REVISED FINAL STORM DRAINAGE REPORT FOR: MOUNTAIN AIRE (MTA) PRD/PLAT & POULSBO MEADOWS (PM) PRD/PLAT LOCATED IN NE ¼, SEC. 24, TWP. 26 N., RGE. 1 E., W.M. Assessor’s Account Nos.: (MTA) 242601-1-015-2008, 242601-1-016-2007, 242601-1-043-2004, 242601-1-044-2003 (PM) 242601-1-033-2006 POULSBO, KITSAP COUNTY, WASHINGTON Dated: February 27, 2013 Rev: April 22, 2013 Rev: June 3, 2013 Rev. October 11, 2016 Rev. December 27, 2016 FOR: Quadrant Corporation 14725 SE 36 th , STE 200 Bellevue, WA 98006 PREPARED BY: Team 4 Engineering 5819 NE Minder Road Poulsbo, WA. 98370 (360) 297-5560 “I hereby state that this Drainage Report has been prepared by me or under my supervision and meets the standard care and expertise which is usual and customary in this community of professional engineers. The analysis has been prepared utilizing procedures and practices specified by the City of Poulsbo and within the standard accepted practices of the industry. I understand that the City of Poulsbo does not and will not assume liability for the sufficiency, suitability, or performance of drainage facilities prepared by me.”
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16.12.27 Final Storm Drainage Report - Poulsbo, …...The 29.75-acre Mountain Aire site is zoned Low Density Residential (4 to 5 dwelling units per acre). The site contains four tax-parcels
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Dated: February 27, 2013 Rev: April 22, 2013 Rev: June 3, 2013
Rev. October 11, 2016 Rev. December 27, 2016
FOR:
Quadrant Corporation 14725 SE 36th, STE 200
Bellevue, WA 98006
PREPARED BY:
Team 4 Engineering
5819 NE Minder Road Poulsbo, WA. 98370
(360) 297-5560
“I hereby state that this Drainage Report has been prepared by me or under my supervision and meets the standard care and expertise which is usual and customary in this community of professional engineers. The analysis has been prepared utilizing procedures and practices specified by the City of Poulsbo and within the standard accepted practices of the industry. I understand that the City of Poulsbo does not and will not assume liability for the sufficiency, suitability, or performance of drainage facilities prepared by me.”
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TABLE OF CONTENTS
I. EXECUTIVE SUMMARY ..................................................................................................................................... 4 II. LOCATION ............................................................................................................................................................. 5 III. DESCRIPTION OF PROPOSED DEVELOPMENTS ........................................................................................ 5 IV. DESCRIPTION OF PROPOSED WATER QUALITY AND QUANTITY MANAGEMENT FACILITIES ..................................................................................................................................................................... 6 V. PRE-DEVELOPMENT SITE CONDITIONS ...................................................................................................... 7 VI. UPSTREAM ANALYSIS ....................................................................................................................................... 8 VII. DOWNSTREAM ANALYSIS ................................................................................................................................ 9 VIII. OVERVIEW OF PROPOSED STORM WATER MANAGEMENT STRATEGIES ...................................... 9 IX. TECHNICAL REQUIREMENTS ....................................................................................................................... 10 X. HYDROLOGIC ANALYSIS ................................................................................................................................ 10 1. PRE-DEVELOPED CONDITIONS: PreDev-MTA (original) ............................................................................... 11 2. PRE-DEVELOPED CONDITIONS: PreDev-PM (original) .................................................................................. 12 3. POST-DEVELOPED CONDITIONS: PostDev-MTA (original) ........................................................................... 13 4. POST-DEVELOPED CONDITIONS: PostDev-MTA (updated) ........................................................................... 13 5. POST-DEVELOPED CONDITIONS: PostDev-PM (original) .............................................................................. 14 6. BASIN: Upstream-MTA (updated)......................................................................................................................... 15 7. BASIN: Upstream-PM (original basin, updated text) ............................................................................................. 16 8. BASINS PreDev-MTA +PreDev-PM = PreDev-Both (original) ............................................................................ 16 9. BASINS PostDev-MTA + PostDev-PM = PostDev-Both (updated) ...................................................................... 17 10. BASINS PostDev-MTA + Upstream-MTA = DevMTA+Up (updated) ................................................................. 17 11. BASINS DevMTA+Up + PostDev-PM = Both+Up (updated) ............................................................................... 18 12. STORMWATER QUANTITY CONTROL FACILITY (updated) ........................................................................ 18 13. STORMWATER QUALITY ENHANCEMENT (original) .............................................................................. 22 14. STORMWATER “SSA” ANALYSIS (POND INLET SIDE) ............................................................................... 22 15. STORMWATER “BACKWATER” ANALYSIS (POND INLET SIDE) .............................................................. 23 16. STORMWATER “BACKWATER” ANALYSIS (POND OUTLET SIDE) .......................................................... 23 XI. EROSION AND SEDIMENT CONTROL .......................................................................................................... 25 XII. OPERATION AND MAINTENANCE MANUAL ............................................................................................. 25
ORIGINAL FIGURES FIGURE 1 VICINITY MAP ............................................................................................................................... A1 FIGURE 2A PREDEVELOPED SITE MAP – MOUNTAIN AIRE ..................................................................... A2 FIGURE 2B PREDEVELOPED SITE MAP – POULSBO MEADOWS ............................................................. A3 FIGURE 3A SOILS MAP – MOUNTAIN AIRE .................................................................................................. A4 FIGURE 3B SOILS MAP – POULSBO MEADOWS .......................................................................................... A5 FIGURE 4A DOWN STREAM MAP – MOUNTAIN AIRE ............................................................................... A6 FIGURE 5A DEVELOPED CONDITIONS SITE MAP – MOUNTAIN AIRE ................................................... A7 FIGURE 5B DEVELOPED CONDITIONS SITE MAP – POULSBO MEADOWS ........................................... A8
ATTACHMENT A PROJECT AREA WORKSHEET – MOUNTAIN AIRE .................................................... C1
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ATTACHMENT B PROJECT AREA WORKSHEET – POULSBO MEADOWS ............................................ C2 ATTACHMENT C POND CORRECTION FACTOR WORKSHEET .............................................................. C3 ATTACHMENT D POND VOLUME STAGE-STORAGE WORKSHEET ...................................................... C4 ATTACHMENT E DETAILED HYDROGRAPH DATA .................................................................................. C5 ATTACHMENT F EMERGENCY OVERFLOW CAPACITY CHECK ........................................................... C6 ATTACHMENT G OVERALL BASIN MAP ..................................................................................................... C7 ATTACHMENT H SSA CAPACITY CHECK ................................................................................................... C8 ATTACHMENT I SUNNYMEDE TRUNK LINE PROFILE ........................................................................... C9 ATTACHMENT J PARK TRUNK LINE PROFILE ....................................................................................... C10 ATTACHMENT K BACKWATER CALCULATION WORKSHEET ............................................................ C11 ATTACHMENT L POULSBO MEADOWS DEVELOPMENT MAP ............................................................ C12 ATTACHMENT M POND OUTLET BACKWATER CALCULATION WORKSHEETS.............................. C13 ATTACHMENT N GRAVEL FILTER WINDOW CONVEYANCE WORKSHEET ..................................... C14
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I. EXECUTIVE SUMMARY
The projects known as Mountain Aire and Poulsbo Meadows proposed a “Regional” Stormwater Detention Pond to serve both projects on an adjacent property known as the Wyatt Parcel. The projects are in various stages of development with Mountain Aire Division 1 constructed as a Final Plat, Mountain Aire Division 2 is under construction, and Poulsbo Meadows is currently being reviewed by the City for Preliminary Plat approval.
The “Final Storm Drainage Report” for the overall “Mountain Aire & Poulsbo Meadows Preliminary Plat” Stormwater Management design, dated June 3, 2013, included the approved design that was constructed in 2015. Portions of this report are identified herein as the “Original” report.
Subsequent to completion of construction of Division 1 of Mountain Aire, and during review of Poulsbo Meadows Preliminary Plat, City staff raised concerns about Stormwater conveyance capacity for the approved design.
The City notified Team 4 Engineering of the concern. This report presents the results of the analysis performed in response to the concerns.
The analysis included the following elements;
Reviewed the impervious surfaces areas allowed for each development. The developer of the Mountain Aire project reviewed the completed construction and remaining home construction and provided accurate values for total impervious surfaces within the project. The City requested that the values provided by the developer be increased by 200 square feet per lot. This revision increased the amount of impervious surface in the Mountain Aire project to an amount larger than the approved design. The projected impervious surface amounts for Poulsbo Meadows was not changed.
Upstream runoff tributary to each development was reviewed. Upstream areas tributary to Poulsbo Meadows were removed from the analysis by provision of an upstream bypass system within Poulsbo Meadows.
Stormwater flow rates were calculated using “Storm and Sanitary Analysis 2015” (SSA), a product of Autodesk. Conveyance capacity of the system was analyzed using the “Backwater Analysis” method as defined by the Kitsap County Stormwater Management Manual. Analysis of the As-Built Stormwater Detention Pond was preformed using StormSHEDTM (Release 6.1.6.8) by Engenious Systems, Inc., which uses the SCS-based Santa Barbara Urban Hydrograph (SBUH) method with Type 1A, 24-hour rainfall distribution.
The analysis revealed that corrections are required to address deficiencies in the original design. The corrections are summarized as follows:
Some storm drains segments conveying storm flows to the detention pond are undersized for the anticipated flow rates to be expected with the larger storm events.
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The increase in impervious surface area per lot in the Mountain Aire development requested by the City requires a revision to the size of the orifice in the control structure in the Mountain Aire Stormwater Detention Pond.
Some segments of the Stormwater outfall pipes are undersized for the anticipated flow rates to be expected from Stormwater discharges from the detention pond associated with larger storm events
Construction Plan Addendum plan sheets to address these revisions will be prepared and submitted to the City for review and approval prior to field corrective measures being implemented.
