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HomeMy WebLinkAboutCDP 16-07; OTA RESIDENCE; PRELIMINARY HYDROLOGY STUDY; 2016-02-16PRELIMINARY HYDROLOGY STUDY for 4090 GARFIELD STREET CARLSBAD, CA 92008 City of Carlsbad, CA PREPARED FOR: Blair and Paula Ota 4 Bellezza Irvine, CA 92620 Date: February 16, 2016 PREPARED BY: Pasco Laret Suiter & Associates 535 N. Highway 101, Suite A Solana Beach, CA 9207 5 (858) 259-8212 BRIAN ARDOLINO, RCE 71651 RECEIVED FEB 2 9 2016 CITY OF CARLSBAD PLANNING DIVISION Z·IJ· H, DATE Ota Residence TABLE OF CONTENTS Executive Summary Introduction Existing Conditions Proposed Project Summary of Results and Conditions Conclusions References Methodology Introduction County of San Diego Criteria Runoff coefficient determination Hydrologic Analyses Pre-Developed Hydrologic Analysis Post-Developed Hydrologic Analysis Hydraulic Calculations Appendix 2 SECTION 1.0 1.1 1.2 1.3 1.4 1.5 1.6 2.0 2.1 2.2 2.3 3.0 3.1 3.2 4.0 5.0 February 2016 Ota Residence 1.0 EXECUTIVE SUMMARY 1.1 Introduction This Hydrology Study for the Ota Residence project has been prepared to analyze the hydrologic and hydraulic characteristics of the existing and proposed project site. This report intends to present both the methodology and the calculations used for determining the runoff from the project site in both the pre-developed (existing) conditions and the post- developed (proposed) conditions produced by the 100 year 6 hour storm. In addition this report will propose the sizing of all necessary storm drain facilities and storm drain piping necessary for the storm drain system to safely convey the runoff from the 100-year rainfall event. 1.2 Existing Conditions The property is geographically located at N 33°08' 45.19" W 117°20'26.55". The site is bordered by residential development to the north and south and east. Garfield Street is located to the west of the proposed development. The project site is located in the Agua Hedionda Hydrologic Area and more specifically, the Los monos Sub-Area (904.31). The existing project site includes 1 residence, detached garage and associated paving and hardscape. The site consists mostly of a gentle slope from the southeast to northwest. Drainage from the existing site sheet flows in the westerly direction onto Garfield Street. The runoff is then conveyed north along the curb and gutter toward existing curb inlets in Date Avenue and ultimately discharges west into the Agua Hedionda Lagoon. 1.3 Proposed Project The intent of the proposed project is to construct 1 new single family residence with associated hardscape improvements and driveways. The proposed drainage design conveys runoff from the east to the west to landscape BMP planter areas for LID storm water treatment. Once treated, the water is discharged through stormdrain curb underdrains onto Garfield Street as it does in the existing condition. We believe the proposed storm drain system will not adversely affect the downstream system negatively. To address the storm water quality goals established for this development, proposed permanent Best Management Practice (BMP) and treatment methods will be incorporated into the storm water runoff design. The proposed BMP's include multiple landscape BMP planter areas, which are intended to mitigate