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HomeMy WebLinkAboutCT 02-16; ROBERTSON RANCH EAST VILLAGE; TEMPORARY DESILTING BASIN REPORT; 2007-09-18/ ROBERTSON RANCH EAST VILLAGE CT 02-16 TEMPORARY DESILTING BASIN REPORT ,- JOB NO. 01-1014 1. dUYISED::SEPTEMBER:18-2007-- PREPARED: SEPTEMBER 20, 2005 Lu 60223 EXP. 6/30108 clW- -, KE1W. EN RCE 60223 D/ EXP. 06/30/08 O'DAY CONSULTANTS, INC. 2710 LOKER AVENUE WEST, SUITE 100 CARLSBAD, CA 92010 TEL: (760) 931-7700 FAX: (760) 931-8680 G:\OI 1014\Hydrology\lempDesiltBasinCalcs-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 Iavg=l.7"/6 hr = 0.28 in/hr 10-year, 6-hour storm event= 1.7" A= area of basin Vs= .00024 ft/s (.01mm 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 thatthe 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 desilting 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: R16.55 p22 = 16.55*(1.3)22 = 29.48 P1.3" (2-year, 6-hour storm event per Erosion and Sediment Control Handbook) G:\O1 I 014\Hyclrology\TempDesiltBasinCalcs-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)-'/ 3600*T*Cd*(g)5 Ao= Surface area of orifice (st) 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/s2 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: L. - i" scwL T Tr1T3/2 - Where: Cscw = 3.27 & 0.4 /HW L= Circumference of Standpipe H= Head above Standpipe Hw Height of Standpipe G:\01 10 14\Hydrology\TempDesiltBasinCalcs-rev3 .doc Temporary Desilting Basin Calculations 10-Year Storm Event = 1.7 in. Basin Area (ac.) C L0 (in/hr) Q(cfs) Vs (ft/s) As reg'd (sfl 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) Q3 (P24-0.25)2/(P24+0.85) S=1000/CN-10 CN=94 (2003 San Diego County Hydrology Manual, Table 4-2) G:\01 1014\HydrologyRREastDesiltBasins-3.xls Dewaterina Orifice Sizina Calculations Basin As (sf) liCtil 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 OAV T (hr) Cd G (ftls2) Ao (sfl Ao (in 21 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- V 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- 1" dia. holes 40 0.6 32.2 0.064 9.19 Use 6- 1 /2 dia. holes 40 0.6 32.2 0.027 3.90 Use 5- 1" dia. holes I P10yr..24hr) (in.) I Tc (mm.) 1100 (in./hr.) I Area (Basm 1) (AC.) I C I Q1 (cfs) I 2.6 I 12.5* 3.79 I 26.47 I 0.35 I 35.11 I Basin Q1QQ MAY Ccr.wj . U_fill Urn I thw8. 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= RxKxLSxCxP=0.69 R= 29.48 K= 0.15 LS= 0.12(100'@l%) C= 1.0 P= 1.3 Streets: A= RxKxLSxCxP=3.05 R= 29.48 K= 0.15 LS= (0.20x 1122)+(1.50x480)+(0.14x223) =053 1122+480+223 C= 1.0 P= 1.3 SloDes: A= RxKxLSxCxP=0.32 R= 29.48 K= 0.15 LS= 5.64(10'@50%) C= 0.01 P= 1.3 Natural Ground Cover (Onen SDace): A= RxKxLSxCxP=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\lempDesiltBasin_lnfo.xps BASIN 2 Soil Loss: Pads: A= RxKxLSxCxP0.69 R= 29.48 K= 0.15 LS= 0.12(100'@l%) 0= 1.0 P= 1.3 Streets: A= Rx Kx 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= RxKxLSxCxP0.32 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%) 0= 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 = R x K x LS x,C x P = 1.90 R= 29.48 K= 0.15 LS = 0.33 (500'@ 2%) 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 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'(0l%) 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= 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 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= Rx 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 Slopes: A= RxKxLSxCxP=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 = 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 Sod Loss: Pads: A= R x Kx 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= RxKxLSxCxP=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= RxKxLSxCxP=0.69 R= 29.48 K= 0.15 LS= 0.12(100'©l%) C= 1.0 P= 1.3 Streets: A= RxKxLSxCxP=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 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 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= R x Kx LS x C x P = 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. I UI; L.STANDPIPE-18'0 PIPEMININUM WITH NO PERFORATIONS #6 REBAR,SPACED 6'O.C. AROUND RISER CIRCUMFERENCE 4' RED PAINT S PLATE 3 12'x STRIPE ALL . ITX AROUND RISER r.