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SECTION 600 STORM DRAINAGE FACILITIES 601.00 GENERAL CONDITIONS 3 610.00 STORM DRAINAGE DESIGN AND TECHNICAL CRITERIA 3 611.00 Design Methods 5 611.01 Minor and Major Design Storms 5 611.02 Storm Return Periods 5 611.03 Rainfall Intensities 6 611.04 Runoff Computations, Rational Method 6 611.05 Runoff Coefficients 6 612.00 Detention 7 612.01 General 7 612.02 Equation Method 8 612.03 Sequential Detention 8 613.00 Design Standards 8 613.01 Open Channels 8 613.02 Storm Sewers and Storm Inlets 11 613.03 Culverts 12 613.04 Street Flow Capacities 14 620.00 GENERAL PROVISIONS 16 621.00 General 16 622.00 Approved Plans 16 623.00 Permits Required 16 624.00 Traffic Control 16 630.00 EROSION CONTROL 16 631.00 General 16 632.00 Requirements 17 633.00 Submittal 17 634.00 Erosion Control 17 635.00 Erosion Control Structures 17 640.00 STORM DRAINAGE CONSTRUCTION 18 641.00 Site Work and 18 641.01 General 18 641.02 Trenching, Backfilling and Compacting 18 642.00 Materials 18 642.01 Pipe 18 642.02 Pipe Joints 19 642.03 Manholes, Inlets and Sidewalk Chases 19 642.04 Manhole Base Slabs & Base Beams 20 642.05 Concrete 20 642.06 Cast and Ductile Iron Fittings 20 642.07 Bedding Material 20 642.08 Riprap and Filter Cloth 20 ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-2 643.00 22 643.01 Alignment and Grade 22 643.02 Protection of Existing Underground Utilities 22 643.03 Wet 22 643.04 (Left Blank Intentionally) 22 643.05 Storm Sewer Pipe Installation 22 643.06 Connections to Existing Manholes 22 643.07 Construction of Manholes, Inlets and Sidewalk Chases 22 643.08 Construction of Open Channels and Special Structures 23 643.09 Riprap and Filter Cloth 23 643.10 24 644.00 Inspections 24 650.00 TRENCHING, BACKFILLING AND COMPACTING 26 660.00 RESTORATION AND CLEANUP 26 670.00 GRADING AND EXCAVATION 26 ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-3 SECTION 600 STORM DRAINAGE FACILITIES 601.00 GENERAL CONDITIONS Refer to Section 100 TITLE, SCOPE AND GENERAL CONDITIONS of these CONSTRUCTION STANDARDS & SPECIFICATIONS for additional requirements that apply to all projects within Elbert County. All enclosed Strom Water Collection Systems, Catch Basins, Curbs, Gutters and Detention/Retention Structures shall be owned and maintained by the governing Metro District or Homeowners Association. All Open Roadside Drainage Channels and Culverts will be maintained by Elbert County. 610.00 STORM DRAINAGE DESIGN AND TECHNICAL CRITERIA All proposed construction plans and submittals shall be accompanied by a storm drainage analysis. Appropriate drainage system design shall be submitted for approval by the DPW Director Or designee for each phase of construction. Approval of the analysis and design is subject to the following conditions: A. Construction of the system shall commence within 365 days of the date of the approval. B. No construction has been completed on any adjacent property that may have affected the drainage pattern within the basin. New submittals may be required by the DPW Director or designee. The proposed construction shall not damage upstream or properties. The planning and design of the drainage system shall not transfer the problem from one location to another. Except where specified in these CONSTRUCTION STANDARDS & SPECIFICATIONS, the procedure, criteria, and standards set forth in the latest revision of the Urban Drainage Flood Control District Storm Drainage Criteria Manual—hereby referred to as the UDFCD Manual—shall be instituted for the analysis of any drainage system. Current engineering practices and drainage methodology, as well as common sense, shall be involved with the analysis of any drainage system. The runoff analysis for a particular area shall be based on the natural, undisturbed land for that area. Any contributing runoff from upstream areas shall be based on the existing land use and topographic characteristics of those areas. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-4 Where a master drainage plan for a given area of Elbert County is available, proposed drainage systems shall conform to that plan. Consideration shall be given as to how the proposed master plan drainage systems shall tie into the existing upstream and drainage system. In areas where a master plan is not available, major drainageways and easements shall be located to provide continuity with existing drainage conditions. These drainageways and easements shall be shown on all drainage plans. A CONSTRUCTION STORMWATER DISCHARGE (CDPS) PERMIT SHALL BE OBTAINED FROM THE COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT FOR DISTURBANCE OF ANY SITE LARGER THAN ONE ACRE. FOR CAPITAL IMPROVEMENT PROJECTS, THE CONTRACTOR IS REQUIRED TO OBTAIN A CDPS PERMIT. Permanent stormwater quality facilities shall be designed and constructed in accordance with the UDFCD Manual (Volume The Federal Emergency Management Agency (FEMA) floodplain boundaries are available from the DPW Director Or designee and shall be shown on all preliminary and final drainage plans. All ponding facilities shall be of the detention type. Retention facilities shall only be allowed with the written approval of the DPW Director or designee Construction that shall impair surface or subsurface drainage shall not be approved. Elbert County reserves the right to issue and enforce more stringent criteria shall adverse conditions exist. Designs that vary from the criteria shall require written approval of a variance by the DPW Director Or designee prior to final approval of the plans. Natural topographic features shall be the basis of location for easements and future runoff calculations. Where defined, existing drainage patterns and slopes shall be used. The drainage facilities shall be able to handle the design flows with no erosion damage to the system. Streets shall not be used as primary floodways for major storm runoff. The amount of runoff in the streets shall not exceed the limits established in Section 613.04 Street Flow Capacities of these CONSTRUCTION STANDARDS & SPECIFICATIONS. For inlet and manhole details, refer to the CDOT M&S Standards. Stormwater detention facilities or natural drainageways are to be used whenever feasible. Any alteration to natural drainage patterns shall not be approved unless a thorough ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-5 investigation and analysis shows no hazard or liability. The DPW Director Or designee shall have final authority over any system design. ALL DRAINAGE FACILITIES SHALL BE IN AN EASEMENT OR PUBLIC R.O.W. All drainage improvements shall be as natural in appearance as possible to be aesthetically pleasing. Maintenance access shall be provided for all drainage and flood control facilities. Irrigation ditches shall not be used as the outfall of any drainage basin. 