Existing Hardscape In The Creek

"Hardscape" is the term used to describe the materials used to fortify the banks or floor of a creek against erosion. Man-made surfaces such as poured concrete, gunite (sprayed cement-mix as used to build swimming pools), cement bags, rock rip-rap, and similar, are all forms of hardscape that have historically replaced the natural dirt and vegetation within this stretch of Adobe Creek.

When a natural creek is modified this way, government regulations require that this environmentally negative practice be "mitigated" by environmentally positive practices that will result in a net overall improvement to the environment. Such mitigations can be achieved through a number of methods, such as planting native trees or making other improvements that will enhance the habitat for fish or wildlife.

The Upper Reach 5 of Adobe Creek has been fortified over the years on an ad hoc basis - there are stretches of cinder-block wall, gunite-sprayed embankments, heavy concrete pourings and other attempts to control erosion in the creek. This is the status quo - as displayed in the pictures below.

When the creek is eventually re-engineered, the intent is to make it as environmentally positive as possible. Ideally the new banks would be completely natural (moderately sloped with natural vegetation), but this ideal can only be achieved at best over limited stretches of the creek because of other constraints. Thus it is anticipated that any "new" creek will require stretches of hardscape to control erosion. Thoughts at this time go toward "gabions" (collections of large rocks anchored within wire baskets) sloped at something less than vertical, or rock rip-rap, large rocks stacked at an even lesser slope. Either method would be designed in such a way as to encourage growth of vegetation and to restore the riparian habitat to as natural a state as possible.

Use of such gabions or rock rip-rap brings with it "mitigation requirements". In this case we expect to be replacing vertical impervious walls (cinder-block, gunite, poured concrete) with gabions or rock that are sloped and that host vegetative growth, providing the shade and cover necessary for a high quality riparian habitat. Thus the use of gabions or rock in this context will of itself be a substantial environmental improvement and qualify as "self mitigation" thus eliminating or heavily reducing the need for off-site mitigative measures.

The pictures below are provided to show the extent of the existing "hardscape" within this stretch of the creek. In general and unless otherwise noted, these pictures were taken looking downstream. The Los Altos (North) bank is on the right, and the Los Altos Hills (South) bank is on the left.

A map showing the location and extent of hardscape in the creek may also be seen by clicking here.

To view these pictures, just scroll down the smaller images below, and click on any of them to see an enlarged version of the picture.

(If you have "Popups disabled" in your browser, then you can instead click in the red button to the right of each picture to see its enlargement - but in that case you will have to use the "Back" button each time to return to this page.)

(1) This is looking downstream at the start of Reach 5 where the creek goes under the Edith bridge.

The white staff with markings is shown to provide some idea of scale. Each of the demarcations on this staff represent 1 ft. From this we can see that there is just under 7ft of clearance between the creek bed and the underside of the bridge. The flat part of creek bed is around 12 ft before the banks slope up on either side.

(2) Moving downstream, here the creek has just passed under the bridge - we have a hard vertical wall on the left, and in the background we can see the main choke-point that restricts creek flow so that it backs up and causes flooding upstream in Reach 6.

(3) Choke-point - This is a close-up of the choke-point seen in the background of the previous picture. Immediately after exiting the bridge, all the water coming under the bridge has to pass through this constriction which is hardscaped on both sides.

On the left we have the end of the vertical concrete wall that is effectively a continuation of the bridge profile, and on the right we have some massive poured-concrete structures. The creek bed makes a sudden drop of between 1 and 2 ft. here (about 4ft. from top of wall on left) and has an effective width of around 5ft at bed level. Closer to the top-of-bank, the effective width tapers out to around 7 or 8ft.

(4) A few yards downstream and looking back (upstream) toward the bridge - here we can see the use of gunite on the northern creek-bank to protect the roots of the redwood trees right at the water's edge. These redwood trees are an irreplacable asset (irreplacable in ours or our children's lifetimes) and must be protected. Any further erosion, such as might occur if the gunite is undermined by flowing water might be fatal for these trees. A thin skin of concrete is not sufficient for the long run - something more substantive and that will resist undermining is needed to protect these trees and to let them further mature over the next few centuries.

On the opposite bank but not visible in this picture, a wooden plank wall has already been eroded and torn out by the creek.

(5) A few yards further downstream the gunite gives way to a cinder-block wall, and the bed of the creek is now also hardscaped against erosion.

