Attachment_0004_-_Hydraulic_Scoping_Memo_with_FIRM_Maps.pdf

APRIL 9, 2021

Hydraulic Scoping Memorandum

Introduction

The Conant Boat Access is located on the South Fork of the Snake River, in Bonneville County, Idaho. The boat access is a public boat launch managed by the Bureau of Land Management (BLM) and consists of a primary parking area and concrete boat ramp on the north, and a secondary parking area for a raft launch on the south. It provides recreational and fishing access, which is essential for the area, and serves on average 200,000 visitors per year. Starting in 2004, severe bank erosion occurred immediately upstream (south) of the boat access along the west (left) bank of the river. The bank erosion has made the raft launch unsafe for use and caused deposition of material at the primary concrete boat ramp. The bank erosion is expected to continue, possibly impacting the parking areas and further impacting the concrete boat ramp.

General Site Conditions

The boat access site is located downstream of the Palisades Reservoir. Below the reservoir, the South Fork of the Snake River has been experiencing bed and bank degradation. As with most dam sites, reservoir acts to deprive the river of the sediment which would naturally be transported by the river downstream. The downstream river now has more capacity to carry additional sediment which encourages clear water scouring of the bed and banks. This geomorphic condition generally causes the river to respond in a number of ways.

The channel bed slope begins to flatten over time as bed degradation occurs. This results in lowering ground water levels which impacts riparian habitat. Flattening of the channel slope requires that the river develop tighter meanders with deeper main waterways. This can expand bends outward resulting in undercut banks such as what can be seen occurring along the south end of the boat access area. Below, Figure 1 depicts current site conditions using a 2017 aerial image and identifies features surrounding the boat access area.

The boat access area is located on the outside downstream edge of a large meander bend. Over time, this meander bend has extended west creating a sharper radius turn and longer main channel flow path which flattens the channel slope. The meander bend has also migrated north into the boat access area. Aerial photos from 1992 show the bank at the raft launch area protected by an upstream peninsula of well vegetated alluvial deposits. Flooding in 1997 caused the river bend to erode a large amount of this alluvial material and shifted the riverbank approximately 40 feet west. Since then, the outer bank at the bend has shifted an additional 125 feet west. Analysis of more recent aerial photography shows the rate of bank loss between 2012 and 2020 to be approximately 3 feet per year. While some areas have eroded at a slightly slower rate due to vegetation and/or topographical constraints, other areas have eroded at an increased rate. A geotechnical evaluation of aerial imagery has been completed and includes channel migration zone mapping (See Geotechnical Scoping Report).

The raft launch area is now fully exposed to the large bend in the river which has eroded and undermined the bank. The bank abruptly drops into fast moving and deep water that has formed along the outside of the river bend, making put-in and take-out of rafts and kayaks hazardous.

The primary concrete boat ramp has not experienced significant bank erosion and is still accessible to the public. This is due to riprap armoring located along the bank immediately upstream of the ramp. There also appears to be stable material at the south end of the riprap that prevents erosion below the water surface. This may be remnant material from an old boat ramp location that can be seen on aerial images from 1987.

Maintaining the primary boat ramp has required increased in-stream work. Material eroded from the upstream banks is transported and deposited on the concrete ramp during higher flows. This requires the BLM to remove the stream substrate material on an annual basis to keep the boat ramp operational.

Erosion Hazards:

It is expected that the river will continue to erode further west, jeopardizing the parking area for the raft launch and possibly the private properties to the south. Although protected by riprap armoring, the primary parking area and concrete boat ramp could also become threatened by the erosion if the riprap becomes undermined. Preliminary geotechnical mapping of possible erosion hazard areas is included in Geotechnical Scoping Report. The mapping shows the erosion hazard area extending well into the raft parking area and impacting the area near the concrete boat ramp.