611.00 Design Methods 611.01 Minor and Major Design Storms Every urban area has two separate and distinct drainage systems whether or not they are actually planned for and designed. One is the initial system, which corresponds to the minor (or ordinary) storm recurring at regular intervals. The other is the major system, which corresponds to the major (or extraordinary storm), which is unlikely to occur more often than once in one hundred (100) or more years (“100-year storm”). Since the effects and routing of stormwaters for the major storm may not be the same as for the minor storm, all storm drainage plans submitted for approval shall be submitted in detail in two separate phases: one indicating the effects of the minor storm and the other showing the effects of the major storm. A. Minor storm provisions: The objectives of the minor storm planning are to minimize inconvenience, to protect against recurring minor damage, to reduce rising maintenance costs, to create an orderly drainage system and to provide a sociological benefit to the urban resident. The minor storm drainage system may include curb and gutter, storm sewer, swales, and other open drainageways and detention facilities. B. Major storm provisions: The major storm shall be considered the 100-year storm. The objectives of the major storm planning are to eliminate substantial property damage or loss of life and shall be as directed and approved by the DPW Director or designee. Major drainage systems may include storm sewers, open drainageways and detention facilities. The correlation between the minor and major storm system shall be analyzed to ensure a well-coordinated drainage system. 611.02 Storm Return Periods ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-6 The minor and major storm design return periods shall not be less than those shown below: DESIGN STORM RETURN PERIODS Land Use or Zoning Design Storm Return Period Minor Storm Major Storm Residential 2-year 100-year Business 5-year 100-year Public Building Areas 5-year 100-year Parks, Greenbelts, etc. 2-year 100-year Open Channels and Drainageways - 100-year Detention Facilities 10-year1 100-year 1A two year storm return period shall be used if the detention facility does not have a water quality outlet. 611.03 Rainfall Intensities The rainfall intensities to be used in the computation of runoff shall be obtained from the Time-Intensity-Frequency Curves shown in the Detail Drawings. 611.04 Runoff Computations, Rational Method The Rational Method shall be utilized for sizing storm sewers and for determining runoff magnitude for all watersheds The procedures for the Rational Method, as explained in the latest edition of the UDFCD Manual (Volume shall be followed in the preparation of drainage reports and storm drainage facility designs in Elbert County. 611.05 Runoff Coefficients Rational method runoff coefficients: The runoff coefficient to be used in conjunction with the Rational Method shall not be less than those listed below: ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-7 RUNOFF COEFFICIENTS FOR RATIONAL METHOD Land Use or Surface Characteristics Percent Impervious Frequency 2 5 10 100 Business Commercial Areas Neighborhood Areas Residential Single-Family Multi-Unit (detached) Multi-Unit (attached) 1/2 Acre Lot or Larger Apartments Industrial Light Areas Heavy Areas Parks, Cemeteries Playgrounds Schools Railroad Yard Areas Undeveloped Areas Historic Flow Analysis Greenbelts, Agricultural Offsite Flow Analysis (when land use not defined) Streets Paved Gravel Drives and Walks Roofs Lawns, Sandy Soil Lawns, Clay Soil 95 85 * 60 75 * 80 80 90 5 10 50 15 2 2 45 100 40 90 90 0 0 .79 .66 * .37 .51 * .57 .57 .71 .08 .11 .34 .14 .81 .68 * .41 .54 * .59 .59 .73 .18 .21 .40 .24 .83 .69 * .46 .58 * .63 .63 .75 .28 .30 .46 .32 .88 .75 * .63 .66 * .70 .70 .81 .52 .53 .60 .54 (See "Lawns") .31 .89 .28 .71 .71 .00 .04 .37 .90 .35 .73 .73 .01 .15 .44 .92 .42 .75 .75 .05 .25 .59 .96 .58 .81 .81 .20 .50 *Refer to the graphs in the Runoff Chapter of the UDFCD Manual. 612.00 Detention 612.01 General Onsite detention is required for all new development, expansion and redevelopment. The required minimum detention volume and maximum release rates at these volumes for the 2-year, 5-year and 100-year storms shall be determined in accordance with the procedure and data set forth in these criteria. If the pond does not include a water quality outlet, which controls about the one and one-half (1½) year storm, the pond shall be designed to release the two year historic rate. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-8 Exemptions from the detention requirement may be granted if it can be demonstrated that the developed area does not adversely affect the major drainageways (assuming the entire drainage area is fully developed) and that the water quality is maintained. Offsite flows cannot be passed through the detention pond. Offsite areas shall be included in the drainage area contributing to the pond. In certain cases, offsite flow can be routed around the detention pond. Parking lots that serve as detention storage ponds shall not have a storage depth of more than one foot. Parking lots that serve as detention storage ponds shall place notification signs that the area ponds during a rainfall event. The signs shall be permanent and high quality and shall meet Elbert County’s specifications for traffic signs. 612.02 Equation Method The equation method found in the Storage section of the UDFCD Manual may be used to design detention ponds for drainage areas smaller than ninety (90) acres; however, if the calculated allowable release is greater than the historic runoff, a different method shall be used to determine the storage requirements. 612.03 Sequential Detention The sequential detention method shall be used for ponds that drain into each other. Use the Sequential Detention Form at the end of Section 600 of these CONSTRUCTION STANDARDS & SPECIFICATIONS to size sequential detention ponds. 