Base Engineering Assessment Program Stormwater Systems

Capacity Assessment

1. Stormwater pipes and pits

10. 
    Pit and pipe stormwater networks are generally required to accommodate a 1 in 10 year ARI storm event in accordance with Defence guidelines (Standard Clauses on Civil Engineering for Inclusion in Functional Design Briefs). The local government typically requires design for a 1 in 10 Year ARI.
    
11. 
    Defence Clauses for Function Design Briefs (Design Brief) provides a benchmark for the capacity and compliance of the stormwater system. An extract is included in. Table 1.1.
    

1. 
   Defence Clauses for Function Design Briefs
   

12. 
    The capacity of gravity stormwater mains and inlets is generally assessed by:
    

• 
  Discussions with base staff ascertain whether any capacity issues are evident (e.g. localised flooding events occurring at regular frequencies)
  
• 
  Undertaking hydraulic modelling of the stormwater trunk system using an appropriate software package and a 1 in 10 year ARI design storm event.
  

1. Condition Assessment

   1. Stormwater Pipes

13. 
    Stormwater pipes including culverts are graded on a scale of 1 to 5, with 1 representing ‘As new’ condition and 5 representing an ‘unserviceable’ condition (as per 1.1.1.1.1Table 1.1).
    
14. 
    Stormwater mains are graded using two criteria:
    

• 
  Structural: The extent and significance of structural defects such as cracking, holes, collapse/partial collapse, and joint displacement. Typically, a stormwater pipe with a poor structural score requires renewal either through re-lining, pipe bursting or either replacement.
  
• 
  Serviceability: The extent and significance of defects that affect hydraulic performance such as tree root intrusion, debris, and silt/ solids deposition. In many instances, a main with a poor serviceability score can be addressed through maintenance (e.g. jetting or jetting and root foaming). Where blockages resulting from root intrusion continues to be frequent even with improved maintenance, or, if the extent of root intrusion causes structural defects, renewal of the stormwater pipe may need to be considered where cost-effective.
  

15. 
    Using the Conduit Inspection Reporting Code of Australia, WSA 05-2013, Water Services Association of Australia (WSAA), a standard score is assigned to each defect using proprietary software such as Wincan Pipe Inspection Software. The scores are aggregated to derive peak and average condition scores. These are also combined into an overall score.
    
16. 
    The Conduit Inspection Reporting Code of Australia grading is consistent with the baseline parameters adopted for condition; however, a grading of 5 is assigned to mains in a very poor condition even though the mains are still functional.
    
17. 
    Stormwater pipes are normally assessed using Closed Circuit Television (CCTV) inspections. Ideally, all stormwater mains should be inspected by CCTV at regular intervals (10 to 15 years) as part of best industry practice. Where no existing CCTV footage is available or a comprehensive survey is cost-prohibitive, a representative sample of approximately 20% of the network should be used. The sample should contain:
    

• 
  Stormwater pipes of different diameters
  
• 
  Stormwater pipes of different materials
  
• 
  Stormwater pipes where problems have been experienced (e.g. poor hydraulic performance or blockages due to debris)
  
• 
  Stormwater pipes in different areas of the base (to ensure the sample includes stormwater mains from different stages of the base development).
  

18. 
    In addition to CCTV inspections, known pipe material and age can be used to estimate stormwater pipe condition and should be confirmed through discussions with the CMS, who may have experience or records indicating the physical condition of certain material types or ages arising from any maintenance undertaken.
    
19. 
    The standard asset life for each network component should be considered in the absence of specific data and will require either the age or material type of stormwater pipe (or both), to estimate the condition based on the percentage of the nominal asset life remaining. The nominal asset life for different pipe materials is included in Appendix C.
    

1. Stormwater Pits, Headwalls, Tidal Flaps and GPTs

20. 
    The pits, maintenance holes, headwalls and tidal flaps are graded on a scale of 1 to 5, with 1 representing ‘As new’ condition and 5 representing an ‘unserviceable’ condition as outlined in (structural) and Table 1.2 (serviceability). Tidal flaps are assessed using the pit covers condition description. Headwalls are assessed using the pit walls condition descriptions.
    

1. 
   Structural condition for pits, headwalls, tidal flaps and GPTs
   

2. 
   Serviceability condition for pits, headwalls, tidal flaps and GPTs
   

1. Open Lined Channels and Unlined Channels

21. 
    The condition assessment of open lined channels and unlined channels is based on Table 1.1 and Table 1.2.
    

1. 
   Structural condition for open channels
   

1. 
   Serviceability condition for open channels
   

22. 
    Pits and maintenance holes are normally assessed through physical inspections from the surface only (i.e. no man entry into the holes). Thorough inspections of the holes can be undertaken when the covers are removed for CCTV inspections, or can be undertaken in conjunction with maintenance staff by removing the cover (the number of operators required to remove the cover is related to the size, configuration and weight of the cover). Mechanical lifting is normally required to remove the cover.
    
23. 
    While the majority of covers can be assessed relatively simply from surface inspections, the internal condition will generally only be assessed for a representative sample of pits and maintenance holes (approximately 20%-50% of the network), as a complete inspection is time and cost prohibitive. The sample should be similar to that outlined for stormwater pipes.
    
24. 
    If a cover cannot be removed or the maintenance hole is buried, the condition is considered to be unserviceable (5) as it cannot function as intended.
    

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