Hydrology

The boat access is approximately at River Mile (RM) 886 which has a basin area of approximately 5,460 square miles. Flows within the South Fork of the Snake River are altered by the operation of Palisades Dam which is located upstream of the site (RM 902). The typical runoff pattern without the dam would be high spring runoff with lower fall and winter runoff. The dam operation has effectively leveled flows out over the year, resulting in a lower sustained flow throughout the year with now higher fall and winter flows for power generation. Approximately 95% of the basin area above the boat access site is regulated by the dam.

There are two gages below the dam which can be used to estimate peak discharges at the boat access site. The Erwin gage is 1.5 miles downstream of Palisades dam and the gage is located 2.4 miles upstream of the town of Heise. Peak discharge estimates for each gauge station are presented in Table 1.

Table 1.

Peak Flood (cfs) 2-yr 10-yr 50-yr 100-yr 500-yr

Gage 13032500 (Irwin) 19,500 26,700 32,400 34,600 39,700 Gage 13037500 (Heise) 22,000 34,100 46,500 52,200 67,100

Flows upstream of these gages are regulated by the Palisades Dam and are affected by irrigation diversions.

Additional basin delineations and analysis of gage data will be needed to determine peak flows at the boat access site.

Floodplain and Flood-rise

Federal Emergency Management Agency (FEMA) flood inundation maps 160027 0300C and 160154 0005B (See attached FIRM Maps) were reviewed to determine what mapped floodplains occur within the project study area. Each of the bank stabilization alternatives are mapped within a Zone A floodplain designation.

Zone A are areas of 100-year flood with base flood elevations and flood hazards factors not determined.

Detailed mapping and study have not been completed in this zone.

Bank stabilization alternatives that are constructed within the flood zone will require a detailed flood study to determine the 100-year flood elevations and any impacts to flooding. Alternatives will need to be designed to cause no rise in the 100-year flood profile. This will be particularly important since private properties with insurable buildings are found in the low-lying floodplain area immediately upstream and south of the study area.

A no-rise certification (https://www.fema.gov/no-rise-certification-floodways), or a Conditional Letter of Map Revision (CLOMR https://www.fema.gov/conditional-letter-map-revision) will need to be submitted to the local floodplain manager. If a flood-rise results from a proposed improvement, further site investigation may need to be conducted to determine upstream impacts and necessary mitigation.

Water Quality

The primary potential pollutant in stormwater runoff, caused by parking area and bank stabilization work is sediment/total suspended solids originating from soils exposed onsite during the construction process.

Sediment may be entrained in stormwater runoff from cut/fill slope areas that are excavated and graded.

Implementing stormwater best management practices (BMPs) during parking area construction should be sufficient in preventing soil erosion and sediment transport from work in the upland parking areas.

The largest impact will be from in-water bank protection work which requires excavation and placement of fill within the waterway. This will require area isolation and channel re-alignment or diversion work. This work will need to be performed during low flow periods (spring or fall). Work area isolation typically involves working from upstream to downstream to smooth transition flows during installation of turbidity barriers.

https://www.fema.gov/no-rise-certification-floodways) https://www.fema.gov/conditional-letter-map-revision)

Turbidity barriers can consist of any number of measures such as berms, sheet piles, concrete barriers, bags filled with ballast material, or water filled bladders. These systems need to be designed to withstand the erosive forces within high velocity conditions with deep and shallow depths. Simple turbidity curtains will likely not be adequate. Materials used for berms or other types of barriers will need to consist of clean material or wrapped to prevent sediment entering the river.

Navigable Waterway

The South Fork of the Snake River is listed as a navigable river under the authority of Idaho State. Bank stabilization alternatives will need to consider providing conditions that are safe for users and sufficient clearance to accommodate river traffic. Timing of construction will need to be sensitive to users and notifications of closures or restrictions will need to be communicated.

Flooding History

The most significant flooding post-dam construction occurred in 1997 when heavy snowmelt combined with a sustained rain event required a record release of flows from the Palisades Dam. The record dam release was a result of underpredicted high flows which resulted in an outflow estimated to be between a 100-year and 500-year reoccurrence event according to FEMA statistics. Flood levels rose above the banks and inundated the lower step of the back deck of the South Fork Lodge.

Bank stabilization measures should be designed to accommodate the 100-year flood and 500-year check flood.

