9 Water Q uality
9.1 Design Objectives
For these guidelines the design criteria for stormwater treatment has been targeted at achieving:
• 75% removal of suspended sediment as required by TP10 (ARC, 2003)
• Compliance with TP10 design requirements
• Removal of all sediments >100Pm and allowance for the capture of fines of up to 20Pm
• Allow for some ‘capture’ of dissolved metals
9.2 Design Approach
9.2.1 Key factors
Permeable pavements have been previously designed as an infiltration device where the pollutant removal capabilities were based on land application and treatment through soils (Debo & Reese after Schueler).
Since infiltration is not applicable to a low permeability sub grade the focus of contaminant removal mechanisms is filtration and sedimentation.
For filtration systems TP10 considers filtration, adsorption and absorption, and settlement as key contaminant removal mechanisms.
9.3 Design Specifications
9.3.1 Pavement Construction
Filtration is designed to capture sediments with particle size >100Pm and this shall be in the form of a sand filter layer with a geotextile placed beneath it.
Settlement of fines shall be for particles of up to 20Pm, and a retention area is incorporated in the base course layer.
• Retention time of at least 1.5 hours shall be provided; and
• The base course shall have a minimum thickness of 200 mm to provide sufficient adsorption space for dissolved chemicals.
For stormwater treatment the following profile is recommended. Pavers, sand bedding and the basecourse
SECTION 9
Water Quality
pavers
Bedding layer / Jointing Sand 50mm
Sub Base 200mm minimum
Retention time 1.5 hours
Subgrade Non woven
Geotextile
Liner
Figure 9-1 - Typical Cross Section Of Permeable Pavement
9.3.2 Geotextile Specification
Geotextiles should be designed as follows:
1. AOS 100 um 2. kgeotechnical kpavement
3. (kgeotechnical/t)*h*Ag >+ qrequired
4. kgeotechnical/t = permittivity 0.5 sec-1
where:
AOS = apparent opening size h = average head
Ag = effective geotextile area qrequired = flow rate
9.3.3 Infiltration Rate
• The design infiltration rate of a permeable paving system shall be one tenth of the infiltration rate of a new system.
• Sufficient storage capacity must be provided in the basecourse voids to allow the flow control and treatment.
• Minimum retention time for water quality objectives is 1.5 hours where extended detention is not
SECTION 10
Construction
10 Constructio n
10.1 Erosion and Sediment Control
The construction sequence is paramount to the successful performance of the permeable paving. The following aspects must be considered:
• No sediment-laden stormwater should be allowed to enter the permeable paving area during construction, as it will clog-up the storage voids and the permeable surface. Runoff from exposed surfaces, if any, (i.e. surfaces not grassed/vegetated/paved) should not be allowed to flow across the permeable paving surface.
• Preferably, all work surrounding the permeable paving should be completed and all surfaces surrounding the permeable paving should be stabilised (i.e. paved or grassed or vegetated, as appropriate) prior to commencing the permeable cell construction.
10.2 Discharge into Basecourse not Permitted
No runoff should be allowed to enter or be piped into the basecourse directly. All water must enter the basecourse through the permeable paving.
10.3 Flexible Membrane Liners
If a Flexible Membrane Liner (FML) is required the following quality control procedures must be applied as a minimum:
• The FML and the protective overlay geotextile should be supplied and installed by a specialist Subcontractor competent in this type of work
• The base and sides of cells should be free of sharp objects and angular objects that may puncture the FML. Cell surfaces must be inspected and approved by the Specialist Subcontractor prior to
installing the FML.
• Daily Test Welds: A test weld, 3 metres long should be made with each welding machine, and each operator, each day prior to welding commencing, and under the same conditions as exist for the FML welding. The testing should be in accordance with ASTM D4437 (ASTM, 2003). As well as on each day of welding, the testing should be repeated any time the machines are restarted after cooling or when weather conditions change.
• If necessary, random weld samples from the installed welded sheeting should be tested.
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Construction
10.4 Joint Sand and Bedding Sand
Random samples of joint sand and bedding sand must be tested to verify compliance with the design grading.
10.5 Basecourse Material
Random samples of basecourse material must be tested to verify compliance with the design grading, voids ratio and compacted permeability.
Due to the coarse nature of the materials, permeable aggregates are prone to segregation during the transportation and construction process. Care should be taken to avoid segregation during handling of the materials.
The pavement layers should be compacted in layers of uniform thickness not exceeding 150mm to ensure that the maximum density is achieved for the particular aggregate type and grading, without crushing the individual particles.
10.6 Commission Testing
Infiltration Test: Based on: Pollution Retention Capability and Maintenance of Permeable Pavements (Dierkes et al., 2002.)
At the commissioning, it is recommended that infiltration tests be carried out to verify the new infiltration rate. A suggested method is as follows:
• Place an open metal cylinder with a minimum diameter of 750mm on the surface of the paving. The bottom end of the cylinder must have a soft thick rubber gasket.
• Weigh down the cylinder to prevent movement and to achieve seal between the bottom end of the cylinder and the paved surface.
• Calculate the new infiltration rate as stated in the design (see example below):
Example:
Test cylinder diameter 750mm
New infiltration rate stated in the design 1200mm/hr Rate per minute 3.14 x (0.75)2/4 x 1.2 x 1,000 / 60 8.84 L/minute
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Construction
• Soak the test area within the cylinder for 10 minutes to saturate the blocks. Clean and mop the surrounding area of the cylinder
• While discharging the hose to a suitable outlet (e.g. catchpit), adjust the discharge rate to the new infiltration rate
• Direct the discharge into the test cylinder and time it for 5 minutes
• To pass the test, there should be no standing water within the test cylinder and no visible leaks through the joints between blocks, at any time within the 5 minutes or after the test.
• If the test fails, reduce the rate by 20% and repeat the test
10.7 Concrete Paving Blocks
The construction of the concrete segmental paving including the placing, preparation of the bedding sand and joint filling should be in accordance with NZS 3116:2002 ‘Concrete Segmental Paving’. Where appropriate, additional specific information or advice on the construction of a permeable pavement should be sought from the concrete block manufacturer.
SECTION 11