nhc technical report - city of pasadena
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Appendix B-1 Intake/Diversion Design (Area 2) Draft Basis of Design
Report, March 2020
ARROYO SECO CANYON PROJECT – INTAKE/DIVERSION DESIGN
(AREA 2)
DRAFT
BASIS OF DESIGN REPORT
Prepared for:
Kennedy/Jenks Consultants Pasadena, CA
On behalf of:
Pasadena Water and Power Pasadena, CA
Prepared by:
Northwest Hydraulic Consultants Inc. Pasadena, CA
2 Mar 2020
NHC Ref. No. 6004878
Prepared by:
Northwest Hydraulic Consultants Inc. Edward E. Wallace, P.E. Gwyn Perry, P.E. Travis Shinkle
Reviewed by:
Paul Chau, P.E. – Kennedy/Jenks Consultants
Barry Chilibeck, P. Eng. – Norhwest Hydraulic Consultants
DISCLAIMER
This document has been prepared by Northwest Hydraulic Consultants Inc. in accordance with generally
accepted engineering practices and is intended for the exclusive use and benefit of Kennedy/Jenks
Consultants and Pasadena Water and Power and their authorized representatives for specific application
to the Arroyo Seco Canyon Project in Pasadena, CA. The contents of this document are not to be relied
upon or used, in whole or in part, by or for the benefit of others without specific written authorization
from Northwest Hydraulic Consultants Inc. No other warranty, expressed or implied, is made.
Northwest Hydraulic Consultants Inc. and its officers, directors, employees, and agents assume no
responsibility for the reliance upon this document or any of its contents by any parties other than
Kennedy/Jenks Consultants and Pasadena Water and Power.
DRAFT Basis of Design Report II Arroyo Seco Canyon Project – Intake/Diversion Design
TABLE OF CONTENTS
1 BACKGROUND AND PURPOSE ............................................................................................................... 1 1.1 PWP Diversion Operation ................................................................................................................ 1 1.2 Environmental Process .................................................................................................................... 1 1.3 Purpose of this Basis of Design Report ............................................................................................ 2
2 HYDROLOGIC SETTING .......................................................................................................................... 2 2.1 Hydrology of Arroyo Seco ................................................................................................................ 2
2.1.1 Flow Frequency and Duration Characteristics............................................................................ 2 2.1.2 Storm Event Characteristics ....................................................................................................... 5
2.2 Geomorphic Characteristics ............................................................................................................ 8 2.3 Fish and Aquatic Habitat ................................................................................................................ 12
2.3.1 Historical Fish Habitat and Presence ........................................................................................ 12 2.3.2 Present Conditions ................................................................................................................... 13 2.3.3 Future Conditions ..................................................................................................................... 14
3 DESIGN OBJECTIVES AND CRITERIA ..................................................................................................... 14 3.1 Diversion Flows .............................................................................................................................. 14
3.1.1 Instream Flow ........................................................................................................................... 15 3.2 Operation of the Diversion Weir ................................................................................................... 15 3.3 Stream Habitat ............................................................................................................................... 15 3.4 Fish Protection ............................................................................................................................... 16 3.5 Aquatic Organism Passage............................................................................................................. 16 3.6 Flood and Erosion Protection ........................................................................................................ 17
4 BASIS OF DESIGN ................................................................................................................................. 18 4.1 Project Elements ............................................................................................................................ 18 4.2 Diversion Weir ............................................................................................................................... 18
4.2.1 Structure and Gate Type .......................................................................................................... 18 4.2.2 Operation and Controls ............................................................................................................ 19 4.2.3 Effects of Flood Flows ............................................................................................................... 19
4.3 Roughened Channel ...................................................................................................................... 25 4.4 Intake Components ....................................................................................................................... 31 4.5 Gates and Controls ........................................................................................................................ 36 4.6 Road Stabilization .......................................................................................................................... 36 4.7 Intake Service Building .................................................................................................................. 37 4.8 Potential Future Components ....................................................................................................... 37
4.8.1 Optional Bypass Pipe ................................................................................................................ 37 4.8.2 Instream Flow ........................................................................................................................... 37 4.8.3 Optional Fishway ...................................................................................................................... 38
5 CONSTRUCTION CONSIDERATIONS ..................................................................................................... 38 5.1 Construction Timing ...................................................................................................................... 38 5.2 Elements of Construction .............................................................................................................. 38
5.2.1 Instream Construction .............................................................................................................. 38 5.2.2 Grading ..................................................................................................................................... 39 5.2.3 Structures ................................................................................................................................. 39
DRAFT Basis of Design Report III Arroyo Seco Canyon Project – Intake/Diversion Design
5.2.4 Piping ........................................................................................................................................ 39 5.2.5 Electrical and Controls .............................................................................................................. 39 5.2.6 Revegetation ............................................................................................................................ 39
5.3 Temporary Construction Effects .................................................................................................... 40 5.3.1 Access and Public Safety........................................................................................................... 40 5.3.2 Control of Water and Water Quality Protection ...................................................................... 40 5.3.3 Material Import and Export ...................................................................................................... 40 5.3.4 Dust Control and Noise............................................................................................................. 40
REFERENCES ................................................................................................................................................ 42
APPENDIX A - Fisheries Review Letter, Dr. Camm Swift
APPENDIX C - Preliminary (30%) Plans
APPENDIX B - Hydraulic Information
DRAFT Basis of Design Report IV Arroyo Seco Canyon Project – Intake/Diversion Design
LIST OF TABLES
Table 2.1 Flow frequencies at Arroyo Seco diversion based on annual peaks at USGS Gage 11098000
for period 1914 to 2015 (from Deere and Ault, 2017) ............................................................ 3
Table 3.1 Potential fish passage design flows ....................................................................................... 17
Table 4.1 Depth and velocity criteria from CDFW and NMFS road crossing guidelines ....................... 26
Table 4.2 Riprap Sizing for Road Stabilization ...................................................................................... 37
LIST OF FIGURES
Figure 2.1 Annual flow duration curve for Arroyo Seco at USGS Gage 11098000 (from Deere and Ault,
2017) ....................................................................................................................................... 3
Figure 2.2 February flow duration curve for Arroyo Seco at USGS Gage 11098000 (from Deere and
Ault, 2017) ............................................................................................................................... 4
Figure 2.3 September flow duration for Arroyo Seco at USGS Gage 11098000 (from Deere and Ault,
2017) ....................................................................................................................................... 5
Figure 2.4 Storm hydrographs for 2010 (top) and 2005 (bottom) events ............................................... 7
Figure 2.5 Arroyo Seco streambed and small bedrock drop between Millard Creek and diversion ....... 9
Figure 2.6 Arroyo Seco streambed and small boulder drop downstream of first bridge upstream of
Millard Creek ........................................................................................................................... 9
Figure 2.7 Profile of Arroyo Seco near PWP diversion, 2015 topography ............................................. 10
Figure 2.8 Stream slope in immediate vicinity of diversion, 2015 topography ..................................... 10
Figure 2.9 Barrier approximately 500 feet downstream of diversion ................................................... 11
Figure 2.10 Diversion dam and sluice notch looking upstream ............................................................... 12
Figure 4.1 2-year flood extent and peak velocities, existing conditions ................................................ 20
Figure 4.2 2-year flood extent and peak velocities, proposed conditions ............................................. 21
Figure 4.3 10-year flood extent and peak velocities, existing conditions .............................................. 22
Figure 4.4 10-year flood extent and peak velocities, proposed conditions ........................................... 23
Figure 4.5 100-year flood extent and peak velocities, existing conditions ............................................ 24
Figure 4.6 100-year flood extent and peak velocities, proposed conditions ......................................... 25
Figure 4.7 Typical Roughened Channel Section ..................................................................................... 26
Figure 4.8 Hydraulic model proposed roughened channel cross section showing flow depth and
velocity distribution at high design flow for juvenile fish passage (61.3 cfs) ....................... 28
Figure 4.9 Hydraulic model proposed roughened channel cross section showing flow depth and
velocity distribution at high design flow for adult anadromous salmonid fish passage (306.5
cfs) ......................................................................................................................................... 29
Figure 4.10 Hydraulic model proposed roughened channel cross section showing flow depth and
velocity distribution at low design flow for juvenile fish passage (1 cfs) .............................. 30
Figure 4.11 Hydraulic model proposed roughened channel cross section showing flow depth and
velocity distribution at low design flow for adult anadromous salmonid fish passage (3 cfs)
............................................................................................................................................... 31
DRAFT Basis of Design Report V Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.12 Screen bay configuration options (not to scale) ...................................................................... 33
Figure 4.13 Intake Hydraulic Profile ......................................................................................................... 35
DRAFT Basis of Design Report 1 Arroyo Seco Canyon Project – Intake/Diversion Design
1 BACKGROUND AND PURPOSE
The City of Pasadena Department of Water and Power (PWP) is conducting a project for improvement of
the diversion and groundwater recharge system in Arroyo Seco Canyon. The Arroyo Seco Canyon Project
includes work in three distinct areas for removal of a non-operational headworks structure, modification
of the storm-damaged diversion structure, and improvements to the groundwater recharge system. A
design for the project was completed in 2017 (Carollo Engineers, 2017). The California Environmental
Quality Act (CEQA) Mitigated Negative Declaration (MND) was challenged, and PWP initiated an
environmental compliance process for preparation of an Environmental Impact Report (EIR), which is
now in progress.
A California Department of Fish and Wildlife (CDFW) Streambed Alteration Agreement was drafted
during the project development and review process (CDFW, 2017a). CDFW also provided additional
comments (CDFW, 2017b) from engineering review of the project regarding HEC-RAS hydraulic
modeling, planted riprap revetment, grouted riprap, fish screens, fish passage, flows below the dam, and
the geotechnical report.
PWP retained Kennedy/Jenks and subcontractor Northwest Hydraulic Consultants (NHC) to develop
conceptual designs to address the topics raised in the draft agreement and CDFW comments and to
develop modifications to the design of the diversion in Area 2 of the project. NHC was assisted in the
fisheries review by Dr. Camm Smith, a leading authority on native fish in the Southern California region.
The modified design must also consider maintaining the yield of the diversion to the extent feasible for
recharge of groundwater resources, operational safety and reliability, and achievement of other
environmental objectives.
1.1 PWP Diversion Operation
PWP currently diverts most of the flow from Arroyo Seco upstream of the diversion weir, when stream
flows are below their full water right of 25 cfs. PWP plans to divert 25 cfs up to a higher threshold of
streamflow before lowering an operable weir and allowing more turbid flow to pass downstream.
Diversion during flood flows is not anticipated due to high turbidity levels and risk to diversion facilities.
A primary benefit of the modified diversion will be to allow diversion during higher flows, at least during
periods when turbidity and suspended sediment concentrations are considered acceptably low.
1.2 Environmental Process
PWP has completed an Initial Study for the project (Dudek, 2019) and conducted a public scoping
meeting on 21 November 2019. Environmental analysis to evaluate potential impacts identified in the
Initial Study, including assessment of biological resources in the area of the diversion, is in progress. The
design of the diversion is being coordinated with the environmental evaluation to identify and minimize
potential impacts.
DRAFT Basis of Design Report 2 Arroyo Seco Canyon Project – Intake/Diversion Design
1.3 Purpose of this Basis of Design Report
The purpose of this report is to present the design criteria and basis of design for the proposed modified
diversion and intake system. A key objective of this report is to provide detailed information that can be
reviewed with CDFW and other stakeholders to confirm the specific design direction for the project.
2 HYDROLOGIC SETTING
2.1 Hydrology of Arroyo Seco
Arroyo Seco is a tributary to the Los Angeles River that drains approximately 18 square miles of the
Angeles National Forest in the San Gabriel Mountains at the site of the diversion. The diversion is
located in the Arroyo Seco canyon approximately one-half mile upstream of the canyon mouth and the
head of the alluvial fan now occupied by the reservoir area behind Devil’s Gate Dam. The dam was
constructed in 1920 and is owned and operated by Los Angeles County Flood Control District. A bridge
crosses the stream at the Jet Propulsion Laboratory (JPL) east gate and is approximately located at the
transition between confinement of the stream in the canyon and the unconfined alluvial fan and
reservoir deposits downstream. Three other bridges are located between the JPL bridge and the
diversion intake, and the Millard Creek tributary joins Arroyo Seco approximately 600 feet upstream of
the JPL bridge.
