MEMORANDUM. TNC Fisher Slough Final Design and Permitting Subject: Large Wood Debris Design. Jenny Baker (TNC) Dave Olson (DD3)

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1 MEMORANDUM TNC Fiher Slough Final Deign and Permitting Subject: Large Wood Debri Deign To: From: Jenny aker (TNC) Dave Olon (DD3) David Cline (Tetra Tech) Jon Olmted (Tetra Tech) Jay lair (Tetra Tech) Date: Sept. 10, 2009 Introduction Thi hydraulic deign technical memorandum i the engineering analye performed for the large wood debri deign for the Fiher Slough Tidal Marh Retoration Project. There are approximately 20 to 30 large cottonwood tree, approximately 3-4ft in diameter and more than 100ft long that will be alvaged a part of the project for optional intallation during contruction. The actual intallation i contingent upon approval of the deign and intallation by The Nature Conervancy and Dike Ditrict #3, and will be decided at an unknown date in the future. Thi technical memorandum include the large wood debri intallation analyi and deign requirement if carried forward. Hydraulic Analyi of LWD Intallation Placement of LWD in the tidal marh area will be ubjected to frequent inundation, with extremely low channel velocitie (le than 1fp on the marh plain) and hydraulic force. Therefore, it i aumed that the deign hall protect againt buoyancy and floatation of LWD, which could then be tranported downtream toward other floodgate, levee, bridge infratructure. uoyancy deign involve developing ome ort of anchorage for the wood ytem. Three type of anchorage/ballat are typically ued, including: Soil ballat covering a ignificant portion of the LWD member Rock anchor attached to the LWD via bolt, cable and/or chain Pile anchor attached to the LWD via bolt, cable and/or chain Recent placement of LWD in tidal marh area with oil ballat ha had poor reult and thi type of anchorage i not conidered appropriate for thi project. Rock anchor are alo not conidered appropriate, a rock i not typically found in the Skagit Delta area naturally. Therefore, the remaining anchorage type deemed mot appropriate i the ue of a pile anchoring ytem. A pile reitance analyi wa performed to provide upporting documentation of the adequacy of the propoed LWD anchorage deign. 1

2 The firt tep in the pile analyi involve etimating the wood buoyancy force. The buoyancy force i determined uing the following Equation 1. The buoyancy force i decribed a the equivalent to the weight of the diplaced fluid acting upward on the LWD member. Equation 2 i the etimate of volume of wood, that i diplacing the water, including the tree rootball and bole. Equation 1. F = γ V w wood Equation 2. V + L wood = LR AR* A Where, F = ouyancy force on wood γ w = Unit weight of water V wood = Volume of LWD wood member L R = Length of rootball A R = Average area of rootball L = Length of LWD bole A = Area of LWD bole baed on DH The buoyancy force for a fully ubmerged, 50ft long cottonwood with a 4ft DH i etimated at 51,214lb of force. The buoyancy force for anchored wood tructure require the ue of pile anchor. The following Equation 3 6 were evaluated to determine the anchor pile configuration for the propoed feature. Table 1 i a ummary of the analyi reult. The overall reult i that uing 2, 1ft diameter timber pile located at either end of the tructure indicate that the pile hall be driven a minimum depth of 30ft to prevent pull-out. Other option would be to intall extra pile or ue larger diameter pile to increae urface area reitance. Equation 3. P = f SA p Equation 4. SA p = πdl Equation 5. f = 1 2 K γ ' Y t p tanφ Where: Kt = 0.5 to 0.7 (USACE, 1991) elected 0.6 2

3 Factor of afety to determine pile izing i a follow: Fb Equation 6. FS = 2. 0 P Table 1. Deign Analyi Summary Wood Parameter Volume LWD LWD Length - L b (ft) 50.0 LWD Diameter - DH (ft) 4.0 LWD Rootball Length - L rb (ft) 5.0 LWD Rootball Diameter - D rb (ft) 10.0 Volume LWD V wd (ft 3 ) uoyancy Force F (lb) 51,214.2 Surface Area of Pile Number of Pile 2.0 Diameter of Pile 1.0 uried Length of Pile 30.0 Surface Area of Pile (f) Pile Skin Reitance (f) Paive oil preure coefficient (Kt) 0.5 uoyant Soil Unit weight (γ') Aumed pile length below bed (ft) 20.0 Soil friction (failure) angle (φ) 34.0 Soil friction angle adjutment (δ) 0.9 Adjuted oil friction (failure) angle (φ' ) 30.6 Pile Skin Reitance (f) (lb/f) Pile Anchor Reitance Force (lb) 103,150.8 Factor of Safety 2.0 A final deign check wa performed on the chain connection. It wa determined that chain with 25,000lb working load hall be wrapped a one time to meet the load requirement of the buoyancy force. Connection hall alo ue through bolt, and not anchor crew, to prevent pull out failure. Wood with different ize can ue different load rating, depending upon the buoyant characteritic. Minimum pile intallation hall be a pecified, 30ft deep, 1ft diameter pile, with two pile anchor. The following Attachment A ha a conceptual deign plan for ingle, large wood debri intallation configuration. 3

4 Attachment A Large Wood Debri Intallation Deign Plan 4

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