Section_J_-_Att_No__005_Salmon_Habitat_Enhancement_at_Muir_Woods_Phase_2.pdf

DRAFT

Salmon Habitat Enhancement at Muir Woods, Phase 2

Draft 90% Construction Designs And

Design Basis Report

Prepared for

Carolyn Shoulders, Project Manager National Parks Service

Golden Gate National Recreation Area Building 201 Fort Mason San Francisco, CA 94123

Caroline Christman, Project Coordinator Golden Gate National Parks Conservancy

Building 201 Fort Mason, 3rd Floor San Francisco, CA 94123

Prepared by

Northern Hydrology & Engineering PO Box 2515

McKinleyville, CA 95519

March 4, 2022 i

TABLE OF CONTENTS

Introduction

Project Location

Project Goals

Project Background

Phase 2 90% Construction Designs

Overview

Riprap removal

Wood addition

Hydraulic Analysis

Habitat Mapping and Analysis

Vegetation

Design Areas

Area 7

Area 6

Area 5

References ii

APPENDICES

Appendix A: Phase 2 Draft 90% Plans

Appendix B: Phase 1 Wood Placement and Small Woody Debris

Appendix C: Model Development

LIST OF TABLES

Table 1. Summary of wood quantity and size utilized in Phase 2 designs Table 2. LWD key piece frequency NMFS (2012) rating.* Table 3. Summary of flood flows modeled in the hydraulic analysis Table 4. Juvenile salmonid winter habitat area estimated for existing and design conditions Table 5. Pool frequency in the Phase 2 project area

LIST OF FIGURES

Figure 1. Velocity shift between existing (EG) and design (DG) conditions at winter habitat flows (5-80 cfs) and the 100-year flood. 6

Figure 2. Difference in water surface elevation at 100-year flow. 7 Figure 3. Change in inundation extent at the 100-year flow for existing and design conditions for

Area 5 (top panel) and wood placement at Creek STA 37+30 (bottom panel) (placeholder figure). 8

Figure 4. Difference in velocity at the 100-year flow. 9 Figure 5. Difference in water surface elevation in Area 7 at the 100-year flow. 10 Figure 6. Difference in inundation extent in Area 7 at the 100-year flow. 11 Figure 7. Difference in velocity in Area 7 at the 100-year flow. 11 iii

LIST OF PREPARERS

Bonnie Pryor, Principal Fluvial Geomorphologist

J. Rose Patenaude, Senior Civil Engineer

Brian Buchanan, Senior Scientist

Brian Draeger, Water Resources Engineer I

Tomi Bradshaw, Engineering Technician I

Prepared under the supervision of:

Jeffrey K. Anderson, RCE 50713

INTRODUCTION

Muir Woods National Monument (MWNM) was established in 1908 to preserve the old-growth redwood (Sequoia sempervirens) forest. The Salmon Habitat Enhancement Project is focused on reducing impacts to natural channel processes that have occurred as part of the earlier management of the Redwood Creek watershed. Habitat improvements are focused on federally and state-listed endangered coho salmon (Oncorhynchus kisutch) and federally listed threatened steelhead trout (O. mykiss) and improving the overall riverine and forest ecology.

The early land management activities led to widespread channel incision, which drove the placement of riprap along the channel banks and ongoing removal of wood from the channel. The collective result of these management actions is that the channel has remained fossilized since riprap was installed in the 1930’s. Riprap has suppressed streamside vegetation recovery and reduced natural recruitment of wood in the channel. Wood was systematically removed from the channel until the late 1980’s. Since the 1980’s, fallen wood has generally been retained, but the frequency of large wood remains low. Wood jams that form along the stream channel network create natural sediment storage reservoirs. Sediment accumulations around wood jams are present at some locations, but their spacing is too wide to build back up the channel bed or provide necessary salmonid habitat. The oversimplified, incised channel that developed in the 1930’s lacks the key attributes necessary to support all life stages of salmonids. In particular, juvenile rearing habitat is extremely limited. The channel lacks low velocity areas for fish to rest even during moderate winter flows and the channel lacks deep pools with complex cover for summer rearing. Natural recovery of the system requires removal of artificial constraints on geomorphic process, vegetation recovery along the stream banks, and the return of abundant large, instream woody debris.

PROJECT LOCATION

The project is located within Muir Woods National Monument (MWNM) and the adjacent National Park Service (NPS) management area within California State Parks (Appendix A, Sheet G3). MWNM is located approximately 24 kilometers (km) (15 miles [mi]) north of San Francisco, in Marin County’s Redwood Creek watershed. MWNM occupies 224 hectares (ha) (554 acres [ac]) and includes ~1 mile of the mainstem of Redwood Creek. The first phase of implementation focused on the upper 0.5 of Redwood Creek and this phase is focused on the lower 0.5 mile (Appendix A, Sheet C1).

PROJECT GOALS

Identified goals for the Salmon Habitat Enhancement Project at Muir Woods include:

1. Enhance winter/spring habitat for Coho fry and juveniles and summer habitat for Coho and steelhead juveniles through in-stream actions, floodplain enhancement and bank revegetation.

2. Mitigate and minimize adverse effects to the cultural resources to the extent possible while also allowing an updated understanding of conservation to be achieved for the health of the channel, salmon populations, and redwood forest.

3. Restore natural geomorphic processes where possible, given constraints to channel function such as the existing trail system and the need to maintain much of the channel bank revetment as a cultural resource.

