The URL can be used to link to this page

Home My WebLink AboutMS 05-19; HEMLOCK PROJECT; DRAINAGE & HYDROLOGY STUDY; 2006-01-07DRAINAGE & HYDROLOGY STUDY for HEMLOCK PROJECT MS 05-19/SOP 05-09/CP 05-10/CDP 05-25 APN: 204-251-06 Prepared for: City of Carlsbad 1635 Faraday Avenue Carlsbad, CA 92008 Telephone: (760) 602-4600 Prepared by: Alta Consultants 4901 Morena Blvd., Suite 409 San Diego, CA 92117 (858) 581-6101, Fax (858) 581-6138 Bartolome J. Pastor, RCE 38606 Project Site Address: 130 Hemlock Avenue Carlsbad, CA 92008 Revision Date: January 7, 2006 Submitted by: GOFAT, LLC 160 Tamarack Ave. Carlsbad, CA 92008 94111~ ~~ Lp/q / 6(p gineer of Work - olome J. Pastor Date RCE 38606, Expires 3/31/2007 ALTA CONSULTANTS PLANNING • ENG1NEERING• SURVEYING 4901 Morena Blvd. Suite 409 San Diego, Ca. 92117 TEL.(858) 581-6101 FAX (858) 581-6138 ALTACONSULTANTS(ãAOL.COM n~5-M-4-4T- # ci Table of Contents Introduction & Project Description...................................................................................Page 2 Scopeof Work ............................................................... . ................................................... Page 2 Methodology.....................................................................................................................Page 2 Existing Site Conditions .................... . ............................................................................... Page 3 Developed Site Conditions .............................. ................................................................. Page 3 RunoffAnalysis................................................................................................................Page 4 HydraulicAnalysis ...........................................................................................................Page 4 Summary..........................................................................................................................Page 5 Hydraulic Calculations ...................................................................................................... Page 6 Additional Attachments AttachmentA.........................................................................................................Vicinity Map AttachmentB ...................................................................................................... Hydrology Map Attachment C........................................................................................Runoff Coefficent Chart Attachment D........................................................Hydrology & Hydraulic Reference Formula Attachment E .............................................................................. 85th Percentile Isopluvial Map 1 INTRODUCTION This preliminary drainage & hydrology study has been prepared to document the design and calculations of the drainage systems associated with the proposed site development located at the Hemlock Project (MS 05-19/SDP 05-09/CP 05-10/CDP 05-25, CE 06-01) within the City of Carlsbad. The existing 0.13 acre site consists of a two story multi-family residence, driveway and retaining walls. The proposed development for this site will include the installation of new retaining walls, the construction of a two-story, two unit, attached multi-family residence with underground parking garage, new driveway paving and drainage improvements. Project runoff will be conveyed through the use of multiple catch basins, cleanouts, storm drains, basement sump pump and sidewalk under- drain as shown in the Hydrology Map in Attachment B. Any run-off leaving the site will be mitigated through this storm drain system to follow the existing flow pattern along Hemlock Avenue. SCOPE OF WORK The purpose of this report is to analyze the impact of 100-year storm flows on the proposed storm drain system. The scope of work for this