Stormwater StudyTo 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

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.

NSCC pays $80,000.00/yr in Stormwater/drainage fees.

Photo courtesy of City of Seattle, www.seattle.gov

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

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.

The glacial till is located 1-4 inches below the top soil.

Photo by Hunter Brown

Photo by Hunter Brown

The test pits ranged in depth from between 12 and 18 inches below the surface.

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

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 BrownPhoto by Hunter Brown

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

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.

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

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.

Table by Hunter Brown

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

The vast majority of the permeable surfaces on campus sit on glacial till which isn’t very permeable. However, over the 2011-

2012 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.

Photo courtesy of Seattle Daily Photo, www.seattle-daily-photo-blogspot.com

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:

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. Photo by Hunter Brown

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.