peak flows as well as serve as settling basins. February 2016 3 Ota Residence 1.4 Summary of Results Upon performing hydrologic analysis of the project site in both the proposed developed and existing condition the following results were produced. One discharge point was analyzed. In the pre-developed condition one discharge point was analyzed. Outlet point indicates that the 100-year peak flow is 0.40 cfs with a time of concentration of 8.1 min based on an area of0.11 AC. In the post-developed condition one discharge point was analyzed. Outlet point indicates that the 100-year peak flow is 0.39 cfs with a time of concentration of 8.9 min based on an area of 0.11 AC. 1.5 Conclusions The overall peak flow leaving the property is decreased by 0.01 cfs. Based on the discussion in this report it is the professional opinion of Pasco Laret Suiter & Associates, Inc. that the existing drainage system on the corresponding Preliminary Grading Plan will function to adequately intercept, contain and convey flow to the appropriate points of discharge. See Sections 3.0 and 4.0 for calculations. 1.6 References ''San Diego County Hydrology Manual': revised June 2003, County of San Diego, Department of Public Works, Flood Control Section. "California Regional Water Quality Control Board Order No. 2009-0009-DWQ, "California Regional Water Control Board, San Diego Region (SDRWQCB). February 2016 4 Ota Residence 2.0 METHODOLOGY 2.1 Introduction The hydrologic model used to perform the hydrologic analysis presented in this report utilizes the Ration Method (RM) equation, Q=CIA. The RM formula estimates the peak rate of runoff based on the variables of area, runoff coefficient, and rainfall intensity. The rainfall intensity (I) is equal to: Where: 1 = 7.44 x P6 x o-0-645 I = Intensity (in/hr) P6 = 6-hour precipitation (inches) D = duration (minutes -use Tc) Using the Time of Concentration (Tc), which is the time required for a given element of water that originates at the most remote point of the basin being analyzed to reach the point at which the runoff from the basin is being analyzed. The RM equation determines the storm water runoff rate (Q) for a given basin in terms of flow (typically in cubic feet per second (cfs) but sometimes as gallons per minute (gpm)). The RM equation is as follows: Where: Q=CIA Q= flow (in cfs) C = runoff coefficient, ratio of rainfall that produces storm water runoff (runoff vs. infiltration/ evaporation/ absorption/ etc) I= average rainfall intensity for a duration equal to the Tc for the area, in inches per hour. A = drainage area contributing to the basin in acres. The RM equation assumes that the storm event being analyzed delivers precipitation to the entire basin uniformly, and therefore the peak discharge rate will occur when a raindrop falls at the most remote portion of the basin arrives at the point of analysis. The RM also assumes that the fraction of rainfall that becomes runoff or the runoff coefficient C is not affected by the storm intensity, I, or the precipitation zone number. In addition to the above Ration Method assumptions, the conservative assumption that all runoff coefficients utilized for this report are based on type "D" soils. Rational Method calculations were performed using the AES 2010 computer