GWINNETT COUNTY STORM WATER MANAGEMENT REVIEW CHECKLIST STORM WATER MANAGEMENT REPORT

Transcription

1 GWINNETT COUNTY STORM WATER MANAGEMENT REVIEW CHECKLIST STORM WATER MANAGEMENT REPORT DEVELOPMENT NAME: DISTRICT/LANDLOT/PARCEL: ENGINEER: FAX NUMBER/ REVIEWER: DATE: COUNTY CASE #: FYI: IT IS THE OWNERS/DEVELOPER S RESPONSIBILITY TO BE IN COMPLIANCE WITH APPLICABLE NATIONAL POLLUTION DISCHARGE ELIMINATION SYSTEM (NPDES) PERMIT AND CLEAN WATER ACT REQUIREMENTS ANNOTATED CHECKLIST IS REQUIRED WITH ALL RE-REVIEWS Walk Through READY FOR ONE STOP Plans have met the criteria for One Stop plan approval process Appointments are received for 8:30am through 10:30am every Thursday Please call , and ask to schedule your appointment Appointment: An appointment may be scheduled with a plan reviewer after the plans have been revised to address all comments Please call to schedule an appointment FYI if all comments are not addressed after the third review, a meeting will be required with County Staff, Designer and Owner/Developer STORM WATER MANAGEMENT REPORT 1) Storm Water Management Report shall contain the following: A Professional Engineer seal, signature and date (UDO TITLE III ) B Narrative explaining the purpose of the report C An executive summary of the report s findings organized similar to the sample tables provided below: Basin Return Frequency Drop-Off, include a copy of these comments and one set of plans, do not drop off money or easement documents Bring to Suite st floor One Justice any day of the week Allow up to 5 business days for rereview Predeveloped property line Flow Summary Routed By-pass flow flow Post developed flow (routed + property line Ponding elevation 10% point predeveloped flow 10% point postdeveloped flow GWINNETT COUNTY SWM REVIEW CHECKLIST STORM WATER MANAGEMENT REPORT Page 1 of 6

2 A Energy Dissipation Summary: Pipe outlet headwall / Detention pond outlet A B C Reference Chapter 7 of GC SSFISS 25 year Non-erosive Froude postdeveloped velocity from Number flow velocity Storm Water at outlet Design headwall Manual Type of Energy Dissipation Measures proposed Downstream receiving conveyance velocity summary: Study point/ hydraulic structure/ Basin A B C 25 year predeveloped flow velocity 25 year postdeveloped flow velocity Non-erosive velocity from Storm Water Design Manual Current condition of the channel (appear stable or is it eroding) Adverse impact expected from proposed project Detention necessary? Times of Concentration Summary: Sub-area Pre/Post Overland flow, minutes Pre/Post Shallow Concentrated flow, minutes Pre/Post Open channel flow, minutes Pre-developed Tc, minutes Postdeveloped Tc, minutes A-1 25/15 35/20 10/ A-2 Curve Number Summary: Sub-area A-1 A-2 Pre-developed Curve Number Post-developed Curve Number Gutter Spread Calculations Summary: (for roadways, max to be 8 ) CB Max spread, ft 1 GWINNETT COUNTY SWM REVIEW CHECKLIST STORM WATER MANAGEMENT REPORT Page 2 of 6

3 2 D A map showing drainage areas used for pipe design(udo TITLE III ) E A map showing drainage areas for all hydrographs Preferably county GIS for basins less than 100 acres (UDO TITLE III ) F A map showing all on-site drainage areas, off-site drainage areas, and all pond bypass areas considered in detention calculations (UDO TITLE III ) G Calculations determining the capacity of existing pipe to carry the proposed discharges(udo TITLE ) H Calculations showing discharge of concentrated flows into the streets do not exceed the flow rates in table 9-G of the the Gwinnett County Stormwater Systems and Facilities Standards and Specifications and UDO TITLE III TABLE 9-G MAXIMUM FLOWS INTO STREETS ALLOWABLE PEAK FLOW STREET CLASSIFICATION RATE FOR A 2-YEAR STORM Local 20 cfs Minor Collector 10 cfs Other 05 cfs I Calculations showing outlet pipe for detention ponds will accommodate 125% of Q 100 routed, if no earthen embankment emergency spillway is proposed(udo TITLE III ) J Time of concentration calculations for all hydrographs Follow the procedures set forth in the Gwinnett County Stormwater Systems and Facilities Standards and Specifications (UDO TITLE III F) K Curve Number calculations for both pre-developed and post-developed conditions for all hydrographs Follow the procedures set forth in the Gwinnett Stormwater Systems and Facilities Standards and Specifications (UDO TITLE III ) L Provide Energy dissipator calculations/designs for outlet headwalls of pipes and detention ponds The following energy dissapators may only be used in the corresponding Froude number range: Energy dissipator Froude Number range Riprap apron Less than or equal to 25 Riprap outlet basins Less than or equal to 25 Baffled outlets 1 to 9 M An analysis of downstream conditions See number 3 below (UDO TITLE III N Detention pond computer output for the 2, 5, 10, 25 and 100-year storms in basins where detention is necessary(udo TITLE III-405) 2) The Rational Method may not be used for detention pond design(udo TITLE III ) GWINNETT COUNTY SWM REVIEW CHECKLIST STORM WATER MANAGEMENT REPORT Page 3 of 6

