Response to Information Request Letter 10-6-2025 · OPW25/0042

Response
ALTA advises that all discrete RIPs have been conservatively sized to contain up to 100 year ARI
events. Should RIPs fail or more intense storms be experienced overflow would be to adjacent
roadway (legal point of discharge).

In the absence of final landscape design ALTA is yet to finalise the exact levels and contours, and
suggest this could this be dealt with via a condition.

6. It is noted that a gabion rock wall is specified as the method of retaining along the edge of the
   overland flow swale in places. Given the ability of water to seep into this type of retaining (large
   voids), it is unclear if this method of retaining is suitable for the edge of a drainage channel and
   if a more suitable method is available that provides better long term performance of the
   channel and adjoining areas.

Response
ALTA advise that the proposed Gabion Walls would be constructed with a heavy geofabric filter
(Bidim A24 or equivalent) along their base and rear faces such that any water entering the gabions
would not export sand from behind or beneath the walls.

Gabions are frequently used in waterways for batter retention (see below link)
https://www.gabionbarriers.com/application/gabion-channel.html

Issue – Overland Flow Channel

The design of the Overland Flow Channel includes roughness assumptions consistent with
mown grass or rock at a maximum batter of 1:2. Council has concerns with the ability to
provide long-term stabilisation for a grass or rock 1:2 batter.

Information Required
7.    Provide additional detail demonstrating how long-term stabilisation will be provided to the
      batters of the Overland Flow Channel. The additional detail must be prepared by a suitably
      qualified person (RPEQ) and include treatment options consistent with the current
      hydraulic design assumptions of the Overland Flow Channel (i.e., turf or rock).
      Where long-term stabilisation of the proposed 1:2 turfed/rocked batter isn’t achievable,
      provide a revised set of drawings amending the Overland Flow Channel to include a
      reduced batter (i.e., a 1:3 vegetated batter).

Response
ALTA advises that the modelled roughness co-efficients were consistent with mown grass (n=.035)
along the channel base with vegetated batters (n=.06) on one side and gabion walls on the other.
An additional run has now been completed with the base roughness coefficient increased to reflect
a vegetated treatment lest that landscape outcome be pursued. An additional channel flow depth
of less than 100mm resulted which is insignificant to the performance or freeboard of the channel
(see graph in attached Channel base roughness comparison).

Inspection of Figures C11 and C12 of the Engeny report suggests channel flow velocities less than
1m/second for 1% AEP flows (with 2100 climate change) where jute mat and landscaped batters
with grassed base are proposed.