Final Drainage Report

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1 Thornton Electric Substation Project Final Drainage Report December 14, 2016 DRAFT Prepared for: Xcel Energy, 1800 Larimer Street, Suite 400, Denver, Colorado Prepared by: 350 Indiana Street, Suite 500, Golden, Colorado 80401

2 Draft Final Drainage Report Xcel Energy Thornton Electric Substation Project Grading and Drainage Design 3. Drainage Design Criteria The criteria developed for the design of the stormwater management system for the Project Site are based on the guidance provided in Volumes 1, 2, and 3 of the 2016 USDCM published by UDFCD and the 2012 City of Thornton Standards and Specifications for the Design and Construction of Public and Private Improvements, Section 400 Stormwater Drainage Design, Grading, and Water Quality Technical Criteria (Thornton Standards and Specifications). In areas where the criteria manuals and Standards and Specifications conflict, the Standards and Specifications shall govern (City of Thornton 1975 [ (c)]). 3.1 Regulations All drainage facilities shall be designed in accordance to the provisions of Chapter 400, "Storm Drainage Design and Technical Criteria," of the Thornton Standards and Specifications (City of Thornton 1975 [ ]). 3.2 Development Criteria Reference and Constraints Previous Drainage Studies Previous drainage studies (i.e., project Master Drainage Plans) for the Project area will not affect or be affected by the proposed drainage design for the Projects Effects of Adjacent Drainage Studies There are no known effects of adjacent drainage studies that will influence or be influenced by the proposed drainage design for the Project Site Constraints Existing constraints (e.g., drainages, slopes, streets, utilities, existing structures, irrigation ditches, etc.) for the Project are generally considered minimal, however, however, there are some that require explanation. The site plan facilities and associated structures have been arranged so that no impacts to existing utilities will occur. The existing/current retention basin was designed and constructed with calculations provided from TAG Engineering Consultants (1985) with an allowable discharge of cubic feet per second. This value was also used by JR Engineering for the design of the storm sewer system downstream of the existing retention basin as part of the Sundown at Skylake Ranch Phase III Drainage Report (1997). Although hydrologic analysis permits an allowable discharge of 24.7 cubic feet per second from the proposed detention basin (see runoff flow from the detention basin in Section 5.2.3), the historic discharge rate of cubic feet per second will be used to design the proposed detention basin outlet structure as part of this Project. In addition, the inlet and outlet elevations of the proposed detention basin are similarly constrained by the existing retention basin configuration. Thus, certain detention basin physical characteristics may not meet the criteria established by City of Thornton and/or UDFCD (see Section 3.5 for variance and a description of deviation and rationale) without unnecessarily burdening the Project Public Service Company of Colorado Thornton Electric Substation Project 3-1

3 Draft Final Drainage Report Xcel Energy Thornton Electric Substation Project Grading and Drainage Design 3.5 Variances from Criteria Requested Variances The existing retention basin bottom currently slopes at 0.5 percent. As identified in the design drawings in Appendix C, the proposed detention basin maintains the current bottom slopes due to the vertical elevation constriction of the upstream and downstream system invert elevations for the inlet and outlet structures. The current design does not meet the 4 percent bottom grading required by the City of Thornton detention pond design criteria (2012). Therefore, the Project will request a variance to deviate from the City of Thornton criteria Justification of Variances The existing storm system vertical elevations on the downstream side of the proposed detention basin have been established as part of the Sundown at Skylake Ranch development. Likewise, the inlet structure to the detention basin cannot deviate excessively from its current elevation without impacting the grades and associated drainage upstream. An analysis was performed in which the proposed outlet structure 24-inch RCP slope was varied from 1 to 3.6 percent and the accompanying elevation gain was compared to the detention basin inlet structure elevation to assess compliance with the basin bottom slope required by the City of Thornton (2012). It was determined that the criteria could not be met unless the detention basin was expanded on the west side and overdesigned volumetrically. The Project will incur excessive financial burden in order to meet this criteria without improving the function or operation of the detention basin in any other significant way. This is the rationale for the proposed variance from the Thornton Standards and Specifications. Public Service Company of Colorado Thornton Electric Substation Project 3-5