report includes analyzing existing and proposed drainage conditions, a review of potential downstream impacts, performing hydrologic and hydraulic calculations and presenting the results. This preliminary report has been prepared in conjunction with the Storm Water Management Plan and the proposed Site Development plans. The objective of this report is to design the size, alignment and grade of proposed drainage improvements for this project. METHODOLOGY This hydrology study is confined within the limits of the development. Runoff from the property located easterly currently flows along the concrete driveway into a 2" storm drain pipe and outlets into Hemlock Ave. This drainage flow will not be altered. The properties located northerly and northwesterly are at a lower grade than the proposed project and runoff flows will not be affected. The runoff calculation chart shown in Attachment C of this report shows the runoff coefficients C used to calculate the designed runoff Q through the proposed on-site storm drain improvements. This report has been prepared in accordance with current City of Carlsbad regulations and procedures. The following have been used in preparation of this study: County of San Diego Storm Water Standards Manual, Revised May 2003 County of San Diego Hydrology Manual, 2003 Edition Handbook of Hydraulics (7th Edition), Brater and King The calculations are based on the Rational Method. The existing site is designated urban with an average slope of 1%. The drainage area for the project is 0.13 acres. The existing condition for a 100-year storm will generate a Q1 00=0.68 cfs total run-off. Remarks Area Precipitation Length of Travel Effective Slope Time of Concentration Runoff Coefficient Intensity A = 0.13 Acres, P6 = 2.5 Inches L= 100 Feet S= 1.0% Tc = 5.51 Minutes C= 0.85 I = 6.17 Inches/Hour Urban Overland Time Chart Soil Group "D" Pre-development Runoff Q = 0.68 cfs DEVELOPED CONDITIONS Based on the data, calculations and recommendations contained in this report, a system can be constructed to adequately convey the proposed Qioo to the appropriate discharge points. All drainage from the site will follow the same overall drainage pattern as the existing condition, with no change in run-off direction leaving the site. The proposed project will produce an additional Q100=0.35 cfs total runoff within the on-site drainage area. This site has been designed to maintain the existing drainage patterns. All runoff shall be mitigated through the proposed storm drain system and is calculated to discharge Q=1.03 cfs to Hemlock Avenue during a 100-year storm event. Because the existing site and the proposed design are almost identical in relation to pervious surface area and runoff variables, the proposed design represents an almost negligible increase in runoff from the existing condition of Q=0.68 cfs. The primary condition of concern is blockage of BUT structures which could cause flooding. A maintenance plan is required and shall be performed by the homeowners. All permanent BMP structures shall be inspected weekly and after any rainfall. Post-Construction Hydrology is shown on page 6 of this report Formulas used are shown in Attachment D. DEVELOPED AND UNDEVELOPED RUNOFF ANALYSIS The runoff analysis for the existing conditions was performed using the Rational Method. The hydrological analysis utilized to determine the run-off at each design point was the Rational Method Equation (Q=C x I x A). The following pages describe the methods used to determine each component of the Rational Method Equation, in which; Q = Run-off (cfs) C = Run-off Coefficient 'A =Rainfall Intensity (in/hr) A = Area (acres) The data for the analysis are presented in the tables at the back of this report. DETERMINATION OF RUN-OFF COEFFICIENT Run-off coefficients are dependant on the proposed land use of the site. The