4. Reestablish floodplain connectivity where feasible, given the varied levels of disconnection and existing constraints such as trails on tops of banks, to provide winter habitat, decrease channel high flow velocities and increase groundwater recharge.

Additional measurable objectives for achieving primary goals are:

· Increase sediment retention and deposition in the project reach to re-aggrade the channel and reconnect the floodplain over the long term.

· Increase the frequency of large woody debris (LWD) and small woody debris (SWD).

· Decrease channel high flow velocities.

· Increase pool depth.

· Increase the frequency of pools.

The 90% construction designs discussed in this report address all objectives.

PROJECT BACKGROUND

Conceptual planning and compliance for the project addressed actions along the full channel throughout Muir Woods, but preparation of designs and implementation was divided into two phases of work.

Conceptual planning included an assessment of site conditions, the development of conceptual designs, impact analysis for all creek actions from the Plaza area to Bridge 4 (NHE, 2017). Compliance consisted of an Environmental Assessment (Horizon, 2017) and Finding of No Significant Impact (FONSI, 2018), with Biological Opinions issued by both by the National Marine Fisheries Service and the U.S. Fish and Wildlife Service and a Memorandum of Agreement issued by the State Historic Preservation Office (SHPO). Construction designs (CDs) for Phase 1 addressed creek restoration actions in the upstream half of Muir Woods, between Bridge 3 and Bridge 4 (NHE, 2019, NPS, 2018) and supplemental design information for riprap burial (NHE, 2018). Phase 1 was implemented in summer/fall of 2019. This document describes construction design and analyses for Phase 2, in the downstream half of Muir Woods.

Phase 2 is expected to occur in 2023.

PHASE 2 90% CONSTRUCTION DESIGNS

OVERVIEW

The Phase 2 project area is 2,780 feet long and extends from the cascade adjacent to the Plaza to Bridge 3 at the upstream end (Appendix A Sheet C1). There are three subareas where most of the activities will occur (riprap removal, large wood placement, floodplain grading). Heavy equipment will be used to conduct the construction in these areas (Area 5, 6, 7). These areas were selected based on opportunities for habitat enhancement where other infrastructure constraints, such as visitor trails and a sewer line, did not occur.

Habitat enhancement actions include riprap removal, expansion of low elevation floodplains and alcoves, and installation of large wood on the floodplain that was suitable for in-stream habitat enhancement.

Between these areas, wood of varying sizes will be added with low impact techniques used in Phase 1 such as griphoists, cables and post drivers.

RIPRAP REMOVAL

Riprap removal immediately restores the potential for channel migration, a primary physical process in a valley bottom that creates habitat for aquatic organisms and terrestrial species (Ward and Stanford, 1995).

Although discussed separately, riprap removal works in concert with wood placement to achieve channel function and complexity essential in providing high quality salmonid habitat.

Restoring full geomorphic function to the stream channel would require removing all sections of riprap.

However, NPS cannot remove all the riprap on channel banks due to NPS infrastructure at the tops of banks and the status of the riprap as a contributing feature to a cultural landscape. Riprap sections are retained adjacent to bridge abutments (Bridges 1 & 2), a sewer line near the entrance, in certain areas targeted for high visibility to visitors, and in most areas where the trail is particularly close the stream bank (within 1 channel width).

The trail system is a vital part of the NPS infrastructure since Muir Woods is visited by more than 1 million people per year. The use of riprap as bank stabilization is highly effective and halts channel migration.

Following construction, channel migration rates in Muir Woods may be initially elevated due to channel incision and the long-term impacts that the riprap has had on the bank (e.g., suppression of vegetation growth). Trails along banks where riprap is removed will have an elevated risk of erosion compared to existing conditions, but the risk of erosion remains relatively low since the riprap sections proposed for removal are generally more than a channel width from the nearest trail.

Phase 2 construction designs (CD’s) entail removing 510 CY of riprap along 742 ft of stream bank. Banks are generally left in as as-is condition following riprap removal except for two locations where the bank is graded to a slope of 1.5H:1V. In each of the three work areas, riprap is disposed of differently: it is repurposed to provide additional protection to infrastructure, buried within the channel bed, or off-hauled to a stockpile area for other park uses (Appendix A Sheet G3). Select boulders in the channel bed will be retained to enhance winter steelhead habitat as defined by Fong et al. (2016). Boulders will be placed in clusters of approximately 3 boulders that are each separated by at least 0.3-0.9 m (1-3 ft) to allow flow to scour in between and boulders. The placement of these boulders will be prioritized in areas that meet velocity and depth habitat criteria but lack suitable cover.

WOOD ADDITION

Large Wood structures Redwood Creek in MWNM has a deficit of wood in the channel due to historic management practices of removing wood from the stream channel (NHE, 2017). Phase 2 CDs include 14 large wood placement sites that include both large and small woody debris and are intended to promote bar and pool formation, provide cover for salmonids, and increase sediment storage. Small woody debris is expected to be less persistent with higher exchange rates between structures than larger wood.

A total of 66 logs will be used in the wood designs (Table 1, Appendix A Sheet C2, C4, C5). Thirty-three logs will be relocated from the floodplain into the channel (including three live alders with rootballs that are withing the grading boundary), 10 are within the channel and will be repositioned, and 23 logs will be imported. Many areas can accommodate more wood than has been identified to-date and will be supplemented if additional logs are located. Numerous piles of slash from fallen trees have been identified and will serve as primary sources for the small woody debris. Small woody debris will be used in conjunction with LWD (Appendix A Sheet W18) to create more complex structures.

