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HomeMy WebLinkAboutCT 02-16; ROBERTSON RANCH; TEMPORARY DESILTING BASIN REPORT; 2007-09-18cr O7t(v ROBERTSON RANCH EAST VILLAGE CT 02-16 TEMPORARY DESILTING BASIN REPORT JOB NO. 01-1014 REVISED: SEPTEMBER 18, 2007 PREPARED: SEPTEMBER 20, 2005 - RCE 60223 EXP. 06/30/08 nq OFESS/o Lu 60223 r; Exp. 6/30/08 CIVIL OF CA 01-fZP7 DATE O'DAYCONSULTANTS, INC. 2710 LOKER AVENUE WEST, SUITE 100 CARLSBAD, CA 92010 TEL: (760) 931-7700 FAX: (760) 931-8680 G:\01 I 014\Hydrology\TempDcsiltBasinCalcs-rev3.doc Temporary Desiltine Basin Calculations Desilting Basin Sizing Desilting basins were sized based on the 10-year, 6-hour storm event. The design particle size used is .01 mm. (fine silt) The equation used to size the basins is: As= 1.2*Qavg/ Vs Qavg= C*Iavg*A C= 0.35 Iavg4.7"/6 hr = 0.28 in/hr 10-year, 6-hour storm event 1.7" A= area of basin Vs= .00024 ft/s (.01nun sized particle) where As is the appropriate surface area for trapping particles of a certain size and Vs is the settling velocity for that size particle. In this case, Vs= .00024 ft/s. Qavg is the average runoff from each basin during a 10-year, 6-hour storm. Temporary Desiltation Basin Capacity per DS-3 Desiltation basin sizes were also checked per the capacity table in the City of Carlsbad Standard Drawing DS-3. All of the basins' capacity is significantly greater than that required by DS-3. Sediment Storage Volume Required The amount of sediment that the temporary desilting basins are designed to store is the amount generated by 5 years worth of 2-year, 24-hour storm events. The two-year storm event was used per City standards. (Chapter 7-Grading and Erosion Control Standards) The universal soil loss equation was used to estimate sediment volumes entering the desiltmg basins. A= R x K x LS x C x P A= soil loss, tons/(acre)(year) R= rainfall erosion index, 100 ft. tons/acre x in/hr K= soil erodibility factor, tons/acre per unit of R LS= slope length and steepness factor, dimensionless P= erosion control practice factor, dimensionless Rainfall Erosion Index, R For a Type I Storm: Rl6.55 p2.2 = 16.55*(1.3)2.2 = 29.48 P=1.3" (2-year, 6-hour storm event per Erosion and Sediment Control Handbook) GAO I I0I4\Hydrology\lempDesiltBasinCalcs.rev3 .doc Soil Erosion Factor K The existing soil was approximately 63.2% sand, 23.0% clay, 13.0% silt and 5.8% gravel. K is approximately 0. 15. Slope Length and Steepness Factor LS See attached Table 5.5 taken from Erosion and Sediment Control Handbook for LS equation used. Cover Factor C Mass Graded Pad: C=1.0 Erosion Control Practice Factor P Pads will be compacted and smooth (P = 1.3) Dewatering Orifice Sizing Calculations The dewatering holes on the standpipes for each basin are designed to allow sediment to settle for 40 hours before outletting through the standpipe. The following equation was used to size the dewatering holes Ao= As (2h)5 / 3600*T*Cd*(g)5 Ao= Surface area of orifice (sf) As= Basin area (sO h= Head of water (ft) T= Time (hrs) Cd= 0.6 (sharp edged orifice) g= Acceleration of gravity= 32.2 ft/s' Standpipe Riser Sizing Calculations The standpipe risers are sized to allow the 100-year storm event to outlet from the basin without completely filling it. The following equation was used to size the standpipe risers: '...' - I' T - SCWId-1 312 Where: Csw = 3.27 & 0.4 /HW L= Circumference of Standpipe H= Head above Standpipe Hw Height of Standpipe G:\O1 1014\Hydrology\TempDesiltBasinCalcs.rev3.doc Temporary Desilting Basin Calculations 10-Year Storm Event P6 = 1.7 in. Basin Area (ac.) C L. (in/hr) Q(cfs) Vs (ftls) As reg'd (sO Width Length As used 1 26.47 0.35 0.28 2.59 0.00024 13,125 60 680 40800 2 13.58 0.35 0.28 1.33 0.00024 6,733 60 400 24000 3 3.15 0.35 0.28 0.31 