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Attached to the e-mail Further information re: stormwater - REC17/0011.
Our Reference: 2017-10 Brisbane Stormwater Management Pty Ltd
ACN 094 878 734 ABN 75 094 878 734
92 Alkoomie Street, Wynnum Nth, Qld
PO Box 8119, Wynnum Nth, Q 4178
Date: 13 October 2017 Ph 07 33963383 Mb 0402409604
Email: [email protected]
Noosa Council
P O Box 141
Tewantin 4565
Attn: Mr Patrick Murphy
Coordinator, Planning & Development Assessment
Dear Sir,
Re: REC 17/0011, 20 EAGLE DRIVE, TEWANTIN
In response to the Council’s 5 October request for additional information, the XPStorm models have been
revised and the following comments are offered in relation to the stormwater quantity aspects and some of the
basin design aspects of the information request. The relevant parts of the request are highlighted in blue script,
whereas the responses (including some comments on discussions with Council’s reviewer, Premise Consulting)
are in black text.
Modelling Methods
BSM has undertaken a stormwater quantity investigation to design the basin at 14 Eagle Drive. An
XPSTORM model was developed to model the impacts to the whole catchment, the downstream boundary
being where stormwater enters the Noosa Caravan Park from Read Street. The model is calibrated to the
Rational Method calculation with ARR2016 rainfall for the post-development case without the basin. Within
the model, some engine parameters have been changed away from the default values. The SWMM Runoff
Non-linear Reservoir Method was employed in XPSTORM.
We note:
XPSTORM v9.28 was used. It was agreed that Version 9 has the advantage of being a particularly
robust version of the software and, although superseded several times over, can continue to be used.
The default contraction loss has been changed to 0.2 and applied across the model. This was a comment
by Premise rather than an issue. Noted that it was applied globally only to the open channel reaches; -
the piped reaches were individually assigned loss coefficients from QUDM manhole/gully charts.
This was acceptable to Premise.
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The DESIGN rainfall applied to the model Mitigate.xp appears to be the 10% AEP 1hr event. Relates to
BSM's preferred manner of running individual storms. Volunteered by Premise that it could be handled a
little easier by ticking the selected storms in the global storms database, but not an issue.
An initial depth of 0.01m has been applied to every node. This was a hangover from earlier versions of
XPStorm’s Extran engine in which it was used to counter model instability, which is greatly reduced
in Version 9 of XpStorm. Initial depths in the models have now been changed to zero.
Evaporation has been set to 0.1mm/day (typically assume 0mm/day). Default value, but insignificant in
the scheme of things – has been changed to zero in the remodelling.
Issues
The hydrology method hasn’t been fully explained within the report but it appears that a mixture of ARR2016
and ARR1987 methods has been applied. One version of ARR should be consistently followed throughout the
study. If ARR2016 rainfall is selected, the full ARR2016 method should be employed. Agreed with Premise
that it will be okay for BSM to use ARR87 rainfalls with ARR87 temporal patterns with zero initial loss.
The report states that the models have been calibrated to the Rational Method. QUDM v4 states:“Utilisation
of the Rational Method, as described within the Manual, may be considered appropriate…as a ‘checking tool’
(not calibration) of numerical models developed for small ungauged catchments (i.e. checking for potential
gross errors)”. The models were previously set-up with appropriate routing parameters and the initial losses
were then varied between the AEP's to achieve a close calibration to the Rational Method across the range of
AEP. Agreed with Premise that the initial losses will be changed to zero and the Rational Method will only be
used as a sanity/comparison check. The Manning’s n values for the subarea runoffs were set at 0.020 and
0.045 for impervious and pervious areas respectively in the original modeling, which we suggest are
appropriate values, and the remodelling has also used these values. The comparison check with the Rational
method, for the “developed case” flows at the model outlet, prior to the addition of the basin, is summarised in
Table 1 below:
Table 1: XPStorm Developed Scenario, Comparison With Rational Method
Event Rational Method XPStorm Model
AEP (ARI) (m3/s) (m3/s)
AEP 1% (100 yr) 6.93 6.76
AEP 10% (10 yr) 4.01 4.23
AEP 63% (1 yr) 1.80 2.90
Initial losses should be 0mm/hr. Applying no initial losses is consistent with the requirements of both
ARR1987 and ARR2016 that confirm the temporal patterns used for design purposes are the ‘burst’ elements
within greater duration events. ARR2016 further states that there is no evidence that infiltration losses change
with respect to the recurrence interval being modelled and that continuing losses can be applied equally to
frequent and rare events. As above, the initial losses have been reduced to zero for the remodelling. A
continuing loss rate of 2 mm/hr has been maintained in the remodelling, across the full range of AEP.
