water can be prevented from running off a parcel, it would lessen the
cost and effort required to treat that water downstream. Green roofs and
vegetated surroundings create what Strecker calls a "sponge,"
simulating, to a degree, natural undisturbed conditions and minimizing
runoff.

Perhaps more of the watershed restoration money ought to be spent for
on-site BMPs that reduce runoff volumes, such as biofiltration systems
(swales with moisture adsorbing soils), stormwater planter boxes and
green roofs. Systems that infiltrate also reduce runoff, but some of
those flows might also reach the lake and some of the dissolved
pollutants might still get to the lake.

Strecker also stresses "that in selecting and designing BMPs for
treating runoff flows, one needs to consider the pollutant types, the
form that they are in (dissolved and/or attached to small particles),
and the runoff rates. Too often, all BMPs are considered the same, but
they are not in terms of performance. For example, many of the
underground systems installed at Lake Tahoe are probably doing very
little to reduce the very fine particulates and nutrients that are the
concern at the lake."

Strecker's work has been focused on improving how BMPs can be selected
and designed to specifically target and remove the pollutants of
concern.

Questions or comments? Send them to basinwatch@sbcglobal.net