Loading...
HomeMy WebLinkAboutM_SEPA DNS Reponse to City_Broodstock_210115_v1.pdf 700 Fifth Avenue | PO Box 34018 | Seattle, WA 98124-4018 | 206-684-3000 | seattle.gov/utilities 1 November 18, 2020 Electronic Transmittal: vdolbee@rentonwa.gov Vanessa Dolbee Planning Director City of Renton 1055 South Grady Way Renton, WA 98057 Subject: Cedar River Sockeye Hatchery Broodstock Collection Facility Replacement Project SEPA Determination of Non-Significance (DNS) Response to City of Renton October 22, 2020 Comments Dear Ms. Dolbee: Thank you for reviewing the SEPA checklist and Determination of Non-Significance for the Cedar River Sockeye Hatchery Broodstock Collection Facility (BCF) Replacement Project. This letter provides responses to the City of Renton’s October 22, 2020 comments. The City’s comments are included below in bold, and SPU responses follow. 1. A Zero Rise Floodway analysis shall be completed to meet the City’s flood regulations, RMC 4-3-050, and The City of Renton’s Storm Water Draining Manual requirements. Mitigation should be implemented to ensure the project does not result in flood level rise or impact any downstream properties. Consistent with the requirements outlined in Renton Municipal Code (RMC) 4-3-050 and City of Renton’s 2017 Surface Water Design Manual, an engineering analysis was completed evaluating whether the project would affect the Base Flood Elevation. The analysis was conducted following the 2013 Federal Emergency Management Agency (FEMA) guidance document to ensure that placement of the BCF weir would not cause any increase in flood levels within the Cedar River floodplain during the occurrence of the base (100-year) flood discharge. This analysis incorporated site conditions following the early 2020 river flood event and will be included with the project’s City of Renton shoreline permit application. Following the City of Renton’s 2017 Surface Water Design Manual and the FEMA regulations, the analysis findings demonstrate a no-rise effect on the 1-percent annual chance flood; Because the design avoids impacts, no additional mitigation is proposed. 2. The SEPA states that imported material would be used to prevent scour only at the concrete sill and improved boat ramp. The City requests that SPU evaluates the potential for increased scour along the banks of the Cedar River through the entire project area due to the addition of the new facilities and potential changes in flow patterns around these facilities. Once the scour study is completed suggested mitigation should be implemented to prevent scour along the banks of the river. The replacement BCF is designed to avoid or minimize scour within the project area. Per page 30 of the SEPA checklist, Attachment A - Vicinity Map presents a general project area where both permanent facilities and temporary construction activities (within SDOT ROW beneath the I405 bridge) will occur. Page 31 of the SEPA checklist, Attachment B – Project Layout provides a closer view of the final project component locations. 700 Fifth Avenue | PO Box 34018 | Seattle, WA 98124-4018 | 206-684-3000 | seattle.gov/utilities 2 On the north side of the river, there is an existing rock wall that will not be disturbed, and the proposed new facilities should not change the scour potential that currently exists along the wall. The northwest side near the I405 bridge will only be used for construction staging/access and will remain the same as is now, thus, no significant changes to existing scour potentials at that location are anticipated. Along the south side of the river, most of the improvements occur above the ordinary high-water mark (OHWM). The project proposes minor changes that include minor widening of the access road, shifting the existing 20 ft (+/-) boat ramp 20 ft east and installing a concrete retaining wall and key wall to prevent its undermining. Additionally, proposed 8- to 16-inch diameter boulders will protect the up and down stream banks. This is an improved design compared to existing conditions. As part of the 60% design package, a scour analysis was prepared by our design consultant for the project. “Technical Memorandum No. 3 - Scour Analysis – Cedar River Broodstock Collection Facility (November 19, 2019)” is attached for your review. Per Section 3.0 of the document: “The study shows that the potential scour hole depth at the project location would be up to 6 feet upstream of the facility and 10 feet downstream of the facility.” Thus, designers calculated the required 8 to 16-inch diameter boulder sizes to protect the riverbed and shoreline based on the river velocities, natural substrate, and scour potential. 