program. To perform the hydrology routing, the total watershed area is divided into sub-areas which discharge at designated nodes. The procedure for the sub-area summation model is as follows: (1) (2) (3) Subdivide the watershed into an initial sub-areas and subsequent sub-areas, which are generally less than 10 acres in size. Assign upstream and downstream node numbers to each sub-area. Estimate an initial Tc by using the appropriate nomograph or overland flow velocity estimation. The minimum Tc considered is 5.0 minutes. Using the initial Tc, determine the corresponding values of I. Then Q = CIA. February 2016 5 Ota Residence (4) Using Q, estimate the travel time between this node and the next by Manning's equation as applied to particular channel or conduit linking the two nodes. Then, repeat the calculation for Q based on the revised intensity (which is a function of the revised time of concentration) 2.2 County of San Diego Criteria As defined by the County Hydrology Manual dated June 2003, the rational method is the preferred equation for determining the hydrologic characteristics of basins up to approximately one square mile in size. The County of San Diego has developed its own tables, nomographs, and methodologies for analyzing storm water runoff for areas within the county. The County has also developed precipitation isopluvial contour maps that show even lines of rainfall anticipated from a given storm event (i.e. 100-year, 6-hour storm). One of the variables of the RM equation is the runoff coefficient, C. The runoff coefficient is dependent only upon land use and soil type and the County of San Diego has developed a table of Runoff Coefficients for Urban Areas to be applied to basin located within the County of San Diego. The table categorizes the land use, the associated development density (dwelling units per acre) and the percentage of impervious area. Each of the categories listed has an associated runoff coefficient, C, for each soil type class. The County has also illustrated in detail the methodology for determining the time of concentration, in particular the initial time of concentration. The County has adopted .the Federal Aviation Agency's (FAA) overland time of flow equation. This equation essentially limits the flow path length for the initial time of concentration to lengths of 100 feet or less, and is dependent on land use and slope. 2.3 Runoff Coefficient Determination As stated in section 2.2, the runoff coefficient is dependent upon land use and soil type and the County of San Diego has developed a table of Runoff Coefficients for Urban Areas to be applied to basin located within the County of San Diego. The table, included at the end of this section, categorizes the land use, the associated development density (dwelling units per acre) and the percentage of impervious area. Low Density Residential coefficient of 0.41 was used for pervious area, and Commercial/Industrial coefficient of 0.87 was used for impervious area. Weighted runoff coefficients for onsite areas were calculated using the existing and proposed impervious area for each basin. See Appendix 5.0 for Coefficient Table and C Value Calculations on the Pre and Post Development Maps. February 2016 6 Ota Residence 3.0 HYDROLOGIC ANALYSES Rational