4 Draft Final Drainage Report Xcel Energy Thornton Substation Project Grading and Drainage Design Off-Site Runoff Compliance USDCM states that the stormwater management system should be designed beginning with the outlet or point of outflow from the project, giving full consideration to downstream effects and the effects of off-site flows entering the system. The downstream conveyance system will be evaluated to ensure that it has sufficient capacity to accept design discharges without adverse upstream or downstream impacts such as flooding, stream bank erosion, and sediment deposition. In addition, the design of a drainage system takes into account the runoff from upstream sites, recognizing their future development runoff potential (UDFCD 2016). As previously stated in Sections and 3.3.3, all channels and the detention basin emergency spillway and outlet have been designed to account for the safe and effective collection, conveyance, and controlled discharge of both on-site and off-site (combined/aggregate) flows for the 100- year stormwater event. The downstream existing system will be evaluated after the 60 percent review by the City of Thornton. Currently, the proposed development will have negligible impact to the downstream existing system, since the 100-year peak flow will be released at equal or less than historical flows, therefore, there are no off-site runoff compliance issues identified for the Project Proposed Drainage Patterns The site plan layout (including the features described in Section 1.2.4), developed/proposed grading, detailed channel/basin/outlet pipe/spillway information, and the existing and developed basins (including land use/surface details, developed condition slopes, hydraulic flow paths, etc.) are provided in the design drawings in Appendix C. The hydrologic and hydraulic design of the Project Site drainage structures have been discussed and summarized in Section 2.2 and Section 5.0. The O&M instructions for the proposed stormwater drainage facilities and erosion control measures will be provided after the 60 percent review by the City of Thornton Riprap and Erosion Protection Riprap erosion protection is expected to be installed at varies locations as shown on the construction drawings in Appendix C. Prior to the 100 percent submittal design will be completed in accordance with UDFCD design requirements. 5.2 Specific Details Hydraulic Structures Swale-1, designed to convey 12.7 cfs or approximately 75 percent of basin D2 (17.0 cfs). Flows conveyed by this swale are routed from north to south to design point 2 as showing on Design Drawing C This swale is V-shape type with 4:1 (H:V) side slopes and varies in depth from 0 to 1 feet depth. Swale-2, designed to convey 4.3 cfs or approximately 25 percent of basin D2 (17.0 cfs). Flows conveyed by this swale are routed from west to east to design point 2 as showing on Design Drawing C This swale is V-shape type with 4:1 (H:V) side slopes and varies in depth from 0 to 1 feet depth. Public Service Company of Colorado Thornton Electric Substation Project 5-2

5 Draft Final Drainage Report Xcel Energy Thornton Electric Substation Project Grading and Drainage Design determining the storage capacity. The peak outflow is confined to the 100-year historical peak outflow (for outlet structure configuration and hydraulic calculations, please refer to Appendix ). The preliminary calculations for the 100-year storm event discharge rate (Q100-yr) through the detention basin outlet pipe discharge orifice is 20.7 cfs, which is 0.33 cfs or 1.62 percent more than the maximum allowable Q100-yr of cfs (see Section 3.2.3), but 4 cfs less than 24.7 cfs maximum required for the existing condition of the retention basin as identified in the Sundown at Skylake Ranch Phase III Drainage Report by JR Engineering (1997). The proposed outlet structure will be finalized for the 90 percent design pending the outcome of the 60 percent City of Thornton review Emergency Spillway The emergency spillway will be located on the northeast corner of the proposed detention basin, which is the apparent location of the existing emergency spillway (for the emergency spillway location see Design Drawing C-1011). The emergency overflow was preliminary sized to determine the appropriate configuration in accordance with the City of Thornton and UDFCD (for the emergency spillway hydraulic calculations, please refer to Appendix ). The emergency spillway has been evaluated based on the two times the 100-year peak inflow of cfs, or cfs, which correlates to the drainage basins areas described in Section 2.2 and 100-year peak inflow calculated by the UD-Detention Model in Appendix. The emergency spillway is currently configured with a broad-crested weir length of 100 feet, 25-foot crest breadth, and sideslope of 4:1 (H:V). Manning s friction coefficient corresponded with gravel that meets the City of Thornton requirement for riprap-lined overflow. ased on this configuration, the headwater elevation was calculated to be ft msl, or a headwater height of 1.16 ft above emergency spillway elevation of ft msl. With 1-ft freeboard requirement from City of Thornton Standards and Specifications, the surrounding detention basin berm will stand at ft msl. The proposed emergency spillway will be finalized for the 90 percent design pending the outcome of the 60 percent City of Thornton review Downstream Effects Analysis It was determined that the historical flows contributing to the downstream existing storm system will be reduced by 12.3 cfs, as described in Section 2.2, and the proposed detention basin outlet structure will be design to allow the historical peak outflow of cfs, as described in Section ased on these two conditions, it is semi-quantitatively determined that the proposed development will not affect the functionality of the existing storm system previously designed since flowrates will be lower after the project development has been constructed. However, the downstream effects analysis will be completed as part of the 90 percent design as required by the City of Thornton. This downstream evaluation will be provided in Appendix Seepage and reach Analysis Seepage and breach analysis will be discussed with the City of Thornton after review of the 60 percent preliminary design to determine if these analyses are required for this Project. Public Service Company of Colorado Thornton Electric Substation Project 5-5

