These days, state highway departments do more than just pave
roads and build bridges. Their responsibilities include environmental
stewardship, and DOTs are sometimes the first line of defense for eliminating
surface water pollution.
The California Department of Transportation (Caltrans)
builds and maintains some of the world's busiest highways, including the
extensive Southern California freeway system. The vast number of roads in and
around Los Angeles collect and channel enormous amounts of water, water that
eventually makes its way to local streams, rivers and, in this case, the
Pacific Ocean.
I-210 is one of these roads. It is a relatively new
eight-lane freeway skirting the northern boundary of the Greater L.A. Basin.
The recently opened six-mile section of the freeway bisects numerous densely
populated suburban communities so traffic is usually heavy, well in excess of
100,000 vehicles per day.
With the beautiful San Bernardino Mountains on the horizon,
the highway was built primarily below grade so the view would not be
obstructed. Essentially, it is a concrete trench nearly six miles in length.
The highway is drained by a reinforced concrete box culvert measuring 3 x 1.5
meters. In L.A.'s arid climate, rainfall events are infrequent but often intense,
and the runoff is usually packed with all kinds of pollutants and trash.
"The Los Angeles Regional Water Quality Control Board
has started to adopt specific total maximum daily loads (TMDLs) for a variety
of water courses in the L.A. area," explained Doug Failing, who as
Caltrans' District 7 director is responsible for the highways in Los Angeles
and Ventura counties. "This includes a 'zero trash' policy. At present,
this policy doesn't apply to the watershed where I-210 is located, but it
probably will soon. This type of environmental policy is the way of the future
because it's a good thing to do; it's good for our state and our region."
According to Failing, the runoff from I-210 contains the
usual debris: cups, cans, plastic and paper. And like most highways, it also
has a toxic mix of metals such as beryllium from tires, aluminum from auto
chassis parts, iron from engines and copper from brake linings. These metals,
along with oil and other materials, settle into the sediment along the highway
and then wash into the storm drain during rain events. Much of this trash and
pollution eventually ends up in the Pacific where it becomes a real
environmental concern.
"Here in L.A. we want to keep our coasts clean,"
said Failing. "It's important to our residents and our economy."
The screens clean
Caltrans looked at several options for cleaning the
highway's storm water runoff. Installing screens or baskets at storm water
drains was quickly ruled out. Such devices require cleaning after almost every
rain event and the drains are located within a few feet of high-speed traffic.
The high level of maintenance and the safety factor for workers made this
option unacceptable.
Clearly, an end-of-line cleaning process was preferred, but
there were other factors affecting the decision. Volumetric-type devices that
catch and hold large amounts of water for a long time were considered, but in
this densely populated area it would have required Caltrans to displace several
homes and it would have cost millions of dollars.
With these limitations, Caltrans engineers began looking at
flow-based treatment devices. These devices accept a flow of water, put it
through a treatment process, then release the water to continue its journey
downstream.
"There are very few flow-based treatment devices
available to us," said Failing. "We had been experimenting with CDS
(continuous deflection separation) units for some time. I like the screening
device. It not only captures trash, you get a high level of sediment removal as
well, and that was very important to us."
One of the biggest benefits of the CDS technology in this
application was that Caltrans could fit a CDS unit into a small patch of land
that could be easily and affordably obtained.
"What we did was find pockets of land away from the
highway where we could put in CDS units," said Failing. "We're very
conscious of maintenance activities where we have freeway flow, for two
reasons: one, when people see maintenance workers they tend to slow down, look
and not pay attention to other cars. Two, exposing our workers to traffic is
dangerous. In this case the CDS unit is about a quarter-mile from the freeway.
It's very safe."
There are actually five CDS units incorporated into the new
I-210 freeway system, one of which is the largest CDS unit ever installed in
the U.S. The pit excavated for the largest unit was 40 ft wide, 40 ft long and
38 ft deep. The cylindrical screen assembly on the unit is 15 ft in diam. by 15
ft tall. The unit is designed to filter a "first flush" runoff event
of 175 cfs (78,400 gpm).
"One of the advantages of this technology is that it is
scalable," said Mark Cuneo, P.E., of CDS Technologies, Long Beach, Calif.
"We can build them as large or as small as needed. The objective is to be
the most economical in terms of construction and long-term maintenance
costs."
The CDS system is entirely self-operating, relying on water
hydraulics, gravity and a non-blocking screen. Raw storm water enters the CDS
diversion chamber where a weir guides the flow into the unit's separation chamber.
A vortex is formed that spins floatables and suspended solids around the screen
cylinder. The screen deflects the pollutants to the center of the separation
chamber. The trash and suspended solids gently settle into a sump where they
remain until they are removed. The water passes through the screen, out of the
separation chamber and into the diversion chamber, then re-enters the storm
water flow headed downstream. Caltrans expects to have to clean the sump of
this particular unit only twice a year.
While most CDS units are precast, the size of the biggest
unit on the I-210 project required that it be cast-in-place. However, the
project presented few difficulties for the contractor.
"The construction wasn't too difficult; it's basically
a large concrete cylinder cast in place on a concrete foundation,"
explained contractor Don Reiter of Yeager Skanska Inc., Riverside, Calif.
"The CDS people were very helpful in answering questions for us. It was a
very interesting project simply because of the size."
Trust worthy
According to Caltrans' Failing, the real issue for most DOTs
is performance. This is what really separates the CDS technology from other
end-of-line storm water treatment devices. The CDS unit removes 100% of the
floatables and has been shown to remove 80% of total suspended solids from
storm water runoff. Plus, it has no moving parts and requires minimal
maintenance.
"This unit performs very well," said Failing.
"It saves us in maintenance costs, it's safe for the workers and it cost
less than a fourth of what it would have cost us to build a volumetric-based
treatment device.
"We have a responsibility as stewards of the public
trust to spend the public's money wisely and minimize the impact on traffic and
the environment," Failing added. "These devices help us work within
the greater public trust."
