Adobe Creek Collaborative Workshop
Hillview Community Center (Room 17)
97 Hillview Ave
Los Altos, California
6:30 to 8:30, Wednesday, May 25, 2005
Workshop Purpose
A. For the District to Receive Comments and Answer Question about the Alternatives
B. Allow for an Open Session for Discussion
Meeting Notes
1. Welcome, Introductions & Workshop Organization – Jitze Couperus
A. Review Meeting Agenda
2. Introduction of Workshop - Katherine Oven
Katherine summarized the work that had been done in the previous two weeks to provide an alternative that protected the Los Altos trees and did not encroach beyond the Los Altos Hills blue line. This is Alternative 6, the Blue Line Geomorphic, which shortens the length of the geomorphic stretch of the creek by about 100 feet.
I. Alternative 6 Blue Line Geomorphic
Tony explained how Alternative 3 was modified to preserve trees at the Wittner and Lippe properties. That shifted the geomorphic design into the Rogez and Astiz properties. It has a 32.5’ twice bankfull width and begins deeper with the sediment removed from under the bridge. To avoid encroaching beyond the blue line in three locations, hardscape was added. The geomorphic channel will end halfway through the Lippe property. The details of the design will be addressed once the preferred alternative is chosen. Right now the goal is to establish the footprint, determine the need for mitigation and where it might be located.
II. Proposed Alternatives downstream of Robleda Storm Drain
Tony identified all the existing hardscape downstream of the Robleda Storm Drain. He described each downstream alternative:
Alternative A – Provides necessary bank protection in areas that are currently deteriorated. Those repairs include installing drop structures in place of the Remmel/Moos waterfall to protect the gunite that’s being undercut on the banks. Approximately thirty feet of rock slope protection would be installed on the Remmel property. Rock would be installed underneath the sacked concrete at the Van Horne property to protect against the scour that is happening there as well as at the storm drain and at one other erosion site on the Van Horne property.
Alternative B – Repairs are the same as Alternative A, with an additional extension of the rock slope protection on the Remmel property through the Robinwood Pool property. The rock lining would be installed to the level of the 100 year flow, about 12 feet up the bank, placed in front of the existing sacked concrete. Additional land would be gained from the fill behind the rocks transitioning to match the existing slope
Alternative C - Installs a vertical flood wall from the end of the existing sacked concrete at the Remmel property through the end of the Robinwood Pool property for about 125 feet.
Alternative D – Installs a vertical 12 foot flood wall through the Lippe property through the Remmel and Pool properties to where the existing sacked concrete is.
Katherine pointed out the preliminary cost estimates on the matrix. She discussed the practical difficulty of erecting a twelve foot wall.
II. Preliminary Construction cost of the Proposed Alternatives
Katherine pointed out the ranges of costs for all the alternatives. More hardscape produces more cost. Construction is budgeted at about $1.5 million.
3. Alternative Reviews - Tony Ndah
Summary Table of Comments Received at Workshop
NAME / COMMENT / RESPONSESteve Seligman / What does geomorphic mean? / It is how the creek would configure itself if left to natural forces.
Jitze Couperus / What does the red line on the map represent? / It represents the limit of construction activity. Water can flow outside that point.
Tom Mandle / Is the forty foot width the width of the finished creek?. / No, it’s the limits of the construction area. Placement of boulders would occur within the forty feet.
Libby Lucas / If you take out the sediment, does that still leave the sewer line too high? Is there a way to take care of the sewer line? / Yes, that is still unresolved. If the sewer is left in place, a drop structure could be installed just downstream of it, or the channel bottom could be sloped with a drop structure placed at the downstream end of the bridge.
Melissa Seligman / The red line abuts a lot of trees. Will their roots be trimmed? / No, the trees will be protected in place. The blue line was created to protect the trees. The red line does not necessarily mean the bank will drop off. Depending where the tree is on the slope, there could be rock protecting it, or there could be packed earth there. That will vary along the creek.