Most design logs in Phase 2 can be retrieved and placed by heavy equipment. Logs that cannot be moved with heavy equipment will be moved with by manual mechanical techniques (e.g., griphoists), similar to Phase 1.

All wood is added to the channel in an unanchored condition and is expected to adjust similarly to wood recruited to the channel by natural processes. The expected mobility of a given piece of wood depends on the size (diameter and length), presence of a root wad, position relative to other large obstructions, and proportion of the piece submerged at a given flow. The natural complexity and stability of wood jams is mimicked by utilizing combination of pin logs, retaining a portion of each log on the stream bank, interlocking logs where feasible, and adding SWD.

Table 1. Summary of wood quantity and size utilized in Phase 2 designs.

Diameter (cm)

Design Quantity to Add to Redwood Creek

(Number of Pieces)

>60 36 30-60 24 15-30 3 + 1 live alder 10-15 2 (live alders) Total 66

NMFS (2012) identified the following rating thresholds for key pieces of large wood debris frequency for channels based on bankfull width. Redwood Creek has a bankfull width of 9 meters, thus classed as a “smaller channel (bankfull width < 10 meters)”. A key piece for this size channel is less is a minimum diameter of 0.55 meters (1.8 feet) and a length of 10 meters (33 feet), or a volume of 2.5 cubic meters or greater. The rating thresholds are as follows:

· Poor = < 4 key pieces/100 meters;

· Fair = 4 to 6 key pieces/100 meters;

· Good = 6 to 11 key pieces/100 meters;

· Very good = > 11 key pieces/100 meters.

The designs increase the overall rating of the Phase 2 project area from poor to fair. Area 7 rating improves to very good due to the abiltiy to import large wood to this area. Area 6 rating remains poor due to the smaller size of most pieces available on the adjacent floodplain. Area 5 increases from poor to fair. This area is also limited by the size of wood available on the adjacent floodplain, but a recent tree fall from the adjacent hillslope provided a larger supply of key pieces than Area 6. Between the main work areas, the rating remains poor due to a lack of available wood.

Additional wood is added in the design that does not meet the NMFS (2012) key piece definition. While the smaller logs will be less stable than the key pieces, the smaller wood has an important role in creating complex habitat.

Table 2. LWD key piece frequency NMFS (2012) rating. *

Location Key Piece Frequency

(#/100 meters) NMFS Rating

Existing Design Existing Design Total Phase 2 2 6 Poor Fair Area 7 (Plaza) 3 20 Poor Very good Area 6 (between Bridge 1 and 2) 1 2 Poor Poor Area 5 (between Bridge 2 and 3) 1 6 Poor Fair Between Main Work Areas 3 3 Poor Poor

* This evaluation is based on pieces which meet the NMFS definition of a key piece; however, additional logs that do not meet this definition will also be installed.

In addition to the large wood placement in the Phase 2 work area (Cascade to Bridge 3), two logs that were intended for placement in Phase 1, will be installed during Phase 2 (Appendix B, Phase 1 Sheet C5).

Small Wood Structures Locations of small wood structures were identified throughout the full project area (Phase 1 and 2 project areas) in Phase 1 (Appendix B Phase 1 Sheet C5 and C29). These structures consist of a series of vertically installed small logs, with interwoven branches. These structures are not included in the habitat analysis for Phase 2. Phase 1 locations and designs will be modified from the original design based on the observed performance of structures installed in Phase 1 and identification of areas where both the depth and velocity habitat criteria are met, but cover is lacking.

HYDRAULIC ANALYSIS

A steady-state 2-dimensional hydraulic model was developed using the U.S. Army Corps of Engineers’ (COE) HEC-RAS River Analysis System Version 6.1 (COE, 2021). The model was calibrated using measured water surface elevations at Bridge 3 at four flows between 5 cfs (low winter habitat flows) and 789 cfs (5–10-year event). Winter habitat for juvenile salmonids was estimated for flows (5 cfs from 80 cfs). Inundation area and flow patterns were estimated for winter peak flows (Table 3). Details on model development and methods are provided in Appendix C.

Table 3. Summary of flood flows modeled in the hydraulic analysis.

Return Interval (yr)

Chance Exceedance (%) Bridge 3 (cfs) Bridge 2 (cfs) Bridge 1 (cfs)

2 50 397 401 426

5 20 699 707 750

10 10 883 892 947

25 4 1,091 1,103 1,170

50 2 1,242 1,255 1,331

100 1 1,398 1,413 1,498

200 0.5 1,543 1,559 1,652

500 0.2 1,741 1,759 1,864

Floodplain Connectivity <in progress>

Low elevation floodplains and alcoves are infrequent in MWNM. The few low elevation floodplains that exist are typically one channel width wide and were likely created by the formation of large wood jams that caused localized channel widening. Existing low elevation floodplains occur in Areas 7 and 6.

In Phase 2, the design features to increase floodplain connectivity to the existing low elevation floodplains include a combination of selectively lowering floodplains, raising riffle crest elevations, and strategically adding large wood to locally increase water surface elevations adjacent to the low elevation floodplains.

Specific changes to inundation of these features are discussed in the “Design Areas” section.

Velocity

Figure 1. Velocity shift between existing (EG) and design (DG) conditions at winter habitat flows (5-80 cfs) and the 100-year flood.

100-year Flow <in progress>

The designs increase the water surface elevations at the 100-year flow (Figure 2) resulting in shallow, but broad inundation across the floodplain at the 100-year flow (Figure 3). Velocity decreases in the central portion of the main channel and increases along the channel margins (Figure 4).

