HomeMy WebLinkAbout6618; AGUA HEDIONDA STREAMBANK RESTORATION; Riprap - Sizing of Rock for Bank Repair; 2016-03-11
215127 Revised Rock Sizing Memo 03-11-2016 1
BALANCE HYDROLOGICS, Inc.
MEMO
To: Wayne Chang, P.E. (Chang Consultants)
From: Edward D. Ballman, P.E.
Date: March 11, 2016
Subject: Calculations for Sizing of Rock for the Bank Repair Project
at the Coman Property, City of Carlsbad
At your request Balance Hydrologics staff completed calculations to support the selection of
rock lining material for the main creek channel and storm drain outfall remediation associated
with the proposed channel bank repair along Agua Hedionda Creek at the Coman property.
Sizing calculations were completed for two separate applications of rock lining to provide
flexibility in the selection of materials should you so choose. The first of these is the proposed
retrofit of the existing storm drain outfall that enters the creek at the site, and the second pertains
to rock that can be used to line the channel of Agua Hedionda Creek itself. Each of the analyses
are summarized below.
Storm Drain Outfall Rock
Sizing of rock for reconstructing the existing storm drain outfall was carried out using force
balance equations embedded in a spreadsheet solver developed by our staff. Input data for the
spreadsheet included parameters for flow depth and velocity based from a trapezoidal channel
calculator (Hydrocalc Hydraulics by Dodson & Associates) based on the following assumptions:
Design flow: 100 cfs (Q100 depicted on storm drain plans provided by the City)
Channel bottom width: 7.5 feet
Channel bed slope: 1:1
Channel side slopes: 1:1
Manning’s n: 0.045 (San Diego County guidance for rock lining)
The resulting design parameters are:
Flow depth: 0.58 feet
Flow velocity: 21.2 feet/second
The above depth and velocity numbers were then entered into the spreadsheet solver, which was
used iteratively to identify a rock diameter that provided a reasonable factor of safety for the
100-year flow rate. The identified minimum rock diameter was found to be 2-foot diameter,
yielding a factor of safety against mobilization of 1.4. This solution was found with an
215127 Revised Rock Sizing Memo 03-11-2016 2
embedment depth of 0.9 feet.1 The spreadsheet solution for this case is included as Attachment
A.
Channel Lining Rock
Sizing of rock for use as lining of the creek channel itself was completed in generally the same
manner. However, in this case, input data for the spreadsheet included parameters for flow depth
and velocity based on previous modeling completed by Chang Consultants using the U.S. Army
Corps of Engineers’ HEC-RAS hydraulic modeling package. The pertinent design parameters
include:
Design flow: 8,080 cfs (Q100 flow rate from HEC-RAS output)
Cross-section as per HEC-RAS with bed slope = 0.011 ft/ft
Manning’s n: 0.045 (San Diego County guidance for rock lining)
Flow depth: 13.5 feet
Flow velocity: 17.5 feet/second
These depth and velocity numbers were entered into the spreadsheet solver, which was used
iteratively to identify a rock diameter that provided a reasonable factor of safety for the 100-year
flow rate. The identified minimum rock diameter was found to be 2.25feet, yielding a factor of
safety against mobilization of 1.2, with an embedment depth of 1.1 feet. In this case, it is clear
that somewhat larger rock would be recommended for a higher factor of safety. The spreadsheet
solution for this case is included as Attachment B.
Specified Rock Material
Per the recently prepared plans for the bank repair project, it appears that a single rock class with
a median diameter of 2.9 feet (1-ton class) will be used for both applications. Calculations for
the storm drain outfall using that rock size are included as Attachment C and show a factor of
safety of 3.9.
Likewise, the calculations for the channel lining rock were updated for the 2.9-foot diameter
material (see Attachment D) and show a factor of safety of 1.5 (with much higher resistance to
mobilization if the rock is embedded beyond half diameter.
1 The spreadsheet functions are set up to allow a maximum embedment of half the rock diameter. Any embedment
greater than that cited above can be assumed to be more stable and yield a higher factor of safety.
