RussianRiverRd_RetainingWallMemo 12-10-2019.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

RUSSIAN RIVER CAMPGROUND ACCESS, PHASE II

HYDRAULICS WALL WATERWAY MEMO

To: Reuben Johnson, WFLHD Project Manager From: Matthew Dillin, P.E., WFLHD Hydraulics Engineer Date: December 10, 2019 Project: AK_FS_CNF_1015(2) – Russian River Campground Access, Phase 2

Purpose

The scope of the Phase 2 project is to reconstruct approximately 0.9 miles of the Russian River Campground Access Road, located in the Chugach National Forest. The Russian River Campground Access Road is a paved two-lane, two-mile loop road. The roadway begins at Milepost 52.5 on the Sterling Highway and terminates at a small day use parking area (Grayling Parking Area). The roadway is important to the Chugach National Forest as it provides campground access and has become a popular access point for sport fishing on the Kenai and Russian Rivers. Western Federal Lands Highway Division is providing the full PS&E package with Environmental Documentation and Construction Engineering for contract work. The roadway will be owned and maintained by the National Forest Service.

Background

In 1930s the Sterling Highway crossed the Kenai River at Schooner Bend with a two lane covered bridge.

In 1955 the covered bridge was replaced with the current bridge crossing, approximately 800-feet upstream.

In 1966 the existing access to the Russian River Campground was constructed. During the construction of the access road, Bin walls were installed adjacent to the Kenai River along the outside of Schooner Bend.

In 1988 the access road and loops were paved. Gabion walls were installed near the bin walls sometime prior to 1989, likely to correct edge settlement of the roadway through the slide area. In 1989 a permit was obtained to place riprap in the river below the existing bin wall. It is assumed that this was to protect the bin wall from active scour. According to permit documents, the armor rock that was installed was 30-inches to 60-inches in diameter (Figure 2).

Recently the Russian River Access Roadway has had a series of failures due to an active landslide near Schooner Bend on the Kenai River, located roughly 1,500-feet south from Sterling Highway (see Figure 1 for Site Location). The roadway reconstruction will include roadway stabilization and widening to reduce maintenance and allow for safer pedestrian passage. In order to stabilize the slide and provide safer access for the visiting public, a wall is proposed near the active landslide. Due to the constraints of the site, the location of the wall will need to be located adjacent to the Kenai River along Schooners Bend. The purpose of this report is the summarize the hydraulic analysis that was done to quantify the impacts of the wall on the existing channel and to provide recommendations for protecting the proposed wall from scour.

Stream Conditions

The Kenai River is located in a moderately steep and moderately confined valley. From the streams origin at Kenai Lake through the project reach the channel is single thread meandering channel with an

Memo to: Reuben Johnson, WFLHD Project Manager December 10, 2019 approximate sinuosity of 1.25. The valley slope through the project reach is approximately 0.0030 ft/ft. The proposed roadway encroachment will be located on the outside of a sharp bend, known as Schooner Bend.

Active channel width (ordinary-high-water) is approximately 200-feet. Active channel depth (ordinary-high-water) is roughly estimated to be 12-feet. The 100-year flood event depth is approximately 20-feet.

The adjacent floodplain is well vegetated with grasses, shrubs, and evergreens. Pool-riffle is the dominant bed form. Based on visual observation, sand, gravel, cobble, and small boulders comprise the streambed material. Based on the available woody debris in the system, bank-side vegetation, and expected flood flow depths, the stream can carry a large amount of large woody debris.

The stream banks appear moderately stable. Based on Goggle Earth imagery, the channel has not migrated appreciably over the past decade. At the project site the channel travels through a tight bend known as Schooner Bend where the river flows along the toe of the Russian River Road embankment. The roadway has been in place since 1966 and repairs along the roadway bin wall were required in 1989. Since that time no repairs of the embankment have been recorded. No erosion was observed on the surface of the embankment; however, no bathymetric data or cross-section survey was collected in order to monitor the embankment under the water surface. Cross-section data from 2001 to 2017 was available from bridge inspection reports for the existing Sterling Highway crossing just upstream of the project. According to the cross-section data, no appreciable long-term aggradation or degradation has occurred within the river over this time period.

