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Home My WebLink AboutPD 2018-0027; Gee Property; Preliminary Drainage Study; 2020-04-04 PRELIMINARY DRAINAGE STUDY GEE RESIDENCE 3700 Alder Avenue Carlsbad, California GRADING PLAN 520-3A GR2019-0035 Dated 4-28-2020 Prepared By: Erin Rist RCE 65122 Prepared For: Chuck Gee 3700 Alder Avenue Carlsbad, California SITE DESCRIPTION The subject project is located at 3700 Alder Avenue, City of Carlsbad, California. The property fronts Alder Avenue to the front and El Camino to the rear (east), with similarly developed single family residences to the north and south. As we understand the scope of the current improvements includes as part of this plan includes the new driveway to the rear garage. The site is currently developed with a single-family residence and detached garage. The topography of site area generally consists of foothill terrain. The total acreage of the property is approximately 2.29 acres, the area of development consists of approximately 0.41 ac and the area disturbed, as covered by these plans is approximately 0.52. The overall topography difference across the site is approximately 105 feet. The overall topography difference in the area of development is approximately 20 feet. EXISTING DRAINAGE PATTERNS Alder Avenue is generally high compared with the property and therefore the property does not drain to the street. In general, the upper portion of the project is developed, draining toward the rear slope that extends down to El Camino Real. This preliminary drainage study was prepared following a code violation associated with the development of additional garage area. We understand the additional garage buildings depicted on this grading plan were approved and permitted as part of proposed building development, however additional site improvements, including driveway extensions were not. Prior to the violation the site was developed with existing single family residence, detached garage, paved driveway and landscape improvements. As part of the 2014 grading permit, site runoff for these improvements, was collected in private storm drain system and routed to three separate rip rap energy dissipators at that discharged runoff from the upper pad area to the slope that descends approximately 90+ feet to El Camino Real. PROJECT SCOPE This hydrology study analyzes the additional runoff associated with both previously permitted (residence and detached garages) as well as more recent unpermitted (driveway) site development, which includes two detached garages, as well as unpermitted improvements, including, impervious driveway area and site landscape retaining walls. BASIN DESCRIPTION AND PROPOSED FLOW PATTERNS For this drainage analysis, the subject site has been divided into two main basins, DMAs 1A & 1B, as part of post development flow analysis. Basins include proposed new development as well as existing improvements. New drainage infrastructure is designed in consideration of proposed and existing. Runoff from these areas are collected with a new private storm drain system and routed to detention for containment of the increase in flow during peak storm events as a result of all improvements. Two bioretention areas are proposed at basin outfalls for containment of increased flows. One of the bioretention basin outlets runoff to be dissipated along an rip rap. The second bioretention area will be tied to existing storm drain improvements in drainage easement north of the property. ANALYSIS & METHODS The hydrologic analysis utilizes the Rational Method of the County of San Diego Hydrology Manuals to calculate peak runoff rates (Q) and time of concentration (Tc). Attached are San Diego isopluvial maps for 100 year (Q100). The runoff coefficients for the basins are calculated in the attached tables based upon the percentage of impervious area. In consideration of the small tributaries’ calculations are run with a time of concentration of 5 minutes. Soil type D was utilized in the preliminary drainage design. CONCLUSIONS As a result of the installation of unpermitted impervious driveway and flatwork surfaces, the peak runoff from the site is calculated to increase by approximately 0.11 cfs and 0.62 cfs, respectively per basin. The attached spreadsheet includes calculations for onsite detention to contain peak flows volumes onsite. Pipe sizing at the bioretention overflows is calculated with flow control, utilizing pre-development Q100 to size pipes. Overflows will be tied to existing storm drain improvements , Basin 1A and rip rap energy dissipators, Basin 1B. DMA 1BDMA 1AHYDROLOGY PRE- DEVELOPMENTDRAINAGE SUMMARY TABLELEGENDDMA 1 ALDERAVENUELOT 8MAP NO. 2647DMA 1BDMA 1ALOT 9MAP NO. 2647HYDROLOGY POST DEVELOPMENTDRAINAGE SUMMARY TABLELEGENDDMA 1A GEE RESIDENCE2700 Alder Avenue, CarlsbadPRELIMINARY HYDROLOGY CALCULATIONSCp 0.35C(composite) C=0.90 x % impervious surfaces + Cp x (1‐%impervious)RATIONAL METHOD FOR TRIBUTARY < 0.5 SQ MILEQ = CIAI (in/hr) - 100 YR7.25= 7.44 P6 D^(-0.645)P62.8SOIL TYPE D85th Percentile Storm 0.65HYDROLOGIC & HYDRAULIC DESIGNBASIN NO. TOTAL AREA (sf) TOTAL AREA (ac)IMPERV AREA (sf)AREA IMPERV (ac)% IMPAREA PERV (sf)AREA PERV (ac)C T Q100 (cfs)V (ft3)=P6 100*A*CPRE‐DEVELOPMENT ONSITEDMA 1A 14,355 0.33 4,115 0.09 0.2910,240 0.24 0.515 1.21 1,670 DMA 1B 12,695 0.294,223 0.10 0.338,472 0.19 0.535 1.13 1,551 SUBTOTAL 27,050 0.628,338 18,712 Subtotal Q2.34 3,221 POST DEVELOPMENT ONSITEDMA 1A 14,355 0.33 5,275 0.12 0.379,080 0.21 0.555 1.32 1,816 delta Q0.11DMA 1B 12,695 0.2910,964 0.25 0.861,731 0.04 0.835 1.74 2,400 SUBTOTAL 27,050 0.6216,239 10,811 delta Q0.624,216 7,901 Subtotal Q3.06 DETENTION VOLUME (cf)Detention Area 1Volume (cf)Vpost-VpreRequired Provided DMA 1A146 225DMA 1B850 850 HYDROLOGY 100 4. 52.75 61 2.75 5 7.2 SITE 33d04’07.67” ; 117d05’38.30” SITE 50 4. 252.25 53 2.25 5 5.9 SITE SITE SITE SITE HYDRAULICS HYDRAULICSCHANNELSLabel Solve For Friction MethodRoughness CoefficientChannel Slope (ft/ft)Normal Depth (in)Left Side Slope (ft/ft (H:V))Right Side Slope (ft/ft (H:V))Discharge (ft³/s)Flow Area (ft²)Wetted Perimeter (ft)Hydraulic Radius (in)Top Width (ft)Critical Depth (ft)Critical Slope (ft/ft)Velocity (ft/s)Velocity Head (ft)Specific Energy (ft)Froude NumberFlow TypeConcrete Brow Ditch 1BNormal Depth Manning Formula 0.013 0.25 3.97 1.00 1.00 1.50 0.11 0.94 1.40 0.66 0.67 0.01 13.68 2.91 3.24 5.93 SupercriticalPIPESLabel Solve For Friction MethodRoughness CoefficientChannel Slope (ft/ft)Normal Depth (in)Diameter (in)Discharge (ft³/s)Flow Area (ft²)Wetted Perimeter (ft)Hydraulic Radius (in)Top Width (ft)Critical Depth (ft)Percent Full (%)Critical Slope (ft/ft)Velocity (ft/s)Velocity Head (ft)Specific Energy (ft)Froude NumberMaximum Discharge (ft³/s)Discharge Full (ft³/s)Slope Full (ft/ft)Flow TypeDMA 1APIPE to BMP 1A from SUB‐BASINNormal Depth Manning Formula 0.01 0.03 2.47 4.00 0.30 0.06 0.60 1.13 0.32 0.30 61.70 0.01 5.31 0.44 0.64 2.24 0.46 0.43 0.01 SuperCriticalDMA 1ADET BASIN TO BOX ‐ 6" ‐ LIMIT PRE‐DEV FLOWNormal Depth Manning Formula 0.01 0.03 4.78 6.00 1.21 0.17 1.10 1.82 0.40 0.49 79.70 0.02 7.21 0.81 1.21 1.97 1.34 1.24 0.03 SuperCriticalDMA 1BDET BASIN to RIPRAP ‐ RESTRICTED TO PREDEV Q ‐ 2 OUTLETSNormal Depth Manning Formula 0.01 0.02 3.12 6.00 0.55 0.10 0.80 1.54 0.50 0.38 51.90 0.01 5.34 0.44 0.70 2.07 1.11 1.03 0.01 SuperCritical