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HomeMy WebLinkAboutFile Documents.701 Gibson Ave.0086-2021-BRES (41)Engineering2_701 Gibson Ave_0086_2021_BRES Page: 11 Author: pjm File Name: 40.DrainageReport.701Gibson.20211102.pdf The existing drywell system can be used for detention of the disturbed area if adequate capacity/functionality is confirmed. Detention or conveyance to the City's system is still required for this project per the URMP. What conveyance mechanism is proposed to verify that runoff leaving the rain garden will reach the drywells for detention? Based on the grading plan it looks like the runoff will sheet flow to the south, how will it enter the grate to the drywell and not flow past? Is erosion of the steep slope a concern as runoff leaves the riprap? 40.DrainageReport.701Gibson.20211102.pdf (4) Page: 25 Author: pjm File Name: 40.DrainageReport.701Gibson.20211102.pdf Do any special soils preparations need to be considered for the rain garden treatment technique? Will infiltration be achievable? Page: 48 Author: pjm File Name: 40.DrainageReport.701Gibson.20211102.pdf This is misleading. The calculation is performed using 60.5% imperviousness, as calculated in the step above, not 100% impervious as stated in this line. This calculation is done using 60.5% impervious. Page: 51 Author: pjm File Name: 40.DrainageReport.701Gibson.20211102.pdf Pipe is required to be sized so that design depth does not exceed 80% of the pipe diameter. Revise to comply with section 4.8. Historic Flow Path: Runoff from basin EX-1 sheet flows southwest from the north property boundary. Design point one has been associated with the basin and is the location of the historic discharge point from the site. Table 1 below is a summary of the existing basin information. Three existing drywells onsite provide stormwater detention. Stormwater discharges to the drywells via pipe flow and a 24-inch inlet grate that tops one of the drywells. The proposed project does not interfere with the existing drywells or their current capability to capture and detain stormwater; therefore, stormwater detention for the site is provided as an existing condition that will not be disturbed. The ability of the drywell inlet grade to capture runoff can be increased with the proposed site grading during landscape improvements. The proposed raise grade around the south side of the grate will help the inlet capture more stormwater runoff. Table 1. Historic Basin Characteristic BASIN AREA, ACRES C, 10YR I, 10YR Q10-YEAR, CFS C, 100YR I, 100 YR Q100-YEAR, CFS EX-1 0.043 0.15 3.42 0.02 0.35 5.47 0.08 C. Proposed and Offsite Basin Descriptions One proposed onsite basin and two offsite basins have been delineated in analyzing drainage at 701 Gibson. The proposed onsite basin matches the existing disturbed basin. Proposed basin PR-1 encompasses the driveway and landscaping located north and west of the residence and along the west property line southwest to the bioretention pond. From the northeast the storm flows will sheet flow across the proposed driveway and into a slot drain that pipes directly to the bioretention pond. Additional stormwater will sheet flow across a grass lawn south of the driveway and into the bioretention pond. Stormwater overflows below the detention pond area in the landscaping will sheet flow down the vegetated hillside. The landscape architects have proposed improved grasses and landscape features that will help stabilize the hillside and provide improved water quality to the stormwater flows. Design point one is located at the The existing drywell system can be used for detention of the disturbed area if adequate capacity/functionality is confirmed. Detention or conveyance to the City's system is still required for this project per the URMP. What conveyance mechanism is proposed to verify that runoff leaving the rain garden will reach the drywells for detention? Based on the grading plan it looks like the runoff will sheet flow to the south, how will it enter the grate to the drywell and not flow past? Is erosion of the steep slope a concern as runoff leaves the riprap? Do any special soils preparations need to be considered for the rain garden treatment technique? Will infiltration be achievable? Total Area: 1,901.3 + 1,286.5 = 3,187.8 𝑓𝑡ଶ New Impervious Area: 1,176.6 + 751.4 = 1,928.0 𝑓𝑡ଶ Imperviousness: ଵଽଶ଼௙௧మ ଷଵ଼଻.଼௙௧మ × 100 = 60.5% Treated as 100% Impervious WQCV (watershed-inches): 0.116 New Impervious Area WQCV (𝑓𝑡ଷ): 0.116(𝑤𝑎𝑡𝑒𝑟𝑠ℎ𝑒𝑑−𝑖𝑛.)× ଵ௙௧ ଵଶ௜௡ × 3187.8 𝑓𝑡ଶ = 30.8 𝑓𝑡ଷ Bioretention Pond: WQCV Depth Capacity = 1𝑓𝑡 Flat Area Required =ଷ଴.଼௙௧య ଵ.଴௙௧ = 30.8 𝑓𝑡ଶ Flat Area Provided =𝟑𝟏.