Kenai River Hydrology

Flows within the Kenai River originate from the runoff of glaciers and icefields within steep mountainous watershed that surrounds Kenai Lake. On the east end of Kenai Lake, near the community of Cooper Landing, the lake narrows to form the Kenai River. The project site is located a Schooners Bend, several miles downstream of its formation. Development within the watershed is sparse and most of the land mass is Federally owned wilderness and forest area. Peak flows near the project site were estimated using peak-discharge regression equations and the statistical analysis of USGS stream gage station data. The values from these analyses were then compared with published values from Methodology and Estimates of Scour at Selected Bridge Sites in Alaska (WRIR 00-4151).

USGS stream gage station data was available for Kenai River roughly several miles upstream from the project (#15258000, Kenai at Cooper Landing, AK). The basin size for the Cooper Landing gage is 643 square miles, accounting for roughly 85% of the total basin area tributary to the project location. The annual peak flows for the gage were analyzed using the PeakFQ program. PeakFQ performs an annual flood-frequency analysis using the Bulletin 17B guidelines. In addition to the gage data analysis, peak discharges were estimated using the USGS regression equations. The USGS StreamStats (Version 3) web-based application was used for delineating the watershed, calculating a drainage area (759 square miles), mean annual precipitation (69 inches), and peak flow rates.

The flood-frequency analysis of USGS stream gage data identified a 100-year peak discharge of roughly 25,400 cfs, the USGS regression analysis identified a composite 100-year peak discharge of 37,200 cfs.

Both of these values were utilized to compute a weighted flow estimate following the procedures outlined in the published 2016 USGS report Estimating Flood Magnitude and Frequency at Gaged and Ungaged Sites on Streams in Alaska and Conterminous Basins in Canada, Based on Data Through Water Year 2012.

A weighted flow estimate of 30,000 cfs was computed for the 100-year event at the project site. Flow estimates were then compared with the 2001 published USGS report Methodology and Estimates of Scour at Selected Bridge Sites in Alaska (WRIR 00-4151). This document used a multiple regression analysis where both the gage data analysis and regression flood estimates were combined using a logarithmic-weighting factor. Within this document the 100-year peak discharge at Schooners Bend was estimated to be 36,900 cfs. In the FEMA Flood Insurance Rate Study (FIS) flows in the Kenai River at Juneau Creek, roughly 3 miles upstream from the project, were estimated as 30,200 cfs.

For the hydraulic floodplain and stream stability analyses, the more conservative values published in the WRIR 00-4151 were utilized for the 100-year return event. For analyses of all other return events, the weighted flow estimates computed from the gage data and regression analysis were utilized. A summary of flows for each hydrologic analysis is available in Figure 5.

Mapping

LiDar mapping for the project was provided by the Chugach National Forest. The LiDar mapping was supplemented with some additional ground survey within the project limits collected by Western Federal Lands. Due to cost and time constraints, no bathymetric survey or cross-sectional survey was collected within the Kenai River. Cross-sectional data from bridge inspection reports was available for the Sterling Highway crossing (Bridge 0673) located roughly 1,500-feet upstream of the project site. For the purposes of modeling the channel hydraulics, this cross-section data was used to approximate the channel bottom through the project reach.

Hydraulic Model

A two-dimensional, non-uniform, steady-state hydraulic model was created using SRH2D with Aquaveo SMS Version 13.0.3. The model was used for estimating flow depths, flow velocities, and water surface elevations within the Kenai River along the project reach. Manning’s n values of 0.02, 0.035, 0.04, and

0.06 were utilized for the roadway, main channel, sand bar, and overbank areas, respectively. The model used a three second time-step and a total run time of three hours, allowing the model to reach a steady-state condition. Both the upstream and downstream boundary conditions were located over a mile from the project site. The upstream boundary condition was modeled as a constant discharge, conveyed across the main channel. The downstream boundary condition was modeled as a constant water surface elevation, corresponding to the normal water surface depth. The model was run using mapping corresponding to the existing LiDar surface data with an approximated low-flow channel based off of the bridge inspection sounding data (Figure 12). The model mesh generally consists of equilateral triangles with 20-foot lengths for each side (Figure 6). For the floodplain and channel stability analyses the 100-year event was modeled.