3. The SEPA checklist identified the potential for sediment and debris to back up behind the new concrete sill and picket panel system. While the proposed debris deflector may be effective at redirecting some of the debris, the City has concerns about the potential for flooding in the area should the deflector either become damaged during a heavy rain event or be less effective than expected at the time of installation. The City requests that SPU propose additional mitigation to ensure that sediment and debris do not build up behind the facility and cause damage to nearby or downstream properties. The existing BCF deflectors consist of two aluminum panels with vertical bars. The deflectors are installed at an angle to the flow, just upstream of the existing BCF to prevent debris from impacting the adult collection box. The intent is to utilize the existing BCF deflectors with the new BCF facility as they are in good condition and meet the needs of the proposed improvements. The existing deflectors will be installed based on the existing configuration and current procedures such as driving “T” posts through the deflector anchor cylinders and into the riverbed. The installation and general location of the BCF deflectors will remain the same for the new BCF. The existing deflectors have provided adequate protection with little impact to the facility or accumulation of debris with proper maintenance from the operators. Next, the debris deflectors are in the river during BCF operation from September through November, and there is 24-hour on-site oversight during this operation period. Should the debris deflectors become damaged, on-site staff will follow written standard operating processes (SOP) to respond accordingly. SPU, WDFW and the design engineer of record will develop these SOPs as we finalize the design and construction of the facility. SPU will also utilize adaptive management practices during use of the proposed BCF to improve all facility SOPs. To date, there are no known cases of damage to nearby or downstream properties because of naturally occurring debris passing through the current facility. 700 Fifth Avenue | PO Box 34018 | Seattle, WA 98124-4018 | 206-684-3000 | seattle.gov/utilities 3 Lastly regarding sediment transport, the riverbed directly upstream of the replacement BCF will rise to meet the elevation of the sill (refer to response to comment 1 regarding no-rise effect to the floodway overall, which accounts for this sediment accumulation). Once a consistent elevation is established, sediment transport will continue through the picket panels when they are raised from September to November. Between December and July, maintenance of the picket panels may occur on an up-to-weekly basis to remove accumulated bedload. This would require raising the picket panels a few inches off the lowered position to dislodge accumulated sediment and debris and allow flow to pass the bedload downstream. This recurring maintenance will be refined through adaptive management practices once the facility is in operation. Importantly, the bedload is naturally occurring; the BCF does not change the amount of sediment transported downstream by the Cedar River, therefore, impacts to downstream properties are not anticipated. 4. SPU should consider the ongoing City of Renton bank stabilization project located just upstream of the project site. The embankment located on the south side of the river experienced extreme erosion and score during the last (and most recent) flooding event. This bank remains unstable. The City is currently working on a project to stabilize portions of this bank. The instability of this riverbank should be considered when evaluating impact to and on your project. SPU acknowledges the bank stabilization effort upstream of the project site, and that mass-wasting events within the Cedar River may potentially impact all downstream resources, including the BCF. The replacement BCF has been designed to withstand the 100-year flood event. The following comments address items that will need to be coordinated with the City during the permit and construction process for the replacement broodstock collection facility. • The Agreement between the City of Renton and Seattle Public Utilities expires on August 21, 2023. A new agreement needs to be negotiated and executed prior to construction improvements. • As the Broodstock Facility is located on grant funded property, Seattle Public Utilities’ proposed expanded use of the property must be approved by the granting agency, the State Recreation and Conservation Office, prior to entering into a new agreement. • At a minimum, the following documents are required to be updated (and included in the new Agreement): o Access and Use Area, including legal description of Renton property and map and delineation of WSDOT property (Exhibit A of executed Second Memorandum of Agreement regarding Cedar River Access Facility and Broodstock Collection, as Amended). o Cedar River Sockeye Hatchery Broodstock Collections Operations Safety Plan (Exhibit B of executed Second Memorandum of Agreement regarding Cedar River Access Facility and Broodstock Collection, as Amended). o Access Facility Management Plan dated July 30, 2008 (included as Exhibit C of executed Second Memorandum of Agreement regarding Cedar River Access Facility and Broodstock Collection, as Amended). • Any proposed mitigation shall be approved by the City and the Recreation and Conservation Office. 