The increase in water surface elevation Area 5 primarily occurs upstream of log L1012 with WSE increases of 1-2 feet (Figure 5).

Figure 2. Difference in water surface elevation at 100-year flow.

Figure 3. Change in inundation extent at the 100-year flow for existing and design conditions for Area 5 (top panel) and wood placement at Creek STA 37+30 (bottom panel) (placeholder figure).

Figure 4. Difference in velocity at the 100-year flow.

An increase in water surface elevation occurs in Area 7 due to raising of the channel bed and addition of wood (Figure 5). Overbank flows under existing conditions occur at >100-year flow and under design conditions occur ~25-year flow. The increase of riffle crests of 1 ft results in an increase in water surface elevation of ~0.5 ft at the 100-year flow. The addition of wood increases water levels an additional 1-1.5 feet in Area 7. The inundation extent transitions from being confined to the channel, to shallow inundation within the new, expanded riparian area (Figure 6). This change represents a condition associated with a less incised channel system. Shifts in velocity patterns at the 100-year flow are largely decreases in velocity along the main flow path, with increases in velocity along the channel margins (Figure 7).

Figure 5. Difference in water surface elevation in Area 7 at the 100-year flow.

Figure 6. Difference in inundation extent in Area 7 at the 100-year flow.

Figure 7. Difference in velocity in Area 7 at the 100-year flow.

HABITAT MAPPING AND ANALYSIS

Habitat mapping and analyses focus on juvenile salmonid summer and winter rearing habitat. Summer habitat was mapped in 2019 and 2020. Winter habitat was mapped on January 31, 2022. The provisional discharge at USGS Gage 11460151 (Redwood Creek at Hwy 1 Bridge at Muir Beach, CA) was 4.2 cfs during the winter habitat survey. Model estimates of winter rearing habitat are estimated for existing and design conditions between 5 cfs and 80 cfs at Bridge 3 in MWNM.

Winter Habitat

Winter habitat for juvenile salmonids is defined as areas which have depths greater than 1.5 feet, velocities less than 1 foot/second and are within 3.3 feet of cover. Cover types include roots, large wood, flooded brush, rocks greater than 0.33 ft (10 cm), undercut, w/roots, overhead cover, emergent marsh, and water depth greater than 4 ft.

Habitat estimates predicted by the 2-dimensional hydraulic model have some important limitations. All wood is represented in the terrain topography, which creates a complete flow obstruction in the model.

Flow cannot pass under the wood due to the inherent limitations of a 2-dimensional model. Therefore, no habitat is predicted under and within complex rootwads/logs that are in pools. The approach also creates a larger obstruction for logs that are ramped down from the stream bank. Given these caveats on the potential errors of the habitat area magnitudes, the same modeling approach was used for existing and design;

therefore, the percent increase in habitat should be a reasonable estimate. In addition, the predicted habitat area results at 5 cfs compared reasonably well field mapping of habitat at 4.1 cfs.

Existing winter habitat areas are limited in the project reach. During low winter habitat flows (5 cfs), velocities are low, but flow depth is generally below the habitat criteria except in pools. As flow depth increases to within habitat criteria range, velocity begins to exceed the habitat criteria. Cover is low at all flows. In most areas, the amount of existing habitat diminishes as flows increases (Table 4).

The design substantially increases the winter habitat in all work areas and in the Phase 2 project area (Table 4). Area 7 has the largest increase in habitat area due to the increase in key pieces. Area 6 has the second highest habitat area per unit length, followed by Area 5.

Table 4. Juvenile salmonid winter habitat area estimated for existing and design conditions.

Habitat Type Flow

Location

Project Reach

(Cascade to Bridge 3)

Area 7 Area 6 Area 5 Between Areas

Length (ft) 2780 430 430 350 1570

Winter Habitat

5 cfs

EG Habitat Area (sq ft) 1059 79 333 442

EG Habitat Per Unit Length (ft)

0.4 0.2 0.8 1.3 0.1

DG Habitat (sq ft) 3427 1311 1223 588

DG Habitat Per Unit Length (ft)

1.2 3.0 2.8 1.7 0.2

Increase 223% 1562% 268% 33% 48%

34 cfs

EG Habitat Area (sq ft) 937 133 194 166

EG Habitat Per Unit Length (ft)

0.3 0.3 0.5 0.5 0.3

DG Habitat (sq ft) 4646 1747 1124 458

DG Habitat Per Unit Length (ft)

1.7 4.1 2.6 1.3 0.8

Increase 396% 1218% 478% 177% 196%

80 cfs

EG Habitat Area (sq ft) 647 59 106 17

EG Habitat Per Unit Length (ft)

0.2 0.1 0.2 <0.1 0.3

DG Habitat (sq ft) 3671 1589 657 170

DG Habitat Per Unit Length (ft)

1.3 3.7 1.5 0.5 0.8

Increase 467% 2575% 522% 902% 170%

Summer Habitat

Pool frequency is used as a measure of summer habitat. Deep pools are defined by NPS as having depths greater than 1.5 feet. CD’s increase pool frequency the most in Area 7 (Plaza), which has no pools greater than 1.5 feet. Four pools are constructed in association added wood. One new pool is excavated in Area 6 and the deep pools at the well-developed meander bends are retained and expanded (Appendix A Sheet C4.

In Area 5, one large pool is excavated in associated with the large log (L1012) (Appendix A Sheet C7).