0.00024 1,562 60 120 7200 4 2.31 0.35 0.28 0.23 0.00024 1,145 60 120 7200 5 17.03 0.35 0.28 1.67 0.00024 8,444 60 400 24000 6 5.43 0.35 0.28 0.53 0.00024 2,692 60 140 8400 7 8.67 0.35 0.28 0.85 0.00024 4,299 60 200 12000 8 2.63 0.35 0.28 0.26 0.00024 1,304 60 120 7200 As= (1.2Qavg)NS Year, 24-Hour Storm Event, 5 Years of Sediment Basin Soil Loss (cy/5vr) Soil Storage Depth (ft) 1 66.20 0.04 2 50.31 0.06 3 8.63 0.03 4 13.50 0.05 5 28.21 0.03 6 9.49 0.03 7 19.85 0.05 8 48.96 0.18 Basin Qa (in) Area (ac.) Volume (cf) Storage Depth (ft) 1 1.30 26.47 124,912 3.1 2 1.30 13.58 64,084 2.7 3 1.30 3.15 14,865 2.1 4 1.30 2.31 10,901 1.5 5 1.30 17.03 80,365 3.4 6 1.30 5.43 25,624 3.1 7 1.30 8.67 40,914 3.4 8 1.30 2.63 12,411 1.7 P24=1.9 in (2-Year Storm) 0a (P24 0.25)21(P24+0.8S) S=1000/CN-10 CN=94 (2003 San Diego County Hydrology Manual, Table 4-2) G:\01 1014\Hydrology\RREastDesiltBasins-3.xls Dewaterinci Orifice Sizinci Calculations Basin As (Sf) 1 40800 3.1 2 24000 2.7 3 7200 2.1 4 7200 1.5 5 24000 3.4 6 8400 3.1 7 12000 3.4 8 7200 1.7 Standoine Riser Sizina Calculations For 0- T (hr) Cd G (ft/s') Ao (sO Ao (in 2) 40 0.6 32.2 0.207 29.84 Use 10- 2" dia. holes 40 0.6 32.2 0.114 16.38 Use 6- 2" dia. holes 40 0.6 32.2 0.030 4.33 Use 6- 1" dia. holes 40 0.6 32.2 0.025 3.66 Use 5- 1" dia. holes 40 0.6 32.2 0.128 18.38 Use 6- 2" dia. holes 40 0.6 32.2 0.043 6.14 Use 8- i" dia. holes 40 0.6 32.2 0.064 9.19 Use 6- 11/2fl dia. holes 40 0.6 32.2 0.027 3.90 Use 5- 1" dia. holes P6(100yr-24hr) (in.) Tc (mm.) I (in.! hr.) Area (in 1) (AC.) C Qioo (cfs) 2.6 12.5* 3.79 26.47 0.35 35.11 Basin MAY C-qrw MIN LLW L(ft I thru8 35.11 3.39 1 10.36 3.30 Use 42" CMP Riser * Ti = 12.5 COMING OFF OF A PAD @ 1% PER TABLE 3-2 OF THE SAN DIEGO COUNTY HYDROLOGY MANUAL G:01 1014\Hydrology\RREastDesiltBasins-3.xls BASIN I Soil Loss: Pads: A= Rx Kx LSx C x P = 0.69 R= 29.48 K= 0.15 LS= 0.12(100'@l%) C= 1.0 P= 1.3 Streets: A= R x K x LS x C x P = 3.05 R= 29.48 K= 0.15 LS (0.20 x 1122) + (1.50 x 480) + (0.14 x 223) = _______________________________ = 0.53 1122 +480 +223 C=. 1.0 P= 1.3 Slopes: A= R x K x LS x C x P = 0.32 R= 29.48 K= 0.15 LS = 5.64 (10 @ 50%) C= 0.01 P= 1.3 Natural Ground Cover (Ooen SDace: A= R x K x LS x C x P = 0.02 R= 29.48 K= 0.15 LS = 5.08 (660' @ 12%) C= 0.01 P= 0.09 A = 0.69 (9.59) + 3.05 (4.70) + 0.32 (2.59) + 0.02 (9.59) 9.59 + 4.70 + 2.59 + 9.59 = 0.83 tons/acre . year Sediment Storage = (0.83 tons/acre year) x 5 years x 26.47 acres = 109.85 tons Average Density = 123 lbs/ft3 = 1.66 tons/C.Y. Sediment Storage Volume = 66.20 C.Y. G:\01 1014\Hydrology\TempDesiltBasin_lnfo.xls BASIN 2 Soil Loss: Pads: A= RxKxLSxCxP0.69 R= 29.48 K= 0.15 LS= 0.12(100'@l%) C= 1.0 P= 1.3 Streets: A= R x K x LS x C x P = 4.37 R= 29.48 K= 0.15 LS = (0.19 x 750) + (0.36 x 730) + (2.02 x 570) = 0.76 750+730+570 C= 1.0 P= 1.3 Slopes: A= R x K x LS x C x P = 0.32 R= 29.48 K= 0.15 LS = 5.64 (10' @ 50%) 0= 0.01 P= 1.3 Natural Ground Cover (Ooen Soace: A= R x K x LS x C x P = 0.02 R= 29.48 K= 0.15 LS = 5.08 (660'@ 12%) C= 0.01 P= 0.09 A = 0.69 (6.32) + 4.37 (2.77) + 0.32 (0.40) + 0.02 (4.09) 6.32 + 2.77 + 0.40 + 4.09 = 1.23 tons/acre . year Sediment Storage = (1.23 tons/acre year) x 5 years x 13.58 acres = 83.52 tons Average Density = 123 lbs/ft3 = 1.66 tons/C.Y. Sediment Storage Volume = 50.31 C.Y. BASIN 3 Soil Loss: Pads: A= RxKxLSxCxP=0.69 R= 29.48 K= 0.15 LS= 0.12(100'@l%) C= 1.0 P= 1.3 Streets: A= RxKxLSxCxP=1.90 R= 29.48 K= 0.15 LS = 0.33 (500' @ 2%) C=. 