This report has been prepared to replace the original Final Storm Drainage Report. The revised report is similar to the ‘original’ report, but includes the City requested impervious area revision to Mountain Aire basins, tables, software references, etc. Some sections have been left in their original state with no changes made (noted as “original” in the section heading).
II. LOCATION
This report provides an analysis of a detention pond/wetpond water quantity/quality management facility designed to accommodate Stormwater drainage from the proposed Mountain Aire Planned Residential Development (PRD)/Plat and the proposed Poulsbo Meadows Planned Residential Development (PRD)/Plat. The proposed development sites are located in the northeast quarter of Section 24, Township 26 North, Range 1 East, W.M., in Kitsap County, Washington. The Mountain Aire site is situate on the east side of the intersection of Noll Road NE and NE Hostmark Street in Poulsbo, Washington. The Poulsbo Meadows site is located on the south side of Noll Road and west side of Tallagson Road NE. (Figure 1 – Vicinity Map).
III. DESCRIPTION OF PROPOSED DEVELOPMENTS
The 29.75-acre Mountain Aire site is zoned Low Density Residential (4 to 5 dwelling units per acre). The site contains four tax-parcels (242601-1-015-2008, 242601-1-016-2007, 242601-1-043-2004, and 242601-1-044-2003) and has buildable areas throughout the entire site except in the southeast corner where a Category III wetland, Wetland A, (Wiltermood & Associates) and associated 80-foot buffer are located. A man-made, Category IV wetland, Wetland B, is located in the northeast corner of site (Wiltermood & Associates). A permit was obtained from the US Army Corps of Engineers to fill this small, man-made wetland. (Reference #NWS-2009-547) and the work was done during the construction of Division one of the plat.
The Mountain Aire proposal is a PRD/Plat consisting of 145 single-family residences. All buildings and pavement will be constructed outside of the required 80-foot Wetland A buffer plus 15-foot impervious surface setback. In addition, Lemolo Creek (Type IV per WDNR) is located east of the project boundary and is subject to a 100-foot buffer and 25-foot setback. All building will be outside of the Lemolo Creek buffer and setback. Access for the project is from two entry points off of Noll Road NE. Other improvements include water service for domestic and fire flow, sanitary sewer service, miscellaneous utilities, and a storm drainage collection and conveyance system. Water for domestic use is from City of Poulsbo mains located in the right of way of Noll Road NE. Sanitary sewer service is provided by a gravity sewer system extending south through to the Blue Heron project and then west to a City of
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Poulsbo sanitary sewer manhole in the Plat of Deer Run. The project will have open space areas consisting of amenities such as a walking trail, a children’s playground, an open play area, and landscaping. The storm drainage collection and conveyance system includes a detention pond for water quantity mitigation and a wetpond for water quality enhancement to be located south of the project.
Note that the project site is located within a Critical Aquifer Recharge Area of Concern, and the City has required that a hydrogeological report be prepared for the project. Infiltration and low impact development best management practices are required where soils permit per the City’s Critical Areas Ordinance. Neither is proposed for this project based on the results of the hydrogeological report which indicates a limited capacity of the near surface soils for infiltration (ESC, February 2008).
The 9.18-acre Poulsbo Meadows site is zoned Low Density Residential (4 to 5 dwelling units per acre). The site contains one tax-parcel (242601-1-033-2006) and has buildable areas throughout the entire site.
The Poulsbo Meadows proposal is to construct a Planned Residential Development/Plat (PRD)/Plat consisting of 46 single-family, detached residences. Lemolo Creek (Type V) is located east of the project boundary within Kitsap County and all buildings and pavement will be constructed outside of the required 75-foot Lemolo Creek buffer plus 25-foot setback. Access for the project will be off of Noll Road NE approximately 250 feet west of its intersection with Tallagson Lane NE. Other improvements include water service for domestic and fire flow, sanitary sewer service, miscellaneous utilities, and a storm drainage conveyance system. Water for domestic use will be from City of Poulsbo mains located in the right of way of Noll Road NE. Sanitary sewer service is proposed to be through a comprehensive gravity sewer system extending south through the Mountain Aire and Blue Heron projects and then west to a City of Poulsbo sanitary sewer manhole in the Plat of Deer Run. The project will have open space areas that are connected by a sidewalk system and consist of open play and seating areas, picnic areas, and a gazebo. Stormwater will be conveyed south to a Stormwater detention pond and wetpond located on the Mountain Aire PRD/Plat (Wyatt). The Wyatt pond and wetpond are designed to accommodate the Stormwater from both the Mountain Aire and Poulsbo Meadows projects. Clean Stormwater will be used to recharge Wetland A located on the Mountain Aire PRD/Plat site.
Please note that the Poulsbo Meadows project site is located within the City’s Aquifer Recharge Area of Concern (ARAC). The City requires projects within ARACs to utilize Stormwater infiltration where soils permit. Envirosound Consulting, Inc. conducted a geotechnical engineering infiltration study for the project (February 2008). Low impact development and infiltration are not proposed for this project based on the results of the study which indicates a limited capacity of the near surface soils for infiltration.
IV. DESCRIPTION OF PROPOSED WATER QUALITY AND QUANTITY MANAGEMENT FACILITIES
The detention pond and wetpond are located on the adjacent parcel directly south of Mountain Aire (Wyatt). The Mountain Aire site receives offsite runoff from an area north of the site. This offsite runoff will be directed to the proposed detention facility. The following summarizes the sub-basins used in the Stormwater conveyance analysis. (See revised basin map noted below.) The assumed 50% coverage per lot in the original Stormwater report for the Mountain Aire project has been revised per the developer to actual lot coverages of 3,330ft2 on 16 of the lots and the remaining 129 lots having an impervious lot coverage of 2,873ft2 for the conveyance analysis. (See revised basin area worksheet noted below.)
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1. MTA-UPSTREAM - Existing basin for the offsite runoff located north of the Mountain Aire project site (7.54 ac).
2. PM-UPSTREAM – Existing basin for the offsite runoff from the area west of the site (0.0 ac).
3. PM – The Poulsbo Meadows project site (9.18ac).
4. Basin 1-1 – Mountain Aire Lots 60 - 69; 128-138; and Tract C (4.54 ac).
9. Basin 3-2 – Mountain Aire Lots 9-11; 46-49; and approximately 80% of Tract F (2.17 ac).
10. Basin 4 – Mountain Aire Lots 12-45; 120-127; and half width of Noll Road adjacent to the property (8.46 ac).
(Attachment A – Project Area Worksheet – Mountain Aire)
(Attachment B – Project Area Worksheet – Poulsbo Meadows)
(Attachment G – Overall Basin Map)
V. PRE-DEVELOPMENT SITE CONDITIONS
The undeveloped site (Figure 2A – Predeveloped Site Conditions) contains established second growth forest, various homes and outbuildings, pastures, and lawns and landscaping. The declivity of the site is from northwest to the wetland in the southeast. The terrain ranges between an 8 to 10 percent slope across the site except for in the northeast corner where the terrain is somewhat shallower than the rest of the site at a 5 percent slope. Isolated pockets of terrain with 15 to 20 percent slopes exist in the western half of the site. Stormwater tends to converge towards the southeast corner of the site.
The site supports a mixed plant community with a variety of coniferous and deciduous trees and groundcovers including shrubs, grasses and other plants. Trees and plants observed include Douglas fir, western hemlock, western white pine, Pacific madrone, red elderberry, holly, Pacific willow, western red cedar, big leaf maple, red alder, bitter cherry, cascara, salmonberry, Himalayan blackberry, Indian plum, Oregon grape, and a variety of ferns and other plants.
The soils on site are listed in the “Soil Survey of Kitsap County Area, Washington” prepared by the United States Department of Agriculture, Soil Conservation Service as (Figure 3A – Soils Map – Mountain Aire):
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MTA (Onsite): 90% #39 Poulsbo Gravelly Sandy Loam, 0-6% Slopes #40 Poulsbo Gravelly Sandy Loam, 6-15% Slopes Hydrologic Group "C" 10% #22 Kapowsin Gravelly Loam, 0-6% Slopes Hydrologic Group "D"
The Poulsbo soil is included in Hydrologic Group “C” and the Kapowsin soil is included in Hydrologic Group “D” (Exhibit 1 – Hydrologic Soil Groups).
The undeveloped site (Figure 2B – Predeveloped Site Conditions) contains established second growth forest and pasture areas. The declivity of the site is from northwest to southeast. The terrain ranges between 5 to 11 percent slopes across the site.
The site supports a mixed plant community with a variety of coniferous and deciduous trees and groundcovers including shrubs, grasses and other plants.
The soils on site are listed in the “Soil Survey of Kitsap County Area, Washington” prepared by the United States Department of Agriculture, Soil Conservation Service as (Figure 3B – Soils Map – Poulsbo Meadows):
The Poulsbo soil is included in Hydrologic Group “C” and the Kapowsin soil is included in Hydrologic Group “D” (Exhibit 1 – Hydrologic Soil Groups).
VI. UPSTREAM ANALYSIS
Mountain Aire (MTA): A 7.54-acre area to the north and west slopes toward the site and some runoff from this property could enter the site. Offsite runoff will be routed to the runoff control facility.
Per Section 5.2.3 of the Kitsap County Stormwater design manual, offsite contributing runoff may be routed through the runoff control facility if the pre-development peak 2-year and 100-year flow rates from the off-site contributing basin area do not exceed the pre-development on-site peak flow rates for the same storm events.
MTA Site PreDev 2-yr = 2.21 cfs MTA Site PreDev 100-yr = 9.14 cfs
Offsite 2-yr = 1.08 cfs Offsite 100-yr = 3.47 cfs
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Therefore, the 7.54-acre offsite area can be routed through the runoff control facility following outlet control sizing to determine stage of additional required freeboard.
Poulsbo Meadows (PM): The property to the west slopes toward the site and some runoff from this property could enter the site. The offsite runoff from this 5.94-acre basin will be bypassed to an existing culvert on Noll Road extending towards Tallagson Road NE.
