Loading...
HomeMy WebLinkAboutFile Documents.430 W Hallam St.0291.2017 (79).ARBK1 Drainage Report 430 WEST HALLAM ASPEN, CO September 5, 2018 Prepared by Richard Goulding, P.E. Roaring Fork Engineering 592 Highway 82 Carbondale, CO 81623 2 Drainage Report 430 WEST HALLAM ASPEN, CO I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 430 WEST HALLAM, ASPEN, CO WAS PREPARED BY ME FOR THE OWNERS THEREOF IN ACCORDANCE WITH THE PROVISIONS OF THE CITY OF ASPEN URBAN RUNOFF MANAGEMENT PLAN AND APPROVED VARIANCES AND EXCEPTIONS LISTED THERETO. I UNDERSTAND THAT IT IS THE POLICY OF THE CITY OF ASPEN THAT THE CITY OF ASPEN DOES NOT AND WILL NOT ASSUME LIABILITY FOR DRAINAGE FACILITIES DESIGNED BY OTHERS. RICHARD GOULDING, P.E. RFE Project # 2017-23 3 Table of Contents 1.0 General .................................................................................................................................................... 4 1.1 Existing Site ......................................................................................................................................... 4 1.2 Proposed Conditions ........................................................................................................................... 4 1.3 Previous Drainage Studies .................................................................................................................. 4 1.4 Offsite Drainage & Constraints ........................................................................................................... 4 2.0 Drainage Basins and Sub-basins .............................................................................................................. 5 2.1 Drainage Basins ................................................................................................................................... 5 2.2 Peak Discharge Calculations................................................................................................................ 5 3.0 Low Impact Site Design ........................................................................................................................... 6 3.1 Principles ............................................................................................................................................. 6 4.0 Hydrological Criteria ............................................................................................................................... 7 4.1 Storm Recurrence and Rainfall ........................................................................................................... 7 4.2 Peak Runoff Methodology .................................................................................................................. 7 5.0 Hydraulic Criteria .................................................................................................................................... 8 5.1 Inlets .................................................................................................................................................... 8 5.2 Pipes .................................................................................................................................................... 9 6.0 Proposed Facilities ................................................................................................................................ 11 6.1 Drywell .............................................................................................................................................. 11 7.0 Operation and Maintenance ................................................................................................................. 12 7.1 Drywell .............................................................................................................................................. 