Method Parameters Runoff Coefficient C=0.87* for pre-developed condition type "D" soils Runoff Coefficient C=0.41 * for post-developed condition type "D" soils 100 Year 6 Hour Storm Precipitation (P6)=2.5 in (see rainfall isopluvial*) Tc=(11.9L3/ LiE)0.385 per Figure 3-4 of the County of San Diego Hydrology Manual (L=miles)* Tt=Ti + Tc I= Intensity in/hr, I=7.44xP6xD-0.645* Duration (D)= Time of Concentration, Tc Q=Peak Runoff, Q=C*I*A (cfs) C= (% IMP x 0.87) + [(1-%IMP) x 0.41] *From San Diego County Hydrology Manual, June 2003 Revision 3.1 Pre-Developed Hydrologic Model Output BASIN A=4,995 sf= 0.11 ac Impervious area= 3,674 sf % IMP= 74% C= (0.74x 0.87) + [(1-0.26)0.41] C= 0.75 Initial Ti for 3% = 7.0 min Tt=(l 1.9(124/5280)3 / 4.7)0-385 Tt=0.019 hours = 1.1 min Tc= 7.0+1.1 Tc=8.1 minutes 1100=7 .44(2.5) (8.1) -0.645 1100=4.8 in/hr Q100= 0.75(4.8 in/hr) 0.11 acres Qioo=0.40 cfs 3.2 Post-Developed Hydrologic Model Output BASIN A=4,995 sf= 0.11 ac Impervious area = 3,997 sf %IMP= 80% C= (0.80x 0.87) + [(1-0.80)0.41] C= 0.78 Initial Ti for 3% = 7.0 min Tt=(l 1. 9(193 / 5280)3 / 4.3)0.385 Tt=0.032 hours = 1.9 min 7 February 2016 Ota Residence Tc= 7.0+1.9 Tc=8.9 minutes Iioo=7.44(2.5)(8.9) -0-645 Iioo=4.5 in/hr Q100= 0. 78( 4.5 in/hr) 0.11 acres Qioo=0.39 cfs February 2016 8 Ota Residence 4.0 HYDRAULIC CALCULATIONS Detention Basin Sizing: LID BMP Sizing Requirement LID BMP area required is equal to 4% of the impervious are being removed and replaced or added Required BMP = 0.04 (4,020 sf) = 161 SF Proposed BMP Area = 170 SF Proposed BMP Area > Required BMP Area February 2016 9 Ota Residence 5.0 APPENDIX February 2016 10 San Diego County Hydrology Manual Date: June 2003 Section: Page: 3 12 of26 Note that the Initial Time of Concentration should be reflective of the general land-use at the upstream end of a drainage basin. A single lot with an area of two or less acres does not have a significant effect where the drainage basin area is 20 to 600 acres. Table 3-2 provides limits of the length (Maximum Length (LM)) of sheet flow to be used in hydrology studies. Initial Ti values based on average C values for the Land Use Element are also included. These values can be used in planning and design applications as described below. Exceptions may be approved by the "Regulating Agency" when submitted with a detailed study. Table3-2 MAXIMUM OVERLAND FLOW LENGTH (LM) & INITIAL TIME OF CONCENTRATION (T1) Element* DU/ .5% 1% 2% 3% 5% 10% Acre LM Ti LM Ti LM Ti LM Ti LM Ti LM Ti Natural 50 13.2 70 12.5 85 10.9 100 10.3 100 8.7 100 6.9 LDR 1 50 12.2 70 11.5 85 10.0 100 9.5 100 8.0 100 6.4 LDR 2 50 11.3 70 10.5 85 9.2 100 8.8 100 7.4 100 5.8 LDR 2.9 50 10.7 70 10.0 85 8.8 95 8.1 100 7.0 100 5.6 MOR 4.3 50 10.2 70 9.6 80 8.1 95 7.8 100 6.7 100 5.3 MOR 7.3 50 9.2 65 8.4 80 7.4 95 7.0 100 6.0 100 4.8 MDR 10.9 50 8.7 65 7.9 80 6.9 90 6.4 100 5.7 100 4.5 MDR 14.5 50 8.2 65 7.4 80 6.5 90 6.0 100 5.4 100 4.3 HDR 24 50 6.7 65 6.1 75 5.1 90 4.9 95 4.3 100 3.5 HDR 43 50 5.3 65 4.7 75 4.0 85 3.8 95 3.4 100 2:7 N.Com 50 5.3 60 4.5 75 4.0 85 3.8 95 3.4 100 2.7 G.Com 50 4.7 60 4.1 75 3.6 85 3.4 90 2.9 100 2.4 O.P./Com 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 100 2.2 Limited I. 