700 Fifth Avenue | PO Box 34018 | Seattle, WA 98124-4018 | 206-684-3000 | seattle.gov/utilities 4 In support of a new or amended agreement for continued use of the site, SPU has reviewed the existing documents, prepared potential draft revisions, and discussed internally how to initiate Memorandum of Agreement negotiations. Leslie Betlach suggested that negotiating a new agreement is likely to take one year, including coordination with the Washington State Recreation and Conservation Office. SPU will submit the materials listed above, including any proposed onsite mitigation, in a timely manner so that a new agreement can be in place prior to the proposed Q2 2022 construction start date. Upon completion of this project specific SEPA process (November 2020), SPU plans to submit the City of Renton shoreline permit application, consistency analysis, and supporting documents. We believe that these materials provide additional and helpful detail and will respond to some of the City’s technical questions. We look forward to answering any additional questions as those materials are reviewed. SPU acknowledges and appreciates the interagency coordination over the past 10 years in support of the BCF, which is a critical component of the Landsburg Mitigation Agreement and Cedar River Watershed Habitat Conservation Plan and provides engaging educational opportunities to area residents. We look forward to continuing this collaboration into the future, for the benefit of our communities and the environment. Respectfully, Fernando Platin (Digitally Signed) SPU BCF Project Manager Cc: Betty Meyer – SPU SEPA Responsible Official Clayton Antieau – SPU Permitting and Environmental Review C. E. “Chip” Vincent, CED Administrator: cvincent@rentonwa.gov Ron Straka, Utility Systems Director: rstraka@rentonwa.gov Kelly Beymer, Community Services Administrator: kbeymer@rentonwa.gov Joe Farah, Utility Engineering Manager: jfarah@rentonwa.gov Leslie Betlach, Parks Planning and Natural Resources Director: lbetlach@rentonwa.gov Erica Schmitz, Parks Planning Manager: eschmitz@rentonwa.gov Alex Morganroth, Senior Planner: amorganroth@rentonwa.gov Rev. 1 / November 2019 1 McMillen Jacobs Associates Technical Memorandum Technical Memorandum 003 To: Michael Norton, PE Seattle Public Utilities Project: Cedar River Broodstock Collection Facility (BCF) From: Derek Nelson, PE McMillen Jacobs Associates cc: File Prepared by: Abil Nazari, Ph.D. Kevin Jensen, P.E. McMillen Jacobs Associates Job No.: 18-101 Date: November 19, 2019 Subject: Scour Analysis – Cedar River Broodstock Collection Facility 1.0 Introduction The major aim of the Broodstock Collection Facility (BCF) Replacement Project is to replace the existing Broodstock Collection facility on the Cedar River because it is under-performing in collection of migrating Sockeye Salmon and is a safety concern for operating personnel. As part of the design process, consideration of scour near the facility is important to ensuring a lasting facility. Scour can be defined as the process by which the particles of soil or rock around an abutment or pier of a bridge or other structure are eroded and removed to a certain depth (called the scour depth). For rivers and open channels with bed material coarser than sand, armoring and slope reduction processes may occur simultaneously. Analysis must be performed to determine which will provide the limiting factor. There are many approaches and equations available to compute the scour depth in rivers and open channels. The scour depth can easily be obtained after a flood by finding the depth of the scour with reference to the surroundings or existing structures. If this is not possible, the mean depth of scour may be easily obtained by mathematical formulas for natural streams in the alluvial depth. Determining potential scour depth is an important first step in designing a stable -in-stream facility. 1.1 Purpose This Technical Memorandum (TM) describes the work completed to determine possible scour hole depths within the Cedar River channel (project location in Figure 1) during the occurrence of the base (100-year) flood discharge and includes additional scope of work to perform a scour analysis for the project. The TM also includes an additional design for engineered scour holes if required to prevent major scour issues at the proposed facility. This TM also includes discussion of the riprap design criteria according to the scour investigation for the project location. Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 2 McMillen Jacobs Associates Figure 1. Scour monitoring site location on the lower Cedar River 2.0 Methodology 2.1 Local Scour Analysis Cascade Environmental Services (CES) has conducted two previous studies on streambed scour in the Cedar River [1]. CES utilized buried neutrally buoyant radio transmitters in an effort to determine the minimum flow at which stream bed scour occurs. Data from recovered accelerometers suggests that initial streambed mobilization occurs when flows reach approximately 2,250 cfs at the Renton Gage. Accordingly, the Instream Flow Commission (IFC) decided to increase the existing scour threshold from 1,800 cfs to 2,200 cfs (USGS Renton gage). Based on the CES investigation results and using the HEC- RAS flood model survey results for the base (100-year) flood discharge [2], this TM improves the applicability of the study results and provides a supplemental study element which addresses potential scour hole depths at the proposed facility. The principal references for design of mid-channel structures for scour such as at bridge piers are: · National Cooperative Highway Research Program Synthesis 5 (1970), · C. R. Neill (1973), · Federal Highway Administration, Training and Design Manual (1975), · Federal Highway Administration (1980), and · S. C. Jain (1981) [3]. Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 3 McMillen Jacobs Associates The numerous empirical relationships for computing scour in rivers, open channels and at bridge piers include one or more of the following hydraulic parameters: pier width, pier skewness, flow depth, velocity, and size of sediment. The many relations available are further broken down by Jain (1981) to two different approaches: (1) regime for degradational scour, and (2) rational for scour around pier. Degradational scour is the general removal of sediment from the river bottom by the flow of the river. This sediment removal and resultant lowering of the river bottom is a natural process, but may remove large amounts of sediment over time. Regime equations are supported by field measurement methods. In gravel bed rivers, bed sills are used to limit bed degradation and to control erosion in the proximity of bridge piers or in upstream and downstream channels of dam stilling basins. Scour prediction equations for upstream and downstream scour have been proposed by researchers (Bormann and Julien 1991; Gaudio et al. 2000; Lenzi et al. 2003a, b; D’Agostino and Ferro 2004; Marion et al. 2004; Comiti et al. 2005; Ben Meftah and Mossa 2006; Marion et al. 2006; Pagliara and Kurdistani 2013). However, almost all these equations are developed for unsubmerged or partially submerged weirs or sills, downstream of which scouring is a result of free over fall plunging jets (or partially submerged impinging jets) [3,4,5]. For a fully submerged weir, flow regimes over the weir can be classified as: (1) surface jet, (2) surface wave, (3) breaking wave(or surface jump), and (4) impinging jet (Wu and Rajaratnam 1996) [6,7]. The first three regimes can be collectively named the surface-flow regime; for these, the flow remains as a jet at the surface in the downstream channel, with its thickness increasing downstream because of turbulent mixing. Assuming constant relative density of sediment and fluid viscosity, a dimensionless expression for the equilibrium scour depth around the submerged weir in a uniform sediment can be presented as follow: for upstream: = 0.384 .+ 0.758 (1) = 1.719 .+ 3.246 (2) for downstream: = 0.592 .+ 1.358 (3) = 3.608 .+ 5.086 (4) where: dsu = scour depth at upstream of the weir (ft) lsu = scour length at upstream of the weir (ft) dsd = scour depth at downstream of the weir (ft) Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 4 McMillen Jacobs Associates lsd = scour length at downstream of the weir (ft) hc = critical depth flow over weir (ft); the flow depth over sill can be estimated with the formula of the critical flow depth, ℎ= !" #$% (5) where: q= design flow discharge per unit width (ft2/s)= 8051.5 So= the initial bed slope (ft/ft)= 0.0001 Vave.= mean velocity at river (ft/s)= 8.9 ∆=’(’) ’), ρs= sediment density= 0.1654 (lb/ft3), ρs= water density= 0.0624 (lb/ft3) g= Acceleration due to gravity (32.2 ft/s2) Based on the foregoing, the calculated scour depth and length at the project location are presented in Table 1. Table 1. Local Scour Depth and Length at the Project Location Upstream Downstream ds (ft) 5 9 ls (ft) 20 32 2.2 General Scour Analysis Per NRCS (2007), the total scour depth needed for design of hydraulic structures is an additive combination of several different types of scour, including local scour and general scour. Local scour has already been described, and results of local scour under existing conditions are provided in Table 1 above. General scour, which reflects changes in bed elevation due to reach-scale erosion, is provided in NRCS (2007) by the following equation: z!