Tom Mandle / What does the matrix mean relative to the creek width? / On the Los Altos side where there is gunite, the trees won’t be impacted. The circles represent the canopy of the tree, not the trunk. The blue line top of bank was based on the old project, which provided a six foot buffer from the trunk of the trees. That was approved by the horticultural specialist.
Steve Seligman / What does it mean if the red line goes through the middle of the tree? / It looks like the tree on the Seligman property with the red line through it may be able to be saved. There is some flexibility in the location of the red line to allow for the protection of trees.
Tom Mandle / Do the dimensions on the matrix represent the size of the hole in the ground, crudely? Does it represent the dimensions that would be on the cross section for that area? / Yes.
Bernice Moos / Are you saying you’re keeping the existing gunite? / If removing the gunite is detrimental to the tree, it will be left and soil will be put in front of it. If the gunite can be safely removed it will be.
Bernice Moos / Why wouldn’t the soil on top of the gunite wash away? / The soil is clay, it would be sloped and planted, and the roots of the vegetation will hold the whole thing in place.
Libby Lucas / Is the loss of so many trees acceptable? / It’s not ideal, but it is acceptable.
Bernice Moos / What is a flood wall? / A flood wall is a concrete structure.
Libby Lucas / Has any of this been presented to the regulatory agencies? / Not yet.
Tom Mandle / What are the dark lines by the bridge? / Those are the existing wing walls that will be removed.
Melissa Seligman / Does the dotted line represent where it’s geomorphic? Wouldn’t it extend beyond the Remmel property? / It represents the lowest point of the channel. The alternative illustrated only addresses upstream of the stormdrain. Downstream of the stormdrain that dotted line would follow the centerline between the toes of the banks as it exists now.
Richard Moll / The jog in the blue line looks artificial. / We need to be able to choose an alternative now in order to progress with the CEQA process. Exceeding the blue line interferes with being able to get approval of an alternative. Refinements of the alternative can be made later.
Tom Mandle / What was the capacity of Alternative 6 before it was made to fit between the blue lines? / It was 1400 cfs, the same as it is now. Any capacity issues could be resolved by making the channel deeper, which may introduce more hardscape.
Bernice Moos / The hardscape on the matrix for Alternative 6 is only 400 square meters. / The hardscape will increase. The number is preliminary and may go up another 120 square meters.
Richard Moll / Is there any change in the shape of the thalweg that causes you concern? / There may be a lower sinuosity which may require more bank protection to stay within the blue lines.
Jitze Couperus / Alternative 6 is pretty close to geomorphic. That is very exciting.
Libby Lucas / Have you built step pools elsewhere like the ones you propose here? / Yes, at Alamo creek and further downstream on Adobe.
Melissa Seligman / Could someone walk up the creek with the drop structures there if there were water in it? / Yes, you could walk up the side.
Richard Moll / Can we build this? Will it work? Will the agencies love it? / The agencies were clear that it will be important to keep track of the hardscape that’s going in relative to the hardscape that’s being taken out. But we’ve done the best we can with a semi-geomorphic design.
Tom Mandle / Is the capacity in the section of the creek nearer the bridge going to be sufficient to carry the overland flows that will enter it there? / We don’t know yet. Once the footprint is established, the alternative can be adjusted to make it fit. But the deeper the water and the steeper the slope the bigger the rock size. A one foot rock on the grade control structures at the top end will work, but you will need a four foot diameter rock at the bottom end. Heterogeneous grade control structures there could provide three to six inch drops for smaller fish to go through.
Melissa Seligman / With Alternative A, is there no plan to do anything to the Robinwood property? / There is rock slope protection to shore up the existing erosion where the two properties meet.
Richard Moll / There is a spring feeding the bottom of the creek that needs to be accounted for in the design process. Please add that to the matrix of comments. / It will be added to the matrix..
Bernice Moos / Where the large rocks are on our side, you’ll need to protect underneath once the waterfall is removed. / Correct. That’s not shown on the drawing of the rock lining for Robinwood Pool.