4 3) Analysis of downstream conditions shall include the following (UDO TITLE III ): A Describe in combination with a topographic map, all culverts, obstructions, existing and potential erosion problems, elevations of existing improvements, and existing drainage complaints, between the downstream property line and the 10% point B Analyze downstream watercourses and receiving conveyances to determine 25-yr flow channel velocities If the non-erosive velocity of the stream is exceeded, detention may be required Provide calculations with crosssection, depth of flow and velocity in channel(udo TITLE III ) C Analyze existing pipe systems and culverts for compliance with current development regulation design criteria Culverts should pass Q 100 If existing pipes are not adequate for increased 100-year flow, detention may be required (UDO TITLE III ) D Hydrograph comparisons for the 1, 2, 5, 10, and 25, 50 and 100-year storms for both the downstream property line study point and the point where the drainage basin equals 10 times the project area E A detailed written description of the first 500 feet off site; at least one photograph looking downstream with some object included in the photograph for scale shall be included F FYI: One method to perform the Hydrograph analysis methodology would be as follows for each study point (UDO TITLE III ): 1 Pre-developed on-site hydrograph 2 Basin area at study point excluding on-site area hydrograph 3 Combine 1 and 2 4 Post-developed on-site hydrograph 5 Combine 4 and 2 6 Compare 5 and 3 to show no increase 4) Since downstream analysis indicates adverse impacts are expected, provide storm water detention in Basin(s) for the 1, 2, 5, 10, and 25-year storms Route the 100-year storm through the pond so that the earth embankment will not overflow 5) Detain the runoff from the 1-year storm for 24 hours Use the SCS runoff equation to size the volume required Use the following equation to size the orifice: A = (V/t)/(06*(644*H/2) 05 ) where t = 86,400 sec A = area of the orifice, ft 2 H= height above the centroid of the orifice V = 1-yr channel protection volume, ft 3 6) Discuss the calculation of comment(s) # with this office 7) All hydrographs shall be based on a 24-hour storm 8) Provide a cross-section of detention pond embankment including outlet pipe, emergency spillway, embankment slopes, minimum embankment top width, outlet control structures, headwalls, and riprap Verify that minimum 1-6 freeboard above maximum water surface elevation is provided for earthen dams 9) Maximum ponding for parking lot detention: 6 on 10-year storm, 9 on 100-year storm In truck parking areas, the maximum depth of parking shall be 12 for the 10-year storm 10) Orifices less than 3 in diameter must be protected from clogging such as Detail 629 A trash rack (maximum mesh opening of d/2) with a surface area of at least 10 square feet shall protect all orifices 15" or less in diameter Provide detail GWINNETT COUNTY SWM REVIEW CHECKLIST STORM WATER MANAGEMENT REPORT Page 4 of 6

5 11) The following outlet control structure pipe sizes are required based on outlet orifice diameters Orifice Diameter Minimum Pipe Diameter < to < to ) For the site, use pre-developed CN less than or equal to 55 unless approved by this office 13) Post-developed flows at every location where run-off will leave the development must be less than orequal to pre-developed flows, unless meeting the conditions of Refer to year storm in basin ( UDO TITLE III ) 14) 24-hour detention of the 1-year storm is required if water quality protection is required, unless meeting conditions of (UDO TITLE III ) 15) Where water quality is treated in a regional stormwater facility approved between April 27, 1999, and January 1,2001, all lots within the treated area shall eather confirm to the original permit requirements and water quality regulations at the time of approval or conform to the currentwater quality regulations A volume verification confirming facility maintenance in accordance with the original design shell be certified by an authorized registered professional an submitted to the Department for review and approval (UDO TITLE III ) 16) Where stormwater detention is treated in a regional facility approved before April 27, 1999, all lots within the treated area shall confirm to current water quality and channel protection regulations A volume verification confirming facility maintenance in accordance with the original design shell be certified by an authorized registered professional an submitted to the Department for review and approval (UDO TITLE III ) 17) Since detention for project is claimed in an existing facility, provide documentaion that 1) existing facility was certified in last 18 months or 2) documentaion that facility meets the development requirements when the facility was approved, or 3) that maintenance/ construction will be preformed to bring facility into compliance (UDO TITLE III ) 18) Prior to approval of the report, the applicant shall certify and provide documentation that all other applicable environmental permits have been acquired for the site Use Environmental Permit Affidavit form a) NPDES General Permit for Construction (Ga EPD) b) COE 404 Permit (wetlands) c) Ga EPD Stream Buffer Variance 19) Provide Storm Water Quality Performance Review form (TSS spreadsheet) or document in report that water quality treatment is not required in accordance with UDO TITLE III ) Base the volume of the 1 year storm on the runoff equation 25 in the Design Manual Do not rely on the computed volume from storm water hydrology software for this volume Show calculations GWINNETT COUNTY SWM REVIEW CHECKLIST STORM WATER MANAGEMENT REPORT Page 5 of 6

6 21) Time interval used in routing must be set to 20% of Time to Peak or smaller This is imperative for Hydraflow software so storm volumes are calculated properly 22) Provide completed Site Development Permits (CDP & MDP) release affidavit form 23) Provide a table of contents and either provide page numbers or tabs referencing sections for the following 1) Executive summary 2) Hydrology 3) Water quality 4) Floodplain 5) Environmental permits 6) Annotated checklists 24) For projects such as parking lot additions and land disturbance permits that do not have Final Plats or Certificates of Occupancy associated with them, a performance surety is required prior to the issuance of a permit in the amount of 110% of the cost of construction of the Storm Water Management system (Effective ) ADDITIONAL COMMENTS 25) 26) 27) 28) GWINNETT COUNTY SWM REVIEW CHECKLIST STORM WATER MANAGEMENT REPORT Page 6 of 6