Description - Karoro/Paroa
Figure 4.25: Karoro/Paroa Sewerage Scheme
Karoro
CD
South Beach
Kowhai St Pump station Oxidation Pond
Jacks Road
Pump station
Paroa
State Highway
Paroa Pump station
TASMAN SEA
Reticulation
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The Karoro/Paroa sewerage scheme (not a combined sewerage/storm water scheme) is shown in Figure 4.25 above and is described as follows:
• The Karoro/Paroa sewer network collects sewage from Karoro, South Beach and Paroa. • The reticulation is made up of four subsystems:
o South Paroa: The South Paroa reticulation collects sewage from the southern end of Paroa between Rutherglen Road and just south of Clough Road. The sewage is discharged at the State Highway/Paroa pump station;
o North Paroa: The North Paroa reticulation collects sewage from the State Highway/Paroa pump station and the area around Clough Road to Mill Creek. The sewage is discharged at the Jacks Road pump station;
o Karoro: The Karoro reticulation collects sewage from the area between Two Mile Creek to Kowhai Drive. The sewage is discharged at the Kowhai Street pump station;
o South Beach: Sewage from the Jacks Road pump station and the Kowhai Drive pump station is pumped via rising mains into the South Beach reticulation. The South Beach reticulation also collects sewage from all of South Beach from Stanton Crescent in the north to Jamieson Street in the south.
• All the sewage from the four sub-systems flows to two oxidation ponds;
• The effluent from the oxidation ponds either flows under gravity or is pumped from the ponds to an ocean outfall.
• The total length of pipe in the reticulation network including the rising mains and ocean outfall is 16,000m; approximately 60% of pipes are PVC and 40% are AC. Details of the pipe in the Karoro and South Beach/Paroa areas are presented in the following table.
Table 4.17: Summary Table - Karoro/Paroa Sewer Network Area Pipe Type Length
(m)
Percentage Size Range Dia. (mm) Year Installed AC 6,597 87.6% 100 - 150 1971 -1985 Karoro Earthenware 63 0.8% 150 1971 PVC 867 11.5% 100 - 300 1971 - 1999 Sth Beach Paroa PVC 8,486 100% 100 - 300 1999 - 2003 Total 16,013
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Deficiencies - Karoro/Paroa
In the event of a major Alpine Fault earthquake such as that described in Section 2.7, the Karoro- South Beach-Paroa area is expected to experience strong intensity MM VIII to MM IX shaking that will induce severe damage to the sewerage system. Most of the area is located on geologically recent alluvial soils, with some ground settlement and distortion to be expected in places. Liquefaction can be expected in areas of looser sandier soil. The oxidation ponds, pump stations, manholes and pipe alignment are likely to be affected particularly where liquefaction occurs.
There is a significant risk that the Karoro-South Beach-Paroa sewerage scheme may fail to transport, treat and discharge sewage due to the following:
• Major pipe failures. Based on the damage assessment chart in Appendix A and the MM values assigned in Figure 2.5 (MM VIII to IX), it is expected that:
o PVC pipes - 25% of entries and junctions will fail generally, and between 50% to 70% in areas that are affected by liquefaction, and
o Earthenware and AC pipes with rubber ring joints - 50% of entries and junctions will fail generally, and between 70% to 80% in areas that affected by liquefaction.
Based on the above failure rates it is likely that between 35% to 66% of entries and junctions in the sewer network will fail, with a higher rate of failure in the AC pipes in the Karoro area.
• As it is almost certain that the power supply grid will fail there will be no power to the four pump stations;
• Manholes and pump stations are likely to move particularly manhole structures that are empty and/or have not been designed for earthquake lifting forces. This movement is likely to disrupt pipe alignment at entries and exists, especially where joints are rigid. This may allow surrounding soil material to be washed into the pipes and structures when pumps are started again;
• Sewers may fail in the vicinity of the CD post (Karoro School) preventing sewage generated at the post from being transported away; and
• Damage is likely to occur to the oxidation pond resulting in sewage breaching the pond banks.
The assessed failure of scheme components and resultant surcharging of sewage (4 locations) are presented in Figure 4.26 and described as follows:
o Surcharge 1: Kowhai Street Pump Station - On loss of power supply the pump station will stop and surcharging will occur at the pump station. The surcharge will discharge to Watsons Creek;
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Total or significant failure - no flow Major Joint failure & alignment damage & blockages - marginal flow Minor to moderate damage - close to normal flow plus I/I flows
Potential areas of surcharge
o Surcharge 2: Oxidation Ponds - The oxidation pond pump will stop on loss of power. Effluent from the pond will discharge at low tide (without the pump). The oxidation pond wall is likely to have subsided during the earthquake due to liquefaction. Effluent will breach the pond wall via the subsided section and will pond outside the oxidation pond before seeping into the surrounding sandy soil;
o Surcharge 3: Jacks Road Pump Station - On loss of power supply the Jacks Road pump station will stop and surcharging will occur at the pump station. The surcharge will discharge to Mill Creek; and
o Surcharge 4: State Highway/Paroa Pump Station - On loss of power supply the pump station will stop and surcharging will occur some distance away at Paroa School. The surcharge will discharge to Saltwater Creek.
o Sewer failure in the vicinity of the CD post may prevent sewage at the post being transported away; and
o Damage to inadequately restrained equipment at the oxidation ponds.13
Figure 4.26: Assessed Karoro/Paroa Sewerage Scheme Failure and Surcharging
Karoro
CD South Beach
1
Kowhai St Pump Station
2
Jacks Road Pump Station Oxidation Pond
3
Paroa
State Highway
Paroa Pump Station
4
TASMAN SEA
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Upgrades & Improvements - Karoro/Paroa
The risks identified in the previous section will be reduced by implementing the improvements outlined in Section 4.4.2 along with undertaking a geotechnical and structural assessment of the oxidation ponds to quantify the earthquake risks at the site and recommend any structural improvements to address these risks.
It is anticipated that recovery of the Karoro/Paroa sewerage system will proceed as follows after a major Alpine Fault earthquake:
• Power to the sewage pump stations will fail at time zero, the time of the earthquake, or very shortly afterwards;
• Significant joint failure is expected - more in Karoro than in South Beach and Paroa. Workmen will check for surcharging at known trouble spots and where sewage is ponding workmen will arrange temporary drains to nearby waterways;
• The community will be informed not to discharge wastewater to the sewer and temporary sanitation measures e.g. Portaloo (where available), pit latrines, etc will be put in place; and • The sewerage scheme will be assessed and contracts let to repair the scheme.
Draft Final 179 December 2007 099