PM Site PreDev 2-yr = 1.15 cfs PM Site PreDev 100-yr = 4.18 cfs
Offsite 2-yr = 0.90 cfs Offsite 100-yr = 2.62 cfs
(Figure 5B – Developed Conditions Site Map – Poulsbo Meadows)
VII. DOWNSTREAM ANALYSIS
Stormwater runoff from the developed site will be directed to a wetpond for detention and release at a predeveloped metered rate. The discharge from the detention pond will be released to wetland systems on the eastern portions of the developed site (including the Mountain Aire site and “Wyatt” parcel) via dispersion trench facilities. Flows will continue easterly approximately 75-feet offsite and then enter Lemolo Creek, which flows south approximately 1.5 miles where it enters Liberty Bay.
An emergency overflow weir is located in the southwest corner of the pond. If the weir is breached in an overflow condition, Stormwater will discharge from the pond in this location. The downstream flow path from this outlet weir generally flows in a southeasterly direction. Flow will travel overland and into a manmade pond (constructed by the previous owner as a backyard amenity) located south of the detention pond. This manmade pond, will likely be full during a peak storm event, but will still provide energy dissipation. Flows leaving the manmade pond continue southeasterly on the “Wyatt” parcel, crossing a pedestrian trail, approximately 45’ north of the southern “Wyatt” property line. Flows will continue easterly approximately 75-feet offsite and then enter Lemolo Creek, which flows south approximately 1.5 miles where it enters Liberty Bay. No existing homes are in either discharge flow path.
(Figure 4 – Downstream Map)
VIII. OVERVIEW OF PROPOSED STORM WATER MANAGEMENT STRATEGIES
The Stormwater Management strategy selected for this project includes the following elements:
Basin PostDev-MTA encompasses Stormwater flows from a 29.88-acre area that will be mitigated by the Stormwater management facilities.
Basin PostDev-PM encompasses Stormwater flows from a 9.18-acre area that will be mitigated by the Stormwater management facilities.
The combined project sites will be developed to collect and route Stormwater flows to a two-celled detention pond for Stormwater quantity control. The pond design is based on Stormwater flows from all related sub-basins from the developed Mountain Aire and Poulsbo Meadows project sites.
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Additional freeboard is added to accommodate the offsite contributing basin Upstream-MTA. The SSA analysis includes the Upstream-MTA Basin rather than treating it simply as additional freeboard.
The pond includes additional volume for sediment storage and a 33.6% correction factor.
The wetpond is designed for the treatment of the mean annual storm volume per Kitsap County Stormwater Management Manual 2009, method 6.11 Wetpond sizing. (Exhibit 8 – MTAPM-Pond Wetpond Worksheet).
The constructed closed conveyance piping from the “Wyatt” pond inlet to CB#5 within Division I of the Mountain Aire subdivision must be upgraded to 36” diameter N-12 or equivalent.
Portions of the Mountain Aire – Division II will be rerouted to avoid the main trunk line north of CB#5. Several sheets within the Approved Construction Plan set must be amended to account for these revisions.
IX. TECHNICAL REQUIREMENTS
The City of Poulsbo Department of Public Works – Stormwater Division Standards in conjunction with the 1997 Kitsap County Stormwater Management Design Manual, define the technical requirements for design of the project Stormwater management facilities.
The Water Quantity Mitigation facility will be a two-celled storm detention pond designed to provide stream bank erosion control (50% of the 2-year storm) and detain 10-year and 100-year, 24-hour, Type 1A storm to predeveloped flow rates. The detention volume as calculated by analysis will be increased by the prescribed correction factor from the Manual (Attachment C – Pond Correction Factor Worksheet).
After the detention pond has been sized for onsite flows, offsite contributing flows will be added and routed through the detention pond as designed. The freeboard of the detention pond will be increased to account for the additional storage from the offsite contributing flows.
The Water Quality Mitigation facility will be a wetpond designed to treat the mean annual storm event from the onsite basins.
The pond inlet conveyance systems will be analyzed for backwater and surcharge conditions for the 100-yr storm event (Attachment H – SSA Capacity Check).
The pond outlet conveyance systems will be analyzed for backwater and surcharge conditions for the 100-yr storm event (Attachment M – Pond Outlet Backwater Calculation Worksheets).
X. HYDROLOGIC ANALYSIS
Runoff computations utilize the SCS-based hydrograph method, Santa Barbara Urban Hydrograph (SBUH) method with Type 1A, 24-hour rainfall distribution.
The hydrological analysis for pond sizing was performed using StormSHEDTM (Release 6.1.6.8) by Engenious Systems, Inc. ***Some of the sections 1-13 below are retained from the Original design (noted as “original”), unless noted otherwise (noted as “updated”). ***
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*** The hydrological analysis for conveyance analysis was performed using Autodesk Storm and Sanitary Analysis 2015 v.9.1.140.1 and Kitsap County Stormwater Design Manual Figures 4.9 and 4.10. New sections have been added that incorporate summaries of the analysis provided by the SSA design. ***
Basin PreDev-MTA parameters are presented below. This information is obtained from the history file for the project from the StormShedTM software and is used in the analysis. PreDev-MTA Event Summary: BasinID Peak Q Peak T Peak Vol Area Method Raintype Event ------- (cfs) (hrs) (ac-ft) ac /Loss PreDev-MTA 2.21 8.67 2.0601 29.87 SBUH/SCS TYPE1A 2 yr PreDev-MTA 4.26 8.67 3.4495 29.87 SBUH/SCS TYPE1A 10 yr PreDev-MTA 9.14 8.50 6.5692 29.87 SBUH/SCS TYPE1A 100 yr
Drainage Area: PreDev-MTA Hyd Method: SBUH Hyd Loss Method: SCS CN Number Peak Factor: 484.00 SCS Abs: 0.20 Storm Dur: 24.00 hrs Intv: 10.00 min Area CN TC Pervious 29.0700 ac 81.53 1.13 hrs Impervious 0.8000 ac 98.00 0.08 hrs Total 29.8700 ac Supporting Data: Pervious CN Data: MTA - Est. Second Growth Forest 78.50 14.6400 ac MTA - Ex Pasture/Landscaping 85.40 12.2600 ac Wyatt - Est. Second Growth Forest 78.50 1.6700 ac Wyatt - Ex Pasture/Landscaping 85.40 0.5000 ac Impervious CN Data:
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MTA - Ex Homes/Asphalt/Concrete 98.00 0.5000 ac Wyatt - Ex Homes/Asphalt/Concrete 98.00 0.3000 ac Pervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Sheet Forest w/Heavy Ground Litter & Meadows 300.00 ft 4.04% 0.4000 46.55 min Shallow Forest w/Heavy Ground Litter & Meadows 17.13 ft 4.01% 3.0000 0.48 min Shallow Forest w/Heavy Ground Litter & Meadows 200.00 ft 6.68% 3.0000 4.30 min Shallow Forest w/Heavy Ground Litter & Meadows 176.00 ft 4.91% 3.0000 4.41 min Shallow Short Grass, Pasture & Lawns 260.00 ft 6.91% 11.0000 1.50 min Shallow Forest w/Heavy Ground Litter & Meadows 419.00 ft 9.13% 3.0000 7.70 min Shallow Forest w/Heavy Ground Litter & Meadows 164.00 ft 8.74% 3.0000 3.08 min Impervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 5.0000 5.00 min
2. PRE-DEVELOPED CONDITIONS: PreDev-PM (original)
Basin PreDev-PM parameters are presented below. This information is obtained from the history file for the project from the StormShedTM software and is used in the analysis. PreDev-PM Event Summary: BasinID Peak Q Peak T Peak Vol Area Method Raintype Event ------- (cfs) (hrs) (ac-ft) ac /Loss PreDev-PM 1.15 8.17 0.7362 9.18 SBUH/SCS TYPE1A 2 yr PreDev-PM 2.08 8.17 1.1938 9.18 SBUH/SCS TYPE1A 10 yr PreDev-PM 4.18 8.17 2.1961 9.18 SBUH/SCS TYPE1A 100 yr
