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Home My WebLink AboutMS 2018-0012; THREE ON CHERRY; DRAINAGE STUDY; 2022-09-16Drainage Study Three on Cherry Lot G, Block 1 of Map No, 1777 160 Cherry Avenue Carlsbad, California Prepared for: CHERRY KMA, LLC 6026 WENRICH PLACE SAN DIEGO, CA 92120 Prepared by: Christensen Engineering & Surveying 7888 Silverton Avenue, Suite “J” San Diego, CA 92126 (858) 271-9901 September 16, 2022 Introduction This project involves removal of the existing single-family residence and appurtenances from the parcel at 160 Cherry Avenue, in the City of Carlsbad. Following its removal, a 3-unit multi-family residential project will be constructed, including landscaping and pervious paving and walkways. The attached drainage area map is from a topographic survey by Teas Land Surveying, Inc., dated November 23, 2017 and revised January 29, 2019. The site, in its existing pre-construction condition, conveys offsite runoff from the northeast property. The combine offsite and onsite runoff drains northwest to a neighboring property. Following construction, offsite runoff remains, will enter the new drainage system and will be conveyed to Cherry Avenue same with the onsite runoff. Runoff will not flow over neighboring properties. Modified Rational Method was used to analyze existing and proposed flow based on the 2003 County of San Diego Hydrology Manual, Appendix B. The pipes and channel (drainage ditch) were sized using Autodesk Civil3D, Hydraflow Express, Appendix D. Antony K. Christensen RCE 54021 Exp. 12-31-23 JN A2018-119 11-11-22 Date Calculations Pre-construction: (From San Diego County Hydrology Manual, Figure 3-3) Tc = Time of concentration = 1.8(1.1-C)Dist. 1/2 ( S 1/3 ) For residential area From Table 3-1 Soil Type = B CP = 0.25 Impervious area: Onsite = 2,029 sf, Offsite = 1,188 sf Total impervious = 3,217 sf = 0.074 ac Total area = 0.160 ac (onsite) + 0.098 ac (offsite) = 0.258 ac Imperviousness pre-construction = 0.074/0.258 = 28.7% C = 0.90 x (% Impervious) + CP x (1-%Impervious) C = 0.90 x 0.287 + 0.25(1-.287) = 0.44 L = 256’ (use 100 maximum per Table 3-2 of the Hydrology Manual to determine Ti ) ∆E= 59 – 55 = 4’ S = (59-55)/256 = 1.6% Tc = 1.8(1.1-C)Dist. 1/2 ( S 1/3 ) = 1.8(1.1-0.44)100 1/2 ( 1.6 1/3 ) Tc = 10.2 minutes IX = 7.44 P6 Duration -0.645 Since D = 10.2 IX = 1.66 P6 from: San Diego County Hydrology Manual Rainfall Isopluvial Map for 100 year Rainfall Event – 6 Hour P6 (100) = 2.6 I100 = 1.66 (P6) I100 = 1.66 (2.6) I100 = 4.32 Q100 = (0.44) (4.32) (0.258) Q100 = 0.49 cfs Post-construction: Nodes 1 – 2 (Overland flow) Area “PCA” = 6,121 sf = 0.141 ac, L =127’ Impervious area: Onsite = 1,470 sf, Offsite = 1,188 sf Total impervious = 2,658 sf = 0.061 ac Imperviousness = 0.061/0.141 = 43.3% C = 0.90 x (% Impervious) + CP x (1-%Impervious) C= 0.90 x 0.433 + 0.25(1-.433) = 0.53 CA = 0.53 (0.141) = 0.075 ∆E= 57.4 – 54.6 = 2.8’ S = 2.8/127 = 2.2% Tc = Time of