2.1.1 Flow Frequency and Duration Characteristics
Deere and Ault (2017) used the USGS stream gage 11098000 on Arroyo Seco to assess hydrologic
characteristics of the stream, adjusting the gage values by about 10 percent to account for additional
watershed area between the gage and the diversion, which is located about one mile downstream of the
gage. Estimated flow frequencies are shown in Table 2.1.
The flood of record at the gage occurred in the 1938 Water Year with a peak flow of approximately 8,600
cubic feet per second (cfs). A peak of approximately 8,500 cfs occurred in the 1969 Water Year. Since
that time, no flood larger than 5,400 cfs has occurred. The most recent large flood events occurred in
January 2005 (3,540 cfs), February 2010 (4,620 cfs) and Dec 2011 (2,260 cfs). The 2010 event followed
the 2009 Station Fire that burned 160,000 acres, including much of the upper Arroyo Seco watershed.
The 2010 event carried high sediment and debris loads, removed much of the riparian vegetation along
the stream corridor, and damaged the diversion and access road. Flood flows and debris in the 2010
event overtopped the diversion intake vault and access road.
DRAFT Basis of Design Report 3 Arroyo Seco Canyon Project – Intake/Diversion Design
Table 2.1 Flow frequencies at Arroyo Seco diversion based on annual peaks at USGS Gage 11098000
for period 1914 to 2015 (from Deere and Ault, 2017)
Recurrence Interval, years
Peak Flow, cfs
2 613
5 1,878
10 3,204
50 7,543
100 9,962
Flow duration characteristics were also compiled by Deere and Ault from mean daily flow gage records
for the full annual record and for specific months. The annual flow duration curve is shown in Figure 2.1.
Figure 2.1 Annual flow duration curve for Arroyo Seco at USGS Gage 11098000 (from Deere and Ault,
2017)
DRAFT Basis of Design Report 4 Arroyo Seco Canyon Project – Intake/Diversion Design
On an annual basis, flows below 1 cfs occur about 35 percent of the time and flows above 25 cfs occur
only about 7 percent of the time. The flow duration analysis indicated that the wettest months are
typically February (Figure 2.2) and March, with flows above 1 cfs about 95 percent of the time and flows
above 10 cfs about 35 percent of the time. The lowest flows occur in summer and fall months and the
gage analysis indicates that zero flow occurs at the gage 24 percent of the time in August and 26 percent
of the time in September (Figure 2.3). Flows less than 1 cfs occur 70 and 75 percent of the time in
August and September, respectively. The gage record shows significant diurnal variation in summer
months, presumably due to evapotranspiration by stream vegetation.
Figure 2.2 February flow duration curve for Arroyo Seco at USGS Gage 11098000 (from Deere and
Ault, 2017)
DRAFT Basis of Design Report 5 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 2.3 September flow duration for Arroyo Seco at USGS Gage 11098000 (from Deere and Ault,
2017)
Adjustments to gage values for watershed area differences are reasonable for flow frequency estimates
but the accuracy of area-based adjustments such as the one applied in the Deere and Ault study may be
less accurate for flow duration analysis because of large variations in lower flows that may occur with
geology, stream substrate, vegetation, and other factors. On 17 August 2019 NHC made rough
estimates of flow at the diversion of about 1 cfs and the gage recorded about 0.8 cfs on the same day,
indicating that for at least one observation, little gain or loss was evident between the gage and the
diversion at low flows. The median mean daily flow for this date according to the USGS gaging statistics
is 0.4 cfs.
Under existing conditions, PWP diverts all flow in the creek at the diversion during periods of low flow,
except for an amount that leaks or seeps past the dam. Millard Creek contributes flow downstream, but
likely does not contribute perennial flow. On 17 August 2019, about 0.4 cfs of flow was estimated below
Millard Creek, with about one third coming from Millard Creek and two thirds from Arroyo Seco. This
flow estimate is very rough due to the difficulty in estimating shallow flow rates over a rough streambed
and is only a single observation that may not be representative of typical conditions.
Flow frequency and flow duration information are relevant to fish passage design and instream flow
objectives, as described in subsequent sections.
2.1.2 Storm Event Characteristics
The Arroyo Seco watershed generates relatively high unit runoff rates due to its geology, hypsography,
and climatic setting. The computed 100-year recurrence peak flow corresponds to a unit flow rate of
DRAFT Basis of Design Report 6 Arroyo Seco Canyon Project – Intake/Diversion Design
about 550 cfs per square mile. Although high runoff may be generated by convective storms, the largest
peaks in the record are generally associated with multi-day frontal storms and atmospheric river events
from the Pacific Ocean. The steepness of the watershed and the potential for intense rainfall rates
associated with some frontal storms produces very rapid increases in flow. Figure 2.4 shows the
hydrographs associated with the 2010 and 2005 annual peak events from USGS Gage 11098000.
Differences in hydrograph shapes are due to antecedent conditions and precipitation patterns, but in
both cases, flows increase and decrease rapidly in response to precipitation. In the 2010 event, flows
increased from about 5 cfs on 5 February to the peak of 4,620 cfs only 24 hours later on 6 February and
receded to about 30 cfs another 24 hours later on 7 February. Rapid changes in stage and flow can be
expected to produce large sediment and debris loads and adjustment of channel beds and banks.
The most productive periods for diversion are associated with prolonged wet periods that are associated
with light or moderate rainfall, such that flows remain above 5 to 10 cfs for prolonged periods of time
and turbidity levels are not excessive.
DRAFT Basis of Design Report 7 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 2.4 Storm hydrographs for 2010 (top) and 2005 (bottom) events
DRAFT Basis of Design Report 8 Arroyo Seco Canyon Project – Intake/Diversion Design
2.2 Geomorphic Characteristics
The streambed in the project area is generally comprised of coarse sediments ranging from sand to
boulders. During an NHC site reconnaissance, bedrock exposure was noted in several locations between
JPL bridge and the diversion in the banks and bed of the channel. Active streambed widths vary
between about 20 and 40 feet. The channel varies between step-pool and plane bed morphology, with
step-pools associated with steeper reaches and very coarse (cobble to boulder) bed material and
bedrock exposures. These two channel morphologies are consistent with the overall slope of the
channel of about 2.5 to 3 percent.
The watershed was severely burned in the 2009 Station Fire and was subject to elevated sediment
debris and sediment loads in the years following the fire. As noted above, peak flows of approximately
4,600 cfs and 2,300 cfs were recorded at the gage in Water Years 2010 and 2011, respectively. At the
present time, there is very little fine-grained material (silt or clay) in the present bed material in the
reach between the JPL bridge and the diversion.
The channel between JPL and the diversion is densely vegetated, with willow dominant in some
segments of the banks and alder dominant in others. There are numerous areas where alders of the
same age class occur in stands along the bank, potentially indicating cycles of scouring and regrowth.
The latest cycle may be associated with post-Station Fire flows. Aerial photography from March 2011
shows a scoured channel and vegetation mostly removed in a swath 60 to 100 feet wide between the
headworks and JPL. Given the magnitude of the scour and deposition areas in 2011, the recovery of
riparian vegetation along this reach of channel in less than 10 years is remarkable. Figure 2.5 and Figure
2.6 show typical sections of stream bed in this reach in August 2019. Vegetation characteristics
upstream of the diversion to the headworks in Area 1 of the project area are similar. Riparian vegetation
likely plays a role in channel stability during some flood flows, but due to the steepness of the channel
and high sediment and debris loads, may be partially or nearly entirely lost in the largest events.
The diversion dam forms a small high point in the bed to facilitate flow into the intake. Figure 2.7 shows
a plot of the stream profile taken from aerial topography developed by PWP in 2015. Figure 2.8 shows a
similar plot in the immediate vicinity of the diversion. The stream slope is flattened upstream of the
diversion but maintains a continuous positive slope of about 1.5 percent.
DRAFT Basis of Design Report 9 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 2.5 Arroyo Seco streambed and small bedrock drop between Millard Creek and diversion
Figure 2.6 Arroyo Seco streambed and small boulder drop downstream of first bridge upstream of
Millard Creek
DRAFT Basis of Design Report 10 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 2.7 Profile of Arroyo Seco near PWP diversion, 2015 topography
Figure 2.8 Stream slope in immediate vicinity of diversion, 2015 topography
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DRAFT Basis of Design Report 11 Arroyo Seco Canyon Project – Intake/Diversion Design
Numerous barriers to aquatic organism passage are present in Arroyo Seco and Los Angeles River
downstream of the project area. In the vicinity of the project, Devil’s Gate Dam is a complete barrier
and the reservoir stream segment above the dam is subject to deposition, channel shifting, and frequent
dry conditions due to loss of flow into the alluvium as the channel emerges from the canyon. Upstream
of the JPL bridge, partial temporal barriers may be present during low flows due to steep channel
segments and step-pool or bedrock drops. An anthropogenic barrier exists at the road crossing
approximately 500 feet downstream of the diversion at a channel spanning weir that apparently was the
site of a historical stream gage. A drop of approximately three feet occurs at this concrete structure,
shown in Figure 2.9.
At the diversion, a drop of approximately 2 feet occurs from the base of the flashboard notch to the
water surface in the scour pool downstream. The top of the diversion dam is approximately 2.5 feet
higher than the notch. A photo of the diversion dam and sluice notch is shown in Figure 2.10.
Figure 2.9 Barrier approximately 500 feet downstream of diversion
DRAFT Basis of Design Report 12 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 2.10 Diversion dam and sluice notch looking upstream
2.3 Fish and Aquatic Habitat
NHC retained Camm C. Swift, PhD, an expert in southern California native fish, to provide input on
fisheries conditions in the project reach. The discussion below primarily summarizes information from a
review letter provided by Dr. Smith, which is also included as Appendix A (Swift, 2019).
2.3.1 Historical Fish Habitat and Presence
The Arroyo Seco historically provided habitat for seven species of native freshwater fish, namely Pacific
brook lamprey, Lampetra spp.; Pacific lamprey, Entosphenus tridentata; rainbow trout/steelhead,
Oncorhynchus mykiss; arroyo chub, Gila orcutti; Santa Ana speckled dace, Rhinichthys cf osculus; Santa
Ana sucker, Catostomus santaanae;and unarmored threespine stickleback, Gasterosteus aculeatus
williamsoni. Judging by their occurrences elsewhere in the Los Angeles and Santa Ana rivers, only the
dace and trout would have been common in Arroyo Seco from JPL upstream and the others would have
primarily been in the lower reaches in habitats with lower gradients.
None except rainbow trout have been recorded as present in Arroyo Seco upstream of Devil’s Gate Dam
for at least 50 years and probably much longer due to major modifications to the watercourse
downstream (Swift, et al, 1993). The rainbow trout is essentially the freshwater resident population of
the species; if and when some go to sea to mature and return to the natal stream, they become
steelhead. Rainbow Trout were present in upper Arroyo Seco for a long time and the stream was
DRAFT Basis of Design Report 13 Arroyo Seco Canyon Project – Intake/Diversion Design
accessible to steelhead at some time in the past. A population had maintained itself there more or less,
possibly augmented by hatchery fish planted intermittently over time, but several surveys since the 2009
Station Fire by CDFW and NOAA biologists have not detected any fish.
No non-native species are known above Devil’s Gate Dam except for possibly the descendants of
introduced trout. Historical records exist of brown trout being placed in upper Arroyo Seco but there
are no recorded observations of brown trout.
2.3.2 Present Conditions
Given the historical presence and successful reproduction of rainbow trout in the watershed upstream
of the diversion, the Arroyo Seco watershed is assumed to include habitat in at least some locations that
is potentially suitable for future establishment/recurrence of rainbow trout. Vegetation and undercut
boulders or rock provide cover, and groundwater upwelling and extensive canopy keeps the water cool
enough for them. Even in the dry season a few large pools may provide refuge to support a small
population.
In the reach downstream of the diversion, recent observations indicate a few deeper pools with
protection of undercut boulders and/or woody debris. However, many reaches are very shallow and
exposed and the bulk of suitable habitat is likely upstream of the diversion, as it has been for a long
time. The habitat currently present downstream of the diversion could support a small number of trout
but clearly only a few tens of small fish, if that. In addition, this habitat is only suitable if pools are
maintained during the summer months and it is not clear from recent observations (in a relatively wet
water year) whether this is typically the case in late summer and fall. In addition, limited pool areas
receive significant recreational use for swimming/bathing, and this disturbance would be adverse for fish
in the limited areas available.