1.0 P= 1.3 Slopes: A= RxKxLSxCxP=0.32 R= 29.48 K= 0.15 LS = 5.64 (10' © 50%) C= 0.01 P= 1.3 A = 0.69 (2.18) + 1.90 (0.67) + 0.32 (0.30) 2.18 + 0.67 + 0.30 = 0.91 tons/acre . year Sediment Storage = (0.91 tons/acre . year) x 5 years x 3.15 acres = 14.33 tons Average Density = 123 lbs/ft3 1.66 tons/C.Y. Sediment Storage Volume = 8.63 C.Y. BASIN 4 Soil Loss: Pads: A= R x K x LS x C x P = 0.69 R= 29.48 K= 0.15 LS= 0.12(100'@1%) C= 1.0 P= 1.3 Streets: A= RxKxLSxCxP=4.37 R= 29.48 K= 0.15 LS = 0.76 (500' @ 4%) C= 1.0 P= 1.3 Slopes: A= RxKxLSxCxP=O.32 R= 29.48 K= 0.15 LS = 5.64 (10' @ 50%) C= 0.01 P= 1.3 A = 0.69 (1.06) + 4.37 (0.83) + 0.32 (0.42) 1.06 + 0.83 + 0.42 = 1.94 tons/acre year Sediment Storage = (1.94 tons/acre year) x 5 years x 2.31 acres = 22.41 tons Average Density = 123 lbs/ft3 = 1.66 tons/C.Y. Sediment Storage Volume = 13.5 C.Y. BASIN 5 Soil Loss: Pads: A= R x Kx LS x Cx P = 0.69 R= 29.48 K= 0.15 LS= 0.12(100'©l%) C=• 1.0 P= 1.3 Streets: A= R x K x LS x C x P = 5.12 R= 29.48 K= 0.15 LS = (1.90 x 840) + (0.49 x 600) + (0.29 x 120) + (0.16 x 280)+ (0.34 x 620) = 0.89 2460 C= 1.0 P= 1.3 SloDes: A= R x K x LS x C x P = 0.32 R= 29.48 K= 0.15 LS = 5.64 (10'@ 50%) C= 0.01 P= 1.3 Natural Ground Cover (ODen SDace: A= R x K x LS x C x P = 0.02 R= 29.48 K=. 0.15 LS = 4.18(720' @11%) C= 0.01 P= 0.09 A = 0.69 (7.65) + 0.89 (3.63) +0.32 (2.62) + 0.02 (3.13) 17.03 = 0.55 tons/acre year Sediment Storage = (0.55 tons/acre . year) x 5 years x 17.03 acres = 46.83 tons Average Density = 123 lbs/ft3 = 1.66 tons/C.Y. Sediment Storage Volume = 28.21 C.Y. BASIN 6 Soil Loss: Pads: A= R x K x LS x C x P = 0.69 R= 29.48 K= 0.15 LS= 0.12(100'©l%) C= 1.0 P= 1.3 Streets: A= R x K x LS x C x P = 2.30 R= 29.48 K= 0.15 LS = (0.40 x 1000) + (0.38 x 250) = 0.40 1250 C= 1.0 P= 1.3 Slopes: A= RxKxLSxCxP=0.32 R= 29.48 K= 0.15 LS = 5.64 (10' @ 50%) C= 0.01 P= 1.3 A = 0.69 (3.49) + 0.40 (1.38) + 0.32 (0.56) 3.49 + 1.38 + 0.56 = 0.58 tons/acre year. Sediment Storage = (0.58 tons/acre . year) x 5 years x 5.43 acres = 15.75 tons Average Density = 123 lbs/ft3 = 1.66 tons/C.Y. Sediment Storage Volume = 9.49 C.Y. BASIN 7 Soil Loss: Pads: A= R x K x LS x C x P = 0.69 R= 29.48 K= 0.15 LS= 0.12(100'@l%) C= 1.0 P= 1.3 Streets: A= R x Kx LS x C x P = 1.21 R= 29.48 K= 0.15 LS = (0.20 x 1000) + (0.26 x 260) = 0.21 1260 C=. 1.0 P= 1.3 Slopes: A= R x Kx LS x C x P = 0.32 R= 29.48 K= 0.15 LS = 5.64 (10' @ 50%) C= 0.01 P= 1.3 A = 0.69 (5.60) + 1.21 (1.91) + 0.32 (1.16) 8.67 = 0.76 tons/acre . year Sediment Storage = (0.76 tons/acre year) x 5 years x 8.67 acres = 32.95 tons Average Density = 123 lbs/ft3 = 1.66 tons/C.Y. Sediment Storage Volume = 19.85 C.Y. BASIN 8 Soil Loss: Pad: A= RxKxLSxCxP=7.53 R= 29.48 K= 0.15 LS = 1.31 (600' © 5%) C= 1.0 P= 1.3 Slopes: A= RxKxLSxCxP=0.46 R= 29.48 K= 0.15 LS = 7.97 (20' @ 50%) C= 0.01 P= 1.3 A = 7.53 (2.13) + 0.46 (0.50) 2.63 = 6.18 tons/acre year Sediment Storage = (6.18 tons/acre . year) x 5 years x 2.63 acres = 81.27 tons Average Density = 123 lbs/ft3 = 1.66 tons/C.Y. Sediment Storage Volume = 48.96 C.Y. ItIImIIJi * din mass U5•.l IPji55 U.*•1 Pm5 •5so huaki15l55*1 ass sa ia Ikq, ra 5 15SLIileuiwnmnm 'a".... U p'•- '4Lar I am ap a... Mai IL 51 '.a p PU•5 -iiik;,emlK5t wool W Man All ii maria Ur -.IBM : .. 1* I" V*; * i;I1 F .5 ' I - -' ''-' JUL 'iUiIili idt1 .'lIhU;a cuss 1Ih'?.a ,.ii,.ji 1 •. 'i'U1.—i . 3II... t _'v vr ".'.' .tf Ili' •' ; !-'-'- -'.'9a. ,. rsi fj1q U ad g1h,pp •T4 lULp -a.!i1y. i111s1 i' •wa. MCA groats&. 5 kaL -- - -..0 Sam, - r - I4. - ••Ui '.io'a'.; I/?, INuuli no IfalIçdIl I i .- Ip,I I lUPPVlTII ii.. U LENGTH PER PLAN I] F 0 L..STANDPIPE-18'0 PIPE,MINIMUM WITH NO PERFORATIONS 6 RESAR,SPACED 6'O.C. AROUND RISER CIRCUMFERENCE S7 PLATE 4' RED PAINT 12'x STRIPE ALL 3 PIPE O.D. AROUND RISER r + PLAN VIEW NTS FACES SIMILAR TO SORS D-7O t 'u".._.-21 OR fl.A11ER IF SO SPECIFIED ON PLANS TYPICAL THROUGHOUT SEE SECTION 300-6 SM. SPECS. SLOPE PER PLAN, 22 MINIMUM-7 SECTIONA-A SONS 0-70'MANNEL OR NTS RIP RAP PER SID. D-40 LAR TO SORS 0-72 INSEE DETAIL B BASIN CAPACITY TABLE L4'+0.D. oçj __________________________ I DRAIN PIPE I c.1 IN CUBIC YARDS) SECTION B-B DETAIL A DCATIONS ON LOTS ADJACENT TO DWELLINGS SHALL BE 3' GUNIIE. 