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 100 2.2 General I. 50 3.7 60 3.2 70 2.7 80 2.6 90 2.3 100 1.9 *See Table 3-1 for more detailed description 3-12 ,o.o 9.0 8.0 70 6.0 5.0 4.0 3.0 2.0 :5 j -~1.0 :;o.9 -~0.8 .sl .50,7 0.6 0.5 0.4 0.3 02 0.1 l',.' ),,..''"'" i". "i-,.1 I I I I I I I I 1' 1' .... ""i,..i :.'N I I N.. 1'. .... r,.. -.....N." ,f"i,. ..... I I '1 ~ ~H.. -~ .. ','{ II EQUATION i-1 J . ", r"!-,, .... 11· I = 7.44 Pa o-0.645 I .. : i,... ~ I • , ....... ""' . . l'' l .... :,.. , "I '"' 1 1 I = Intensity (m/hr) I 'f.._ I"'-.,, ', i,. " .. ,.. P5 = 6-Hour Precipitation (in) K 11 , .. 17' ~, t I I l D = Duration(min) i I ' I', ,J I', .... 1 :,,f' .. H,. I ~ . l ........ 1 , II l I I I I 1 I I" ... I li .. ~ i 1 .... ~.. 1 ' . I i I ! I I i l I"'" I ' I r"' i I .. I J i~ r ', 11 11 l l' I I ' iJ. I 1-N,. I I I I I ~ .. l 1~ t 1' i I I ! i I"', I 11 "1 ~J .. i 11 ' I l .. ~ 11 ~~:,,.. l I t I Ii I II I I ~... I ~ ' I . I I I, !,NJ ... .... I j I ')...l :,,~ \I~· 1l 1l .... .. ii 1' 1::~ ~:< .. ~ II . ,. !o. I :,, N.::,,' '" N, 'S, I . I .. , .. W l 11m I I 111 I. 11', 1 ...... ~ .. 1~ I l I I 11 I 111~ Ill I" .. II l'I I 11 '! 1, .,.. ...... I l I I I i I ! I II :1 I I Ill 11 .... , "":-. I~ ·~ , I I I j 11 I I !I I 11 t!, 111 I I .11 I' .. ~ .. I' II . ! I 1 l i I 11 I .. ~ .. 11 1111 I ", .... I .. T 1 1 111 1 · 1 I 1111 1 I 1 • .. .. .... 1 .. r I I I I \ I I I I I I '1 ,.,, r. .. , .. ' i l ! I I I I I II I l'h. .. 11 I Ii I I I I I I I .I ,, .. 11 ~+. "-_.._ !._J,_ 1 _ I -1 I O I I ' ii ' 1~.t-,,___..__ ,., -· • r-f-- . I I j ! l _j_,_,_,_ --···1-W-.J I I f • _ i. I ; i l I I I I 5 6 7 8 9 10 15 20 30 40 50 2 3 4 5 6 Minutes Hours Duration en t " 8. 6.0 -g, 5.5 !! 5.0 g· 4.5 '§' n 4.0 i 3.5 _e 3.0 2.5 2.0 1.5 1.0 Intensity-Duration Design Chart -Template Directions for Application: (1) From precipitation maps determine 6 hr and 24 hr amounts for 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 (not applicaple to Desert). (3) Plot 6 hr precipitation on the right side of the chart. (4) Draw a line through the point parallel toJhe plotted lines. (5) This line is the intensity-duration curve for the location being analyzed. Application Fonn: (a) Selected frequency ___ year (b) P5 = in. P24 = P5 = o;p> --I --'P24 -- (c) Adjusted p6<2) = __ in. (d) tx = __ min. (e) I= __ inJhr . Note: This chart replaces the Intensity-Duration-Frequency curves used since 1965. P6 1 1.5 . 2 2.5 3 3.5 . 4 4.5 5 S.S 6 Duration I I I I I I I I I I I -5 2,63 3.95 5.27 6.59 7.90 9.22 10.54 11.86 13.17 1449 15.81 1 2.12 ·3,1a: 42,( s.so 6.36 7.42~ 8.48 9.54 10.so 11.66· 12.12 10 "us · 2.sf 3.37' 4.21 · 5.os 5.90; 6.74 · 1.sa s.42 · 9.27 10.11 15 1.30 , 1,95' 2.59. 3.24 3.89. 4:54-5.19 . 5.84 . 6.49. i. 13. 7.78 20 -;:os · 1.S2:-2.,s: 2.6(:i.2( a.n'. 4.3_1 : 4.~. $:39: s:93 ~ s.46 25 0.93 1.40 1.87 2.33 2.80:3.27 3.73 4.20 4.67 5.13 5.60 30 o.83 : 1.24_ 1.ai( 2 012.49'. uo: 3.a2; 3.73: 4. 1s >.ss · 4.98 40 0.69 1.03 1.38 1 72 2.07 2 41, 2.76 .'.1.10 3.45 3 79 4. 13 50 ·o.oo ·0.90··,;,g· 1.49 1.19·2.09· 2.39 2.69 2.95 328 · 3.ss 60 o.53 o.so: 1.os: t.33 · 1.sg: 1.86; 2.12 2.39. 2.ss: 2.92 · 3.18 ... !O _0.4 t . o,~1: 0.82. _1.,02 . 1.23: ua. 1.63 . 1.84 • 2.04 . 2.2s • 2.45 120 0.34 0.51 0.68 0.85 1.02 1.19 1.36 1.53 1.70 t.87 2.04 150 0.29 ·0.44' 0.59 0.73 0.88 1.03. 1.18. 1.32. 1.47 1.62 1.76 ,so ·o .. 