Relating to the XPSTORM model the routing method applied in the model is not endorsed by either version of
ARR. It is important that basis for the basin design enables confidence that the detention system will be
sufficient for the development. Noted by Premise that the SWMM "Runoff" routing method is a US-derived
method, whereas Premise would have preferred an Australian derived method (preferably, Laurenson's). BSM
have been using the SWMM Runoff routing method in XPStorm and in its predecessor UDD for almost 20
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years and have thoroughly checked its calculations of peak runoff rates from individual subareas against
Rational Method calculations of the same runoff rates and found it to be a satisfactory method. In this instance
it has been applied equally to the pre- and post-development scenarios. Agreed with Premise that the method
can continue to be used.
Information Required
Ensure the stormwater quantity study employs methods that are consistent with one version of ARR. Apply
appropriate routing methods and parameters to the XPSTORM model. The Rational Method can be used as a
‘checking tool’ but not for calibration of numerical models. See relevant comments above.
Considering the length of the basin’s design life, ensure the freeboard provided is sufficient under a climate
change scenario to protect adjacent residential properties. Premise noted that for a 60 year lifespan (i.e. to
about 2080) the increase in the rainfall intensities should be 15 percent. BSM previously checked the spillway
for a PMF equal to 400 percent of the Q100 and found it to have sufficient capacity to accommodate the PMF.
Remodelling the system with 115% of the 100 year rainfall indicates that the extra runoff will utilize just 0.02
metre of the 0.30 metre freeboard.
Ensure only major flows exit the basin over the weir and that water exiting the basin in this manner flows to
the parkland and does not enter adjacent residential properties. Premise have clarified that they were not
seeking to restrict the basin overflows to major storms only - instead BSM can use whatever overflow
frequency they prefer. However, Premise were wanting to keep the overflows out of the adjacent residential
properties. The design of the basin inlet at the sag in Eagle Drive, with its accompanying filling of driveways
and verges, provides for complete capture of all flows up to AEP1% and thereby eliminates the existing
overflows from Eagle Drive to the properties adjoining the basin.
The underground pipe outlet for the basin substantially reduces the quantum of overland flows through the
park adjoining the Tait Street properties, as documented in Table 3 on the following pages of this response.
However the topography of the park is such that the existing major overflows through the park probably
spread out onto No 21 Tait Street and, even with the reduced overflows, some part of those overflows might
still continue to spread out onto that property albeit at a much reduced rate. If this is a critical issue, the
applicant could construct a low bund along the boundary between the park and No 21 Tait Street to confine
the overflow to the park.
Revision To Impact Mitigation
The revised modelling results in some changes to the basin configuration. The rectangular low-level orifice in
the outlet box is enlarged from 100 mm x 90 mm to 200 x 100 mm. The 1 metre wide weir in one side of the
outlet box is lowered by 0.005 metre (5 mm), to EL 4.86, and the other three sides are raised by 0.03 metre to
EL 5.14. The spillway in the embankment crest is raised by 0.02 metre, from EL 4.98 to EL 5.00. The revised
basin and its inlet/outlet details are shown in Figure 5B attached hereto. The 100 year storage volume is
increased from 510 cubic metres at a water level of EL 5.11 to 523 cubic metres at EL 5.14. The key impacts
with this arrangement are described hereafter for the following locations:
- Water levels at node N36, at the Eagle Drive sag.