2) ALL STEEL PIPE AND ETAIL B DIP GALVANIZED AFTER FABRICATION. TEMPORARY DESILTATION BASIN OUTLET AND CAPACITY TABLE 44 6_6-C )TY ENGINEER DM1 SUPPLEMENTAL STANDARD NO. — COARSE AGGREGRA1E 11 560-C-3250 CONCRETE ANCHORS PER PLAN 'SEE DETAIL A 3'+OD. OF DRAIN PIPE I 31 NTS 1 1/2'x 2 A-36 STL ANGLE NTS (ACRES ) AVERAGE SLOPES 2% 5% 8% 10% 12% 15% '0 MB W/ NUT & WASHER 10 270 350 370 400 450 500 15 NOTES: 400 420 460 600 675 750 20 1) DESILTATION BASINS BUILT 540 700 1 740 800 1 900 1000 40 1080 14061 1480 1600 1800 2000 80 COMPLETELY LINED WITH 2160 28001 2960 3200 3600 4000 100 2700 35001 3700 4000 4500 5000 150 HARDWARE TO BE HOT 1 4000L 42001 4600 6000 6750 7500 200 1 5400 70001 7400 8000 9000 10000 San Diego County Hydrology Manual Date: June 2003 Section: 4 Page: 11 of 60 Table 4-2 çContinued) RUNOFF CURVE NUMBERS FOR PZN CONDITION = 2.0 Average Percent Curve Numbers for Cover Treatment Hydrologic Impervious Hydrologic Soil Groups: Cover Description or Practice Condition Area A B C D Close-seeded legumes or rotated pasture..................................Straight row................................Poor.....................................................66 77 85 89 Good....................................................58 72 81 85 - Contoured...................................Poor.....................................................64 75 83 85 Good....................................................55 69 78 83 Contoured and terraced ..............Poor.....................................................63 73 80 83 Good....................................................51 67 76 80 Cultivated land .........................................................................Without conservation treatment................................................................72 81 88 91 With conservation treatment .....................................................................62 71 78 81 ) Fallow.......................................................................................Bare soil ....................................................................................................77 86 91 Crop residue cover.....................Poor.....................................................'76 85 90 92 Good....................................................74 83 88 90 Farmsteads (buildings, lanes, driveways, and surrounding lots)...................................................................................................................59 74 82 86 Irrigatedpasture ............................................................................................................................... Poor ......... ............................................ 58 74 83 87 Fair .......... ............................................ 44 65 77 82 Good....................................................33 58 72 79 Orchards (deciduous) ......................................................................................................................(see glossary description) Orchards(evergreen)......................................................................................................................Poor.....................................................57 73 82 86 Fair......................................................44 65 77 82 Good....................................................33 58 72 79 Rowcrops ........................................................ ......................... Straight row................................Poor.....................................................72 81 88 91 Good....................................................67 - 78 85 89 Contoured...................................Poor.....................................................70 79 84 88 Good....................................................65 75 82 86 4-11 8 a= 7 UflO ( a) Curves on this shoot are for the Ix case laO.2S,so that Infiltration —Curve a +0O28) 6 l - Time .0;4 Initial la 7. 1 9 4 (1 4 1 L I 3L 2 15 iT 4 '•.•• 4 Rainfall (P) in Inches a4v. f.c1M -— 4 SOURCE: TR-55, Second Ed., June 1986 FIGURE NRCS Solution of the Runoff Equation LOCATION I MAXIMUM DENSITY f..' (Oct) OPTIMUM MOISTURE HB-1 @ 5-10' 127.0 10.5 TP-26 @ 2'-3' 114.0 13.0 *flD0@7l 120.5 13.0 *B..2 @ 5' 128.0 10.0 *B@4I 126.0 11.0 * Location and testing completed in preparation of GSI (2001 C and 2002b) Av JA,K. - 12.35 p '* Expansion Index (E I) testing was performed on representative soil samples of colluvium and terrace deposits in general accordance with Standard No 18-2 of the UBC/CBC (ICBO, 1997 and 2001). The test results are presented below as well as the expansion classification according to UBC/CBC (ICBO, 1997 and 2001) LOCATION [ SOIL TYPE E I EXPANSION POTENTIAL TP-1 @ 0-3' SANDY CLAY 61 Medium TP-1 @ 4151 SANDY SILT 25 Low TP-2 © 3'5' CLAY 60 Medium TP-38 © 3151 SAND 4 Very Low *TP1 @ 1-2' SILTY SAND 1 Very Low *TP..lo @ T-8' SANDY CLAY 102 High *B..2 © 5' SANDY CLAY 32 Low *B © 4' SILTY SAND 19 Very Low * Location and testing completed in preparation of GSI (2001c and 20020) Shear testing was performed on a remolded sample of site soil in general accordance with ASTM test method D-3080. Results of shear testing (GSI, 2001c and 2002b) are presented as Plates C-i through C-12 in Appendix C. 'f • , .•' I - Calayera Hills, LLC W.O. 5353-A-SC Robertson Ranch, East Village January 15, 2007 ee:\wp9\53oo\5353a.uge Page 14 GeoSoils, 1.;JS w. j. 4, At Estimating Soil Loss 5.15 x. (ExaaT,'e Ji ••i' .1' ..•° so Al ..f\IA '.•\ '...\ .5, 4 z• '"•' \.//\.. \ ". ./ •\ 40 : , \ ' / .•,.• •. ,,. -#--.. .T' A ,....••i\ •\ / 025, 1/ .'(#••• /•..'\,, •.)(/ / \j,..' O zwo 10 IiN , I - .- purcad sand Fig. 5.6 Triangular nomograph for estimating K value. (8) Se. Table 5.3 for adjust- • ments to K value under certain conditions. EXAMPLE 5.4 Given: A soil with the following particle size distribution. Component Size, man Fraction, % Sand 2.0-0.1 30 Very fine sand 0.1-0.05 10 Silt 0.054.002 20 Clay Less than 0.002 40 Find: Texture and K value. Solution. Entering Fig. 5.1 with 40 percent total sand and 20 percent silt, the texture is found to be on the border between clay and clay loam. Entering Fig. 5.6 with the same percents (see bold lines), the K value is found to be 0.19. Table 5.3 describes adjustments to the K factor. Adjustment 1 is a correction for very • 3.10 Eroaton and Sediment Oonirol H a n d b o o k control practices than construction in areas with low R values. If a more precise value for R is needed, other references (10, 20, 2 1 ) t h a t e x p l a i n h o w t o c a l c u l a t e R for individual storms and years from local data should be consulted. An "isoezodent" map, prepared by Wlschmeler for t h e U S D A ( 2 0 ) a n d s h o w n in Fig. 5.2, is used to find the R value for sites east, of the Rocky Mountains (approximately 104' west longitude). R can be interpolated for points between the lines. Contact local soil conservatio n service offices for more detailed Infor.. mation on R values In areas covered by this map. Wed of the 104th west merid-ian, irregular topography makes use of a generalized map Impractical For the western states, R is calculated by using rainfall data. Results of investigations at C) Fig. 5.3 Distribution of storm types in the we.tern United States. (4) I'yp e I I s t o r m s CCOW in Arizona, Colorado, Idaho, Montana, .'eaii, New Mexico, Utah, and Wyo m i n g .Iso. 5.12 Erosion and Sediment Control Handbook 700 500 J300 20C