Drainage Area: PreDev-PM Hyd Method: SBUH Hyd Loss Method: SCS CN Number Peak Factor: 484.00 SCS Abs: 0.20 Storm Dur: 24.00 hrs Intv: 10.00 min Area CN TC Pervious 9.0200 ac 84.32 0.60 hrs Impervious 0.1600 ac 98.00 0.08 hrs Total 9.1800 ac Supporting Data: Pervious CN Data: PM - Est. Second Growth Forest79.70 2.8000 ac PM - Ex Pasture/Landscaping 86.40 6.2200 ac Impervious CN Data: PM - Ex Homes/Asphalt/Concrete 98.00 0.1600 ac Pervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Sheet Forest w/Heavy Ground Litter & Meadows 300.00 ft 11.00% 0.4000 31.19 min Shallow Lawn & Landscaping 246.00 ft 5.30% 11.0000 1.62 min Shallow Forest w/Heavy Ground Litter & Meadows 76.00 ft 7.90% 3.0000 1.50 min Shallow Short Grass, Pasture & Lawns 76.00 ft 7.90% 11.0000 0.41 min Shallow Forest w/Heavy Ground Litter & Meadows 83.00 ft 11.00% 3.0000 1.39 min Impervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 5.0000 5.00 min
Basin PostDev-MTA parameters are presented below. This information is obtained from the history file for the project from the StormShedTM software and is used in the analysis. PostDev-MTA Event Summary: BasinID Peak Q Peak T Peak Vol Area Method Raintype Event ------- (cfs) (hrs) (ac-ft) ac /Loss PostDev-MTA 6.23 8.00 2.2514 29.88 SBUH/SCS TYPE1A 6 mo PostDev-MTA 11.38 8.00 4.0295 29.88 SBUH/SCS TYPE1A 2 yr PostDev-MTA 16.38 8.00 5.7574 29.88 SBUH/SCS TYPE1A 10 yr PostDev-MTA 26.68 8.00 9.3221 29.88 SBUH/SCS TYPE1A 100 yr
Drainage Area: PostDev-MTA Hyd Method: SBUH Hyd Loss Method: SCS CN Number Peak Factor: 484.00 SCS Abs: 0.20 Storm Dur: 24.00 hrs Intv: 10.00 min Area CN TC Pervious 12.3200 ac 86.00 0.17 hrs Impervious 17.5600 ac 98.00 0.08 hrs Total 29.8800 ac Supporting Data: Pervious CN Data: MTA - Lot - Lawn & Landscaping 86.00 7.1000 ac MTA - ROW - Lawn & Landscaping 86.00 0.9400 ac MTA - Tracts A-D, F-J - Lawn & Landscaping 86.00 0.8800 ac MTA - Tract E&K - Lawn & Landscaping 86.00 0.0200 ac MTA - Tract H&I - Designated Park 86.00 2.2100 ac Wyatt - Lot - Lawn & Landscaping 86.00 1.1700 ac Impervious CN Data: MTA - Lot - Roofs/Drives 98.00 9.9700 ac MTA - ROW - Drives/Sidewalks 98.00 6.0100 ac MTA - Tract E&K 98.00 0.1200 ac MTA - Tract H&I - Designated Park 98.00 0.1600 ac Wyatt - Lot Improvements 98.00 0.3000 ac Wyatt - Storm Pond Surface 98.00 1.0000 ac Pervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 10.0000 10.00 min Impervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 5.0000 5.00 min
Basin PostDev-MTA parameters are presented below. This information is obtained from the history file for the project from the StormShedTM software and is used in the analysis. Additional 200 SF per lot is included in this developed basin. This updated basin is used in place of the original in all design and analysis of this revised report. PostDev-MTA Event Summary: BasinID Peak Q Peak T Peak Vol Area Method Raintype Event ------- (cfs) (hrs) (ac-ft) ac /Loss PostDev-MTA 6.20 8.00 2.2411 29.86 SBUH/SCS TYPE1A 6 mo
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PostDev-MTA 11.33 8.00 4.0155 29.86 SBUH/SCS TYPE1A 2 yr PostDev-MTA 16.33 8.00 5.7408 29.86 SBUH/SCS TYPE1A 10 yr PostDev-MTA 26.62 8.00 9.3014 29.86 SBUH/SCS TYPE1A 100 yr
Drainage Area: PostDev-MTA Hyd Method: SBUH Hyd Loss Method: SCS CN Number Peak Factor: 484.00 SCS Abs: 0.20 Storm Dur: 24.00 hrs Intv: 10.00 min Area CN TC Pervious 12.4500 ac 86.00 0.17 hrs Impervious 17.4100 ac 98.00 0.08 hrs Total 29.8600 ac Supporting Data: Pervious CN Data: MTA - Lot - Lawn & Landscaping 86.00 7.2200 ac MTA - ROW - Lawn & Landscaping 86.00 0.6800 ac MTA - Tracts - Lawn & Landscaping 86.00 0.3600 ac MTA - Tracts - Designated Park 86.00 3.0200 ac Wyatt - Lot - Lawn & Landscaping 86.00 1.1700 ac Impervious CN Data: MTA - Lot - Roofs/Drives 98.00 9.7300 ac MTA - ROW - Drives/Sidewalks 98.00 6.1500 ac MTA - Tracts 98.00 0.0700 ac MTA - Tracts - Designated Park 98.00 0.1600 ac Wyatt - Lot Improvements 98.00 0.3000 ac Wyatt - Storm Pond Surface 98.00 1.0000 ac Pervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 10.0000 10.00 min Impervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 5.0000 5.00 min
Basin PostDev-PM parameters are presented below. This information is obtained from the history file for the project from the StormShedTM software and is used in the analysis. The “Closed Conveyance Tie” shown in the Tc routing accounts for the conveyance of the Stormwater from this project area to the storm pond. PostDev-PM Event Summary: BasinID Peak Q Peak T Peak Vol Area Method Raintype Event ------- (cfs) (hrs) (ac-ft) ac /Loss PostDev-PM 1.77 8.00 0.7087 9.18 SBUH/SCS TYPE1A 6 mo PostDev-PM 3.20 8.00 1.2596 9.18 SBUH/SCS TYPE1A 2 yr PostDev-PM 4.58 8.00 1.7933 9.18 SBUH/SCS TYPE1A 10 yr PostDev-PM 7.43 8.00 2.8920 9.18 SBUH/SCS TYPE1A 100 yr
Drainage Area: PostDev-PM Hyd Method: SBUH Hyd Loss Method: SCS CN Number Peak Factor: 484.00 SCS Abs: 0.20 Storm Dur: 24.00 hrs Intv: 10.00 min Area CN TC Pervious 3.5200 ac 86.00 0.30 hrs Impervious 5.6600 ac 98.00 0.21 hrs
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Total 9.1800 ac Supporting Data: Pervious CN Data: PM - Lot - Lawn & Landscaping 86.00 2.1900 ac PM - ROW- Lawn & Landscaping 86.00 0.0000 ac PM - Tracts A & B 86.00 1.0500 ac PM - Tracts C - E 86.00 0.2800 ac Impervious CN Data: PM - Lot - Roofs/Drives 98.00 3.0400 ac PM - ROW - Drives/Sidewalks 98.00 2.1100 ac PM - Private Road Tract 98.00 0.4500 ac PM - Tracts A & B 98.00 0.0600 ac PM - Tracts C - E 98.00 0.0000 ac Pervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 10.0000 10.00 min Channel Closed Conveyance Tie 1400.00 ft 0.50% 42.0000 7.86 min Impervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 5.0000 5.00 min Channel Closed Conveyance Tie 1400.00 ft 0.50% 42.0000 7.86 min
6. BASIN: Upstream-MTA (updated)
Basin “Upstream-MTA” parameters are presented below. This information is obtained from the history file for the project from the StormShedTM software and is used in the analysis. This is the offsite upstream area that contributes flow to the onsite basin. This runoff will be routed through the storm pond following its design. Additional freeboard will be added to the ponding depth to account for this flow volume. Upstream-MTA Event Summary: BasinID Peak Q Peak T Peak Vol Area Method Raintype Event ------- (cfs) (hrs) (ac-ft) ac /Loss Upstream-MTA 1.11 8.00 0.6328 7.54 SBUH/SCS TYPE1A 2 yr Upstream-MTA 1.87 8.00 1.0021 7.54 SBUH/SCS TYPE1A 10 yr Upstream-MTA 3.61 8.00 1.8138 7.54 SBUH/SCS TYPE1A 100 yr
Drainage Area: Upstream-MTA Hyd Method: SBUH Hyd Loss Method: SCS CN Number Peak Factor: 484.00 SCS Abs: 0.20 Storm Dur: 24.00 hrs Intv: 10.00 min Area CN TC Pervious 6.2900 ac 81.74 0.58 hrs Impervious 1.2500 ac 98.00 0.17 hrs Total 7.5400 ac Supporting Data: Pervious CN Data: N - Est. Second Growth Forest 78.50 2.0300 ac N - Ex Pasture/Landscaping 86.40 2.5800 ac S - Est. Second Growth Forest 78.50 1.6800 ac Impervious CN Data: N - Ex Homes/Asphalt/Concrete 98.00 1.0000 ac S - Ex Homes/Asphalt/Concrete 98.00 0.2500 ac Pervious TC Data:
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Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 35.0000 35.00 min Impervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 10.0000 10.00 min
Basin “Upstream-PM” parameters are presented below. This information is obtained from the history file for the project from the StormShedTM software and is used in the analysis. This offsite runoff will be routed around the Poulsbo Meadows site and released at the downstream project boundary. Upstream-PM Event Summary: BasinID Peak Q Peak T Peak Vol Area Method Raintype Event ------- (cfs) (hrs) (ac-ft) ac /Loss Upstream-PM 0.90 8.00 0.5368 5.94 SBUH/SCS TYPE1A 2 yr Upstream-PM 1.43 8.00 0.8361 5.94 SBUH/SCS TYPE1A 10 yr Upstream-PM 2.62 8.00 1.4869 5.94 SBUH/SCS TYPE1A 100 yr
Drainage Area: Upstream-PM Hyd Method: SBUH Hyd Loss Method: SCS CN Number Peak Factor: 484.00 SCS Abs: 0.20 Storm Dur: 24.00 hrs Intv: 10.00 min Area CN TC Pervious 4.7400 ac 82.55 1.04 hrs Impervious 1.2000 ac 98.00 0.08 hrs Total 5.9400 ac Supporting Data: Pervious CN Data: W - Est. Second Growth Forest 79.10 2.5000 ac W - Ex Pasture/Landscaping 86.40 2.2400 ac Impervious CN Data: W - Ex Homes/Asphalt/Concrete 98.00 1.2000 ac Pervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Sheet 2nd Growth 105.08 ft 4.76% 0.4000 18.84 min Sheet 2nd Growth 82.65 ft 6.05% 0.4000 14.12 min Sheet 2nd Growth 47.92 ft 10.43% 0.4000 7.34 min Sheet 2nd Growth 64.35 ft 7.20% 0.4000 10.78 min Shallow 2nd Growth 5.14 ft 7.20% 3.0000 0.11 min Shallow 2nd Growth 321.56 ft 7.74% 3.0000 6.42 min Shallow 2nd Growth 30.08 ft 16.62% 3.0000 0.41 min Shallow 2nd Growth 37.44 ft 13.36% 3.0000 0.57 min Shallow 2nd Growth 38.93 ft 12.84% 3.0000 0.60 min Shallow 2nd Growth 55.56 ft 9.00% 3.0000 1.03 min Shallow 2nd Growth 123.15 ft 9.33% 3.0000 2.24 min Impervious TC Data: Flow type: Description: Length: Slope: Coeff: Travel Time Fixed Fixed 0.00 ft 0.00% 5.0000 5.00 min