The flow duration curve in Figure 2.3 indicates that flows at the gage upstream are less than 1 cfs almost
80 percent of the time in September, and the gage records indicate that zero flow occurs a large fraction
of the time in August and September. Any trout downstream of the diversion would presently be unable
to migrate upstream during drying conditions due to long areas of shallow flow and the barrier at the
bridge downstream of the diversion. Migration downstream would be impossible due to dry conditions
at the head of the reservoir. No fish have been observed in this reach in recent biological surveys.
Very high flows may wash fish downstream to the reservoir where they will likely perish under current
conditions and remove riparian vegetation, as noted in recent observations of single age class stands of
riparian trees. In addition, sediment delivery associated with mass wasting processes or wildfire may
bury suitable substrates and vegetation, eliminate cover, and change turbidity and temperatures in the
stream. The observations of fish in the Arroyo Seco watershed before the Station Fire and no
observations almost a decade later are indicative of the significance of these types of events to fish
populations.
Although suitable habitat may be present for reestablishment of rainbow trout in the upper watershed,
anadromy is currently prevented for steelhead trout by multiple significant barriers downstream,
DRAFT Basis of Design Report 14 Arroyo Seco Canyon Project – Intake/Diversion Design
including the Los Angeles River flood control channel and Devil’s Gate Dam. Access to upstream
spawning and rearing habitat may also be limited by Brown Mountain Dam, located about 3 miles
upstream of the diversion.
2.3.3 Future Conditions
The Southern California Steelhead Recovery Plan (NOAA, 2012) describes priority actions for recovery of
steelhead in the region, including the Los Angeles River basin. The Los Angeles River is listed as a Core 3
population, indicating lower priority than some other regional streams with Core 1 and Core 2
populations. The Recovery Plan lists removal of barriers at dams, diversions, roads, and other structures
as a priority action in the Los Angeles River biogeographic region. Steelhead recovery in Arroyo Seco
would involve modification of multiple flood control facilities and removal of multiple barriers between
the project area and the ocean, but efforts are underway to investigate the potential of restoring
anadromous access in the Los Angeles River. For example, the Council for Watershed Health is currently
working under a grant from the Wildlife Conservation Board to investigate and design passage and
habitat features in a portion of the Los Angeles River. Access to tributaries and headwater areas for
spawning and rearing would be a vital part of any recovery strategy for steelhead in the watershed.
Speckled dace or arroyo chub might also be considered for re-establishment on lower gradient portions
of the stream but are considered unlikely to survive in the mountainous conditions of the upper Arroyo
Seco.
3 DESIGN OBJECTIVES AND CRITERIA
3.1 Diversion Flows
The proposed surface diversion is intended to provide a reliable source of surface water diversion
throughout the year and over a range of flows. Diversion during flood flows is not anticipated due to
high turbidity levels and risk to diversion facilities. PWP currently diverts most of the flow when flows
are below their full water right of 25 cfs and is planning to divert 25 cfs up to a higher threshold before
shutting down the diversion and allowing more turbid flow to pass downstream. Potential thresholds
considered have included 50 cfs, 100 cfs and higher values. For the purposes of this report, a threshold
of 100 cfs is referenced, but this may be adjusted in the future based on a better understanding of the
relationship between turbidity and stream flow, and refinement of the fish protection and sediment
management design approaches. A primary benefit of the modified diversion will be to allow diversion
during higher flows, at least during periods when turbidity and suspended sediment concentrations are
considered acceptably low. The previous design for the diversion (Deere and Ault, 2017) utilized an
operable weir in a modified diversion structure, and this basic configuration was retained for the current
design effort.
DRAFT Basis of Design Report 15 Arroyo Seco Canyon Project – Intake/Diversion Design
3.1.1 Instream Flow
CDFW draft permit conditions and comments on the previous design refer to the need to maintain
instream flows downstream of the diversion, but do not provide specific targets. PWP conducted a
study to address CDFW concerns and concluded that the diversion does not impact habitat in the
downstream reach such as to require a release of surface water for instream flow, as long as the flow of
groundwater remains unobstructed (Psomas, 2018). The present condition of seepage past the dam was
intended to be maintained by constructing a modified structure at similar depth. No instream flow
release criteria have been used in the preliminary design, but the diversion design includes gates and
piping that would allow for release of surface water for instream flow in the future, if such requirements
are developed. Instream flow releases would not, however, prevent flows from becoming very low or
zero when natural inflows are low. Based on the flow duration analysis, this occurs during most years in
the summer and fall.
3.2 Operation of the Diversion Weir
The proposed design for the diversion uses an operable weir to pond water at the intake during flows
below an operating threshold. The proposed diversion weir will be operated to divert up to 25 cfs
through the intake and pass all remaining flow downstream. The weir will be fully raised during low
flows and will modulate during stream flows between 25 cfs and 100 cfs to maintain 25 cfs through the
intake. Once the high flow threshold of 100 cfs stream flow is reached, the weir will be fully lowered.
Operating criteria will need to be established for lowering the weir at a controlled rate to avoid rapid
changes in upstream water surface that might result in stranding or other adverse conditions for aquatic
organisms, and rapid increases in flow downstream that exceed the naturally high rates due to storm
events.
The operable weir concept has advantages for managing sediment and minimizing effects on the
channel morphology. Flow in the channel is relatively shallow at the diversion flow rates and a small
range of streamside operating water levels is desirable to manage sediment, minimize drop to the
downstream channel, and minimize the size of the operable weir required. It is desirable to set the open
weir crest elevation slightly below the intake invert to allow for some sediment accumulation upstream
of the dam without entraining it in the diversion and screens. During periods of high flows, lowering the
weir would allow removal of accumulated sediment by erosion and transport downstream, to restore
the streambed elevation to the crest of the notch. An operational plan is needed to turn the diversion
out and flush accumulated sediment, if needed prior to stream flows reaching the threshold at which the
crest gate would be lowered. The crest gate can be divided into sections to facilitate level control and
sediment removal. An objective of the design is to provide operational flexibility with the crest gates to
avoid the need to excavate sediment out of the stream bed in the vicinity of the intake.
3.3 Stream Habitat
In NHC’s August 2019 site observations in the reach downstream of the diversion, flows on the order of
0.5 cfs were maintaining isolated pool habitats and relatively cool water. However, this is based on a
DRAFT Basis of Design Report 16 Arroyo Seco Canyon Project – Intake/Diversion Design
single site observation and may not be characteristic of all years. The physical habitat associated with
the channel bed and banks includes a mixture of coarse sediments and riparian vegetation that could
support aquatic species, including fish. Limiting factors for fish include low or zero flows in the late
summer and fall, associated loss of connectivity and isolation of pools, and numerous bedrock and
boulder drops that would prevent mobility during low flows.
Upstream of the diversion, physical habitat conditions appear to be similar and dry weather flows are
higher, although still subject to low or zero flows in late summer.
3.4 Fish Protection
Fish are typically protected from entrainment in diversion flows using a screening system. CDFW (2002)
and NMFS (1997) have published screen design criteria for anadromous salmonids. The criteria
established by the two agencies are similar, and include requirements for screen size opening, screen
approach velocity, and sweeping velocity. CDFW guidance expresses a preference for on-channel
screens but provides criteria for off-channel screens as well. Numerous screen types and configurations
have been used for exclusion of fish including end of pipe screens, cone screens, cylinder screens, drum
screens, vertical screens, and horizontal screens.
A key criterion for screen sizing is approach velocity, the velocity vector component perpendicular to the
screen, as this determines the screen area required. CDFW and NMFS criteria for salmonid fry set design
approach velocity at 0.33 fps for on-channel screens and 0.4 fps for off-channel screens. Much lower
values apply if the screens are not self-cleaning (passive). Sweeping velocity, the velocity vector
component parallel to the screen, is typically designed to be greater than the approach velocity.
Although off-channel screens are considered less desirable in the CDFW guidance, potential advantages
include lower potential for damage during floods and better control over depth and velocity distribution
on the screens.
Based on the review of fisheries conditions in the project area, the screens will be designed for approach
velocities consistent with protection of salmonid fry as described in the CDFW and NMFS guidance.
3.5 Aquatic Organism Passage
The existing diversion structure presents a barrier to passage due to the drop height, limited flow width
through the flashboard slots, and high velocities during high flows.
Passage of fish downstream during diversion operations would result in fish being transported to the
downstream reach and reservoir area in low flows. Under current conditions, fish may perish in this
reach due to isolation or stranding in low or zero flow periods. The preliminary design focuses on
excluding fish from the diversion and downstream reach while considering potential for future
downstream passage during diversions if connectivity is re-established for passage through Devil’s Gate
Dam and downstream channels to the ocean.
DRAFT Basis of Design Report 17 Arroyo Seco Canyon Project – Intake/Diversion Design
The diversion weir will be lowered during periods of higher flows, and some passage of fish downstream
may occur during these periods. The preliminary design will include features to allow upstream passage
when the weir is lowered, and diversions are not occurring.
CDFW (2002) and NMFS (2019) standards for adult salmonid high fish passage design flows are typically
the 1 percent exceedance flow or 50 percent of the 2-year flow. Juvenile salmonid high fish passage
design flow is typically the 10 percent exceedance flow or 10 percent of the 2-year flow. Table 3.1 shows
potential fish passage design flows based on these criteria and analysis of the stream flow records for
Arroyo Seco. It should be noted that low design flows provided by the criteria may not be passable in
adjacent stream reaches due to the natural bed morphology. Additional analysis may be necessary to
establish appropriate low fish passage design flows based on hydraulic characteristics of critical riffles or
drops in adjacent reaches, but preliminary designs were evaluated based on the standard criteria listed.
Table 3.1 Potential fish passage design flows
High Design Flows % 2-Year Q, cfs
Adult Anadromous Salmonid 50 306.5
Juvenile Salmonid 10 61.3
Low Design Flows Q, cfs
Adult Anadromous Salmonid 3
Juvenile Salmonid 1
As noted above, the design considers the need for potential downstream passage at the diversion if safe
passage to the ocean is re-established downstream. The design should also consider the potential need
for future upstream passage at the facility during diversions, if connectivity from the Pacific Ocean is re-
established and steelhead are able to ascend the stream.
3.6 Flood and Erosion Protection
The design should allow flood flows to pass the intake, while minimizing potential for substantial
damage to the facilities, and without significant adverse impact on the roadway or other infrastructure
compared to existing conditions.
Very high velocities occur during flood flows and native bed materials include large rock, boulders, and
bedrock. Some use of rock is needed in the design to stabilize the bed and bank against future flood
events. Assuming an ample source of native rock, natural materials can be used to design fish and
wildlife friendly biotechnical bed and bank stabilization that matches natural conditions in the stream as
much as possible and minimizes the use of structural concrete and grouted rock. However, because of
the high velocities during flood flows, some use of these materials is anticipated.
DRAFT Basis of Design Report 18 Arroyo Seco Canyon Project – Intake/Diversion Design
Where feasible, vegetation should be incorporated into the design to reduce velocities and boundary
shear stresses, especially in areas adjacent to the main channel such as the left overbank downstream of
the diversion. The design will be based on hydraulic model output, geomorphic assessment of potential
profile and lateral adjustments, computation of stability for various material types, and incorporation of
scour protection.
4 BASIS OF DESIGN
4.1 Project Elements
All project elements are shown on the preliminary (30%) plans included in Appendix B and are described
in the following sections. It should be noted that final design plans will include a survey map showing
current topography, existing structures and trees within the project area. Project elements include the
following: diversion weir, downstream roughened channel, diversion intake and fish screens, gates and
controls, the intake service building, and potential future elements including instream flow release, fish
bypass and fishway.
4.2 Diversion Weir
4.2.1 Structure and Gate Type
The proposed diversion control structure spans the entire width of the existing channel in the same
general location as the existing structure. A 30-foot long operable weir crest gate is located in a notch
section of the structure. The diversion weir may be designed to operate using cable, hydraulic, or
inflatable operating mechanisms. The design is investigating the potential for the weir to be split into
two crest gate sections, 10 feet and 20 feet in length, so that the shorter section can be used for normal
operations. The invert of the gated notch section is set at an elevation of 1177.8 feet, approximately 4
feet lower than the existing diversion weir, and the crest gates will raise to a maximum elevation of
approximately 1183.0 feet. The structure section is 7.5 feet wide in the stream longitudinal direction
and has a top elevation of 1183 feet.
