The URL can be used to link to this page

Home My WebLink AboutFile Documents.124 W Hallam St.0025.2017 (25).ARBK1 Drainage Report 124 W. HALLAM STREET ASPEN, CO March 8, 2017 Prepared by Richard Goulding, P.E. Roaring Fork Engineering 592 Highway 133 Carbondale, CO 81623 Received 4/12/2017 10:23:42 AM 2 Drainage Report 124 W. HALLAM ST. ASPEN, CO I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 124 W. HALLAM ST. WAS PREPARED BY ME FOR THE OWNERS THEREOF IN ACCORDANCE WITH THE PROVISIONS OF THE CITY OF ASPEN (COA) URBAN RUNOFF MANAGEMENT PLAN (URMP) 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 # 2016-42 Received 4/12/2017 10:23:42 AM 3 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 ...................................................................................................................................... 5 2.1 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 Storage Volumes Methodology .......................................................................................................... 8 5.0 Hydraulic Criteria ................................................................................................................................... 8 5.1 Piping .................................................................................................................................................. 8 5.2 Inlet Sizing ........................................................................................................................................ 10 6.0 Proposed Facilities ................................................................................................................................ 10 6.1 Proposed Structures .......................................................................................................................... 10 7.0 Operation and Maintenance .................................................................................................................. 11 7.1 Drywell ............................................................................................................................................. 11 8.0 Appendices ............................................................................................................................................ 11 Drawings 11x17 ...................................................................................................................................... 11 Received 4/12/2017 10:23:42 AM 4 1.0 General 1.1 Existing Site The site is located at 124 West Hallam Street in the West End of Aspen, Colorado. Currently, there are two lots that make up the existing property with an historic Victorian house spanning the two lots. 1st st. is to the west, with West Hallam Street to the south. To the north of the two lots is an unpaved alleyway. To the east are more lots that have been developed. The lot is home to many large trees and landscaping. The lot proposed for redevelopment is the eastern of the two lots. The topography of the lot slopes to the north where the alley meets the north lot line. HP Geotech performed a field exploration on September 11, 2015. A sub-surface soils report was produced on September 25, 2016. The soil profile consists of 12 to 18 inches of topsoil overlaying 2 to 4 feet of slightly gravelly sandy silty clay overlaying silty sandy gravel with cobbles and probable boulders. No free water was encountered at the time of drilling and the sub soils were slightly moist to moist. A final percolation rate of 3 inches per hour or 20 minutes per inch (mpi) was observed. 