The Hard Stuff

The bridges are being manufactured and the poured-in-place concrete abutments for the 5 new bridges have been approved by the City.  The longer the concrete allowed to sit, the stronger the abutment will be.  So the contractor is taking advantage of the holiday lull to get ready to pour concrete.  The photo above shows crew members marking out where the abutments will go.

The point bar is starting to take shape as cobbles are brought in and placed.

There are a few places along the stream where we want to let visitors get right up to the water, but we also need to provide some extra-strong protection for the bank.  In those places, the contractor has placed large, angular boulders that lock together to form a solid structure.  There is no grout or concrete holding the boulders together, just gravity and friction--two things that will outlast the strongest grout.

Bioengineering 101

On Friday (12/18/09), Drew Goetting, Principal designer at Restoration Design Group, came down from Berkeley to give us all a lesson in bioengineering.  First, dig a 3-4" deep trench at the toe of the bank slope.  Toss the dirt from the trench upslope to make it easier to fill the wattle.

Next, lay out the coir fabric so the edge extends a few inches upslope past the lip of the trench.  Secure the fabric to the soil with biodegradable staples (in this case, Bio-Stakes made from corn).

Shovel some of the soil from the trench over the coir fabric, then place willow pieces (branches, twigs, and leaves) over the soil.  Add more willows and more soil, making sure that the willows are generally oriented along the streambank and no branches are sticking out perpendicular to the direction of the water flow.  More willows and more soil, until the whole thing is the right height (see below for what "right" means).

Next, imagine that you are making a giant California roll, using the coir fabric as your sushi mat and the willows and dirt as your rice, crab (or krab), and avocado.  Kick and push the mixture into place--it is pretty rugged and will withstand some rough treatment.  The goal is an evenly compacted roll whose top is fairly level.  The finished top of the willow wattle should be above the water surface when the stream is flowing.

Pull the coir fabric fairly tightly over the soil and willows.  Secure the edge of the coir fabric with more biodegradable staples.

Pound some willow stakes straight through the entire thing.  The stakes should go at least a couple of feet into the ground under the wattle. 

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The result is a willow wattle.  The fabric will hold the soil and protect the bank on a temporary basis.  Meanwhile, some of the willows inside the wattle and some of the willow stakes will sprout and their roots will grow to hold the streambank.   After a while, the coir fabric and the staples will biodegrade and all that will be left will be willows, creating a sustainable streambank and a vegetative screen of riparian habitat.

Fine Grading

About half of the stream has dried out enough so that heavy equipment can go back in.  The contractor is doing the fine grading, getting the elevation of the streambed just right and contouring the banks so they are ready for installation of the bioengineering bank strengthening.  The contractor has also installed a couple of areas of boulder reinforcement like what is shown in the photo above.  The stream restoration consultants will come in tomorrow to look at this boulder reinforcement to make sure it is properly done.  Once the restoration consultant is satisfied with the contractor's work, this bank treatment will be replicated at a few more locations along the stream.  The boulder treatment is being used at spots where erosion potential is greatest--immediately upstream and downstream of the pedestrian bridges, and at a couple of places where the walking trail comes very close to the stream.  The restoration consultant will also be giving the contractor detailed instructions in planting willow poles and stakes, installing coir logs, and building willow wattles.  Some of the willows will even be harvested from within the Nature Center itself.

And the rains came...

The weekend rains put quite a bit of water into the streambed.  The contractor's crew was wading through water and mud today pumping the water out.  Once the water is gone, it will take another couple of days for the streambed to dry out enough to take equipment back in.  Hopefully, we won't lose too many more days to rain.

In Spite of the Rain

It's been a rainy week at the Nature Center, but work continues on the stream.  Last week I posted a photo of the point bar cobble mock-up.  You can see the mock-up in the center-right of this photo.  This week the exact location for the cobbles was defined and graded.  The cobbles will be about one foot deep, so the slope needs to be lowered by that amount to allow for a smooth transition between the surrounding grade and the cobbles. 

The stream grading is almost finished, with just a small area upstream of the south lake remaining.  When the muck was dredged out of the stream, the contractor spread it in an unused part of the Nature Center south of Willow Street.  The material was turned periodically, enabling it to dry out.  The contractor has used that dried out dredge material mixed with clean soil (yes, we tested it) to build up the streambed to the specifications on the plans.  The photos below show the big pile of material that was taken out of the streambed in October (left) and what that pile looks like now (right).  The woman with the muck in her hand is Jessica Hall from Restoration Design Group.

Mocking up the Point Bar

The contractor mocked up a small section of cobbles so we could get an idea of how the point bar area will look.  The area will use cobbles that are approximately 6" in diameter, with coarse sand spread between them for stability.  The mock-up shown above used materials on hand rather than the materials we want in the final product, so is has road sub-base instead of sand.  This worked well enough, but didn't look very good.  Even with the larger sub-base material, the field of cobbles was quite stable.  The area will be uneven to maintain a natural look, but will be fairly easy to walk on (though not for those in high heels or toddlers just learning to walk).

Field Design

Because the exact condition of the stream could not be determined in advance, this project is being done as a design-build. The contractor is working from 30% construction plans with frequent input from the designers and City staff. Here, the project team is discussing when and how to remove willows that lean over the trail.  These willows will be cut where the orange line has been painted across the trunk.  The trunks, branches, and leaves will be used for staking and in willow wattles; the root ball will be used for further bank stabilization and to give the stream a more natural look.

Surveying

The surveyors are out today, establishing the elevations for the stream from the north lake to the south lake. In concert with the stream restoration consultants, we will determine exactly how much to raise the elevation of the north lake. The decision will balance improving the stream flow with innundating the edges of the north lake.

Entwined No More

On Nov. 5, I posted a photo of a sycamore and pepper tree twisted together and seemingly forming a single hybrid tree. Above is another "before" shot, and below is the same tree today. You can see a small bit of the pepper that was left attached to the sycamore -- to have removed that would have damaged the sycamore unnecessarily. The sycamore is kind of spindly and leggy right now, but it likely will fill out and regain a more healthy shape now that the pepper has been removed and the sycamore has access to light and air.