The lowered crest elevation of the structure is based on review of stream profile and existing diversion
information. The lowered crest is intended to reduce the structure’s effects on hydraulics and sediment
transport in the stream when the gate is lowered, while allowing diversion to the existing pipeline that
conveys flows to the recharge facilities. The notch width in the diversion structure was set by estimating
the active bed width of the stream upstream and downstream of the diversion and by evaluation of two-
dimensional (2D) hydraulic modeling results to attempt to provide a velocity distribution similar to
adjacent stream reaches.
Additional geotechnical investigation is required to support the design of the new diversion structure,
including the foundation for the structure, effect of the structure on seepage flows, foundation for the
right abutment, and the tie-in to the existing valley slope to prevent flanking.
DRAFT Basis of Design Report 19 Arroyo Seco Canyon Project – Intake/Diversion Design
4.2.2 Operation and Controls
Operation of the crest gates will be controlled by water depth measurements from a transducer located
immediately upstream of the diversion. The transducer will communicate with a programmable logic
controller (PLC) housed in the Intake Service Building. The PLC will control the modulation of the
diversion weir gates to accurately deliver flow through the intake.
The diversion weirs will remain fully raised until the total streamflow reaches 25 cfs at an upstream
water surface elevation of 1183.45 feet. The diversion weirs can then be modulated to maintain this
water surface elevation, and the full 25 cfs diversion, up to a total streamflow of 100 cfs, at which point
the weirs will be fully lowered and the intake will be isolated from the stream.
A diversion gate on the downstream side of the screens has the capability for automatic and remote
operation such that the diversion flows can be fully shut down when the crest gates lower. Full isolation
of the intake would be accomplished by manual installation of bulkhead flashboards upstream of the
screens.
4.2.3 Effects of Flood Flows
A preliminary evaluation of hydraulic characteristics of Arroyo Seco in the project vicinity under existing
and proposed conditions was completed using the HEC-RAS computational package developed by the
Hydrologic Engineering Center of the U.S. Army Corps of Engineers (USACE). A 2D model of existing
conditions was developed using 2016 LiDAR topography, and was modified for proposed conditions. For
purposes of preliminary comparison, the Manning’s roughness value for both models was set to 0.06 for
all cells in the 2D flow area, representing the composite roughness of the floodplain vegetation and
channel bed material. In the next phase of design, 2D modeling will be refined to include additional land
surface categories with varying roughness values.
Peak 2-year, 10-year and 100-year peak velocities through the project vicinity for existing and proposed
conditions are shown in Figures 4.1 through 4.6 (flow direction is from top to bottom). These results
indicate that under proposed conditions, when the gates are fully down and the crest elevation is
approximately 4 feet lower than the existing weir elevation, flood water surface elevations (indicated by
extent of inundation in the figures) are at or below existing conditions and maximum velocities are
similar to existing condition. The figures show flood flows greater than about the 10-year flow spilling
out of the channel under existing conditions, and 100-year flows spilling out of the channel in both
existing and proposed conditions. These results will be refined as the design progresses but are
generally consistent with observed behavior in the 2010 event.
DRAFT Basis of Design Report 20 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.1 2-year flood extent and peak velocities, existing conditions
VELOCITY
SCALE, feet per
second
EXISTING DIVERSION STRUCTURE
DRAFT Basis of Design Report 21 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.2 2-year flood extent and peak velocities, proposed conditions
DIVERSION STRUCTURE
VELOCITY SCALE, feet
per second
DRAFT Basis of Design Report 22 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.3 10-year flood extent and peak velocities, existing conditions
FLOW OUTSIDE
CHANNEL
VELOCITY SCALE, feet
per second
DRAFT Basis of Design Report 23 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.4 10-year flood extent and peak velocities, proposed conditions
VELOCITY SCALE, feet
per second
DIVERSION STRUCTURE
DRAFT Basis of Design Report 24 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.5 100-year flood extent and peak velocities, existing conditions
FLOW OUTSIDE
CHANNEL
DRAFT Basis of Design Report 25 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.6 100-year flood extent and peak velocities, proposed conditions
4.3 Roughened Channel
The existing diversion structure creates a drop in stream profile and is a barrier to aquatic organism
passage under most flows. In the proposed design, a roughened channel section downstream of the
diversion structure is proposed to allow upstream passage when the weir crest gates are lowered.
Roughened channels are an accepted method of providing passage for steelhead trout in CDFW’s
Salmonid Stream Restoration Manual (CDFW, 2010), requiring site specific design and review. The
roughened channel configuration proposed is a channel approximately the width of the active channel in
adjacent stream segments, with an asymmetric v-shaped cross section to concentrate lower flows and
provide adequate depth. Compared to other potential types of upstream passage features, the
roughened channel type has the advantage of providing a range of velocities and depths at a given flow
rate. Thus, smaller or weaker swimming native fish and other aquatic organisms can pass along the
margins of the channel or near the bottom, using spaces in the bed material for hydraulic cover.
The roughened channel design follows the depth and velocity criteria established by CDFW (2002) and
NMFS (2019) guidance for fish passage at road crossings, shown in Table 4.1. These criteria are typically
FLOW OUTSIDE
CHANNEL
VELOCITY SCALE, feet
per second
DRAFT Basis of Design Report 26 Arroyo Seco Canyon Project – Intake/Diversion Design
assumed relevant for design at features other than road crossings, but roughened channel designs are
considered on a case-by-case basis for specific sites.
Table 4.1 Depth and velocity criteria from CDFW and NMFS road crossing guidelines
Species Min Depth, ft Max Velocity, fps
(<60 ft passage
length)
Max Hydraulic
Drop, ft
Notes
Adult Anadromous
Salmonid
1.0 6 1
Juvenile Salmonid 0.5 1
1 (2 short distance)
0.5
1
CDFW
NMFS
The proposed roughened channel profile slope downstream of the diversion weir is 4%. This is about
twice the average bed slope and thus would be reasonably similar to a natural steep section or chute in
the adjacent reaches of channel. The proposed typical channel section is an asymmetrical v-section with
dimensions as shown in Figure 4.7.
Figure 4.7 Typical Roughened Channel Section
Potential passage conditions were assessed using HEC-RAS over a range of flows based on channel cross
sections constructed of Engineered Streambed Material (ESM). The size of a stable ESM was estimated
using the CDFW guidance (2009) for roughened channels. The CDFW equation is based on the USACE
(EM 1110-2-1601 Hydraulic Design of Flood Control Channels, 1994) equation for steep slopes, but is
modified to produce a well-graded mixture of rock, gravel and sand similar to natural streambeds. One
of the key parameters in the CDFW equation is slope. Using the bed slope (4%), the D84 rock size to
resist motion in the 100-year flood event was estimated at approximately 6 feet, and the D50 at 2 to 3
feet. These sizes are probably impractical or at least very costly for construction, and extremely large
0
1
2
3
4
5
6
7
8
0 10 20 30 40 50 60 70 80 90
Hei
ght
abo
ve in
vert
, fee
t
Station, ft
10
1
2
1
1FT AT 2H:1V
MIN. 5FT
DRAFT Basis of Design Report 27 Arroyo Seco Canyon Project – Intake/Diversion Design
rock will potentially make fish passage difficult at lower flows. However, at the 100-yr flow, the slope of
the energy grade line is approximately 2.6%, which is closer to the overall slope of Arroyo Seco. Using
the energy grade slope, the D84 rock size was estimated at approximately 4.7 feet, and the D50 at
approximately 2 feet. These sizes are more practical for use in construction and were adopted for the
preliminary design. It should be noted that some potential for damage during extreme events will
remain, and maintenance of the roughened channel may be required after large events to repair
damage and maintain conditions for fish passage. Details for the ESM sizing calculations, gradations and
assumptions can be found in Appendix C.
A one-dimensional (1D) HEC-RAS model was used to simulate the high and low design flows for juvenile
and adult anadromous salmonid fish passage for cross sections representing the roughened channel.
For this assessment, depth-dependent roughness values were estimated using the Mussetter equation,
following methods provided in the CDFW (2009) guidance.
Figures 4.8 and 4.9 show the depth of flow and velocity distributions from the 1D model in a typical
channel section representing the roughened channel under high design flow conditions for juvenile
salmonids and adult anadromous salmonids. Maximum channel velocities under proposed conditions
are approximately 4 fps for the juvenile high design flow and 8 fps for the adult passage flow. The
velocity plots indicate the potential importance of channel margins for passage at higher flows, where
lower velocity zones meeting the passage design criteria are maintained.
Figures 4.10 and 4.11 show the depth of flow and velocity under minimum low design flow conditions
for juvenile salmonids and adult anadromous salmonids. The 1D model does not fully represent the
complex shape of the low flow portion of the channel formed by intersecting rock surfaces, but the
roughness estimate is intended to approximate the effect of an irregular flow path. Results indicate that
depth and velocity conditions consistent with design criteria could be met.
At flows above 25 cfs, there is some potential that fish would pass over the weir, and a cushion pool
downstream of the weir at the head of the roughened channel would be desirable to prevent injury.
DRAFT Basis of Design Report 28 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.8 Hydraulic model proposed roughened channel cross section showing flow depth and
velocity distribution at high design flow for juvenile fish passage (61.3 cfs)
DRAFT Basis of Design Report 29 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.9 Hydraulic model proposed roughened channel cross section showing flow depth and
velocity distribution at high design flow for adult anadromous salmonid fish passage
(306.5 cfs)
DRAFT Basis of Design Report 30 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.10 Hydraulic model proposed roughened channel cross section showing flow depth and
velocity distribution at low design flow for juvenile fish passage (1 cfs)
DRAFT Basis of Design Report 31 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.11 Hydraulic model proposed roughened channel cross section showing flow depth and
velocity distribution at low design flow for adult anadromous salmonid fish passage (3 cfs)
The crest gate sill elevation was selected to more closely represent the general profile of the stream
without the influence of the existing diversion. This will promote sediment transport continuity through
the diversion reach during larger flows when bed material is mobilized and when the crest gate will be
down. Grading upstream of the structure during construction will be limited to the immediate area of
the diversion, and some evolution of the stream profile can be expected after construction. Bed material
that has aggraded upstream of the structure is expected to be transported downstream, establishing a
more uniform profile upstream of the diversion.
4.4 Intake Components
The proposed diversion intake is located on the left bank of the stream immediately upstream of the
diversion weir. The intake is equipped with a trash rack mounted parallel to the stream flow and a
bulkhead, allowing the diversion to be shut off during flood flows. Two configurations for the intake and
screen bay were developed. The first configuration places the screens along the left bank of the stream
immediately behind the trash rack. The second configuration uses an intake angled upstream and a
tapered off-channel screen bay. Figure 4.12 shows the conceptual layout for the two options. After
review with the City, the tapered screen bay option was selected for use in the preliminary plans for its
ability to flush sediments through the bay, protection of the screens, and potential adaptability in future
DRAFT Basis of Design Report 32 Arroyo Seco Canyon Project – Intake/Diversion Design
conditions for passage of fish downstream. Potential disadvantages are that any fish that enter the
screen bay under present conditions would be further from the stream and might not find their way
back to the channel as easily. The tapered bay would ultimately allow a sweeping velocity to meet
design criteria when used with a downstream fish bypass, but prior to that time the sweeping velocity
would decrease through the bay. Review with CDFW is anticipated prior to advancing the design.
DRAFT Basis of Design Report 33 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.12 Screen bay configuration options (not to scale)
DRAFT Basis of Design Report 34 Arroyo Seco Canyon Project – Intake/Diversion Design
The intake shown in the drawings in Appendix B is an angled approach channel to a tapered fish screen
bay. The tapered bay ends at a 12-inch diameter slide gate and flushing pipe for removing sediment and
small debris build-up in front of the screens. Potential for future fish bypass at this location is described
in Section 4.6.
The screening system will be designed to meet current NMFS and CDFW design guidelines for approach
velocity. For a 4-foot diameter drum screen and a maximum approach velocity of 0.4 fps at the
maximum diversion flow of 25 cfs, the minimum screen length required is approximately 20 feet. For
ease of fabrication, installation and maintenance, two 10-foot-long drum screens are proposed.
Vertical screens and drum screens were considered for use at the site, and the preliminary design is laid
out for use of drum screens. Drum screens are frequently used in small irrigation diversions and consist
of a cylinder of screen material that rotates in the flow. The rotating action is used to clean small debris
from the screen. The drum screen was initially conceived based on the configuration with the screens
immediately behind the trash rack but was retained for use in the tapered screen bay. Additional review
with the screen manufacturers and CDFW will influence the final selection of screen type.