+ PIPE O.D. SLOPE FACES SI1i BE HYDROSEEDED PLAN VIEW NTS SIMILAR TO SDRS D-70 I COARSE AGGREGRAIE III I 31 SECTION A-A SDRS 0-70' CHANNEL OR NTS RIP RAP PER STD. 0-40 AR TO SDRS 0-72 TwSEE DETAIL 9 4'+O.D. OF ? BASIN CAPACITY TABLE L I PIPE I _________ DRAIN (IN CUBIC YARDS) SECTION B-B DETAIL A NTS NTS I 1/2'x 2' A-36 STh ANGLE A110NS ON LOTS ADJACENT TO DWELLINGS SHALL BE 3' GUNITE. 2) ALL STEEL PIPE AND ETIL B DIP GALVANIZED AFTER FABRICATION. 560-C-3250 CONCRETE ANCHORS PER PLAN SEE DETAIL A 14 (ACRES) AVERAGE SLOPES 2% 5% 8% 10% 12% 15% MB WI NUT & WASHER 10 270 350 370 400 450 500 NOTES: 15 400 420 460 600 675 750 1) DESILTATION BASINS BUILT 20 540 700 740 800 900 1000 40 1080 1400 1480 1 1600 1800 2000 80 COMPLETELY LINED WITH 2160 2800 2960 3200 3600 4000 100 2700 3500 3700 4000 4500 5000 150 1 HARDWARE TO BE HOT 4000 4200 4600 6000 6750 1 7500 200 1 54001 70001 7400 1 80001 9000 10000 __21 OR FLATTER IF SO SPECIFIED ON PLANS TYPICAL THROUGHOUT SEE I TION 300-6 66;LSM LOPE PER PLAN, 2% MINIMUM -p' 3'-sO.D. OF DRAIN PIPE (7 LENGTH PER PLAN Cl}Hiifk I I ,PER PLAN, 6' MINIMUM I ri WIDTH, LENGTH, RIP-RAP SIZE ul AND FILTER ROCK SIZE PER PLAN TEMPORARY DESILTATION BASIN OUTLET 6-C CITY ENGINEER DAli SUPPLEMENTAL STANDARD NO. - 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 Condition3 Area 4 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 Time wunoff(a) Initial 7 I I >7 / I.L!iL11 1 h:' .TtT1 'tjii rr1.i li iI 1' 1• kLk Curves on this shoot are for (I, 0 C (5 a 0 C C.) 3 77, 0 2 4-5 it - 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Rainfall (P) in Inches 1,7 CVE- ?4 SOURCE: TR-55, Second Ed., June 1986 Abs is traction a -4-1-;- FIGURE NRCS Solution of the Runoff Equation I 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 @ 3-5' SAND 4 Very Low *TP1 @ 1-2' SILTY SAND 1 Very Low *TP.1 0 @ 7'-8' SANDY CLAY 102 High *132 @ 5' SANDY CLAY 32 Low *B @ 4' SILTY SAND 19 Very Low * Location and testing completed in preparation of GSI (2001 c and 2002b) 77 LOCATION MAXIMUM DENSITY (pc ' OPTIMUM MOISTURE CONTENT (%)1• HB-1 @ 5-10' 127.0 10.5 TP-26 @ 2'-3' 114.0 13.0 *TP10 @ 7' 120.5 13.0 *13.2 @ 5' 128.0 10.0 *B @ 4' 126.0 11.0 * Location and testing completed in preparation of GSI (20016 and 2002b) AIj.JAK. I'5j Expansion Index Testing Expansion Index (E.l.) testing was performed on representative soil samples of colluvium kana terrace deposits in general accordance with Standard No. 18-2 of the UBC/CBC 1997 and 2001). The test results are presented below as well as the expansion classification according to UBC/CBC (ICBO, 1997 and 2001) iréct Shear Tests Shear testing was performed on a remolded sample of site soil in general accordance with STM test method D-3080. Results of shear testing (GSI, 2001 c and 2002b) are presented as Plates C-i through C-12 in Appendix C. avera Hills, LLC W.O. 5353-A-SC ertson Ranch, East Village January 15, 2007 :\wp9\530o\5353a.uge Page 14 GeoSotls, Inc. ;?IIR Estimating Soil Lose /\ go 04¼tWT0 54) so 0 t..\ ..i\ .•I \ 4rç IssL%_c .....o, 1 8' if\.'fi' '2 .• .•°' /y'--ic7, \.e. I Al , > I / \ •. p 40 i 20 -f—k-.. —v- •-.A,ç:. I: .'.;1 •', ' I' •\ / ...\ /1 ' ' / ..• '..... \ / .•f ' ' .. (:t": I A;<\ ••,••• -. 