A short case study on some possible economic advantages of using numerical modelling versus simplified methods for local government authorities in areas such as large cross drainage structures. In light of very recent announcements on the AR&R releases, this also gives further merit in reducing the use of techniques such as the Rational Method

1. SIMPLIFYING STORMWATER DESIGN IS COSTING US MONEY –
A SHORT CASE STUDY
Adam Berry - Floodplain Management Engineer
Abstract
Most engineers are obviously aware of the alternative to utilising the basic methods of the Rational
Method which involves the use of flood modelling software. The scope of this paper will not enter
the realm of explaining the methods or workings of the software as it is well understood,
documented and generally best practice. This paper makes a direct comparison of the Rational
Method verses a combined hydrologic and hydraulic model through the case studies. The hydrology
and hydraulic software brands are not mentioned as the key component of the paper is highlighting
from a practical point of view the benefits of using a flood model in the first place.
Introduction
Cost effective engineering due to increasing materials and services costs coupled with constrained
budgets has become critical in todays practice. Whilst it is important to balance the books with
regard to reduced capital works budgets, it is also obviously important to meet engineering
standards, align with guidelines, utilise available technology and act in the best interests of the
community with regard to safety and most effective use of public money.
This opens the door and aligns with the case study: in some cases we can adequately design with
engineering conservatism when it comes to stormwater and flooding whilst still reducing the
significant cost of roadworks with regard to cross drainage works.
A common method for some government authorities to design cross drainage structures for
roadworks is using a combination of the Rational Method and simple hydraulic methods for sizing.
In particular, the use of the Rational Method is commonly recognised as being limited ain application
and becoming increasingly “out of date” in complex modern stormwater assessment. Regardless of
this recognition, many government authorities continue to use the practice for hydrological design
of culverts and often not allowing enough emphasis on the stated Queensland Urban Drainage
Manual (QUDM) limitations on applying the Rational Method.
Rational Method Limitations:
The Rational Method has been around for well over 100 years and is used due to its common
understanding throughout the engineering community, its ease and speed of use and most
importantly in the context of this paper, its low cost of application.
QUDM 2013 provisional (and previous versions) specifies that the Rational Method should not
generally be used with any of the following scenarios:
 Overland flow path passing through a low gradient oval or park
that provides significant detention storage during major storm
events
 Catchments where travel time for the minor drainage system is
significantly different from that of the major drainage (overland

2. flow) system
 Relief drainage works incorporating split pipe flows
 The upstream catchment is zoned for urban usage, but is
currently undeveloped
 Catchments containing significant on-site stormwater detention
(OSD)
 Sub-catchments containing one or more large lakes, wetlands or
detention/retention basins
 Catchments containing a major water supply dam or weir
 Catchments with an upper rural area containing a farm dam
 Urban catchments with an area greater than 500 ha.
 Catchments developed using the principles of Water Sensitive
Urban Design
 Partially urbanised, ungauged catchments
 Irregular shaped catchments
 Catchments with a significant change in catchment slope or
stream slope
Considering most culvert design usually encompasses a fairly large catchment, it is likely that many
or even all of this scenarios will be present in most culvert design scenarios. It would still seem
apparent however that the practice of using the Rational Method is still prevalent, regardless of
these exceptions stated in QUDM, most likely due to convenience and low cost. In excess of the
limitations above, the fact the Rational Method does not allow any temporal rainfall variation,
defined floodplain storage or accurate sub catchment definition gives further support to limit its use
on most culvert design.
It is also noted that through new research projects from Australian Rainfall and Runoff (AR&R),
techniques may become more reliant on software based applications encompassing improved
techniques for hydrologic and hydraulic methods. In particular, one of the conclusions extracted
from Project 13 Draft of the Australian Rainfall and Runoff revisions states:
“Without carrying out similar studies to the Part I study undertaken in the ACT on a significant
number of additional gauged urban catchments then it is the view of the authors that continued use
of the Rational Method for urban drainage analysis and design can no longer be justified”
The Obvious Alternative
Flood models of course. Again the benefits and realisation of using a flood model are nothing new
and will not be repeated in specific detail, however the main simplified benefits of this combination
for the sake of summary are:
 A more realistic representation of the floodplain and resultant flood discharges
 Ability to more accurately define catchment and floodplain storage such as localised
depressions, varying channel grades, detention basins, embankments and upstream
hydraulic structures through digital elevation models.
 A better representation of rainfall via spatial and temporal variation and rainfall losses
 Considering most local government culverts are usually designed between 10% to 2% Annual
Exceedance Probability (AEP) immunity, the flood models can generally provide a more
optimistic discharge value for these lower storm events

3. Case Study One
This case study focusses on a project that was constructed in 2012 and included the following
characteristics:
 Kerb and channel
 Full Pavement upgrade
 Culvert upgrade
Current analysis used for culvert design:
The design team used the Rational Method for all stormwater calculations involving longitudinal and
cross drainage structures. The basic extracts of the Rational Method components included:
 A 30 hectare catchment
 A time of concentration of 16 minutes
 An impervious fraction of 55%
The discharge associated with this assessment was 15.2m3/s. Figure 1 below shows the catchment
utilised in the calculations.
Figure 1. Culvert location and catchment
PC Drain was then utilised to size the culverts based on 15.2m3/s. The final configuration of the
culverts was to be designed to meet the required immunity level of 10% AEP for a collector road.