Under current conditions, no flow would normally be bypassed from the upstream side of the screen bay
and sweeping velocity will decrease along the screens. In this configuration, sweeping velocity is not
relevant to downstream passage, as fish would be excluded from the downstream reach. At the 25 cfs
design flow, sweeping velocity is approximately 1 fps at the bay inlet, and approximately 0.4 fps after
traveling three-quarters of the length of the screens. See Appendix C for sweeping velocities along the
length of the screen across a range of design flows without a bypass flow.
A second 12-inch diameter slide-gated cleanout pipe is located at the back of the fish screens to remove
any sediment build-up behind the screens. This pipe could also be used to convey any future instream
flows determined to be needed, as described in Section 4.8.
The final component of the intake structure is the outlet weir, located behind the fish screens. The
drums screens must be designed to operate with submergence of 65 to 85 percent of diameter, and
tailwater control is needed for this purpose. The design uses an 8-foot-long outlet weir that would be
fitted with an adjustable weir plate set at an elevation of approximately 1182.5 feet. In conjunction
with the diversion weir, this height maintains a minimum depth on the screens of 2.5 feet, or
approximately 70% of the total diameter. Flow over the weir spills to a small vault and a 30-inch
diameter slide-gated outlet pipe that conveys the diverted flow to existing Tunnel #4 and the spreading
basins downstream.
The hydraulic profile of the system is shown in Figure 4.13 below. This figure displays the calculated
water surface elevation profile over a range of design flows.
DRAFT Basis of Design Report 35 Arroyo Seco Canyon Project – Intake/Diversion Design
Figure 4.13 Intake Hydraulic Profile
DRAFT Basis of Design Report 36 Arroyo Seco Canyon Project – Intake/Diversion Design
4.5 Gates and Controls
The gate on the 30-inch diameter outlet pipe will be controlled by the PLC and automated based on
water surface level readings immediately upstream of the diversion. The gate will close once stream flow
reaches the 100 cfs threshold. High flow events will be forecasted and flow into the intake structure will
be shut off pre-emptively and manually, using the bulkhead behind the trash rack, to prevent damage
and large amounts of sediment and debris from entering the structure. The two 12-inch diameter
cleanout pipes will be manually operated for the removal of sediment build-up, during routine
maintenance of the intake.
4.6 Road Stabilization
The methods described in USACE Engineer Manual 1110-2-1601 were followed to develop the gradation
of rock slope protection for the road stabilization on the left overbank (USACE, 1994). This rock slope
protection will protect the roadway and shoulder and will be separated from the roughened channel by
a regraded and replanted area. To the extent feasible, this overbank area would be stabilized by
vegetative or biotechnical methods, but additional hydraulic analysis will be required to establish design
parameters in subsequent design phases.
For the preliminary design of the road stabilization, velocity, depth, and channel curve radius to width
ratio were developed from the preliminary 2D modeling results and applied in the USACE rock sizing
equation for D30. A previous geotechnical report (Converse Consultants, 2013) recommended use of a
Cast-In-Drilled-Hole (CIDH) pile-supported retaining wall along the west side of the road and included
general recommendations for rock slope protection. However, this protection was associated with
grading that created a slope set back from the stream adjacent to the road. The proposed design would
fill the area between the roughened channel to restore vegetated floodplain. This area is proposed to
be stabilized by biotechnical methods, with additional rock slope protection along the road, with a toe
down into the fill to protect against scour. The geotechnical report included a boring near the southern
end of this area that encountered granitic bedrock at less than 5 feet of depth, but site observations
indicate that the road is located on a greater depth of alluvial material for most of its length. Additional
subsurface exploration may be needed to define the limits of shallow bedrock.
2D modeling for 100-year flow conditions show a depth of approximately 3 feet and a velocity of
approximate 9 fps on the left bank. The road stabilization is relatively straight, however the overall
channel curves at a radius to width ratio of 4/1, which will direct flows toward the left bank. Using the
USACE equation and these input parameters, the D30 rock size to resist motion in the 100-year flood
event was estimated at approximately 1.85 feet. See Appendix C for additional details on the sizing
calculations. This sizing falls within the range for the USACE typical 42” riprap gradation, shown in Table
4.2 below. Although this rock could be vegetated, the slope protection is very close to the road and
vegetation might reduce roadway clearance. Instead of joint planting the rock, dense planting or
biotechnical stabilization is recommended along the toe and transverse to flow between the rock slope
protection and the roughened channel.
DRAFT Basis of Design Report 37 Arroyo Seco Canyon Project – Intake/Diversion Design
Although the preliminary modeling indicates depths and velocities that can be addressed using
vegetated floodplain and rock slope protection along the road, more detailed 2D modeling will be
performed as the design progresses to represent the floodplain area and consider potential risks of
erosion during extreme events. Additional measures such as subsurface groins or a return to the
retaining wall concept might be considered if the risks are considered excessive.
Table 4.2 Riprap Sizing for Road Stabilization
Rock Size (ft)
D30 min 1.70
D50 min 2.47
D90 min 2.04
D100 min 2.57
D100 max 3.46
4.7 Intake Service Building
The Intake Service building will house electrical and control equipment and potentially a compressor for
operation of the crest gates. The structure could potentially be a prefabricated metal building, concrete
masonry unit structure, or wood frame structure. Concrete masonry construction may offer the most
protection from vandalism. The need for elevation above the roadway grade or floodproofing will be
evaluated in subsequent phases of design.
4.8 Potential Future Components
4.8.1 Optional Bypass Pipe
Downstream fish passage is not considered beneficial in low flows under current conditions. However,
the intake design can accommodate an optional 24” diameter slide-gated pipe from the screen bay to
bypass fish downstream of the diversion while the diversion is in operation, if safe passage is re-
established downstream to the ocean. If a bypass is required in the future, the design would conform to
CDFW and NMFS criteria.
4.8.2 Instream Flow
PWP has determined that instream flow releases beyond seepage and leakage at the diversion are not
needed to maintain stream habitat downstream. If instream flows are determined beneficial in the
future, the proposed design can accommodate release of surface water for instream flows from the
downstream side of the screens.
DRAFT Basis of Design Report 38 Arroyo Seco Canyon Project – Intake/Diversion Design
4.8.3 Optional Fishway
Under existing conditions, no fish have been observed in the downstream reach and anadromous
passage from the ocean is not possible due to various infrastructure barriers. However, the proposed
design can accommodate a future bypass fishway to allow for upstream passage while the diversion is
operating, if access for steelhead should be re-established. Preliminary hydraulic calculations for the
sizing of a bypass roughened channel pool and chute fishway with a maximum flow of 10 cfs was used to
size and develop a preliminary layout for a potential future fishway. Adequate length is provided if the
fishway begins immediately upstream of the intake structure and runs to the east, around the outside of
the structure, then along the left bank and down the hillslope to connect with the stream at the end of
the proposed roughened channel section. Other types of fishways may be considered in the future, but
layout of this fishway type provides a conservative check on space requirements. The fishway is not
proposed to be designed at this time.
5 CONSTRUCTION CONSIDERATIONS
5.1 Construction Timing
The project will require instream work and will benefit from construction during the lowest flows
possible, which would be in late summer and early fall. Some work is outside the limits of the channel
and could be performed earlier in the year if other environmental protections are in place in accordance
with project permits and mitigation measures. A key schedule constraint may be surveys and
monitoring for nesting birds. To the extent feasible, vegetation removal outside the nesting bird season
may reduce the risk of delays. Except for potential vegetation removal phasing, the project is expected
to be constructed in a single construction season. Some mechanical components, such as fish screens
and crest gates may have long lead times for preparation of shop drawings, submittals, and fabrication
which should be considered in construction scheduling. Coordination between construction activities in
Areas 1 and 3 may also influence construction timing and scheduling and could provide some
construction and cost efficiencies in shared or phased use of equipment, materials, and staging areas.
5.2 Elements of Construction
5.2.1 Instream Construction
A temporary stream bypass system, via gravity or pump, will need to be implemented to isolate the
project reach from streamflow prior to and throughout construction. If the instream portion of the work
is conducted in late summer, typical flows would be feasible to pump around the excavation with a
portable pump. Monitoring of weather forecasts and provision for a larger gravity bypass may be
needed to address the potential for higher flows that could occur due to rain events as part of a rain
event action plan. Dewatering will likely be required for excavation and installation of the diversion
structure. Water quality protection measures will be needed for control of turbidity in the dewatering
flows and release back to the stream or adjacent infiltration area.
DRAFT Basis of Design Report 39 Arroyo Seco Canyon Project – Intake/Diversion Design
5.2.2 Grading
Grading for the roughened channel design and roadway/hillslope protection is estimated at
approximately 2000 CY of cut and 2010 CY of fill. A portion of the cut may be suitable for reuse in the
ESM or as other fill on the site. However, because the ESM requires a specific gradation, some or most of
this material may be required to be imported. For the purpose of preliminary estimation of haul
quantities, approximately 20 percent of the excavated material was assumed to be reused as fill,
resulting in a preliminary estimate for import material of 1,600 cubic yards and export of about the same
amount. A portion of the export may be used for other areas of the project.
5.2.3 Structures
The intake and diversion structures will be constructed of reinforced concrete. Structural design will be
performed in subsequent design steps and wall and footing thicknesses and configurations shown in the
drawings are approximate. The intake structure will be designed for access from the ground level
adjacent to the roadway, similar to the existing intake vault, and the top is anticipated to be partly solid
and partly constructed as removable grating or access cover.
A geotechnical investigation will be required to support design of the diversion structure and intake. A
previous geotechnical assessment (Converse Consultants, 2013) addressed stabilization of the roadway
at the site but did not include exploration or recommendations for the diversion structure and intake.
5.2.4 Piping
Installation of new piping from the intake vault to the conveyance line is anticipated. The new piping
would extend from the outlet vault downstream of the screen bay to an existing vault at the head of the
tunnel section of conveyance piping on the east side of the road. Additional piping will be required for
flushing pipes from the screen bay and for electrical and control lines. The deepest trenching will likely
be associated with the outlet pipe, which is estimated to be about ten feet deep based on the profile in
Figure 4.13.
5.2.5 Electrical and Controls
Electrical power will be required to operate the crest gates and outlet slide gate. The other small gates
and bulkhead are expected to be operated manually. Overhead power supply presently exists to the
location of the former traveling screen site and would need to be extended to the diversion site.
Automatic control is anticipated for the crest gates and remote control for the crest gates and pipeline
outlet gate will be investigated during subsequent phases of design. Methods for providing site security
and protection of mechanical and control equipment will also be investigated.
5.2.6 Revegetation
Revegetation for the project would include riparian planting on the left and right banks along the
roughened channel and right bank key for the diversion structure. The left overbank area is vulnerable to
DRAFT Basis of Design Report 40 Arroyo Seco Canyon Project – Intake/Diversion Design
erosion during large floods and more intensive biotechnical stabilization may be needed in this area.
The preliminary design would restore this area as high vegetated floodplain between the roughened
channel and the roadway stabilization slope protection. Revegetation species will be selected based on
native species present in the project vicinity, including willow, alder, and sycamore. Willow species may
be emphasized in areas with high erosion potential to take advantage of high roughness, low stature,
and extensive root systems for reduction of flood velocities and increased stability of bank and
floodplain surfaces. Revegetation plans would also consider trees and species that are required to be
removed for construction to mitigate these losses.
5.3 Temporary Construction Effects
5.3.1 Access and Public Safety
Construction access to and from the project site will be exclusively via the paved roadway of Gabrieleno
Trail, a route used heavily by hikers, equestrians and bikers year-round. Due to the limited width of the
existing trail and the intake area, temporary closures and use restrictions may be required during
construction. However, the roadway is not anticipated to be fully closed during the entire construction
period, and daily access to the USFS residential area should be feasible to maintain. The roadway/trail
can be open during times when construction is not active, such as weekends, and when construction
activities do not require equipment to be positioned in the roadway. Construction activities would be
isolated with construction fencing, and temporary traffic control may be required for some activities. An
access plan and project schedule will be required as part of construction contract administration to
minimize closure and restrictions on use of the roadway.