't I.' •" / io h T'7 .e — Ps,csnt an4 Fig. 5.8 Triangular nomograph for estimating K value. (6) See Table 5.3 for adjust. monte to K value under certain conditions. EXAMPLE 5.4 Given: A soil with the. following particle size distribution. Component Size, mm Fraction, % Sand 2.0-0.1 30 Vary fine sand 0.1-0.05 10 Silt 0.05-0.002 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 lie a correction for very 5.10 Erosion and Sediment Control Handb o o k control practices than construction in a r e a s w i t h low 11 values. If a more precise value for R is needed, other references (10, 20, 21) that explain bow to calculate R for individual storms and years from lo c a l d a t a should be consulted. An 9soerodent" map, prepared by Wlachmeier for the U S D A ( 2 0 ) a n d shown in Fig. 5.2, is used to find the R value for sites e4 of the Rocky Mountains (approximately 104' wed Longitude). R can be Interpolated for points between the lines. Contact Local soil conservation service offices for more detailed Infor-mation on 11 values In areas covered by this map. West of the 104th west mend-Ian, irregular topography makes use of a generalized map Impractical. For the western states, R is calculated by using rainfall data. Results of investigations a t Fig. 5.3 Oistribtttion of storm types in the western Urntd States. (4) 'Fype II storms in Arizona, Colorado, LFtho. Montana, .'evadi, New Mexico, Utah, and Wyoming ,iiso. 5.12 Erosion and Sediment Control Handbook 700,__ ___________ _ _ _ _ _ _ _ ONMEN No MMENE No MENEM ON MEMM No MEENG ME moons "a MEN WO -O-OR ...WON W-09HOMMEN 5.5 - 2yor, 6-tr rein. In 25 50 75 IOU - 2-year. 6-hr rain, mm Fig. 5.5 Relations between average annual erosion index and 2-year, 6-hr rainCall in California. (14) J The differences in peak intensity are reflected in the coefficients of the equa-tions for the rainfall factor. Figure 5.5 is a graphical represent a t i o n o f t h e e q u a - tions. The equations, also shown on the curves for each i n d i v i d u a l storm type, are: R 27p t2 type 11 R 16•55pU type I <— R 10.2 p5.' type IA where p is the 2-year, 6-hr rainfall in inches. (If p is in inillimutars, the uquations become: R 00219p2Z, type IL; R 0.0134p, type I; R 0.00828p5.2, t>pe IA.) The R aIue is rounded to the nearest whole number. Whun the rainfall time distribution curves (Fig. 5.4) and the corresponding R value ion- tire .m- pircd, it is evident that the stronger the peak intensity of the typical storm, the hi,her the rainfall er.sion index.. U Estimating Soil Less -1 5.23 TABLE 5.6 C Values for Soil Lou Equation' Soil Los. Type of cover C factor reduction, % Non. tO 0 Native vegetation (undisturbed) 0.01 99 Temporary seedinge 90% cover, annual gruaas, no mulch 0.1 90 Wood fiber mulch. % tonfacre (1.1 tIlt.), with seedt 05 50 Excelsior mat, jutet 0.3 70 Straw mulcht 1.5 toni/acm (8.4 Vhs) tacked down 0.2 80 4 tons/acre (9.0 tIlts), tacked down U5 95 'Map .40cm 8.0. 11,16, and 23 fPor slopes up to 2:1. if a complete cover of newly seeded annual grasses is well establishe d b e f o r e t h e onset of rains. In many area., seed. and wood fiber mulch are applied hydraulically shortly before the rainy season. The early rains cause the seeds to germinat e , but a com-plete grass cover Is not established until at least 4 weeks later. During the ger-mination and early growth period, the wood fiber mulch pro v i d e s o n l y m a r g i n a l ) protection. A C value of 0.8 is an appropriate average representing little p r o t e c - tion Initially and more thorough protection when the grass I. well established. On bare soils mulch can provide immediate reduction In soil toes, and It per-forms better than temporary seedings in some cases. Straw mulch Is more effec-tive than wood fiber mulch; it reduces loss about 80 percent (C value, 0.2) when it Is applied at the rate of 3000 lb/acre (3.4 tIlts) and tacked down. Additional reduction Is obtained with 8000 lb/acre (90 t/ha) of straw, but this rate may not be cost-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 intermediate level of protection; the C value equals approximately 0.3. Teat results of various mulch treatments are presen t e d i n C h a p . 