09 ch7 stormwater drainage

Chapter 7 - Stormwater Drainage Design Table of Contents

April 1999

Chapter 7:

STORMWATER DRAINAGE


April 1999

7.0 STORMWATER DRAINAGE DESIGN Table of Contents Clause Subject Page No 7.1 GENERAL........................................................................................................................ 7-1 7.2 DESIGN STANDARDS.................................................................................................... 7-1 7.3 HYDROLOGY .................................................................................................................. 7-2 7.3.1 Design Rainfall Data ........................................................................................................ 7-2 7.3.2 Catchment Area ............................................................................................................... 7-3 7.3.3 Rational Method............................................................................................................... 7-3 7.3.4 Other Hydrological Models............................................................................................... 7-4 7.4 HYDRAULICS.................................................................................................................. 7-4 7.4.1 Hydraulic Grade Line ....................................................................................................... 7-4 7.4.2 Minor System Criteria ...................................................................................................... 7-5 7.4.3 Pits ................................................................................................................................... 7-6 7.4.4 Hydraulic Losses.............................................................................................................. 7-7 7.5 OVERLAND FLOWPATHS.............................................................................................. 7-7 7.5.1 General ............................................................................................................................ 7-7 7.5.2 Freeboard......................................................................................................................... 7-8 7.6 OPEN CHANNELS .......................................................................................................... 7-8 7.7 MAJOR STRUCTURES................................................................................................. 7-10 7.8 RETARDING BASINS.................................................................................................... 7-10 7.9 ON-SITE STORMWATER DETENTION ....................................................................... 7-12 7.10 INTERALLOTMENT DRAINAGE................................................................................... 7-12 7.10.1 Interallotment Drainage Pits........................................................................................... 7-14 7.11 CONDUITS, LOCATION AND COVER ......................................................................... 7-14 7.12 EASEMENTS................................................................................................................. 7-15 7.13 STORMWATER DISCHARGE....................................................................................... 7-16 7.14 MISCELLANEOUS ........................................................................................................ 7-17 7.14.1 Trenchstops and Bulkheads .......................................................................................... 7-17 7.14.2 Subsoil Drainage............................................................................................................ 7-17 7.14.3 Kerb and Gutter Discharge ............................................................................................ 7-17 7.14.4 Habitable Floor Levels ................................................................................................... 7-17


April 1999

7.0 STORMWATER DRAINAGE DESIGN Table of Contents (Continued) Clause Subject Page No 7.15 DOCUMENTATION ....................................................................................................... 7-18 7.15.1 Easements and Agreement ........................................................................................... 7-18 7.16 STORMWATER QUALITY............................................................................................. 7-18

Chapter 7 - Stormwater Drainage Design

April 1999 Page 7 - 1

7.0 STORMWATER DRAINAGE DESIGN 7.1 GENERAL

Stormwater drainage systems shall be designed to satisfy the following objectives: 1 The inundation of private and public land and buildings is minimised and that

surface flow routes convey floodwaters below the prescribed velocity/depth limits where practical and possible.

2 The convenience and safety for pedestrians and traffic affected by frequent

stormwater flows is maximised by controlling those flows within prescribed limits.

3 The retention within each catchment as much incident rainfall and runoff as is

appropriate for the planned use and characteristics of the catchment. 4 That the proposed system is designed with consideration of all ultimate and

interim upstream and downstream characteristics and that the proposal operates as a total system and does not adversely affect any systems or property within the catchment.

5 That stormwater is adequately treated to remove pollutants and systems

provided to replenish subsurface flows in appropriate locations. 6 Erosion and sedimentation is minimised to acceptable levels. 7 To minimise maintenance requirements and enhance the urban landscape

were possible. 7.2 DESIGN STANDARDS

In pursuit of these objectives, the following principles shall apply: 1 New Developments are to provide a stormwater drainage system in

accordance with the "major/minor" system concept set out in Chapter 14 of Australian Rainfall & Runoff, 1987 (AR&R 1987); that is, the "major" system shall provide safe, well-defined overland flow paths for rare and extreme storm runoff events while the "minor" system shall be capable of carrying and controlling flows from frequent runoff events.

Refer point 3 Clause 7.3.1 for design events for flow calculations.

2 Redevelopment - Where the proposed development replaces an existing

development, the on-site drainage system is to be designed in such a way that the estimated peak flow rate from the site for the design average recurrence interval (ARI) of the receiving minor system is no greater than that which would be expected from the existing development. Such stormwater drainage systems to be designed in accordance with AR&R 1987 or Australian Standard 3500.3 National Plumbing and Drainage Code where applicable unless otherwise detailed herein.

3 The requirements of Council's current Flood Prone Land Policy shall be

adhered to for all developments unless otherwise advised.


