Response to information request REC13/0095

      4. Information Required.


Attached are our drainage calculations for the ephemeral gully between lots 5, 6 and 7, which
show the flow depths in the 1 %AEP. Refer to Plan 1225-19B which shows a green Q100 line for
the local short time flooding.


A summary is as follows for Local Flooding:


                   .     Lot 7: Local Q100 in gully has the highest relative level upstream of culvert at RL.                            .




                         1 03.475(driveway weir) + 0.063(maximum weir flow H) = 103.538
                         RL 103.538 vs. house site FFL at RL 104.25.
                         Minimum freeboard is 104.25  103.538 = 0.712 freeboard.
                                                                           -




                   .     Lot6: Local Q100 in gully has the highest relative level upstream of culvert at RL
                         1 02.883(driveway weir) + 0.155(maximum weir flow H) = 103.04
                         RL 103.04 vs. house site FFL at RL 103.34.
                         Minimum freeboard is 103.34  103.04 = 0.300 freeboard.
                                                                           -




                   .     Lot 5: Local Q 1 00 in gully has the highest relative level upstream of culvert at RL
                         102.579 (driveway weir) + 0.111 (maximum weir flow H) = 102.690
                         RL 102.690 vs. house site FFL at RL 102.990.
                         Minimum freeboard is 102.990  102.690 = 0.300 freeboard.
                                                                             -




The proposed filling and FFL have a minimum of 300mm freeboard in accordance iNith QUDM.



      5. Information Required.


The adopted flood study for this area uses an average n value of 0.064 (n = 0.06 for bed, n=
0.03-0.05 for cleared flood plain). We have had an approximate sensitivity assessment of the
flood data in relation to Lot 5 carried out by BSM (Brisbane Stormwater Management) which is
enclosed and as shown on Plan 1225-19B.


As noted previously, our representation of existing model Q1 00 shows that a small portion of the
gully parallel to Wilgee Court fronting lot 5 has some existing Q100 inundation (Plan 1225-19B).

The house sites are to be filled to increase the freeboard and a minimum floor level as shown on
Plan 1225-198.


As an additional measure to account for possible increase in n values and flood levels we have
shown allotment filling levels to suit an n value of 0.10 and sensitivity to n = 0.15.


A summary of Lot 5 Q1 00 levels is as per attached BSM letter.


The effective allotment extent is considered to be the filled portion west of the ephemeral gully
which will be flood free based on sensitivity analysis of n values. A drainage easement will cover
the remaining portion of the lot in the ephemeral gully and prevent future owners from using this
part of the land for structures.




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