Pools are not excavated in association with other placed wood in Area 5 because a pool is expected to form on the outside of the meander bend where riprap is removed.

Table 5. Pool frequency in the Phase 2 project area.

Location Deep Pool Frequency (#/100 meters)

Existing Design Total Phase 2 1.8 2.1 Area 7 (Plaza) 0.0 3.1

Area 6 (between Bridge 1 and 2) 1.4 2.1

Area 5 (between Bridge 2 and 3) 2.8 3.7

Between Main Work Areas 2.1 1.5

VEGETATION

Vegetation will remain undisturbed except within grading boundaries and access routes identified on the plans (Appendix A), but will be fully revegetated by NPS after the work The extent of riprap buried in the bank is uncertain; therefore, save trees are noted when they occur close to an estimated grading boundary or are rooted within a riprap section where riprap removal will be selective to protect the tree (Appendix A Sheet C2, C4, C7). If large roots from any tree not specifically identified for removal is encountered, grading will be modified to protect the tree roots. Trees will only be removed if they are specifically identified on the plans. Within grading boundaries, where vegetation impacts cannot be avoided, NPS will salvage and replant vegetation. A salvage plant list and locations are provided in Appendix A Sheet C6 Table 7.

Expanded vegetation areas occur within Area 7 and are discussed within that section.

DESIGN AREAS

AREA 7

Existing Area 7 is adjacent to the Entrance Plaza. The reach is 430 feet in length and located at Project STA 20+90 to 25+20. The creek is approximately 30 feet wide and flows from northwest to southeast through the valley floor which is roughly 210 feet wide. The river left side of the valley/floodplain (~130 feet wide for the top of bank) is occupied by NPS infrastructure including Entrance Plaza, visitor center, bathrooms, parking, and utilities. This portion of the valley/floodplain is about 6 feet above the channel bed and is only inundated by the creek when during flows greater than the 100-year flow with localized inundation just downstream of the middle rock check dam. Downstream of the bathrooms, flows above the 500-year flow are contained within the channel. The right side of the valley is ~50 feet from the top of the channel for the upper 200 feet of the reach and is free from infrastructure except for an abandoned leach field and pipe. The valley floor elevations are similar on both sides of the channel, apart from a lower elevation floodplain that is inundated at flows larger than the 2-5-yr flow. Downstream of the low elevation floodplain, the creek flows against the valley wall for the final 230 feet of the reach and exposes bedrock occurs in select locations (Appendix A Sheet C2).

Area 7 channel is located between two boulder grade control structures that were installed in 1930’s in response to channel incision. The upstream structure, Middle Rock Check Dam is built as a boulder drop structure and appears to be keyed into the adjacent riprap. The riprap extends upstream to Bridge 1 and ~100 feet downstream on river right and ~ 190 feet downstream of river left. The downstream boulder structure is referred to as the “Cascade”. The channel between the boulder grade controls is generally a straight and plane bed, except for three shallow pools in the upper 170 feet of the reach

Vegetation in the reach includes alder, bay, maple, redwood, elk clover, a variety of ferns, blackberry, and carex. The vegetation on river left is younger and sparser than in other areas of MWNM. Much of its floodplain was part of a parking lot until NPS pulled back the edge of the parking lot in the early 2000’s and later converted the area into a pedestrian plaza. The river right floodplain is more densely vegetated with a wider diversity of vegetation. Where riprap occurs, vegetation on the stream bank is limited.

NPS has worked to reduce infrastructure that constrains the project. The sewer line at the top of the bank was rerouted away from the channel in 2020 and plugged and grouted. A portion of the former parking area was removed and converted to a riparian area. Additional infrastructure modifications that are being planned as part of the Sustainable Access Project include relocations of the bathrooms away from the channel, and removal of the pay phone.

Design

Riprap

Portions of riprap sections on the left and right banks will be removed downstream of the Middle Rock Check Dam. R1 is removed about 30 feet downstream of the Middle Rock Check Dam (1 channel width) and L1 is removed about 68 feet downstream of the Middle Rock Check Dam. Upstream, a longer section of riprap is retained on river left to protect the sewer line. The risk of damage to the sewer line resulting from this project is low, given the length of riprap that has been retained and the distance from the channel.

As an additional layer of protection, rock will be buried within the bank, behind the remaining section of L1 (Appendix A C2). This rock is intended to slow bank erosion toward the sewer line in the event of a large, future, channel adjustment. However, this rock does not provide engineered protection of the sewer line or plaza area. Engineered protection of the sewer line and plaza area within the footprint of this project would require more material than available and create additional impacts to the existing natural resources.

On river right, riprap will be carefully removed, and the bank will remain in an as-is condition. This approach is preferred in in this area to retain the existing root strength and minimize vegetation disturbance on the right bank. The bank may be close to vertical or slightly undercut after rock is removed. The bank will be opportunistically planted where feasible. Rock will be similarly removed on river left, but portions of the left bank and floodplain will be graded to create a secondary channel and alcove (see section Channel Bed for more information). The soil from the bank grading will be either stockpiled at Conlon (Appendix A Sheet G3).

Channel Bed

Channel bed modifications are intended to increase summer and winter rearing habitat. Actions include excavation of pools, raising the riffle crest elevation, constructing a flow split, and confinement of the low flow channel (Appendix A, Sheet C2). Four new pools will be excavated in Area 7. Material from these pools will be used to raise the riffle crest elevation, build bars in the plane bed channel, and confine the low flow channel to ~7 feet wide.