concentration = 1.8(1.1-C)Dist. 1/2 ( S 1/3 ) = 1.8(1.1-0.53)127 1/2 ( 2.2 1/3 ) Tc = 8.9 minutes IX = 7.44 P6 Duration -0.645 Since D = 8.9 P6 (100) = 2.6 I100 = 7.44 (2.6)(8.9 -0.645 ) I100 = 4.72 Q100 = 0.075 (4.72) = 0.354 cfs Nodes 2 – 3 (Pipe flow) Area “PCB” = 139 sf = 0.003 ac, L =21’ Impervious area = 66 sf Imperviousness = 66/139 = 47.5% C = 0.90 x (% Impervious) + CP x (1-%Impervious) C= 0.90 x 0.475 + 0.25(1-.475) = 0.56 CA= 0.56 (0.003) = 0.002 ∆E= 52.0 – 51.6 = 0.4’ S = (52.0 – 51.6)/21’ = 1.9% Q100 @ node 2 = 0.666 cfs V100 = 3.99 fps (see attached exhibit) Tt = (21/3.99) (1/60) = 0.09 minutes Tc = 8.9 + 0.09 = 8.99 minutes I100 = 7.44 (2.6)(8.99 -0.645 ) I100 = 4.69 ∑CA = 0.075 + 0.002 = 0.077 Q100 = CIA = (0.077 x 4.69) = 0.361 cfs Nodes 3 – 4 (Pipe flow) Area “PCC” = 257 sf = 0.006 ac, L =21’ Impervious area = 66 sf Imperviousness = 66/257 = 25.7% C = 0.90 x (% Impervious) + CP x (1-%Impervious) C= 0.90 x 0.257 + 0.25(1-.257) = 0.42 CA= 0.42 (0.006) = 0.003 ∆E= 51.6 – 51.4 = 0.4’ S = (52.0 – 51.6)/21’ = 1.9% Q100 @ node 3 = 0.361 cfs V100 = 4.07 fps (see attached exhibit) Tt = (21/4.07) (1/60) = 0.09 minutes Tc = 8.99 + 0.09 = 9.08 minutes I100 = 7.44 (2.6)(9.08 -0.645 ) I100 = 4.66 ∑CA = 0.077 + 0.003 = 0.080 Q100 = CIA = (0.080 x 4.66) = 0.373 cfs Nodes 8 – 7 (Overland flow) Area “PCF” = 647 sf = 0.015 ac, L =21.4’ Impervious area = 66 sf Imperviousness = 66/647 = 10.2% C = 0.90 x (% Impervious) + CP x (1-%Impervious) C= 0.90 x 0.102 + 0.25(1-.102) = 0.32 CA = 0.32 (0.015) = 0.005 ∆E= 57.5 – 54.8 = 2.7’ S = (57.5-54.8)/21.4 = 12.6% Tc = Time of concentration = 1.8(1.1-C)Dist. 1/2 ( S 1/3 ) = 1.8(1.1-0.32)21.4 1/2 ( 12.6 1/3 ) Tc = 2.79 minutes, use 5 minutes minimum IX = 7.44 P6 Duration -0.645 Since D = 5 P6 (100) = 2.6 I100 = 7.44 (2.6)(5 -0.645 ) I100 = 6.85 Q100 = 0.005 (6.85) = 0.034 cfs Nodes 7 – 6 (Pipe flow) Area “PCE” = 165 sf = 0.004 ac, L =22’ Impervious area = 0 sf Imperviousness = 0% C = 0.90 x (% Impervious) + CP x (1-%Impervious) C= 0.90 x 0 + 0.25(1-0) = 0.25 CA= 0.25 (0.004) = 0.001 ∆E= 53.8 – 53.3 = 0.5’ S = (53.8 – 53.3)/22’ = 2.3% Q100 @ node 13 = 0.034 cfs V100 = 2.12 fps (see attached exhibit) Tt = (22/2.12) (1/60) = 0.17 minutes Tc = 5.0 + 0.17 = 5.17 minutes I100 = 7.44 (2.6)(5.17 -0.645 ) I100 = 6.70 ∑CA = 0.005 + 0.001 = 0.006 Q100 = CIA = (0.006 x 6.70) = 0.040 cfs Nodes 6 – 5 (Pipe flow) ∆E= 53.3 – 50.0 = 3.3’, L = 64’ S = (53.3 – 50.0)/64’ = 5.1% Q100 @ node 12 = 0.040 cfs V100 = 3.0 fps (see attached exhibit) Tt = (64/3.0) (1/60) = 0.36 minutes Tc = 5.17 + 0.36 = 5.53 minutes I100 = 7.44 (2.6)(5.53 -0.645 ) I100 = 6.42 ∑CA = 0.006 Q100 = CIA = (0.006 x 6.42) = 0.039 cfs Nodes 4 – 5 (Pipe flow) Area “PCB” = 3,910 sf = 0.090 ac, L = 61’ Impervious area = 1,476 sf Imperviousness = 1,476/3,910 = 37.7% C = 0.90 x (% Impervious) + CP x (1-%Impervious) C= 0.90 x 0.377 + 0.25(1-0.377) = 0.49 CA= 0.49 (0.090) = 0.044 ∆E= 51.2 – 50.0 = 1.2’ S = (51.2 – 50.0)/61’ = 