060486 LEON WHITNEY DAM SITE, SCS GEOLOGY REPORT; [apparently by Lyle J.
Steffan, State Geologist, USDA-SCS and Michael Rogers, Geologist Trainee,
USDA-SCS."
- "Purpose of Investigation
- - The purpose of this site investigation was to determine the
feasibility of constructing a large earth dam on Leon Whitney's ranch in
Glenn County, California (Figure 1). The dam would provide storage for
irrigation water. The rock structure in the foundation and emergency
spillway, the amount of suitable earth material for construction and the
amount of runoff from storm events and annual precipitation were all
investigated.
- Conclusions
- - 1. Foundation materials at the proposed centerline of the dam and
emergency spillway are adequate.
- - 2. Adequate borrow material is not readily available within a 1/4
mile of the site. Only 20,000 cubic yards of material suitable for core
fill were located in this investigation. An additional 40,000 cubic
yards of core material and 169,000 cubic yards of shell material need to
be located.
- - 3. Average annual runoff to the site ranges from 573 to 986 acre-feet
depending on the method of calculation. A 2-year, 24-hour rainfall event
generates from 355 to 430 acre-feet of runoff. It appears that about 500
acre-feet of runoff per year may be expected to occur at the site as a
minimum (less in dry years and more in wet years). With about 100
acre-feet of evaporation each year, only about 400 acre-feet of water would
be available for irrigation each year.
- - 4. Peak flow from a 50-year, 24-hour storm is estimated to be about
947 to 1260 cfs.
- - 5. A major Coast Range thrust fault is mapped by CDMG just upstream
of the dam centerline.
- - 6. About one acre feet [sic] of sediment per year is expected to
accumulate in the reservoir.
- - 7. Seepage losses from the pool area should not be significant.
- Recommendations
- - 1. The proposed dam should not be built until adequate fill materials
for the core and shell are located and are available for use.
- - 2. The dam could be reduced in size since the runoff each year will
only be about 500 acre-feet.
- - 3. Although no recent movement has been detected on the fault at the
site, its nearness to the embankment fill indicates special features should
be designed into the dam to prevent failure if movement occurs. The design
and constuction of the special features will significantly increase the
costs of this dams [sic]. [to p. 2]
- - 4. Extra fill should be placed over the channel area to counteract
differential settlement between the bedrock area in the abutments.
- - 5. Landowner should be aware that some erosion in the emergency
spillway may occur after it functions and the erosion should be repaired
to prevent spillway failure."
- Site Description
- - The site is located on Leon Whitney's ranch in Glenn County,
California at the eastern edge of the Coast Range mountains, in the" SE 1/4
SE 1/4 NW 1/4 S7 T22N R6W. "The watershed for the dam is in the" NE
corner of T22N R7W. "Leon Whitney had the dam area surveyed and staked
along the proposed dam centerline, spillway and pool areas. [para] The
drainage area for the site is about 2240 acres of moderate to steeply
rolling hills. The slopes range from 20 to 60 percent. Elevation ranges
between 3377 feet at the watershed divide to 880 feet in the channel at
the dam site. Average annual rainfall is 21.18 inches [where?]. This
value is based on 56 years of data obtained from the Stonyford Ranger
Station [which is sheltered by Snow Mountain & St. John Mountain?] and
published in the Climatological Data Summary of California - 1984; v.88,
No. 13. [para] Vegetation in the lower half of the watershed consists of
rangeland (predominantly annual grasses with some perennials), and
woodland-grass with oaks and digger pines. The upper reaches consist
mainly of woodland and native brush. The ground cover was assumed to be
in fair condition in the lower reaches and fair to poor in the upper
reaches. Vegetation is sparse at the top of hills where bedrock is often
exposed. [from historic overgrazing? para] The soils are generally
poorly developed. The parent material is the underlying bedrock of
shales, sandstones, and conglomerates. Soil textures consist largely of
silty clays, clayey silts, and gravelly clays and silts. Depths to
bedrock typically range from 6 to 20 inches in the dam area, although some
may be up to 60 inches in depth in very localized colluvial areas. Most
soils are of low plasticity to non-plastic.