Variable name Symbol Value Units Value UnitsBoulder diameterD 2 ft 0.610 mBoulder radiusr 1 ft 0.305 mBoulder densityρL165lb·ft-32643.046kg·m-3Buriel depthk 0.9 ft 0.274 mSubmerged cross sectional areaAsub--ft20.095m2Submerged volumeVsub--ft30.048m3Submerged surface areaSAsub--ft20.339m2Water densityρw62.43lb·ft-31000kg·m-3Gravitational accel.g 32.17ft·s-29.8m·s-2Bed friction coefficientµbed0.47 -- 0.47 --Drag coefficient (Rep)CD-- -- 1.500 --Drag coefficient (Gippel et al, 1996)CD-- -- 0.9 --Lift coefficient (Scheidegger; Munson)CL-- -- 0.178 --Dynamic viscosity (at 10°C)µ -- -- 0.00131N·s·m-2Kinematic viscosity (at 10°C)ν-- -- 0.00000131m2·sParticle Reynold's NumberRep-- -- 3006938.089 --Design eventQ 100 cfs 2.832 cmsFlow depth submerging boulderdw0.58 ft 0.177 mFlow velocity near boulderU 21.2ft·s-16.462m·s-1Bed slope at boulder locationα1ft·ft-145.000 degreesManning's roughness coefficientn 0.075 -- 0.075 --Dragld0.29 ft 0.09 mGravity (Resisting)lgr0.99 ft 0.30 mGravity (Driving)lgd0.10 ft 0.03 mLift (Resisting)lLr0.10 ft 0.03 mLift (Driving)lLd0.99 ft 0.30 mNormal ForceFN--lbf1837.83 NLift Force (Resisting)FLr--ft·lbf7.64 N·mLift Force (Driving)FLd--ft·lbf75.97 N·mEffective WeightWeff--lbf2599.08 NGravity Force (Resisting)Fgr--ft·lbf557.36 N·mGravity Force (Driving)Fgd--ft·lbf56.02 N·mDrag ForceFdrag--ft·lbf281.15N·mIs the boulder stable?-- -- -- YES--Attachment A: Storm drain outfall with rock diamter = 2.0 feetGreen cells are calculated by spreadsheet tool.Light blue cells are calculated by spreadsheet tool and are the resultant moments that determine the particle stability.Rock characteristicsPhysical constantsHydraulic inputs (from HEC or otherwise)Moment ArmsResultant Forces and MomentsBlue cells contain values that must be entered by user.Note that moment arms are specific to boulder burial depth and location of center of rotation; magnitudes may be different than depicted here.Burial depth, k, is limited to boulder radius, r. Greater burial depth leads to a stable, embedded boulder, regardless of ambient forces.+Y+X-Y-XFN, FLdWeff,x; FdragWeffαkdwDWeff,yCenter of rotationlldd, l, lgdgd,,llLrLrllLdLd, l, lgrgrDrFLrAsubFLForce magnitudes not drawn to scale.215127 SD Outfall Rock Sizing Calcs, Spherical Rock, n=0.045
Variable name Symbol Value Units Value UnitsBoulder diameterD 2.25 ft 0.686 mBoulder radiusr 1.125 ft 0.343 mBoulder densityρL165lb·ft-32643.046kg·m-3Buriel depthk 1.1 ft 0.335 mSubmerged cross sectional areaAsub--ft20.190m2Submerged volumeVsub--ft30.087m3Submerged surface areaSAsub--ft20.755m2Water densityρw62.43lb·ft-31000kg·m-3Gravitational accel.g 32.17ft·s-29.8m·s-2Bed friction coefficientµbed0.47 -- 0.47 --Drag coefficient (Rep)CD-- -- 1.500 --Drag coefficient (Gippel et al, 1996)CD-- -- 0.9 --Lift coefficient (Scheidegger; Munson)CL-- -- 0.178 --Dynamic viscosity (at 10°C)µ -- -- 0.00131N·s·m-2Kinematic viscosity (at 10°C)ν-- -- 0.00000131m2·sParticle Reynold's NumberRep-- -- 2792410.076 --Design eventQ 8080 cfs 228.800 cmsFlow