2.0% Q100 @ node 4 = 0.373 cfs V100 = 4.2 fps (see attached exhibit) Tt = (61/4.2) (1/60) = 0.24 minutes Tc = 9.08 + 0.24 = 9.32 minutes I100 = 7.44 (2.6)(9.32 -0.645 ) I100 = 4.58 ∑CA = 0.080 + 0.044 = 0.124 Q100 = CIA = (0.124 x 4.58) = 0.568 cfs Combined flow @ Node 10 Use Tc = 9.32 minutes, I100 = 4.58 ∑CA = 0.124 + 0.006 = 0.13 Q100 = CIA = (0.13 x 4.58) = 0.596 cfs 1. Discussion Pre-construction the site runoff flows northwesterly across the neighboring properties. Following development, the site will convey its runoff to Cherry Avenue through curb outlet. There is a calculated increase of 0.106 cfs for the 100-yr return frequency event following development. Runoff will flow along Cherry Avenue, turn left on Carlsbad Boulevard then enter the curb inlet as it was pre-construction. 2. Test for Adequacy The attached program was used to test for adequacy of the curb outlet. They were found to be adequate to convey the runoff. The proposed system requires the use a pump to convey 0.596 cfs (100 year storm) of runoff from A-4 central catch basin onsite to the curb outlet. The pump will need to be capable of conveying 0.596 cfs to the curb outlet. Pump needs to overcome head loss from elevation changes, friction and small bends. Entrance and exit losses are ignored since they are insignificant. The total elevation change is (54.2’ – 46.2’) = 8’. To determine other head losses, the velocity in each pipe must be known. To provide conservative values for each head loss it will be assumed that the flow from the pump is at the approximate TDH value. For the 1.5 HP Goulds pump the maximum flow for a static head of 16 feet is 300 gpm. This is equivalent to 0.668 cfs. V=Q/A A= r2 For a 3” pipe r = .17 A = (0.17)2 A= 0.091 ft2 V= 0.668/0.091 V= 7.34 fps The friction loss for the length of pipe can be calculated using the following Hazen – Williams formula: hf = 3.02LD-1.167 (V/Ch)1.85 for a 4” pipe L = 92 ft (from catch basin to curb oulet) D = 4” = 0.33’ V = 7.34 Ch = 140 (plastic pipe) hf = 3.02(92) (0.33)-1.167 (7.34/140)1.85 hf = 4.33’ Therefore, the elevation and frictional headloss is TDH = 8 + 4.33 = 12.33’ Since the Q = 0.596 cfs = 7.48 gal/ft3(0.596) (60sec/min) = 267 gpm Therefore, pump must be capable of conveying 267 gpm with a total dynamic head of 13.77 feet. 1.5 hp Goulds pump is capable of conveying 300 gpm at a head of 16 feet and is therefore adequate. Even assuming some loss for the manifold in the system the pumps will be adequate. The pumps will be placed in a catch basin and an alarm system will be needed to alert the development owner to the failure of the pump(s). A check valve will be needed to keep the runoff from flowing back into the catch basin, once the pumps shuts off. APPENDIX Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Friday, Nov 11 2022 Curb outlet Rectangular Bottom Width (ft)= 3.00 Total Depth (ft)= 0.25 Invert Elev (ft)= 55.20 Slope (%)= 1.50 N-Value = 0.013 Calculations