Gosnells

STORMWATER DISPOSAL SITES WITH A LOT CONNECTION

SANDY SOIL SITES <350m²

REFER TO STANDARD DRAWING ES 40

Any development brings about an increase of impervious or slow draining areas, when compared to the previous use of the land. This results in increased stormwater runoff rates and a decreased time for the excess stormwater, that can now no longer be infiltrated on the development site, to reach the City’s drainage system. As a result and given the accumulation of flows, the comprehensive drainage system can no longer function properly and flooding may occur.

On-site infiltration facilities provide temporary storage for infiltration of stormwater runoff from developments and restricts the discharge from the site at a rate which the existing drainage system is capable of accommodating, mimicking pre-development conditions. Please read these guidelines in conjunction with Standard drawing ES 39. The following standard advice notes and requirements shall be adhered to when constructing stormwater disposal systems specific to lots less than 350m², with predominantly sandy soil conditions, room to provide sufficient numbers of soakwells and good infiltration potential. Variation to these requirements is possible, based on site location and soil environmental conditions and where considered justified, and should be discussed with the City of Gosnells’ (CoG) Technical Service Branch. Please contact them on 9397 3000.

1. If the subdivisional development has allowed for a connection to the street drainage system, then the soakwell design is to be based on retaining & infiltrating the critical 1 in 100 year storm event on site. Unless otherwise determined by the CoG technical services branch, phone 9397 3000. Required capacities - accounting for clogging safety factors, should be calculated from the spreadsheet available on the City’s website – www.gosnells.wa.gov.au. A printout of the completed spreadsheet is to be submitted with the building application

2. If a lot connection pit is not provided please contact CoG

3. All soakwells are to be interconnected on site

4. All soakwell clearances from footings and boundaries to meet BCA and geotechnical requirements

5. Soakwells should generally not be deeper than 1.2m. If deeper than 1.2m step irons must be installed

6. A silt pit is required or a soakwell positioned to act as silt pit, with outlet 300mm minimum from base of pit

7. Runoff from roof areas is to be evenly distributed into soakwells at front and rear of the property

8. Grated inlets are to be installed at the base of all downpipes. (See detail on Standard Drawing ES 40)

9. When pipes are located under the slab they must be a minimum 150mmD, SN8 sewer class UPVC

10. 2% grade on all pipes used to interconnect soakwells

11. Where there is an existing spigot from a neighbouring lot and a lot connection pit is provided on that lot, then the silt pit may be omitted on the subject lot

12. An overflow connection to CoG's drainage system for larger storm events is required

13. The lowest lid invert level in the interconnected system is to have a grated access opening for emergency overflow paths to the road via the driveway or an alternative CoG approved flood path

14. Where an overland flow path is not possible, pumping may be required. Liaise with CoG technical services for this situation

15. When a soakwell or silt pit lid is in a trafficable area it must be fitted with a trafficable lid and have a 150mm precast concrete base

16. Rainwater tanks should be considered to compliment retention and for non-potable use. Dispose of overflows to soakwells

17. Additional sub-soil drainage might be required where shallow groundwater or low permeability subgrade (clayey subgrade/coffee rock) is present

18. Calibrated correction of the ground water level (gwl) should be made for groundwater that is measured in summer to identify winter peak gwl's or average annual maximum groundwater levels (AAMGL)

19. No topsoil or over burden is to be buried on site. Maximum 100mm topsoil depth

20. The pad must not be cut into the existing finished lot levels, nor should it be left lower than the remaining part of the lot.

STORMWATER DISPOSAL WITHOUT A LOT CONNECTION

SANDY SOIL SITES 350m²

REFER TO STANDARD DRAWING ES 39

Any development brings about an increase in impervious or slow draining areas, when compared to the previous use of the land. This results in an increased stormwater runoff rate and a decreased time for the excess stormwater, which can no longer be infiltrated on the development site, to reach the City’s comprehensive drainage system. As a result, and given the accumulation of flows, the comprehensive system can no longer function properly and flooding may occur.

On-site infiltration facilities provide temporary storage for infiltration of stormwater runoff from developments and restricts the discharge from the site at a rate which the existing drainage system is capable of accommodating, mimicking the pre-development conditions. Please read these guidelines in conjunction with Standard drawing ES 39. The following standard advice notes and requirements shall be adhered to when constructing stormwater disposal systems specific to lots equal or less than 350m², with predominantly sandy soil conditions, room to provide sufficient numbers of soakwells and good infiltration potential. Variation to these requirements is possible, based on site location and soil environmental conditions and where considered justified, and should be discussed with the City of Gosnells’ (CoG) Technical Service Branch. Please contact them on 9397 3000.