613.00 Design Standards 613.01 Open Channels Except as modified herein, open channels shall be designed for the 100-year storm and shall conform to the UDFCD Manual. In addition, the channel design shall also be analyzed with respect to minor storm runoff. Whenever practical, the channel shall have slow flow characteristics, be wide and shallow, and be natural in its appearance and functioning. Channels shall be designed so that critical depth and super-critical flows are avoided. Channel capacities shall be computed from Manning's Formula for uniform flow, except at crossings and transitions where the design shall account for backwater effects. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-9 The channel cross-section may be any type suitable to the location; however, the limitations for design for the major storm and minor storm design flows shall include: A. Capacity: The channel and overbank areas shall have adequate capacity for the 100-year storm runoff. B. Side slopes: Side slopes shall be as flat as practical. Side slopes of 4:1 (run:rise) shall be considered a normal minimum. Under special conditions, slopes of 3:1 may be utilized with written approval of the DPW Director or designee. The practical slope for mowing equipment is 4:1 or less. C. Depth: The maximum design depth of flow for the major storm shall be limited to five feet of depth in the channel cross section outside of the low-flow or trickle channel. Any design variation exceeding the maximum depth of flow shall be submitted in writing for approval by the DPW Director or designee. Critical depths and velocities shall be investigated and reported for both the major and minor storm runoffs. D. Freeboard: Except where localized overflow in certain areas is desirable for additional ponding benefits or other reasons, the minimum allowable freeboard shall be one foot. E. Bottom width: The bottom width shall be designed to satisfy the hydraulic capacity of the cross-section recognizing the limitations on velocity, depth and Froude number. F. Slope of channel: Grass-lined channel slopes are dictated by velocity and Froude number requirements. Grass-lined channels normally shall have slopes of 0.2% to 0.6%. Where the natural topography is steeper than desirable, drops may be utilized. G. Curvature: The centerline curvature shall have a radius of not less than twice the design flow top width, but not less than one hundred (100) feet. H. Trickle channels: Concrete trickle channels to carry low flows may be required for all new urban grassed channels. The capacity of a trickle channel shall be approximately two percent of the major design flow. The shape of concrete trickle channels shall be parabolic. Where two percent of the major design flow exceeds ninety (90) cfs, a low flow channel shall be required. Low flow channels shall be in accordance with the UDFCD Manual (Volume All concrete trickle channels shall have a minimum slope of one percent. I. Design velocity: The maximum velocity for the major storm design runoff shall not exceed seven feet per second for grass- lined channels, except in sandy soil where the maximum velocity shall not exceed five feet per second. The Froude number (turbulence factor) shall be less than 0.8 for grass-lined channels. Grass-lined channels having a Froude number greater than 0.8 ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-10 shall not be permitted. Minimum velocities for all channels shall not be less than two feet per second for the minor storm runoff. J. Erosion: All channels shall be designed with the proper and adequate erosion control features. K. Grass lining: The grass lining for channels shall be in accordance with the UDFCD Manual (Volume L. Water surface profile: A water surface profile for the major storm runoff shall be computed for all channels and clearly shown on the construction plans submitted for approval. Computations of the water surface profile shall utilize standard backwater methods such as HEC-2 and shall take into consideration all losses due to velocity changes, drops, bridge and culvert openings, and other obstructions. A Computations Report shall be submitted along with the construction plans. The energy gradient line shall be shown on the construction plans. M. Roughness coefficient The value of the roughness coefficient to be used in Manning's Formula shall not be less than those listed below: MINIMUM VALUES OF ROUGHNESS COEFFICIENT Type of Channel and Description Closed Conduits: Minimum Concrete Pipe: Culverts with bends, connections & debris 0.013 Storm sewer 0.013 Subdrain with open joints 0.016 PVC Pipe 0.011 Concrete Surfaces (bottom & sides): Smooth finish 0.015 Unfinished 0.017 Concrete Bottom (with sides of): Mortared stone 0.020 Dry rubble or riprap 0.030 Gravel Bottom (with sides of): Formed concrete 0.020 Dry rubble or riprap 0.040 Excavated or Dredged Channels and Ditches: Earthen, Straight & Uniform, no brush or debris: Grassed, less than 6" high with: Depth of flow < 2.0 feet 0.035 Depth of flow > 2.0 feet 0.030 Grassed, approx. 12" high with: Depth of flow < 2.0 feet 0.060 Depth of flow > 2.0 feet 0.035 Grassed, approx. 24" high with: ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-11 613.02 Storm Sewers and Storm Inlets Except as subsequently modified, the design of storm sewers and inlets shall conform to the criteria set forth in the UDFCD Manual. Storm sewers and inlets shall be of sufficient capacity to adequately carry the expected runoff from the initial design storm. Computer programs such as UDFCD’s UDSewer and UDInlet are encouraged in the design of the storm sewer system. THE STORM DRAINAGE SYSTEM DESIGN FORM IN THE BACK OF SECTION 600 OF THESE CONSTRUCTION STANDARDS & SPECIFICATIONS SHALL BE USED IN THE DESIGN OF STORM SEWERS AND INLETS. The completed form shall be included in the drainage report. The storm sewer system shall be installed at all locations where the allowable street capacity is exceeded or wherever ponding of water is likely to occur. The invert elevation of storm sewer outfalls into channels shall be at least one foot above the channel invert. The minimum allowable pipe size for storm sewer systems shall be as follows: MINIMUM ALLOWABLE PIPE SIZE Type of conduit Min. Inside Pipe Dia. Main Trunk Sewer 21” Individual laterals/Driveway Culverts 18” Pipe