Compute by:Known Q Known Q (cfs)= 0.60 Highlighted Depth (ft)= 0.08 Q (cfs)= 0.600 Area (sqft)= 0.24 Velocity (ft/s)= 2.50 Wetted Perim (ft)= 3.16 Crit Depth, Yc (ft)= 0.11 Top Width (ft)= 3.00 EGL (ft)= 0.18 0 .5 1 1.5 2 2.5 3 3.5 4 Elev (ft)Depth (ft)Section 54.75 -0.45 55.00 -0.20 55.25 0.05 55.50 0.30 55.75 0.55 56.00 0.80 Reach (ft) 'Ill; 7 -- Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Friday, Nov 11 2022 NODES 2-3 Circular Diameter (ft)= 0.50 Invert Elev (ft)= 52.00 Slope (%)= 1.90 N-Value = 0.012 Calculations Compute by:Known Q Known Q (cfs)= 0.35 Highlighted Depth (ft)= 0.23 Q (cfs)= 0.354 Area (sqft)= 0.09 Velocity (ft/s)= 3.99 Wetted Perim (ft)= 0.75 Crit Depth, Yc (ft)= 0.31 Top Width (ft)= 0.50 EGL (ft)= 0.48 0 1 Elev (ft)Section 51.75 52.00 52.25 52.50 52.75 53.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Friday, Nov 11 2022 Nodes 3 - 4 Circular Diameter (ft)= 0.50 Invert Elev (ft)= 51.60 Slope (%)= 1.90 N-Value = 0.012 Calculations Compute by:Known Q Known Q (cfs)= 0.36 Highlighted Depth (ft)= 0.23 Q (cfs)= 0.361 Area (sqft)= 0.09 Velocity (ft/s)= 4.07 Wetted Perim (ft)= 0.75 Crit Depth, Yc (ft)= 0.31 Top Width (ft)= 0.50 EGL (ft)= 0.49 0 1 2 Elev (ft)Section 51.00 51.50 52.00 52.50 53.00 Reach (ft) -/ ' I ' V I -- - Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Friday, Nov 11 2022 Nodes 4 - 5 Circular Diameter (ft)= 0.50 Invert Elev (ft)= 51.20 Slope (%)= 2.00 N-Value = 0.012 Calculations Compute by:Known Q Known Q (cfs)= 0.37 Highlighted Depth (ft)= 0.23 Q (cfs)= 0.373 Area (sqft)= 0.09 Velocity (ft/s)= 4.20 Wetted Perim (ft)= 0.75 Crit Depth, Yc (ft)= 0.31 Top Width (ft)= 0.50 EGL (ft)= 0.50 0 1 Elev (ft)Section 50.75 51.00 51.25 51.50 51.75 52.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Friday, Nov 11 2022 Nodes 7 - 6 Circular Diameter (ft)= 0.33 Invert Elev (ft)= 53.80 Slope (%)= 2.30 N-Value = 0.012 Calculations Compute by:Known Q Known Q (cfs)= 0.03 Highlighted Depth (ft)= 0.07 Q (cfs)= 0.030 Area (sqft)= 0.01 Velocity (ft/s)= 2.25 Wetted Perim (ft)= 0.32 Crit Depth, Yc (ft)= 0.10 Top Width (ft)= 0.27 EGL (ft)= 0.15 0 1 2 Elev (ft)Section 53.00 53.50 54.00 54.50 55.00 Reach (ft) ~ l I. V ' ,-./ --- Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Friday, Nov 11 2022 Nodes 6 - 5 Circular Diameter (ft)= 0.33 Invert Elev (ft)= 53.30 Slope (%)= 5.10 N-Value = 0.012 Calculations Compute by:Known Q Known Q (cfs)= 0.04 Highlighted Depth (ft)= 0.07 Q (cfs)= 0.040 Area (sqft)= 0.01 Velocity (ft/s)= 3.00 Wetted Perim (ft)= 0.32 Crit Depth, Yc (ft)= 0.11 Top Width (ft)= 0.27 EGL (ft)= 0.21 0 1 Elev (ft) 53.00 53.25 53.50 53.75 54.00 AE Feet SDDO 4D00 EQUATION Tc = (1~t)0.385 Tc == Time of concentration (hours) L = Watercourse Distance (miles) .6.E = Change in elevation along effective slope line {See Figure 3-S)(feet) 3000 3D00 0.5 ' ' ' ' 30 20 300 5 200 AE L SOURCE: Callfomia Division of Highways (1941) and Kirplch (1940) Nomograph for Determination of Tc Hours Minutes 4 3 2 1 ' ' ' Tc so 40 30 20 18 16 14 12 10 9 8 7 6 5 4 3 Time of Concentration (Tc) or Travel Time (Tt) for Natural watersheds FIGURE ~ . 