1.2 Proposed Conditions This project is classified as a ‘Major Project’ in Table 1.1 of the URMP, as the proposed development is over 1000 square feet and disturbs an area greater than 25% of the site footprint. The intent of this report is to demonstrate compliance with the requirements of the COA URMP. The Low Impact Design (LID) Principles in the introduction of the manual were used as a guide throughout the design process. The proposed development involves the relocation and renovation of the existing structure and the construction of a new addition and basement. Grading around the new structure will also take place to accommodate new walkways, lawn area and patios. No changes to land use or soil types are planned. Cut depths of up to 16 feet are expected and site grading cuts and fills are very minor compared to the foundation excavation. The runoff from impervious surfaces will be collected in a system of roof drains, trench drains and slot drains, which will convey runoff to a drywell at the South East corner of the property. The drywell will be sized for total detention. If the storm system were to be overloaded, the final receiving waters would be the Roaring Fork River. A foundation drain system will collect runoff that seeps into the ground from around the building foundation and pipe it to an internal drywell located under the mechanical room in the basement. 1.3 Previous Drainage Studies The property is not within a mudflow area as defined by the COA Storm Drainage Master Plan. The latest URMP does not show any infrastructure near the property. The curb and gutter is the primary means of conveyance for the CoA’s storm system in this area. 1.4 Offsite Drainage & Constraints There appears to be no offsite drainage flowing onto the property. The gravel alley to the north is uneven with ponding occurring in ruts and potholes. To the east, and west, pervious lawn areas and landscaping border the property. Hallam St. is lined with curb and gutter on the south side of the property. Received 4/12/2017 10:23:42 AM 5 2.0 Drainage Basins The entire site is routed to one overall point of concentration, the drywell, and the entire site was modeled as a single drainage basin to determine the total detention volume required. This basin was then divided into sub basins comprised of the roof area, driveways, walkways and patios. The sub basins were then used to calculate 100-year peak flow conditions for sizing individual inlets and piping. 2.1 Basins Basin 1 is comprised of all the impervious area on the lot. This includes roof area, the driveway on the alley the patios and walkways. This basin is 3,989 square feet (sf), and is 100% impervious. 2.2 Peak Discharge Calculations Peak flows were calculated for 10- 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 0.77 inches for the 10-year event and 1.23 inches for the 100-year event. Equation 2.1 was referenced when solving for the rainfall intensity (I). I = 88.8P1/(10+Td )1.052 Runoff coefficients (C), a function of the hydrologic soil group (in this case, B) and the percentage of impervious area within each sub basin were developed using Figure 3.2. The runoff coefficient was then multiplied by the rainfall intensity (I) and the acreage of each major basin (A) to determine the peak discharge for the Major Basin. Q allowable was calculated the same way, except the basin was treated as undeveloped, or 100% pervious. The Peak Discharge (Qp) in cubic feet per second (cfs) is given by equation 3.1. Qp= CIA Qp= Peak Discharge (cfs) A= Area (acres) I= Rainfall intensity (inches per hour) C= Runoff Coefficient (unitless) The tables below contains the peak flows for developed and undeveloped conditions for 10- and 100-year storm events. Received 4/12/2017 10:23:43 AM 6 3.0 Low Impact Site Design 3.1 Principles Principle 1: Consider storm water quality needs early in the design process. The grading and drainage design was coordinated with the architects early in the design. Comments from owners were considered and analyzed. Multiple site visits ensured proper understanding of existing conflicts. Principle 2: Use the entire site when planning for storm water quality treatment. The entire site was analyzed when determining the total detention volume. The drywell is sized for the required total