diameters of less than eighteen (18) inches may be allowed; however, a variance request shall be submitted to the or designee in writing, and approval shall be obtained from the DPW Director Or designee prior to final design. Arch pipes may be allowed where design conditions dictate, provided that the minimum cross-sectional areas are not less than those specified above. All storm sewer conduits shall have sufficient structural strength to withstand an H-20 design load. Depth of flow < 2.0 feet 0.070 Depth of flow > 2.0 feet 0.035 Earth bottom with riprap on sides 0.040 Rock or Shale Cuts: Smooth and uniform 0.035 Jagged and irregular 0.040 Curb and Gutter (concrete) 0.016 ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-12 Manholes shall be a minimum of sixty (60) inches for lines eighteen (18) inches to twenty-one (21) inches diameter, and seventy-two (72) inches for lines twenty- four (24) inches to thirty (30) inches diameter. For storm pipe larger than thirty (30) inches diameter, the DPW Director Or designee shall approve the manhole design size. Where two or more pipes enter a manhole, the or designee shall approve the manhole design size. The maximum allowable distance between manholes or other suitable appurtenances for cleanouts shall not exceed those listed below: MAXIMUM ALLOWABLE MANHOLE SPACING Inside Diameter Maximum Allowable Distance Between Manholes and Cleanouts 18" - 36" 400 feet 36" - 60" 500 feet 60" & Larger 750 feet The velocity for the minor flows in conduits shall not be less than two feet per second. Storm inlets shall be utilized at all points where ponding or sump conditions exist. Inlets shall be curb opening inlets, type or grated inlets with curb openings. All inlets shall be similar and equal to those in the Detail Drawings or as approved by the DPW Director or designee. Grated inlets shall be recommended for bicycle traffic. Grated Inlets shall be Neenah Foundry model number R-3157A or R-3233 or an approved equivalent. The theoretical capacity and spacing of storm inlets shall be analyzed using the criteria—including reduction factors—set forth in the UDFCD Manual. The size of outlet pipes from stormwater inlets shall be based on the theoretical capacity of the inlet. All pipe outlets shall be protected in accordance with the Detail Drawings. 613.03 Culverts Culvert capacities shall be at least equal to the capacities of culverts designed in accordance with the procedures outlined in Federal Highway Administration Hydraulic Design Series Number 5, “Hydraulic Design of Highway Culverts”. The DPW Director Or designee shall approve the shape, location and type of construction of culverts. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-13 Culverts shall be sized to have sufficient capacity to pass all of the runoff from the major storm if twenty (20) percent of the pipe is plugged. The following design criteria shall be utilized for all culvert design: A. The culvert, including inlet and outlet structures, shall properly convey water and debris at all stages of flow. B. Culvert inlets shall be designed to minimize entrance and friction losses. Inlets shall be provided with either flared end sections or head walls with wing walls. Projecting ends are not acceptable. Large structures shall be designed to resist hydrostatic uplift forces. C. Culvert outlets shall be designed to avoid sedimentation, undermining of culvert, or erosion of channels. Outlets shall be provided with either flared end sections or headwalls, with wingwalls and riprap. Projecting ends are not acceptable. Additional outlet control, in the form of riprap, channel shaping, etc., may be required. D. Culvert slopes shall be selected to eliminate excessive velocities and scour. Generally, the minimum slope of culverts shall be limited to one-half percent. E. Headwater ponding above culvert inlets shall not be acceptable if such ponding appears likely to cause property or roadway damage, culvert clogging, saturation of fills, detrimental upstream deposits of debris, or inundation of existing or future utilities and structures. F. Tailwater height at the outlet shall be subject to approval by the DPW Director or designee. G. Culverts shall be analyzed to determine whether discharge is controlled by inlet or outlet conditions for both the minor storm discharge and the major storm discharge. Computations for selected culvert sizes shall be submitted to the DPW Director Or designee for approval. Computer programs such as the FHWA HY8 may be used to design culverts. H. Minimum Allowable Size: The required size of a culvert shall be based on adequate hydraulic design analysis. 1. Circular culverts under roadways/driveways shall have a minimum diameter of twenty-four (24) inches. 2. Oval culvert dimensions shall be forty-three (42) inches by twenty-seven (27) inches or larger. 3. Box culverts shall have a minimum height of four feet. Smaller culvert sizes may be approved by the DPW Director or designee. I. An overflow path shall be provided in case the culvert becomes plugged. J. Where physical conditions dictate, multiple culvert installations may be approved by the DPW Director or designee. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-14 K. The structural design of culverts shall conform to the methods and criteria recommended by the manufacturer of a specific type of culvert for the specified embankment conditions. 613.04 Street Flow Capacities Except as modified herein, the criteria set forth in the UDFCD Manual shall be used to analyze and to determine the adequacy of streets as a function of the drainage system. Both the minor storm runoff and major storm runoff shall be considered, and calculations showing such runoff at critical sections shall be submitted. The following criteria shall apply in the determination of allowable street flow capacities: A. Street, curb and gutter, sidewalks, cross pans and curb cuts shall conform to all applicable Sections of these CONSTRUCTION STANDARDS & SPECIFICATIONS. B. Street encroachment for the minor design storm shall not exceed the limitations set forth below: ALLOWABLE STREET ENCROACHMENT AND DEPTH OF FLOW FOR MINOR STORM RUNOFF Street Classification Maximum Encroachment Local If curb present, no curb overtopping. Flow may spread to crown of street. Connector If curb present, no curb overtopping. Flow spread shall leave the equivalent of one 10-foot driving lane clear of water. Arterials If curb present, no curb overtopping. Flow spread shall leave the equivalent of two 10-foot driving lanes clear of water – one lane in each direction. No more than two lanes in each direction shall be flooded. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-15 Where no curb exists, street encroachment shall not extend past the public R.O.W. A storm sewer system shall be installed at all points where the maximum allowable street encroachment occurs. C. The allowable depth of flow and inundated area for the major design storm shall not exceed the following limitations: ALLOWABLE DEPTH OF FLOW AND INUNDATED AREA FOR MAJOR STORM RUNOFF Street Classification Allowable Depth and Inundated Areas Local & Connector Lowest entry to residential dwellings and public, commercial, and industrial buildings shall not be less than twelve (12) inches above the 100-year water surface elevation. The depth of water over the gutter flowline shall not exceed twelve (12) inches. Arterial Lowest entry to residential dwellings and public, commercial, and industrial buildings shall be not less than twelve (12) inches above the 100-year water surface elevation. The depth of water at the street crown shall not exceed six inches to allow operation of emergency vehicles. Depth of water over gutter flow line shall not exceed twelve (12) inches. D. Cross-street flow occurs when runoff flowing in a gutter flow across the street to the opposite gutter or inlet. Allowable cross- street flow is summarized in the following table: ALLOWABLE CROSS-STREET FLOW Street Classification Initial Storm Flow Major Storm Flow Local Six inches depth in crosspan Twelve (12) inches of depth above gutter flow line Collector Six inches of depth in crosspan Twelve (12) inches of depth above gutter flow line Arterial None No cross-street flow. Maximum depth of upstream gutter of twelve (12) inches. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-16 620.00 GENERAL PROVISIONS 621.00 General All storm drainage construction in the Elbert County R.O.W./Dedicated Easements shall comply with these CONSTRUCTION STANDARDS & SPECIFICATIONS. These standards shall include new storm drainage construction and repairs and maintenance of existing facilities within Elbert County. 622.00 Approved Plans All storm drainage construction shall be in accordance with engineered construction plans prepared under the direction of a Colorado Registered Professional Engineer. Storm drainage plans shall include an Area Grading Plan and an Erosion Control plan as defined in Section 161.09 Area Grading Plan Details and Section 161.10 Erosion Control Plan Details of these CONSTRUCTION STANDARDS & SPECIFICATIONS. Where work is to be performed over, under or in an irrigation ditch, written approval of the ditch owner is required prior to written approval by the DPW Director or designee. 623.00 Permits Required A PPIP or Grading Permit issued by Elbert County shall be required and shall not be issued until the DPW Director Or designee has approved the storm sewer plans. A NPDES permit shall be obtained for any disturbance of one acre or more. Refer to Section 150.00 PERMITS AND INSPECTIONS of these CONSTRUCTION STANDARDS & SPECIFICATIONS for additional requirements. 624.00 Traffic Control Traffic control shall comply with Section 141.12 Traffic Control, Barricades and Warning Signs of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 630.00 EROSION CONTROL 631.00 General Erosion and sedimentation are natural processes, the intensity of which is increased by land disturbing activities that reduce or destroy the aesthetic and practical values of neighboring properties, streams and lakes. The purpose of these erosion control criteria is to reduce intensified erosion—caused by either wind or water—to an acceptable level without placing undue burdens on the landowner, builder or community. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-17 632.00 Requirements Erosion control measures shall be designed in conformance with the UDFCD Manual (Volume All land-disturbing activities within Elbert County shall comply with the Colorado Department of Public Health and Environment Regulations. 633.00 Submittal A discussion that summarizes erosion control methods shall be submitted as part of the preliminary and final drainage reports, as required in Section 162.00 Engineering Reports of these CONSTRUCTION STANDARDS & SPECIFICATIONS. A detailed erosion control plan shall accompany the Area Grading Plan and approved Drainage Plan, as required in Section 161.10 Erosion Control Plan Details of these CONSTRUCTION STANDARDS & SPECIFICATIONS. The erosion control plan shall be approved by Elbert County prior to receiving a PPIP or Grading Permit. 634.00 Erosion Control Measures A CONSTRUCTION STORMWATER DISCHARGE PERMIT (NPDES PERMIT) SHALL BE OBTAINED FROM THE COLORADO DEPARTMENT OF HEALTH AND ENVIRONMENT FOR SITE DISTURBANCE LARGER THAN ONE ACRE. Detailed erosion control measures shall be provided to protect the following: A. Inlets and culverts B. Drainageways C. Streams or other water bodies immediately adjacent to land disturbed by construction activity D. Cut and fill areas E. Properties and improved streets adjacent to construction activity F. Others as required by the Road & Bridge Superintendent / Elbert County Engineer Temporary erosion control measures such as sediment traps, straw bales or silt fence shall be properly placed in accordance with the Colorado Department of Public Health and Environment approved Stormwater Management Plan (SWMP) (CDPS Permit) prior to any earthmoving on the site. Erosion control measures shall be kept in good repair and fully functional until erosion potential from the site no longer exists. Permanent erosion control (sod, seed, mulching, etc.) shall be in place prior to the request for a Certificate of Occupancy or Letter of Final Acceptance. 