Model results for flow depths, flow velocities, and water surface elevations are presented in Figures 7, 8, and 9 respectively.

Kenai River Floodplain

The site is adjacent to a Federal Emergency Management Agency (FEMA) special flood hazard area Zone A, (FIRM 020012 2125 A, Figure 4). A detailed FEMA mapping study has not been completed for the site and no base flood elevations have been determined. Floods near the project site can occur from snowmelt and heavy precipitation. Additionally, flooding can also occur from the sudden release of a glacier-dammed lake (jokulhlaup) and the ice jams. Due to the sporadic nature of the ice jam formations, these flooding sources are not considered in the development of the Flood Insurance Rate Maps. For the purpose of estimating flooding at the project site, flows published in the WRIR 00-4151 document were used to model the 100-year return event.

Based on the hydraulic modeling of the existing conditions, it is anticipated that the proposed roadway improvements will be outside of the 100-year base flood elevations. If improvements are required within the floodplain, they will only impact the outer fringe of the floodplain, along the southern bank of the Kenai River. This impact is not anticipated to significantly increase the base flood elevations within the Kenai

River and will not negatively impact insurable structures. See Figure 9 for the modeled floodplain limits and existing base flood elevations. As the design progresses, further modeling will be conducted as required to quantify the impacts of the improvements on the existing floodplain.

Embankment Erosion Protection

The riprap was sized for a 100-year event using the approach from USACE EM 1110-2-1601, June, 1994.

Computation were based on the two-dimensional hydraulic analysis results for the 100-year flood event. A stability factor of 0.3 was used for angular rock. Specific gravity of the stone was assumed to be 2.65. The slope of the existing colluvium near the channel toe of 2(h):1(v) was used to calculate the revetment bank angle. The recommended riprap size along the project roadway can be found in Figure 11.

The location of the largest calculated riprap corresponds closely with the permit application for riprap submitted in 1989. The size of the calculated D50 to D100 matches closely with the sizes proposed in the 1989 permit drawing. Additionally, the calculated riprap sizes upstream of the 1989 placement site matches field observations for areas along the outside of Schooner Bend. No active erosion to the outside of Schooner Bend was observed during the site visit in the fall of 2019. According to flow data from the Cooper Landing gage, the riprap placed in 1989 has seen several moderate floods corresponding to the 5-year to 10-year events. The largest flood event since 1989 occurred in 1995 with flows corresponding to a 25-year event.

Summary & Recommendations

Wall Alignment

• Where possible the layout of the proposed walls should be placed outside 100year water surface elevations (Figure 9).

• In general, disturbances to the existing riprap embankment should be minimized.

• Any disturbances to the existing embankment should be repaired using riprap sizing corresponding to the minimum calculated riprap sizing (Figure 11).

Embankment Erosion Protection

• Existing embankment material did not show any signs of erosion or scour above the water surface.

• Observed existing riprap sizing is commensurate with the proposed riprap sizing calculations.

• In general, if disturbances to the existing riprap embankment are minimized and the proposed walls are placed outside of the 100-year water surface elevation then the design should be to preserve the existing embankment and monitor over the long term.

• Any disturbances to existing bank armoring during construction shall be repaired in accordance with the riprap sizing presented in Figure 11. Material salvaged on-site may also be used to repair the disturbed embankment.

• No underwater investigation or survey was conducted to monitor the existing embankment under the water surface; therefore, issues under the surface could not be identified.

• Due to the location of the proposed wall, continuous monitoring will be required.

General Scour

• Given the available data, the channel appears stable through the project reach. No long term degradation or channel migration trends appear to be an imminent threat to the proposed embankment. However, the wall is located at the outside of a sharp bend were erosive forces within the channel will be high. It is recommended that the project owners continue to monitor the improvements following the maintenance guidance below.