Page 7 - 2 April 1999

7.3 HYDROLOGY 7.3.1 Design Rainfall Data

1 Design Intensity-Frequency-Duration (IFD) Rainfall - IFD relationships shall be derived in accordance with Chapter 2, Volume 1 AR&R 1987, for the particular catchment unless IFD tables are provided, refer below.

2 Design IFD rainfall tables are provided for specific locations. Refer Appendix

"G". 3 Design Average Recurrence Interval (ARI) - For the "major/minor" systems are

given below for specific development types.

Minor System ARI Land Use:-

Urban Residential (single allotments) 5 Year Urban Residential (medium-high density see below) 20 Year Commercial 20 Year Industrial 20 Year Rural Developments (Refer Clause 7.7.2)

10 Year

Major System ARI Overland Flow Paths and Trunk Drainage

100 Year

Medium and high density residential developments such as villas, townhouses, units etc shall have a minor system designed for the 20 year ARI event. Major overland system flowpaths within developments such as pathways or roadways shall cater for the 100 year ARI Event minus 50% of the minor flow (piped system, eg 5 year ARI, see above). This is generally referred to as the "gap-flow". Refer to the Australian Road Research Board Special Report 34 (ARRB SR 34). Note, trunk drainage is excluded from the above “gap-flow” design procedure. 4 Irrespective of the above, where a development is designed in such a way that

the major system flows involve surcharge across private property, then the underground system (both pipes and inlets with designated blockage factors) shall be designed to capture and contain flows having an ARI of 100 years from the upstream catchment. An emergency overland flow path shall also be provided for these systems. Easements are to be provided in private property over such pipe systems and overland flow paths. Restrictions shall also be placed on the property so as not to permit changes in surface levels or the construction of certain structures within these easements.



7.3.2 Catchment Area

1 The catchment area of any point is defined by the limits from where surface runoff will make its way, either by natural or man made paths, to this point. Consideration shall be given to likely changes to individual catchment areas due to the full development of the catchment.

2 Where no detailed survey of the catchment is available, 1:4000 orthophoto

maps as a minimum are to be used to determine the catchment and to measure areas.

3 Catchment area land use shall be based on current available zoning

information or proposed future zonings, where applicable ie, the ultimate developed state of each catchment including all contributing catchments.

4 Provisions shall be made where the likelihood of higher density developments

will take place in the future. 5 Catchment area details are to be approved by the Engineer prior to the

commencement of detail design.

7.3.3 Rational Method

1 Rational Method calculations to determine peak flows shall be carried out in accordance with Chapter 14, AR&R 1987 and the requirements of this Specification.

2 All calculations shall be carried out by a qualified person experienced in

hydrologic and hydraulic design. 3 Co-efficients of Run-off shall be calculated in accordance with Section 14.5 of

AR&R 1987 and full details of co-efficients utilised shall be provided. 4 The absolute minimum impervious percentage for single residential lots shall

be 50%, with values for medium and high density residential, commercial or industrial developments determined after consultation with the Engineer or refer Clause 7.10.

5 The time of concentration of a catchment (tc) is defined as the time required

for storm runoff to flow from the most remote point on the catchment to the outlet of the catchment. tc shall be for the ultimate developed catchment as in Clause 7.3.2 generally catering for the greatest flow.

Consideration shall be given that natural or developed catchments have defined flowpaths and sheet flows generally occur for short distances only. The tc should be calculated using a combination of the Kinematic Wave and Manning’s formula to determine as accurately as possible the actual tc. Unless sufficient evidence is provided, the Kinematic Wave equation shall be used for a maximum 50 metre length in determining the tc.

6 Where the flow path is through areas having different flow characteristics or

includes property and roadway, then the flow time of each portion of the flow path shall be calculated separately.


- 4 April 1999

7 The maximum tc in an urban area shall be 20 minutes unless sufficient evidence is provided to justify a greater time. The tc in rural areas shall be calculated in accordance with Section 5.4 of AR&R 1987.

8 Flow paths to pits shall be representative of the fully developed catchment

considering such things as fencing and the likely locations of buildings and shall be shown for each collection pit on the catchment area plan. Consideration shall be given to likely changes to individual flow paths due to the full development of the catchment. Refer point 5 above.

9 Surface roughness co-efficients "n" shall be adopted from information

provided in Chapter 14 of AR&R 1987 and should be verified by the Engineer. Values applicable to specific zoning types and overland flow path types are given below:

Surface/Zoning Roughness Co-efficient "n"

Flow across Parks 0.35

Flow across Rural Residential land 0.30

Flow across Residential (2a) 0.21

Flow across Residential (2b) 0.11

Flow across Industrial 0.06

Flow across Commercial 0.04

Flow across Asphalt Roads 0.012

Flow across Paved Areas 0.01

Flow across Gravel Areas 0.02

These values are only to be used where upstream areas are not developed therefore accurate flow lengths and surfaces are unknown. Flow lengths are to be kept to a minimum prior to assuming entry into a gutter or piped system. This will provide conservative design flows that should be adopted.