Detailed hydrograph data can be found in Attachment E. ADDHYD [PostDev-MTA] AS [2 yr] [PostDev-PM] AS [2 yr] TO [PostDev-Both (2yr)] Peak Flow: 14.5247 cfs Peak Time: 8.00 hrs Hyd Vol: 229787.00 cf - 5.2752 acft ADDHYD [PostDev-MTA] AS [10 yr] [PostDev-PM] AS [10 yr] TO [PostDev-Both (10yr)] Peak Flow: 20.9093 cfs Peak Time: 8.00 hrs Hyd Vol: 328185.02 cf - 7.5341 acft ADDHYD [PostDev-MTA] AS [100 yr] [PostDev-PM] AS [100 yr] TO [PostDev-Both (100yr)] Peak Flow: 34.0490 cfs Peak Time: 8.00 hrs Hyd Vol: 531142.49 cf - 12.1934 acft Hydrograph ID: PostDev-Both (2yr) Area: 39.0400 ac Hyd Int: 10.00 min Base Flow: Peak Flow: 14.5247 cfs Peak Time: 8.00 hrs Hyd Vol: 5.2752 acft Hydrograph ID: PostDev-Both (10yr) Area: 39.0400 ac Hyd Int: 10.00 min Base Flow: Peak Flow: 20.9093 cfs Peak Time: 8.00 hrs Hyd Vol: 7.5341 acft Hydrograph ID: PostDev-Both (100yr) Area: 39.0400 ac Hyd Int: 10.00 min Base Flow: Peak Flow: 34.0490 cfs Peak Time: 8.00 hrs Hyd Vol: 12.1934 acft
Detailed hydrograph data can be found in Attachment E. ADDHYD [PostDev-MTA] AS [2 yr] [Upstream-MTA] AS [2 yr] TO [DevMTA + Up(2yr)] Peak Flow: 12.4345 cfs Peak Time: 8.00 hrs Hyd Vol: 202481.70 cf - 4.6483 acft ADDHYD [PostDev-MTA] AS [10 yr] [Upstream-MTA] AS [10 yr] TO [DevMTA + Up(10yr)] Peak Flow: 18.1997 cfs Peak Time: 8.00 hrs Hyd Vol: 293722.51 cf - 6.7429 acft
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ADDHYD [PostDev-MTA] AS [100 yr] [Upstream-MTA] AS [100 yr] TO [DevMTA + Up(100yr)] Peak Flow: 30.2277 cfs Peak Time: 8.00 hrs Hyd Vol: 484177.66 cf - 11.1152 acft Hydrograph ID: DevMTA + Up(2yr) Area: 37.4000 ac Hyd Int: 10.00 min Base Flow: Peak Flow: 12.4345 cfs Peak Time: 8.00 hrs Hyd Vol: 4.6483 acft Hydrograph ID: DevMTA + Up(10yr) Area: 37.4000 ac Hyd Int: 10.00 min Base Flow: Peak Flow: 18.1997 cfs Peak Time: 8.00 hrs Hyd Vol: 6.7429 acft Hydrograph ID: DevMTA + Up(100yr) Area: 37.4000 ac Hyd Int: 10.00 min Base Flow: Peak Flow: 30.2277 cfs Peak Time: 8.00 hrs Hyd Vol: 11.1152 acft
Detailed hydrograph data can be found in Attachment E. ADDHYD [DevMTA + Up(2yr)] AS [2 yr] [PostDev-PM] AS [2 yr] TO [Both + Up(2yr)] Peak Flow: 15.6304 cfs Peak Time: 8.00 hrs Hyd Vol: 257351.75 cf - 5.9080 acft ADDHYD [DevMTA + Up(10yr)] AS [10 yr] [PostDev-PM] AS [10 yr] TO [Both + Up(10yr)] Peak Flow: 22.7814 cfs Peak Time: 8.00 hrs Hyd Vol: 371838.47 cf - 8.5362 acft ADDHYD [DevMTA + Up(100yr)] AS [100 yr] [PostDev-PM] AS [100 yr] TO [Both + Up(100yr)] Peak Flow: 37.6565 cfs Peak Time: 8.00 hrs Hyd Vol: 610152.11 cf - 14.0072 acft Hydrograph ID: Both + Up(2yr) Area: 46.5800 ac Hyd Int: 10.00 min Base Flow: Peak Flow: 15.6304 cfs Peak Time: 8.00 hrs Hyd Vol: 5.9080 acft Hydrograph ID: Both + Up(10yr) Area: 46.5800 ac Hyd Int: 10.00 min Base Flow: Peak Flow: 22.7814 cfs Peak Time: 8.00 hrs Hyd Vol: 8.5362 acft Hydrograph ID: Both + Up(100yr) Area: 46.5800 ac Hyd Int: 10.00 min Base Flow: Peak Flow: 37.6565 cfs Peak Time: 8.00 hrs Hyd Vol: 14.0072 acft
12. STORMWATER QUANTITY CONTROL FACILITY (updated)
The Stormwater quantity control proposed for this project is a two-celled detention pond located on the Wyatt parcel. This will provide detention for flows from Basins PostDev-MTA, PostDev-PM and associated offsite. In the inflow cell of the pond, an additional 0.5 feet of depth will be added to allow sedimentation. Inflow and outflow from the pond are from opposite cells. The pond will contain an orifice control structure with a riser, which will release Stormwater at metered rates to mimic predeveloped conditions and meet all design criteria of the City of Poulsbo Public Works – Engineering Division.
The allowable discharge from the site at the point of analysis is 50% of the 2-year predeveloped flow, and matched predeveloped rates for the 10- and 100-yr storms. To match predeveloped runoff rates, as calculated above, the predeveloped discharge from basin PreDev-Both (PreDev-MTA plus PreDev-PM) is 3.27 cfs and results in an allowable discharge rate for the 2-year storm from the detention pond of
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1.64 cfs. Allowable release rates for the 10-year and 100-year storm are 6.22 cfs and 13.28 cfs, respectively.
An analysis of the As-Built pond with updated basin information is presented. The following data from the History File of StormSHEDTM for this project presents the parameters and results of the analysis: Running Y:\StormShed\RLP-Both Report.pgm on Wednesday, December 28, 2016 HYDLIST SUMMARY [PreDev-Both (2yr)] [PreDev-Both (10yr)] [PreDev-Both (100yr)] [PostDev-Both (2yr)] [PostDev-Both (10yr)] [PostDev-Both (100yr)] [Out2yrBoth] [Out10yrBoth] [Out100yrBoth] LSTEND HydID Peak Q Peak T Peak Vol Cont Area ------- (cfs) (hrs) (ac-ft) (ac) PreDev-Both (2yr) 3.27 8.50 2.7963 39.0500 PreDev-Both (10yr) 6.22 8.17 4.6433 39.0500 PreDev-Both (100yr) 13.28 8.17 8.7653 39.0500 PostDev-Both (2yr) 14.52 8.00 5.2752 39.0400 PostDev-Both (10yr) 20.91 8.00 7.5341 39.0400 PostDev-Both (100yr) 34.05 8.00 12.1934 39.0400 Out2yrBoth 1.64 24.00 5.2741 39.0400 Out10yrBoth 4.05 12.33 7.5322 39.0400 Out100yrBoth 13.28 9.00 12.1890 39.0400
STORLIST [Pond-Both] LSTEND Node ID: Pond-Both Desc: Manhole structure Start El: 236.0000 ft Max El: 241.4500 ft Contrib Basin: Contrib Hyd: Stage Area Volume Volume 236.00 34504.00 0.00 cf 0.0000 acft 238.00 39707.00 74211.00 cf 1.7037 acft 240.00 45691.00 159609.00 cf 3.6641 acft 241.45 49618.00 228708.02 cf 5.2504 acft DISCHLIST [combo-Both] [morif-Both] [rweir-Both] LSTEND Control Structure ID: combo-Both - Combination Control Structure Descrip: Multiple Orifice Start El Max El Increment 236.0000 ft 242.0000 ft 0.10 ID List: morif-Both rweir-Both Control Structure ID: morif-Both - Multiple Orifice Structure Descrip: Multiple Orifice Start El Max El Increment
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236.0000 ft 242.0000 ft 0.10 Orif Coeff: 0.62 Bottom El: 234.00 ft Lowest Diam: 5.7642 in Control Structure ID: rweir-Both - Rectangular weir Descrip: Rectangular Weir Start El Max El Increment 239.2900 ft 242.0000 ft 0.10 Length: 3.7300 ft RLPCOMPUTE [RLP-Both] SUMMARY 2 yr Match Q: 1.6352 cfs Peak Out Q: 1.6360 cfs - Peak Stg: 239.29 ft - Active Vol: 2.9691 acft 10 yr Match Q: 6.2214 cfs Peak Out Q: 4.0466 cfs - Peak Stg: 239.62 ft - Active Vol: 3.2945 acft 100 yr Match Q: 13.2822 cfs Peak Out Q: 13.2812 cfs - Peak Stg: 240.26 ft - Active Vol: 3.9470 acft
The control structure as described above is an orifice structure with a circular orifice for the 2-year storm and a rectangular weir on a riser for the 10-year and 100-year storms. The placement of the circular orifice for the 2-year storm is 2 feet below the outlet elevation. The detailed information is as follows:
The “updated” design analysis used a stage-storage defined pond for simulated calculations. The active storage volume requirements prior to application of the correction factor are 2.97 ac-ft for the 2-yr, 3.30 ac-ft for the 10-yr, and 3.95 ac-ft for the 100-yr storm.
The Manual requires a correction factor be applied to the storage required by calculation. The area within the storm basin has 59.1% impervious surface ground cover resulting in a correction factor of 33.6% (See Exhibit 7 – Volume Correction Factor).
With a correction factor of 33.6%, the volume of the pond was increased. This is accomplished by increasing the pond volume by 33.6 percent. The volume of the design pond as shown on the plans was calculated using average areas of contours with the aid of an Excel spreadsheet (Attachments C & D).