detention volume. No runoff from an impervious surface will be discharge without being collected and routed to the drywell. Principle 3: Avoid unnecessary impervious area. 10 Year Peak Discharge Developed Calculations 1 Hour(P1)0.77 Return Period 10 Basin ID Total Area Imp. Area Impervious C Value Time of CIntensityQ Max See(D1) (ft 2)(ft2)(%)From Table (Td) I=88.8P1/(10+Td)1.052 (ft3/sec) 1 3989.00 3989.00 100.00% 0.916 5 3.96 0.33 10 Year Peak Discharge Pre Development Calculations 1 Hour(P1)0.77 Return Period 10 Basin ID Total Area Imp. Area Impervious C Value Time of CIntensityQ Max See(D1) (ft 2)(ft2)(%)From Table (Td) I=88.8P1/(10+Td)1.052 (ft3/sec) 1 3989.00 0.00 0.00% 0.150 5 3.96 0.05 100 Year Peak Discharge Developed Calculations 1 Hour(P1)1.23 Return Period 100 Basin ID Total Area Imp. Area Impervious C Value Time of CIntensityQ Max See(D1) (ft 2)(ft2)(%)From Table (Td) I=88.8P1/(10+Td)1.052 (ft3/sec) 1 3989.00 3989.00 100.00% 0.950 5 6.33 0.55 100 Year Peak Discharge Pre Development Calculations 1 Hour(P1)1.23 Return Period 10 Basin ID Total Area Imp. Area Impervious C Value Time of CIntensityQ Max See(D1) (ft 2)(ft2)(%)From Table (Td) I=88.8P1/(10+Td)1.052 (ft3/sec) 1 3989.00 0.00 0.00% 0.350 5 6.33 0.20 Received 4/12/2017 10:23:43 AM 7 Green roof will be utilized to reduce the impervious area on the structure. Gaps in the modular blocks of walkways are proposed to break up and minimized directly connected impervious area and allow for infiltration. Principle 4: Reduce runoff rates and volumes to more closely to match natural conditions. All runoff from impervious surfaces will be collected in a drywell, which has capacity for total detention. This will eliminate runoff leaving the site allowing maximum infiltration. Principle 5: Integrate storm water quality management and flood control. All inlets and piping are sized to accommodate the 100-year peak flows. Pipes were sized to ensure 100-year flows do not exceed 80% of the pipe capacity. The drywell is sized for total detention and should not overflow during the 100 year event. Principle 6: Develop storm water quality facilities that enhance the site, the community and the environment. An oil and sand separator is specified for the driving surface while all inlets have 6 inch sumps to help trap sediment. All runoff from impervious surfaces is routed to the drywell eliminating any stress on the surrounding areas. Principle 7: Use treatment train approach. A green roof has been implemented over the new addition to reduce runoff rates and help filter storm water before it is piped to the drywell. A two part drywell will be used to settle out contaminants before runoff is infiltrated. Sumps help collect unwanted debris that may create blockages. Principle 8: Design sustainable facilities that can be safely maintained. Inlets and piping will be vacuumed or flushed periodically to maintain adequate flow. Proper grading reduces dangerous slopes and proper drainage reduces ice buildup. All facilities are easily accessible and are installed with cleanouts when applicable. Principle 9: Design and maintain facilities with public safety in mind. The proposed storm system adds gutters and drains to roofs. This reduces ice buildup and the damages that follow. There are no drop-offs or steep grades proposed. 4.0 Hydrological Criteria 4.1 Storm Recurrence and Rainfall Due to the constrained site and grade limitations, the runoff from the site cannot discharge into the City of Aspen storm system. Therefore, total detention is required. This volume is obtained by multiplying 100 year rain fall depth by the amount of impervious area. Received 4/12/2017 10:23:43 AM 8 4.2 Storage Volumes Methodology The total detention volume calculation is shown below. 