depth submerging boulderdw13.5 ft 7.000 mFlow velocity near boulderU 17.5ft·s-15.334m·s-1Bed slope at boulder locationα0.011ft·ft-10.630 degreesManning's roughness coefficientn 0.05 -- 0.05 --Dragld0.58 ft 0.18 mGravity (Resisting)lgr1.12 ft 0.34 mGravity (Driving)lgd0.03 ft 0.01 mLift (Resisting)lLr0.03 ft 0.01 mLift (Driving)lLd1.12 ft 0.34 mNormal ForceFN--lbf3519.10 NLift Force (Resisting)FLr--ft·lbf0.04 N·mLift Force (Driving)FLd--ft·lbf164.85 N·mEffective WeightWeff--lbf3519.32 NGravity Force (Resisting)Fgr--ft·lbf1206.40 N·mGravity Force (Driving)Fgd--ft·lbf0.29 N·mDrag ForceFdrag--ft·lbf847.29N·mIs the boulder stable?-- -- -- YES--Attachment B: Creek channel with rock diameter = 2.25 feetGreen cells are calculated by spreadsheet tool.Light blue cells are calculated by spreadsheet tool and are the resultant moments that determine the particle stability.Rock characteristicsPhysical constantsHydraulic inputs (from HEC or otherwise)Moment ArmsResultant Forces and MomentsBlue cells contain values that must be entered by user.Note that moment arms are specific to boulder burial depth and location of center of rotation; magnitudes may be different than depicted here.Burial depth, k, is limited to boulder radius, r. Greater burial depth leads to a stable, embedded boulder, regardless of ambient forces.+Y+X-Y-XFN, FLdWeff,x; FdragWeffαkdwDWeff,yCenter of rotationlldd, l, lgdgd,,llLrLrllLdLd, l, lgrgrDrFLrAsubFLForce magnitudes not drawn to scale.
Variable name Symbol Value Units Value UnitsBoulder diameterD 2.9 ft 0.884 mBoulder radiusr 1.45 ft 0.442 mBoulder densityρL165lb·ft-32643.046kg·m-3Buriel depthk 1.4 ft 0.427 mSubmerged cross sectional areaAsub--ft20.146m2Submerged volumeVsub--ft30.104m3Submerged surface areaSAsub--ft20.491m2Water densityρw62.43lb·ft-31000kg·m-3Gravitational accel.g 32.17ft·s-29.8m·s-2Bed friction coefficientµbed0.47 -- 0.47 --Drag coefficient (Rep)CD-- -- 1.500 --Drag coefficient (Gippel et al, 1996)CD-- -- 0.9 --Lift coefficient (Scheidegger; Munson)CL-- -- 0.178 --Dynamic viscosity (at 10°C)µ -- -- 0.00131N·s·m-2Kinematic viscosity (at 10°C)ν-- -- 0.00000131m2·sParticle Reynold's NumberRep-- -- 4360060.228 --Design eventQ 100 cfs 2.832 cmsFlow depth submerging boulderdw0.58 ft 0.177 mFlow velocity near boulderU 21.2ft·s-16.462m·s-1Bed slope at boulder locationα1ft·ft-145.000 degreesManning's roughness coefficientn 0.075 -- 0.075 --Dragld0.29 ft 0.09 mGravity (Resisting)lgr1.45 ft 0.44 mGravity (Driving)lgd0.05 ft 0.02 mLift (Resisting)lLr0.05 ft 0.02 mLift (Driving)lLd1.45 ft 0.44 mNormal ForceFN--lbf5901.85 NLift Force (Resisting)FLr--ft·lbf5.86 N·mLift Force (Driving)FLd--ft·lbf169.91 N·mEffective WeightWeff--lbf8346.48 NGravity Force (Resisting)Fgr--ft·lbf2606.83 N·mGravity Force (Driving)Fgd--ft·lbf89.94 N·mDrag ForceFdrag--ft·lbf407.66N·mIs the boulder stable?