hydraulic soil group classification for this site is "Group D". Coefficients for this project were calculated from the County Hydrology Manual, 2003 Edition. DETERMINATION OF INTENSITY .Rainfall Intensity (I) is based on the "Intensity-Duration-Design Chart" shown in the County Hydrology Manual, 2003 Edition. TIME OF CONCENTRATION Overland Time is the period required for run-off to travel from the farthest edge of a drainage area to the lowest point. The method of determining Overland Time is dependant on the type of watershed. The Urban Overland Time of Flow Formula-was used to determine the Time of Concentration. DETERMINATION OF AREAS The areas (A) of each drainage basin were determined from topographic field survey. HYDRAULIC ANALYSIS The hydrology report is based on the County Hydrology Manual (2003 Edition), Rational Method (Q=CIA), and Handbook of Hydraulics (7th Edition) by Brater & King. Hydraulic calculations (as shown on Page 6) have been performed to confirm capacities of the proposed storm drain system and proposed grades. This project will not significantly increase runoff from the project site.. The proposed storm water system and conveyances will provide an improved Water Quality environment. Calculations listing the 1 00-year flow, time of concentration, slope, and site watershed areas for all proposed storm water conveyances are presented in the tables at the back of this report. SUMMARY Based on the information and calculations contained in this report, the proposed storm drain system will function to adequately intercept, contain and convey Qioo to appropriate points of discharge. The developed condition substantially conforms to the existing condition. Hydrologic calculations in this report show that the residential development project for the Hemlock Projeët does not substantially increase run-off to the existing Hemlock Avenue. The chosen drainage mitigation measures will satisfactorily handle storm water runoff. This report has been based on the Site Development project located at Hemlock Avenue (MS 05-19/SDP 05-09/CP 05-10/CDP 05-25, CE 06-01). 5 PROPOSED HYDROLOGY CALCULATIONS Project: Hemlock Project Precipitation (P6) = 2.5 Location: Hemlock Avenue, Carlsbad Soil Classification: "D" Date: September 8, 2004 Storm Frequency: 100 years Remarks Node 1 ->2 Area (Al) = 0.03 Acres Driveway runoff to slotted drain and sump pump Travel Distance (L) = 65 Feet Slope (5) = 14.0% Time of Concentration (Tc) = 1.51 Minutes Intensity (1) = 14.25 Inches/Hour Runoff Coefficient (C) = 0.85 Designed Runoff (Q) = 0.36 C.F.S. Pumped to Node 3-> Node 4 and outlet to Hemlock Ave. via sidewalk underdrain Node 3 ->4 Area (A2) = 0.04 Acres Roof runoff and front patio area Travel Distance (L) = 62 Feet Slope (5) = 1.0% Time of Concentration (Tc) = 3.54 Minutes Intensity (1) = 8.23 Inches/Hour Runoff Coefficient (C) = 0.85 Urban Area Watershed Designed Runoff (Q) = 0.28 C.F.S. Runoff to Hemlock Ave. at Node 4 via 6" PVC storm drain. Node 5 ->6 ->3 Area (A3) = .06 Acres Runoff through landscaping at rear of property, roof runoff, some paving/patio areas Travel Distance (L) = 75 Feet Slope (S)= 1.0% Time of Concentration (Tc) = 3.90 Minutes Intensity (I) = 7.74 Inches/Hour Runoff Coefficient (C) = 0.85 Urban Area Watershed Designed Runoff (Q) = 0.39 C.F.S. Runoff to Hemlock Ave. via sidewalk underdrain Concentration @ Node 4 Total Runoff at Node 4 (Qt) = 1.03 C.F.S. Outlet to Hemlock Ave, via proposed sidewalk underdrain, 2-3" outlet pipes. SPECIAL NOTE: All Catch Basins to be sized 12"x12"x12" minimum. At that size, catch basin capacity is 1.77cfs. 100-year storm event has been calculated to produce a maximum of 1.03 cfs with a maximum of 0.75cfs at any one catch basin at any one time. HEMLOCK PROJECT THOMAS BROTHERS PAGE 1106 VICINITY MAP NOT TO SCALE ATTACHMENT A ALTA CONSULTANTS PLANNING ENGINEERING SURVEYING 4901 Morons Blvd. St.. 409 San Diego, CA 92117 (858) 581-8101 Paz (858) 581-8138 56.50 1W 53.00 TR' 40.50 BR ' Dt\/ACV '-' D WALL .L. SURROUNDiNG PRP€ L_z' ,.j ' cj .;LL APPROX. EX1511N0 THREE STORY APARTMENT BULDNG APPROX, FF45.50 HEUiI L OCI( MINOR SUB D I VIL Iu'w'z N WYO L nGY lu'Aff \\ ..L JT--)P _ YEA R S TORM RV ENT \<AO \\\\ \ \56.5 / EX. 6' SDcE \7 g \ - r LA5cJVL 4UD BRW ) WI)Ofl FENCE - - \ CLEANOUT : • y ricpin (TO BE REMOVED) 7 W/CON CRETE I (To p TG=50.50 E=49. 