3989݂ݐଶ ∗1.23݅݊ 12 ݂ݐ ݅݊ ൌ 408.87݂ݐ ଷ 5.0 Hydraulic Criteria 5.1 Piping Pipes used in all drainage systems will be standard dimension ratio (SDR) 35 PVC with a Manning’s coefficient (n) of 0.01. The pipes were sized to accommodate peak flows for a 100 year event at 80% full. If the water level in the pipe exceeds the 80% full criteria, then the pipe is deemed inadequate. Sub- basins where delineated to isolate specific peak flows. Below is a table of sub-basin peak flows. Two separate systems were established to covey storm water down the east and west sides of the property to the drywell. Below is a table showing which sub basins contribute runoff to each pipe and the total flow (Q) accumulated during a 100-year peak flow event. Total Detention Storage Basin Total Area Impervious Area Impervious Full Detention Depth Required Storage BMP (ft2)(ft2) (%) (in) (ft 3) 1 3989.00 3989.00 100.00% 1.23 408.9 DRYWELL Sub‐Basin Peak Discharge ‐ Developed 1 Hour(P1)1.23 Return Period 100 Basin ID Total Area Imp. Area Impervious %C Value Time of CIntensityQ Max See(D1)ft2 ft2 Percentage From Table (Td) I=88.8P1/(10+Td)01.052 ft3/sec 1 214.83 214.83 100.00% 0.950 5 6.33 0.03 2 1220.89 1220.89 100.00% 0.950 5 6.33 0.17 3 661.00 661.00 100.00% 0.950 5 6.33 0.09 4 520.00 520.00 100.00% 0.950 5 6.33 0.07 5 447.00 447.00 100.00% 0.950 5 6.33 0.06 6 58.70 58.70 100.00% 0.950 5 6.33 0.01 7 142.30 142.30 100.00% 0.950 5 6.33 0.02 8 307.00 307.00 100.00% 0.950 5 6.33 0.04 9 267.00 267.00 100.00% 0.950 5 6.33 0.04 Received 4/12/2017 10:23:43 AM 9 Calculated pipe sizes were tested for hydraulic capacity at 80%. Values for depth of flow for each pipe were calculated. Design charts giving Q design / Q full were downloaded from Federal Highway Administration (FHWA) and the equations in Section 4.8.4 of the URMP were used as the basis for these calculations. Calculated pipe sizes and internal depth of flow within the pipes is shown below. Pipe System Pipe Contibuting Sub‐Basins Peak Flows (CFS) Eastern E1 1 0.03 E2 1, 2 0.20 E3 1, 2, 30.29 E4 1, 2, 3, 40.36 Western W1 6 0.01 W2 7 0.02 W3 6, 70.03 W4 6, 7, 80.07 W5 6, 7 ,8 ,9 0.11 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) E1 0.17 2.00% 0.01 0.25 3.04 4.0 E2 0.26 2.00% 0.01 0.30 3.58 8.0 E3 0.33 2.00% 0.01 0.33 3.92 8.0 E4 0.39 2.00% 0.01 0.35 4.18 8.0 W1 0.02 2.00% 0.01 0.11 1.36 4.0 W2 0.04 2.00% 0.01 0.15 1.81 4.0 W3 0.06 2.00% 0.01 0.17 2.09 4.0 W4 0.10 2.00% 0.01 0.21 2.49 4.0 W5 0.10 2.00% 0.01 0.21 2.49 4.0 Pipe Sizing K=0.462 Pipe Design Flow Rate Proposed Slope Manning Coefficie nt Required Pipe Diameter Equation 4‐31 Required Pipe Diameter Proposed Pipe Diameter 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) S (%) n d (ft) = {nQdes/K√S}3/8 Dreq (in) Dpro (in)Qfull (ft3/s) = A(1.49/n)((Dpro/48)2/3)S1/2 Qdes/Qfull (from Chart) d (in) = (d/D)*Dpro (Yes/No) E1 0.17 2.00% 0.01 0.25 3.04 4.0 0.001 154.17 0.90 0.02 Yes E2 0.26 2.00% 0.01 0.30 3.58 8.0 0.002 139.75 0.90 0.02 Yes E3 0.33 2.00% 0.01 0.33 3.92 8.0 0.002 195.46 0.90 0.02 Yes E4 0.39 2.00% 0.01 0.35 4.18 8.0 0.002 247.16 0.90 0.02 Yes W1 0.02 2.00% 0.01 0.11 1.36 4.0 0.002 8.03 0.90 0.02 Yes W2 0.04 2.00% 0.01 0.15 1.81 4.0 0.002 23.10 0.90 0.02 Yes W3 0.06 2.00% 0.01 0.17 2.09 4.0 0.002 39.00 0.90 0.02 Yes W4 0.10 2.00% 0.01 0.21 2.49 4.0 0.001 74.06 0.90 0.02 Yes W5 0.10 2.00% 0.01 0.21 2.49 4.0 0.001 74.06 0.90 0.02 Yes Pipe Sizing Received 4/12/2017 10:23:43 AM 10 5.2 Inlet Sizing Inlets were sized to accommodate peak flow conditions for each basin. Equations 4-17 through 4-20 were used to size inlets, which incorporate a 50% clogging factor (Cg) and a 40% opening in grates (m). A water depth of 0.05 feet was assumed and all the inlets were treated as sumps as they will be set a minimum of 0.05 feet below the flow lines. The slot and trench drains were treated as rectangular sumps. Inlets set in landscaping will be circular. Below are the sump calculations for circular inlets. 