1. The soakwell design is to be based on retaining & infiltrating the critical 1 in 100 year storm event on site, unless otherwise determined by the CoG technical services branch. Pre-development conditions for the purpose of this application refer to an undeveloped natural site. Required capacities - accounting for clogging safety factors, should be calculated from the spreadsheet available on the City’s website – www.gosnells.wa.gov.au. A printout of the completed spreadsheet is to be submitted with the building application

2. All sites are to have interconnected soakwell pits with minimum 100mm diameter pipes connecting the pits within an area. Where the pipes are placed under the house foundation pad and between areas they should be a minimum 150mm diameter Type SN 8 Sewer Class UPVC

3. All soakwell clearances from footings and boundaries are to meet BCA and geotechnical requirements. Typical minimum offset from house footings and other structures to soakwells in free draining sand is 1.5m

4. The base of a soakwell should generally not be deeper than 1.2m. When deeper than 1.2m, step irons must be installed. An allowance must be made for cover and lid depth requirements over soakwells as this will impact on the usable volume

5. Runoff from roof areas is to be evenly distributed into soakwells at front and rear of property

6. 2% grade on all pipes used to interconnect soakwells

7. The lowest lid invert level in the interconnected system shall be located at the road reserve side of the property and is to have a grated access opening for emergency overflow path to road via driveway or alternative CoG approved flood path

8. Grated inlets are to be installed at the base of all downpipes (see detail A on drawing ES39)

9. No overland overflow or piped connection to CoG's drainage system is permitted up to the design storm event

10. The option of rainwater tanks should be considered to compliment stormwater infiltration and for not-potable use of rainwater. Dispose overflow into soakwell pits

11. No topsoil or over burden is to be buried on site. Maximum 100mm topsoil depth

12. FFL and lot grades should allow for safe overland flow path for storm events greater than 1:100 year ARI to the road or an open drain

13. The house foundation pad must not be cut into the existing finished lot levels nor should it be left lower than the remaining part of the lot

14. A design engineer is to be engaged when subsoil drains are considered necessary due to high groundwater levels or where low permeability subgrade is present (clayey subgrade/coffee rock).

STORMWATER DISPOSAL

CLAY / SILTY SANDY SOIL SITES  350m²

REFER TO STANDARD DRAWING ES 41

Any development brings about an increase in impervious or slow draining areas, when compared to the previous use of the land. This results in increased stormwater runoff rates and a decreased time for the excess stormwater, which can no longer be infiltrated on the development site, to reach the City’s comprehensive drainage system. As a result and given the accumulation of flows, the comprehensive system can no longer function properly and flooding may occur.

On-site detention facilities provide temporary storage of stormwater runoff and restrict the discharge from the developed site, at a rate which the existing drainage system is capable of accommodating, mimicking pre-development conditions. Please read these guidelines in conjunction with Standard drawing ES 41. The following standard advice notes and requirements shall be adhered to when constructing stormwater detention systems specific to lots equal or greater than 350m², with predominantly clayey sand or silty sand soil conditions and poor infiltration potential. Variation to these requirements is possible, based on site location and soil environmental conditions and where considered justified, and should be discussed with the City’s Technical Service Branch. Please contact them on 9397 3000.

1. The detention system design is to be based on retaining the balance between the pre and post-development critical 1 in 100 year storm event on site. Pre-development conditions for the purpose of this application refer to an undeveloped natural site. Required capacities should be calculated from the spreadsheet available on the City’s website – www.gosnells.wa.gov.au. A printout of the completed spreadsheet is to be included with the building application

2. A design engineer is to be engaged when subsoil drains are considered necessary due to high groundwater levels or re-active soils

3. All sites are to have interconnected pits with minimum 100mm diameter pipes connecting the pits within an area. Where the pipes are placed under the house foundation pad and between areas they should be a minimum 150mm diameter Type SN 8 Sewer Class UPVC

4. Pits and pipes must be designed and graded to completely drain after storm events, so as to provide full storage capacity within the next 24 hours

5. Outlets are to be at the base of all pits, allowing for sufficient pipe grades

6. Grated inlets are to be installed at the base of all downpipes (see detail A on drawing ES 41)

7. An overland overflow via the driveway or similar route is required in combination with a low level controlled outlet to the City’s drainage system

8. FFL and lot grades should allow for safe overland flow path for storm events greater than 1:100 year ARI to the road or an open drain

9. Where overland flow path is not possible, pumping may be required. Liaise with CoG technical services for this situation

10. A silt pit is required on site - see drawing ES 50 - lot connection pit, prior to connection to the City’s drainage system

11. All clearances from footings and boundaries are to meet BCA and geotechnical requirements

12. The house foundation pad must not be cut into the existing finished lot level nor should it be left lower than the remaining part of the lot

13. No topsoil to be buried onsite. Maximum 100mm topsoil depth

14. The option of rainwater tanks should be considered to compliment stormwater detention and for not-potable use of rainwater. Dispose overflow into stormwater pits

15. Designs are to be checked and approved by the City of Gosnells before construction.