- Site Geology
- - Bedrock is exposed primarily along streambanks and at the tops of
hills. The rocks in this area consist of interbedded, slowly to nonpermeable
sandstones and conglomerates of the Mesozoic Knoxville and Franciscan
formations (Figure 2). A large north-south trending thrust fault divides
the Knoxville to the east and Franciscan to the west. The fault is marked by
ultrabasic intrusive igneous rock [!] also of Mesozoic age that consist
largely of serpentine and peridotite. California Division of Mines and
Geology maps show the igneous intrusion to be very nearly pinched out in
the dam site area. Field investigation along tributaries leading to the
dam area and beyond failed to produce evidence of the intrusion or the
fault [uh, the entire scarp?].
According to California Division of Mines and Geology map 1975,
the fault is condidered [sic] to be pre-Quaternary (older than 2 million
years), with no historic record of displacement. However, the fault
cannot be considered 'dead'. some displacement may still occur along the
original fault plane. [to p. 3] The bedding of the Knoxville formation at
the center line of the dam dips very nearly verically to the east. The dip
of the bedding is perpendicular to the direction of flow in the creek at the
site. Some widely spaced jointing (5 feet) occurs in the massive
conglomerates and sandstones. The joints trend in an east-north easterly
direction and dip 47 degrees to the west. The joints are not open however,
so seepage should be minimal. [para] The stream channels and tributaries
around the site appear to be neither aggrading nor degrading. The channel
bottom sediment consists of cobbles and boulder-sized metasedimentary
fragments of schists and phyllites that have been eroded from older
alluvium forming the banks of the channels. [para] Based on the Stoney
[sic] Creek Preauthorization Report (SCS, 1984), sediment yeild to
the site will be about 1 acre-foot per year. Without any major fires in
the watershed, sedimentation in the pool should not be a serious problem
[but major fires are a serious & too-frequent problem?]. About 60 acre-feet
of storage should be assigned to sediment storage. Thsi volume will be
unavailable for water storage after about 50 years of operations of the
pool.
- Site Hydrology
- - Annual runoff to the dam site was estimated by three different
methods in order to determine whether enough water would be provided to
the site. [para] The first estimate is based on the average annual
precipitation, effective precipitation and water lost to deep percolation.
The Stonyford Range Station has recorded an average annual rainfall of 21.18
inches, based on 56 years of rainfall data. A calculated estimate of the
effective rainfall (water that is used by vegetation) is 8 inches per
year. Assuming another 8 inches is lost to deep percolation of ground
water, this leaves 5.28 inches or 986 acre-feet of annual runoff to the
reservoir. [para] The second estimate is taken from the SCS Engineering
Field Manual, Chapter 2, Exhibit 2 through 8 (estimation of annual water
yields from ungaged watersheds of 10 to 2000 acres). This graph uses the
average annual temperature and precipitation, the type of vegetation,
and the size of the watershed to determine the annual runoff. This
method results in 2.0 inches or 573 acre-feet of annual ronoff [sic]. [para]
The third method is a hydrologists 'rough estimate' or 'rule of thumb'
calculation of runoff using only the annual precipitation data (annual
precipitation squared and divided by 100). This estimate yields 4.48
inches or 837 acre-feet per year of runoff. [para] Surface evaporation
of the pond area must also be considered. Assuming there is 17.5 acres
of pond surface area when there is 600 acre-feet of water (survey estimate),
about 128 acre-feet of water could be lost annually due to pond evaporation.
this estimate is based on annual evaporation data taken from the Solano
lake weather station, Solano County (88 inches per year). [4 miles west
of Winters? a bit less hot than Whitney's] [para] Actual runoff volumes
from storm events were also calculated using the procedure provided in
Chapter 2, SCS Engineering Field Manual. The average of the high
(50-year storm) and low (2 and 10-year storms) volume estimates is about
[to p. 4] 230 acre-feet per year. Although this method is not an accurate
estimate of average annual runoff in the watershed, it indicates that dry
years will have significant impact on the amount of water being supplied
to the site. [para] The following peak flow rates and volumes were
determined:
------------Storm Frequency------------
2-year 10-year 50-year
Peak Q (cfs):
Fair Condition* 412 605 947
Poor Condition 485 874 1,260
Volume (acre-feet):
Fair Condition 355 504 766
Poor Condition 430 598 840
* Condition refers to the degree of vegetative ground cover and canopy
cover intercepting rainfall
- Proposed Structure
- - Proposed dam would be a zoned, earth fill structure with a maximum
height of 115 feet. Most of the fill would not exceed 65 feet in height
because the centerline is at a deep, narrow canyon cut by the creek in
hard, sandstone and conglomerate beds [sic, punctuation]. The dam crest
would be about 600 feet long and the total fill would average 300 feet
from toe to toe (maximum width of base is about 550 feet. Estimated total
fill required would be about 229,000 cubic yards. Out of this, about
61,000 cubic years of plastic fines are required for the core and the
remaining 168,000 cubic yards of shell material could be less plastic
and contain more coarse material. [para] Spillway crest would be at 990
feet elevation in the north abutment (left abutment when looking downstream).