4. This resulted in the existing 3 x 1350mm diameter pipes remaining plus an additional 2 x 1200mm
diameter pipes constructed.
The hydrology and hydraulic model
An existing flood study (calibrated to one historic event) for the sub regional area that was modified
to incorporate the following:
 A 2m digital elevation model incorporating a 12D tin of the proposed road upgrade
 Extending the length of the existing 3 x 1350mm diameter pipes to incorporate the widened
road.
 More accurately defining the Mannings roughness values and terrain around the culverts.
 It is noted that during detailed design that further work should be performed on the model
to incorporate specifics about the area.
The 60 minute storm was found to be critical for the culvert location and the 10% AEP hydraulic run
resulted in a discharge of 10.71m3/s. As it can be seen below from Figure 2, the existing 3 x
1350mm diameter pipes provided sufficient capacity to allow the road 10% AEP immunity, thus not
requiring replacement. Additionally Figure 3 shows that the road actually provides close to a 5% AEP
immunity. It should be noted that the water crossing the road in the right of picture is associated
with road flow and other catchments.
Figure 2. 10% AEP immunity

5. Figure 3. 5% AEP immunity
Analysis of the catchment
As stated above the Rational Method has numerous limitations and some of these were obvious
within the catchment and were highlighted in the hydraulic model. Figure 4 below the upper semi-
rural catchment, various branch flow paths and hydrological differences that would have likely
reduced the discharge due to flood peak timing. Figure 4 also shows various locations of storage and
hydraulic structures through the catchment that result in reduced outflow at these locations.

6. Figure 4. Floodplain Storage
Impact on project costs
As it can be seen from the assessment above, the reduction in discharge combined with more
accurate hydraulic modelling, results in very noticeable differences between the two methods. The
culverts were not required to be upgraded at all to meet QUDM requirements with regard to
immunity levels utilising the flood model.
Whilst the culverts did not require upgrading, the pipes had to be extended to match the new width
pavement and additional items such as headwalls required. The culvert was in sound condition and
with the extension of the culvert it was estimated the asset would have another 30 years of design
life before requiring replacement.
Table 1 below shows the total incurred costs for the entire project and the savings that could have
been recognised with the culverts. As the table shows, a saving of $80,000.00 would have been
realised.
Table 1. Potential Cost Savings
Actual
Cost
Predicted
Cost
Saving %
Saving
$520,000 $440,000 $80,000 15.38

7. Case Study Two
This case study focusses on a potential upcoming project that may be constructed in the future and
consist of a major culvert upgrade.
Future analysis used for culvert design:
The design team will use the Rational Method for all stormwater calculations involving longitudinal
and cross drainage structures. The basic extracts of the rational method components include:
 A very large 308.8 hectare catchment
 A time of concentration of 38 minutes
 An impervious fraction of 55%
The discharge associated with this Rational Method assessment for a 5% AEP event was 67.25m3/s.
Figure 5 below shows the location of the culvert and the catchment.
Figure 5. Culvert location and catchment
Simple hydraulic methods were then utilised to size the culverts based on 67.25m3/s. The final
configuration of the culverts were e designed to meet the required immunity level of 5% AEP and
sized at approximately 6 x 3600mm wide x 1800mm high box culverts.
The hydrology and hydraulic model:
An existing flood study (linked to a calibrated creek regional model) for the sub regional area that
was modified to incorporate the following:
 A 2m digital elevation model and more accurate representation of the road crossing

8.  More accurately defining the approach and departure of the flowpath in proximity to the
culvert including terrain changes and mannings roughness refinement.
 It is noted that during detailed design that further work should be performed on the model
to incorporate specifics about the area.
The 60 minute storm was found to be critical for the culvert location and the 5% AEP hydraulic run
resulted in a discharge of 40.95m3/s. Iterations of culvert arrangements resulted in a 4 x 3600mm
wide x 1800mm high. Perusal of the 1D results indicated that the proposed culverts were
discharging 38.131m3/s through the barrels which is reflective of the large volume of embankment
storage behind the culverts.
Figure 6 shows the proposed box culvert arrangement provided sufficient capacity to allow the road
5% AEP immunity.
Figure 6. Road immunity
Analysis of the catchment:
It is noted that the catchment analysed with the Rational Method is well outside of its applicable use
(considering limitations) and resulted in a significant difference in discharges between the two
methods. Figure 7 below shows vast components of storage otherwise missed in the Rational
Method associated with large upstream detention basins and significant storage behind road
embankments etc.

9. Figure 7. Floodplain storage
Impact on project costs
The estimated future cost of the culvert upgrades utilising both methods is shown below in Table 2.
Table 2. Potential Cost Savings
Simplified
Design
Advanced
Design
Saving %
Saving
$725,000 $480,000 $245,000 33.79

10. Conclusion
The Rational Method has commonly referred limitations and as recommended in QUDM should be
restricted and more stringently enforced to simple design applications such as car parks, small
developments and longitudinal road drainage (road pit and pipe network).
Often hydrologic and hydraulic models are not used because of the cost and time. At least in the
example of this case study, not using the flood models often has the opposite effect resulting in
significant additional capital expenditure. Each local government authority should carefully consider
the real tangible benefits of using hydrological and hydraulic models for culvert design, allow
sufficient lead time for this to occur and in some cases the recognition of savings to the bottom line
of some major road projects will soon become apparent. If not for costs savings, the flood modelling
approach provides a more usable and robust method of multiple catchment designs.
References
Engineers Australia, Project 13 Stage 3: Urban Rational Method Review Draft, Australian Rainfall &
Runoff, February 2014.
Queensland Urban Drainage Manual, Third Edition 2013 – Provisional, Department of Energy and
Water Supply, 2013.