12 8.0 Appendices ............................................................................................................................................ 12 Drawings 11x17 ....................................................................................................................................... 12 4 1.0 General 1.1 Existing Site The following report is an evaluation of a residential site located to the north of West Hallam Street between North 3rd and North 4th Street in Aspen, Colorado. The property is addressed at 430 West Hallam Street, and is located between two residential homes. To the north is the Block 35 Alley Right-of-Way, and to the south is West Hallam Right-of-Way. The lot consists of 6,439 square feet and consists of an existing 2,840 square footprint two-story structure with a gentle slope down to the northeast. Large mature trees are scattered along the eastern and southern side of the existing structure. No sidewalk is installed in front of the residence, but curb and gutter is in place. All utilities are located in the alley, excluding the water line, which is located in West Hallam Street. At the time of this report, no sanitary sewer line has been located. A Geotechnical Report was produced on March 31st, 2017. 1.2 Proposed Conditions This project is classified as a ‘Major Project’ as per Table 1.1 of the URMP. This is because the proposed development is over 1,000 square feet (sf) and disturbs an area of approximately 6,400 sf. This has implications for the design. The intent of this report is to demonstrate compliance with the requirements of the URMP. The Low Impact Design (LID) Principles in the introduction of the manual were used as a guide throughout the design process. Onsite storm infrastructure has been sized for conveyance and full detention storage of a 100-year event. The proposed residence is 6,210 square feet (sf) with a 3,305 square foot footprint. Extensive landscaping will extend to the property line surrounding the residence. An above patio will be located on the south side of the structure. A two-car garage will be accessed from the alley with a concrete strip. Cuts of up to 12’ deep are expected for the sub-level excavation. For drainage, a screened rock bed will capture all runoff from roofs and hardscape and is sized to have capacity for full detention. Utility connections will be made to the existing infrastructure. 1.3 Previous Drainage Studies The City of Aspen updated their URMP in 2001 and the property is within the boundaries of the study. The study indicates that the property is not within a Mudflow Area. 1.4 Offsite Drainage & Constraints No offsite basins effect the site, so no analysis was required. 5 2.0 Drainage Basins and Sub-basins The site was divided into one major drainage basin, which was then subdivided into smaller sub-basins. A Drainage Exhibit in the appendices illustrates the basin and sub-basin delineations. It lists Impervious Areas, Runoff Coefficients, and Peak Flows. The sub-basins were created to calculate the concentrated flow from each impervious area, including patios, decks and roofs. These sub-basin peak flows were then used to size the proposed infrastructure. 2.1 Drainage Basins Basin 1 is 6,439 square feet (sf), 69% impervious, and consists of roof drains, inlets, trench drains, and the landscaping surrounding the residence. Runoff from this basin is collected and conveyed to the drywell. This drywell has capacity for full detention for the entire basin. 