The riffle crest elevation will be raised by 1 foot at STA 196 (Appendix A Sheet C2 and C8). This location was selected to increase the frequency of inundation of the existing low elevation floodplain in river right.

The riffle crest will be reinforced with a channel spanning wood jam. Logs that span the channel will be installed at an angle to maintain flow depths at summer low flows.

The channel is widened from STA 250 to 350 to create a flow split that will be reinforced with large wood.

The main channel will remain in the existing alignment (river right) and a secondary channel will be created on river left. The portion of the widened channel that was formerly floodplain, will be over-excavated by 6 inches and backfilled with bed material. The secondary channel will be connected to the main channel during the winter base flows by an alcove at the downstream end of the secondary channel. Water will enter the upstream end of the secondary channel at flows greater than 5 cfs.

Wood Placement

Large and small wood will be strategically added to the channel to improve cover, promote bar and pool formation, increase low velocity refugia, and reinforce increases in the channel bed grade. Seven logs in the channel will be reconfigured and combined with 23 imported logs to create additional habitat. Imported wood will have a typical diameter of 2 feet and length of 30 feet. Fifteen of the imported logs will have rootballs.

Five wood placement sites are designed (Appendix A Sheet C2 and W1). The upstream site (STA 320, Appendix A Sheet W7) includes logs with rootballs in pools to promote pool scour and add habitat, while the trunks reinforce the channel thalweg elevations. A wood jam is designed at the head of a mid-channel bar to encourage a flow split. Alders removed as part of the riprap removal are used as a whole tree to add roughness to the secondary channel in the flow split. The site at STA 230 (Appendix A Sheet W6) is located at the downstream end of the flow split and intended to promote pool scour and expand winter rearing habitat and utilizes a cluster of alders to provide additional stability to the installed wood. The site at STA 190 is designed to reinforce a raised riffle crest (Appendix A Sheet W5). The two downstream placement sites (STA 90 Appendix A Sheet W4) are intended to promote pool scour and provide winter habitat by the creation of low velocity both in the pool and on the adjacent bar.

Infrastructure

Infrastructure modifications include the removal of the AT&T cable and conduit and removal of the abandoned section of plugged and grouted sewer line.

Vegetation

The riparian corridor will be expanded on river left along 288 linear feet of channel with an average width of 30 feet. The area planned for expansion includes a portion of the existing riparian area where a parking lot was recently removed and extends into the footprint of the existing entrance plaza area and bathrooms.

This action will be completed as part of the Sustainable Access Project.

Expected Site Evolution The channel is expected to be converted from a plane bed channel to a forced bar-pool morphology due to the addition of wood. Constructed bars and pools will evolve in response to the sediment supply from upstream, local hydraulics, persistence of placed wood, and accumulation of new wood. Where feasible, the added wood will be positioned between live trees, embedded in the stream channel, and secured with pinning logs to increase the stability of individual pieces; however, all placed wood will remain mobile during large floods, similar to naturally delivered wood. Future wood sources to the channel that may replace mobilized wood include tree fall from the adjacent floodplains and hillslopes and mobile wood transported from upstream reaches. Since this area is the most downstream area, it will continue to benefit as wood moves through the system. The new riparian area on river left will also serve as a future source of wood to the channel (Appendix A Sheet C2).

The widened channel area (STA 250-350, Appendix A Sheet C2) is expected to be a depositional area. The constructed main channel and secondary channel may grow or shrink depending on local depositional patterns and the accumulation or loss of wood and establishment of vegetation.

AREA 6

Existing Area 6 is located between Bridge 1 and 2. The reach is 430 feet in length and located at Project STA 29+80 to 34+10. The creek is approximately 30 feet wide and flows from northwest to southeast through the valley floor which is roughly 260 feet wide and supports an old growth redwood forest. This portion of the valley/floodplain is about 4.5 feet above the channel bed and the most frequently inundated portion of the floodplain. The creek overtops its banks at flows greater than 2-5-yr flow. The valley floor elevations on both sides of the channel are similar, apart from a low elevation floodplain that is inundated at flows larger than ~80 cfs flow (<2-year flow). During the summer of 2021, the riffle near Bridge 1.5 (STA 100, Appendix A Sheet C4) was dry and pools were stagnant. While disconnected flow is a common occurrence in downstream reaches of Redwood Creek, this was the first time it was documented in MWNM.

Trails occur on both sides of the channel. The trail on river left is about 50 feet from the top of bank. The trail on river right was recently abandoned and rerouted further from the channel to the edge of the valley.

The asphalt trail, base rock, and small bridge that crosses a wet area (Bridge 1.5) remains near the top of bank.

A small, intermittent tributary enters Area 6 near the downstream end of the project area. The tributary appears to spread across the valley floor as overland flow. Recent trail work defined one of the possible flow paths on the valley floor with straw wattles that are ~3 feet wide and <6 inches deep. The defined flow path is directed to an 8” diameter ceramic culvert that transitions to a 12” diameter culvert on the downstream side. The culvert enters the channel at STA 40 (Appendix A Sheet C4). The position where the culvert passes under the trail is slightly raised.

Area 6 channel is located downstream of a log grade control structure that was installed in 1930’s in response to channel incision (NHE, 2019). The log grade control has been the location of a large, complex wood jam that has been losing accumulated wood in recent years. Flows have cut under the grade control log, and it is not uncommon for log to be dry during summer low flow. The channel in Area 6 is has a gentle curvature downstream of the log grade control, followed two tight meander bends. Pools occur at the log grade control and at the two meander bends. The log grade control is secured by riprap on river right. No riprap is visible on river left at the log grade control. Riprap is nearly contiguous on river right through the reach. Riprap between R3 and Bridge 1.5 was identified by probing the ground surface, due to limited visibility. A 130-foot section of riprap occurs on river left (L2) on the outside of the meander bend.

