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Marlgorough Massachusetts
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Drainage Report Requirements
Bound report with cover stating the project title, name of the development, developer, owner, location, design engineer, date of submittal etc.

Pre and post-development plans on 24”x36” sheets in AutoCAD format

Detail sheets showing pertinent details

Drainage Report
Description of Project
Locus Map
Description of Existing Conditions
Description of Proposed Conditions

Summary table showing changes in pre and post-development flows for the 2, 10, 25, and 100-year storm events
·       Post-development flows shall not exceed pre-development flows

Pre-Development Drainage Conditions (describe in report and delineate on plan)
·       Show existing subcatchment areas and label impervious and pervious areas within each
·       Tc flow paths with slopes and distances for all subcatchment areas
·       Existing point source discharge points for all subcatchment areas
·       CN number calculations
·       Tc calculations
·       Flow Calculations for the 2, 10, 25, and 100-year storm events showing time-stage relationship (peak flow, peak hour)
·       Soil Conservation Service Maps
·       Test Pit Locations
·       Groundwater Elevations
·       Percolation Test Results

Post Development Drainage Conditions (describe in report and delineate on plan)
·       Show proposed subcatchment areas and label impervious and pervious areas within each
·       Tc flow paths with slopes and distances for all subcatchment areas
·       Proposed point source discharge points for all subcatchment areas
·       CN number calculations
·       Tc calculations
·       Flow Calculations for the 2, 10, 25, and 100-year storm events showing time-stage relationship (peak flow, peak hour)

Detention Basins
Description of basin (details, method of construction, sizing etc.)
Calculations showing flow entering detention basin
Calculations showing time-stage relationship inside basin (peak flow, peak elevation, peak hour)
Outfall structure detail showing all outfall elevations
Recommend outfall structure with a low flow discharge and grated top
Show all outfall structure discharge elevations and 100-year storm event elevation
Detention basin sections showing all elevations including 100-year storm event elevation
Emergency overflow spillway made of rip rap
Basin discharge shall have flared end with rip rap apron
Recommend 3:1 basin side slopes
Recommend 10 foot wide level access around entire basin for maintenance
Recommend enclosing basin with gated fence
Verify groundwater elevations do not impact operation of basin

Hydraulic Calculations
Hydraulic calculations based on the 25-year storm event
Hydraulic summary table
All catch basins shall have 4 foot deep sumps
Recommend all drainage pipes shall be RCP
Recommend all drainage pipes be 12 inches in diameter (minimum)
Recommend minimum cover over pipe shall be 2.5 feet
Recommend minimum preferred slope of drainage pipe is 1.0% (0.5% absolute minimum)
Recommend maximum slope of drainage pipe is 9%
Recommend velocity of flow within the pipe not be less than 2 ft/s (cleaning) or greater than 10 ft/s (scour)

Stormwater Management
Completed Stormwater Management Form from the latest edition of the Massachusetts Department of Environmental Protection Stormwater Management Policy

Summary of pre and post-development flows
·       Infiltration flows from recharge structures can not be subtracted from post-development flow calculations

Water quality
·       Describe Best Management Practice used
·       Water quality volume calculations (supported by SCS soil data)
·       Roof drain calculations if applicable
·       TSS removal calculations
·       Sizing of structures
·       Oil separation

Recharge to groundwater
·       Recharge to groundwater volume calculations
·       Roof drain calculations if applicable
·       All subsurface structures shall be accessible for maintenance
·       All recharge rates shall be supported by actual percolation test results

Operation and maintenance section
·       All drainage structures shall be inspected after every storm
·       Inspection reports shall be filed for each inspection
·       Inspection reports shall be filed with the Conservation Commission and City Engineer yearly for compliance
·       All drainage structures shall be cleaned as required to remove debris and sediment
·       Parking lots shall be swept as required to remove debris and sediment



 

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