𝟔 𝒇𝒕𝟐 This is misleading. The calculation is performed using 60.5% imperviousness, as calculated in the step above, not 100% impervious as stated in this line. This calculation is done using 60.5% impervious. Highlighted Depth (ft) = 0.29 Q (cfs) = 0.250 Area (sqft) = 0.08 Velocity (ft/s) = 3.14 Wetted Perim (ft) = 0.80 Crit Depth, Yc (ft) = 0.28 Top Width (ft) = 0.21 EGL (ft) = 0.44 Pipe is required to be sized so that design depth does not exceed 80% of the pipe diameter. Revise to comply with section 4.8. The rain garden will provide enough detention for basin PR-1 to maintain the historical flowrate of basin EX-1, as well as the required WQCV. The drainage report has been updated to include stormwater detention to the narrative and calculations. The hillside is the historic release for this area and we are improving the sheet flow surface Notes have been added to verify percolation of subgrade soils in rain garden area. If soils are not percolating additional depth and washed gravels will be added to get to depth of suitable percolation. See notes on revised sheet C.1.002 The 100% impervious line is outdated/typo. The calculations of WQCV using 60.5 are correct. The revised biopond calcs no longer show 100% impervious. Pipe calculations have been revised by increasing the pipe slope, so the design depth does not exceed 80% of the pipe diameter. *increase slope to 1.1% (currently 1.0%) -remove line Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf Clarify if this parking area is surrounded by a retaining wall or landscape curbing? The grading plan doesn't clarify if the curb is elevated. New retaining walls are not permitted in the ROW. 90.Civil.701Gibson.20211102.pdf (16) Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf The storm infrastructure in the water main easement needs to be minimized. The pipe for the planters runs down the easement. Revise the alignment so this pipe creates a minimized conflict if/when the Water Department needs to access this line. Can this pipe go straight across the easement or do grades not allow? The response to comments letter states the storm drain insulation is called out on the grading plan, I couldn't find this call out. It looks like heat tape is proposed instead? Water Department will need to approve heat tape in the easement. Insulation may be preferred. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf Sightlines in the ROW need to be maintained. Height of landscaping needs to meet COA standards. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf The soils report calls out fill soils are in the area, how will the subsoil be prepared to ensure infiltration at the rain garden. The soils report recommends compaction however compaction typically doesn't permit infiltration. Per the URMP Appendix A, provide profiles of the proposed pipes. 8. 1 2 ' Clarify if this parking area is surrounded by a retaining wall or landscape curbing? The grading plan doesn't clarify if the curb is elevated. New retaining walls are not permitted in the ROW. S0 0 ° 5 2 ' 3 2 " W 48 . 1 2 ' 24 . 9 7 ' N6 6 ° 1 6 ' 2 2 " W 54. 2 1 ' S2 7 ° 4 1 ' 0 8 " W The storm infrastructure in the water main easement needs to be minimized. The pipe for the planters runs down the easement. Revise the alignment so this pipe creates a minimized conflict if/when the Water Department needs to access this line. Can this pipe go straight across the easement or do grades not allow? The response to comments letter states the storm drain insulation is called out on the grading plan, I couldn't find this call out. It looks like heat tape is proposed instead? Water Department will need to approve heat tape in the easement. Insulation may be preferred. 22 . 3 4 ' GI B S O N A V E N U E S6 4 ° 1 5 ' 5 7 " E Sightlines in the ROW need to be maintained. Height of landscaping needs to meet COA standards. S3 2 ° 2 1 ' 3 1 " W 1 1 0 . 9 7 ' N 4 9 ° 0 9 ' 3 0 " W 4 4 . 5 3 ' The soils report calls out fill soils are in the area, how will the subsoil be prepared to ensure infiltration at the rain garden. The soils report recommends compaction however compaction typically doesn't permit infiltration. Per the URMP Appendix A, provide profiles of the proposed pipes. A 6" tall landscape curb ties into existing retaining wall as discussed during conference calls with the City of Aspen. A callout and spot elevations have been added to revised sheet C.1.002 to clarify. Storm infrasture in the easement has been minimized and reviewed by the Aspen Water Department. The two pipe alignments provide the shortest possible lengths in the easement and avoid horizontal bends directly above approximate water main location. The pipes cannot go straight across easement due to the close proximity of the property line Heat tape has been approved provided the heat tape is added to easement agreement. Heat tape has been added to the agreement and remains part of the design. Sightlines will be maintained with new landscaping. Revised landscape plans show sightlines are maintained to meet COA standards. Rain garden area will be verified at base depth for percolation and deepened if necessary to achieve adequate percolation. Notes have been added to sheet C.1.002 to verify percolation and avoid compaction in infiltration area. City of Aspen Engineering has agreed to not require pipe profiling due to the pipe size and the suitable amount of vertical separation from the water main. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf This 30" boulder retaining wall will need to be approved by the Water Department and written into the easement agreement that calls out all encroachments in the easement. It will be the property owner's responsibility to replace this infrastructure when the Water Department needs access to the main line in the easement. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf The rain garden, riprap, storm pipes from the planters/trench drain, heat tape, concrete paver driveway and existing areas of the building encroaching into the easement shall be written into the easement agreement (attached to the comment letter email). COA Water and Attorney will review the proposed agreement and encroachments for approval. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf These dogwoods are proposed directly above the water main. This is not permitted. Small shrubs are permitted in the easement however nothing can be planted above the water main. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf How can the alignment be shifted so the pipe in the easement is minimized? Is it possible to run the pipe perpendicular to the easement rather than at an angle? The response to comments letter states the storm drain insulation is called out on the grading plan, I couldn't find this call out. It looks like heat tape is proposed instead? Water Department will need to approve heat tape in the easement. Insulation may be preferred so frost depth is not changed. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf What is the estimated/expected vertical separation between the proposed storm pipes and water main/service lines? Previously Water preferred insulation to heat tape on storm pipes above the water main. This is pending approval by Water via the easement agreement. This 30" boulder retaining wall will need to be approved by the Water Department and written into the easement agreement that calls out all encroachments in the easement. It will be the property owner's responsibility to replace this infrastructure when the Water Department needs access to the main line in the easement. 1"W 1 1 0 . 9 7 ' The rain garden, riprap, storm pipes from the planters/trench drain, heat tape, concrete paver driveway and existing areas of the building encroaching into the easement shall be written into the easement agreement (attached to the comment letter email). COA Water and Attorney will review the proposed agreement and encroachments for approval. S3 2 ° 2 1 ' 3 1 " W 1 1 0 . 9 7 ' N 4 9 ° 0 9 ' 3 0 " W 4 4 . 5 3 ' These dogwoods are proposed directly above the water main. This is not permitted. Small shrubs are permitted in the easement however nothing can be planted above the water main. How can the alignment be shifted so the pipe in the easement is minimized? Is it possible to run the pipe perpendicular to the easement rather than at an angle? The response to comments letter states the storm drain insulation is called out on the grading plan, I couldn't find this call out. It looks like heat tape is proposed instead? Water Department will need to approve heat tape in the easement. Insulation may be preferred so frost depth is not changed. What is the estimated/expected vertical separation between the proposed storm pipes and water main/service lines? Previously Water preferred insulation to heat tape on storm pipes above the water main. This is pending approval by Water via the easement agreement. The 30" retaining wall has been written into an easement agreement and accepted by the water department. The property owner is aware of the responsibly to replace landscaping infrastructure. All mentioned items have been added to the easement agreement. Landscape plans have been revised to remove plantings above water main. Revised civil plans to reflect these landscape updates. The vertical separation between storm pipes and the water main is estimated to be around 12 feet or more. The revised grading plan calls out this separation. Heat tape has been added to the easement agreement and approved Pipes in the easement have been reviewed by the Aspen Water Department. Pipe alignments provide the shortest possible lengths in the easement and avoid horizontal bends above the water main location. The pipes cannot run perpendicular easement due to the close proximity of the property line size of easement Heat tape has been added to the easement agreement and approved by the water department Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf State that an encroachment will be obtained for existing items in the ROW. