nhc technical report · arroyo seco canyon project – intake/diversion design (area 2) draft basis...

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


velocity distribution at high design flow for adult anadromous salmonid fish passage (306.5

cfs) ......................................................................................................................................... 29


velocity distribution at low design flow for juvenile fish passage (1 cfs) .............................. 30


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.


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.


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)


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)


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


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.


Figure 2.4 Storm hydrographs for 2010 (top) and 2005 (bottom) events


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.


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


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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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


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


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,


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.


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


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.


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.


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.


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.


Figure 4.1 2-year flood extent and peak velocities, existing conditions

VELOCITY

SCALE, feet per

second

EXISTING DIVERSION STRUCTURE


Figure 4.2 2-year flood extent and peak velocities, proposed conditions

DIVERSION STRUCTURE

VELOCITY SCALE, feet

per second



FLOW OUTSIDE

CHANNEL


per second




per second

DIVERSION STRUCTURE



FLOW OUTSIDE

CHANNEL



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


per second


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

(


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.



velocity distribution at high design flow for juvenile fish passage (61.3 cfs)



velocity distribution at high design flow for adult anadromous salmonid fish passage

(306.5 cfs)



velocity distribution at low design flow for juvenile fish passage (1 cfs)



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


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.


Figure 4.12 Screen bay configuration options (not to scale)


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.


Figure 4.13 Intake Hydraulic Profile


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.


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.


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.


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


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


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.


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.


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 fi

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