- Overflow rates through the park, between nodes N34 and N22.
- Overflow rates across Tait Street, between nodes N22 and N17.
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- Overland flows through the Tait Street properties, around nodes N45 to N50.
- Water levels in Tait Street at Node 45, at the point of inflow for roofwater from the new lots 1 to 9.
- Water levels at Node N14B at the sag in Tait Street.
- Water levels at Node 11, midway along the creek between Tait Street and the caravan park.
- Water levels and flow rates in the creek at Node N2, at the point of discharge to the caravan park.
Water Levels At Node N36, At The Eagle Drive Sag
The pre-and post-development water levels in the Eagle Drive sag are summarised in Table 2, below.
In the pre-development scenario, the overflow level into No 14 Eagle Drive at EL 5.77 is just overtopped in
the AEP 20% (5 yr) storm and overtopped by 0.10 metre in the AEP 1% (100 year) storm which, because of
the almost flat overflow profile, generates substantial overflow rates into the property. The adjoining
properties, with ground levels of 5.82 and 5.84 at their Eagle Street frontages, are also at risk in the AEP 10%
and higher events.
In the absence of any mitigation, the post-development diversion of developed site runoff to Eagle Drive
would adversely affect water levels in the sag. However, with the proposed additional underground piping at
the sag and with an inlet chute cut into the verge, the water levels in the sag are significantly reduced and
overflows through the properties adjoining the sides of the basin are eliminated.
Nett Result: Complete impact mitigation; substantial benefit
Table 2: Water Levels At Node N36 In The Eagle Drive Sag
Event Pre-Development Post Development
AEP (ARI) EL (m) EL (m)
AEP 1% (100 yr) 5.87 5.80
AEP 2% (50 yr) 5.87 5.78
AEP 5% (20 yr) 5.86 5.77
AEP 10% (10 yr) 5.85 5.74
AEP 20% (5 yr) 5.84 5.72
AEP 50% (2 yr) 5.75 5.66
AEP 67% (1yr) 5.71 5.63
Overflow Rates Through The Park, Between Nodes N34 and N22
In the pre-development scenario, the relevant flows here are the overflows from the Eagle Drive sag. In the
post-development scenario, in the absence of the detention basin and its inlet and outlet, the diverted flows to
Eagle Street would cause an increase in overland flow rates at this location. However, with the basin in place,
the relevant flows are restricted to the overflows from the weir in the basin embankment, which will only occur
in events of AEP 20% or higher – lesser event flows will be piped underground through to the creek on the
downstream side of Tait Street.
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The pre- and post-development overflow rates are summarised in Table 3 below
Nett Result: Complete impact mitigation; substantial benefit
Table 3: Overflow Rates Through The Park, Nodes N34 to N22
Event Pre-Development Post Development
AEP (ARI) (m3/s) (m3/s)
AEP 1% (100 yr) 1.70 1.60
AEP 2% (50 yr) 1.37 1.23
AEP 5% (20 yr) 1.13 0.92
AEP 10% (10 yr) 0.76 0.51
AEP 20% (5 yr) 0.36 0.13
AEP 50% (2 yr) 0.02 0.00
AEP 67% (1yr) 0.01 0.00
Overflow Rates Across Tait Street, Nodes N22 To N17.
In the pre-development scenario, these include the flows from the Eagle Drive sag (as in Table 3 above) plus
surface runoff from the park and Tait Street, less flows collected into the underground system at the Tait
Street kerb. In the post-development scenario, the flows from the Eagle Drive sag are replaced by the
overflows from the detention basin. The impacts broadly mimic those in Table 3 above, with details as
summarised in Table 4, below.
Nett Result: Complete impact mitigation; substantial benefit
Table 4: Overflow Rates Across Tait Street, Nodes N22 To N17.