(6) A little further down, the cinder-block wall on the north bank is now matched by a concrete wall on the south bank, and the bed of the creek is also hardscaped. The vertical walls are around 6ft high and the bed varies from about 5 to 7 ft wide.

(7) Note proximity of redwood tree to cinder-block wall on right. So far it survives...

(8) The creek widens a little bit, and the wall is now only 5ft high. Nevertheless, high flows pour over the northern (right-hand) wall in at this turn in the creek, and flow over the neighbor's yards before reentering the channel farther downstream

(9) The vertical-walled "alley" continues with cinder-block on the one side, concrete on the other.

(10) At this point, the concrete on the south bank appears to have been constructed in two phases. There is a continuous lower ledge topped by a taller wall - consisting at some points of cement bags molded and set in situ, at other points poured concrete.

(11) The massive lower ledge continues topped by a poured concrete wall. What appears at first sight to be a wall made of wooden planks is merely the imprint on the concrete of the original planks used to build the concrete mold.

(12) At this point, the concrete wall transitions from merely massive to seriously massive.

(The drain pipe visible in this picture has been abandoned. Its upper end was plugged when a new home was developed on this property.)

(13) A close up of the same point.

Connoisseurs of Classical European Architecture will recognise homage being paid to the style known as "Rhine-bridge Bunker".

(14) Continuing downstream, the south bank eventualy reverts to a simple flat concrete wall. However, near its end, we can see that the immovable object is losing out to the irresistible force of Eucalyptus tree roots.

At this point the hardscape on the creek-bed stops, and is immediately followed by a drop-off.

(15) The right-hand curve of the creek transistions into a left-hand curve, and accordingly it is now the turn of the north bank to require extra armour against erosion. Here we see the gunite treatment being resumed on that side.

(16) A close-up of the same point. The gunite has been extended to the creek-bed in an attempt to prevent the wall being undermined by water creeping underneath and washing out behind it. This merely shifts the problem slightly...

(17) The gunite continues downstream, protecting the stand of redwood trees on the northern bank. The bed is wider here, and the bank has dropped to between 3 and 4 ft above the bed.

Note the discrepancy between north and south bank wall heights, with the north side being considerably lower. The south bank still has some old eucalyptus stumps forming a wall, but water will spill over and flow through the yards on the north bank during high-flow periods.

(18) But there is clearly not much leeway. Here the gunite actually covers the base of one of the trees.

(19) Now we reach some fairly large stepped "drop offs" in the creek - each protected by concrete hardscaping in the bed to prevent erosion of the bed "travelling" back upstream. The south bank is protected here by a wall of planks - not as long-lasting perhaps as concrete, but nevertheless man-made hardscape - and a candidate for a more environmentally friendly design.

Sunlight showing between the planks is mute testimony to their (lack of) efficacy and the resulting erosion that has occurred behind them.

(20) This view is taken looking back up the creek (the direction from which we came) and provides a longer perspective on this part of the creek. It is approximately double the width here of the narrower sections upstream, and the banks are 3 to 4ft high rather then the 6 to 7ft encountered upstream.

Again, with the lower banks on the north side, water floods over into residents' yards here during high-flow periods.(North is to the left in this picture.)

(21) Here we are looking downstream at the point where the flow from Robleda joins the creek via underground culvert. The creek takes a right-hand jog approximately where we are standing, and the culvert opening is in the creek wall just below the bottom-left corner of this picture.

The extra volume of water entering the creek at this point is clearly significant - note the height of the hardscaped banks here as much greater than immediately upstream - despite the creek being wider at this point and the non-vertical (tapering out) of the walls.

(22) This is a close-up of the south bank at the same point where the creek makes a right dog-leg and the culvert delivering water from the Robleda source is just visible. (The grey curved structure poking out opposite the 3rd and 4th sandbags from the bottom of the picture.)

Height of the creek wall at this point is estimated around 12 ft, and width between 8ft (at the bottom) and 24ft (at the top). This compared to the roughly 6-by-6ft cross-section of the creek as seen in picture (6) above. This increase in capacity reflects the water that "joined" the creek after it entered Reach 5 downstream of Edith Bridge - both via overland flow from Fremont and Edith Roads and the Robleda culvert.

(23) This image shows the old black-top driveway that parallels the southern creek-bank close to the Edith Bridge. While this "hard" surface was not intended to prevent creek erosion, nevertheless it is "hardscape" whose removal and subsequent replacement with suitable vegetation will also contribute to the overall environmentally positive impact we hope to achieve with this project.