Richard Moll / How tall is the vertical wall? / About 12 feet, to the height of the 100 year flow.
Bernice Moos / I would be looking at a 12 foot tall wall.
Richard Moll / You could put plantings in the sloped rock of concept B, but it would be more difficult to do that in the wall. / We would try to vegetate the rock.
Libby Lucas / What is the difference in life of the rock versus the wall? / The wall would be more permanent, and the rock would require more routine maintenance.
Richard Moll / Do you have a recommendation on the alternatives? / No. Based on the requests that have been made, these are the alternatives we’ve come up with.
Jitze Couperus / Would construction of the vertical wall provide for recovery of the same additional property as the rock lining? / Yes, approximately.
Melissa Seligman / How would the Remmels and the Lippes feel about a vertical wall? / The Lippes priority is preservation of their trees and backyard as is. The Remmels haven’t been asked yet.
Bernice Moos / A vertical wall is a step backward to 1988. We did not want walls in the channel.
Melissa Seligman / Would the rocks provide as much stability as the wall? / Rocks will provide as much erosion protection.
Tom Mandle / Would the regulatory agencies prefer the rock to the concrete or the steel? / Most likely.
Tom Mandle / What’s the disadvantage of the rock? / Jitze Couperus-Rock requires some maintenance. Concrete walls probably don’t require any.
Jitze Couperus / Animals can live in rock. They can’t live in concrete walls.
Richard Moll / Maybe the spring here will keep the footing of a concrete vertical wall from being on solid ground. If you put in a vertical sheet of steel you might rupture the aquifer and have water coming up on both sides. / This point is well taken. No matter what alternative is chosen, the spring will be addressed during the design.
Melissa Seligman / Alternatives A and B don’t address the Lippes or the Remmels at all, right? / They do address the Remmels. The Lippes are taken care of as part of the upstream portion.
Melissa Seligman / Does it matter erosion wise at the Lippes and Remmels whether the solution there is a wall or rocks? / Their bank will be protected to the best extent possible within the blue lines.
Steve Seligman / What is the advantage of the wall? It’s two to six times as expensive. / The wall would bring more confidence that it’s going to be in place for the next 40-50 years.
Richard Moll / I think you should add that as a consideration to the matrix. / That would be good.
Libby Lucas / There are several parks upstream. If children were washed into the creek, with rock they would have the capability to haul themselves out. With a wall there is no way to get any traction. / That’s a very good point.
Bernice Moos / We love the rock on our property. Things grow in it. We can walk down to the creek. / The agencies will look at rock as hardscape the same as anything else. From the District’s point of view, rock hardscape is environmentally and aesthetically preferable to concrete.
Richard Moll / Paul Amato has seen the downstream portion and said that hardscape is the only answer. The channel is so narrow and deeply incised you can’t have a geomorphic solution. I think they’ve accepted that. / If we go over the hardscape “budget”, there will be mitigation costs associated with that. The agencies are looking for the overall environmental benefit.
Melissa Seligman / Are the neighbors who are not at this meeting satisfied with alternatives 1 through 5? Did they know about alternative 6? / Richard Moll- Absence is not acceptance. Alternative 6 was just presented tonight.
Richard Moll / Do all of the solutions downstream of the Robleda storm drain have the waterfall changed into the drop structures? / Yes.
Susan Mandle / The Lippes would like to discuss with the District and D.J. on site in their yard the different alternatives and the location and impact of the red line. / The District didn’t want to start staking until we know exactly what we’re going to do.
Tom Mandle / What’s missing still is the overland flow, which is a critical element of the overall system design which needs to be handled by the Water District and the Town somehow. That needs to be part of the presentation of the alternatives, otherwise we’re signing up for something that’s not complete. If we need to choose an alternative in June, the Town and the District have some work to do together before then. / Overland flow modeling won’t be done in June. The decision will be about the footprint without the detailed knowledge of the overland flow.