Posted by: Deirdre Des Jardins | January 30, 2020

Groups criticize Del Puerto Canyon Reservoir site, environmental report

Patterson, CA

On Monday, January 27, Friends of the River, Save Del Puerto Canyon, California Water Research, and environmental and fishing groups filed comments highly critical of the Del Puerto Canyon Reservoir proposed site and the project’s Draft Environmental Impact Report.

Del Puerto Water District and the San Joaquin River Exchange Contractors are proposing to construct the project to store agricultural water supplies. The city of Patterson (population 22,352) is just below the main dam, and local residents are concerned about public safety.

Friends of the River’s senior advocate, Ron Stork, joined Modesto area geologist Garry Hayes and geologist / environmental compliance expert, Dr. Tom Williams, in expressing concerns about the reservoir inundating an area with active landslides.  Stork is widely recognized for warning the California Department of Water Resources of the potential failure modes of the Oroville dam emergency spillway.

Garry Hayes commented:

The discussion in the DEIR [Draft Environmental Impact Report] of the possibility of mass wasting, also known as a landslide, is wholly inadequate. In the event of mass wasting, debris flows may slide into the reservoir, resulting in a lake tsunami.  The tsunami could exceed the dam height, resulting in overtopping of the dam. Seven landslides are within the reservoir inundation zone. The DEIR has no specifics about the age of the failures, or the volume and length, and nothing about the current activity.

The DEIR also does not describe the large slump/earth flow complex, which to the uninformed eye simply looks like a hill, visible to the north from the site of the proposed reservoir, which forms a hummocky topography in the small side canyon.  The toe of the slump is eroded by Del Puerto Creek, which has led to continuing reactivation of the slide.  The public is not informed that standing at the entry into the canyon one can see another large active slump which originated at the south side of the canyon.  That it is still active, i.e. moving, is evidenced by repeated road repairs at either end of the slump.

Garry Hayes’ Geotripper blog post on the landslides in the reservoir inundation zone is available here.  A photo of one of the landslides is reproduced below.

Del Puerto slide

Large landslide in Del Puerto Canyon dam inundation zone       Source: Garry Hayes

Hayes is also concerned that the series of faults in the area are poorly studied and poorly understood.  There was an 1880 earthquake in the area, but it is poorly documented.  The rocks in the area are so soft that erosion removes evidence of past earthquakes.

The Draft Environmental Impact Report proposes to mitigate the landslide risk in the future by preparing a “design level Geotechnical Investigation and Report” and by following the report.  Ron Stork commented:

The DEIR provides no geotechnical analysis supporting the conclusion that any landslides would be slow and at a scale that would not form a lake tsunami or seiche wave of significant magnitude. Landslides are a known risk for dam failure. A large slope failure into the Vajont reservoir in Italy caused a large tsunami which overtopped the dam.

Although the Vajont reservoir had different geotechnical conditions, the lessons learned apply. A detailed geotechnical evaluation of the stability of any areas susceptible to sliding under reservoir conditions should be done before selecting the dam site.  Deferring the necessary geotechnical analysis until after the Water District has approved the environmental review for the site — on the assumption that there will be cost-effective engineering solutions to address this risk — is not good process and assumes facts that may not prove to be true.

Dr. Tom Williams concurs with the opinions of Garry Hayes and Ron Stork, stating:

The geological formations and current topography are conducive to mass movements, slumps, and landslides at present and when their lower supporting slumps are wetted and lose strength to carry the loads of the “dry” materials. Many landslides are waiting to happen when they get wet. Numerous slides/slumps may interfere with the efficient and reasonable storage and operations of the reservoir.

The comments note that the Draft EIR rejects an alternative location near Ingram that would not be above the city of Patterson. The location appeared to be rejected because it would store 67,000 acre-feet of water, 13,000 acre-feet less than the Del Puerto Canyon site.

Fishing groups expressed concerns that Del Puerto Creek is the main source of gravel for spawning sites used by White sturgeon in the San Joaquin River.  The comments note that the DEIR does not specify the extent of the impact the project on this substrate, or how it will affect spawning reaches.

Del Puerto Canyon is also the only real public access to the Diablo mountain range from the Northwest San Joaquin Valley. Del Puerto Creek is one of the few westside streams in the San Joaquin Valley with perennial stretches. The creek has rare riparian habitat and contains more than 160 bird species, making Del Puerto the third most diverse bird habitat in the entire county. It has unique geology and has been visited by hundreds of geology students as well as the National Association of Geoscience Teachers, the Geological Society of America and other national organizations. Residents have formed a Save Del Puerto Canyon Facebook group.

The comments note that the DEIR states that additional conservation by the agricultural water districts is not feasible, although the statement appears to be contradicted by the Westside-San Joaquin Integrated Regional Water Management Plan. The DEIR also does not consider alternative water supplies available to the agricultural water districts. The North Valley Regional Recycled Water Program, which is being implemented by Del Puerto WD and the Cities of Modesto and Turlock, is currently expected to deliver up to 26,000 acre feet a year of recycled water.  According to the Westside-San Joaquin Integrated Regional Water Management Plan, the districts are also implementing agricultural tailwater reuse projects.

 


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