Archive for January, 2010

Eastern knockers

31 January 2010

Recently I visited some of the knockers in the steep hillside east and south of Mountain View Cemetery. These three are chert of deep-sea origin; the middle photo best shows its typical “ribbon” bedding.




This area borders the steep gorge of Moraga Canyon and harbors a number of deer and turkeys, among other wildlife.


Chabot gorge

24 January 2010

As you walk up to Chabot Dam the valley carved by San Leandro Creek opens up below, and it’s quite impressive (click photo for bigger version):

chabot gorge

At the top, the dam is buttressed against a ridge of the volcanic rock of Leona Quarry. Before the dam was built, there must have been quite a defile here:

chabot dam

The rock is nicely exposed just next to the dam, worth taking a close look at:

chabot rock

The gorge is more than 100 meters deep and only a couple hundred meters wide. Like other Oakland stream valleys, it seems large for the stream running through it today. I think much of the hardest work carving the canyons was done during periods of much wetter climate than today. Or maybe it was drier, with the land being more prone to desert-style flash floods.

Old-fashioned water filtration

16 January 2010

On the path up to Chabot Dam, you pass this row of big tanks (click for big version).

water filtration

They were used to filter the water from Lake Chabot reservoir, and they’re still filled with fine sand plus, I suppose, the decades’ worth of slime and crap they kept out of Oaklanders’ stomachs and food.

This is roughly the same purification method we rely on when we use well water. The fine pores between mineral grains purify groundwater in two ways. First, of course, they physically trap the crap. Second, the minerals themselves chemically attach to many dissolved contaminants. Clay minerals are especially important for that.

The days are long gone when Oakland was served by wells. The water table has been pulled down all over the city by human intervention. The headwaters of Lion Creek were once called Laundry Canyon because there was so much good water coming down. Fruitvale irrigated orchards for many years. The rains we get here can’t keep up, so now we pipe our water in from the Sierra.

One important feature of the new state water compact, if they can ever get it nailed down, will be a new, wide-scale program of monitoring groundwater. We need that to get a handle on the whole resource. Everyone knows, except the statutes, that surface water and groundwater are intimately connected. Western water law is an ass.

Tsunamis in Oakland

6 January 2010

The authorities have released new tsunami inundation maps, one of which includes Oakland:

tsunami map

Click it for the 2200×2200 version that I created just for Oakland.

There is no one tsunami that will wash over this much of the city; it’s a composite of a bunch of different possible tsunamis. In general, Oakland is like the rest of the Bay in being quite well sheltered from the kind of tidal waves that struck Sumatra five years ago. The Golden Gate keeps out the worst of the water. But it’s conceivable that some waves could wash up to about the 10-foot elevation level, especially in the parts of town nearest to the ocean. For residents, the worst threat would be in farthest West Oakland and the Jack London Square area. It also looks like the Webster Tube might briefly flood at the Alameda entrance, but probably not enough to make the tunnel totally impassable. The bridges would be fine, of course.

The time to worry about this is whenever there is a great earthquake in Cascadia or coastal Alaska. By “great” I mean an event of magnitude 8 or larger. But a lot depends on the details, like exactly where the quake occurs. If it moves a lot of seafloor, that would spawn the biggest waves. Even then, we would have several hours’ warning. In sum, tsunamis are a minor worry for Oaklanders—except for you living in boats.