Archive for July, 2011

I’ve become a fan of the blog Hydraulically Inclined (http://ayresriverblog.com) written by two Ayres Associates staff, Anthony Alvarado and Dusty Robinson.  Recently, they put together a two part blog about the technical considerations that went into mapping a section of the San Joaquin River’s bathymetry.

I live in Massachusetts so my familiarity with this site and the river’s history and management is limited.  Nevertheless, given all the tools and information on the internet, there are some ways to start to gain some familiarity with the San Joaquin River from my desk thousands of miles away.

Some of the tools I used to get some background on this site were Google Earth, and two handy KML files: FEMA’s National Flood Hazard Layer and a USGS point file of realtime gages.  I also used FEMA’s Map Service Center to read the Flood Insurance Study for the San Joaquin River.

Looking at the site in Google Earth, it became readily apparent that the hydrology is tricky, namely due to all the diversion canals, dams (flood control and storage), levees and land use changes that all alter the hydrology in different ways.  The other thing that I found quite interesting was by using the time slider in Google Earth, it became quite apparent how much land use change has occurred at the site in just the past 10 years.

San Joaquin Google Earth 2002 imagery

San Joaquin 2010 Google Earth image, note amount of development that has occurred

Using a KML file that has point locations of all the realtime USGS gages revealed that the nearest gage is #11303500 San Joaquin River near Vernalis, CA which is roughly 13 miles upstream of the site.  Taking a look at the realtime data indicated that the river has been experiencing above average flows for the past few months. I was also impressed with the frequency with which the USGS has been making measurements at this site.

USGS data reveal above average flows and several stage discharge measurements in the past few months.

Flows going from ~10,000 cfs to ~30,000 cfs resulted in roughly 9 feet of stage change at the gage site.  I’d be curious to know how much the width and depth changed at the site during this high flow period. My guess is that perhaps some of that inset forested meander was inundated.

Using the path measurement tool in Google Earth, it appears as though a diversion canal was built ~11 miles downstream of the gage at Paradise Dam and little over 2 miles upstream of the study site.  To what extent that reduces peak flows at the site I am not sure, but I initial guess is that it is responsible for some diminution in peak flows to the site.

The Flood Hazard Layer provides some helpful information as well. For starters, the channel is heavily flood controlled.  The channel width for the 100-yr flood is not much wider than the wetted channel in the photo due to all the dikes.  I’d be curious to know what sort of stream power and shear stress values get generated during those types of flows.  Using the distance tool I did a quick measurement of the slope during the base flood. Very roughly, upstream of the site the slope is 1:4000 (0.025%) and downstream the slope is 1:3000 (0.033%). Given these low slopes, my initial hunch is that the bed is pretty fine grained, probably mostly sand and finer.

Base flood, cross sections and flood control structures shown

The FIS is an interesting read. The San Jaoquin has quite a history!  Kudos to the team that was able to stitch together all the previous studies and get all the data onto a common datum.  Here’s a bit of text from that report:

Cross section surveys in in the restudy were provided by Ayers Associates. Results from a completed sounding survey of the channel floor by boat in February 1999, a cross section survey by Ground Point Station (GPS) in 2000, a 1998 section survey produced by contract with GeoTopo Inc., and a linear interpolation between cross sections were used in the analysis.

For countywide revisions, no updated hydrologic and hydraulic analyses were prepared. MAP IX-Mainland compiled existing data to convert the previous study into digital format. MAP IX-Mainland completed this work in October 2007…

Base map information sown on this FIRM was derived from multiple sources. This information was compiled from the USGS, 2002, National Geodetic Survey, 2002, City of Lathrop, 1997, City of Manteca, Department of Public Works, 2001, and San Joaquin County Community Development Department, 2008.  Additional information was photogrammaticaly compiled at a scale of 1:12,000 from USGS aerial photography dated 1989, 1993 and 2002.

Given all the hydrologic alterations that have occurred within the County, the report is a bit lengthy and goes into detail about the methods used to estimate peak flows.  The punchline from all of this is in Table 10.

Increasing drainage area, decreasing peak flows

The take home message I get from this table is that my initial hunch was correct: dams and diversions play a major role in the hydrology of this river.  Mossdale (more or less the location of the study site I believe) has a drainage area about 1% bigger than at the Vernalis gage, yet the 0.01AEP flow is about 31% less than at the gage.

One question I would have for Anthony and Dusty is how much confidence they have in this table.  Do they have reason to believe these numbers should be adjusted up or down for some reason?

If I was bidding on a project at this site, these are some of the desktop tools I would use to gain some level of familiarity with the site.   I’m sure there is quite a bit more to learn, but I just wanted to highlight the utility of some of the hydrologic resources that are out there.


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