Interoffice
Technical
MEMORANDUM
To:
Scott Downie, Bill Condon, and Kris Vyverberg
From:
Mark Smelser, CEG 2192
Engineering Geologist
HabCon Region 1
Date:
October 14, 2008
Subject:
Feliz Creek Reconnaissance and Draft Proposed Scope of Work
A reconnaissance level traverse of the Feliz Creek channel up- and downstream
of the dam on the Hognestad property was undertaken on Wednesday October
8, 2008. The traverse was attended by Steve Allen and Paul Wisniewski
(Winzler and Kelley), Michael Love (Michael Love and Associates), and Kris
Vyverberg and Mark Smelser (Department of Fish and Game). Approximately
4,300 feet of channel up- and downstream of the dam was walked and another
2,600 feet of channel further upstream was reconnoitered with vehicles.
Collectively, nearly 6,000 feet of channel was evaluated between two existing
bridges; one located approximately 1,000 feet downstream of the dam and the
other approximately 5,000 feet upstream from the dam. The traverse confirmed
the presence of a fault underlying this segment of channel and a diverse group of
rocks (peridotite, serpentinite, schist, meta-volcanics, and interbedded sandstone
and shale). In addition, a suite of alluvial surfaces adjacent to Feliz Creek was
also observed. The purpose of the traverse was to develop a scope of work for
an engineering geologic characterization of this channel segment to support the
development of engineering alternatives that will improve fish passage through
the dam site and not adversely impact existing residential structures and nearby
bridges.
It is generally understood that improving fish passage through the dam site will
require some, or all, of the dam to be removed. Any level of dam removal has
the potential to 1) release sediment stored behind the dam, 2) cause an
upstream migrating headcut, and 3) affect the toe of the slope below the
Hognestad property. In order to develop alternatives that improve fish passage
and protect the existing structures, the geologic, geomorphic, and hydrologic
constraints need to be accurately characterized. More specifically, the volume
and upstream extent of sediment stored behind the dam needs to be quantified,
the type and strength of the various earth materials needs to described, and the
recent geomorphic history of Feliz Creek needs to be deciphered. In addition,
the spatial relationships between the existing anthropogenic structures (houses,
Interoffice Technical Memorandum
Feliz Creek
Page 2
October 14, 2008
bridges, roads, cutslopes, and fillslopes) and the various geologic and
geomorphic features and processes needs be understood. All of those elements
then need to be integrated with a quantitative understanding of the contemporary
hydrology and hydraulics affecting streamflow and fish passage through the dam
site.
Following the traverse, consensus was reached that the scope of additional work
be developed in a phased approach with specific deliverables. Winzler & Kelley
is currently authorized to prepare a geologic and geomorphic map of the channel
segment lying within the Hognestad property boundaries. The primary purpose
of that map is to characterize the type and distribution of rock units and geologic
structures, provide a qualitative description of the earth materials and their
engineering properties, and map and describe the distribution of alluvial surfaces
up and downstream of the dam site. It is my understanding that the geologic and
geomorphic mapping will rely upon both field mapping as well as analysis of
aerial photographs. The primary deliverable for this effort is anticipated to be a
geologic map sheet that accurately depicts the various anthropogenic structures,
geologic elements (rock units and structures), channel controls, and alluvial
surfaces. Included on the map sheet should be detailed descriptions of the earth
materials and detailed descriptions of the suite of alluvial surfaces. Most
importantly, this map will identify specific cross-sections to be surveyed with a
total station during subsequent phases of work.
Following completion of the geologic and geomorphic map, the following tasks
are recommended:
Task 1:
Technical Meeting to Review the Geologic and Geomorphic Map
A technical meeting between interested parties will be conducted to
review the geologic and geomorphic map. The purpose of this meeting
will be to discuss the findings and reach consensus on specific
elements of the following tasks.
Task 2:
Total Station Survey of the Longitudinal Profile and Select CrossSections
A longitudinal profile of the channel thalweg between the two bridges
will be surveyed. During that survey, select cross sections will also be
surveyed. The purpose of the cross section surveys is to quantify the
relief and spatial distribution of: 1) the suite of alluvial surfaces; 2)
pertinent geologic and geomorphic features; and 3) anthropogenic
structures (e.g., bridges, residential structures). A minimum of 12
cross sections are likely to be required and several more may be
required depending on the results of the geologic mapping and the
potential need to support hydraulic modeling. The actual number and
location of the necessary cross sections will be determined at the
technical meeting identified as Task 1. Furthermore, these survey
efforts will require property access from the upstream landowner.