10.0 9.0 8.0 7.0 .,...,_, ,.._ .... , ' ...., I I 1' ...... 6.0 ' 1 "-5.0 I' I ; 4.0 '4-. I 3.0 I I 2.0 1' I I -;::- 5 1 -§ c1.0 ;o.9 -~0.8 QI :go.7 0.6 i 0.5 0.4 I 0.3 I ' 0.2 I I I I I l f I I I I I I -,- -1-I I i", 1' 'k. ·l ':..... ' ,, ... I I I ' I i ... 0.1 .... , .... , .... , .... .... , ·-· - ...... ' I .... , .... , , .... """"' .. ... ' ' .. ' .......... .. .. ~ '"" ....... "" ' . ' "" ...... ' ,, ... .. "" .. I= 3.2in/hr I'-, .. ' .. '"" .... .. r-... I"',. I'-.,., . ' .. ,, .... ~ .. ""'i... "", .. ·--~ - -----· 1---.. ----• 1c = 20min 111111111 l 11111111 5 6 7 8 9 10 15 20 30 Minutes ! I I I I l I I I .. I .. t I .... ~ I .... i .. ~ .. ~ I ~ .. I l .... I I .. I I I ·~ I I i ; -- I I - l I I I I f I I I I i 11 I I I I I l 11 EQUATION I = 7.44 P5 o·0.645 ! I I = intensity (in/hr) i Ps = 6-Hour Precipitation (in) I I D = Duration (min} ' I 11111111111 I I I I I ! I ' 'i.. I l I I ' .. ,, ...... I I ' ,' ,, .. ... • .. I ' ' :-., :1 ~ I " ,, ... j I I, ,, ,, ~ r-i. I " I ,, ' ,, • 1 -l I, i ' J ,, ' 1"' I I . .... , .... t I J. ' N.. I I "r-. I , .. I I I ""'" .... I ~ 17 I ' I I ' r ~ ' I .... I I I ,, I ~ I ! I ', I I ; ' I ; I I ' I I --·-f---· -,_ -+· . l·J I I I I I ' ; I I I I ! I I I I I i a, :i: a C -. '1J iil o. 6.0 '2. 5.5 §!: 5.0 g· 4.5 '§' n 4.0 i 3.5~ 3.0 2.5 2.0 1.5 1.0 11111111 111111111111111 I I lllllil 40 50 1 2 3 4 5 6 Hours Duration Intensity-Duration Design Chart -Example Directions for Application: (1) From precipitalfon maps determine 6 hr and 24 hr amounts ror the selected frequency. These maps are included in the County Hydrology Manual {10, 50, and 100 yr maps included in the Design and Procedure Manual}. (2) Adjust 6 hr precipitation (if necessary) so that it is within the range of 45% to 65% of the 24 hr precipitation (nol applicaple to Desert). {3) Plot 6 hr precipitation on the right side of the chart. {4) Draw a line through lhe point parallel to the plotted lines. (5) This line is the intensity-duration curve for the location being analyzed. Appllcatfon Form: (a) Selected frequency _filt_ year (b) Ps = 3 in. P24 = 5.5 ~ = 54.5 %12) --' --'P24 -- (c) Adjusted p6<2> = _3_ in. (d) Ix= ~ min. (e) I= _R__in.lhr. Nole: This chart replaces the Intensity-Duration-Frequency curves used since 1965. PG , 1 1.5 · 2· . 2.5 3 3.S 4 4.5 S 5,5 : 6 Duration, l I ; l I I 1 · i I l I . ·1 ·s 2.63 ;=!,9515.27; 6.59 J.90 9.22p0.54_ 11.86! 13.17: 14.49, IS,81 7 2.12 3.18;4.24!5.30 6.36·7.42: 8.48. 9.54 10.60111.66'12.72 10 1.sa ;2.s3ia.~?14.2i_),~;s.£1!ll ~.74: t,5-~j t~?..: 9-.21: 10.J.1 15 1:30 _ 1.95: ?,SJ; 3.24, 3.89'.4.54: 5.19 __ 5.~.: 6..,_4~_, 1,13 i .7.l~ 20 1.08 ·1.62:2.15'2.69 3.23 3.77; 4.31 · 4.85 ! 5.39, S.9316.-16 25 o.9a :1.40: 1.a1;2.33(2.ao;3~211 :iia: ;r20 i":61 ··s:fa: s:so 30 O.B3 ; 1.24; 1.66, 2.07 ;2.49. 2.90( 3.