Storm Water Drainage Design Checklist etc.

The robust design and quality implementation of storm water drainage system is sin qua non for the longevity of road network and environmental protection of a new development.

The drainage design of storm water should follow the applicable design standards and backed by adequate design calculations. The typical design practices and criteria used in the design are reflected in the following lines.

Potential Deliverables

• Drainage Design Report (potentially including hydrology report and hydraulic model)
• Existing and proposed drainage plan including overland drainage outfalls
• Overall site grading plans with enlarged view, where applicable
• Plan of storm sewer with catch basin, manhole, connections with invert elevations, low points, covers, separation from other utilities, sleeves for future utility crossings and other required details
• Storm sewer profiles (plan & profile on same sheet)
• Pipe and Access chambers (types. Levels etc. in form of a schedule)
• Cross sections at specific locations
• Existing and proposed culvert with ditch/box culvert profiles if any
• Typical Standard Details
• Specifications

Checklist of Storm Water Drainage Design

• Confirm overall drainage layout principles, as they are different on rural and trunk roads than on non-trunk roads. Also different in cuttings than on embankments.
• Consider re-routing of offsite flow impacting proposed development
• Provide adequate protection from offsite runoff
• Grading and drainage shall be designed allowing safe passage to 100-yr flood through proposed development.
• Existing drainage pattern to be shown on the plan
• Finished Floor Level (FFL) of all proposed buildings labelled
• Coordinate FFL elevation with architectural drawings.
• FFL of all existing buildings within development and adjacent properties are identified and label
• Make sure that proposed building FFL are higher than 100-yr flood elevation
• Minimum FFL shall be at least 300 mm higher than adjacent low kerb elevation
• Label all slopes around building pad ensuring positive drainage away from the building.
• Existing and proposed contours to be shown and labelled on the plan
• Existing grades along entire perimeter of proposed development to be indicated.
• Proposed grades for all parking areas, streets, landscape and hardscape areas to be shown.
• Proposed grading to be tied to existing grading along property boundary.
• Where difference in elevation between existing and proposed elevation exceed 20 cm, propose retaining wall
• Top of retaining wall to be indicated on the plan
• Typical cross section of retaining wall to be prepared
• Ultimate project outfall to be indicated on the plan
• Erosion protection to be proposed at the outfall
• Drainage details such as ditch and swale shapes, invert elevations, alignment, and location dimensions to be indicated
• Channels in both plan and profile view to be indicated. Callout stationing, flow line elevation, slope, top of ditch elevation at every 50m interval
• Hydraulic grade line and freeboard to be indicated
• Channel flowing in subcritical flow regime are preferred
• Consider erosion protection such as grouted riprap or concrete lining, as needed
• If channel side slope steeper than 4:1, consider channel lining.
• Consider depth of water table while designing detention/retention ponds
• Depth of detention/retention pond shall not exceed 1.0 m
• Storm water retention time shall not exceed 36 hrs
• Minimum size of storm water main is 500 mm
• Minimum size of storm water lateral pipe is 300 mm
• Check minimum cover on all storm water pipes. If cover is less than 1.2m (applicable only to uPVC or GRP pipes), consider concrete encasement or any other type of protection.
• Show all storm water pipes in plan and profile view with all crossing utilities.
• Provide minimum separation of 500 mm with all crossing utilities.
• The class and gauge of reinforced concrete storm drain pipes to be appropriate for adequate cover for the specified load.
• That pipe size, type, class, gauge, length, and inverts are shown on plans. If extensive storm drainage is required, check to see that drainage structures and pipe schedules are shown on plans.
• Install Manhole (MH) where storm pipe alignment or storm drain pipe size is changing.
• Match crown elevations where pipe size is changing.
• Maintain maximum spacing of MH and catch basins according to the applicable standards/guidelines.
• Add adequate number of cross section through site in order to clarify the design intent.
• Provide a Drainage Area Map that delineates all drainage areas that impact the site, on-site and off-site.?Drainage areas are to correspond to points of concentration/design as example; 1) where off-site runoff enters the site (street, channels, etc.), 2) where runoff enters a storm systems (inlet, channel, street, etc.), 3) where runoff from one system joins another system and 4) at the point the runoff exits the site.?Design flow rates should be labelled at these points.?These points should correspond to design details depicted in the drainage report.
• All necessary drainage features to be identified
• All drainage facilities (e.g., gully spacing, flat spots, cross fall, ditches) to be adequate
• Features such as gullies shouldn’t obstruct cycle routes or footpaths
• Desirable minimum earth cover to be provided to the pipes
• Minimum clearance from the other utilities to be provided
• Adequate design of box culverts, headwalls, wing walls to be provided
• All low points to be identified and provided adequate drainage system
• MH covers to be flushed to finish ground avoiding pedestrian/cyclist obstruction
• Ultimate outfall location to be identified
• Hydraulic model and design calculations to be prepared along with the design drawings etc.
• Debris and clogging of storm drain inlet to be adequately addressed?
• Flood inundation of storm drain system to be appropriately analysed
• Pollution control measures to be provided
• For inlets on grade, is any runoff bypasses the inlet and how that runoff is taken care off before it leaves the site
• For catchment area less than 1 square kilometre, use the rational method.?For catchment area larger than 1 square kilometre, use the hydrograph method
• Evaluation of Existing, Interim, and Ultimate drainage conditions
• Determine any adverse drainage impact to adjacent property
• Storm inlet calculations must show that roadway drylane criteria is satisfied. Inlet sizing and spacing must be based on required interception, not just arbitrarily spaced along a roadway
• Confirm that proposed drainage design of site is in general agreement with drainage patterns and alignments specified in the Master Plan.

