GUIDE TO THE PROVISION OF STORMWATER SOAKWELLS IN SANDY SOILS AT THE LOW POINT OF A DRAINAGE CATCHMENT NOT EXCEEDING 10,000M2
A. The Guide shall not be used for any other purposes except the disposal of stormwater on a single residential/ commercial/ industrial site.
B. If a catchment area exceeds 10,000m2, the stormwater drainage sump shall be designed in accordance with “Australian Rainfall and Runoff”, the Institution of Engineers, Australia 1987 and/or any reputable drainage calculation program that is satisfactory to the City of Cockburn such as the “PCSump” software program, by Main Roads Western Australia.
C. Regular removal of debris and silting material from soakwells at an interval not more than 3 years is required. This statement must be included in all drainage management plans submitted to council for approval.
1. Define stormwater drainage catchments within the property boundaries and mark them on a drawing preferably at a scale of 1:200, showing the existing and proposed contour levels, buildings and car parks.
2. Calculate the impervious area (e.g. rooftops, bitumen/concrete/brick paving or hardstanding areas including car parks) of each drainage catchment as defined above.
V = A 80 Where A is the impervious area of a catchment measured in square metres; m2
And V is the required soakage volume of that catchment measured in cubic metres; m3 4. Decide what type and size of soakwells are to be installed; for example 1800mm
diameter and 1800 mm deep. 5. Therequirednumberofsoakwellscanbecalculatedasfollows:N = 1000 x V S
Where N is the number of soakwells. And S is the volume of a single soakwell
Notes: 1. Thenumberofsoakwellsisalwaysroundeduptothenearestwholenumber. 2. WhereNisgreaterthan1,allsoakwellswithinthecatchmentshallbelinkedtogetherby
drainage pipes, the diameter of which shall not be less than 100mm.
The rooftop area is 15.24m x 36.50m = 556m2 The required soakage volume is 556 80 = 6.95m3 Select 1200mm diameter x 1200mm deep soakwells Table 1 shows that this soakwell has a volume of 1357 litres The required number of soakwells is 6.95 x 1000 / 1357 = 5