The vegetation in Area 6 has a diverse mix of vegetation including alder, bay, redwood, elk clover, azalea, hazelnut, a variety of ferns, blackberry, and carex in addition to old-growth redwood trees. Where riprap occurs, vegetation on the stream bank is limited.

Summer and winter rearing habitat is limited to the two pools on the meander bends. The pools are 2.6 feet and 3.2 feet. Winter habitat totaled 272 square feet.

Design

Riprap

The majority of riprap will be removed in Area 6 to restore natural channel processes (Appendix A Sheet C4). The downstream segment of R2 will be retained to protect the adjacent trail and limit disturbance to an existing deep pool. Short segments of R3 will remain in place to protect selected trees. A rock key-in will be constructed at the upstream end of remaining riprap at R2. Following careful removal of riprap at R3, R3A, and L2, the bank will remain in an as-is condition to retain the existing root strength and minimize vegetation disturbance. The bank may be close to vertical or slightly undercut after rock is removed. The bank will be opportunistically planted where feasible. The bank at R3 will be graded following riprap removal to a slope of 1.5H:1V (Appendix A Sheet C12). This grading will ensure a smoother transition between the soil bank and remaining riprap bank. The soil from the bank grading will used on the floodplain for trail decommissioning. If large tree roots are encountered during grading, grading will cease, and the bank will be left in a similar condition to other banks.

Riprap removed from the channel banks will be buried at least 3 feet below the average bed elevation of the channel, as occurred in Phase 1 of implementation (Appendix A Sheet C17 and C18). Given the typical rock size is 2 feet, a 5-foot minimum excavation depth is required to bury the rock. Tile probes were insert into the bed at two locations near R2 and R3 to a depth of 5 feet to confirm that the necessary excavation depths were achievable. Construction techniques will be the same as those described in Phase 1 (NHE, 2018) apart from an added specification to ensure any deposits of silt or clay that are encountered is buried at least 2 feet below the bed surface. This burial requirement will aid in the recovery of the invertebrate population.

Infrastructure

A portion of the asphalt trail that is adjacent to the creek on river right was abandoned in 2020 and a new boardwalk was constructed along the right valley wall. The asphalt trail, base rock, culvert, and a small wooden bridge (Bridge 1.5) and associated abutments and rock will be removed. The trail footprint will be decompacted and soil generated from bank grading will be used to backfill the area to the adjacent floodplain grade and blend with the surrounding topography. In areas where trail fill forms the right bank of the channel, the floodplain will be left at the elevation of the native floodplain material allowing for more frequent inundation of these low elevation floodplains. The rehabilitated trail area will be covered with redwood duff and revegetated, similar to the Cathedral Grove trail segment removal in Phase 1.

Channel Bed

Channel bed modifications are intended to increase summer and winter rearing habitat. Actions include excavation of pools, raising the riffle crest elevation, and constructing a flow split and alcove, and lowering a floodplain (Appendix A Sheet C4). Since the reach is already sinuous with two well developed pools at the meander bends, only one new pool will be excavated adjacent to R3 riprap removal where channel curvature is expected to increase, and large wood will be added. The upstream portion of the pool adjacent to the portion of R2 that is removed will be deepened. Material from pool excavation will be used to raise the riffle crest elevation and enhance the bar in the upstream section of Area 6.

The riffle crest elevation will be raised by 1 foot at STA 264 (Appendix A Sheet C9). This location was selected to increase the frequency of inundation of the existing low elevation floodplain in river left. The riffle crest will be reinforced with a channel spanning wood jam. Logs that span the channel will be installed at an angle to maintain flow depths at summer low flows (Appendix A Sheet W11).

Riprap removal at R3B will create a void on the inside bend of the first meander bend. This area will be regraded to create a secondary channel (Appendix A Sheet C4). The flow split that will be reinforced with a wood jam. The main channel will remain in the existing alignment (river left). The new secondary channel will be connected to the main channel during the winter base flows by an alcove at the downstream end of the secondary channel. Water will enter the upstream end of the secondary channel when flows are between 5 and 34 cfs.

The river left floodplain will be lowered to increase connectivity during lower winter flows. The excavated portion of the floodplain will be connected at ~5 cfs and the entire low elevation floodplain is inundated by the 2-year flow.

Wood Placement

Since large wood cannot be imported to this work area, large wood can only be sourced from existing channel and the adjacent floodplain. Four logs from the channel and thirteen logs from the floodplain will be configured into five wood jams (Appendix A Sheet C4). All wood jams include small woody debris (not shown on drawings) to increase cover and increase jam complexity. These jams are strategically added to provide cover, promote bar and pool formation, and reinforce riffle crest elevations.

Wood placements STA 300, 130, 50 (Appendix A Sheets W12, W9, W8) are intended to promote pool scour and provide cover in existing and constructed pools. The largest wood pieces are used in the upstream pool where the channel lacks sufficient channel curvature to create a deep meander bend pool. Wood jam STA 210 (Appendix A Sheet W11) is intended to reinforce the raised riffle crest elevation and wood jam STA 170 (Appendix A Sheet W10) is intended to reinforce the flow split.