Spillway would be cut into shale and sandstone bedrock. The cut could be
about 800 feet long and the alignment would be along the contours of a
ridge trending north-south. The outlet would be into a north-south trending
tributary which empties into the main drainage just below the site. [para]
Principal spillway would be a pipe that would outlet about 1200 feet
down-stream. This pipe would be the source of irrigation water to
gravity-irrigate pasture land between the steep slopes of the Coast Range
and the Whitney Ranch shop. [para] Capacity at the emergency spillway
crest would be 1309 acre-feet. This would represent a surface area of
32 acres." [to p. 5]
- Site investigation
- - On Tuesday morning June 3, Leon Whitney, ranch owner and Bill
Pellersells from the Willows SCS office accompanied the geologist to the
site. Mr. Whitney pointed out the dam and emergency spillway centerlines
and the approximate water line in the pool at 990 feet elevation. The owner
felt he could find suitable fill material for the dam in the emergency
spillway cut and along the small drainages entering the pool area. He
saw no problems with constructing the fill in the confines of the narrow
canyon. He also felt that runoff at this site was above normal due to the
poor vegetative cover, steep slopes, and the impermeable nature of the
bedrock. Mr. Whitney has been a contractor for almost 30 years and has
built dams on his property east of the Coast Ranges and on other ranches
in the area. [para] Tuesday afternoon was spent walking the site and
drilling exploratory hand auger holes to check for suitable fill material.
Sandstone and conglomerate rock at the foundation area is competent,
nonpermeable to slowly permeable and high strength. Soil thicknesses were
20 inches over small areas. In many areas weathered bedrock is at the
surface. The emergency spillway centerline, the tributary to be used as an
outlet for the emergency spillway, and the pool area were all examined for
suitable fill material. Soil thicknesses were 20 inches over small areas
but in most areas, weathered bedrock is at the surface. [para] Salt
precipitates were seen on shale outcrops in the creek banks where ground
water seeps and springs were encountered. It appears the ground water
in the vicinity may be high in salt content so the surface runoff could
also have higher than normal salt content. [para] The owner had a D-7
caterpillar tractor on the site Wednesday morning. Ten test pits were
dug along the emergency spillway centerline, both abutments and the
pool area (Figure 1). These pits were supplemented with some additional
hand auger test holes. Logs of the test pits and pertinent auger holes are
attached. [para] Based on the site reconnaisance [sic], test pits and
auger holes, potential borrow areas were outlined on the soils map
(Figure 3). The areas and depths of suitable material for the core of
the dam are shown in the following table. Total cubic yards of suitable
fill are 20,960. About 61,000 cubic yards are needed to construct the
core of the proposed dam.
-------------------------Areas of Potential Borrow---------------------
Depth of Cubic Yards
Number On Area in Suitable Soils of Potential
Figure 3 Square Feet in Feet Borrow Material
1 74,050 1.33 3,660
2 43,560 2.13 3,440
3 30,490 2.40 2,710
4 46,170 2.40 4,100
5 2,610 2.13 210
6 10,450 2.13 820
7 43,560 2.13 3,440
8 8,710 4.00 1,290
9 8,710 4.00 1,290
-------
20,960
=======
The HgxB soil (Hillgate Loam, gullied) 0.05 [sic] miles south of the site may
have up to 54 inches of CL, ML or CH material at the surface. This soil was
not tested in the field due to its distance from the site and because it is
outside Mr. Whitney's property line. If half this soil was actually suitable
for fill, about 63 acres of open rangeland would need to be stripped to get
229,000 cubic yards of fill.
/s/ Lyle G. Steffen, State Geologist, USDA-SCS
/s/ Michael Rogers, Geologist Trainee, USDA-SCS