Stream Impacts

• Existing flow conveyance, sediment transport, sediment deposition, and woody debris conveyance through the project reach are expected to be remain unchanged post project.

Floodplain and Flood-Rise Impacts

• It is highly recommended that walls be placed outside of the 100-year floodplain.

• If walls need to be placed within the 100-year floodplain due to site constraints, the impacts will need to be quantified with further analysis.

Construction Construction activities for the new wall and roadway fill will be conducted outside of the active channel;

therefore, no flow diversions or dewatering are expected to be required. Contractor should limit disturbances to the existing embankment. Any disturbances to existing embankment armoring during construction shall be repaired with the minimum calculated riprap size (Figure 11). Material salvaged on-site may be used to repair the areas of disturbed embankment in place of imported riprap. If off-site riprap material is imported for repairs, the contractor should be instructed to pull the conserved embankment material back over the placed riprap.

Operation and Maintenance Embankment should be inspected annually and after each extreme flood event. Replace or reposition dislodged stones. The presence of woody debris trapped upstream, beneath, and downstream of walls should be removed as needed for maintaining flow conveyance capacity and preventing logs deflecting flow towards embankment.

References

Curran, J.H., Barth, N.A., Veilleux, A.G., and Ourso, R.T., 2016, Estimating flood magnitude and frequency at gaged and ungaged sites on streams in Alaska and conterminous basins in Canada, based on data through water year 2012: U.S. Geological Survey Scientific Investigations Report 2016–5024, 47 p., http://dx.doi.org/10.3133/sir20165024.

Dowl Engineers, Hydrology Report Russian River Campground Access Road Mile Post 52.5 Sterling Highway Cooper Landing, AK (March 2006).

Federal Emergency Management Agency, Flood Insurance Study Number 02122CV001B, Kenai Peninsula Borough, Alaska and Incorporated Areas, Volumes 1 & 2. Revised October 20, 2016.

Federal Emergency Management Agency, Flood Insurance Rate Map 020012 2125 A, Kenai Peninsula Borough, Alaska and Incorporated Areas. Effective Date May 19, 1981.

HMH (December 6, 2017). Bridge Scour Evaluation, Bridge No. 0673.

Lim, Eric (August 16, 2019). Geotechnical Memo 25-19, Preliminary Geotechnical Recommendations Russian River Campground Phase II, AK FS CNF 1015(2) [Memorandum]. Vancouver, WA: Western Federal Lands.

U.S. Department of the Interior: U.S. Geological Survey (2001). Methodology and Estimates of Scour at Selected Bridge Sites in Alaska (WRIR 00-4151). Retrieved from http://ak.water.usgs.gov.

http://dx.doi.org/10.3133/sir20165024 http://ak.water.usgs.gov/

Attachments: Figure 1 – Site Location Figure 2 – 1989 Revetment Protection Typical Section Figure 3 – Kenai River Drainage Basin Figure 4 – FIRM Map Figure 5 – Hydrology Figure 6 – 2D Hydraulic Model Figure 7 – Flow Depth Figure 8 – Flow Velocity Figure 9 – 100-year Water Surface Elevations Figure 10 – Sterling Bridge Highway Bridge Cross-Sections Figure 11 – Riprap Protection Sizing Figure 12 – Existing Surface Elevations

Cc: Sven Leon, Hydraulics Team Lead

Project Site USGS Gage Locations

Map from Google Earth , image date 4/17/2021.

Approximate Roadway Reconstruction Limits

N

FIGURE 1

RUSSIAN RIVER ROAD

Site Location

0 2000 feet

Coopers Landing

Approximate Slide Location

Schooner Bend

Approximate location of Riprap Installation, 1989

Project Description "Discharge 1,700 cubic yards of rockfill below the ordinary high water mark along a 370 foot length of the Kenai River to install a revetment. 37 cubic yards of material will be dreged from a 5' wide by 5' high by 40' long trench in the river bed at the upstram end of the revetment and the trench will be filled with rock to construct supportive keyway."

