Transcript perc test

Stormwater Study
To determine the effectiveness of the campus’
existing stormwater infrastructure, if NSCC
qualifies for a stormwater credit and to propose
future measures that will enable NSCC to save
money and the environment.
Hunter Brown, Laura McFarlane, Krasimira Pesheva
NSCC pays $80,000.00/yr
in Stormwater/drainage fees.
Photo courtesy of City of Seattle, www.seattle.gov
Seattle Public Utilities (SPU) has a Stormwater Facility Credit
Program wherein NSCC can receive a credit for up to 50% off of their
annual bill by managing all of its stormwater onsite.
Photo by Hunter Brown
Currently, NSCC has a
number of rain gardens,
greenbelts and retention areas
built to manage and slow the
flow of water off of its
impermeable surfaces.
Photo by Hunter Brown
Photo by Hunter Brown
Our goal was to determine the
infiltration rate of the retention pond
for the stormwater runoff from the
south east parking lot in order to
determine the ponds management
capabilities.
Image by Hunter Brown
To calculate the infiltration rate we used the Falling
Head Percolation Test Procedure from the King County
Surface Water Design Manual.
As per the County’s
instructions we dug 3
test pits, evenly
distributed within the
basin of the retention
area.
Image by Hunter Brown
The test pits were dug
down to the depth of the
most limiting soil horizon
beneath the surface which
was glacial till; dense clay
inundated with rocks.
Photo by Hunter Brown
The glacial till is located 1-4
inches below the top soil.
Photo by Hunter Brown
The test pits ranged in depth
from between 12 and 18 inches
below the surface.
Photo by Hunter Brown
A 6 inch diameter pipe, 12
inches in length, was used in each
test pit to maintain a consistent
surface area and volume for
calculating the infiltration rate.
Photo by Hunter Brown
Because the limiting horizon
was glacial till, we saturated each
test pit with water for 23
continuous hours prior to running
our test.
Photo by Hunter Brown
Photo by Hunter Brown
Measurements were taken at 30 minute
intervals until the drop in water did not vary by
more than a 1/16th of an inch for two successive
water level drops within a 90 minute time period.
Test Pit 0-30 min 30-60 min 60-90 min 90-120 min
120-150 min 150-180 min
1
1/64
1/64
1/64
1/64
1/64
1/64
2
1/16
5/32
3/16
1/8
5/32
5/32
3
3/16
3/32
3/16
1/8
1/8
3/32
Table by Hunter Brown
The Percolation Rate for each test pit was calculated
by dividing the time interval used between measurements
(30 min) by the magnitude of the last water level drop (our
final measurement, 150-180 min).
Test Pit
Percolation Rate
1
1920 minutes/inch
2
192 minutes/inch
3
320 minutes/inch
Table by Hunter Brown
The percolation rates of the three test pits vary by more
than 20 minutes/inch, thus they cannot be averaged and
we are unable to determine the actual infiltration rate of
the retention pond.
In the process of conducting our test it was
discovered that the retention pond was
connected to a catch basin as a fail safe in
the event of overflow. The benefit of this is
that a further study of the retention ponds
infiltration rate can now be done on a large
scale by utilizing water flow meters. Meters
should be placed at both of the inlet pipes as
well as at the outlet pipe where it drains into
the catch basin. This study should also utilize
information from the on campus weather
station in order to quantify and determine the
effectiveness of the retention pond and
compare the expected input with the
measured input. The study should take place
throughout fall, winter and spring.
Further research must also be done to
determine all of the sources flowing into
the retention pond. While it may be safe
to assume that only the Outfall drains
feed the pond it is possible that one or
more the catch basins to the south may
also flow into the pond.
Figure Modified by Hunter Brown
Photo courtesy of Seattle Daily Photo, www.seattle-daily-photo-blogspot.com
The vast majority of the permeable surfaces on campus sit on
glacial till which isn’t very permeable. However, over the 20112012 school year geologist John Figge and his class will be taking
core samples of the soil and mapping them on a campus grid.
Working with his results NSCC should be able to determine which,
if any, of the additional proposed future studies and/or projects on
the following pages will be worth exploring.
Secondary south eastern parking lot.
The photograph on the right
was taken from the north end
of this parking lot, facing
south. As shown in the photo
this parking lot has a
substantial grade.
Ergo, it would be relatively easy and
inexpensive to install a series of asphalt
berms approximately 1-1/2 inches tall
and 3 inches in width along the length
of the parking lot to guide the storm
water into a series rain gardens, that
would also need to be constructed, to
the west.
Image by Hunter Brown
Converting the existing lawns:
Photo by Hunter Brown
To the south of the DSHS and College Center
buildings are thousands of square feet of
lawn that could easily be converted into rain
gardens capable of handling most or all of the
rain fall landing on these buildings. The most
difficult aspect of this project would be how
to divert the water into the gardens. This is
something that NSCC should keep in mind
for all of its future building projects.
Triangle in need of curb cutouts:
If future studies revel that the south
eastern retention pond is not effectively
managing all of the stormwater coming
off of the southeastern parking lot then
some of the storm water should be
diverted to the triangle to the west. This
could be accomplished by cutting out
sections of the existing curb surrounding
the area on the east and southwest sides
as well as by creating a larger berm on
the western corner.
Image modified by Hunter Brown
Northeastern Triangle:
This area was engineered to manage the
stormwater coming off of both the
buildings and the parking lots to the
southwest. While we do know that this
retention are works, because it has never
overflowed, we do not yet know how
much storm water it is managing and how
much is going into the cities stormwater
system. A study of this area utilizing
water meters and data from the weather
station through fall, winter and spring
will be necessary.
Image modified by Hunter Brown
Finally, there are also several other areas in and around the various
parking lots that could easily have small rain gardens retro fitted into
place however their effectiveness would pale in comparison to the
previously mentioned suggestions. Additionally, it is our
recommendation that all research, work and labour be performed by
students at the college to reduce expenses while simultaneously
providing hands on experience for the students.