On January 8, 2014 a team of scientists and other members of Tualatin Riverkeepers Citizen Action Committee took a field trip to view three sites where significant erosion has been caused by development that increased stormwater runoff from Bull Mountain.  The reason for this field trip was to familiarize the members with issues that should be addressed by the natural resources and stormwater plans for South Cooper Mountain and West Bull Mountain (aka River Terrace).

Mud plume in the Tualatin River from Bull Mountain runoff.

Our first stop showed a deeply incised creek on the property of Mike Meyer.  This site was featured in an article last year in the Tigard Times and other community newspapers (Mike Meyer’s Bull Mountain Washout – May 16, 2013).  A hydrological study of this creek commissioned by Clean Water Services determined that the volume of water flow from Bull Mountain has increased 30 to 40 percent since 1980, with a 10 percent increase in peak flow.  As a result, erosion has caused a canyon to form and continue to migrate upstream.

It was raining the day we visited the site and a plume of mud was obvious where the creek spilled into the Tualatin River as can bee seen in the photograph.

Steeply sloped, Essex Drive can generate large
 volumes of  high velocity stormwater runoff.
The second stop on our tour was higher up on Bull Mountain on part of the Ascension Trail. The trail runs through a beautiful wooded natural area purchased by the City of Tigard with funding from Metro.  The area is significant for its lack of invasive plant species that are problematic in so many other natural areas.

Soon after development of luxury homes on Essex drive, the inadequacy of the storm sewers and drainage plan became obvious.  With the first big storm, manhole covers were blown of the storm sewers and water ran down Essex Drive into the natural area, eroding a gully that is 10 feet deep in some places.  

Ecologist Paul Whitney PhD examines the gully on the Ascension Trail caused by runoff from Essex Drive.

To prevent further erosion, the gully was lined with large rip-rap and a large above ground sewer pipe was installed.  The pipe runs down the slope and discharges into a small tributary of summer creek at the base of the slope.

A homeowner has covered the ground with black plastic in an attempt to prevent his luxury home from sliding further down the hill.  Structural damage has already occurred to the home.

The third stop on the tour was at East Bull Mountain Nature Park.  We scrambled down the slope to see erosion working its way up the creek.  Soils in this area are highly erodible and runoff from development could prove disastrous to some of the homes lining both sides of the ravine.

Geologist Charles Carter PhD and Ecologist Paul Whitney PhD examine the bowl falls that has
formed on a tributary of Summer Creek just down slope from East Bull Mountain Nature Park.

In 2013 the City of Tigard spent over $650,000 on stream bank stabilization on Benchview Creek on Bull Mountain.  The 2014 budget includes $1.3 Million for the same project. Multiple sites on Bull Mountain have yet to be addressed.