After the pond has been sized and volume corrected, the offsite upstream basins was added to onsite developed basin Both+Up (This basin is a combination of Hydrographs added together, including PostDev-MTA, PostDev-PM and their associated upstream basins.) The combined basins will be routed through the detention pond and outlet control as designed. The addition of the offsite basin area will cause the level in the pond to increase over the corrected peak stage, which may require more freeboard. After routing the upstream offsite through the detention pond, the 100-year storm level reached a stage of 240.15, which is lower than the design peak stage of 240.26. (Pending further analysis, which may alter the peak stage. See Backwater analysis below in sections 14, 15 & 16.) Running Y:\StormShed\RLP-BothUP Report.pgm on Wednesday, December 28, 2016 HYDLIST SUMMARY [PreDev-Both (2yr)] [PreDev-Both (10yr)] [PreDev-Both (100yr)] [Both + Up(2yr)] [Both + Up(10yr)] [Both + Up(100yr)] [Out2yrBothUP] [Out10yrBothUP] [Out100yrBothUP]
STORLIST [Pond-BothCF] LSTEND Node ID: Pond-BothCF Desc: Manhole structure Start El: 236.0000 ft Max El: 241.4500 ft Contrib Basin: Contrib Hyd: Stage Area Volume Volume 236.00 45417.00 0.00 cf 0.0000 acft 238.00 52266.00 97683.00 cf 2.2425 acft 240.00 60142.00 210091.00 cf 4.8230 acft 241.45 65311.00 301044.42 cf 6.9110 acft DISCHLIST [combo-Both] [morif-Both] [rweir-Both] LSTEND Control Structure ID: combo-Both - Combination Control Structure Descrip: Multiple Orifice Start El Max El Increment 236.0000 ft 242.0000 ft 0.10 ID List: morif-Both rweir-Both Control Structure ID: morif-Both - Multiple Orifice Structure Descrip: Multiple Orifice Start El Max El Increment 236.0000 ft 242.0000 ft 0.10 Orif Coeff: 0.62 Bottom El: 234.00 ft Lowest Diam: 5.7642 in Control Structure ID: rweir-Both - Rectangular weir Descrip: Rectangular Weir Start El Max El Increment 239.2900 ft 242.0000 ft 0.10 Length: 3.7300 ft RLPCOMPUTE [RLP-BothUP] SUMMARY 2 yr Match Q: 1.6352 cfs Peak Out Q: 1.5967 cfs - Peak Stg: 239.14 ft - Active Vol: 3.7084 acft 10 yr Match Q: 6.2214 cfs Peak Out Q: 3.9842 cfs - Peak Stg: 239.62 ft - Active Vol: 4.3290 acft 100 yr Match Q: 13.2822 cfs Peak Out Q: 11.4222 cfs - Peak Stg: 240.15 ft - Active Vol: 5.0399 acft
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The control structure as described above is an orifice structure with a circular orifice for the 2-year storm and a rectangular weir on a riser for the 10-year and 100-year storms. The placement of the circular orifice for the 2-year storm is 2 feet below the outlet elevation. The detailed information is as follows:
The selected Stormwater quality enhancement method is a wetpond. The wetpond is designed for the treatment of the mean annual storm volume per Kitsap County Stormwater Management Manual 2009, method 6.11 Wetpond sizing. (Exhibit 8 – MTAPM-Pond Wetpond Worksheet) The wetpond as shown on the plans meets these requirements.
*** SPECIAL NOTE *** Sections 14, 15 & 16 utilized a heavily iterative process. Portions of this analysis can seem out of order and must be reviewed as a whole.
14. STORMWATER “SSA” ANALYSIS (POND INLET SIDE)
The purpose of the Stormwater analysis utilizing the Autodesk Storm and Sanitary Analysis 2015 v.9.1.140.1 (SSA) was to perform a capacity check on the existing storm trunk line under NE Sunnymede Street in the Mountain Aire Plat – Division 1. The conveyance is further described as being from CB#13 at the north limits of the Mountain Aire project site to the pond on the “Wyatt” parcel south of the Mountain Aire site.
This analysis found that portions of the constructed conveyance were undersized and would need to be replaced with larger diameter pipe. In order to limit the amount of pipe that would need to be upsized, portions of the interior site of Mountain Aire have been re-routed away from the main Sunnymede trunk line, to rejoin the main trunk line at CB #5. Also, the upstream for the Poulsbo Meadows site was removed from the basin analysis (this revision is reflected in the “updated” portions of the pond design in earlier sections for overall design and analysis consistency). This upstream will be routed through the Poulsbo Meadows site and discharged at the downstream project margin rather than be routed through the “Wyatt” pond.
(Attachment L – Poulsbo Meadows Development Map)
The peak stage initially utilized to perform the SSA analysis in the pond is 240.13. This is slightly lower than the 240.22 from the original design, corrected pond peak stage. Backwater effect, analyzed in Sections 15 & 16 below, indicate a peak stage of 240.18. This was utilized as the tailwater depth for the final “Backwater” check.
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The results of the capacity check determined the storm drains from CB#5 to the pond need to be upsized to 36” diameter N-12, or equivalent. Amended Construction Plans will be submitted to the City for review to accomplish this and other revised work to affect the overall capacity design from the SSA results. Profiles of the trunk lines being analyzed for capacity are included as attachments.
The SSA analysis performed in section 14 above provided calculations for the Hydraulic Grade Line (HGL) within the inlet conveyance system. A second method of analysis was performed, called a “Backwater” analysis which also tests the conveyance capacity but is based on the tail water depth of each run.
The more conservative of both sets of analysis was used to determine the overall capacity of all tested conveyance systems.
(Attachment K – Backwater Calculation Worksheet) Note: Attachment K includes supporting documentation.
The outfall was included in the “Backwater” analysis for a capacity check. The results of this backwater check included revisions to the design outlet control. See conclusions at the end of this Section 16, below for details of the revisions to the outlet control. The discharge conveyance pipes are currently all 12” diameter. These pipes are routed to dispersion trenches which discharge to the surrounding wetland buffers. The analysis of the capacity check indicated that the storm lines discharging from the pond are undersized from the pond outlet to CB#D2. These storm drains need to be upsized to 24” diameter N-12 or equivalent. And the storm line from CB#D2 to the “North” dispersion trench needs to be upsized to 18” diameter N-12 or equivalent.
The Stormwater Detention Pond on the “Wyatt” parcel has an outlet control orifice/riser flow control device in CB#D1. The design peak allowable discharge from this facility is 13.28 cfs under the original report. The actual reported discharge is 11.42 cfs. The discharge from the pond passes through an outlet pipe to the control structure prior to passing through a flow splitter structure (CB#D2). From the flow-splitter, 12” conveyance pipes route Stormwater to dispersion trenches near an onsite wetland buffer. The discharge is meant to be directed to this buffer/wetland to maintain wetland hydrology and provide recharge to these critical areas. This document provides an analysis of the effects of the peak discharge on the conveyance and release mechanisms of the pond discharge facilities. Peak discharge will be reported as 11.41 (remaining less than the 13.28 cfs, allowable discharge) due to revisions to the outlet control, based on backwater pressure, noted below.
Control Structure: The control structure (CB#D1) includes a 5.76” diameter orifice, located in a control outlet riser tee, 2 feet below the outlet discharge IE. The effective elevation of this orifice is at the
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discharge invert elevation (~236.00). The control structure also includes a 24” diameter riser with a rectangular weir (Length of 3.73’, elevated at 239.29) and a top elevation of 240.18. This control structure will be reviewed for backwater effects below.
Conveyance Pipes: The discharge pipe from the Pond to the control Structure (CB#D1) is currently 12” diameter N-12. The conveyance pipe from CB#D1 to CB#D2 is 12” diameter N-12. The pipes from CB#D2 to the dispersion trenches are both 12” diameter N-12. The pipes from the pond to the flow splitter (CB#D2) are very flat (SL=~0.0%). The pipes from CB#D2 have slope on them. The “north” leg has SL=3.94%, the “south” leg has a SL=8.93%.
Evaluation of Conveyance Pipes: Utilizing Backwater analysis, the “flat” pipes from the pond to CB#D2 are required to be upsized to 24” diameter N-12. The 12” diameter N-12 pipe from the flow-splitter (CB#D2) to the “North” dispersion trench also needed to be upsized to 18” diameter while the pipe to the “South” dispersion pipe was found to be adequately sized using calculations in a “Backwater Analysis” spreadsheet.
Dispersion Trenches: There are two dispersion trench systems onsite. They are described as a “North” and a “South” facility and are fed by the respective north and south conveyance pipes from the CB#D2 flow splitter. The “North” dispersion trench is 2’ deep by 2’ wide by 269.3’ long. The “South” dispersion trench is 2’ deep by 2’ wide by 96.8’ long. The discharge flows are distributed through the dispersion trench elements via perforated N-12 pipes, by Prinsco. Stormwater is distributed throughout each of the dispersion trench facilities, roughly evenly, by the carrying capacity of the perforated pipe, located within the dispersion trench section. The slope of the dispersion/distribution pipe varies. The discharge from these facilities is held at a constant grade elevation at the top of the dispersion trench section, above the dispersion/distribution pipe, by a “grade board”. The elevation of the grade board is at 234.00. The backwater effect on the discharge is taken into account in the “Discharge Flow Elevation” section below.
Maintenance Standards Considerations: Volume 5 – Runoff Treatment BMPs found in the DOE Stormwater Management in Western Washington Manual includes “Maintenance Standards” in Table 4.5 for Dispersion Trenches. A key consideration included here is regarding “plugged perforations”. If over half of the perforations are found to be plugged, the facility will be required to be cleaned or replaced. We therefore assume that half of the perforations are plugged, so that normal operations can continue up to this design point.
Porosity Analysis: The dispersion trenches are gravel-filled trenches. The gravel filling the dispersion trench section is poorly-graded, clean, round gravel. The gravel is approximately 1-1/4” in size, although particle size does not affect the porosity, as long as the material is poorly-graded. The void ratio is approximately 30%. The porosity of such a material is very high. For purposes of analysis, the low end of the porosity spectrum for this material was evaluated to determine if the gravel within the dispersion section would have an effect on the Stormwater flowing through the facility. (10,000 cubic-feet per square-feet per day) The total flow capacity through this material, within the combined “north” and “south” dispersion trenches, is approximately 25 cfs. This pass-through rate is much higher than the peak discharge rate (13.28 cfs). No negative affect is anticipated.