= T − -. (6) where: T = thickness of the active layer of the bed (ft) -. = smallest armor material size (ft) NRCS (2007) provides relationships of T and -. in terms of the porosity of bed material, flow depth, energy slope, shear velocity, median grain size within the reach, and other variables. Assuming a 100- year flow depth of 13.41 feet, an energy slope of 0.00113 ft/ft, a D50 of 14 mm, and a fraction of bed material equal to or coarser than that calculated for Dx of 28%, the calculated general scour depth is approximately 1.0 feet. Total scour depth for design purposes is provided below in Table 2. Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 5 McMillen Jacobs Associates Table 2. Total Scour Depth and Length at the Project Location Upstream Downstream zt (ft) 6 10 ls (ft) 20 32 2.3 Riprap Design In 1998, based on 2017-2018 post-dredged Cedar River scour monitoring report, the Seattle District of the U.S. Army Corps of Engineers implemented the Cedar River Section 205 Flood Damage Reduction Project to provide 100-year flood protection along the lowermost 1.25 miles of the Cedar River (USACE, 1997) [5]. The project included dredging approximately 158,000 cubic yards of sediment from the river channel and construction of adjacent floodwalls and levees. Since 1998, the City of Renton (City) has assumed responsibility for annual monitoring of sediment accumulation and flood protection levels through the reach. To fulfill this obligation, the City retained Northwest Hydraulic Consultants (NHC) to survey cross-sections along the lower two miles of the river (Figure 1) and evaluate remaining sediment storage capacity. NHC was retained by the City 22 times (1991, 1997 to 2012, and 2014 to 2018) to survey approximately 40 channel cross-sections along the lower two miles of the Cedar River. In 2001, NHC prepared a detailed report that investigated past rates of sediment accumulation and predicted future changes to the channel profile (NHC, 2001). One of the objectives of this study was to estimate the allowable bed profile, defined as the average channel bed profile computed at 100-year flood levels. As a result, to help the City assess when to begin permit applications for the next dredging, a warning bed profile was defined 1.5 feet below the allowable bed profile. More recently, Tetra Tech has conducted hydrologic and hydraulic analyses for the City as part of a recertification effort for the Cedar River Section 205 Levee System (Tetra Tech, 2017; Tetra Tech, 2018). An updated flood frequency analysis, utilizing 17 years of additional gage records, was used to replace flood frequency discharges computed by King County’s Department of Natural Resources (King County, 2000). The new frequency analysis yielded a 100-year discharge of 10,900 cfs, which was adopted to replace the County’s previous estimate of 12,000 cfs (Tetra Tech, 2017). Based on the 2018 bed profile comparison between Tetra Tech (2018) and NHC (2001), there is room to store approximately 59,500 and 99,500 cubic yards of sediment within the revised warning and allowable limits, respectively. For comparison, the respective storage capacities were 63,600 and 103,000 cubic yards in 2017. The annualized volume of sediment deposited, eroded, or dredged in the reach downstream from Williams Avenue is summarized in Appendix A. For reference, the maximum mean daily and maximum instantaneous flows that were recorded during the previous 12-month period are also listed. The computed average sediment deposition rate downstream of Williams Avenue, since 1998, is approximately 8,100 cubic yards per year. If sediment were to deposit at this average annual rate, the warning and allowable bed profiles would be effectively reached in approximately 7 and 12 years, respectively. However, the remaining storage may be effectively full over a shorter period if the river Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 6 McMillen Jacobs Associates experiences “above average” flood seasons (e.g., 2007 and 2011, when 21,200 and 19,200 cubic yards of sediment were deposited, respectively). Currently, the channel downstream from Williams Avenue has room to store approximately 99,500 cubic yards of sediment before it can no longer safely contain the 100-year flood (i.e., until the bed profile reaches the allowable bed profile). If sediment were to deposit at the average rate of 8,100 cubic yards per year, it will effectively reach the warning and allowable bed profiles in approximately 7 and 12 years, respectively. 