~2. 3,73: 4.15 4.56 ! 4.98 4_0 0.69. 1.03_ 1.38_ 1.72 _2.01:2.41 i 2.76, 3.10 i 3.45 3.79; 4.13 50 0.60 .0.90, 1.19. 1.49 1.79"2.09, 2.39. 2.69, 2.98 3.28. 3.58 so o.53 ·o.eo: 1.00· 1.33: 1.ss· 1.95; 2.12 · 2;39; 2.65 2.!i2; a.18 ~~ 0.41 ;o.s1 iQ.tl2: 1.02. 1.2( 1.4a: t.63 ! 1.84 ; 2.04 ?-25: 2.~5 120 0.34 10.51:0.68,0.85'1.02 1.19' 1.36 1.53, 1.70 1.87: 2.04 1so o.29 ·o.44io.s9' 0.13:0.88: 1.03' us· 1.32' 1.47 1.s2 · 1.16 .•. -··-····· . • . • -. ~ -·• _, . • -. . • ! t • . ! . . 180 0.26 ,0.39: o.s2; o._65 J>.78;0.91 ! 1.04 _ 1.18, 1_,a_1 1,44 J t.5? 240 0.22 :o,33)0,43j o.54 :0.65,0.76, o.87: o.98 ! 1.0_8 1.1_\l. i,:ro 300 0.1s ;o.2s;o.38: o.47,0.56!0.66! 0.1s; o.~ L o.94 1,0:l: 1.13 360 0.17 ,0.2~:o.a:J•OAg_,o.sp o.sa. o.67: o.7li~84 0.92, 1.00 FIGURE @] San Diego County Hydrology Manual Date: June 2003 Table 3-1 Section: Page: RUNOFF COEFFICIENTS FOR URBAN AREAS Land Use I Runoff Coefficient "C" Soil Type NRCS Elements Count Elements % IMPER. A B Undisturbed Natural Terrain (Natural) Pennanent Open Space O* 0.20 0.25 Low Density Residential (LOR) Residential, 1.0 DU/A or less IO 0.27 0.32 Low Density Residential (LOR) Residential, 2.0 DU/A or less 20 0.34 0.38 Low Density Residential (LDR) Residential, 2.9 DU/A or less 25 0.38 0.41 Medium Density Residential (MOR) Residential, 4.3 DU/A or less 30 0.41 0.45 Medium Density Residential (MOR) Residential, 7.3 OU/A or less 40 0.48 0.51 Medium Density Residential (MDR) Residential, l0.9 DU/A or less 45 0.52 0.54 Medium Density Residential (MOR) Residential, 14.5 DU/A or less 50 0.55 0.58 High Density Residential (HOR) Residential, 24.0 DU/A or less 65 0.66 0.67 High Density Residential (HDR) Residential, 43.0 DU/A or less 80 0.76 0.77 CommerciaVlndustrial (N. Com) Neighborhood Commercial 80 0.76 0.77 Commercial/Industrial (G. Com) General Commercial 85 0.80 0.80 Commercial/Industrial (O.P. Com) Office Professional/Commercial 90 0.83 0.84 Commercial/Industrial (Limited l.) Limited Industrial 90 0.83 0.84 Commercial/Industrial {Genera_l J.) General Industrial 95 0.87 0.87 C 0.30 0.36 0.42 0.45 0.48 0.54 0.57 0.60 0.69 0.78 0.78 0.81 0.84 0.84 0.87 3 6 of26 D 0.35 0.41 0.46 0.49 0.52 0.57 0.60 0.63 0.71 0.79 0.79 0.82 0.85 0.85 0.87 *The values associated with 0% impervious may be used for direct calculation of the runoff coefficient as described in Section 3.1.2 (representing the pervious runoff coefficient, Cp, for the soil type), or for areas that will remain undisturbed in perpetuity. Justification must be given that the area will remain natural forever (e.g., the area is located in Cleveland National Forest). DU/ A = dwelling units per acre NRCS = National Resources Conservation Service 3-6 33•·30· 3:1'15' 33'0Q' 32"4ti' ano· Orange County g ~ 0 "' "' ~ S: ... g ~ ,,, ·"' :~ ,., M ~ fo ~ 33·30• .. ··• ,..lJ . •c·'e County .. ,.. .:,, .. , Rivers" .. , ·· .. , i1i>, ,.·) ,. ········•: ....... - '\ .. 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