By THOMAS W. HOLZ SCA Consulting Group
Evidence is incontrovertible that
development destroys fish habitat. For over three decades the literature has
documented the devastation of streams in watersheds when they are urbanized.
The latest in a long string of papers
on the subject is by Chris May and Richard Horner (“Salmon In the City
Conference," I998). They
corroborate the near total destruction of stream habitat when a watershed reaches
impervious surface percentages between 10 and 20 percent. The steepest decline
in habit quality begins when the first few percent of forest are converted to
pavement.
Traditionally, urbanization implies
impervious percentages of about 30 percent for residential development to 80
percent or more for commercial uses. Because of its requirements for high
densities, the Growth Management Act may drive impervious surface percentages
even higher.
To date, we have depended on
stormwater design manuals required by regulation to protect habitat Though the
storage volume to contain runoff from new development has been steadily rising
for over 20 years, evidence shows that engineering approaches have not been
effective at pre- serving receiving waters
In a paper presented by Beyerlein and
Brascher (1998) at the “Salmon In the City" conference, it was shown that
such methods have little chance of simulating forest cover, regardless of
storage provided. It is clear that if we are to save salmon habitat our options
are few: stop development or find a way to develop that doesn't impact habitat.
Although we have known for some time
of the devastating impacts of development on streams, this knowledge alone has
not provided sufficient motivation to change, in fundamental ways, the way we
develop land. Urbanization is occurring in the Puget Sound basin at
record-breaking levels, even though it is well known that each acre developed
is harmful to salmon habitat.
Yet our attempts to mitigate the
effects of development are still pinned on discredited, "end-of-
pipe," engineering solutions. But now it seems to be dawning, albeit
slowly, that what we do to the land in the process of development can't be
mitigated.
The federal Endangered Species Act may
provide motivation for change it provides that threatened or endangered species
may not be “taken.” The definition of "take" includes destruction of
habitat. If a development project will "take" such species, the law
provides no latitude to claim "economic hardship" or "vested
rights" just because permits have already been granted. The courts can
stop that project immediately and permanently.
In the Northwest, vegetation in a
pristine watershed will evaporate/transpire about half of the annual
precipitation. The other half, for the most part, is stored in the thick duff
and soils on the forest floor to be infiltrated to deep groundwater and
released over weeks, months and years to surface water.
After development vegetation is
cleared and the duff and soil on the forest floor are removed. Large percentages of the site are made
impervious. Rainfall that previously
evapotranspired now becomes runoff and thus nearly all of the annual
precipitation runs sheet flow off the land and is diverted to surface water.
And it is released in a matter of hours instead of months devastating streams
with enormously higher flows. Groundwater recharge is significantly reduced.
Removal of the forest effectively
doubles annual runoff volume and pavement causes a dramatic increase in peak
flows. Consequently, stream channels
are dredged as if a fire hose had been directed al them. Salmon spawning and refuge habitat
disappear, pools disappear, and the channel gets wider and deeper.
In the summer, flows that were fed by
groundwater are reduced and are spread thin in the newly dredged and widened
channel. Water temperature soars. Quality of water declines precipitously.
Few will argue that development, as it
has been practiced to date, does not "take" salmon.
Speakers at the Salmon in the City
Conference defined the minimum characteristics of a watershed, which must be
maintained if habitat is to be preserved. They are:
What
Does Zero-impact Development Look Like?
Current site design largely ignores
existing land contours and vegetation. A grid system of streets and building
sites, usually intended for one or two story structures, is imposed regardless
of the shape of the ground. Land is cleared and grubbed, graded, benched and
terraced with the goal of maximizing buildable space. Generally, one owner will
create lots and prepare the site. Lots are then sold and subsequent owners will
build on the lots. Usually little or no planning of structures or landscaping
is done at the site plan stage.
By contrast, “zero impact development”
(ZID) is intended to maintain characteristics of a healthy watershed and to
turn a forested site to human uses with no measurable impact on receiving
waters, such a design requires planning of site and structures to act in unison
to meet this goal.
SCA Engineering has been developing
procedures and designs for a ZID project. For example, design would begin with
an inventory of significant trees, careful measurement of existing contours,
and location of natural drainage." features. Next, roads would be designed
to follow existing con- tours to the maximum extent possible. Structures would
be located last.
Roads would be the minimum width to
meet "functional' requirements (not necessarily the current
“regulatory" requirements). Imperviousness of roads would be further
reduced through the use of innovative surfaces.
Structures would be tall rather than
rambling (to reduce footprint) and might have runoff attenuation as part of the
design (such as garden rooftops). Rain
gutters and downspouts would be eliminated as they concentrate flow - a
violation of' ZID principles.
Structures would be widely spaced to allow natural vegetation to buffer
them and to “disconnect" their impacts from downstream habitat or
property. Structures might span over roadways to minimize impervious
surfaces. Covered bridges would not be
just a quaint artifact to attract tourists.
They would support garden roofs to mitigate runoff. Parking that
couldn't be placed under structures would be made of pervious materials or
covered with garden roofs to the maximum extent possible.
Vegetation unavoidably removed would
not be taken from the site. It would be chipped and used for staging
areas, bedding, mulch, or simply added to the duff of the “leave" areas.
Native vegetation in the designated “leave" areas would be considered part
of the water management system and would be protected in perpetuity by
covenant.
No stormwater collection system would
be constructed. The forest would remain largely intact and would be the
“stormwater management system".
The cost of zero impact development has
not been rigorously estimated but it is believed that cost savings (because of
narrow roads, no storm drainage collection) should offset much of the higher
costs associated with innovative construction practices and materials.
Furthermore, if the cost of stream
restoration is added to the equation, zero impact comes off far less expensive.
The first ZID project to be
constructed will demonstrate that land can be turned to human purposes without
devastating impacts on receiving waters. The benefits of having made such a
demonstration in the face of extinction of species dependent on such habitat
can't be overstated. If fish habitat is to be preserved in watersheds with fish
bearing streams, it is SCA’s opinion that zero impact design offers the only
alternative to halting of development.
Thomas W. Holz is Hydrologic Services
Manager for SCA Consulting Group in Olympia.