2.3.1 Design Guidelines Two methods or approaches have been used historically to evaluate a material’s resistance to particle erosion. These methods are the permissible velocity approach and the permissible tractive force (shear stress) approach. Under the permissible velocity approach the channel is assumed stable if the computed mean velocity is lower than the maximum permissible velocity. The tractive force (boundary shear stress) approach focuses on stresses developed at the interface between flowing water and materials forming the channel boundary. Permissible velocity procedures were first developed in the 1920’s. In the 1950’s permissible tractive force procedures became recognized, based on research investigations conducted by the Bureau of Reclamation. Design Relationships The hydrodynamic force of water flowing in a channel is known as the tractive force. The basic premise underlying riprap design based on tractive force theory is that the flow-induced tractive force should not exceed the permissible or critical shear stress of the riprap [8]. Assuming a specific gravity of 2.50, equations (7) and (8) can be used to determine D50 of the riprap by the tractive stress method. Use of a design method based on tractive stress is still preferred for final design. The following equations use tractive stress design guidelines: -/0(23) = 14.2 67 ℎ89:;69 /=# (7) where: D50 = median riprap size in (ft) SF = stability factor, Table 3 hmean = maximum section depth (ft) Se = average energy grade line slope (ft/ft) K1 = [1-{sin2 θ /sin2 φ }] 0.5 = bank angle modification factor, Figure 3 θ = bank angle with horizontal, degrees φ = riprap material angle of repose, degrees, Figure 2 -/0(>) = 0.0059?99@$/(ℎ89:;0./=##./) (8) Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 7 McMillen Jacobs Associates where: D50 = median riprap size in (m) hmean = maximum section depth (m) Vdeep = the average velocity in deep of the main channel (m/s) Figure 2. Optimal Riprap Side Slope for Given Size Riprap Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 8 McMillen Jacobs Associates Figure 3. Bank Angle Correction Factor (k1) Nomograph The stability factor, SF, is used to reflect the level of uncertainty in the hydraulic conditions at a particular site. The stability factor is used to increase the design rock size when these conditions come to bear. The design rock size (D50) increases linearly with the stability factor. Table 3 presents guidelines for selection of an appropriate value for the stability factor. Table 3. Criteria for Selection of the Stability Factor Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 9 McMillen Jacobs Associates Using values of mean depth and velocity at the BCF location that are borrowed from the 100-year no-rise HEC-RAS model, results of Equations 7 and 8 are summarized in Table 4 below. From the table, the estimated stable D50 ranges from about 0.23 feet to 0.34 feet, or from about 2.8 inches to 4.1 inches. Table 4. Design Parameters Riprap Protection at Project Location Equ. SF hmean Se θ ϕ K1 Vdeep D50 (6) 1.4 13.41 ft 0.00113 3:1 41.5 0.876 --- 4.1 in (7) --- 4.09 m --- 3:1 41.5 0.876 2.71 m/s 2.8 in In addition to the methods just described, the well-known Shield’s approach was also undertaken. With this method, the incipient grain size is given by: - = A A∗(C (C) (9) In this equation, D is the critical bed shear stress, D ∗ is the dimensionless Shield’s parameter, and EF and E are the specific weight of sediment and water, respectively. Assuming a Shield’s parameter value of 0.044 for medium gravels, a specific gravity of sediment of 2.65, a depth of water during the 100-year flow of 13.41 feet, and a bed slope equal to the energy slope of 0.00113 yields an incipient grain size of 2.5 inches, which matches well with the values presented in Table 4. Perhaps not surprisingly, the larger of the two D50’s presented in Table 4 is quite close to the D100 of the native material found in the Cedar River near the project location. To prohibit mobilization of the D50 particle size class, an additional factor of safety FOS of 2.0 has been applied, such that the effective D50 used for design shall be 8.3 inches. Rock Gradation The gradation of stones in riprap revetment affects the riprap’s resistance to erosion. The stone should be reasonably well graded throughout the riprap layer thickness. After a D50 has been determined for the location, the gradation should be stated using the guidelines in Table 5. Table 5. Riprap Gradation Guidelines Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 10 McMillen Jacobs Associates Normally a gradation envelope is specified to allow for more flexibility in manufacturing the material to meet specified gradations. For designing riprap in the lower Cedar River, Figure 4 represents the material gradation. Using this gradation class, a standard design size can be selected, if design criteria and economic considerations permit. Figure 4. Cumulative Frequency Curve Generated from