Tributary

Although the tributary may follow multiple flow paths through relatively unconfined flow, recent trail work defined a flow path that directs the flow to a culvert that will be removed as part of the trail decommissioning. The culvert and trail may have acted as grade control, inhibiting incision of the channel.

To retain this potential function, a small grade control feature will be installed where the tributary intersects the decommissioned trail (Appendix A Sheet C4).

Vegetation

Several save trees have been identified within the project area. Save trees located along R3 will result in patches of riprap that are retained within the roots of the trees.

Expected Site Evolution Removal of riprap at R3 will allow for increased channel curvature and pool scour. While some patches of riprap will be retained in association with save trees, the channel will have the capability of cutting around these patches and will not likely limit the future channel complexity. Riprap removal at R3 and meander bend development may contribute to future destabilization of the riprap adjacent to the log grade control, and eventually the grade control log.

Partial riprap removal at R2 is intended to enhance the existing meander bend and promote deeper pool scour on the outside of the meander bend. The key-in installed at the upstream end of the remaining segment of R2 is intended to increase the longevity of R2 as the upstream channel adjusts. The R2 riprap segment protects the portion of the trail that remains close to the channel. The remaining segment of R2 may fail by other mechanisms because the riprap section is not keyed into the bed.

The removal of the upstream portion of R2 and R3A and the creation of a secondary channel will create a highly dynamic zone. The constructed main channel and secondary channel may grow or shrink depending on local depositional patterns and the accumulation or loss of wood and establishment of vegetation.

The raised riffle crest elevation is intended to increase the frequency of inundation of the upstream floodplain; however, it is not engineered to remain at this elevation. Wood is added to the stream bed to reinforce the riffle and a deposition area is designed downstream, but the riffle elevation may not persist at this elevation due to the local steepening of the water surface elevation.

The area of the raised riffle crest and the flow split have been documented to have disconnected flow during the summer. The designs will not reduce the risk of flow disconnection and may extend the duration of the low flow disconnection. This potential impact has been deemed acceptable given the increased winter habitat within the reach.

AREA 5

Existing Area 5 is located between Bridge 2 and 3. The reach is 350 feet in length and located at Project STA 43+30 to 46+80. The creek is approximately 30 feet wide and flows from north to south along the east side of the 130 ft wide valley floor. This portion of the valley/floodplain is about 6 feet above the channel bed. The creek overtops its banks at flows greater than 25-year flow. The valley floor elevations on both sides of the channel are similar.

Asphalt trails are built on the floodplain on both sides of the channel. The trail on river left occupies the area between the channel and the valley wall and is within 5 feet of the top of channel in some locations.

The trail on river right is approximately 38-75 feet from the top of bank. These trails are considered critical to the accessible trail network. No options were identified to move these trails further from channel due to the relatively narrow valley floor in this location and steep adjacent hillslopes.

Area 5 is located between two well developed meander bends with good pool development. While the meander bends upstream and downstream of the reach have a relatively tight curvature, the channel in between is relatively straight with a gentle meander bend that has failed to develop additional sinuosity due to riprap (R6).

Three sections of riprap occur in the project reach. Riprap occurs on river right at the gentle bend (R6) and on river left on the outside of the downstream meander bend (L5). A concrete abutment and a small section of riprap associated with the abutment (L6) occurs on the left bank, across from R6.

A log grade control structure was installed in 1930’s in response to channel incision at the downstream end of the reach. The log grade control is secured by riprap on river left (L5). No riprap is visible on river right at the log grade control.

Tree fall from the left valley wall delivered large and small woody debris to this area in WY2017. A permeable wood jam was formed at STA 50 (Appendix A, Sheet C7); however, most of the wood in the channel was transported downstream during subsequent high flows. A large log (L1012) that did not fall into the channel has been staged for placement at the top of the bank along with several other logs since

WY2017.

Summer and winter habitat is limited in Area 5. Summer habitat features included three pools ranging from 1.9-3.6 feet deep. The deepest pool is located on the outside of the upstream meander bend. Winter habitat was limited in Area 5 to four discreet locations totaling 392 square feet and was associated with undercut banks, rocks, and wood. More than half of the winter habitat was associated with pool in the upstream meander bend.

Design The project actions in this area are intended to increase channel complexity and improve salmonid rearing habitat by removing riprap to allow for increased channel curvature on the outside of the bend and increase pool scour at the meander bend, increasing vegetation on the channel bank, and adding wood to provide additional cover and promote more bar and pool formation in the otherwise plane bed channel.

Riprap

Riprap removal will occur on river right (R6) where the trail system is more distant from the channel. Two different treatments are applied to the bank following riprap removal. Riprap will be carefully removed from the upstream section (Appendix A Sheet C7) and the bank will remain in an as-is condition. This approach is preferred in areas where the top of bank is vegetated to retain the existing root strength and minimize vegetation disturbance. The bank may be close to vertical or slightly undercut after rock is removed. The bank will be opportunistically planted where feasible. The downstream section of bank will be graded to a 1.5H:1V slope and revegetated. This portion of the bank has less vegetation and taller banks that will benefit from more intensive planting. The soil from the bank grading will be hauled to Area 6 and used as part of the trail decommissioning. If large tree roots are encountered during grading, grading will cease, and the bank will be left in a similar condition to the upstream section.

Riprap on river left (L6 and L5) will be retained and L5 will be extended to reduce the risk of erosion of the adjacent trail. Due to material and site constraints, the section is not designed to meet all engineering standards for riprap design. The new riprap section repurposes the riprap from river right (R6).