-- -- -- YES--Green cells are calculated by spreadsheet tool.Light blue cells are calculated by spreadsheet tool and are the resultant moments that determine the particle stability.Attachment C: Storm drain outfall with rock diamter = 2.9 feetRock characteristicsPhysical constantsHydraulic inputs (from HEC or otherwise)Moment ArmsResultant Forces and MomentsBlue cells contain values that must be entered by user.Note that moment arms are specific to boulder burial depth and location of center of rotation; magnitudes may be different than depicted here.Burial depth, k, is limited to boulder radius, r. Greater burial depth leads to a stable, embedded boulder, regardless of ambient forces.+Y+X-Y-XFN, FLdWeff,x; FdragWeffαkdwDWeff,yCenter of rotationlldd, l, lgdgd,,llLrLrllLdLd, l, lgrgrDrFLrAsubFLForce magnitudes not drawn to scale.215127 SD Outfall Rock Sizing Calcs, Spherical Rock, n=0.045 D = 2.9
Variable name Symbol Value Units Value UnitsBoulder diameterD 2.9 ft 0.884 mBoulder radiusr 1.45 ft 0.442 mBoulder densityρL165lb·ft-32643.046kg·m-3Buriel depthk 1.4 ft 0.427 mSubmerged cross sectional areaAsub--ft20.320m2Submerged volumeVsub--ft30.190m3Submerged surface areaSAsub--ft21.270m2Water densityρw62.43lb·ft-31000kg·m-3Gravitational accel.g 32.17ft·s-29.8m·s-2Bed friction coefficientµbed0.47 -- 0.47 --Drag coefficient (Rep)CD-- -- 1.500 --Drag coefficient (Gippel et al, 1996)CD-- -- 0.9 --Lift coefficient (Scheidegger; Munson)CL-- -- 0.178 --Dynamic viscosity (at 10°C)µ -- -- 0.00131N·s·m-2Kinematic viscosity (at 10°C)ν-- -- 0.00000131m2·sParticle Reynold's NumberRep-- -- 3599106.321 --Design eventQ 8080 cfs 228.800 cmsFlow depth submerging boulderdw13.5 ft 7.000 mFlow velocity near boulderU 17.5ft·s-15.334m·s-1Bed slope at boulder locationα0.011ft·ft-10.630 degreesManning's roughness coefficientn 0.05 -- 0.05 --Dragld0.75 ft 0.23 mGravity (Resisting)lgr1.45 ft 0.44 mGravity (Driving)lgd0.05 ft 0.02 mLift (Resisting)lLr0.05 ft 0.02 mLift (Driving)lLd1.45 ft 0.44 mNormal ForceFN--lbf7502.36 NLift Force (Resisting)FLr--ft·lbf0.14 N·mLift Force (Driving)FLd--ft·lbf358.21 N·mEffective WeightWeff--lbf7502.82 NGravity Force (Resisting)Fgr--ft·lbf3313.77 N·mGravity Force (Driving)Fgd--ft·lbf1.26 N·mDrag ForceFdrag--ft·lbf1857.95N·mIs the boulder stable?-- -- -- YES--Green cells are calculated by spreadsheet tool.Light blue cells are calculated by spreadsheet tool and are the resultant moments that determine the particle stability.Attachement D. Creek channel with rock diameter = 2.9 feetRock characteristicsPhysical constantsHydraulic inputs (from HEC or otherwise)Moment ArmsResultant Forces and MomentsBlue cells contain values that must be entered by user.Note that moment arms are specific to boulder burial depth and location of center of rotation; magnitudes may be different than depicted here.Burial depth, k, is limited to boulder radius, r. Greater burial depth leads to a stable, embedded boulder, regardless of ambient forces.+Y+X-Y-XFN, FLdWeff,x; FdragWeffαkdwDWeff,yCenter of rotationlldd, l, lgdgd,,llLrLrllLdLd, l, lgrgrDrFLrAsubFLForce magnitudes not drawn to scale.215127 Channel Rock Stability Calcs, Spherical Rock, Q100 D = 2.9