Wolman Pebble Counts and Material Gradation for Riprap Design at Project Location The material gradation curve indicates the following rock characteristics: D15 riprap = 3.3 inches (63.5 mm) D50 riprap = 8.3 inches (157.5 mm) D85 riprap = 12.8 inches (243.8 mm) D100 riprap = 16.5 inches (315.0 mm) Layer Thickness Research [9] indicates that increasing rock layer thickness improves the riprap stability. The increase in stability becomes smaller as the median rock size increases. The study examined only rock gradations where the median rock size was 6 inches or less. All stones used should lie within the riprap blanket to provide the maximum resistance against erosion. Protruding stones can alter the flow net across the channel. Oversize rocks, even in isolated spots, may cause riprap failure by precluding mutual support between individual rocks, providing large voids that expose the filter and bedding materials, and creating excessive local turbulence that removes smaller stones. Small amounts of oversize stone should be removed individually and replaced with proper size stones. The following criteria apply to the riprap layer thickness: 0 10 20 30 40 50 60 70 80 90 100 110 0 1 10 100 1000Cumulative sediment distribution (%)Particle size category (mm) Cedar River Riprap Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 11 McMillen Jacobs Associates 1. The thickness should not be less than 1.25 times the diameter of the upper limit D100 rock (1.25 x 16.5 inches = 20.6 inches) 2. The thickness should not be less than 12 inches for practical placement. For effective protection, the thickness should not be less than 9 inches. 3. The thickness determined by either 1 or 2 above should be increased by 50 percent in all sections when the riprap is placed underwater in water deeper than 3 feet to provide for uncertainties associated with this type of placement (i.e. no less than 31.0 inches). 4. An increase in thickness of 6 to 12 inches, accompanied by an appropriate increase in rock sizes, should be provided where riprap revetment will be subject to attack by floating debris or ice or by waves from boat wakes or wind. This condition is not expected at the facility. 5. Based on the general scour analysis discussed above in Section 2.2, the channel bed surrounding the facility can be expected to degrade as much as 1.0 feet in the worst case. Thus, in order to protect the riprap layer from being undercut, an additional 1.0 feet should be added to the layer thickness. From the above conditions, therefore, a riprap layer of approximately 31 inches will be stable. However, for added safety, it is recommended that the riprap protection layer shall be 4.0 feet thick. Experiences [9] have shown that these thicknesses are adequate regardless of whether a granular filter or a geotextile is used with riprap. Both of these design options are discussed in the following sections. 2.4 Filter Design A filter is a transitional layer of gravel, small stone, or fabric placed between the underlying soil and the structure. The filter prevents the migration of the fine soil particles through voids in the structure, distributes the weight of the armor units to provide more uniform settlement, and permits relief of hydrostatic pressures within the soils. For areas above the water line, filters also prevent surface water from causing erosion (gullies) beneath the riprap. A filter should be used whenever the riprap is placed on non-cohesive material subject to significant subsurface drainage (such as in areas where water surface levels fluctuate frequently and in areas of high groundwater levels). The proper design of granular and fabric filters is critical to the stability of riprap installations on channel banks. If openings in the filter are too large, excessive flow piping through the filter can cause erosion and failure of the bank material below the filter. On the other hand, if the openings in the filter are too small, the build-up of hydrostatic pressures behind the filter can cause a slip plane to form along the filter resulting in massive translational slide failure. Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 12 McMillen Jacobs Associates 2.4.1 Granular Filters For rock riprap, a filter ratio of 5 or less between layers will usually result in a stable condition. The filter ratio is defined as the ratio of the 15 percent particle size (D15) of the coarser layer to the 85 percent particle size (D85) of the finer layer. An additional requirement for stability is that the ratio of the 15 percent particle size of the coarser material to the 15 percent particle size of the finer material should exceed 5 but be less than 40 [8]. These requirements can be stated as: GHI JK KLJ GMI NOKPJ KLJ < 5 < GHI JJ KLJ GHI NOKPJ KLJ < 40 (10) The left side of the inequality in Equation 10 is intended to prevent piping