75 V i.1r rrAl tP IMTERSHED AREA DESIGNATION \ \ \ \ 6 PRIVACY WALL N. \ SURROUNDING PROP'ERTY ••. \ V Ole END 6' WALL \..V >1 E .J..J WALL H /Tdo tw F / 50.50 TRW / 40-5O BRW I CATCH BASIN ( /s / Ei95 /7 VEX. 1 RAISED F ~ NxI,% $NTEP P STORM DRAIN F AA I AtJilIJIL#11 pQ=Q. 68 DESIGNED RUNOFF i IIrCTC cj rlIA! r'IDcfT!c\AI i\fl 'iJI I I i...tJVV L..L V v y END 6' WALL BEGIN 3.5 FENCE* 5-4.50 W 49.00 TRW 40.50 BRW - X. 6 SDG&E EASEMENT V 52.50 1W V EX. GRAVEL •:.:: 49.00 TR 4050 BR 10. oui .. PACOELL RSERTO BE RELOCATED \\ CURD OUTLET /7 V ELEC BOX \ V - 'WATFR METrO 2r \ £ & LAERAL - ALL 3.5w FENcEfWAçL1THN FRONT YARD SETB*K TO BE 3 SIDE WA HiGH S8iUN. MATERALS (PLEXiGLAS$*R0UGHT IRON JJLET i=3 OR StMII) /7 POLE 0 NEER METER 0 !U bt LPL/\UIU ,(' -y\tJ<At \V ) V — \ \ - V L 1 n u V 7 \ "no CURB OUTLET SEWER MO VV \ \ v TER Al GH - Dr D r\ k1 4 U SCALE 10 20 40 .J V 7 F (IN FEET I 1 inch = 10 ft. \ V - F V V V UW —$ POLE V N V I AC V HEMLOCK 257-08 Undisturbed Natural Terrain (Natural) Low Density Residential (LDR) Low Density Residential (LDR.) Low Density Residential (l,DR) Medium Density Residential (MDR) Medium Density Residential (MDR) Medium Density Residential (MDR) Medium Density Residential (MDR) High Density Residential (HDR) High Density Residential (HDR) Commercial/Industrial (N. Corn) Commercial/Industrial (0. Corn) Commercial/Industrial (OP. Corn) Commercial/Industrial (Limited I.) Permanent Open Space Residential, 1.0 DU/A or less Residential, 2.0 DU/A or less Residential, 2.9 DU/A or less Residential. 4.3 DU/A or Less Residential, 7.3 DU/A or less Residential. 10.9 DU/A or less Residential, 14.5 DU/A or less Residential, 24.0 DWA or less Residential, 43.0 DU/A or Less Neighborhood Commercial General Commercial Office Professional/Commercial Limited Industrial San Diego County Hydrology Manual Date: June 2003 Section: 3 Page: 6of26 Table 3-1 RUNOFF COEFFICIENTS FOR URBAN AREAS 4PER. A Soil Type B C D 0 0.20 0.25 0.30 0.35 10 0.27 0.32 0.36 0.41 20 0.34 0.38 0.42 0.46 25 0.38 0.41 0.45 0.49 30 0.41 0.45 0.48 0.52 40 0.48 0.51 0.54 0.57 45 0.52 0.54 0.57 0.60 50 0.55 0.58 0.60 0.63 65 0.66 0.67 0.69 0.71 80 0.76 0.77 0.78 0.79 80 0.76 0.77 0.78 0.79 85 0.80 0.80 0.81 0.82 90 0.83 0.84 0.84 0.85 90 0.83 0.84 0.84 0.85 *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 perilous runoff coefficient, Cp, for the soil type), or fir 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 Axhmenl C. 3-6 HYDROLOGY & HYDRAULIC REFERENCE FORMULA TIME OF CONCENTRATION: Natural Watershed - County Dwg.: Appendex X-A ,11.9L 355 Where: Tc Minutes + Initial 10 Minutes - H / L = Miles of Watershed Along Natural Channel H = Difference in Elevation along effective slope Line Tc = 1.8(1.1 -C) j Urban Area Overland Flow Fig. 3-5 Where: Tc = Minutes C = Runoff Coefficient L Overland Travel Distance S = Effective Slope (%) INTENSITY (InsJhr.): 1 - 744P D °'4 - 6 Intensity-Duraton-Frequency - County Design and .Procedure Manual (Appendix XI-A) Where: I = Intensity (insJhr.) P6=6 hours Precipitaton in (Ins.) D Duration in minutes MA1'NING'S FOR PIPE: (Designed Runoff) Q fl Basic Formula.: For Capacity Calculation, S should be Sf- for Pipe Flowing Full So - for Pipe Partially Full Q Handbooks for Hydraulics by Baxter & Kings ________ 0.0737 (e,-sine/ ) 17 0,14 d S Pipe on channel flow o = 2arccos (I - —;-) a Q =9. 739DrHe h6ohme4 b