7.3.4 Other Hydrological Models

1 Other hydrological models may be used as long as the requirements of AR&R 1987 are met, summaries of calculations shall be provided as well as details of all program input and output.

2 Where computer analysis programs are used, copies of the final data files of

the design shall be provided to the Accredited Certifier with the final drawings for approval by the Accredited Certifier. Details on the use of specific programs, catchment parameters and any other relevant information shall also be submitted.

7.4 HYDRAULICS

7.4.1 Hydraulic Grade Line

1 Hydraulic calculations shall generally be carried out in accordance with AR&R 1987 and shall be undertaken by a qualified person experienced in hydrologic and hydraulic design. The calculations shall substantiate the hydraulic grade line adopted for design of the system and shown on the drawings.



Summaries of calculations shall be shown on the plan and details of all

calculations shall be submitted including listings of all programme input and output.

2 The "major" system shall provide safe, well-defined overland flow paths for the

100 year ARI storm runoff events while the "minor" system shall be capable of carrying and controlling flows from frequent runoff events.

3 Downstream hydraulic grade line level requirements for design are as follows:-

i Known hydraulic grade line level from downstream calculations including pit losses at the starting pit in the design event.

ii Where the downstream starting point is a pit and the hydraulic grade line

is unknown, a level of 0.15m below the pit inlet in the downstream pit is to be adopted.

iii Where the outlet is an open channel and the design storm is the minor

event the top of the outlet pipe shall be the downstream control. iv Where the outlet is an open channel, the design storm is the 100 year

ARI event and downstream flood levels are not known, the top of the outlet pipe shall be the downstream control unless otherwise nominated by the Engineer where the consultant shall estimate the downstream level taking into account backwater affect.

v Where the outlet is an open channel, the design storm is the 100 year

ARI event and downstream flood levels are known, the downstream control shall be the 100 year ARI design flood level. Where future 100 year ARI flood levels are lower than the existing, the higher interim flood level shall be adopted.

vi Where the outlet is at a river or lake the design 100 year ARI flood level

shall be the downstream control unless nominated by the Engineer. For example, the nominated design water level for Tuggerah Lake shall be 1.1m AHD, however the 100 year ARI flood level is generally in the range of 2.2m to 2.4m AHD. The Engineer can generally provide design flood levels for the Tuggerah Lakes System and most major watercourses in the Shire.

4 The hydraulic grade line level in drainage pits shall be limited to 0.15m below

the gutter invert and 0.15m below the underside of the lid for junction pits unless otherwise approved.

7.4.2 Minor System Criteria

1 The acceptable gutter flow width shall be a maximum that will permit the top of kerb to be visible for the nominated minor event recurrence interval irrespective of crossfall or kerb type.



2 Minimum conduit sizes for areas to be in Council control are given below:

• The minimum pipe size shall be 375mm diameter.

• The minimum box culvert size shall be 600mm wide x 300mm high. 3 Minimum and maximum velocity of flows in stormwater pipelines shall be

0.6m/sec and 8m/sec respectively, however, where the likelihood of the catchment surface material being transported into the pipe system is high and may cause excessive scouring, the maximum velocity permitted shall be considerably reduced.

7.4.3 Pits

1 Inlet Pits shall be located to prevent ponding and to limit flow widths and depths to acceptable levels in accordance with this specification. Preference shall be given to the location of drainage pits at the upstream side of lots, pedestrian crossing points and kerb returns.

2 Pits shall also be provided:

• To enable access for maintenance. • At changes in pipeline direction, grade, size, level or class of pipe. • At junctions of pipelines.

3 Pits are to conform to Council's current Standard Drawings and nominated RTA Standards where no Council standard exists.

4 Step irons shall be provided in all pits deeper than 1.2m, unless otherwise

warranted. 5 The maximum recommended spacing of pits where flow widths are not critical

are given below:

Pipe Size (mm) Spacing (m)

Generally less than 1200 80

1200 or larger 150

In tidal influence all 80

5 Kerb inlet lengths (lintels) are to be a desirable maximum of 3.0m. With an absolute minimum of 1.2m. Kerb inlet lengths refer to clear opening. There shall be a minimum 150mm step down to the race from the gutter invert level at the lintel.

6 All grates within road reserves and pathways shall be bicycle friendly and

secured in an approved fashion to permit access only to maintenance personnel.

7 All grate loading requirements shall conform to the intended use of the grate

or adjacent area. 8 Refer Appendix H for pit inlet capacity graphs and explanations.



9 Pits should collect as much stormwater as possible while maintaining safe

designs for pedestrian, vehicular and cycle traffic. Openings must not be large enough to admit a child.

7.4.4 Hydraulic Losses

1 The pressure change co-efficient "K" shall be generally determined from the "Missouri Charts" and the "Hare Equations". The "Hare Equations" shall only be used as a guide and the "Missouri Charts" shall be used were discrepancies between the two exist.

