9Structural Controls - Department of Water - The Western Australian ...

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Annual rainfall pattern: Denmark
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32 Stormwater Management Manual for Western Australia: Structural Controls
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Decile 5 (median) monthly rainfall (mm) Mean number of days of rain >= 1 mm
Figure 1. Denmark annual rainfall pattern. (Source: Bureau of Meteorology 2007.)
The year-round rainfall experienced in this region means that less emphasis needs to be placed on treating
the first flush events that occur in the drier areas of the State. First flush rains generally carry higher
pollutant loads that have built up in urban catchments over the dry season. In regions where year-round
rainfall occurs, it is believed that these pollutants are flushed more regularly and so there is less of a
concentrated load in the first flush rains. In the southern coastal region, pollutants mobilised by the first
major rains of the season are still significant and must be addressed, but it is more important to develop
stormwater management systems that will function effectively year-round.
Approaches implemented
The best management practices introduced in this stage of the subdivision were based on knowledge
gathered from earlier stages, site investigations undertaken and the latest practices and principles of WSUD.
The implemented stormwater management system consisted of the following structural (Figure 2) and nonstructural
techniques:
Structural:
• kerbed roads elevated above natural surface levels, to not interfere with subsurface flows
• side entry pit collection systems that bubble-up into adjacent detention areas (Figure 3)
• flow retention and detention in soakwells and swale systems capturing up to the 1 in 1 year ARI
event
• piped conveyance system for flows exceeding the 1 in 5 year ARI event (Figure 4)
• extended detention basins and flow dissipation structures at the end of each piped system (Figures 9
and 10)
Non-structural:
• maintenance of the stormwater system to ensure effective operation of the BMPs
The stormwater management approach that has been developed for this site relies heavily on infiltration of
stormwater as close to its source as possible, to maintain the pre-development hydrology of the catchment.
Infiltrating stormwater at or near its source has minimised the impact on the water balance of the perched
water table system. The at-source treatment of stormwater also reduces the transfer of any pollutants
associated with urban development to the final receiving water body, the Walpole Inlet. No single BMP
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