[Fosc] Bye-bye acacias and a new landslide in the neighborhood

Chris Burmester christopher at formulate.com
Mon Jan 14 08:27:53 PST 2008


Hi All,

a balanced bit of news (with pictures). Some great news - the 5 
massive Acacias at the end of Benevides are finally gone (thank you 
Martin and the City of Oakland Tree Crew - and Kathren for asking 
again!!) and some not so great news - a landslide across the creek 
from the turnaround due to the Park street drain overflow onto the 
trail (overflow now re-routed).

Acacias first:

I wish I had a "before" picture handy, but the "gap" looking into the 
creek canopy from the end of Benevides was formerly completely filled 
with wall-to-wall Acacias.



There's a young Oak front and center (about 10' tall) that will be 
very happy about the change of scenery.

The City Tree crew left two massive 20'+ trunk segments behind, 
though, right at the top of the slope where we're restoring. Any 
chance to get them to come back and cross cut these into smaller 
pieces so we can move them?




And - finally - the larger trunk of the group of five (foot for scale):



And then the landslide. After our first day of torrential rains last 
week, I noticed that the  first stretch of the Benevides trail had 
turned into a torrent of water coming all the way down from the San 
Luis "glade" (where the Park Street storm drain dumps its water) to 
the base of the big Oak at the start of the Benevides trailhead.

I immediately temporarily fixed this issue by digging a trench and 
creating a waterbar to divert the water in the "glade" off the trail 
and down slope along along it's historical route. This should hold 
for a bit - I'll keep an eye on it.

Needless to say, this overflow severely eroded the trail - we've some 
work to do here - but most of the damage was done to the slope below 
the big Oak where the falling water cut a fairly deep gully and - 
apparently - triggered a significant landslide (see below).



In this picture you are looking up from the turnaround in the creek 
(on the creek bank), across the river up towards the end of 
Benevides. The slide looks about 15' tall (about half the slope) by 
20 ' wide (rough estimate).

At the top right of the slope, you can see where the Park Street 
storm water was descending the slope and the gulley it cut (mostly 
buried in cape ivy). At the top left of the slope, you can see 
another depression/gulley (buried in cape ivy). This second gulley 
has been being cut by the broken storm drain coming off the end of 
Benevides. It is also just dumping on the open slope and causing 
significant erosion damage.

I suspect that this slide is the culmination of both these erosion sources.

I'd like to see if we can get someone look at this who knows 
something about geology. Not only is this dumping a huge silt load 
into the stream (see below - not good), but I'd like someone to 
comment on the general stability of this slope. At the very least, we 
should make sure that the broken storm drain culvert gets fixed so 
it's not dumping right into the "fresh" slide area. I think this 
slope is underset by a very hard mudstone and some sort of bedrock, 
so it should be OK. But I'd like to get the raw water off this slope 
to keep the silt out of the creek.

I'd like to clear the San Luis Glade (again) and create a very deep 
channel through the glade to route the stormwater down the rock 
channel it's cut over the years and not through this new area.

The slide slid into the creek and pretty much dammed it up - but the 
storm flow quickly cut a route around the side, but it did leave a 
new pool behind it.



Regards to everyone!

Chris
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