Key Design Parameters

• Storm return period e.g., 100 years
• M5-60 – Rainfall depth expected at the site for a 60-minute storm with 5-year return period
• Ratio-R – M5-60/M5-2 days
• Time of Entry (mins)
• Maximum Rainfall Rate (mm/hr)
• Volumetric Run-off Coefficient (Cv)
• Minimum Pipe Depth – 0.9 m to 1.2 m
• Minimum Self Cleansing Velocity – generally 0.75 m/s
• Allowance for global warming/climate change - 30-50%
• Level Soffit – top of pipe
• Invert level – bottom of pipe

Highway Design Parameters relating to Drainage Design

• Cross slope/camber – 2 to 3 percent (depending on the type of road, rainfall intensity)
• Vertical Alignment – for motorway desirable maximum is 3%; for dual carriageway it is 4% and for single carriageway its limit is 6%. Any gradient more than 8% requires a departure. Minimum gradient is imposed on design to ensure that effective drainage can be maintained. A minimum longitudinal gradient of 0.5% is the desirable standard
• To assist with drainage, sag curves are best located around the centre or well within the horizontal curve such that the maximum cross falls aids the carriageway drainage.

Pollution Control Measures in Highway Drainage System

• Oil Separators
• Ponds and Wetlands (through filtering effect of vegetation)
• Swales (or shallow grassed channels)
• Bio-treatment channels (filled with materials like limestone, zeolite etc.)
• Filtration Trenches (allow the water within a drain to pass through a trench filled with filter stone before entering another pipe at a lower level.?This process removes some pollutants.
• Soakaways (used on small drainage schemes)

Sustainable Urban Drainage Systems (SUDS)

SUDS, or Sustainable Urban Drainage Systems are a sequence of water management practices and facilities designed to drain surface water in a manner that will provide a more sustainable approach than what has been the conventional practice of routing run-off through a pipe to a watercourse.

A few examples of SuDS are shown below:

• Permeable surfaces
• Filter and filtration tanks
• Swales
• Detention basins
• Ponds

Types of Carriageway Drainage

• Kerb and gully (can be combined with a fin or narrow filter drains)
• Combined surface water and ground water filter drains (widely used on rural roads where kerbs are not required. This is very simple form of drainage surface water running off the edge of the carriageway and into a stone filled trench. A perforated pipe at the bottom of the trench collects the water and removes it from the carriageway).
• Surface water channel (e.g., v shaped or trapezoidal shaped including grassed surface water channel)
• Combined kerb and drainage blocks (e.g., Beany blocks)
• Linear drainage channels (e.g., slot drains, ACO drains. They are normally constructed of slip form concrete. They are most commonly used over long lengths where space is restricted e.g. in central reserve adjacent to concrete barriers).