Increase infiltration
- Walks and Driveways
- This strategy received a moderate
effectiveness rating due to concerns expressed in practitioner
interviews and the results of a simulated rain study that
showed a 14% runoff rate for a gravel driveway, compared to 0%
for a mulched landscape (Ross & Dillaha, 1993). Different
crushed aggregate materials should be investigated with
attention given to the presence of fines.
- Deck over permeable surface
- This is a strategy proposed in a
document published by the Center for Urban Water Resources
Management (CUWRM) at the University of Washington. Although no
data are included in the recommendation it appears to be an
interesting strategy to research further as it would be easily
implemented at the residential site scale (Konrad, et
al. 1995).
- Driveway paving only under
wheels
- This strategy is a simple one. In fact
it is not uncommon to see its use in the driveways of older
Seattle homes, likely utilized as a cost saving measure.
According to a San Francisco stormwater document a driveway
with two concrete wheel tracks can reduce impervious surface
coverage by 60-70% compared with a conventional concrete
driveway (Tom Richman & Associates, 1997).
- Drywells and infiltration tanks,
(driveway/ Parking lot)
- A hole in the ground containing a tank
or catch basin of some type, placed on a layer of washed rock.
The tank can also be filled with washed rock. The top of the
tank is closed with the bottom open, and it is tight-lined to a
drainage source. Drywells and infiltration tanks require
relatively permeable soils. Infiltration tanks can be of any
size or shape and are well suited to the residential site
scale, as they require less area than surface infiltration
strategies (Konrad, et al. 1995). Note: drywells and
infiltration tanks are also used to detain and infiltrate roof
runoff.
- A permeable pavement demonstration
project is being carried out by CUWRM that includes tests of
three plastic and one cement block/ systems, within which grass
is grown. Only the first year’s results are available, but
initial findings indicated significant reductions in stormwater
runoff on the test surfaces (Booth, D., Leavitt, J., and
Peterson, K., 1996). Anyone interested in proposing the use of
this strategy should contact CUWRM to obtain information about
subsequent findings at those test sites.
- According to the CUWRM paving study,
some of the same cement and plastic block systems can also be
filled with gravel. This may reduce issues of grass die back
under vehicle use. Concerns were raised in an interview with
one practitioner regarding the build up of silt and fines in
the spaces between pavers and the potential reduction in
permeability over time. Since 1982, the City of Yokohama has
been studying the effects of storm water runoff control
measures at a residential site where two types of runoff
control techniques were installed, permeable pavement and
infiltration pipes. Study results revealed that 15 to 20
percent of the peak runoff control effect was achieved by the
use of the installed runoff control facilities. Throughout the
study period, no deterioration of the strength and features of
the permeable pavement in comparison with ordinary pavement was
observed (Watanabe, S., 1995). Further research of this
strategy is needed.
- Pervious concrete & porous
asphalt
- This strategy is only effective if soils
and geology allow for minimum necessary rates of infiltration.
Failure typically results when the surface is not protected
from sedimentation and compaction during the ensuing
construction phase. For this system to continue to function, it
must be regularly maintained. One successful maintenance
technique is to commercially vacuum twice each year (Cahill,
1994).
- Vegetation
- Replace lawn & impervious w/
plantings and lawn alternatives (i.e. meadow, forest community,
and native associations)
- In a simulated rainfall study a meadow
cover exhibited a 0% runoff rate (Ross & Dillaha, 1993).
Schueler provides a very good lawn alternative resource list
(Schueler, Fall 1995. p. 311).
- Increase soil storage capacity
- According to a San Francisco stormwater
document, a single street tree can have a leaf surface area of
several hundred to several thousand square feet, depending on
species and size (Tom Richman & Associates, 1997). Trees
may take up between 300 and 450mm of water per year via the
transpiration stream (roots to leaves to air) or via direct
interception by the foliage and subsequent evaporation to the
air. The amount and timing of these things depends upon
species, soil moisture, and time of year. (Obtained through
Email correspondence with Tom Hinkley, who is researching water
uptake by trees at the University of Washington in the College
of Urban Horticulture. If interested in utilizing info, contact
Ken Yocom to coordinate this effort.)
- Research of this strategy suggests that
the use of mulches and other soil amendments increased
infiltration. In one simulated rainfall study a mulched
landscape exhibited a 0% runoff rate (Ross & Dillaha,
1993). A Seattle study monitored test plots of amended and
unamended soils over ten storm events and reported that compost
amended soils reduced surface runoff by 29 to 50%, depending on
the amount and type of compost used (Kolsti et al., 1995). It
was estimated that stormwater detention basin volumes could be
reduced by 5 to 15% if compost amendments were incorporated
into new subdivisions in glacial till soils near Seattle
(Chollak and Rosenfeld, 1998).
- Tilling soil, combined with compost
amendment, followed by addition tilling is a strategy
recommended in the CUWRM document to increase infiltration.
However, tilling alone showed only a .00 to .02
(gms/cc)decrease in bulk soil density in studies published in a
Watershed Protection Techniques article (Randrup, 1998)
(Patterson and Bates, 1994).
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