Scour

A scour and sediment transport analysis has not been performed as part of this initial scoping. As the project progresses, bank protection measures will need to be designed to account for scour and channel degradation.

Additional geotechnical investigations, including borings and material gradations, will be required to perform this analysis.

Aquatic Organism Passage

Fish are present within the South Fork of the Snake River and Prichard Creek. The vertical drop (3 to 4 feet) that has resulted from the west bank erosion and channel degradation along the South Fork Snake River has created a likely barrier to fish. This condition will be further aggravated as bank erosion and channel degradation continue as anticipated.

Under natural conditions, the Prichard Creek channel bottom would degrade and erode down to match the main river channel degraded elevations over time. However, the roadway culverts located immediately upstream limit the creek’s ability to vertically adjust. This may result in a long-term fish barrier concern within Prichard Creek. Further review of the culverts will be needed to determine if future channel adjustments will impact these structures or result in a permanent fish barrier. The channel and structures may need to be modified or replaced to accommodate passage.

Restoration and stabilization work along the west bank of the South Fork Snake River may extend to the confluence of Pritchard Creek. Under this scenario, work to improve channel grading for fish passage in Pritchard Creek will need to be considered. As part of the bank restoration, re-establishing the northerly flowing creek channel that was lost should be considered. This would eliminate the existing vertical barrier and allow the creek more room to naturally adjust to future degradation in the river.

Hydraulic Design Considerations

A preliminary hydraulic analysis (using two-dimensional hydraulic modeling) has been performed to evaluate potential changes to flow velocities, erosive forces, and flood rise. Proposed improvements shall intend to eliminate high velocity currents and ensure safe conditions at the primary boat ramp and raft launch areas.

Channel excavation is anticipated to prevent constriction of main channel flows and possible increases in erosion or flood conditions along upstream properties. Improvements shall intend to ensure no rise in the 100-year flood elevations upstream or downstream.

Rock armoring will likely be needed below low flow levels for newly constructed banks. Above low flow levels, combinations of rock armoring (of varying sizes), large cobbles, willow pole plantings, live plantings, and large wood may be used to provide appropriate erosion protection. Placement of large wood or logs would be limited to configurations that do not pose a hazard to boaters or river users. Large deflective wood jams are not anticipated to be viable at this location.

Armoring on its own tends to produce higher velocities along banks. To prevent this effect, features such as stream barbs, rock spurs, and/or boulder clusters may be installed. These submerged deflective and energy dissipating features help alleviate high velocities while producing a recognizable hydraulic condition that boaters are familiar with navigating. Such features would need to be installed in conjunction with bank protection to be effective and would not function successfully as stand-alone elements under the velocity conditions expected at this site. By reducing velocities, the extent and size of bank armoring material could potentially be reduced.

Bank protection measures should function as adjustable systems capable of accounting for potential scour and channel degradation. Protective armoring would need to extend below expected scour and degradation levels or include sufficient volume and size to settle as the channel shifts.

To further promote fish habitat, increase bank roughness, and improve channel complexity, additional random boulder clusters or log habitat features may be incorporated. Conserved natural material from on-site may also be used for proposed stream substrate.

Constructed floodplain and upland areas may utilize materials consistent with on-site conditions. These areas should be graded with gradual or flat slopes. Organic soil may be placed within rock armoring above the ordinary high water and over planting zones. Areas may be planted with trees, live willow poles, shrubs/sedges, and grasses to provide riparian buffer, shade, biofiltration, habitat, wood recruitment potential, and improved aesthetics.

Constructability

Initial diversion work will need to be performed during low flow periods (spring or fall) and will need to control turbidity. Work will also need to consider boater restrictions and safety.

Modifications and refinement of designs at the time of construction should be expected to accommodate changes in river conditions resulting from pre-construction spring runoff.

Cost and Schedule concerns

Short in-water work periods will be a challenge for constructing new banks and excavating the channel realignment.

Attachments:

FIRM Maps (2 pages)

Hydraulic Memorandum_FIRM Maps