Interoffice Technical Memorandum
Feliz Creek
Task 3:
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October 14, 2008
Preparation of an Engineering Geologic Map of the Hognestad
Residence, Swimming Pool, and Existing Dam.
A detailed engineering geologic map and at least 3 engineering
geologic cross sections of the Hognestad residential compound and
the existing dam need to be prepared in order to show the
relationships between earth materials, slope angles, channel
configuration, and existing structures. The map and cross sections
shall be created at a scale no smaller than 1-inch = 40 feet, and the
cross sections shall extend all the way across the channel so that
slopes outside of the channel are depicted. The engineering geologic
map and cross-sections are essential for evaluating and mitigating
potential risks to the Hognestad’s residential structures associated with
removing the dam and/or a natural failure of the dam. The engineering
geologic map shall also be of sufficient detail, scale, and topographic
control to support the development and evaluation of conceptual
engineering alternatives focused on fish passage and slope stability
beneath the Hognestad’s residence.
The deliverable for this task will be the engineering geologic map and
cross sections and any other appropriate illustrations along with a brief
technical report signed and stamped by a licensed geologist. The
technical report shall describe the integrity of the residential structures
(e.g., house and swimming pool) within the context of the existing
geologic conditions and in conjunction with the existing slope stability
issues (including channel erosion) and any other limiting factors that
need to be considered with regard to the existing dam structure and its
potential to be fully or partially removed. The report shall also include
appropriate recommendations to be considered with regard to the full
or partial dam removal effort as well as recommendations for a
subsurface investigation necessary to support construction of a
retaining wall below the Hognestad residence.
Task 4:
Analysis of Longitudinal and Cross Section Survey Data
The survey data acquired during Task 2 will be combined with geologic
and geomorphic data as well as water surface data (e.g., bankfull and
100-year flood flow levels) and appropriately depicted in drawings.
The primary purpose of the analysis is to: 1) quantify the up- and
downstream distribution (i.e., continuity) of the alluvial surfaces relative
to the thalweg and reference water surface levels; and 2) quantify the
volume of sediment stored behind the dam. Important secondary
purposes include understanding the history and pattern of channel
aggradation several hundred feet upstream from the dam, channel
incision below the dam, and geometric relations of the surrounding
slopes.
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Feliz Creek
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October 14, 2008
The deliverable for this task will be the longitudinal profile and cross
sections and any other appropriate illustrations along with a brief
technical report signed and stamped by a licensed civil engineer and/or
geologist. The technical report shall describe the distribution (i.e.,
continuity) of the alluvial surfaces relative to surface water levels and
also describe the volume and upstream extent of sediment stored
behind the dam. The report shall also include appropriate
recommendations to be considered with regard to the full or partial
dam removal effort (e.g., what is the potential for headcutting and
where are appropriate locations for grade control structures). The
report may also need to include recommendations for a subsurface
investigation to better constrain the depth of alluvial sediments and/or
the strength parameters of subsurface materials.
Task 5:
Technical Meeting to Evaluate the Engineering Geologic
Characterization
A technical meeting between interested parties will be conducted to
discuss the findings and recommendations presented in the reports
described under Tasks 3 and 4. The goals of this meeting will be to: 1)
identify three conceptual engineering alternatives that provide
unimpeded fish passage, protects the Hognestad’s residential
structures, and mitigates upstream headcutting; 2) define a suitable
geotechnical investigation (subsurface exploration and strength
testing) necessary to support the engineering alternatives; 3) define
the limits of a detailed topographic survey to support the development
of final design plans; 4) determine the need for hydraulic modeling; and
5) develop a scope of work that will culminate in a set of final design
plans and an engineer’s cost estimate for implementation.
It is recognized that the lack of a specific time line and unknown costs associated
with the additional tasks are disconcerting. However, given the complexities of
the geologic conditions, the hydraulic controls, and the potential threats to
existing structures, it appears prudent to move forward in a systematic manner.