6.0 Proposed Facilities 6.1 Proposed Structures A single drywell is being utilized to meet the total detention requirement for the entire site. Roof area will be collected via roof drains that will then be internally piped to lamb’s tongue downspouts. Below these downspouts will be inlets to capture the runoff. The snowmelted driveway will flow into a 4-inch wide trench drain piped to an oil/sand separator. Other snowmelted patios and impervious surface will be collected by slot drains. Everything will be piped to the drywell. Analysis of the conveyance structures is contained in section 5.0 of this report. The drywell was sized for the total detention calculated in section 4.2. The drywell structure has a diameter of 6 feet, a perforation depth of 8 feet, and an effective sump depth of 14.5 feet. The soil on the site was determined to have a percolation rate of 3 inches per hour or 20 mpi as stated in the Geotechnical Report. Calculations for determining the perforated area with the drywell were performed using this percolation rate. Sub Basin and Rectangular Inlet Calculations 1 Hour(P1)1.23 m=40% Ys=.05 (Depress inlet by 0.05') 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) (ft 2)(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) North Patio Slot Drain 1 214.83 214.83 100.00% 0.950 5 6.33 0.030 0.5" x 17.25' 0.5 414 0.288 0.335 Yes Main Patio Slot Drain 3 661.00 661.00 100.00% 0.950 5 6.33 0.091 6" x 11.25' 0.5 450 0.313 0.364 Yes Driveway Trench Drain 6 58.70 58.70 100.00% 0.950 5 6.33 0.008 6" x 11.25' 4 213 1.183 1.380 Yes West Walkway Slot Drain 7 520.00 520.00 100.00% 0.950 5 6.33 0.072 6" x 11.25' 6 135 1.125 1.312 Yes Sub Basin and Circular Inlet Calculations 1 Hour(P1)1.23 m=40% Ys=.05 (Depress inlet by 0.05') 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 Capacity See(D1) (ft 2)(ft2)(%)From Table (Td) I=88.8P1/(10+Td)1.052 ft3/sec Wo (inches) Ae=(1‐Cg)mA Q=CoAe√2gYs (Yes/No) Downspout Inlet 1 2 1220.89 1220.89 100.00% 0.950 5 6.33 0.168 12" Round 12 0.157 0.183 Yes Downspout Inlet 2 4 520.00 520.00 100.00% 0.950 5 6.33 0.072 8" Round 8 0.070 0.073 Yes Drywell Grated Lid 5 447.00 447.00 100.00% 0.950 5 6.33 0.062 24" Round 24 0.628 0.655 Yes Downspout Inlet 3 and 4 8 307.00 307.00 100.00% 0.950 5 6.33 0.042 8" Round 8 0.070 0.073 Yes Downspout Inlet 5 9 267.00 267.00 100.00% 0.950 5 6.33 0.037 8" Round 8 0.070 0.073 Yes Drywell Basins Diameter Storage Depth Internal Volume Total Capacity Required Capacity (Name) (#) (ft) (ft)π*H*(D/2)2) (ft3)(ft3)(ft3) Drywell 1 6 14.5 410 410 408.9 Received 4/12/2017 10:23:43 AM Show drain time calculations and summary for the drywell. Include some discussion of drywell PVC Liner. 11 7.0 Operation and Maintenance 7.1 Drywell Drywells must be inspected and maintained quarterly to remove sediment and debris that has washed into them. A maintenance plan shall be submitted to the City in the Drainage Report 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:  Inspect drywells at least four times a year and after every storm exceeding 0.5 inches.  Dispose of sediment, debris/trash, and any other waste material removed from a drywell at suitable disposal sites and in compliance with local, state, and federal waste regulations.  Routinely evaluate the drain-down time of the drywell to ensure the maximum time of 24 hours is not being exceeded. If drain-down times are exceeding the maximum, drain the drywell via pumping and clean out the percolation area (the percolation barrel may be jetted to remove sediment accumulated in perforations. If slow drainage persists, the system may need to be replaced.  Inspect the 6 inch perforated pipe to ensure the surrounding filter fabric has not become clogged. 8.0 Appendices Drawings 11x17 Drywell Needed Perforation Area Infitration Rate 3 in/hr. Hydraulic Cond. (K) 0.0001 ft/sec Required Detention 410.00 ft3 Required Perforation Area (AP)136.67 ft2 Drywell Diameter 6.0 ft Perforation Depth 8.0 ft Proposed Perforation Area (AP) 150.80 ft2 Received 4/12/2017 10:23:43 AM Include Sand Oil separator and Green Roof in Operations and Maintenance section.