6 Draft Final Drainage Report Xcel Energy Thornton Substation Project Grading and Drainage Design Appendix : Hydraulic Computations Xcel Energy Thornton Substation Project

7 E:\4832_Thornton_Area_Substation\CAD\2016 Files\Thornton\Thornton Substation - Grading and Drainage\Hydrology & Hydraulic Calculations\CHANNEL\CHANNEL.dwg, JORGE.CAMPOS, 11/27/ :20 A Scale: AS SHOWN Designed by: Drawn by: Checked by: Approved by: Revision A Depth Q Area Veloc Wp Yc TopWidth Energy (ft) (cfs) (sqft) (ft/s) (ft) (ft) (ft) (ft) Issued by: Issued for: TETRA TECH R Title: XCEL THORNTON SUSTATION PROPOSED CHANNEL - 1 Project: XCEL THORNTON SUSTATION Location: CITY OF THORNTON, COLORADO Project no.: Date: 2016/11/29 Figure no.: 1

8 DETENTION ASIN STAGE-STORAGE TALE UILDER Project: 120th & Holly, Thornton, CO asin ID: PROPOSED DETENTION ASIN Example Zone Configuration (Retention Pond) Depth Increment = 0.1 ft Stage - Storage Description Required Volume Calculation Micropool Selected MP Type = ED Watershed Area = acres Watershed Length = 1,800 ft , Watershed Slope = ft/ft , Watershed Imperviousness = 60.00% percent , , Percentage Hydrologic Soil Group A = 0.0% percent , , Percentage Hydrologic Soil Group = 0.0% percent , , Percentage Hydrologic Soil Groups C/D = 100.0% percent , , Desired WQCV Drain Time = 40.0 hours , , Location for 1-hr Rainfall Depths = Thornton - Thornton City Office , , Water Quality Capture Volume (WQCV) = acre-feet Optional User Input , , Excess Urban Runoff Volume (EURV) = acre-feet 1-hr Precipitation Zone 1 (WQCV) , , Optional Override Area (ft^2) 2-yr Runoff Volume (P1 = 0.95 in.) = acre-feet inches , , yr Runoff Volume (P1 = 1.35 in.) = acre-feet inches , , yr Runoff Volume (P1 = 1.61 in.) = acre-feet inches , , yr Runoff Volume (P1 = 2.03 in.) = acre-feet inches , , yr Runoff Volume (P1 = 2.32 in.) = acre-feet inches , , yr Runoff Volume (P1 = 2.66 in.) = acre-feet inches , , yr Runoff Volume (P1 = 3.33 in.) = acre-feet inches , , Approximate 2-yr Detention Volume = acre-feet , , Approximate 5-yr Detention Volume = acre-feet Zone 2 (EURV) , , Approximate 10-yr Detention Volume = acre-feet , , Approximate 25-yr Detention Volume = acre-feet , , Approximate 50-yr Detention Volume = acre-feet , , Approximate 100-yr Detention Volume = acre-feet , , Stage (ft) Optional Override Stage (ft) Length (ft) Width (ft) Area (ft^2) Area (acre) Volume (ft^3) Volume (ac-ft) , , UD-Detention-Proposed, asin 11/28/2016, 5:05 PM