2 Computer program default pressure change co-efficient "K" shall not be

acceptable unless they are consistent with "1" above.

3 Going from larger upstream to smaller downstream conduits is not permitted without approval of the Engineer prior to detailed design. In going from smaller to larger pipes benching shall be provided in pits to enable a smooth flow transition.

4 Drainage pipe systems shall be designed as an overall system, with due

regard to the upstream and downstream system and not as individual pipe lengths. Drainage pipeline systems shall generally be designed as gravity systems flowing full at design discharge, but may be pressurised. Pipe friction losses and pipe sizes in relation to discharge shall be determined using the Colebrook-White formula with the following roughness co-efficients being 0.6mm for concrete pipes, 0.06mm for FRC pipes and UPVC pipes.

5 Requirements for private pipes entering Council's system are given below:-

i All pipe inlets, including roof and subsoil pipes, shall enter the main pipe system at pits unless otherwise approved by the Engineer. These shall be finished off flush and be grouted into the pit wall.

7.5 OVERLAND FLOWPATHS

7.5.1 General

Overland flowpaths shall be provided where necessary to convey runoff from rainfall events that exceed the capacity of the minor system to systems that cater for the design 100 year ARI flows, generally trunk drainage systems. Refer Clause 7.3, 7.6, 7.7 and 7.8. Such flowpaths are generally roadways and pathways linked together that contain and convey these flows at acceptable depths and velocities. Occasionally it may be necessary to either widen roads, pathways or even increase the size of the minor system to cater for such flows. To ensure public safety within overland flowpaths, the depth of flow should generally be less than 0.2 metres and the product of velocity and depth should not exceed 0.4m2/sec for the major event. Where public safety is not an issue higher velocity depth products may be permitted, subject to approval by the Engineer.



Calculations sufficient to substantiate the design satisfying the above requirements with the required freeboard are to be submitted to the Principal Certifier - Subdivision. Refer Drawings SD 7B, SD 8B, SD 9A, SD 10A and particularly SD 65 for flowpath treatments of pathways and across footpath areas. It should be noted that it is critical to adequately design the inlet to such overland flow pathways to ensure the design flows are conveyed into the flowpaths and the flows do not bypass the system and inundate property.

7.5.2 Freeboard

i A minimum 100mm freeboard will be required between the calculated 100 year ARI flow level in the gutter area and the high point in the footpath to all properties, specifically properties at a lower level than the adjacent road formation. Driveway construction in these instances must ensure this requirement is satisfied. Refer Drawing SD 4C.

ii Overland flow paths, (eg at the end of cul-de-sacs that drain trapped

lowpoints) shall have a minimum 300mm freeboard from the calculated 100 year ARI flow level to any adjacent residential allotment and preferably 500mm freeboard to habitable floor levels.

Where overland flow pathways are located generally parallel to the contour, not depressed and adjacent to building lots that will be probably cut and/or filled a minimum 100mm freeboard from the design 100 year ARI flow level shall be provided. The adjacent low side dwelling shall have floor levels a minimum of 300mm above the surrounding area draining towards the street eg, no trapped low points.

7.6 OPEN CHANNELS

1 Generally, open channels form part of the trunk drainage system that cater for

major event (100 year ARI) flows and shall be designed to have smooth transitions with adequate access provisions for maintenance and cleaning.

2 Design of open channels shall be generally in accordance with Chapter 14 of

AR&R 1987 unless otherwise specified herein, and shall be designed with safety requirements as set out in Section 14.10.4 of AR&R 1987 as a primary criterion. Open channel systems shall be designed to contain the major flow with the required freeboard (refer point 10 below). Refer Drawing SD 28B.

3 Friction losses in open channels shall be determined using Manning’s "n"

values (see below) with due consideration of vegetation and sediment build up:-



Manning’s "n" Roughness Co-efficients for open channels shall generally be

derived from information in Chapter 14 of AR&R 1987. Mannings "n" values applicable to specific channel types are given below:-

Concrete Pipes or Box Sections 0.011 Concrete (trowel finish) 0.014 Concrete (formed without finishing) 0.016 Sprayed Concrete (gunite) 0.018 Bitumen Seal 0.018 Bricks or pavers 0.015 Pitchers or dressed stone on mortar 0.016 Rubble Masonry or Random stone in mortar 0.028 Rock Lining or Rip-Rap 0.028 Corrugated Metal 0.027 Earth (clear) 0.022 Earth (with weeds and gravel) 0.028 Rock Cut 0.038 Short Grass 0.033 Long Grass 0.043

Natural Channel Designs 0.065 to 0.085

4 Designs should be provided to:-

• Prevent persons falling or being swept into them. • Permit easy escape from flood waters. • Include catchrails, side bays or other escape devices as required. • Minimise velocities or average velocity depth products to safe limits.