2.2 Peak Discharge Calculations The peak flows were calculated for each Major Basin for the 5 and 100-year storm events. Rainfall intensity was calculated using a Time of Concentration (Td) of 5 minutes. Actual time of concentration on the site is significantly less than 5 minutes, but according to the City of Aspen URMP, equations used to calculate rainfall intensity are only valid for a Time of Concentration of greater than 5 minutes. So, the smallest valid Time of Concentration value was used. The 1-hour Rainfall depth (P1), given in Table 2.2 as 0.64 inches for the 5-year event and 1.23 inches for the 100-year event. Equation 2.1 was referenced when solving for the Rainfall Intensity. I = 88.8P1/(10+Td )1.052 Runoff Coefficients (C), a function of the Soil Group (in this case B) and the percentage of impervious area within each basin were developed using Figure 3.2. The Runoff Coefficient (C) was then multiplied by the Rainfall Intensity (I) and the acreage of each Major Basin (A) to determine the peak discharge for each Major Basin. Q allowable was calculated the same way except each basin was treated as undeveloped or 100% pervious. The Peak Discharge (Qp) is given by equation 3.1. Qp= CIA Qp= Peak Discharge (cfs) C= Runoff Coefficient (Unitless) I= Rainfall intensity (inches per hour) A= Area (Acres) Peak flow values were used to calculate the size of the proposed detention and conveyance structures, such as drywells, inlets and piping. The tables below contain the peak flows for developed and undeveloped conditions for 5 and 100-year storm events. 6 5 Year Peak Discharge Developed Calculations 1 Hour(P1)0.64 Return Period 5 Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max See(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) 1 6439.00 4467.00 69.37%0.450 5 3.29 0.22 5 Year Peak Discharge Pre Development Calculations 1 Hour(P1)0.64 Return Period 5 Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max See(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) 1 6439.00 3838.00 59.61%0.370 5 3.29 0.18 3.0 Low Impact Site Design Low Impact Development (LID) aims to mimic the natural pre-development hydrologic pattern. The goal is to manage storm water as close to its source as is possible. This entire developed site is approximately 69% impervious. The treatment train approach is used on all runoff to increase water quality and percolation. 3.1 Principles Principle 1: Consider storm water quality needs early in the design process. The Grading and Drainage design was coordinated with the architect during the design phase. Due to the lack of space for the project, coordination in the design process was key. Principle 2: Use the entire site when planning for storm water quality treatment. Because of the size and limitations of the parcel, it was necessary in the design process to use the site efficiently. 100 Year Peak Discharge Developed Calculations 1 Hour(P1)1.23 Return Period 100 Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max See(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) 1 6439.00 4467.00 69.37%0.590 5 6.33 0.55 100 Year Peak Discharge Pre Development Calculations 1 Hour(P1)1.23 Return Period 100 Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max See(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) 1 6439.00 3838.00 59.61%0.540 5 6.33 0.50 7 Principle 3: Reduce runoff rates and volumes to more closely match natural conditions. The runoff will all be infiltrated into the ground, as the screened rock bed is sized for full detention. There will be no runoff leaving the site. Principle 4: Integrate storm water quality management and flood control. The Drywells are being used for water quality, which in itself increases flood control. The screened rock bed will eliminate the peak flow as there is no runoff leaving the site. Principle 5: Develop storm water quality facilities that enhance the site, the community and the environment. The proposed design encourages replenishing groundwater, and does not introduce any runoff into the city infrastructure. This reduces the flows being introduced to the Roaring Fork