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HomeMy WebLinkAboutNSD_DRC_BrinnonHydraulics_073025Lower Dosewallips River – Brinnon Reach Evaluation of Restoration Actions and Berm Treatments DRC | July 30, 2025 0 Overview 1 1 100-Year Flood – Current Conditions 2 2 Conceptual Actions 3 Project Elements Low profile ELJs (1a) and side channel enhancement (1b) included within proposed conditions hydraulic modeling terrain Wood placements (2) included in modeling as heightened roughness Levee improvements through raising and extending (6) included within hydraulic model terrain Flood impoundment (5) addressed through increased capacity of Twana Creek culvert. Improving existing ELJs (7) not included in current round of model iterations Completed multiple flood modeling iterations to examine the hydraulic effects of potential restoration and berm modification actions. 3 Conceptual Restoration Elements 4 ELJs modeled as roughness + surface modification ELJs modeled as roughness only Side channel/inlet excavation ~ 3-6’ included in model surface ELJ surface modifications are typically 2-3 ft above adjacent ground elevation (8-10 ft above thalweg) Typically 3-6 ft excavation for side channel/inlet excavation Backwater connection if allowed for more frequent connection/greater depth within side channel 4 Berm Elements 5 Approximate extents of existing berm Modeled berm extension to block flood channels 5 Existing Berm Raise Testing 6 Berm improvement modeled within hydraulic model terrain Raising berm height by 1-ft and 2-ft within current extents tested – hydraulic model results were similar between the two options, so 1-ft berm raise was assumed for subsequent model iterations 6 100YR Change in WSE – Restoration Treatments Only 7 Twana Cr culvert appears to be at/near capacity Rise generally below 0.1 ft, areas with greater rise (still mostly below 0.3 ft) around ELJs Restoration treatments include side channel inlet excavation and logjam placements Private parcels US-101 Baseline rise with restoration only 7 100YR Change in WSE – Berm Raise (1 ft within current extents) Only 8 Twana Cr culvert appears to be at/near capacity Rise generally below 0.4 ft, with greatest change in depth upstream of US-101 road prism in Dosewallips State Park area Some instances of rise impacting private structures Extent of berm raise Private parcels US-101 Flow trapped between berm and US-101 road prism seems like greater influence on rise Baseline rise with berm raise (within current footprint) only 8 100YR Change in WSE – Restoration Treatments + 1-ft Berm Raise (Current Extents) + Raising Structures within Dosewallips State Park 9 Twana Cr culvert appears to be at/near capacity Rise of up to 0.4 ft in Dosewallips State Park Area Some instances of rise impacting private structures Restoration treatments include side channel inlet excavation and logjam placements Private parcels Berm raised by 1-ft within current footprint Structures within Dosewallips State Park property to be raised by 1-8 ft above 100-year WSE US-101 9 100YR Change in WSE – Restoration Treatments + 1-ft Berm Raise (Current Extents) + Upsizing Twana Creek Culvert 10 Twana Cr culvert upsized to 29-ft span to limit upstream backwater Rise of up to 2 ft in Dosewallips State Park Area downstream of Twana Creek culvert Some instances of rise impacting private structures Restoration treatments include side channel inlet excavation and logjam placements Private parcels Berm raised by 1-ft within current footprint US-101 10 100YR Change in WSE – Restoration Treatments + 1-ft Berm Raise (Expanded Extents) + Raising Structures within Dosewallips State Park 11 Twana Cr culvert appears to be at/near capacity Rise of up to 0.4 ft in Dosewallips State Park Area Some instances of rise impacting private structures downstream of US-101 Restoration treatments include side channel inlet excavation and logjam placements Berm raised by 1-ft within footprint expanded upstream Structures within Dosewallips State Park property to no be raised by 1-8 ft above 100-year WSE US-101 11 100YR Change in WSE – Restoration Treatments + 1-ft Berm Raise (Expanded Extents) + Upsizing Twana Creek Culvert 12 Twana Cr culvert upsized to 29-ft span to limit upstream backwater Rise of up to 2 ft in Dosewallips State Park Area downstream of Twana Creek culvert Restoration treatments include side channel inlet excavation and logjam placements Private parcels Berm raised by 1-ft within footprint expanded upstream US-101 12