Base Engineering Assessment Program Stormwater Systems

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Compliance Assessment

Engineering Development Standards prepared by the local government in which the base is located
AS3500.3 Stormwater Drainage
Standard Clauses on Civil Engineering for Inclusion in Functional Design Briefs, Department of Defence (http://www.defence.gov.au/im/policy/technical/standard_inclusions/civil_feb06.doc

The capacity and compliance of piped drainage systems are intrinsically linked. Pipes and pits are considered non-compliant if they cannot adequately convey the 1 in 10 year ARI.
Pits should also be assessed for compliance with AS3500 requirements regarding safety and access. AS3500 notes minimum pit cover sizes and minimum internal dimension for man access into the pit for cleaning and maintenance. The cover, internal dimensions and the arrangement of step irons should be assessed as part of the investigations.

The pit and pipe stormwater network is required to accommodate a 1 in 10 year ARI storm event. Overland flow paths (which usually consist of roads, car parks and open channels) are designed to appropriately convey overland flows in excess of this event. The 100 year ARI storm event is an accepted design limit for overland flow paths (and is used by the local authority in this instance and the governing criteria).

The stormwater system data collection questionnaire (Appendix E) includes specific questions relating to the stormwater network. Data collection forms (Appendix D) also include prompts for specific data requirements that are needed to undertake the assessment.
Data require for the flood immunity assessments includes site surface levels, layout plans for the site, details of any detention basins, ponds, weirs etc., previous hydraulic modelling studies and any flood hazard studies. Projections of future sea level rises will be required for coastal bases.
The extent of available data should be confirmed during Phase One to establish what studies have already been undertaken, what data is available and where data gaps exist.
Discussions with base staff will provide some indication of recent flood events.
Accurate ground level data is required for flood modelling. The availability of LiDAR data for each site should be verified during Phase One, as historical survey information may be sufficient to complete the assessment.

Where recent flood studies have been undertaken, these will be reviewed and used as a basis for assigning a capacity grade.
If previous studies are limited, do not exist, or do not address the relevant design criteria, then hydraulic modelling will be undertaken using an appropriate software package for the 10, 50 and 100 year ARI design storm events. The analysis will identify areas of localised ponding and flood inundation.

Condition assessments will be undertaken on flow paths within stormwater catchments and will assess whether barriers or obstructions to stormwater flow exist. A serviceability grading will be assigned to each major flow path or catchment. This will be undertaken at a high level as the response will typically involve maintenance activities such as vegetation clearing, which is relatively low cost.

Serviceability - Vegetation

Vegetation has no effect on open conduit

Remediation expected to be required within 3 months

Overgrown, appears choked with weedy vegetation, waterway affected by thick plants and shrubs within channel – Remediation required

Serviceability - Debris

No significant debris in flow path

The extent of debris/ barriers may have limited impact on overland flow

Debris or barriers exists within the flow path which will significantly impact on run-off - remediation required

The Defence Clauses for Function Design Briefs (Design Brief) provide a benchmark for the capacity and compliance of the stormwater system. An extract is included in Table 1.1.
Compliance is assessed by undertaking hydraulic modelling for the 10, 50 and 100 year ARI events.

Location	Criteria
Operational Buildings	Floor level 0.3m above the 100 year ARI flood level.
Other buildings	Floor level 0.2m above the 50 year ARI flood level.
Main access road	Depth of flooding during a 50 year storm should be less than 0.1m.
Other roads, car parks, etc.	Depth of flooding during 10 year ARI storm should not exceed 0.1m for roads and 0.03m for car parks.

Flood Hazard Classes, as outlined in Table 1.2 (Melbourne Water) should also be used to evaluate flood hazard within the study area. A risk class of moderate (2) or above is indicative of the point at which operations by able bodied adults would become impeded. This is also the approximate threshold at which normal passenger cars begin to float.

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