5 Where constraints do not permit the provision of open channel design

specified herein or adequate safety treatments, manproof fencing of the channel section shall be provided with due regard for maintenance and emergency access.

6 Maximum side slopes on conventional grass lined open channels shall be 1 in

6. 7 Low flow provisions in open channels (man-made or altered channels) will

require low flows to be contained within an approved lining generally at the invert of the main channel. Subsurface drainage may be required in grass lined channels to prevent waterlogging of the channel bed. The width, shape and material/s used in the channel shall be approved by the Engineer depending on channel size, maintenance requirements, channel type and the capacity as nominated below.

8 The low flow system shall be designed to convey low or trickle flows. The low

flow capacity will be determined by the Engineer depending on invert type, catchment characteristics, low flow velocities etc, however the minimum size of the low flow treatment shall be a 1.0m wide 100mm deep "vee" section. Generally low flow pipes will not be permitted. Adequate scour protection shall be provided adjacent to the above ground low flow system and any other areas as required ie drop structures etc.



9 The following should be avoided or designs carried out to adequately

accommodate the following:-

• Hydraulic jumps/supercritical flows. • Transitions and constrictions of the channel - backwater effect. • Superelevated flows (around bends). • Freeboard/provisions for debris under structures, mine subsidence etc.

10 A minimum 300mm freeboard shall be provided to the top of the channel and

600mm in mine subsidence areas. Adjacent dwellings should have habitable floor levels a minimum of 300mm above the freeboard requirements.

7.7 MAJOR STRUCTURES

1 All major structures shall be designed for the 100 year ARI event unless

otherwise advised without afflux in urban areas. Some afflux and upstream inundation may be permitted in certain rural areas provided the increased upstream flooding is minimal and does not inundate private property.

2 Major structures within rural areas shall be designed to accommodate flood

events as nominated by the Engineer after individual assessment based on importance of road, the structure, the watercourse, possible increase in flooding, emergency access, safety etc.

3 A minimum clearance of 300mm between the 100 year ARI flood level (also

refer point 4 below) and the underside of any major structure superstructure is required to allow for passage of debris without blockage, however the freeboard will be determined on the size of the debris that is likely to be passed by the structure.

4 All bridges shall be designed for the 100 year ARI flood event as a minimum

without afflux in urban areas unless otherwise advised. Where nominated by the Engineer the effect of extreme floods on the structure shall be determined.

5 Certified structural design shall be required on bridges and other major culvert

structures and may be required on some specialised structures. Structural design shall be carried out in accordance with AUSTROADS (1992) Bridge Design Code.

6 All culverts shall be designed for the 100 year ARI flood event without afflux in

urban areas unless the adjacent area including freeboard requirements will not be adversely affected.

7 Culverts (either pipe or box section) shall be designed with due regard being

given to inlet and exit losses, inlet and outlet control and scour protection. Refer Chapter 7 Austroads "Waterway Design" 1994.

7.8 RETARDING BASINS

1 For each ARI a range of storm events shall be modelled to determine the peak

flood level and discharge from the retarding basin. Storm patterns shall be those given in AR&R 1987 Volume II. Sensitivity to storm pattern should be checked by reversing these storm patterns where required, particularly for large basins.



2 The critical storm duration with the retarding basin is likely to be longer than

without the basin. A graph showing the range of peak flood levels in the basin and peak discharges from the basin shall be provided for the storms examined.

3 Flood Routing should be modelled by methods outlined in AR&R 1987, and

shall include whether the basin has an adverse impact on the total catchment main stream flooding due to the development site within the catchment.

4 The high level outlet to any retarding basin should have a capacity to contain

the 100 year ARI flood event. Additional spillway capacity may be required due to the hazard category of the structure. The hazard category should be determined by reference to ANCOLD (Australian National Conference on Large Dams 1986).

5 The spillway design shall generally be in accordance with the requirements for

Open Channel Design in this Specification and should be approximately 500mm lower than the rest of the embankment. Adequate reinforcing and scour protection shall be provided to the spillway to minimise the possibility of embankment failure.

6 Pipe systems shall contain the design flow through the Retarding Basin wall

and be suitably protected to prevent infiltration of water between the pipe outer surface and the basin wall.

7 The pipe outlet structure shall be protected to prevent blockages. 8 Freeboard - Minimum floor levels of dwelling shall be 500mm above the

design water level at the emergency spillway when in operation. Refer point 4 above. An additional 600mm freeboard shall be provided in mine subsidence areas.

9 Public Safety Issues - Basin design is to consider the following aspects

relating to public safety.

• Side slopes are to be a maximum of 1 in 6 to allow easy egress. • Water depths shall be, where possible, less than 1.2m in the 20 year

ARI storm event. Where either requirement is not practical or economic, greater depths may be acceptable. In that case the provision of safety refuge mounds should be considered.