River. Principle 6: Design sustainable facilities that can be safely maintained. Screens will be placed over downspouts to provide a barrier against vermin and debris. Drainage systems were simply designed so maintenance is minimized. Infrastructure will be just below grade providing little labor for maintenance. The Owner will sign a maintenance agreement as part of their Certificate of Occupancy. Principle 7: Design and maintain facilities with public safety in mind. Proper drainage and grading of the driveway and walkways reduces ice buildup and dangerous icy conditions. All grading was done with safety in mind. 4.0 Hydrological Criteria 4.1 Storm Recurrence and Rainfall The property is not in the commercial core and is served by city curb and gutter so this property classifies as a “Sub-urban area served by public storm sewer”. However, due to limitations on the site, the curb and gutter cannot be utilized, so the site cannot disperse into the curb and gutter. Due to this, the 5 and 100-year events were analyzed. 4.2 Peak Runoff Methodology This site could drain to city storm infrastructure, however due to site limitations and the inability to disperse into the curb and gutter, full detention is necessary. To determine these capacities, the rainfall from a 100-year storm that is collected on all impervious areas must be detained. No detention is required for pervious areas. Below is a summary of the required storage. 8 5.0 Hydraulic Criteria Sub-basins were delineated per the design points of concentrations created by roof drains and inlets. Pipe networks were then created connecting the sub-basins and conveying the flows to the overall point of concentration for the basin. The 100-year peak flow for each sub-basin was calculated. 5.1 Inlets The 100-year peak flows were used in the sizing of inlets. Equations 4-17 to 4-20 from the URMP were used in the analysis. They incorporate a 50% clogging factor and 40% opening in the grates. A water depth of 0.04’ was assumed and all the inlets were treated as sumps as they will be set a minimum of .04‘(½ Inch) below the flow lines. Below is a summary of each square inlet being tested for capacity against their tributary basin, and below that is every circular inlet calculation. Full Detention Storage Basin Total Area Impervious Area Impervious Full Detention Depth Factor of Safety Required Storage BMP (ft2)(ft2)(%)(in)F.O.S.(ft3) 1 6439.00 4467.00 69.37%1.23 1 458 SCREENED ROCK BED 100 Year Sub Basin Peak Discharge Developed Calculations 1 Hour(P1)1.23 Return Period 100 Sub Basin Total Area Imp. Area Impervious C Value Time of C Intensity Sub Basin Flow Rate (Name)At (ft2)Ai (ft2)Ai/At (%)From Table (Td)I=88.8P1/(10+Td)01.052 Qsub (ft3/sec) 1.1 106.00 106.00 100.00%0.950 5 6.33 0.01 1.2 95.00 95.00 100.00%0.950 5 6.33 0.01 1.3 238.00 238.00 100.00%0.950 5 6.33 0.03 1.4 407.00 407.00 100.00%0.950 5 6.33 0.06 1.5 95.00 95.00 100.00%0.950 5 6.33 0.01 1.6 552.00 552.00 100.00%0.950 5 6.33 0.08 1.7 334.00 334.00 100.00%0.950 5 6.33 0.05 1.8 413.00 413.00 100.00%0.950 5 6.33 0.06 1.9 168.00 168.00 100.00%0.950 5 6.33 0.02 1.10 745.00 745.00 100.00%0.950 5 6.33 0.10 1.11 356.00 356.00 100.00%0.950 5 6.33 0.05 1.12 178.00 178.00 100.00%0.950 5 6.33 0.02 1.13 606.00 606.00 100.00%0.950 5 6.33 0.08 1.14 172.00 172.00 100.00%0.950 5 6.33 0.02 Sub Basin and Rectangular Inlet Calculations 1 Hour(P1)1.23 m=40%Ys=.04 (Depress inlet by 0.04') Return Period 100 Cg=50%Co=0.65 Inlet ID Basin ID Total Area Imp. Area Impervious C Value Time of Concentration Intensity Q Max Inlet Type Inlet Width Inlet Length Effective Open Area (EQ. 4-20)Inlet Capacity (EQ 4-19)Has Capacity See(D1)(ft2)(ft2)(%)(From Table) (Td)I=88.8P1/(10+Td)1.052 (ft3/sec)Rectangular Wo (inches)Lo (inches)Ae=(1-Cg)mWoLo Q=CoAe√2gYs (Yes/No) TRENCH DRAIN-B6 1.1 106.00 106.00 