lOC ONEENO No I MENNE''PREE 'AMEN E 0 E 0 0 0 0 EFIN IVA , P-M N MMOND ONEERF, JOAA MENi PRENHEMMME 0.3 1.0 IS 2.0 2.5 3.0 15 40 4.5 - - - p - 2-year, 0-hr rain, In I I I I- 25 50 is too a - 2-year. 6-hr rain, mm Fig. 5.5 Relations between average annual erosion index and 2-yea r , 5 - h r r a i n f a l l i n California. (14) The differences in peak intensity are reflected in the c o e f f i c i e n t s o f t h e e q u a - tions for the rainfall factor. Figure 5.5 is a graphical repre s e n t a t i o n o f t h e e q u a - tions. The equations, also shown on the curves for each in d i v i d u a l s t o r m t y p e , are: R type 11 R 16.55p&2 type I - R 10.2p'4 type IA where p is the 2-year, 6-hr rainfall in inches. (If p is in m i l l i m e t e r s , t h e e q u a t i o n s become: R 0,0219p2J, typo U; R 0.0134p, type I; R 0.00828p2 , t>pe IA.) The R 'alie is rounded to the nearest whole number. When the r a i n f all time distribution curves (Fig. 5.4) and the corresponding R val u e ationi Am an- prcd, it is tvident that the ltrnner the peak intensity of the typical sterm, the hi,her the rainfall ertsion index. Estimating Soil Less - 5.23 TABLE 5.8 C Values for Soil Loss Equation' Soil toss Type of cover C factor reduction, % None 1.0 0 Native vegetation (undisturbed) 0.01 99 Temporary seedluge 90% cover, annual grasses, no mulch 0.1 90 Wood fiber mulch, % ton/acre (1.1 t/ha), with aeedt 0.6 50 Excelsior mat, jutet 0.3 70 Straw inulcht 1.6 tons/acre ($4 t/ba), tacked down 0.2 80 4 tons/acre (9.0 1/ha), tacked down 0.05 95 Adapted= R.& It, l5 and 2O tPorsopes up to 2:1. if a complete cover of newly seeded annual greases I. well established before the onset of rain.. In many areas, seed and wood fiber mulch are applied hydraulically shortly before the rainy season. The early raim cause the see& to germinate, but a com-plete grass cover is not established until at Least 4 week. later. During the ger-mination and early growth period, the wood fiber mulch provides only marginal ) protection. A C value of 0.5 Is an appropriate average representing little. protec-tion Initially and more thorough protection when the grass I. well established. On bare soils mulch can provide immediate reduction In soil Loss, and It per-forms better than temporary seedinp in some cases. S t r a w m u l c h I s m o r e e f f e c - tive than wood fiber mulch; it reduce. Loss about 80 percent (C value, 0.2) when it I. applied at the rate of 3000 lb/acre (3.4 1/ha) and tacked down. Additional reduction to obtained with 8000 lb/acre (90 tlha) of straw, but this rate may no t be coat-effective. Wood fiber mulch alone (without seed) provides very little soil loss reduction; it primarily helps seeds to become established so that the new grass can provide the erosion control. Other products, such as jute, excelsior, and paper matting, provide an intermedfate.tevet of protection; the C value equals approximately 0.3. Test results of various mulch treatments are present e d i n C h a p . 6 . 5.2f Erosion Control Practice Factor P The erosion control practice factor P is defined as the ratio of soil loss with a given surface condition to soil loss with up-and-down-hill plowing. Practices that reduce the velocity of runoti and the tendency of ru n o f f t o f l o w directly down-slope reduce the P factor. in agricultural uses of the USLE, P in used to describe plowing and tillage practices. In construction site applicat i o n s , P reflects the roughening of the soil surface by tractor treads or by ro u g h g r a d i n g , r a k i n g , o r disking. 