through the filter, the center portion provides for adequate permeability for structural bedding layers, and the right portion provides a uniformity criterion. If a single layer of filter material will not satisfy the filter requirements, one or more additional layers of filter material must be used. The filter requirement applies between the bank material and the filter blanket, between successive layers of filter material if more than one layer is used, and between the filter blanket and the riprap cover. In addition to the filter requirements, the grain size curves for the various layers should be approximately parallel to minimize the infiltration of fine material from the finer layer to the coarser layer. Not more than 5 percent of the filter material should pass the No. 200 sieve. The thickness of the filter blanket should range from about 6 inches to about 15 inches for a single layer, or from about 4 inches to 8 inches for individual layers of a multiple layer blanket. Where the gradation curves of adjacent layers are approximately parallel, the thickness of the blanket layers should approach the minimum. The thickness of individual layers should be increased above the minimum proportionally as the gradation curve of the material comprising the layer departs from a parallel pattern. The material gradation curve (Figure 5) indicates the following rock characteristics: D15 base =0.1 inches (2.8 mm) D50 base =0.6 inches (15.4 mm) D85 base = 2.5 inches (63.0 mm) D100 base = 5.4 inches (137.5 mm) and D15 riprap = 3.3 inches (63.5 mm) D50 riprap = 8.3 inches (157.5 mm) D85 riprap = 12.8 inches (243.8 mm) D100 riprap = 16.5 inches (315.0 mm) Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 13 McMillen Jacobs Associates Based on the gradations above, the criteria given in Equation 10 are satisfied based on the riprap layer and native alluvium found in the project reach, such that the native alluvium will serve as an adequate filter layer. 3.0 Conclusion Four common empirical relationships have been used for computing scour hole depth at the facility along the Cedar River. The study shows that the potential scour hole depth at the project location would be up to 6 feet upstream of the facility and 10 feet downstream of the facility. Design methods have been used based on tractive stress for final riprap design to determine the required size of rock at the project location. The gradation of stones in riprap revetment affects the riprap’s resistance to erosion. Based on guidance from the literature, the layer thickness of the riprap blanket shall be no less than 4.0 feet. The D50 of the design riprap is approximately 8.3 inches, with a D15 of 3.3 inches, a D85 of 12.8 inches, and a D100 of 16.5 inches. Based on granular filter layer computations, it was concluded that the native alluvial material will serve as an adequate filter layer for the design riprap. An approximate sketch of the proposed riprap layer is shown in the figure below. Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 14 McMillen Jacobs Associates Figure 5. Proposed Layout and Details of the Riprap Scour Protection Blanket at the BCF Seattle Public Utilities – Broodstock Collection Facility Scour Analysis Rev. 1 / November 2019 15 McMillen Jacobs Associates 4.0 References [1] Cascade Environmental Services (2011), Flow Regulation and Protecting Incubating Cedar River Salmon From Streambed Scour, City of Renton, WA. [2] Northwest Hydraulic Consultants Inc. (2018), Cedar River 207 Annual Sediment Report, City of Renton, WA. [3] Guan, D, Friedrich, H. and Melville B.W., (2015), Scour at Submerged Weirs, Dep. of Civil and Environmental Eng., University of Auckland, New Zealand. [4] Meftah M.B. and Mossa, M. (2006), Scour holes downstream of bed sills in low-gradient channels, Journal of Hydraulic Research Vol. 44, No. 4 (2006), pp. 497–509. [5] Marion, A.,Lenzi, M.A. and Comiti, F. (2004). “Effect of Sill Spacing and Sediment Size Grading on Scouring at Grade-Control Structures”. Earth Surface Processes Landforms 29, 983–993. [6] Lenzi, M.A., Marion, A.,Comiti, F. and Gaudio, R. (2002). “Local Scouring in Low and High Gradient Streams at Bed Sills”. J. Hydraul. Res. IAHR 40(6), 731–739. [7] USDA (2007), Technical Supplement 14B- Scour Calculations. [7] Hoffmans, G.J.C.M. and Verheij, H.J. (1997). Scour Manual. A.A. Balkema, Rotterdam, Brookfield. [8] Richardson, E.V. and Davis, S.R. (1995) Evaluating Scour at Bridges. U.S. Department of Transportation, Federal Highway Administration Hydraulic Engineering Circular 18, Publication FHWA-IP-90-017, 204 p. [9] Cedar River (2018), 2018 Annual Sediment Report, Final Report, City of Renton, Renton, WA. [10] Koerner, R.M. 2005b. Commentary on geosynthetic reduction factors – Part III – Drainage with geonets, geocomposites and geospacers. GFR magazine, Vol. 23, No. 2, pp. 14–15.