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf Provide C&G detail compliant with the Engineering Standards. It looks like a Type A barrier curb is proposed. Provide a detail for the beveled edge at the driveway entrance. It looks like detail #ENG-202H is proposed for the driveway entrance. Include this detail in the civil sheets as well. Is a T top patch proposed or will the new concrete butt up against the existing asphalt edge? COA standard is a T top patch. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf Show the 25' utility easement on this sheet so conflicts with storm infrastructure are shown. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf What happens to the runoff caught in the flowline at the end of the C&G? Creating negative drainage impacts on a neighboring property is not permitted. Show no negative impacts to downstream properties occurs. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf Two water lines are shown in this area. Were two lines located during the main line investigation? State that an encroachment will be obtained for existing items in the ROW. 8. 1 2 ' Provide C&G detail compliant with the Engineering Standards. It looks like a Type A barrier curb is proposed. Provide a detail for the beveled edge at the driveway entrance. It looks like detail #ENG-202H is proposed for the driveway entrance. Include this detail in the civil sheets as well. Is a T top patch proposed or will the new concrete butt up against the existing asphalt edge? COA standard is a T top patch. S3 2 ° 2 1 ' 3 1 " W 1 1 0 . 9 7 ' N 4 9 ° 0 9 ' 3 Show the 25' utility easement on this sheet so conflicts with storm infrastructure are shown. What happens to the runoff caught in the flowline at the end of the C&G? Creating negative drainage impacts on a neighboring property is not permitted. Show no negative impacts to downstream properties occurs. Two water lines are shown in this area. Were two lines located during the main line investigation? A callout has been added to the revised sheet C.1.001 to state the encroachment shall be obtained for the existing retaining wall. A note has been added to sheet C.1.001 to state an encroachment shall be obtained for existing items in ROW. A new details sheet (C.6.003) has been added to the submittal set to include the details mentioned. The revised Site Plan sheet C.1.001 shows proposed asphalt patching. The 25' utility easement is now shown on revised plans. A proposed gutter inlet that ties to the City storm system has been added to the design to improve impacts to the neighboring property. A pipe plan and profile sheet has been added to the submittal set and further analysis of this infrastructure has been included in the revised drainage report Multiple utility locates were conducted for survey information and water main investigation. Two lines were shown as possible alignments. Potholing found one water main and plans have been revised to show one line. Pothole findings have been added to plans. Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf The landscape plans show plantings above the riprap area. Will these trees be planted outside the riprap and the dripline is just shown over the riprap? How will these be planted? Page: [1] 24x36 Author: pjm File Name: 90.Civil.701Gibson.20211102.pdf Clarify, are there 3 water mains in this easement or is this showing potential alignments? City records indicate that only one main is present. S3 2 ° 2 1 ' 3 1 " W 1 1 0 . 9 7 ' N The landscape plans show plantings above the riprap area. Will these trees be planted outside the riprap and the dripline is just shown over the riprap? How will these be planted? 4 4 . 5 3 ' Clarify, are there 3 water mains in this easement or is this showing potential alignments? City records indicate that only one main is present. Revised landscape plans no longer show plantings in riprap area. Civil plans have been updated to reflect landscape revisions. Multiple utility locates were conducted for survey information and water main investigation. Lines from locates were shown as possible alignments. Potholing confirmed one water main. Revised plans show one existing water main.