635.00 Erosion Control Structures ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-18 Refer to the Detail Drawings for erosion control installation. When applicable, details of additional erosion control measures may be obtained from the UDFCD Manual (Volume 640.00 STORM DRAINAGE CONSTRUCTION 641.00 Site Work and Earthwork 641.01 General Site work and earthwork shall be performed in accordance with Section 300.00 SOILS AND EARTHWORK of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 641.02 Trenching, Backfilling and Compacting Trenching, backfilling and compacting shall be performed in accordance with Section 350.00 TRENCHING, BACKFILLING AND COMPACTING of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 642.00 Materials 642.01 Pipe Reinforced Concrete Pipe (RCP) shall be manufactured to comply with ASTM C76. All applicable portions of Section 706 Concrete and Clay Pipe of the CDOT Standard Specifications for Road and Bridge Construction shall apply. Polyvinyl Chloride Pipe (PVC) shall be manufactured to comply with ASTM F794/F949 (ribbed) or ASTM D3034, SDR-35 (smooth). All joints shall be factory prepared compression type (elastomeric gasket joint), providing a watertight seal. Non-Reinforced Concrete Pipe (NRCP) shall be manufactured to comply with ASTM C14. NRCP shall be specified under the criteria of Section M-603-2 of the CDOT M&S Standards for all sizes thirty-six (36) inches diameter and smaller. Polymer Coated Steel Pipe (PCSP) shall be manufactured to comply with AASHTO Section 36. All applicable portions of Section 707 Metal Pipe of the CDOT Standard Specifications for Road and Bridge Construction shall apply. The corrugation of the pipe shall be ¾” x ¾” x 7 ½” spiral rib. Polymer coating, grade 250/250, shall be applied prior to corrugating in conformance with AASHTO 26.3.7. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-19 Corrugated aluminum pipe (CAP) shall be manufactured to comply with all applicable portions of Section 707 Metal Pipe of the CDOT Standard Specifications for Road and Bridge Construction. The pipe sizes shall be as shown on the project plans. The corrugation profile of the pipe shall be ¾” x ¾” x 7 ½” spiral rib. At all locations where corrugated aluminum pipe is proposed to be installed, a corrosion resistance level test shall be performed and a test report detailing any corrosion protection requirements shall be submitted to the DPW Director Or designee for approval. Corrugated polyethylene pipe (CPP) or High-Density Polyethylene Pipe (HDPEP) shall be manufactured to comply with ASTM D3350, with the minimum cell classification of 315412C. Requirements for test methods, dimensions and markings shall comply with AASHTO Designation M-294. The minimum pipe stiffness shall be 46 psi to comply with ASTM D2412 at five percent deflection. Corrugated steel pipe (CSP) shall be manufactured to comply with all applicable portions of Section 707 Metal Pipe of the CDOT Standard Specifications for Road and Bridge Construction. The pipe sizes shall be as shown on the project plans. These conduits and coupling bands shall conform to the requirements of AASHTO M 36M (M 36). Other storm sewer pipe materials may be approved at the discretion of the DPW Director or designee Pipe class designation or gauge shall be as shown on the approved plans or as designated by the DPW Director Or designee for each individual project. Pipe material shall be selected based on strength and soil conditions. All pipe shall be inspected by the Elbert County Inspector/Representative in order to allow for rejection of pipe that fails to conform to the requirements of these CONSTRUCTION STANDARDS & SPECIFICATIONS. Defects shall be marked so as not to disfigure the rejected pipe. Rejected pipe shall be removed from the job site within twenty-four (24) hours. 642.02 Pipe Joints All pipe joints shall be watertight. RCP and NRCP joints shall comply with ASTM C443. CPP joints shall comply with ASTM D3212. PCSP and CAP joints shall comply with AASHTO Section 26.4.2.4.f. Cement mortar joints shall be constructed with mortar mixture composed of one part Portland cement to three parts sand and enough water to produce a workable mix. Mortar that has started to set shall be discarded and a new batch prepared. 642.03 Manholes, Inlets and Sidewalk Chases ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-20 Manhole bases, vaults and inlets may be constructed of cast-in-place or pre-cast concrete. Manhole materials, including access ring and cover sets for all inlet types, shall comply with all applicable portions of Section 532.03 Manholes of these CONSTRUCTION STANDARDS & SPECIFICATIONS. Inlets, except as modified above, shall be constructed in accordance with the Detail Drawings and all CDOT M&S Standards. 642.04 Manhole Base Slabs & Base Beams Refer to Section 532.04 Manhole Base Slabs and Base Beams of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 642.05 Concrete Concrete shall conform to Section 800.00 CONCRETE MIX DESIGN AND CONSTRUCTION of these CONSTRUCTION STANDARDS & SPECIFICATIONS. Type II cement shall be used except where sulfate resistant cement is required. Concrete encasement of pipe shall conform to the details shown on the approved plans. 642.06 Cast and Ductile Iron Fittings Refer to Section 532.06 Cast and Ductile Iron Fittings of these CONSTRUCTION STANDARDS & SPECIFICATIONS. Lids shall be furnished with the words “STORM SEWER” cast on top. 642.07 Bedding Material All applicable portions of Section 353.00 Bedding for Pipelines and Service Lines of these CONSTRUCTION STANDARDS & SPECIFICATIONS shall apply. 642.08 Riprap and Filter Cloth Riprap and filter cloth shall be installed at locations shown on the approved plans, or in locations designated by the DPW Director or designee. Rock used for riprap shall be hard, durable, angular in shape, and be free from cracks, overburden, shale and organic matter. Neither breadth nor thickness of a single stone shall be less than one-third its length, and rounded stone shall not be approved. The rock shall sustain the abrasion test (Los Angeles machine - ASTM C0535-69) and shall sustain a loss of not more than ten (10) percent after twelve (12) cycles of freezing and thawing (AASHTO test 103 for ledge rock procedure The rock shall have a minimum specific gravity of 2.50. Classification and gradation for riprap are shown below. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-21 The riprap designation and total thickness of riprap shall be as specified on the approved plans. The maximum stone size shall not be larger than the thickness of the riprap. CLASSIFICATION AND GRADATION OF RIPRAP Riprap Designation % Smaller Than Given Size By Weight Intermediate Rock Dimension (Inches) d(50) * (Inches) Type VL 70-100 12 50-70 9 35-50 6 2-10 2 Type L 70-100 15 50-70 12 35-50 9 2-10 3 Type M 70-100 21 50-70 18 35-50 12 12 2-10 4 Type H 70-100 30 50-70 24 35-50 18 18 2-10 6 Type VH 70-100 42 50-70 33 35-50 24 24 2-10 9 *d (50) = Mean particle size To minimize vandalism, mix Types VL and L riprap with thirty (30) percent (by volume) topsoil and bury it with a minimum of six inches of topsoil, vibration compacted and revegetated. Filter cloth shall be manufactured especially for the stability of erosion control construction and made from polyethylene, polypropylene or polyester yarns in accordance with the following: Filter cloth shall be manufactured especially for the stability of erosion control construction and shall meet the requirements of CDOT Class B drainage geotextile as specified in Section 712 of the CDOT CONSTRUCTION STANDARDS & SPECIFICATIONS for Road and Bridge Construction. Filter material which is to be placed on top of the filter cloth (at specified thickness) prior to placement of the riprap shall meet the bedding requirements in of the UDFCD Manual (Volume ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-22 When requested by the DPW Director or designee the Contractor shall furnish copies of test reports from a certified testing laboratory for the following: A. Gradation and soundness of riprap B. Gradation of filter material C. Strength and characteristic tests for filter cloth D. Compaction tests of the prepared subgrade 643.00 Installation Refer to Section 533.01 General of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 643.01 Alignment and Grade Refer to Section 533.02 Alignment and Grade of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 643.02 Protection of Existing Underground Utilities Refer to Section 533.03 Protection of Existing Underground Utilities of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 643.03 Wet Trench Refer to Section 352.00 Trench Excavation for Pipelines and Service Lines of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 643.04 (Left Blank Intentionally) 643.05 Storm Sewer Pipe Installation Refer to Section 533.04 Sewer Pipe Installation of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 643.06 Connections to Existing Manholes Refer to Section 533.06 Connections to Existing Manholes of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 643.07 Construction of Manholes, Inlets and Sidewalk Chases Manholes and inlets shall be constructed in accordance with applicable portions of Section 533.07 Construction of Manholes and Clean-outs of these CONSTRUCTION STANDARDS & SPECIFICATIONS. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-23 643.08 Construction of Open Channels and Special Structures All work shall conform to details in the approved plans and supplemental specifications. Construction shall comply with Section 533.02 Alignment and Grade of these CONSTRUCTION STANDARDS & SPECIFICATIONS. When approved, sidewalk chases shall be constructed in accordance with the Detail Drawings. 643.09 Riprap and Filter Cloth Excavation for riprap shall conform to Section 300.00 SOILS AND EARTHWORK of these CONSTRUCTION STANDARDS & SPECIFICATIONS. Filter cloth shall be placed according to the manufacturer’s specifications. Holes, rips or other damage to the filter cloth shall be repaired at the Contractor’s expense, in accordance with the manufacturer’s recommendations. Stabilization material, as described in Section 340.01 Definitions of these CONSTRUCTION STANDARDS & SPECIFICATIONS, shall be placed on top of the filter cloth (where filter cloth is used) to the required thickness. The material shall be placed using equipment that shall not rip, tear or otherwise damage the filter cloth. Any damaged areas shall be repaired at the Contractor's expense. The material shall be leveled to a finished surface that is within one inch of the specified thickness. Riprap shall be placed to conform to the details shown on the approved plans. The larger size stones shall be placed first and roughly arranged in close contact. The toe trench and foundation course shall be closed first. The spaces between the larger stones shall then be filled with smaller stone of suitable size, and placed as to leave the surface evenly stepped, conforming to the contour required. The finished surface shall be even and tight and shall not vary from the planned surface grade by more than three inches per foot of depth. The material may be machine placed with sufficient handwork to conform to these CONSTRUCTION STANDARDS & SPECIFICATIONS. All riprap shall be grouted. The stones shall be laid with care to prevent earth and sand from filling the joints. Joints shall be filled with grout and the surfaces swept with a stiff broom. The work shall be protected and kept moist during hot weather for at least three days after grouting or coated with a clear membrane curing compound. Grout shall consist of one part cement and three parts aggregate, by volume. The Portland Cement shall be Type II and aggregate shall be two parts sand and one part gravel passing a inch square mesh screen. Grout shall be mixed with enough water to permit gravity flow of grout into the interstices with limited spading and brooming. A six ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-24 inch by six inch concrete mow strip is required around the edges of riprap structures. When concreting is permitted during cold weather, the temperature of the mix shall not be less than sixty (60) degrees Fahrenheit at the time of placing. Filter cloth, stabilization material, or riprap shall not be placed on frozen ground. Concrete grout shall not be placed when there is frost in the subgrade. 643.10 Testing 643.10.01 Pipe – Water Tightness All pipe shall be tested for water tightness in accordance with manufacturer’s requirements. A. Reinforced concrete pipe (RCP) shall be tested in accordance with ASTM C443. B. High density polyethylene pipe (HDPE) shall be tested in accordance with ASTM D3212. C. Polyvinyl chloride pipe (PVC) shall be tested in accordance with ASTM D3212. D. Spiral-ribbed aluminized steel pipe (CMP, ASP) does not require watertight joints. 