01-01-2023, 03:27 PM
(01-01-2023, 10:40 AM)jacket3ree Wrote: Fishman -
I went straight to the USGS site: SAN FRANCISQUITO C A STANFORD UNIVERSITY CA[/url]
This is a live link, so I don't know how to copy and paste the hydrograph. There are radio buttons for flow and stage. The data are provisional (vetting will take months), and the number I reported yesterday was from the memory of looking at my phone while standing on the creek bank in the rain, so it appears 4,300 cfs was low on my part - sitting at my desk this morning on a laptop, I read 6,350 cfs at 9:00 on Saturday. This is equivalent to the 40-year event. That's better - it means there is a higher level of protection under existing conditions.
Here is a link to the annual peak streamflow data: [url=https://nwis.waterdata.usgs.gov/usa/nwis/peak/?site_no=11164500]Peak Streamflow for the Nation USGS 11164500 SAN FRANCISQUITO C A STANFORD UNIVERSITY CA
02/03/1998 7,200 cfs
Our flood frequency analysis shows the one-percent peak discharge as 8,000 cfs (YMMV)
Make sure you are looking at peak streamflow data, not maximum daily discharge, which would be lower.
Triangle -
Newell Street is one of those "and then" bottlenecks I described earlier, along with University Avenue. From upstream to downstream, historically Middlefield Road, Pope-Chaucer, University, Newell, West Bayshore, 101, and East Bayshore cause problems. If all those bridges were removed, with relatively minor improvement, the creek could pass the flow. Removing, raising or enlarging each of those structures comes with varying degrees of difficulty. That's probably what the City person meant by saying the solution would need to be "elsewhere". One has to be careful in how one removes a bottleneck, or the problem is indeed moved elsewhere. The bridge restrictions between Middlefield and Bayshore needed to be remedied simultaneously if possible, or temporary flow restrictions may be necessary if available funding doesn't allow it all to be done at once.
There are four basic things that can be done on any system like this:
1. Increase flow conveyance (enlarge creek, remove vegetation, and/or improve bridge hydraulics)
2. Reduce the flow by storing water upstream.
3. Reduce the flow by diverting a portion of it (bypass).
4. Nothing. But even then, if the creek is not maintained, flow conveyance will lessen and flooding may increase.
Some of the public and NGOs like Friends of the Creek favor the fourth option. As crass as this is to say, we need events like this to remind everyone that doing nothing leads to sorrow. Alternative 2 (on stream or off-stream storage) and Alternative 3 (giant concrete underground bypass from Middlefield to the Bay through.....EPA?) have been studied to death and appear to be off the table forever due to ineffectiveness and/or expense. That leaves Alternative 1.
Work should progress from downstream to upstream to avoid induced flooding, and it is. The reach from Bayshore to the Bay is complete, and with some remaining nuance, Bayshore itself is ready to go. That leaves the reach between Middlefield and Bayshore. The JPA has labeled the reach between Pope-Chaucer and Bayshore as "Reach 2". Kevin does a better job of succinctly explaining it than I can:
Kevin Murray Explains
If anyone has further interest, I encourage you to read about it here: The Reach 2 “Upstream Project” When this work is done, the capacity bottleneck moves to Middlefield Road. I haven't heard anything about fixing that, and the downstream capacity is being set to that of Middlefield, which spills in a 100-year event. It appears that a residual 100-year floodplain will remain, but it will less extensive than under current conditions.
Valley Water put together a flyover after the 1998 flood. Full disclosure: They retained my firm to help them reconstruct what happened and we did some modeling. We categorically DID NOT select the background music.
1998 Flood
Remember, a lot of the flooding you see is local - drainage can't get into the perched or leveed creek, so it backs up on the streets.
Jacket3ree, this is great information. Really appreciate you taking the time to share it.



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