Event Pre-Development Post Development
AEP (ARI) (m3/s) (m3/s)
AEP 1% (100 yr) 1.73 1.59
AEP 2% (50 yr) 1.23 1.04
AEP 5% (20 yr) 0.87 0.65
AEP 10% (10 yr) 0.48 0.30
AEP 20% (5 yr) 0.16 0.08
AEP 50% (2 yr) 0.00 0.00
AEP 67% (1yr) 0.00 0.00
Overland Flows Through The Tait Street Properties, Around Nodes N45 to N50
In the pre-development scenario, these flows comprise the overland sheet flows from the subject development
site and the neighbouring property to the east of the site, apart from a small proportion of that flow that
follows the existing driveway swale out to Eagle Drive.
In the post-development scenario, the contributing catchment area is reduced by about two thirds and the
overland flow rates are reduced even further by the underground piping of roofwater from the remaining area.
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The pre- and post-development overland flow rates are summarised in Table 5 below, where they are seen to
be greatly reduced as a result of the development.
Nett Result: No adverse impact; substantial benefits
Table 5: Overland Flows Through The Tait Street Properties, Nodes N45 to N50
Event Pre-Development Post-Development
AEP (ARI) (m3/s) (m3/s)
AEP 1% (100 yr) 1.02 0.36
AEP 2% (50 yr) 0.88 0.32
AEP 5% (20 yr) 0.80 0.30
AEP 10% (10 yr) 0.67 0.26
AEP 20% (5 yr) 0.62 0.22
AEP 50% (2 yr) 0.37 0.13
AEP 67% (1yr) 0.29 0.09
Water Levels In Tait Street At Node 45, At The Point of Inflow For Roofwater From New Lots 1 to 9.
The piping of roofwater from the new lots 1 to 9 will provide an initial surge of runoff to Tait Street.
However, the initial surge will be completely offset by the greatly reduced catchment area further upslope,
such that the water levels in Tait Street will be significantly reduced, as in Table 6 below.
Nett Result: Substantial benefits; no significant adverse impact; no mitigation required.
Table 6: Water Levels In The Tait Street Kerb/Channel At Node N45
Event Pre-Development Post Development
AEP (ARI) EL (m) EL (m)
AEP 1% (100 yr) 3.67 3.63
AEP 2% (50 yr) 3.66 3.62
AEP 5% (20 yr) 3.66 3.61
AEP 10% (10 yr) 3.65 3.60
AEP 20% (5 yr) 3.62 3.59
AEP 50% (2 yr) 3.59 3.58
AEP 67% (1yr) 3.58 3.57
Water Levels At Node N14B At The Sag In Tait Street.
Node 14B is located in the kerb/channel at the Tait Street sag. The pre-and post-development water levels in
the sag are summarised in Table 7, on the following page.
In the absence of any mitigation, the increased imperviousness of the development site would adversely affect
the water levels in the Tait Street sag, across the full range of AEP, and this would be further affected (either
way) by the relative changes in hydrograph timing.
With the basin in place, the impacts in the 1 to 2 year events are limited to an afflux of 0.01 metre, and the
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impacts in the 10 to 100 year events are completely mitigated.
Nett Result: Useful mitigation; with benefits in the major storms but minor (inconsequential) disbenefits in the
minor storms.
Table 7: Water Levels At Node N14B At The Tait Street Sag
Event Pre-Development Post Development
AEP (ARI) EL (m) EL (m)
AEP 1% (100 yr) 3.14 3.12
AEP 2% (50 yr) 3.12 3.11
AEP 5% (20 yr) 3.11 3.09
AEP 10% (10 yr) 3.09 3.07
AEP 20% (5 yr) 3.05 3.05
AEP 50% (2 yr) 2.98 2.99
AEP 67% (1yr) 2.95 2.96
Water Levels At Node 11, Midway Along The Creek Between Tait Street And The Caravan Park.
Node 11 is located in the floodway of the creek. The pre- and post-development water levels are summarised
in Table 8, below.