643.10.02 Pipe – Deflection All flexible pipe shall be tested for deflection in accordance with Section 534.03 Deflection Testing Pipe of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 643.10.03 Manholes All manholes shall be tested in accordance with Section 534.02 Vacuum Testing Manholes of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 643.10.04 Infiltration and Exfiltration If deemed necessary by the DPW Director or designee, the storm sewer system shall be tested in accordance with Section 534.04 Infiltration and Exfiltration Testing of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 644.00 Inspections ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-25 Refer to Section 154.00 Inspections of these CONSTRUCTION STANDARDS & SPECIFICATIONS. Adequate inspections assure compliance to Elbert County requirements and are the basis for Elbert County’s recommendation that said improvements be accepted for maintenance and for release of performance guarantees. It is the responsibility of the Contractor to contact the DPW Director Or designee a minimum of one full working day (twenty-four [24] hours) in advance of the required inspections. Required inspections shall include: A. Stockpiled Materials – Verify that materials meet CONSTRUCTION STANDARDS & SPECIFICATIONS and approved submittals, including but not limited to: bedding material, pipe, fittings, valves, valve boxes, and fire hydrants. Verify that pipe meets roundness specifications and that bells and spigots are not cracked or chipped. B. Excavation – Verify proper trench depths, shoring, spoil pile location, dewatering, and location and protection of existing utilities. C. Installation – Verify proper bedding depth, alignment and grade, clean pipe and lubricants. Verify “slicing in” of bedding at haunches and that all lifting holes in RCP are plugged. D. Backfill and Compaction – Verify proper methods of backfill and compaction, depths of lifts, moisture control, backfill material free of large rock and organic or frozen material, and proper compaction effort and passing tests. E. Testing – Verify that testing methods comply with these CONSTRUCTION STANDARDS & SPECIFICATIONS. Verify that Elbert County has witnesses all low-pressure air tests, joint testing, vacuum testing of manholes and any other testing requirements. F. Construction Acceptance – Refer to Section 200 ACCEPTANCE PROCEDURES of these CONSTRUCTION STANDARDS & SPECIFICATIONS. General items include: 1. All temporary structures, debris, mud and waste materials shall be removed from public property. 2. All relative testing certifications and documentation shall be submitted to Elbert County, including all compaction tests. Copies of originals are acceptable. 3. All storm sewer manholes and inlets are at construction grade, clean, and grouted, ladders straight, inlet protection installed, and storm sewer lines jetted. Verify that storm sewer drainage swales have erosion and sediment control measures installed in accordance with the approved plans. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-26 Prior to requesting a Construction Acceptance inspection, the Contractor shall clean storm sewer mains and shall have the lines inspected with TV video equipment. A copy of the videotape and written report shall be submitted to Elbert County for review. Video shall also include an audio description of pipe and manhole deficiencies, and camera location during the inspection. Any sections that contain debris or obstructions shall be cleaned and re-videotaped. Video shall be continuous from manhole to manhole, and all notations shall correspond to the approved construction plans. If, after visual inspection of the storm sewer lines the DPW Director Or designee suspects that there is a problem, alignment, infiltration, exfiltration and/or deflection tests may be required at the Contractor’s expense. G. Final Acceptance – Refer to Section 200 ACCEPTANCE PROCEDURES of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 1. Verify that all temporary structures, debris, mud, and waste materials are removed from public property. 2. Verify that all storm sewer manholes and inlets are clean. 650.00 TRENCHING, BACKFILLING AND COMPACTING Refer to Section 350.00 TRENCHING BACKFILLING AND COMPACTING of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 660.00 RESTORATION AND CLEANUP Refer to Section 370.00 RESTORATION AND CLEANUP of these CONSTRUCTION STANDARDS & SPECIFICATIONS. 670.00 GRADING AND EXCAVATION Refer to Section 330.00 SITE PREPARATION of these CONSTRUCTION STANDARDS & SPECIFICATIONS. ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-27 TIME OF CONCENTRATION SUBDIVISION: CALCULATED BY: DATE: t c = t i+ t t SUB-BASIN DATA INITIAL / OVERLAND TIME (t i) TRAVEL TIME (t t) t c CHECK (URBANIZED BASINS) FINAL t c REMARKS DESIGN C 5 AREA Ac LENGTH Ft SLOPE % t i Min LENGTH Ft SLOPE % VEL. FPS t t Min TOTAL LENGTH Ft t c = (L/180) +10 Min Min (10) (11) (12) (13) ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-28 STORM DRAINAGE SYSTEM DESIGN (RATIONAL METHOD PROCEDURE) CALCULATED BY: JOB NO: DATE: PROJECT: CHECKED BY: DESIGN STORM: STREET DESIGN POINT DIRECT RUNOFF TOTAL RUNOFF STREET PIPE TRAVEL TIME REMARKS AREA DESIGN AREA (AC) RUNOFF COEFF C * A (AC) t c (MIN) I IN/HR Q (CFS) t c (MIN) I (IN/HR) * A) (AC) Q (CFS) SLOPE STREET FLOW (CFS) DESIGN FLOW (CFS) SLOPE PIPE SIZE LENGTH (FT) VELOCITY (FPS) t t (MIN) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) 1 2 3 4 5 6 7 ---PAGE BREAK--- STORM DRAINAGE FACILITIES SECTION 600 CONSTRUCTION STANDARDS & SPECIFICATIONS 2019 Edition PAGE 600-29 SEQUENTIAL DETENTION CALCULATION CALCULATED DATE: FACILITY NUMBER BASIN AREA Ac Q i CFS IMP % K Ft Q i /A CFS/Ac ∑ Q CFS Z Ac S m Ac-Ft Q m CFS (10) 10 - YEAR 100 - YEAR Facility Number: Designated number of the detention facility being analyzed. Basin Area: Area of basin (sub-basin) tributary to the detention facility not including any area tributary to an upstream detention facility (3)Qi: Peak inflow in cfs from the area described in Column 2 Imp Percent imperviousness of the area described in Column 2 K: K100 = (1.78I - 0.002I2 - 3.56)/1000; K10 = (0.95I -1.9)/1000 Qi/A: Peak inflow (Qi) in column 3 divided by the area in Column 2 Peak inflow into detention facility computed by summation of the peak inflow in Column 3 and the maximum release rate in Column 10 Z: Equivalent inflow area computed by dividing Column 7 by Column 6 (∑Q/Qi/A) Minimum Sm: Minimum allowed storage volume for the respective detention facility V=KA, where K is from Column 5 and A = Z from Column 8 (10) Maximum Qm: Maximum allowed release rate for the respective detention facility Q10 = 0.24Z, Q100 = 1.00Z, where Z is from Column 8