5.3.2 Control of Water and Water Quality Protection
As noted above, a temporary stream bypass and dewatering facilities with water quality protection for
discharges back to the stream will be required. In addition, the project will require development and
implementation of a Storm Water Pollution Prevention Plan (SWPPP) for construction Best Management
Practices (BMPs) for erosion control and water quality protection.
5.3.3 Material Import and Export
Hauling of import and export materials is anticipated to be in conventional road legal dump trucks with
loads not to exceed standard highway limits. Approximately 1600 yards each of import and export is
currently estimated, which may require approximately 200 truck trips. This estimate will be refined in
subsequent phases of design. Stockpiling area is very limited at the site and the opportunity for back
hauling to reduce truck trips and cost may be limited.
5.3.4 Dust Control and Noise
Sweeping and watering will need to be used for dust control during construction. Construction activities
should be limited to normal working hours on weekdays, unless otherwise approved by PWP, and noise
levels from construction equipment would be limited to 85 dBA at 100 feet as provided in the City
DRAFT Basis of Design Report 41 Arroyo Seco Canyon Project – Intake/Diversion Design
Municipal Code. If a pumped stream bypass is used, night operation of a generator and pumps may be
needed, and a sound enclosure should be considered for this application in the subsequent phases of
design.
5.3.5 Operations and Maintenance
A detailed operations and maintenance plan will be developed during the detailed design phase to
address normal operation of the crest gate, fish screens, sediment management, mechanical
components, control system, security, structural integrity, and other elements of the project. The
operations plan will include procedures for shutting down the diversion during high flows and for post-
flood inspections and repairs.
DRAFT Basis of Design Report 42 Arroyo Seco Canyon Project – Intake/Diversion Design
REFERENCES
Carollo Engineers, 2017. 100% Design Submittal for Arroyo Seco Canyon Project. Prepared by Carollo
Engineers and Deere and Ault Consultants for Pasadena Water & Power. April 2017.
CDFW, 2002. Culvert Criteria for Fish Passage. Tech. report, California Department of Fish and Game.
CDFW, 2009. Fish Passage Design and Implementation. Part XII in Salmonid Stream Habitat Restoration Manual 3rd edition. California Department of Fish and Game.
CDFW, 2010. California Salmonid Stream Habitat Restoration Manual 4th edition. California Department of Fish and Game.
CDFW, 2017a. Draft Streambed Alteration Agreement. Notification Number 1600-2016-088-R5. May
2016.
CDFW, 2017b. Email from Victoria Tang, CDFW to Gary Tanaka, PWP regarding Arroyo Seco Canyon
project supplemental info response.
Converse Consultants, 2013. Geotechnical Feasibility Report: Proposed Public Restroom, Roadway
Improvement and Stormwater Sediment Basin Project Arroyo Seco Canyon. Prepared for Carollo
Engineers, Inc. August 2013.
Deere and Ault, 2017. Final Basis of Design Report, Arroyo Seco Intake. Prepared by Deere and Ault Consultants for City of Pasadena and Carollo Engineers.
Dudek, 2019. Arroyo Seco Canyon Project Areas 2 and 3 Initial Study. Prepared by Dudek for City of
Pasadena Department of Water and Power.
NMFS, 1997. Fish Screening Criteria for Anadromous Salmonids. National Marine Fisheries Service
Southwest Region. January 1997.
NMFS, 2019. Guidelines for Salmonid Passage at Stream Crossings. For Applications in California at
Engineered Stream Crossings to Facilitate Passage of Anadromous Salmonids. Original Issue 2001.
Amended September 2019.
NOAA, 2012. Southern California Steelhead Recovery Plan. National Oceanic and Atmospheric
Administration. Southwest Regional Office National Marine Fisheries Service. January 2012.
Psomas, 2018. Arroyo Seco Canyon Project Diversion: Hydraulics, Sediment Transport, and Groundwater
Analysis. Prepared by Psomas for City of Pasadena Water and Power.
Swift, C. C., T. R. Haglund, M. Ruiz, and R. N. Fisher. 1993. The status and distribution of the freshwater
fishes of southern California. 92(3):101-167.
DRAFT Basis of Design Report 43 Arroyo Seco Canyon Project – Intake/Diversion Design
Swift, C. C., 2019. Fish and fish passage related to Pasadena Water and Power Department diversion on
Arroyo Seco above Pasadena. Prepared for Northwest Hydraulics Consultants and City of Pasadena
Department of Water and Power.
USACE, 1994. Hydraulic Design of Flood Control Channels. EM 1110-2-1601.
DRAFT Basis of Design Report Arroyo Seco Canyon Project – Intake/Diversion Design
APPENDIX A
FISHERIES REVIEW LETTER, DR. CAMM SWIFT
DRAFT Basis of Design Report Arroyo Seco Canyon Project – Intake/Diversion Design
1
October 29, 2019
To: Ed Wallace
Northwest Hydraulics Consultants
200 S Los Robles Avenue, Suite 405
Pasadena, CA 91101
From: Camm C. Swift, PhD, 6465 Elmo Road, Cumming, Georgia 30028
Subject: Fish and fish passage related to Pasadena Water and Power Department diversion on Arroyo Seco above Pasadena.
Pasadena Water and Power Department (PWP) has a water diversion on Arroyo Seco that requires upgrading and repair. Part of this work involves consideration of possible protection of, and accommodation for, potential native fishes of the Arroyo Seco, tributary to the Los Angeles River. The following assessment/analysis is based on extensive knowledge of the original fish fauna of the area as well as review of the recent plans for upgrading this diversion, comments from the California Department of Fish and Wildlife (CDFW), and communications with National Marine Fisheries Service (NMFS) and CDFW biologists specifically working with the fishes in the area.
The section of Arroyo Seco from the narrowing at the Jet Propulsion Laboratory (JPL) bridge upstream, including the PWP diversion, is a relatively small upland extent of the entire Arroyo Seco, a major tributary of the Los Angeles River. The whole Los Angeles River system is identified as one of the core populations of steelhead in the Mojave Rim Biogeographic Population Group identified in the NMFS Southern California Steelhead Recovery Plan. As such, the Los Angeles River system is considered essential for recovery of the Southern California Distinct Population Segment of Steelhead, a federally endangered species. Planning efforts by several agencies are underway to promote steelhead recovery. The Bureau of Reclamation has done a preliminary study of modifying the concrete portion of the Los Angeles River to facilitate steelhead and other anadromous fish passage. The (State of California) Wildlife Conservation Board recently awarded a grant to the Council for Watershed Health for design of channel modifications to the Los Angeles River. The City of Los Angeles and project partners are working with multiple agencies to increase biodiversity and restore native fish habitat and fish passage migration corridors to upper tributaries with high intrinsic potential spawning and rearing habitat for not only steelhead/ rainbow trout, but also arroyo chub, speckled dace, unarmored threespine stickleback, and Santa Ana sucker. This native fish project is aligned with approved and ongoing plans for the Los Angeles River watershed and tributaries (http://lariver.org/ ) including the City of Los Angeles Biodiversity Motion (Mayor’s Sustainability Plan, LA Olympics 2028 biodiversity implementation), Greater LAR IRWMP, CDFW native fish habitat priorities and limiting factors analyses, minimum stream flow and water temperature studies, and related multi-benefit projects with ecosystem restoration components.
Seven species of native freshwater fishes historically occurred in Arroyo Seco as documented by Culver and Hubbs (1917): Pacific Brook Lamprey, Lampetra spp., Pacific Lamprey, Entosphenus tridentata, Rainbow Trout/Steelhead, Oncorhynchus mykiss, Arroyo Chub, Gila orcutti, (California species of special concern), Santa Ana Speckled Dace, Rhinichthys cf osculus, Santa Ana Sucker, Catostomus santaanae (federally threatened), and Unarmored Threespine Stickleback, Gasterosteus aculeatus williamsoni (federally endangered). The Speckled Dace ostensibly occurs widely over much of the western United
2
States, but the Los Angeles basin populations are among the most distinctive genetically and may be a distinct and separate species (Smith et al. 2017). The seven native fish species identified by Culver and Hubbs constitute a distinct, endemic part of the aquatic fauna in the Los Angeles River basin, as part of what was called the Santa Ana system of streams which also included the San Gabriel and Santa Ana River basins and a few nearby areas. The fish were widespread from the base of the mountains to the ocean in the originally relatively well-watered systems (Swift et al. 1993; Mendenhall 1908).
Based on historical information, observations, and studies on other streams in the Los Angeles River?
Basin (Swift et al. 1993; Swift and Drill 2008), most of the seven species occurred largely downstream of
the canyon mouths where gradients decreased, more consistent water supplies upwelled from the
ground, and riparian habitat was more extensive. In the case of the Arroyo Seco, the species likely to
have penetrated farther upstream in the steeper gradients of the canyon are the Rainbow Trout and
Speckled Dace, and these would have been the only species consistently present in the Arroyo above
the JPL bridge. Analogous situations apply across the Los Angeles Basin system of streams where
Rainbow Trout and sometimes dace or Arroyo Chubs exist in the upper part of tributaries. These include
the streams, above Pacoima and Tujunga dams, above Cogswell Reservoir in the west Fork San Gabriel
River, above Seven Oaks Dam in the Santa Ana River, in the Santiago Creek drainage above Irvine Lake,
and in upper Trabuco Creek and other tributaries of San Juan Creek. The dace legitimately inhabit such
upstream areas, whereas the Arroyo Chub usually has established populations in the reservoirs,
occasionally moving upstream in the lower gradient areas in and above the reservoir footprint. Arroyo
Chubs usually do not penetrate upstream in higher gradient areas inhabited only by Rainbow Trout and
Speckled Dace.
Upstream of the JPL bridge, partial temporal barriers may be present during low flows due to steep
channel segments and step-pool or bedrock drops. An anthropogenic barrier exists at the road crossing
approximately 500 feet downstream of the diversion at a channel spanning weir that may have been
installed for flow measurement. A drop of approximately three feet occurs at this concrete structure.
Chubs are less able to migrate through locally steep gradients and away from drying conditions than
Rainbow Trout, and in the reach downstream of the diversion would be stranded in pools during low or
zero flow periods.
In the project area only Rainbow Trout have been present within the last 50 years (Swift, et al 1993) and probably much longer due to major modifications to the watercourse downstream. Rainbow Trout were still surviving in the upper Arroyo Seco until the flooding and scouring of the canyon after the large 2009 Station Fire. Currently, however, no fish are known to inhabit the Arroyo Seco above Devils Gate Dam, according to surveys and observations made by NOAA Steelhead Recovery coordinator Mark Capelli In August, 2018 (email 22 August 2019) and by California Fish and Wildlife Fishery Biologist John O’Brien “…in the last few years.” (email 22 August 2019). While Rainbow Trout/Steelhead are not currently present, the design of the diversion should concentrate on this species because it is the only one with a possibility to reoccur and survive until major restoration efforts occur.
The multi-agency plans and projections noted above for the restoration of aquatic habitats for fishes and other native fauna would necessarily involve removal of, or passage around, Devils Gate Dam to provide access for steelhead returning from the ocean. Returning to Arroyo Seco above Devils Gate Dam is now impossible. Such capability as well as actually making passage possible from Devils Gate Dam to the ocean will be a major undertaking to be accomplished many years in the future. The main strategy for the present would presumably be to re-establish genetically identified southern steelhead populations in the upper Arroyo Seco until such time as viable connectivity is established to the ocean.
3
Speckled Dace an Arroyo Chub might also be considered, but are unlikely to survive in the mountainous conditions of the upper Arroyo Seco. Several similar upper watershed areas are have been considered desirable for re-establishing native fishes because of their remaining natural features, despite the fact that the evidence indicates most of the target fish would not survive there very long. The fact that pushing these fish farther upstream is relatively easy compared to trying to bring back the original and more optimal habitat in the lowlands does not mean it will be beneficial to the fish in the long run and likely threatens the likelihood of success.