8. 5.2f Erosion Control Practice Factor P The erosion control practice (actor P is defined as the ratio of soil Loss with a given surface condition to soil loss with up-and-down - h i l l p l o w i n g . P r a c t i c e s t h a t reduce the velocity of runoff and the tendency of runoff to flow directly down-slope reduce the P factor. In agricultural uses of the USLE. P is used to describe plowing and tillage practices. hr construction site applica t 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. Th 1) 524 Erosion and Sediment Control Handbook TABLE 5.7 P Factors for Construction Site, (Adapted from Rat. 15) Surface condition P value Compacted and smooth 1.3 Treckwalked along contour' 1.2 TrackwaIke4 up and down alopa? 0.9 Punched straw 0.9 Rough, irregular cut 0.9 Loose to 12-in (30-cm) depth 0.8 'rusd marks cd.*t.d up cod dwwn g,, $Tresd maske oriented parallel to contain, as In Pip. 5.9 and 5.10 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 value of 1.3. Trackwa)klng is given a value of 1.2 If the vehicle traverses along the contour. The P value in relatively high because the depressions left by cross-slope track-big resemble up-and-down furrows and worsen runoff conditions. Trackwalking 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 Is dished or otherwise loosened to a depth of 1 ft. a slightly Lower P value of 0.5 may be used. This condition is unlikely to occur on a construction site because compaction, not loosening, is required when fill 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 esta b l i s h . nient (see Chap. 6) and thus also reduces the C factor. 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, .4opa length and gradient, cover, and, to a limited extent, surface condition ,.an be m a n i p u - lated. Slope length and vegetative cover are the most effective and easil y i i n p l o - niented measures. Fable 5.3 compares the elfect on the soil loss estimates of varying LS, C, and P. For example, a building pad with a 1 percent slope, smooth surfice, ind no crxer has a fractional wil loss potential. A 2: L slope, CCmflmTh)fl betwccn t.rraL'e l l ABLE 5.5 LS Values (10) LS values for following slope lengths 1, ft (m) 1 LS values for following slope lengths 1, ft (m) Slope ope gradient 10 20 30 40 50 60 70 80 90 100 . 150 200 250 300 350 400 450 500 600 760 800 900 1000 ttio a, % (3.0) (6.1) (9.1) (12.2) (15.2) (18.3) (21.3) (24.4) (21.4) (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 0.320.350.380.400.420A30.450.46 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 1.65 1.75 1.84 2.02 2.18 2.33 2.47 2.61 9:1 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 6.46 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 9.5i 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 4 7.21 8.33 9.31 10.20 11.02 11.78 12.49 13.17 14.43 16.58 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 9.95 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 14.4.6 16.16 17.70 19.12 20.44 21.68 22.86 25.04 27.04 28.91 30.67 32.32 %: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 . ':. 18.62 21.50 24.03 '26.33 28.44 30.40 32.24 33.99 37.23 40.22 42.99 45.60 48.01 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 28.91 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 .}4:1 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 1 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 .: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 .i 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 63.63 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 2 + 4.56 X $ +0.065 (_-!- i..s - topographic factor 82 + 10.000 v? + 10,000 / \72.5/ jslope length, ft (in X 0.3048) s - elope steepness, in - exponent dependent upon slope steepness (0.2 for slopes < 1%, 0.3 for slopes Ito 3%, 0.4 for slopes 3.5 to 4.5%, and 0.5 for slopes> 5%)