tag:blogger.com,1999:blog-1488098001848302815.post8780900465207310708..comments2024-01-09T00:28:09.127-08:00Comments on Harmonic Tremors: Pull-apart basins of the Inland EmpireJulianhttp://www.blogger.com/profile/14946326483548193256noreply@blogger.comBlogger5125tag:blogger.com,1999:blog-1488098001848302815.post-70789790207106207162008-08-19T12:43:00.000-07:002008-08-19T12:43:00.000-07:00One more comment: The flood control agencies/auth...One more comment: The flood control agencies/authorities in CA (where I am now, there are none to precious few) have significant restrictions on construction in the 100-yr floodplain. At this juncture, the levees in between San Jacinto and Lamb Canyon are at about the 50-year event. When I left the SoCal area, the local responsible agency was in the very initial stages of planning to construct set-back levees behind the existing ones to provide protection against a 100-yr event, and try to utilize the area between the two as a riparian habitat mitigation. It is unlikely that any project would have been approved within the 100-yr FP by the area planners since CEQA's passage. <BR/><BR/>Typically, that county requires that any arroyo/stream/channel through a development pass a 100-yr event with an appropriate Manning's roughness coefficient for the expected vegetation. If there is riparian habitat existing or proposed for the arroyo/stream/channel, the hydrologic calculations should include a Manning's corresponding to mature riparian vegetation and the channel still pass a 100-yr event with no overtopping.coconinohttps://www.blogger.com/profile/12050625735753145388noreply@blogger.comtag:blogger.com,1999:blog-1488098001848302815.post-22731976389342108202008-08-19T11:44:00.000-07:002008-08-19T11:44:00.000-07:00Silver Fox: No mini Death Valleys here! Nor is eve...Silver Fox: No mini Death Valleys here! Nor is every mountain a pressure ridge. (Though plenty of them are, around here.) I do have to wonder how much of the hazard risk is let on for prospective buyers. Someone in the department told me that, with regards to faults at least, sellers just have to tell people that their house is in a "zone," with no further detail!<BR/><BR/><BR/>Coconico: Only 700,000? I'd heard an age of 1 million for the San Jacinto (though I honestly can't remember where I got that figure), which is still pretty young, but dropping it down to six digits...woah. Count me 300,000 years more in awe of how quickly the San Jacinto has shaped the landscape. The San Andreas' hyperactive little brother indeed!<BR/><BR/>It doesn't surprise me in the least that the decision of where to build is so political, no matter what the hazard reports say. Though I'd think that'd leave room for far more political kvetching and flailing when something bad does happen...<BR/><BR/>That is very cool about the vernal pool ecosystem in the basin! Just shows I really need to learn more about local plants, if they can tell such stories about the geology. I'll also have to check out that EIS report.Julianhttps://www.blogger.com/profile/14946326483548193256noreply@blogger.comtag:blogger.com,1999:blog-1488098001848302815.post-33753372251330386362008-08-19T09:42:00.000-07:002008-08-19T09:42:00.000-07:00I should add that CA panning regulations are much ...I should add that CA panning regulations are much stricter than most of the rest of the SW, but are not infallible. Different counties apply them more or less stringently, and it depends on public comments and lawsuits as to whether the less stringent counties get on board with the more stringent ones. It is also somewhat to fairly political, depending on the Boards of Supervisors and the political winds that blow thru the county in question.coconinohttps://www.blogger.com/profile/12050625735753145388noreply@blogger.comtag:blogger.com,1999:blog-1488098001848302815.post-14874080591230049972008-08-19T09:17:00.000-07:002008-08-19T09:17:00.000-07:00Julian - I worked fairly extensively in that area ...Julian - I worked fairly extensively in that area a few years back. The EIRs required by CEQA and the extensive planning reviews conducted under CA planning laws usually require some sort of geotech analysis. Any issues arising must be mitigated under CEQA requirements (ie., like landslide hazards). That doesn't necessarily mean the development would not get constructed, but it does mean there is an assumption of a certain amount of risk that is usually identified in the geotech report. Indications of faults are usually trenched and logged or downhole logging is conducted. <BR/><BR/>That aside, my anecdotal evidence is that the subsidence is primarily caused by groundwater withdrawal for domestic/ag use. When San Jacinto was first settled, apparently the San Jacinto River was perennial in the reach upstream of the Soboba Reservation. Flumes constructed of redwood would carry the water to fields and the town. If you can get your hands on the EIS for Eastern Municipal Water District's Hemet San Jacinto Recharge and Recovery Program, you'll note some interesting groundwater basin tidbits about the San Jacinto River.<BR/><BR/>Downstream of Lamb Canyon Road, in the reach from around Mystic Lake to the entrance to Railroad Canyon, the landscape gets very interesting from a hydrogeomorphological standpoint. As I understand it, that reach was a lakebed in Quaternary, gathering lots and lots of very fine grained lacustrine seds. The river in that reach ends up having a gradient of about a foot per mile, resulting in a very wide 100-yr floodplain (~1 mile) and large flood events trun the area more into a very large lake. This has resulted in hydric (frequently flooded) soils and a plant community that is less riparian than it is vernal pool-related. There are three endemic T&E plant species, spreading navarettia, SJ crownscale, and thread-leafed brodeia that inhabit the area as a result of the combination of geology and hydrology of the area. <BR/><BR/>As a side note, I believe research conducted by Becky Dorsey (U of O) has shown that the San Jacinto fault is 700,000 years old or less. I always thought it quite fascinating that this landscape with which I had so become so intimate was so young.coconinohttps://www.blogger.com/profile/12050625735753145388noreply@blogger.comtag:blogger.com,1999:blog-1488098001848302815.post-86703995069605237422008-08-19T05:45:00.000-07:002008-08-19T05:45:00.000-07:00Sounds like it was an interesting field trip: find...Sounds like it was an interesting field trip: finding out you can't find faults at the base of every mountain, a bunch of little mini Death Valleys, it sounds like. As for houses, surely they have some kind of hazard maps? That doesn't always stop development, but then a buyer can maybe pick and choose from bad and worse.Silver Foxhttps://www.blogger.com/profile/03131032620978696727noreply@blogger.com