• The low flow system within the basin shall be designed in accordance

with the low flow requirements of Clause 7.6. • Depth indicators should be provided indicating maximum depth in the

basin. • The desirable minimum slope of the basin floor is to be 1%. Slightly

higher grades 1% to 2% should be adopted where sporting fields are located within basins.



• Protection of the pipe outlet structure shall be undertaken to reduce

hazards for people trapped in the basin. • Signage of the spillway and basin is necessary to indicate the hazard.

Refer Drawing SD 19. • No planting of trees or large shrubs in basin walls is permitted. • No basin spillway is to be located directly upstream of dwellings and

shall be controlled to prevent runoff entering private property. • Submission of design plans to the Dam Safety Committee for comment

where the Committee's guidelines require it.

7.9 ON-SITE STORMWATER DETENTION

1 On-site stormwater detention will be required where considered necessary by the Engineer on redevelopment sites to attenuate the runoff to discharge levels expected from the predeveloped site, for the minor and major events.

2 Calculations are to include any upstream catchments which contribute to the

runoff. 3 Various storm durations are to be modelled to determine the critical duration

both pre and post development. 4 The temporary storage of water is to be contained within the site, and is not to

encroach on adjacent properties or public and road reserves. Pedestrian access is not to be included within the storage area unless readily available alternative routes are provided.

5 Enclosed structures are not recommended due to maintenance problems and

possible health problems. 6 The maximum water depth for the 100 year ARI event is 200mm for carparks,

and 600mm for gardens. 7 An overland flowpath (or spillway) must be provided for the on-site detention

area. A minimum freeboard of 300mm must be provided to floor levels (including adjacent properties) for the 100 year ARI event, assuming 100% blockage of the piped discharge.

8 The piped discharge from the detention area is to connect directly to the street

drainage system where possible. However other discharge locations may be considered.

7.10 INTERALLOTMENT DRAINAGE

1 Interallotment Drainage shall be provided for every allotment which does not

drain directly to its frontage street.

2 Interallotment drainage shall be contained within an easement not less than 1.5m wide , and the easement shall be in favour of the benefiting allotments.



3 Pipe Capacity - The interallotment drain shall be designed to accept

concentrated drainage from buildings and paved areas on each allotment for flow rates having a design ARI the same as the "minor" street drainage system unless otherwise specified in AS 3500.3 or in Clause 7.3.1.

4 In lieu of more detailed analysis, the following areas of impervious surface are

assumed to be contributing runoff to the interallotment drain:- Development Type Minimum % of Lot Area

• Residential (2a) 50 • Residential (2b) 70 • Industrial 80 • Commercial 90 5 Pipes shall be a minimum diameter of 150mm and designed to flow full at the

design discharge without surcharging of pits. Consideration shall be given where connections are proposed to other systems designed to be under pressure.

6 Pipes shall have a minimum cover of 300mm, ensuring that it serves the total

lot, taking into consideration future site regrading or terracing of lots. 7 Pipes - Minimum Grade - The interallotment drainage system shall have a

desirable minimum longitudinal grade of 1%. 8 Interallotment Drainage Pipe - Relationship to Sewer Mains - Where

interallotment drainage and sewer mains are laid adjacent and parallel to each other they are to be spaced 0.75 metres between pipe centrelines unless pipelines are greater than 300mm diameter where the minimum clearance between pipes shall be 450mm. The sewer shall be located closest to the dwelling being served.

9 Interallotment drainage shall be required for high side properties fronting roads

with either kerbing unable to satisfactorily accommodate kerb outlets or with one way crossfall.

10 Refer Chapter 8 for the requirements of direct discharge to kerb and

gutter where interallotment drainage is not provided. 11 Existing lots that discharge stormwater onto proposed developments shall be

provided with either:

i An interallotment drainage system within the existing properties as detailed herein. This will necessitate the creation of drainage easements and negotiations by the Developer; or

ii An interallotment drainage system within the proposed development and

the provision of pipe stubs into each adjoining lot. This will necessitate the creation of easements in favour of the existing properties.

All works shall be to the satisfaction of the existing owners and at full cost to the Developer.



7.10.1 Interallotment Drainage Pits

1 Interallotment drainage pits shall be provided to the low corner of each lot unless otherwise approved. The pits are to be a minimum internal size of 450 x 450mm for residential lots (600 x 750mm where pits are deeper than 0.7m) and 750 x 750mm for industrial lots being either cast insitu or approved precast, with a grated surface inlet.

2 Pits will also be provided at changes of pipe size, changes in grade, changes

in pipe type or class and changes in direction of 45° or greater. Such pits are to be a minimum internal size of 600 x 600mm approved precast or cast insitu.

3 Pits shall be covered by an approved grate to provide an adequate surface

water inlet. All grates are to be provided with either "J" bolts or secured galvanised chains to prevent removal. Pit surrounds will be turfed and pegged or netted to a minimum width of 900mm to provide adequate scour protection. Couch turf shall be provided unless the lot is to be totally turfed by the Developer.