Currently, conditions below the diversion and out onto the surface of the sediment-filled basin behind Devils Gate Dam are marginal or non-existent to support fishes largely due to the very low flows in the Arroyo Seco above the diversion for much of the year. Lack of water is the primary deficiency from about the JPL bridge upstream where substrate and cover is good but water is lacking and barriers to upstream movement are present. Below the JPL bridge the flatter sediment load of the reservoir is open to solar heating, is dominated by finer substrates, and cover and pools are rare or absent. This means fish that get past the diversion due to screening facilities face poor conditions downstream and may not be helped by such passage. During winter high flows fish would go over Devils Gate Dam only to perish over the long concrete-lined channels of Arroyo Seco and Los Angeles River. Even if they reached the ocean (actually the upper end of Los Angeles Harbor) it is unlikely the harbor would provide the beneficial growth conditions provided by relatively natural estuaries and lagoons farther north. During the summer/dry periods of the year, sections of the Arroyo Seco run dry and would leave fish stranded in isolated pools. Also, if significant amounts of sediment were removed from behind Devils Gate, almost certainly the water body would intentionally or unintentionally become home to a variety of exotic aquatic species that adversely affect survival of the native species. This is already true at several other sites in the Los Angeles basin where native stream fish populations interface downstream with flood control reservoirs. Thus, any downstream passage in the next few years at least would send fish into suboptimal conditions and any survivors would need a way to return upstream as the conditions dry out downstream. Another barrier likely may develop when the sediment is removed and the flow into the reservoir area above Devils Gate Dam becomes more precipitous at the mouth of the canyon above the reservoir footprint.
In summary, the primary concern for the diversion should be for minimizing adverse effects on Rainbow Trout/Steelhead. This species would be protected by incorporating screens in the diversion/intake structure designed to prevent Rainbow Trout fry from being entrained with the intake of stream water, and without bypassing the fish downstream where their survival is unlikely. If the introduction of Arroyo Chub and Speckled Dace is to be given consideration in the future, despite the constraints to their existence as noted above, their juvenile and adult sizes are similar to steelhead and the screens would accommodate these later life stages as well. Some calculated loss of very small larvae of the minnows would likely result as is the case in many other places where minnows occur with larger salmonids. Dace larvae occupy margins of the flowing streams, whereas Arroyo Chub larvae drift downstream more and invade standing waters as well (Feeney and Swift 2008). Thus the Arroyo Chub larvae are more likely to be impinged in any diversion screen. As noted above, however, consideration should be given as to the overall likelihood of reintroducing chub in this area before designing to mitigate for a small loss that may not occur.
It is also advisable to provide passage at the diversion dam during higher flows when the diversion is not operating and the flow volume will enable fish to return upstream before flows decline. As much as possible, the diversion should be designed for potential future change in operations to accommodate actual anadromous steelhead arriving from the ocean.
4
References:
Culver, G. B. and C. L. Hubbs. 1917. The fishes of the Santa Ana system streams in southern California. Lorquinia, 1, 82-83.
Feeney. R. F. and C. C. Swift. 2008. Description and ecology of larvae and juveniles of the native
cypriniforms of coastal southern California. Ichthyological Research, 55(1):65-77.
Mendenhall, W. C. 1908. Ground waters and irrigation enterprises in the Foothill Belt, southern California. U. S. Geological Survey, Water-Supply Paper 219, 180 pp., 9 plates
Smith, G. R., J. Chow, P. J. Unmack, D. .F. Markle, and T. E. Dowling. 2017. Evolution of the Rhinichthys osculus Complex (Teleostei: Cyprinidae) in western North America. Pp. 45-84. IN: Fishes of the Mio-Pliocene western Snake River Plain and Vicinity. Miscellaneous Publications of the Museum of Zoology, University of Michigan, No. 204, No. 2.
Swift, C. C. and S. Drill. 2008. The fish study. Pp. 5-22. IN: State of the River 2, Friends of the Los
Angeles River, Los Angeles, CA
Swift, C. C., T. R. Haglund, M. Ruiz, and R. N. Fisher. 1993. The status and distribution of the freshwater fishes of southern California. 92(3):101-167.
DRAFT Basis of Design Report Arroyo Seco Canyon Project – Intake/Diversion Design
APPENDIX B
PRELIMINARY (30%) PLANS
DRAFT Basis of Design Report Arroyo Seco Canyon Project – Intake/Diversion Design
La Canada
T1
ARROYO SECO COVER
Job Number
Drawing Name
Drafter Checked
Sheet Number
Designer
Date
Vicinity Map
SHEET INDEX
LEGEND
PRELIMINARY
NOT FOR CONSTRUCTION
Project Location Map
Drawing Status
5 March 2020 11:00
Title Sheet, Location Map, and Vicinity
City of Pasadena
Department of Water and Power
150 South Los Robles Avenue, Suite 300
Pasadena, California 91101
(626) 744-4005
www.cityofpasadena.net/water-and-power
Sheet 1 of 8
6004878
65% Submittal
tvs/gp tvs eew
PROJECT MANAGER
Arroyo Seco Canyon Project
Area 2 Diversion/Intake Replacement
MARCH 2020
City of Pasadena
Department of Water and Power
CONSTRUCTION PLANS FOR
Project
Location
300 North Lake Avenue, Suite 1020
Pasadena, California 91101
(626) 568-4300
www.kennedyjenks.com
Kennedy Jenks
Project
Location
INTERSTATE
210
CALIFORNIA
Altadena
Pasadena
CALIFORNIA
2
CALIFORNIA
2
E-1818
D.S 206
207
Sheet of 8
6004878
PRELIMINARY
City of Pasadena
Department of Water and Power
150 South Los Robles Avenue, Suite 300
Pasadena, California 91101
(626) 744-4005
www.cityofpasadena.net/water-and-power
Arroyo Seco Canyon Project
Area 2 Diversion/Intake Replacement
300 North Lake Avenue, Suite 1020
Pasadena, California 91101
(626) 568-4300
www.kennedyjenks.com
Kennedy Jenks
Revisions
DateNo.
Description
Job Number
Date
Drafter
Checked
Sheet Number
Designer
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General Notes Sheet
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TYPICAL ABBREVIATIONSA E M S
ABAN ABANDONED EA EACH MH MANHOLE SS SANITARY SEWER& AND E EAST MAX MAXIMUM S SOUTH∠ ANGLE EG EXISTING GROUND MI MILES SD STORM DRAINAB AGGREGATE BASE EL ELEVATION MIN MINIMUM SF SQUARE FEETABA ARCHITECTURAL BARRIERS ACT ELEC ELECTRIC MM MILLIMETER SHT SHEETAC ASPHALT CONCRETE EP EDGE OF PAVEMENT MPH MILES PER HOUR SLP SLOPEADA AMERICANS WITH DISABILITIES ACT ESM ENGINEERED STREAMBED MATERIAL N SPEC SPECIFICATIONAIS AQUATIC INVASIVE SPECIES EW EDGE OF WATER N NORTH SQ SQUAREALT ALTERNATIVE EX EXISTING # NUMBER SRL SMALL ROOTWAD LOGAPN ASSESSOR PARCEL NUMBER F ≠ NOT EQUAL SSTL STAINLESS STEELAPPROX APPROXIMATE FT FEET NA NOT APPLICABLE STA STATIONASCII AMERICAN STANDARD CODE FG FINISHED GROUND NE NORTHEAST STD STANDARDASPH ASPHALT FH FIRE HYDRANT NTS NOT TO SCALE STL STEELAUX AUXILIARY FIG FIGURE NW NORTHWEST SWPPP STORM WATER POLLUTION PREVENTION PLAN
B FL FLOW LINE O SYM SYMBOLBL BASELINE G O.C. ON CENTER TBLK BLOCK GAL US GALLONS OH OVERHEAD TEMP TEMPORARYBM BENCHMARK GALV. GALVANIZED OPT OPTIONAL TBC TOP BACK OF CURBBMP BEST MANAGEMENT PRACTICE GB GRADE BREAK P TBM TEMPORARY BENCH MARKBOT BOTTOM GL GUTTER LIP P PROTECT (MAY BE PRUNED) TC TOP OF CONCRETE
C GND GROUND ± PLUS/MINUS TKPOA TAHOE KEYS PROPERTY OWNERS ASSOCIATIONC COTTONWOOD GPM US GALLONS PER MINUTE LB POUND TOE TOE OF SLOPECF CUBIC FEET H PC PILOT CHANNEL TOP TOP OF SLOPECFS CUBIC FEET PER SECOND HAZ HAZARDOUS PE POLYETHYLENE TP TOP OF PAVEMENTCLR CLEAR H/V HORIZONTAL/VERTICAL PERF PERFORATED TRPA TAHOE REGIONAL PLANNING AGENCYCM CENTIMETER HORIZ HORIZONTAL PERP PERPENDICULAR TSOM TOPSOIL, SOD, ORGANIC MATERIALCMP CORRUGATED METAL PIPE HP HORSEPOWER PROP PROPERTY TW TOP OF WALLCONC CONCRETE HWL HIGH WATER LEVEL PSF POUNDS PER SQUARE FOOT TYP TYPICALCONN CONNECTION I PSI POUNDS PER SQUARE INCH UCONT CONTINUOUS IN INCHES PT POINT UNK UNKNOWNCP CONTROL POINT INTX INTERSECTION PVC POLYVINYL CHLORIDE PIPE UNO UNLESS NOTED OTHERWISECRNR CORNER INV INVERT PVMT PAVEMENT UTIL UTILITYCTC CALIFORNIA TAHOE CONSERVANCY ISA INTERNATIONAL SYMBOL OF ACCESSIBILITY Q UTM UPPER TRUCKEE MARSHCTR CENTER J QTY QUANTITY UTR UPPER TRUCKEE RIVERCULV CULVERT JCT JUNCTION R VCY CUBIC YARDS JP JEFFREY PINE R RADIUS VB VALVE BOX℄ CENTERLINE L RB RIGHT BANK VEL VELOCITY
D LB LEFT BANK RC ROSE/CURRANT CLUMP VOL VOLUME° DEGREES LF LINEAR FEET RC RELATIVE COMPACTION W⌀ DIAMETER LO LOG ONLY REF REFERENCE WC WILLOW CLUMPDG DECOMPOSED GRANITE LOC LOCATION REL RELATIVE W WESTDI DRAIN INLET LP LODGEPOLE PINE REQ REQUIRED W/O WITHOUTDIA DIAMETER LRL LARGE ROOTWAD LOG REV REVISION WSEL WATER SURFACE ELEVATIONDWG DRAWING LT LEFT RGB RED GREEN BLUE YΔ DELTA LVL LEVEL RT RIGHT YR YEAR
Sheet of 8
6004878
PRELIMINARY
City of Pasadena
Department of Water and Power
150 South Los Robles Avenue, Suite 300
Pasadena, California 91101
(626) 744-4005
www.cityofpasadena.net/water-and-power
Arroyo Seco Canyon Project
Area 2 Diversion/Intake Replacement
300 North Lake Avenue, Suite 1020
Pasadena, California 91101
(626) 568-4300
www.kennedyjenks.com
Kennedy Jenks
Revisions
DateNo.
Description
Job Number
Date
Drafter
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Designer
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Access & Staging Sheet
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Jet
Propulsion
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Pasadena
INTERSTATE
210
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EXIT 22B
EXIT 22A
nhc
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Devil's Gate Reservoir
Arro
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Sheet of 8
6004878
PRELIMINARY
City of Pasadena
Department of Water and Power
150 South Los Robles Avenue, Suite 300
Pasadena, California 91101
(626) 744-4005
www.cityofpasadena.net/water-and-power
Arroyo Seco Canyon Project
Area 2 Diversion/Intake Replacement
300 North Lake Avenue, Suite 1020
Pasadena, California 91101
(626) 568-4300
www.kennedyjenks.com
Kennedy Jenks
Revisions
DateNo.
Description
Job Number
Date
Drafter
Checked
Sheet Number
Designer
A B C D E F G H I J K L M N O P
1
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Drawing Information
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Sheet of 8
6004878
PRELIMINARY
City of Pasadena
Department of Water and Power
150 South Los Robles Avenue, Suite 300
Pasadena, California 91101
(626) 744-4005
www.cityofpasadena.net/water-and-power
Arroyo Seco Canyon Project
Area 2 Diversion/Intake Replacement
300 North Lake Avenue, Suite 1020
Pasadena, California 91101
(626) 568-4300
www.kennedyjenks.com
Kennedy Jenks
Revisions
DateNo.
Description
Job Number
Date
Drafter
Checked
Sheet Number
Designer
A B C D E F G H I J K L M N O P
1
Plotted Scale
File Name
Drawing Information
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ARROYO SECO
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NOT FOR CONSTRUCTION
C1
05 March 2020 (11:23)
Site Plan Sheet
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6004878
PRELIMINARY
City of Pasadena
Department of Water and Power
150 South Los Robles Avenue, Suite 300
Pasadena, California 91101
(626) 744-4005
www.cityofpasadena.net/water-and-power
Arroyo Seco Canyon Project
Area 2 Diversion/Intake Replacement
300 North Lake Avenue, Suite 1020
Pasadena, California 91101
(626) 568-4300
www.kennedyjenks.com
Kennedy Jenks
Revisions
DateNo.