Dispersion Trench Orifices: All Stormwater discharge must pass through the perforated dispersion/distribution pipe located within the dispersion trenches. There is a total of 366.1 lineal feet of
25
perforated pipe within the dispersion trenches. The perforated pipe has 48 perforations per lineal foot or 17,572 total perforations. Each perforation is a 0.28” diameter hole. We will assume for this analysis that half of these perforations are plugged. The total area of the free-flowing holes and corresponding elevation head required to pass Stormwater through these facilities is calculated for each storm event for each facility. The additional head required at the north facility for the 2-yr event, 10-year event and peak, 100-yr event is 0.00, 0.03 and 0.29, respectively. The additional head required at the south facility for the 2-yr event, 10-year event and peak, 100-yr event is 0.02, 0.12 and 0.66, respectively. This information is utilized to determine overall backwater effects. See below.
Discharge Flow Elevation: The peak discharge leaves the dispersion facilities as sheet flow over the grade boards (north and south). The average depth of the flow over each facility (north and south) has been calculated by treating the grade board as a “broad-crested weir”, including impacts from the orifices in the perforated dispersion trench pipes. The depth of flow at the north facility for the 2-yr event, 10-year event and peak, 100-yr event are 0.01, 0.02 and 0.04, respectively. The depth of flow at the south facility for the 2-yr event, 10-year event and peak, 100-yr event are 0.02, 0.03 and 0.05, respectively.
Backwater pressure effect on the outlet control from the pond outlet discharge conveyance must be considered. The pond discharge passes through the “design” outlet control orifice/riser and then continues in closed conveyance through a flow-splitter, through additional closed conveyance to two separate dispersion facilities. The original design assumed “free-flow” design. The backwater pressure from the outlet conveyance does not provide “free-flowing” conditions. Each element in the discharge system was analyzed for backwater effect on the outlet control and overall pond stage for each design event. The result of this was a slight modification to the design outlet control to achieve the design discharge while limiting the peak pond stage and overall volume retention of the storm pond to that necessary for Stormwater management compliance.
Backwater Conclusions: All pond discharge systems have been analyzed for capacity, etc., and with some upgrades required, the systems will handle the peak discharge flows through all required storm events, and up through all regular maintenance intervals.
(Attachment M – Pond Outlet Backwater Calculation Worksheets) Note: Attachment M includes supporting documentation.
XI. EROSION AND SEDIMENT CONTROL
A Temporary Erosion and Sedimentation Control Plan has been prepared for the project and will continue to be applied during these modifications and all future construction plans associated with these projects.
XII. OPERATION AND MAINTENANCE MANUAL
An Operation and Maintenance Manual has been previously prepared and submitted along with the “as-built” construction plans as required by ordinance. Updates to the O&M Manual will be supplied as needed.
146 909.7 0.02 Tract A - Landscaped Open Space147 8107.2 0.19 Tract B - Landscaped Open Space148 58953.8 1.35 Tract C - Desiganted Park149 2127.1 0.05 Tract D - Landscaped Open Space150 1933.9 0.04 Tract E - Landscaped Open Space151 61155.4 1.40 Tract F - Designated Park152 2737.5 0.06 Tract G - Designated Park153 2712.3 0.06 Tract H - Landscaped Open Space154 118359.0 2.72 Tract I - Designated Park155 3000.0 0.07 Tract J - Access & Utility TractTotal 259995.9 5.97
OPEN SPACE AREASSF AC Description
Tract A 909.7 0.02 Tract A - Landscaped Open SpaceTract B 8107.2 0.19 Tract B - Landscaped Open SpaceTract C 58953.8 1.35 Tract C - Desiganted ParkTract D 2127.1 0.05 Tract D - Landscaped Open SpaceTract E 1933.9 0.04 Tract E - Landscaped Open SpaceTract F 61155.4 1.40 Tract F - Designated ParkTract G 2737.5 0.06 Tract G - Designated ParkTract H 2712.3 0.06 Tract H - Landscaped Open SpaceTract I 118359.0 2.72 Tract I - Designated ParkTotal 256995.9 5.90
DESIGNATED PARK AREASF AC
Tract C 58953.8 1.35Tract F 61155.4 1.40Tract G 2737.5 0.06Tract I 15883.2 0.36Total 138729.9 3.18
WETLAND AREASSF AC Notes
WETLAND A 55179.0 1.27 Category III - Located in SE Corner of SiteWETLAND A BUFFER 47346.8 1.09 Associated Buffer to Remain Largely IntactWETLAND B 9946.9 0.23 Category IV Located in NE Corner of Site - To Be Filled and DevelopedWETLAND B BUFFER 27096.6 0.62 Associated Buffer to be Abandonded and Developed.
Note: Wetland B and Assocated Buffer Area not included in the Site Area Table.
RIGHT-OF-WAY AREASSF AC Notes
Dedicated Frontage 29715.0 0.68 Calculated Frontage Area Based on 30' Dedication Width (990.5 * 30)Onsite Roads 267892.4 6.15Total 297607.4 6.83
SITE AREA TABLESF AC Notes
Lots 738332.1 16.95 All Lot AreaMisc. Open Space 15790.2 0.36 All Tracts Designated as "Landscaped Open Space" OnlyROW 297607.4 6.83 All Right-of-Way, Including FrontageTract J 3000.0 0.07 Tract J - Access & Utility TractDesignated Park 138729.9 3.18 All Designated Park AreasWetland A & Buffer 102525.8 2.35 Category III - Located in SE Corner of SiteTotal Site 1295985.4 29.75
ONSITE AREA BREAKDOWN
SF AC SF AC Percent Impervious CoverageLots 423897.0 9.73 314435.1 7.22 57.4%Misc Open Space 0.0 0.00 15790.2 0.36 0.0%ROW 267846.7 6.15 29760.7 0.68 90.0%Tract J 3000.0 0.07 0.0 0.00 100.0%Desiganted Park 6964.0 0.16 131765.9 3.02 5.0%Wetland A & Buffer 0.0 0.00 102525.8 2.35 0.0%Totals 701707.7 16.11 594277.7 13.64 54.1%
Overall Area = 29.75 AC
PreDev - OFFSITE AREA BREAKDOWN (WYATT)IMPERVIOUS PERVIOUS
AC ACEx Homes/Asphalt/Concrete 0.30 -Ex Pasture/Landscaping - 0.50Est. Second Growth Forest - 1.67Totals 0.30 2.17
Overall Area = 2.47 AC
PostDev - OFFSITE AREA BREAKDOWN (WYATT)IMPERVIOUS PERVIOUS
OFFSITE AREA BREAKDOWN (UPSTREAM)IMPERVIOUS PERVIOUS
AC ACN - Ex Homes/Asphalt/Concrete 1.00 -N - Ex Pasture/Landscaping - 2.58N - Est. Second Growth Forest - 2.03S - Ex Homes/Asphalt/Concrete 0.25 -S - Est. Second Growth Forest - 1.68Totals 1.25 6.29
Overall Area = 7.54 AC
IMPERVIOUS PERVIOUS
STORM BASIN - PreDev*includes Storm Pond on Wyatt & excludes Wetland A & Associated Buffer
IMPERVIOUS PERVIOUSAC AC
MTA - Est. Second Growth Forest - 14.64MTA - Ex Pasture/Landscaping - 12.26MTA - Ex Homes/Asphalt/Concrete 0.50 -Wyatt - Est. Second Growth Forest - 1.67Wyatt - Ex Pasture/Landscaping - 0.50Wyatt - Ex Homes/Asphalt/Concrete 0.30 -Totals 0.80 29.07
Overall Area = 29.87 AC
STORM BASIN - PostDev*includes Storm Pond on Wyatt & excludes Wetland A & Associated Buffer
Tract A 16255.2 0.37 Designated ParkTract B 31935.9 0.73 Designated ParkTract C 3797.3 0.09 Landscaped Open SpaceTract D 6229.9 0.14 Landscaped Open SpaceTract E 2241.3 0.05 Landscaped Open SpaceTotal 60459.6 1.39
SITE AREA TABLESF AC Notes
Lots 227700.8 5.23 All Lot AreaROW 92059.1 2.11Private Road Tract 19788.5 0.45Tracts A & B 48191.1 1.11 Designated ParkTracts C-E 12268.5 0.28 All Tracts Designated as "Landscaped Open Space" OnlyTotal Site 400008.0 9.18
ONSITE AREA BREAKDOWN
SF AC SF AC Percent Impervious CoverageLots 132250.4 3.04 95450.4 2.19 58.1%ROW 92059.1 2.11 0.0 0.00 100.0%Private Road Tract 19788.5 0.45 0.0 0.00 100.0%Tracts A & B 2409.6 0.06 45781.5 1.05 5.0%Tracts C-E 0.0 0.00 12268.5 0.28 0.0%Totals 246507.5 5.66 153500.5 3.52 61.6%
Overall Area = 9.18 AC
OFFSITE AREA BREAKDOWN (UPSTREAM)IMPERVIOUS PERVIOUS
AC ACW - Ex Homes/Asphalt/Concrete 1.20 -W - Ex Pasture/Landscaping - 2.24W - Est. Second Growth Forest - 2.50Totals 1.20 4.74
Overall Area = 5.94 AC
STORM BASIN - PreDevIMPERVIOUS PERVIOUS
AC AC
PM - Est. Second Growth Forest - 2.80PM - Ex Pasture/Landscaping - 6.22PM - Ex Homes/Asphalt/Concrete 0.16 -Totals 0.16 9.02
Overall Area = 9.18 AC
STORM BASIN - PostDevIMPERVIOUS PERVIOUS
AC ACPM - Lot - Roofs/Drives 3.04 -PM - Lot - Lawn/Landscaping - 2.19PM - ROW - Drives/Sidewalks 2.11 -PM - ROW - Lawn/Landscaping - 0.00PM - Private Road Tract 0.45 -PM - Tracts A & B 0.06 1.05PM - Tracts C - E 0.00 0.28Totals 5.66 3.52
Overall Area = 9.18 AC
Percent Impervious = 61.6%
IMPERVIOUS PERVIOUS
ATTACHMENT C - POND CORRECTION FACTOR WORKSHEET
MTAPM-Pond As-Built Correction Factor Worksheet
Mountain Aire17.41 Impervious Area29.87 Basin Area
Poulsbo Meadows5.66 Impervious Area9.18 Basin Area
MTAPM-Pond 23.07 Impervious Area39.05 Basin Area
59.1% Percent Impervious
Correction Factor(0.4 * Imp%) + 0.1 * 100 = 33.6%
MTA-Pond Stage-Storage InfoStage Area Precorrected Area
Runoff (Dry Weather) Time Step .................Runoff (Wet Weather) Time Step ................Reporting Time Step ...................................Routing Time Step .......................................