4 Pit inlet grates are to be depressed 100mm below surrounding ground level to

assist surface water collection. Note: The 300mm minimum cover applies below the 100mm depression.

5 Step irons are to be provided in all pits deeper than 1.2 metres. Refer

Drawing SD 49A. Such pits are to be a minimum internal size of 750 x 600mm.

6 Connection for roof water for single residential dwellings shall be provided via

a 150mm dia stub into the side of the pit at the invert with a 90° elbow, riser with cap. This requirement only applies for cast insitu pits. Precast pits do not require stubs. Diameters for other developments to be designed.

7.11 CONDUITS, LOCATION AND COVER

1 Pipe Bedding and Cover - Requirements shall be determined from the

Concrete Pipe Association "Concrete Pipe Guide" or AS 3725 Loads on Buried Pipes.

2 Refer Chapter 8 for construction cover/protection requirements, these

will supersede the cover requirements in 1 above. 3 Conduit Location - drainage lines in road reserves shall generally be located

95mm out from the face of kerb and parallel to the kerb. This is applicable up to 525mm diameter only. Refer Drawing SD 48D for larger diameter pipes. Drainage lines in easements shall generally be centrally located within easements.



7.12 EASEMENTS

7.12.1 Easement Widths

Easements are to be provided in private property over pipe systems and overland flowpaths. The minimum width of easement will generally be as follows: a Interallotment drainage 1.5m (300mm pipe maximum) b Pipes 375mm and 450mm 2.0m c Pipes 525mm and 600mm 2.5m d Pipes 675mm to 1200mm 3.0m e Pipes 1350mm to 2400mm 3.5m For larger diameter and multiple pipes a minimum clearance of 500mm from each side of the pipes will be required for easements, with the easement width increasing in 500mm intervals. The above easement widths will necessitate widening where excessive pipe depths occur. Refer Clause 7.3.1 where property restrictions are required in conjunction with overland flowpath drainage easements. All pipes are to be laid centrally within the designated easement. Interallotment drainage easements shall be created in favour of the benefiting properties. All other easements shall be in favour of Council unless advised otherwise.

7.12.2 Building Adjacent to or Over Easements

1 Building over Council or interallotment drainage easements is prohibited except where approved by the Engineer for extremely light buildings such as open carports. Where building works over interallotment drainage is considered, permission from all the owners of the benefiting lots is required. In individual cases, applications for buildings other than the abovementioned may be taken into consideration by Council.

2 In cases where Council has permitted building over an easement, or where a

building is to be erected adjacent to an easement, the following conditions apply:

3 Where an overland flowpath exists or is proposed over a drainage easement

the approval to construct any structure will not be permitted. Generally the lot title will have restrictions placed upon it ensuring that the overland flowpath levels are maintained as constructed and no structures are erected within the flowpath.



4 In cases where a building adjoins an open lined or unlined stormwater

channel, conditions will be imposed in accordance with the situation encountered in each case individually.

5 Where a building adjoins an easement with a piped system: The depth of the

footing at the boundary of the drainage easement is to be level with the invert of the piped drain as a minimum. The footing depth may decrease by 500mm for every 1m increment in distance from the edge of easement depending on soil types.

6 In special cases where there is no possibility of overland flow and where

approval has been given to build over an easement which has a piped drain, pier and beam construction shall be used. The underside of the footing beams are to be a minimum clear distance of 0.60m from the top of the pipe, and all piers shall be located clear of the easement. Concrete encasing of the pipe may also be required and in this instance 0.6m clearance to the underside of the beams is required.

7 Details of the design of piers, beams and footings prepared by a qualified civil

(or structural) engineer shall be submitted for approval with the application. Certification of the design and construction will be required to be submitted by the consulting Engineer.

8 Council will not allow concrete driveways over easements unless it is part of

the approved overland flowpath but will permit block paving.

9 In cases where an open unlined Council stormwater drain is situated in a development site, design and construction of a piped system in accordance with Council’s Engineering Requirements for Development at the applicant's expense may be required by Council.

10 In cases where a Council watercourse or drainage structure is not covered by

easement rights, an easement in favour of Council shall be created over the drainage works prior to the release of building plans.

7.13 STORMWATER DISCHARGE

1 Scour protection at culvert or pipe system inlets and outlets shall be provided

in accordance with the approved engineering plans and current best industry practice unless outlet conditions dictate the use of more substantial energy dissipation.

2 At points of discharge of gutters or stormwater drainage lines or at any

concentration of stormwater from or to adjoining properties, Council will require the Developer to enter into a Deed of Agreement with the adjoining owner(s) granting permission to the discharge of stormwater drainage and the creation of any necessary easements with all costs being met by the Developer prior to development approval.

3 Where the drainage is to discharge to an area under the control of another

statutory authority eg, Public Works, the design requirements of that Statutory Authority are also to be met.



