PNW 98 Session - Full Abstracts

Full Abstract

GREG MAZER

University of Washington, Seattle, WA & Pentech Environmental, Edmonds, WA

Environmental limitations to vegetation establishment and growth in biofiltration swales

To combat nonpoint source water pollution federal and local governmental
agencies throughout the country have required construction of ‘in pipe’
stormwater quality improvement facilities. One such facility, the
biofiltration swale (also called bioswale or biofilter), is an open channel
possessing a dense cover of grasses and/or wetland plants through which
runoff is directed during storm events. Aboveground plant parts (stems,
leaves and stolons) physically filter particulates and their associated
pollutants as runoff passes slowly through the channel. Herbaceous cover
is considered to be well correlated with treatment efficiency.

Environmental conditions and construction history were examined for eight
biofiltration swales in King County, WA to determine relative importance of
and threshold values for the various factors influencing vegetation
establishment and growth. Three of these swales were retrofitted with new
soil and hydroseeded in September, 1996. Because limiting factors proved
overwhelming for two swales, only one swale supported more abundant
vegetation a year later.

Vegetation and plant litter biomass from soil surface to 10 cm height (that
which is most important to sediment filtration) was positively correlated
to insolation and negatively correlated to mean summer base flow depth. For
the one-third of the plots which possessed mean summer soil moisture
potentials less than -15 MPa, biomass was strongly correlated with soil
depth (r2 = 0.76). Plant and litter biomass was also negatively correlated
with mean peak flow velocity during the spring - especially within undammed
plots (r2 = 0.68). Degree of channelization within undammed plots,
assessed with a categorical system used by Wydzga (pers comm, 1997), was
positively correlated to longitudinal slope and both mean and high peak
flow velocity.

A nested two-factorial greenhouse experiment tested the response of four
turfgrass species commonly seeded in bioswales to three inundation regimes
plus a control. Festuca arundinacea (Tall Fescue) accrued significantly
more biomass than Agrostis alba (Redtop), Poa pratensis (Kentucky
Bluegrass) and Alopecurus geniculatus (Meadow Foxtail) over four weeks in
three of the four treatments. The treatment in which seeds were inundated
12 of 14 days for two consecutive periods produced equally minimal
germination amongst all species.

To improve future biofiltration swale design and performance, more
consideration to providing seeded grasses with environmental conditions
conducive to germination and establishment is recommended. However, even
when installed properly, bioswales may not perform as anticipated in the
long term if base and storm flows become channelized and/or base flow
(inundation) persists through the growing season. Therfore, I recommend
restricting biofiltration swales to very low flow (discharge and velocity)
conditions and contructing large retention ponds and wetlands to improve
stormwater quality in areas subject to high discharge.

not sure how to categorize this talk - not certain if its appropriate for
the habitat theme, but thought I'd submit it anyway