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the use of the Rational Method is commonly recognised as being limited ain application and becoming increasingly “out of date” in complex modern stormwater assessment.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

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

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.

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

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.

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

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

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.

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

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.