"All work will be performed in accordance with the enclosed plans, 5 sheets dated 1-31-89"

US Army Engineer District, AK

Permit 1-890027 (Kenai River 227)

FIGURE 2

RUSSIAN RIVER ROAD

Revetment Protection Typical Section

Map from Google Earth, image date 3/1/2007.

N

FIGURE 3

RUSSIAN RIVER ROAD

Kenia River Drainage Basin

Basin Area = 759 sq mi

0 10 miles

Project Location

FEMA Online

FIGURE 4

FIRM

USGS Gage Station: 15258000 Stream Stats Weighted Estimate WRIR 00-4151 based on gage data and regression Scour at Selected Bridge Sites in AK

Annual Exceedence Probability

Return Period (years)

PeakFQ Bull.

17B (cfs)

Annual Exceedence Probability

Return Period (years)

StreamStats Regression

(cfs)

Annual Exceedence Probability

Return Period (years)

Discharge Table 2

(cfs)

Annual Exceedence Probability

Return Period (years)

Discharge Table 2

(cfs)

0.995 1.01 - 0.995 1.01 - 0.995 1.01 - 0.995 1.01 -

0.99 1.01 - 0.99 1.01 - 0.99 1.01 - 0.99 1.01 -

0.95 1.05 - 0.95 1.05 - 0.95 1.05 - 0.95 1.05 -

0.9 1.11 - 0.9 1.11 - 0.9 1.11 - 0.9 1.11 -

0.8 1.25 - 0.8 1.25 - 0.8 1.25 - 0.8 1.25 -

0.667 1.5 - 0.667 1.5 - 0.667 1.5 - 0.667 1.5 -

0.5 2 11080 0.5 2 18400 0.5 2 13828 0.5 2 -

0.4292 2.33 - 0.4292 2.33 - 0.4292 2.33 - 0.4292 2.33 -

0.2 5 14470 0.2 5 23300 0.2 5 17800 0.2 5 -

0.1 10 16880 0.1 10 26700 0.1 10 20606 0.1 10 -

0.04 25 20120 0.04 25 30900 0.04 25 24267 0.04 25 -

0.02 50 22680 0.02 50 34000 0.02 50 27098 0.02 50 -

0.01 100 25370 0.01 100 37200 0.01 100 30062 0.01 100 36900

0.005 200 28210 0.005 200 40400 0.005 200 33140 0.005 200 -

0.002 500 32240 0.002 500 44500 0.002 500 37347 0.002 500 46900

Years of Data 70 PRECPRISOO 69 inches

Basin Area 643 SQ Mi Basin Area 759 SQ Mi Basin Area 759 SQ Mi Basin Area 758 SQ Mi

Kenai R at Cooper Landing AK Kenai River at Project Site Kenai River at Project Site Kenai River at Schooners Bend

FIGURE 5

Hydrology

SMS 13.0 Two-Dimensional hydraulic model.

FIGURE 6

RUSSIAN RIVER ROAD

2-D Hydraulic Model Mesh

Schooner Bend

Approximated Low-Flow Channel

Downstream Boundy Condition Normal Depth

Upstream Boundary Condition Constant flow, Q=36,900

SMS 13.0 - Two-Dimensional hydraulic model.

FIGURE 7

RUSSIAN RIVER ROAD

Flow Depth

Scooner Bend

FIGURE 8

RUSSIAN RIVER ROAD

Flow Velocity

N

FIGURE 9

100-Year Water Surface Elevations

Soundings aquired from bridge inspection report data

FIGURE 10

Sterling Highway Bridge Sections

Revetment riprap designed using approach from USACE EM 1110-2-1601, June, 1994.

N

FIGURE 11

RUSSIAN RIVER ROAD

Riprap Protection Sizing

SMS 13.0 Two-Dimensional hydraulic model.

Upstream Boundary Condition Constant flow, Q=36,900

FIGURE 6

RUSSIAN RIVER ROAD

Existing Surface Elevations

Schooner Bend

Approximated Low-Flow Channel