Enable Overflow Ponding at Nodes .............Skip Steady State Analysis Time Periods ...
Start Analysis On .........................................End Analysis On ..........................................Start Reporting On .......................................Antecedent Dry Days ...................................
File Name ....................................................
Flow Units ....................................................Elevation Type .............................................Hydrology Method ........................................Time of Concentration (TOC) Method .........Link Routing Method ....................................
Subbasin SummarySN Subbasin Area Impervious Impervious Pervious Total Total Total Peak Time of
ID Area Area Curve Area Curve Rainfall Runoff Runoff Runoff ConcentrationNumber Number Volume
Node SummarySN Element Element Invert Ground/Rim Initial Surcharge Ponded Peak Max HGL Max Min Time of Total Total Time
ID Type Elevation (Max) Water Elevation Area Inflow Elevation Surcharge Freeboard Peak Flooded FloodedElevation Elevation Attained Depth Attained Flooding Volume
Link SummarySN Element Element From To (Outlet) Length Inlet Outlet Average Diameter or Manning's Peak Design Flow Peak Flow/ Peak Flow Peak Flow Peak Flow Total Time Reported
ID Type (Inlet) Node Invert Invert Slope Height Roughness Flow Capacity Design Flow Velocity Depth Depth/ Surcharged ConditionNode Elevation Elevation Ratio Total Depth
Area (ac) ........................................................................ 4.54Impervious Area (%) ...................................................... 58.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 86.00Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 4.54 92.96
Tc = Time of Concentration (hr) n = Manning's roughness Lf = Flow Length (ft) P = 2 yr, 24 hr Rainfall (inches) Sf = Slope (ft/ft)
Shallow Concentrated Flow Equation :
V = 16.1345 * (Sf^0.5) (unpaved surface) V = 20.3282 * (Sf^0.5) (paved surface) V = 15.0 * (Sf^0.5) (grassed waterway surface) V = 10.0 * (Sf^0.5) (nearly bare & untilled surface) V = 9.0 * (Sf^0.5) (cultivated straight rows surface) V = 7.0 * (Sf^0.5) (short grass pasture surface) V = 5.0 * (Sf^0.5) (woodland surface) V = 2.5 * (Sf^0.5) (forest w/heavy litter surface) Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hr) Lf = Flow Length (ft) V = Velocity (ft/sec) Sf = Slope (ft/ft)
Channel Flow Equation :
V = (1.49 * (R^(2/3)) * (Sf^0.5)) / n R = Aq / Wp Tc = (Lf / V) / (3600 sec/hr)
Where :
Tc = Time of Concentration (hr) Lf = Flow Length (ft) R = Hydraulic Radius (ft) Aq = Flow Area (ft²) Wp = Wetted Perimeter (ft) V = Velocity (ft/sec) Sf = Slope (ft/ft) n = Manning's roughness
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 3.69Peak Runoff (cfs) ........................................................... 3.02Weighted Curve Number ............................................... 92.96Time of Concentration (days hh:mm:ss) ........................ 0 00:30:50
Subbasin : BASIN1-1
Subbasin : BASIN1-2
Input Data
Area (ac) ........................................................................ 2.19Impervious Area (%) ...................................................... 58.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 86.00Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 2.19 92.96
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 3.71Peak Runoff (cfs) ........................................................... 1.74Weighted Curve Number ............................................... 92.96Time of Concentration (days hh:mm:ss) ........................ 0 00:16:48
Subbasin : BASIN1-2
Subbasin : BASIN2-1
Input Data
Area (ac) ........................................................................ 3.35Impervious Area (%) ...................................................... 58.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 86.00Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 3.35 92.96
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 3.71Peak Runoff (cfs) ........................................................... 2.67Weighted Curve Number ............................................... 92.96Time of Concentration (days hh:mm:ss) ........................ 0 00:16:45
Subbasin : BASIN2-1
Subbasin : BASIN2-2
Input Data
Area (ac) ........................................................................ 3.20Impervious Area (%) ...................................................... 58.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 86.00Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 3.20 92.96
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 3.70Peak Runoff (cfs) ........................................................... 2.50Weighted Curve Number ............................................... 92.96Time of Concentration (days hh:mm:ss) ........................ 0 00:17:56
Subbasin : BASIN2-2
Subbasin : BASIN3-1
Input Data
Area (ac) ........................................................................ 3.10Impervious Area (%) ...................................................... 58.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 86.00Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 3.10 92.96
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 3.70Peak Runoff (cfs) ........................................................... 2.43Weighted Curve Number ............................................... 92.96Time of Concentration (days hh:mm:ss) ........................ 0 00:17:41
Subbasin : BASIN3-1
Subbasin : BASIN3-2
Input Data
Area (ac) ........................................................................ 2.17Impervious Area (%) ...................................................... 58.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 86.00Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 2.17 92.96
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 3.70Peak Runoff (cfs) ........................................................... 1.63Weighted Curve Number ............................................... 92.96Time of Concentration (days hh:mm:ss) ........................ 0 00:20:18
Subbasin : BASIN3-2
Subbasin : BASIN4
Input Data
Area (ac) ........................................................................ 8.46Impervious Area (%) ...................................................... 58.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 86.00Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 8.46 92.96
Time of Concentration
User-Defined TOC override (minutes): 10
Subbasin Runoff Results
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 3.72Peak Runoff (cfs) ........................................................... 7.47Weighted Curve Number ............................................... 92.96Time of Concentration (days hh:mm:ss) ........................ 0 00:10:00
Subbasin : BASIN4
Subbasin : MTA-UP-NORTH
Input Data
Area (ac) ........................................................................ 5.61Impervious Area (%) ...................................................... 18.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 81.72Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 5.61 84.6504
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 2.84Peak Runoff (cfs) ........................................................... 2.36Weighted Curve Number ............................................... 84.65Time of Concentration (days hh:mm:ss) ........................ 0 00:46:09
Subbasin : MTA-UP-NORTH
Subbasin : MTA-UP-SOUTH
Input Data
Area (ac) ........................................................................ 1.93Impervious Area (%) ...................................................... 13.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 81.72Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 1.93 83.8364
Subarea Subarea SubareaShallow Concentrated Flow Computations A B C Flow Length (ft) : 221 0.00 0.00 Slope (%) : 10 0.00 0.00 Surface Type : Woodland Unpaved Unpaved Velocity (ft/sec) : 1.58 0.00 0.00 Computed Flow Time (min) : 2.33 0.00 0.00Total TOC (min) ..................34.73
Subbasin Runoff Results
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 2.78Peak Runoff (cfs) ........................................................... 0.89Weighted Curve Number ............................................... 83.84Time of Concentration (days hh:mm:ss) ........................ 0 00:34:44
Subbasin : MTA-UP-SOUTH
Subbasin : PM
Input Data
Area (ac) ........................................................................ 9.18Impervious Area (%) ...................................................... 62.00Impervious Area Curve Number .................................... 98.00Pervious Area Curve Number ........................................ 86.00Rain Gage ID ................................................................. POULSBO
Composite Curve Number Area Soil CurveSoil/Surface Description (acres) Group NumberComposite Area & Weighted CN 9.18 93.44
Time of Concentration
User-Defined TOC override (minutes): 20
Subbasin Runoff Results
Total Rainfall (in) ............................................................ 4.49Total Runoff (in) ............................................................. 3.75Peak Runoff (cfs) ........................................................... 7.04Weighted Curve Number ............................................... 93.44Time of Concentration (days hh:mm:ss) ........................ 0 00:20:00
Pipe Segment Q Length Pipe Size "n" IE IE Area Velocity Velocity Head Elevation Loss HGL Elevation Head Loss Head Loss Control Elevation Control Elevation Velocity Head Head Loss Head Loss Elevation Check UNSUBMERGED SUBMERGED ELEVATION(cfs) (ft) (inches) Value OUT IN (sq‐ft) (fps) (ft) (ft) (ft) (ft) (ft) (ft) (ft) (ft) (ft) (ft) (ft) (ft)
NOTES(16) Inlet Control Elevation calculation varies depending on submergence condition of the pipe end. See Columns 21‐25 for submerged condition results.(17) Assumed aproach velocity in last pipe matched to previous pipe as a conservative approach.(21) This column includes a test caclulation for determining unsubmerged vs submerged condition. We are assuming all results over 3.5 are "submerged". (22) Unsubmerged pipes on pond inlet side will be assumed full to crown; on pond outlet side will use calculated flow depth, assuming free‐flowing.(24) HW is height of water over IE at pipe end.(25) HW Elevation is the calculated column (16) "Inlet Control Elevation".
Introduction Prinsco offers 3-inch through 60-inch diameter pipe in solid and perforated configurations. The table provided in this technical note provides the standard perforations available for single-wall (SW) and dual-wall (DW) Prinsco pipe. In some cases, there are more than one available perforation pattern, so please verify the perforation pattern needed prior to ordering.
Single-Wall (SW) and Dual-Wall (DW) Pipe Perforations
Perforation Pattern Descriptions: A: 8 rows spaced 45 degrees apart, perforations in alternating corrugation valleys B: 8 rows spaced 45 degrees apart, perforations in every corrugation valley C: 4 rows spaced 90 degrees apart, perforations in every corrugation valley D: 12 rows spaced 30 degrees apart, perforations in every corrugation valley E: 3 rows spaced 120 degrees apart, perforations every 4 inches 1: Pipe available from Midwest manufacturing plants 2: Pipe available from Fresno, CA manufacturing plant