26 :o.3(o.s(o.6s:o.78 o.s,: i.04: us .. : 1.31: 1.44 :1.57 240 _ci,22 .o.3~J(!.~3 .• o.54 .o,GS Q,76. o.87 0.98, 1.os. 1.19. 1.30 300 0.19 0.28'0.38 0.47 0.56 0.66 0.75 0.85 0.94 1.03 1.13 360 -0.11 ·o.2s· 0.33· o.42 · o.so o.ss· os1 o.75 · o.84 · o.s2 · 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 T~e NRCS Elements Coun Elements %IMPER. A B Undisturbed Natural Terrain (Natural) Permanent Open Space O* 0.20 0.25 Low Densi Residential LDR Residential 1.0 DU/A or less 10 027 0.32 Low Density Residential (LDR) 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 (MDR) Residential, 4.3 DU/A or less 30 0.41 0.45 Medium Density Residential (MDR) Residential, 7.3 DU/A or less 40 0.48 0.51 Medium Density Residential (MDR) Residential, 10.9 DU/ A or less 45 0.52 0.54 Medium Density Residential (MDR) Residential, 14.5 DU/A or less 50 0.55 0.58 High Density Residential (HDR) 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 Commercial/Industrial (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 I.) Limited Industrial 90 0.83 0.84 Commercial/Industrial (Genei:al I.) 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 6of26 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 C> io C> ('t) .... 0 0 r 0 0 t--t--t--.... .... .... .... .... .... I I io , co .... .... I C> ('t) 0 co .... .... 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I I ' I ' . . .. . t t I ' • 't I \ '•• •' ·. •, •, ..... •••• n-;t':r · .. -,.-a.,· ·32°45' 32°30' ~ ~ .... - io .... ~ -.... r· ... \. IMPERIAL ~EAC~ '.i CI ,., ••• I I li \r-·:-:·'l:!__ ...... . 8 ~ .... .... .~--.... ' . wrl-- . • .. · -, •. '!.---~·-r•''" ~ co -.... '. \ •, .. •, '· ··-··· . . . .. . .. ~ •, M e X . ' C ~ 0 co .... .... . . io .... 0 co .... .... .. 0 0 ; C: I :::, ,.If)~'< ~2°45' 32"30' County of San Diego Hydrology Manual Rainfall Isopluvials 100 Year Rainfall Event-6 Hours l ........ ~~)1 f t.D ~ 2.s DPW ~GIS N _,, __ _._.,,.__.- ~-~~if;.' Af·..,,~~GIS ~~;':). '1i!"" ~ ~,;fr '':i !,J' %__,.¥,, ,cy,,,$-4 l We Ha,·.: San Diego Covered! -+E THIS MAP 18 PROVIDED Wl1lff)UT WARRANTY OF NlV KNl, EITHER EXPRESS OR IMPUED. INQ.LCING, BUT NOT LIMITED TO, lHE IMPLIED WARIWITlES OF MERCHNITAIIIUTY NID FITNESS FOR A PNITICUlAR PURPOSE . Cowlllhl-,. Rlghla-. Thll praduclamoy __ .._ ... SANDAG Raglonel -Syoiorft--be_...__ ... ---of-. -..-------......---......-.-11r--Mop1. s 3 0 3 MIies -I -====- ' . I Pl Pl [Tl Pl I I I G1 I (J1 --,, IL-- ~ :e: \ OUTL T =481 100= 0 C I ( \\', \~ ~· I I , I I I \ Q \ :;' APN 208--091-0., HYDROLOGIC NODE MAP 4090 GARFIELD STREET PRE-DEVELOPMENT MAP ) - APN 2()8.()91.fJS ', ', ', HIGH POINT EL-52.8 "' --\ APN20UBHJ8 APN 2Q8.091-0S LEGEND BASIN BOUNDARY FLOW LINE IMPERVIOUS AREA ~J I / ;; ! 3,674 SF -C" CALCULATIONS C= (%If.1PERVIOUS x 0.87) + [ (1-%If.1PERVIOUS)x0.41) C= (0.74x0.87) + (0.26x0.41.) C=0.75 SCALE: 1•=20' . . . .,.. f TL~T =48:j CHOO= 9 CFS \ \\ \ \ I \ APN 20l#J91--03 \ ' .·· . ' - :,\, 'l_·• ......... -~ I 'f' ··· l.k.t·-~-,,.% I"' •::.· _ • • ~ .. • •)4'. I I ~~ ~ HYDROLOG/C NODE MAP 4090 GARFIELD STREET POS~DEVELOPMENTMAP - I APN 20l#J91-CS APN 20l#J91.«J APN 208-C91.0S LEGEND BASIN BOLNJARY FLOW LINE IMPERVIOUS AREA 1,. ,, , -CU CALCULATIONS 3.997 SF C= (%IMPERVIOUS x 0.87) + [ (j-%IMPERVIOUS)x0.4j) c-(0.80x0.87) + (020x0.4j) 0=0.78 SCALE: 1•=20'