9 Detention asin Outlet Structure Design Project: 120th & Holly, Thornton, CO asin ID: PROPOSED DETENTION ASIN Stage (ft) Zone Volume (ac-ft) Outlet Type Zone 1 (WQCV) Orifice Plate Zone 2 (EURV) Rectangular Orifice Zone 3 (100-year) Weir&Pipe (Restrict) Total User Input: Orifice at Underdrain Outlet (typically used to drain WQCV in a Filtration MP) Calculated Parameters for Underdrain Underdrain Orifice Invert Depth = N/A ft (distance below the filtration media surface) Underdrain Orifice Area = N/A ft 2 Underdrain Orifice Diameter = N/A inches Underdrain Orifice Centroid = N/A feet User Input: Orifice Plate with one or more orifices or Elliptical Slot Weir (typically used to drain WQCV and/or EURV in a sedimentation MP) Calculated Parameters for Plate Invert of Lowest Orifice = 0.00 ft (relative to bottom of basin at Stage = 0 ft) WQ Orifice Area per Row = 2.806E-02 ft 2 Depth at top of Zone using Orifice Plate = 1.18 ft (relative to bottom of basin at Stage = 0 ft) Elliptical Half-Width = N/A feet Orifice Plate: Orifice Vertical Spacing = 4.70 inches Elliptical Slot Centroid = N/A feet Orifice Plate: Orifice Area per Row = 4.04 sq. inches (use rectangular openings) Elliptical Slot Area = N/A ft 2 User Input: Stage and Total Area of Each Orifice Row (numbered from lowest to highest) Row 1 (required) Row 2 (optional) Row 3 (optional) Row 4 (optional) Row 5 (optional) Row 6 (optional) Row 7 (optional) Row 8 (optional) Stage of Orifice Centroid (ft) Orifice Area (sq. inches) Stage of Orifice Centroid (ft) Orifice Area (sq. inches) Row 9 (optional) Row 10 (optional) Row 11 (optional) Row 12 (optional) Row 13 (optional) Row 14 (optional) Row 15 (optional) Row 16 (optional) User Input: Vertical Orifice (Circular or Rectangular) Calculated Parameters for Vertical Orifice Zone 2 Rectangular Not Selected Zone 2 Rectangular Not Selected Invert of Vertical Orifice = 1.18 N/A ft (relative to bottom of basin at Stage = 0 ft) Vertical Orifice Area = 0.17 N/A ft 2 Depth at top of Zone using Vertical Orifice = 2.00 N/A ft (relative to bottom of basin at Stage = 0 ft) Vertical Orifice Centroid = 0.08 N/A feet Vertical Orifice Height = 2.00 N/A inches Vertical Orifice Width = inches User Input: Overflow Weir (Dropbox) and Grate (Flat or Sloped) Calculated Parameters for Overflow Weir Zone 3 Weir Not Selected Zone 3 Weir Not Selected Overflow Weir Front Edge Height, Ho = 2.00 N/A ft (relative to bottom of basin at Stage = 0 ft) Height of Grate Upper Edge, H t = 4.50 N/A feet Overflow Weir Front Edge Length = 6.00 N/A feet Over Flow Weir Slope Length = N/A feet Overflow Weir Slope = 4.00 N/A H:V (enter zero for flat grate) Grate Open Area / 100-yr Orifice Area = N/A should be > 4 Horiz. Length of Weir Sides = N/A feet Overflow Grate Open Area w/o Debris = N/A ft 2 Overflow Grate Open Area % = 70% N/A %, grate open area / total area Overflow Grate Open Area with Debris = N/A ft 2 Debris Clogging % = 50% N/A % User Input: Outlet Pipe w/ Flow Restriction Plate (Circular Orifice, Restrictor Plate, or Rectangular Orifice) Calculated Parameters for Outlet Pipe w/ Flow Restriction Plate Zone 3 Restrictor Not Selected Zone 3 Restrictor Not Selected Depth to Invert of Outlet Pipe = 0.00 N/A ft (distance below bottom of basin at Stage = 0 ft) Outlet Orifice Area = 2.45 N/A ft 2 Outlet Pipe Diameter = N/A inches Outlet Orifice Centroid = 0.81 N/A feet Restrictor Plate Height Above Pipe Invert = inches Half-Central Angle of Restrictor Plate on Pipe = 2.05 N/A radians User Input: Emergency Spillway (Rectangular or Trapezoidal) Calculated Parameters for Spillway Spillway Invert Stage= 4.90 ft (relative to bottom of basin at Stage = 0 ft) Spillway Design Flow Depth= 0.59 feet Spillway Crest Length = feet Stage at Top of Freeboard = 6.49 feet Spillway End Slopes = 0.17 H:V asin Area at Top of Freeboard = 2.35 acres Freeboard above Spillway = 1.00 feet Routed Hydrograph Results Design Storm Return Period = WQCV EURV 2 Year 5 Year 10 Year 25 Year 50 Year 100 Year 500 Year One-Hour Rainfall Depth (in) = Calculated Runoff Volume (acre-ft) = OPTIONAL Override Runoff Volume (acre-ft) = Inflow Hydrograph Volume (acre-ft) = Predevelopment Unit Peak Flow, q (cfs/acre) = Predevelopment Peak Q (cfs) = Peak Inflow Q (cfs) = Peak Outflow Q (cfs) = Ratio Peak Outflow to Predevelopment Q = N/A N/A N/A Structure Controlling Flow = Plate Vertical Orifice 1 Vertical Orifice 1 Overflow Grate 1 Overflow Grate 1 Outlet Plate 1 Outlet Plate 1 Outlet Plate 1 Outlet Plate 1 Max Velocity through Grate 1 (fps) = N/A N/A N/A Max Velocity through Grate 2 (fps) = N/A N/A N/A N/A N/A N/A N/A N/A N/A Time to Drain 97% of Inflow Volume (hours) = Time to Drain 99% of Inflow Volume (hours) = Maximum Ponding Depth (ft) = Area at Maximum Ponding Depth (acres) = Maximum Volume Stored (acre-ft) =