100.00%0.950 5 6.33 0.015 4" x 25'4 25 0.139 0.139 Yes Sub Basin and Circular Inlet Calculations 1 Hour(P1)1.23 m=40%Ys=.04 (Depress inlet by 0.04')Return Period 100 Cg=50%Co=0.65 Inlet ID Basin ID Total Area Imp. Area Impervious C Value Concentration Intensity Q Max Inlet Type Diameter Area(EQ. 4-20)Inlet Capacity (EQ 4-19)Has CapacitySee(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 ft3/sec Wo (inches)Ae=(1-Cg)mA Q=CoAe√2gYs (Yes/No) INLET-A3.2 1.8 413.00 413.00 100.00%0.950 5 6.33 0.057 8" Round 8 0.070 0.081 Yes 9 5.2 Pipes The pipes were analyzed by calculating the flow from the sub basins entering them. Below is table which groups what sub basins are conveyed in each pipe. The TOC is below 5 minutes for all sub-basins, so a reduction was not taken for the intensity. They were tested for hydraulic capacity at 80% of pipe diameter. Depth of flow was also calculated in the spread sheets below. The pipes are all SDR 35 PVC with a manning’s coefficient of .01. Design Q design / Q full charts were downloaded from FHWA. The equations in Section 4.8.4 was used as the basis for these calculations. Storm System Pipes Pipe System Pipe Contibuting Sub-Basins Design Flow Rate Qdes A A1-A0 1.2, 1.5,1.6,1.8-1.10, 1.12-1.14 0.42 A2-A1 1.5,1.6,1.8-1.10, 1.12-1.14 0.40 A3-A2 1.6, 1.8-1.10, 1.12-1.14 0.39 A3.1-A3 1.6,1.8,1.10 0.24 A3.2-A3.1 1.8, 1.10 0.16 A3.3-A3.2 1.10 0.10 A4-A3 1.9, 1.12-1.14 0.16 A5-A4 1.12-1.14 0.13 A6-A5 1.13, 1.14 0.11 A7-A6 1.13, 1.14 0.11 A8-A7 1.14 0.02 B B1-B0 1.1, 1.4, 1.7, 1.11 0.17 B2-B1 1.7, 1.11 0.10 B3-B2 1.7, 1.11 0.10 B4-B3 1.11 0.05 B5-B0 1.3 0.03 B6-B0 1.1 0.01 A0-B0 1.1-1.14 0.62 10 K=0.462 Pipe Design Flow Rate Proposed Slope Manning Coefficient Required Pipe Diameter Equation 4-31 Required Pipe Diameter Proposed Pipe Diameter Qdes (ft3/sec) S (%)n d (ft) = {nQdes/K√S}3/8 Dreq (in) Dpro (in) A1-A0 0.42 9.00%0.01 0.27 3.23 6.0 A2-A1 0.40 1.00%0.01 0.40 4.81 6.0 A3-A2 0.39 1.00%0.01 0.40 4.75 6.0 A3.1-A3 0.24 1.00%0.01 0.33 3.93 6.0 A3.2-A3.1 0.16 1.00%0.01 0.28 3.40 4.0 A3.3-A3.2 0.10 1.00%0.01 0.24 2.88 4.0 A4-A3 0.16 1.00%0.01 0.28 3.36 4.0 A5-A4 0.13 1.00%0.01 0.26 3.16 4.0 A6-A5 0.11 1.00%0.01 0.24 2.93 4.0 A7-A6 0.11 1.00%0.01 0.24 2.93 4.0 A8-A7 0.02 1.00%0.01 0.14 1.66 4.0 B1-B0 0.17 1.00%0.01 0.29 3.45 4.0 B2-B1 0.10 1.00%0.01 0.23 2.80 4.0 B3-B2 0.10 10.00%0.01 0.15 1.82 4.0 B4-B3 0.05 2.50%0.01 0.15 1.84 4.0 B5-B0 0.03 1.00%0.01 0.16 1.88 4.0 B6-B0 0.01 1.00%0.01 0.12 1.39 4.0 Pipe Sizing Pipe Design Flow Rate Proposed Pipe Diameter Slope 80% of Proposed Pipe Diameter Manning Coefficient Full Pipe Cross Sectional Area Full Pipe Flow Rate Q Design / Q Full d/D Hydraulic Grade Line(Depth of Flow)Depth of Flow Less Than 80% of Pipe Diameter Qdes (ft3/sec) Dpro(in)S (%)Dpro*.8 (in)n A (ft) = π (Dpro/2)2 Qfull (ft3/s) = A(1.49/n)((Dpro/48)2/3)S1/2 Qdes/Qfull (from Chart)d (in) = (d/D)*Dpro (Yes/No) A1-A0 0.42 6.0 9.00%4.8 0.01 0.196 2.193 0.19 0.34 2.01 Yes A2-A1 0.40 6.0 1.00%4.8 0.01 0.196 0.731 0.55 0.60 3.60 Yes A3-A2 0.39 6.0 1.00%4.8 0.01 0.196 0.731 0.53 0.59 3.51 Yes A3.1-A3 0.24 6.0 1.00%4.8 0.01 0.196 0.731 0.32 0.43 2.58 Yes A3.2-A3.1 0.16 4.0 1.00%3.2 0.01 0.087 0.248 0.64 0.65 2.58 Yes A3.3-A3.2 0.10 4.0 1.00%3.2 0.01 0.087 0.248 0.41 0.50 2.00 Yes A4-A3 0.16 4.0 1.00%3.2 0.01 0.087 0.248 0.63 0.65 2.58 Yes A5-A4 0.13 4.0 1.00%3.2 0.01 0.087 0.248 0.53 0.59 2.34 Yes A6-A5 0.11 4.0 1.00%3.2 0.01 0.087 0.248 0.43 0.52 2.06 Yes A7-A6 0.11 4.0 1.00%3.2 0.01 0.087 0.248 0.43 0.52 2.06 Yes A8-A7 0.02 4.0 1.00%3.2 0.01 0.087 0.248 0.10 0.24 0.94 YesB1-B0 0.17 4.0 1.00%3.2 0.01 0.087 0.248 0.67 0.66 2.64 Yes B2-B1 0.10 4.0 1.00%3.2 0.01 0.087 0.248 0.38 0.49 1.94 Yes B3-B2 0.10 4.0 10.00%3.2 0.01 0.087 0.784 0.12 0.26 1.05 YesB4-B3 0.05 4.0 2.50%3.2 0.01 0.087 0.392 0.13 0.28 1.10 Yes B5-B0 0.03 4.0 1.00%3.2 0.01 