4 Piped stormwater drainage discharging to or through recreation reserves if

permitted under the relevant plan of management is to be taken to a natural water course and discharged through an approved outlet structure or alternatively taken to the nearest stormwater line. The creation of easements may be required.

Generally these piped systems are under overland flow pathways. Such outlets shall extend a minimum of 5m into the reserve to permit pedestrian and vehicular access where applicable. The flowpath shall extend to the pipe discharge point in an approved manner to prevent scouring. Suitable approved safety fencing shall be provided at the outlet point.

5 Where no piped drainage system exists the maximum permissable site

discharge from a development to either the kerb and gutter or table drain shall be 60 litres/sec unless otherwise advised by the Engineer. Refer Clause 7.9 for on-site stormwater detention requirements.

7.14 MISCELLANEOUS

7.14.1 Trenchstops and Bulkheads

The provision of trenchstops or bulkheads will be required a minimum every fourth pipe joint where the pipe grade exceeds 10% and behind the kerb and gutter where the line crosses a road. Consideration shall be given to the ground conditions and length of line. Refer Drawing SD 37D.

7.14.2 Subsoil Drainage

1 Subsoil drainage in Pipe Trenches - Subsoil Drainage shall be provided in pipe trenches as outlined below.

2 In cases where pipe trenches are backfilled with sand or other pervious

material, a 3.0m length of subsoil drain shall be provided in the bottom of the trench immediately upstream from each pit or headwall. The subsoil drain shall conform to the requirements of Drawing SD 18A and as below.

3 The upstream end of the subsoil drain shall be sealed with cement mortar or

other approved means, and the downstream end shall discharge through the wall of the pit or headwall. The outlet shall be provided with galvanised mesh to prevent vermin access.

7.14.3 Kerb and Gutter Discharge

Termination of Kerb and Gutter and Associated Scour Protection - Kerb and Gutter shall be extended to a drainage pit or approved natural point of outlet. At the kerb and gutter discharge point approved scour protection shall be provided of length and type sufficiently adequate to cater for design flows.

7.14.4 Habitable Floor Levels

Should be a minimum 300mm above the surrounding ground levels to negate the possibility of street stormwater and overland sheet flows entering dwellings.



7.15 DOCUMENTATION

7.15.1 Easements and Agreements

1 Evidence of any Deed of Agreement necessary to be entered into as part of the drainage system will need to be submitted to the Engineer prior to any approval of the engineering plans. Easements will need to be created prior to approval of the linen plan of subdivision.

2 Where an agreement is reached with an adjacent landowner to increase flood

levels on his property or otherwise adversely affect his property, a letter signed by all the landowners outlining what they have agreed to and witnessed by an independent person shall be submitted to the Engineer prior to any approval of the engineering plans.

7.16 STORMWATER QUALITY

All developments in the Shire shall ensure that stormwater discharge from the site satisfies environmental requirements by way of restricting increases in pollutants during and post construction. As a guide, the minimum treatment objectives based on Ecologically Sustainable Development (ESD) shall be in accordance with table 8.1 of the E.P.A.’s Managing Urban Stormwater - Council Handbook. In areas that are controlled by regional constructed wetlands, at source treatment is required to carry out the initial cleansing of stormwater by targeting suspended solids, litter, coarse sediments and where specific pollutants are generated by the nature of the catchment, such as hydrocarbons, heavy metals and litter from carparks, specific treatments shall be provided. Manufacturers details for particular devices should include all relevant information on pollutants they treat, flow rates (treatable flows), percentage removal of the targeted pollutants. In areas without such facilities a constructed wetland designed in accordance with Council’s requirements shall be provided. Where the end of pipe treatment is only required to trap gross pollutants, an approved underground, easily maintained precast device shall generally be required. The sizing of the device is to be as designed by the manufacturers technical experts. Where the facility is required to be a constructed wetland the size shall be determined in accordance with Chapter 16.3 of the Department of Land and Water Conservation (DLWC) Constructed Wetlands Manual. Other treatment devices will be considered on merit and the design consultant is encouraged to actively seek out any advancement of knowledge in the stormwater pollution treatment area. The proximity and sensitivity of the receiving waters will also play a significant role in the type/s of stormwater quality treatments required.



In some instances the requirements of table 8.1 of the E.P.A.’s abovementioned Council Handbook may be increased as appropriate. Where appropriate, roads and driveways shall be designed to incorporate swales with capacities as nominated (refer Clauses 7.3.1 and 7.4.2) in lieu of kerb and guttering but should not be used where: • On-street parking is required or the area is subject to high use.

• Roads and driveways have numerous accesses.

• Insitu soils have high clay content or are susceptible to heavy compaction. • Roads area either less than 1% grade and greater than 5% or where catchments

are greater than 2 Ha.

Advice should be sought from Consultants who are adequately experienced and are aware of the rapidly expanding knowledge being gained in this relatively new field regarding initial planning and detail design.

09 ch7 stormwater drainage

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