1) 524k Erosion and Sediment Control Handbook TABLE 5.1 P Factors for Construction Sites (Adapted from Ref. 16) Surface condition p value Compacted andemooth 1.3 Trackwalked along contour* 1.2 Trackwalked up and down slopef 0.91 Punched straw 0.91 Rough, Irregular cut 0.9! Loose to 12.!. (30-cm) depth 08 "Tread math wlout.d up and down slops. rrr.sd math odsntsd parallel to contours, as In Pegs. U and 0.10 P values appropriate for construction sites are listed in Table 5.7. • A surface that Is compacted and smoothed by grading equipment Is highly sus-ceptible to sheet runoff and Is assigned a P value of 1.& Trackwalklng Is given a value of 1.2 If the vehicle traversej along the contour. The p value!, relatively high because the depressions left by cross-slope track-ing resemble up-and-down furrows and worsen runoff conditions. Trackwalklng up and down slope reduces P to 0.9. The tread marks act as slope benches; they reduce runoff velocity and trap soil particles (see Fig. 6.10). Punched straw is assigned a P value of 0.9 because the action of punching the straw into the soil roughens the surface and creates a trackwalking effect. When the soil surface I. disked or otherwise Loosened to a depth of 1 ft, a slightly lower P value of 0.8 may be used. This condition is unlikely to occur on a construction site because compaction, not loosening, is required when till slopes are constructed.. Clearly, changing the surface condition does not provide much direct reduc- tion in soil loss; all the P values are close to 1.0. However, roughening the soil surface is essential before seeding because it greatly increases plant establish.. ment (see Chap. 6) and thus also reduces the C (actor. Vegetation, mulch, slope length, and gradient have far more significant effects on the erosion process and provide greater opportunities to reduce soil loss. 5.2g Combined Effects of IS, C, and P Of the five factors in the USLE, the R, LS, and C factors have the widest range. Although R for a site is constant and K is essentially a constant, SLope length and gradient, cover, and, to a lirnitcd extent, surface condition can be manipu- lated. Slope length and vegetative cover are the most effective and easily imple- mented measures. rable 5.5 compares the eifcct on the soil loss estimates of varying LS, (, and P. For example, a building pad with a 1 percent slope, smooth surface, ind no c,ver has a fractional soil loss putcntLsl. A 2: 1 sIipe, ccmmn between trraceI U ABLE 5.5 LS Values* (10) LS values for following slope lengths 1, ft (in) 1 LS values for following slope lengths 1, ft (in) Slope ope gradient 10 20 30 40 50 60 70 80 90 100 150 200 250 300 350 400 450 500 600 700 800 900 1000 itio a, % (3.0) (6.1) (9.1) (12.2) (15.2) (18.3) (21.3) (24.4) (27.4) (30.5) (46) (61) (76) (91) (107) (122) (137) (152) (183) (213) (244) (274) (395) 0.5 0.06 0.07 0.07 0.08 0.08 0.09 0.09 0.09 0.09 0.10 0.10 0.11 0.11 0.12 0.12 0.13 0.13 0.13 0.14 0.14 0.14 0.15 0.15 )0:1 1 0.08 0.09 0.10 0.10 0.11 0.11 0.12 0.12 0.12 0.12 0.14 0.14 0.15 0.16 0.16 0.16 0.1.7 0.17 0.18 0.18 0.19 0.19 0.20 2-- 0.10 0.12 0.14 0.15 0.16 0.17 0.18 0.19 0.19 0.20 0.23 0.25 0.26 0.28 0.29 0.30 0.32. 0.33 0.34 0.36 0.37 0.39 0.40 3 0.14 0.18 0.20 0.22 0.23 0.25 0.26 0.27 0.28 0.29 0.32 0.35 0.38 0.40 0.42 0.43 0.45 0.46 0.49 0.51 0.54 0.55 0.57 4 0.16 0.21 0.25 0.28 0.30 0.33 0.35 0.37 0.38 0.40 0.47 0.53 0.58 0.62 0.66 0.70 0.73 0.76 0.82 0.87 0.92 0.96 1.00 !0: 1 5 0.17 0.24 0.29 0.34 0.38 0.41 0.45 0.48 0.51 0.53 0.66 0.76 0.85 0.93 1.00 1.07 1.13 1.20 1.31 1.42 1.51 1.60 1.69 6' 0.21 0.30 0.37 0.43 0.48 0.52 0.56 0.60 0.64 0.67 1 0.82 0.95 1.06 1.16 1.26 1.34 1.43 1.50 1.65 1.78 1.90 2.02 2.13 7 0.26 0.37 0.45 0.52 0.58 0.64 0.69 0.74 0.78 0.82 1.01 1.17 1.30 1.43 1.54 165 1.75 1.84 2.02 2.18 2.33 2.47 2.61 !Y.: l 8 0.31 0.44 0.54 0.63 0.70 6.77 0.83 0.89 0.94 0.99 . 