0.087 0.248 0.13 0.28 1.10 Yes B6-B0 0.01 4.0 1.00%3.2 0.01 0.087 0.248 0.06 0.18 0.70 Yes Hydraulic Grade Line and Pipe Capacity 11 6.0 Proposed Facilities 6.1 Screened Rock Bed The proposed screened rock bed collects all runoff from the site and is designed to have capacity for full detention. The screened rock bed is 10’ wide 34’ long with a minimum depth of 4.75’, the capacity of the screened rock bed is 460 cf. Infiltration times for the screened rock bed can be found in the tables below. Infiltration rates were found in the geotechnical report. Pipe Design Flow Rate Proposed Pipe Diameter Slope d/D Manning Coefficient Rh/D Hydraulic Radius Exit Velocity (ID)Qdes (ft3/sec) Dpro(in)(%)(from Chart)n (from Chart)Rh (ft) = (Rh/D) Dpro V (ft/sec) = [1.49/n] Rh2/3 √S A1-A0 0.417 6.0 9.00%0.34 0.01 0.18 0.18 14.50 A2-A1 0.404 6.0 1.00%0.60 0.01 0.28 0.28 6.34 A3-A2 0.391 6.0 1.00%0.59 0.01 0.27 0.27 6.27 A3.1-A3 0.236 6.0 1.00%0.43 0.01 0.23 0.23 5.52 A3.2-A3.1 0.160 4.0 1.00%0.65 0.01 0.29 0.29 6.47 A3.3-A3.2 0.103 4.0 1.00%0.50 0.01 0.25 0.25 5.91 A4-A3 0.155 4.0 1.00%0.65 0.01 0.29 0.29 6.47 A5-A4 0.132 4.0 1.00%0.59 0.01 0.27 0.27 6.27 A6-A5 0.107 4.0 1.00%0.52 0.01 0.25 0.25 5.96 A7-A6 0.107 4.0 1.00%0.52 0.01 0.25 0.25 5.96 A8-A7 0.024 4.0 1.00%0.24 0.01 0.14 0.14 3.95 B1-B0 0.166 4.0 1.00%0.66 0.01 0.29 0.29 6.53 B2-B1 0.095 4.0 1.00%0.49 0.01 0.24 0.24 5.81 B3-B2 0.095 4.0 10.00%0.26 0.01 0.15 0.15 13.40 B4-B3 0.049 4.0 2.50%0.28 0.01 0.16 0.16 6.84 B5-B0 0.033 4.0 1.00%0.28 0.01 0.16 0.16 4.33 B6-B0 0.015 4.0 1.00%0.18 0.01 0.10 0.10 3.30 Exit Velocities Screened Rock Bed Storage Storage System Basins Area Depth Void Ratio Total Capacity Required Capacity (Name)(#)(sf)(ft)(ft3)(ft3) Screened Rock Bed 1 323 4.75 0.3 460.28 458 Full Detention Infiltration BMP Max Volume Infiltration Area Infiltration Rate Time To Drain Volume Infiltrated in 24 Hours (name)V (ft3)A (ft2)I (in/hr)(hr)Vtotal (ft3) = V*24/T Screened Rock Bed 460.00 195 30 0.94 11700.00 12 7.0 Operation and Maintenance 7.1 Screened Rock Bed The screened rock bed beneath and piping must be maintained periodically and inspected to ensure proper operation. A maintenance plan shall be submitted to the City describing the maintenance schedule that will be undertaken by the owners of the new residence or building. Minimum inspection and maintenance requirements include the following: - During the first year draw down should be checked for every event over 0.25” of precipitation to ensure no significant backups are occurring. - The maximum drain down time is 24 hours. - The fabric surrounding the bed will be the first component to clog. If the fabric becomes clogged replacement will be needed. - Piping systems and sumps should be checked during and after storms routinely. - Clean out and drainage basins and provide access for hoses and vacuum equipment. - After the first year the system should be cleaned out at least once a year and more if the first-year inspections prove more maintenance is required. - More frequent cleaning reduces the amount of debris entering the system and reduces the need for more intense maintenance. - Remove debris from the gravel bed routinely. If the gravel has been contaminated by soil and sand, clean or replace gravel as necessary. Gravel will possibly have to be replaced every 5-10 years for proper perforation into the lawn. - Clean the inside of the perforated pipe with a 6” or 12” pipe cleaner accessed through cleanouts. This should be done yearly, or as necessary if the system is not infiltrating properly or if the system has become contaminated. - Ensure heat tape is functioning before colder months to prevent damage to piping. - If the storm system is not maintained properly, replacement of parts or of the entire system may be necessary. 8.0 Appendices Drawings 11x17