In the absence of any mitigation, the increased imperviousness of the development site would adversely affect
the water levels in the creek, across the full range of AEP, and this would be further affected (either way) by
the relative changes in hydrograph timing, particularly with respect to the interaction of hydrograph timing
from the intensively developed Riverlands and Noosa Nursing centres alongside the creek.
With the basin in place, the impacts in the 5 and 10 year storms are limited to an afflux of 0.01 metre. In the
other storms, impacts are completely mitigated.
Nett Result: Useful mitigation; but with some minor (inconsequential) disbenefits in the moderate storms.
Table 8: Water Levels At Node N11, Midway Along The Creek
Event Pre-Development Post Development
AEP (ARI) EL (m) EL (m)
AEP 1% (100 yr) 2.41 2.40
AEP 2% (50 yr) 2.35 2.35
AEP 5% (20 yr) 2.29 2.29
AEP 10% (10 yr) 2.21 2.22
AEP 20% (5 yr) 2.11 2.12
AEP 50% (2 yr) 1.96 1.95
AEP 67% (1yr) 1.91 1.91
Water Levels And Flow Rates At Node N2, At The Point Of Discharge To The Caravan Park.
The post-development model without the basin indicated that the development would increase the flow rates at
this location by as much as 8 percent in a 1 year storm, and by lesser amounts at other storm severities.
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The pre- and post-development water levels are summarised in Table 9 on the following page, and the pre- and
post-development flow rates are summarised in Table 10. With the basin in place, mitigation is practically
complete. There remains only a 0.04 m3/sec increase in the 5 year flow rate, but this increase is so small that it
is not reflected in the water levels (to the nearest 0.01 metre) and is therefore of no consequence.
Nett Result: Practically complete mitigation; with minor benefits in most storms but a remaining disbenefit of
no real consequence in the 5 year storm.
Table 9: Water Levels At Node N2, Approaching Caravan Park
Event Pre-Development Post Development
AEP (ARI) EL (m) EL (m)
AEP 1% (100 yr) 2.04 2.03
AEP 2% (50 yr) 1.99 1.98
AEP 5% (20 yr) 1.94 1.93
AEP 10% (10 yr) 1.84 1.84
AEP 20% (5 yr) 1.79 1.79
AEP 50% (2 yr) 1.65 1.64
AEP 67% (1yr) 1.61 1.60
Table 10: Flow Rates From Node N2 to N3, Approaching Caravan Park
Event Pre-Development Post Development
AEP (ARI) (m3/s) (m3/s)
AEP 1% (100 yr) 6.78 6.66
AEP 2% (50 yr) 5.98 5.79
AEP 5% (20 yr) 4.98 4.86
AEP 10% (10 yr) 4.09 4.08
AEP 20% (5 yr) 3.62 3.66
AEP 50% (2 yr) 3.01 2.97
AEP 67% (1yr) 2.70 2.63
Please do not hesitate to contact me if you require clarification of this advice. In the meantime, I remain,
Yours faithfully,
Denis Ogle, RPEQ 2405, Director.
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TOP OF OTHER THREE SIDES
AT EL 5.14 (100 YR WL IN BASIN)
1.0 m WIDE WEIR AT EL 4.86
(ONE SIDE OF BOX, ONLY)
1.0 m
VERGE AND DW AY'S FILLED TO RL 5.95
200 mm W x 100 mm H ORIFICE
IL 4.00
E XG.
TOP OF SAND, EL 3.80
RPB
K&C
PROVIDE GRATES/BARRIERS
AS REQ'D, TO ORIFICE AND
WEIR AND TOP OF BOX
750 DIA EXG. TELSTRA PIT EXPECTED W ATER LEVELS IN SAG
OUTLET OUTLET BOX DETAIL STORM PRE-DEV POST-DEV
100 YR 5.87 5.79
0 SCALE 2m EXCLUSION FENCING, WITH LOCKABLE GATE 84 5 YR 5.81 5.68
ACROSS INLET CHUTE FOR MAINTENANCE ACCESS 5. x
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FIGURE 5B: DETENTION BASIN AND OUTLET DETAIL
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