1.21 1.40 1.57 1.72 1.85 1.98 2.10 2.22 2.43 2.62 2.80 2.97 3.13 9 0.37 0.52 0.64 0.74 0.83 0.91 0.98 1.05 1.11 1.17 1.44 1.66 1.85 2.03 2.19 2.35 2.49 2.62 2.87 3.10 3.32 3.52 3.71 .0:1 10 0.43 0.61 0.75 0.87 0.97 1.06 1.15 1.22 1.30 1.37 1.68 1.94 2.16 2.37 2.56 2.74 2.90 3.06 3.35 3.62 3.87 4.11 4.33 11 0.50 0.71 0.86 1.00 1.12 1.22 1.32 1.41 1.50 1.58 . 1.93 2.23 2.50 2.74 2.95 3.16 3.35 3.53 3.87 4.18 4.47 4.74 4.99 8:1 12.5 0.61 0.86 1.05 1.22 1.36 1.49 1.61 1.72 1.82 1.92 ; 2.35 2.72 3.04 3.33 3.59 3.84 4.08 4.30 4.71 5.08 5.43 5.76 6.08 15 0.81 1.14 1.40 1.62 1.81 1.98 2.14 2.29 2.43 2.56 3.13 3.62 4.05 4.43 4.79 5.12 5.43 5.72 6.27 6.77 7.24 7.68 8.09 6:1 16.7 0.96 1.36 1.67 1.92 2.15 2.36 2.54 2.72 2.88 3.04 3.72 4.30 4.81 5.27 5.69 6.08 6.45 6.80 7.45 8.04 8.60 9.12 9.62 5:1 20 1.29 1.82 2.23 2.58 2.88 3.16 3.41 3.65 3.87 4.08 5.00. 5.77 6.45 7.06 7.63 8.16 8.65 9.12 9.99 10.79 11.54 12.24 12.90 J(:1 22 1.51 2.13 2.61 3.02 3.37 3.69 3.99 4.27 4.53 4.77 5.84 6.75 7.54 8.26 8.92 9.54: 10.12 10.67 11.68 12.62 13.49 14.31 15.08 4:1 25 1.86 2.63 3.23 3.73 4.16 4.56 4.93 5.27 5.59 5.89 7.21 8.33 9.31 10.20 11.02 11.78 12.49 13.17 14.43 15.58 16.66 17.67 18.63 30 2.51 3.56 4.36 5.03 5.62 6.16 6.65 7.11 7.54 7.95 9.74 11.25 12..57 13.77 14.88 15.91 16.87 17.78 19.48 21.04 22.49 23.86 25.15 3:1 33.3 2.98 4.22 5.17 5.96 6.67 7.30 7.89 8.43 8.95 9.43 .' 11.55 13.34 14.91 16.33 17.64 18.86 20.00 21.09 23.10 24.95 26.67 28.29 29.82 35 3.23 4.57 5.60 6.46 7.23 7.92 8.55 9.14 9.70 10.22 12.52 14.46 16.16 17.70 19.12 20.44 21.68 22.86 25.04 27.04 28.91 30.67 32.32 i4:1 40 4.00 5.66 6.93 8.00 8.95 9.80 10.59 11.32 12.00 12.65 15.50 17.89 20.01 21.91 23.67 25.30 26.84 28.29 30.99 33.48 35.79 37.96 40.01 45 4.81 6.80 8.33 9.61 10.75 11.77 12.72 13.60 14.42 15.20 '26.33 18.62 21.50 24.03 28.44 30.40 32.24 33.99 37.23 40.22 42.99 45.60 48.07 2:1 50 5.64 7.97 9.76 11.27 12.60 13.81 14.91 15.94 16.91 17.82 21.83 25.21 28.18 30.87 33.34 35.65 37.81 39.85 43.66 47.16 50.41 53.47 56.36 55 6.48 9.16 11.22 12.96 14.48 15.87 17.14 18.32 19.43 20.48 . 25.09 28.97 32.39 35.48 38.32 40.91 43.45 45.80 50.18 54.20 57.94 61.45 64.78 .%:1 57 6.82 9.64 11.80 13.63 15.24 16.69 18.03 19.28 20.45 21.55 26.40 30.48 34.08 37.33 40.32 43.10 45.72 48.19 52.79 57.02 60.96 64.66 68.15 60 7.32 10.35 12.68 14.64 16.37 17.93 19.37 20.71 21.96 23.15 28.35 32.74 36.60 40.10 43.31 46.30 49.11 51.77 56.71 61.25 65.48 69.45 73.21 j4:1 66.7 8.44 11.93 14.61 16.88 18.87 20.67 22.32 23.87 25.31 26.68 32.58 37.74 42.19 46.22 49.92 53.37 56.60 59.66 65.36 70.60 75.47 80.05 84.38 70 8.98 12.70 15.55 17.96 20.08 21.99 23.75 25.39 26.93 28.39 34.77 40.15 44.89 49.17 53.11 56.78 60.23 63.48 69.54 75.12 80.30 85.17 89.78 75 9.78 13.83 16.94 19.56 21.87 23.95 25.87 27.66 29.34 30.92 37.87 43.73 48.89 53.56 57.85 61.85 65.60 69.15 75.75 81.82 87.46 92.77 97.79 .Y1:1 80 10.55 14.93 18.28 21.11 23.60 25.85 27.93 29.85 31.66 33.38 , 40.88 47.20 52.77 57.81 62.44 66.75 70.80 74.63 81.76 88.31 94.41 100.13 105.55 85 11.30 15.98 19.58 22.61 25.27 27.69 29.90 31.97 33.91 35.74 ; 43.78 50.55 56.51 61.91 66.87 71.48 75.82 79.92 87.55 94.57 101.09 101.23 113.03 90 12.02 17.00 20.82 24.04 26.88 29.44 31.80 34.00 36.06 38.01 J 46.55 53.76 60.10 65.84 71.11 76.02 80.63 84.99 93.11 100.57 107.51 114.03 120.20 95 12.71 17.97 22.01 25.41 28.41 31.12 33.62 35.94 38.12 40.18 49.21 56.82 63.53 69.59 75.17 80.36 85.23 89.84 98.42 106.30 113.64 120.54 127.06 1:1 100 13.36 18.89 23.14 26.72 29.87 32.72 35.34 37.78 40.08 42.24 : 51.74 59.74 66.79 73.17 79.03 84.49 89.61 94.46 103.48 111.77 119.48 126.73 133.59 a1cu1ated from (65.41 X s2 + 4.56)( s +0.065 (...r LS - topographic factor \s2 +10.000 \182 +10,000 /\72.5/ t.. slope length, ft(mX0.3048) a - slope steepness, m - exponent dependent upon slope steepness (O.2 for slopes < 1%, 0.3 for slopes 1 to 3%, I 0.4 for slopes 3.5to4.5%,and I 0.5 for slopes> 5%)