10 DIST E DIST E The difference between the 100yr WSE and the spillway invert does not need to be this far apart. You could provide more freeboard if it was lowered. 80' FEET 0 80' 160' CR#6: 100-year WSE and emergency spillway has been revised. See Exhibit 9, Appendix A, Design Drawings. 240' 1"=80' XCEL ENERGY THORNON SUSTATION LOCATION R AS SHOWN ISSUED FOR REVIEW (CITY OF THORNTON) 12/14/2016 A 60% SUMITTAL - ISSUE FOR REVIEW 12/01/2016 JS 11/28/ /28/ /28/2016 THORNTON SUSTATION PROPOSED DETENTION ASIN SECTIONS C-1005

11 111.34'~2' X 4' OX 1.50% ' 0 50' FEET 100' 4" 1.46% '~ CR#7: Design drawings have been revised per the on-site detention basin that is being proposed. See Exhibit 9, Appendix A for proposed on-site detention Why not just build out the the 24" RCP to it's ultimate outfall on the other side of the future commercial access? 150' 1"=50' XCEL ENERGY THORNON SUSTATION LOCATION R AS SHOWN ISSUED FOR REVIEW (CITY OF THORNTON) 12/14/2016 A 60% SUMITTAL - ISSUE FOR REVIEW 12/01/2016 JS 11/18/ /18/ /18/2016 THORNTON SUSTATION 2'X4' OX CULVERT P&P 24" DIA OUTLET PIPE P&P C-1011

12 R XCEL ENERGY THORNON SUSTATION LOCATION ISSUED FOR REVIEW (CITY OF THORNTON) 12/14/2016 AS SHOWN JS 11/21/2016 THORNTON SUSTATION OUTLET STRUCTURE DETAILS A 60% SUMITTAL - ISSUE FOR REVIEW 12/01/ /21/ /21/ C-1012

13 CR#10: Former Drawing C-1013, now Drawing C-1006, has been revised. The on-site detention basin beign proposed will not be directly impacted or influenced by the upstream drainage basin X1 flows or influence the downstream storm system, since the 100-year storm event had been designed constrainted/controlled to cfs. See additional information in Exhibit 9, Drainage Report, Section 3.2. I'll attach the drainage study for this commercial subdivision, which is in construction. Please incorporate their basins/flows into your background work. 100' FEET 0 100' 200' 300' 1"=100' XCEL ENERGY THORNON SUSTATION LOCATION R AS SHOWN ISSUED FOR REVIEW (CITY OF THORNTON) 12/14/2016 A 60% SUMITTAL - ISSUE FOR REVIEW 12/01/2016 JS 10/31/ /31/ /31/2016 THORNTON SUSTATION EXISTING DRAINAGE AREAS C-1013

14 Should there be a design point here reflecting the confluence of channel 2 and channel 3? CR#11: Former Drawing C-1014, now Drawing C-1007, has been revised per City of Thornton comments. See Exhibit 9, Appendix A - Design Drawings. 50' 0 50' 100' 150' 1"=50' FEET XCEL ENERGY THORNON SUSTATION LOCATION R AS SHOWN DRAFT 60% - ISSUE TO THE CITY OF THORNTON 12/14/2016 A 60% SUMITTAL - ISSUE FOR REVIEW 12/01/2016 JS 11/01/ /01/ /01/2016 THORNTON SUSTATION PROPOSED DRAINAGE AREAS C-1014