Description
Job Number
Date
Drafter
Checked
Sheet Number
Designer
A B C D E F G H I J K L M N O P
1
Plotted Scale
File Name
Drawing Information
A B C D E F G H I J K L M N O P
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0 1
1/2
11:09:45
ARROYO SECO
PRELIMINARY
NOT FOR CONSTRUCTION
C2
05 March 2020 (11:23)
Roughened Channel Profile Sheet
6
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eew
ROUGHENED CHANNEL PROFILE
Sheet of 8
6004878
PRELIMINARY
City of Pasadena
Department of Water and Power
150 South Los Robles Avenue, Suite 300
Pasadena, California 91101
(626) 744-4005
www.cityofpasadena.net/water-and-power
Arroyo Seco Canyon Project
Area 2 Diversion/Intake Replacement
300 North Lake Avenue, Suite 1020
Pasadena, California 91101
(626) 568-4300
www.kennedyjenks.com
Kennedy Jenks
Revisions
DateNo.
Description
Job Number
Date
Drafter
Checked
Sheet Number
Designer
A B C D E F G H I J K L M N O P
1
Plotted Scale
File Name
Drawing Information
A B C D E F G H I J K L M N O P
2
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10
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Status
0 1
1/2
11:09:45
ARROYO SECO
PRELIMINARY
NOT FOR CONSTRUCTION
C3
05 March 2020 (11:23)
Roughened Channel Cross Sections Sheet
7
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eew
SECTION STA 11+52
SECTION STA 10+90
Sheet of 8
6004878
PRELIMINARY
City of Pasadena
Department of Water and Power
150 South Los Robles Avenue, Suite 300
Pasadena, California 91101
(626) 744-4005
www.cityofpasadena.net/water-and-power
Arroyo Seco Canyon Project
Area 2 Diversion/Intake Replacement
300 North Lake Avenue, Suite 1020
Pasadena, California 91101
(626) 568-4300
www.kennedyjenks.com
Kennedy Jenks
Revisions
DateNo.
Description
Job Number
Date
Drafter
Checked
Sheet Number
Designer
A B C D E F G H I J K L M N O P
1
Plotted Scale
File Name
Drawing Information
A B C D E F G H I J K L M N O P
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10
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0 1
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10:42:21
ARROYO SECO DETAILS
PRELIMINARY
NOT FOR CONSTRUCTION
D1
05 March 2020 (11:23)
Details Sheet
8
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INTAKE STRUCTURE PLAN VIEW - (COVER NOT SHOWN)
D1
A
COVER PLAN
D1
B
DRAFT Basis of Design Report Arroyo Seco Canyon Project – Intake/Diversion Design
APPENDIX C
HYDRAULIC INFORMATION
DRAFT Basis of Design Report Arroyo Seco Canyon Project – Intake/Diversion Design
INTAKE/DIVERSION HYDRAULIC DESIGNArroyo Seco
Normal depth from Manning's eq. Normal depth from Manning's eq.Bottom Width (FT) 4 Radius (FT) 1.25Side Slopes (H:V) 0 Invert Slope 0.05Invert Slope 0.005 n 0.015n 0.015Inv Elevation (FT) 1174.47 Inv Elevation (FT) 1179.46 Inv Elevation (FT) 1179.96
Qd (CFS) A (SF) R (FT) d (FT) WSE (FT) Q (CFS) Theta A (SF) R (FT) d (FT) WSE (FT) d (FT) WSE (FT)
25 4.45 0.71 1.11 1175.59 25 2.68 1.74 0.52 0.96 1180.42 0.96 1180.92
WSE at Diversion Weir (FT)
Outlet 1 Q (CFS) Outlet 2 Q (CFS) Total Q (CFS) Q (CFS) A (SF) R (FT) d (FT) WSE (FT) Q (CFS) Theta A (SF) R (FT) d (FT) WSE (FT) d (FT) WSE (FT)
1181.61 1.50 0.00 1.50 1.50 0.71 0.16 0.18 1174.65 1.50 1.25 0.24 0.15 0.24 1179.70 0.24 1180.201181.79 5.00 0.00 5.00 5.00 1.52 0.32 0.38 1174.85 5.00 1.70 0.55 0.26 0.42 1179.88 0.42 1180.381181.99 10.00 0.00 10.00 10.00 2.39 0.46 0.60 1175.07 10.00 2.04 0.90 0.35 0.60 1180.06 0.60 1180.561182.16 15.00 0.00 15.00 15.00 3.13 0.56 0.78 1175.26 15.00 2.29 1.20 0.42 0.73 1180.19 0.73 1180.691182.31 20.00 0.00 20.00 20.00 3.81 0.65 0.95 1175.43 20.00 2.49 1.48 0.47 0.85 1180.31 0.85 1180.811182.45 25.00 0.00 25.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.921182.45 25.00 5.00 30.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.921182.45 25.00 10.00 35.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.921182.45 25.00 20.00 45.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.921182.45 25.00 30.00 55.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.921182.45 25.00 40.00 65.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.921182.45 25.00 50.00 75.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.921182.45 25.00 60.00 85.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.921182.45 25.00 70.00 95.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.921182.45 25.00 75.00 100.00 25.00 4.45 0.71 1.11 1175.59 25.00 2.67 1.74 0.52 0.96 1180.42 0.96 1180.92
Design for Max Qd -->
Tunnel Entrance Diversion Pipe Outlet Diversion Pipe InletOutlet 1 - Diversion
INTAKE/DIVERSION HYDRAULIC DESIGN (CONTINUED)
Weir Equation Exit Head LossQ=C L H^3/2 HL=K(V^2)/2gWeir Coeff (C) 3.2 Max appoach velocity (FPS) 0.4Weir Width 8 Screen Length (FT) 20
K 1.5Top of Weir Elevation (FT) 1181.46 Inv Elevation (FT) 1178.79
US DS Drop Bottom of Screen (FT) 1178.96 Top of Screen (FT) 1182.96Optional Fishway IE (FT) 1180.66 1174.80 5.86 Low Flow depth on Screen 2.50 % of Total Height 63% High Flow % Screen 87%
Q (CFS) A (SF) H (FT) WSE (FT) Depth (FT)Check Screen Length (FT)
Screen Area (SF)
25 7.87 0.98 1182.44 3.48 18.18 63.33
Q (CFS) A (SF) H (FT) WSE (FT) Depth (FT)Check Approach
VelSweep Vel
Point 1Sweep Vel
Point 2Sweep Vel
Point 3Sweep Vel
Point 4HL (FT) WSE US (FT)
1.50 1.21 0.15 1181.61 2.65 0.03 0.1 0.1 0.1 0.0 0.000 1181.615.00 2.69 0.34 1181.79 2.84 0.09 0.2 0.2 0.2 0.1 0.000 1181.79
10.00 4.27 0.53 1181.99 3.03 0.16 0.4 0.4 0.3 0.2 0.001 1181.9915.00 5.60 0.70 1182.16 3.20 0.23 0.6 0.6 0.5 0.3 0.001 1182.1620.00 6.79 0.85 1182.30 3.35 0.30 0.8 0.8 0.6 0.4 0.002 1182.3125.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.4425.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.4425.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.4425.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.4425.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.4425.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.4425.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.4425.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.4425.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.4425.00 7.87 0.98 1182.44 3.48 0.36 1.0 1.0 0.7 0.4 0.003 1182.44
Intake Weir and Diversion Gate Fish ScreenOutlet 1 - Diversion
5ft 5ft 5ft 5ftP1 P2 P3 P4
INTAKE/DIVERSION HYDRAULIC DESIGN (CONTINUED)
Trash Rack Length (FT) 10.0 Weir Top Elev (FT) 1177.82 Dam Top Elev (FT) 1183Sill El (FT) 1178.79 Crest Gate Top Elev (FT) 1182.95 Weir EquationIntake El (FT) 1177.82 Weir Equation Q=C L H^3/2Head loss as a function of velocity Q=C L H^3/2 Weir Coeff (C) 3.2Trash rack and entrance = k1*V^2/2g k1 0.6 Weir Coeff (C) 3.2 Weir Width 26Sill = k2*V^2/2g k2 0.15 Weir Width 10
Total 0.75
Assume fraction of trash rack area open 0.85
Depth (FT) WSE DS (FT) V (FPS) HL WSE US (FT)Q (CFS) H (FT) WSE (FT) Gate EL (FT) WSE (FT) H (FT) A (SF) Q (CFS)
2.82 1181.61 0.05 0.000 1181.61 0.00 0.0000 1181.61 1182.95 1181.61 0.00 0.00 0.003.00 1181.79 0.17 0.000 1181.79 0.00 0.0000 1181.79 1182.95 1181.79 0.00 0.00 0.003.20 1181.99 0.31 0.001 1181.99 0.00 0.0000 1181.99 1182.95 1181.99 0.00 0.00 0.003.37 1182.16 0.45 0.002 1182.16 0.00 0.0000 1182.16 1182.95 1182.16 0.00 0.00 0.003.51 1182.30 0.57 0.004 1182.31 0.00 0.0000 1182.31 1182.95 1182.31 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 0.00 0.0000 1182.45 1182.95 1182.45 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 5.00 0.2901 1182.45 1182.66 1182.45 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 10.00 0.4605 1182.45 1182.49 1182.45 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 20.00 0.7310 1182.45 1182.22 1182.45 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 30.00 0.9579 1182.45 1181.99 1182.45 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 40.00 1.1604 1182.45 1181.79 1182.45 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 50.00 1.3465 1182.45 1181.60 1182.45 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 60.00 1.5206 1182.45 1181.43 1182.45 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 70.00 1.6851 1182.45 1181.26 1182.45 0.00 0.00 0.003.65 1182.44 0.68 0.005 1182.45 75.00 1.7644 1182.45 1181.18 1182.45 0.00 0.00 0.00
Outlet 2 - Crest Gate and Dam CrestTrash Rack and Sill Diversion Weir and Crest Gate DamOutlet 1 - Diversion
Arroyo Seco DiversionRoughened Channel Assessment
ESM SizingCDFW Part XI, 2009 recommends use of USACE eq for shallow steep channelsUSACE EM 1110-2-1601
2D HEC-RAS modeling results under 100-year flow conditions: unit flows for 24.5 ft/s at 9.5 ft depthq 235.2 cfs
S 0.04 use bed slope
D30s= 1.95*S^0.55*(1.25*q)^0.666/32.2^0.333
D30s= 3.97 ft min
other sizes based on D30 calculatedD84= 1.5*D30s 5.95 ft min
D50= 0.4*D84 2.38 ft min
D100= 2.5*D84 14.88 ft
S 0.026 use energy grade slope
D30s= 1.95*S^0.55*(1.25*q)^0.666/32.2^0.333
D30s= 3.11 ft min
other sizes based on D30 calculated
D84= 1.5*D30s 4.66 ft minD50= 0.4*D84 1.86 ft minD100= 2.5*D84 11.66 ft
Riprap SizingArroyo Seco Intake/DiversionRock Sizing for Bank Protection (Road Stabilization)
Corps EM 1110-2-1601In BendD30=Sf*Cs*Cv*Ct*d*[(unit weight of water/(unit weight of stone - unit weight of water))^0.5*v/(K1*g*d)^0.5]^2.5
Selected ValuesSf 1.1 min, see 1601 text 1.1Cs 0.3 for angular rock, 0.375 for rounded 0.3Cv 1.283-0.2log(R/W) 1.2 overall curve R= 400 W= 100Ct based on blanket thickness in relation to D85/D15 ratio 1.0d from RAS 3.00 ft on left bank terraceunit weight of water 62.4 pcfunit weight of stone 165.0 pcfV from RAS 9.00 fps on left bank terraceVss Vss=Vavg*1.74-0.52log(R/W) 12.8 fpsK1 use Plate 39, 40 0.71 use K1 from recommended curve, plate 39, cot(theta)=1.5g 32.2 ft/sec2
ft inD30= 1.85 22.21use USACE 42" gradation
D30 min = 1.70 20.40
D90 min = 2.47 29.64
D50 min= 2.04 24.49
D100 min = 2.57 30.78
D100 max = 3.46 41.47