Council Report - 132005.638.03 - OTHER CHANGE TO AN EXISTING APPROVAL FOR MATERIAL CHANGE OF USE - EXPANSION OF SPECIAL PURPOSE (WASTE DISPOSAL AND RECYCLING FACILITY) & ONGOING CLEARING PURPOSES AT 561 EUMUNDI NOOSA ROAD, DOONAN - Finale

The applicant has prepared a Stormwater Management Plan (Topo, 7/02/2022) for the purposes
of this application. This Stormwater Management Plan addresses stormwater quality. Council’s
independent stormwater consultant has reviewed the applicant’s Stormwater Management Plan
(SWMP) and have provided the following feedback:
•     The SWMP is based on the former master plan (2021) but it is considered that the catchment
      area and disturbance areas are effectively the same.
•     The report also examines a range of options for the position, sizing, and design of the HES
      basin rather that fixing on a particular final approach.
•     The SWMP presents current best practice approaches and includes a range of viable options
      for the expansion which are consistent with both the ERA licence conditions and relevant
      best practice guidelines.
The SWMP proposes a Type A HES basin in a new location along the northern site boundary. This
position has been selected due to the well-known issues with the current basin’s position, as it
suffers from elevated tailwater issues which impact the basins performance.
The IECA (2018) guidelines direct the use of a Type A basin (includes low flow pipe) for activities
such as landfills, however it is noted that similar performance can be achieved by Type B HES
basin (spillway only).
The applicant’s report provides 2 options for a successful use of the Type A basin:
1.    Elevated low level flow outlet above the Q10 flood level so it is always free draining (this
      would require significantly more fill for the RRA pad)
2.    Only have a spillway above the Q10 level and have the low flow pipe with flag gate to prevent
      backwatering. This means that the basin may frequently not drain via the low flow pipe.
Council’s independent stormwater consultant advised:
Option 2 will essentially function as a Type-B basin whenever the downstream water levels are
elevated. Personally, I believe the benefit of having the low-flow pipe in the Type-A configuration
at this site is marginal and introduces additional risk of the flag-gate failing and water flowing back
into the basin from downstream and compromising performance. This is however subjective, and
the condition provides an option but not a direction in this regard.
It also recommends that the report be updated to better reflect the activities and finalised design
details for the HES basin. It is recommended that this can be included as a condition.
Council’s independent stormwater consultant advised that no conditions relating to ongoing
environmental performance/ water quality are necessary as this matter is addressed through the
ERA licence.
Flooding
The landfill is located outside any regional flood mapping; however the site is subject to local
drainage associated with the catchments within and immediately adjacent to the site.
A flood assessment and technical memo have been prepared by the applicant to document and
address flooding at the site.
Council’s independent stormwater consultant has advised that the applicant’s report demonstrates
the extent of flood impacts associated with the works. This involves some impacts to the vegetated
areas to the north of the proposed works but still contained well within the broader site boundary.
Whilst this is consistent with flood hazard overlay code, Council’s independent consultant
recommends a condition that the detailed design should consider a system for flow distribution
along the northern boundary of the fill pad to better replicate existing flow patterns and minimise
impacts on adjoining vegetation due to change in wetness or scour associated with larger flood
events.
Council’s independent stormwater consultant also notes that it would be appropriate that all waste
use areas are located at least 300mm above the 1%AEP (2100) flood level to protect waste being
washed downstream.