Attachment C - Stormwater report

 7.            Summary and Recommendations
This SMP has taken into consideration the historic and proposed land use of the site, and the hydrologic
and flooding behaviour. It also considers potential pollutant loads for the site and the catchment as a
whole. Based on these considerations, the following conclusions are made.

         The previously approved turf farm operated for 30 years and applied considerably higher
          quantities of nutrients (in the form of fertiliser) to the site. Based on the data available it is
          expected the sediment and nutrient loads exiting the site will reduce considerably as a result of
          the proposed change in land use due to decreased application of fertilisers.
         While the irrigation water is high in nutrients, the hydroponic setup is a closed system. Due to the
          lining of the trenches in which the plants and growbags are located, no irrigation water can enter
          the soil profile. Furthermore, as the system is located inside the greenhouse, rainfall and overland
          flows cannot convey nutrients associated with the hydroponics operations to the receiving
          watercourse under normal conditions.
         While the site is subject to inundation during major storm events, the likelihood of nutrient rich
          irrigation water being conveyed to the receiving water is minimal. As indicated by the regional
          hydrograph for the 1% AEP event, there is considerable time before the peak of the hydrographs
          reaches the site. Therefore, operators at the site will be aware of the risk of flooding well in
          advance of the site being inundated. A flood management plan has been developed to ensure the
          operators are able to extract the irrigation water from the system and flush the growbags with
          fresh water prior to inundation of the site. The irrigation is a closed loop system and the irrigation
          water can be extracted and stored in a tank located above the water levels associated with the
          major flood event.
         The pollutant loads generated within the entire catchment, including neighbouring properties far
          exceed those that would potentially leave the site during its operation as a tomato farm. The
          results of this assessment indicate that the annual nutrient loads applied to the tomato crop
          through the irrigation system represents only 6.3% and 4.4% of Total Nitrogen and Total
          Phosphorus loads generated within the entire catchment. Given that TN and TP levels mentioned
          are annual percentages, at any one time the nutrient loads in the irrigation system would be far
          less than these levels, and considering that the irrigation water can be extracted from the system
          in the event of a major flood event the potential for harm is minimal.
         Hydrologic and hydraulic modelling of the site for the pre- and post-development scenarios
          indicates the construction of the greenhouse results in no increase in peak flow rate from the site.
          Therefore, no mitigation measures (i.e. detention basins or tanks) are required to maintain pre-
          development peak flow rates from the site. The requirements of QUDM are therefore satisfied.
         Results of the modelling indicate in the post-development scenario an additional 540m³ will be
          generated during the critical duration (i.e. 180 minute) 1% AEP storm event.
         To satisfy the requirements of the Queensland State Planning Policy’s Water Quality Code, Water
          Sensitive Urban Design (WSUD) features would be required to capture and treat runoff from the
          roof area of the greenhouse. Any runoff for the roof will be low in TSS loads, and the nutrients
          contained will only be associated with atmospheric deposition.


Stormwater Management Plan – Lot 1 on SP108094, 305 Lake Macdonald Drive                                 Page 31

D’VINERIPE
R003370