Hydrologic and Hydraulic Modelling


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Technical Guideline
Hydrologic and Hydraulic Modelling
October 2019

Copyright
© The State of Queensland (Department of Transport and Main Roads) 2019.
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Technical Guideline, Transport and Main Roads, October 2019

Contents

1 Introduction ....................................................................................................................................1

2 Hydrologic and hydraulic modelling specifications for departmental projects......................1

3 Minimum modelling requirements ...............................................................................................1

3.1 Survey requirements....................................................................................................................... 1

3.2 Hydrology........................................................................................................................................ 1

3.3 Hydraulics ....................................................................................................................................... 3

3.4 Bridge design considerations.......................................................................................................... 4

3.5 Recommended initial TUFLOW form loss values........................................................................... 4

3.5.1 3.5.2

Bridge losses ..................................................................................................................4 Guardrail losses..............................................................................................................5

4 Delivery specification ....................................................................................................................6

4.1 Reporting requirements .................................................................................................................. 6

4.2 Supply of hydraulic model and result files ...................................................................................... 7

4.3 Model GIS data ............................................................................................................................... 7

4.4 Modelling log and naming conventions to be used on departmental projects................................ 8

4.5 Completion and handover of a departmental modelling project ..................................................... 8

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Hydrologic and Hydraulic Modelling
1 Introduction
The guideline defines the minimum requirements which must be met when undertaking hydrologic and hydraulic modelling and/or design work for Transport and Main Roads. This covers delivery of modelling work undertaken by either external consultants or internal staff. This includes cross drainage and flooding assessments but does NOT cover road surface drainage / longitudinal drainage. This guideline is also intended to inform the Principal's Designer when developing a contract to engage HD2/HD3 consultants. All hydraulic design reports including the models, result files and any mapping must be submitted to the department's Representative in hard copy and electronic form.
2 Hydrologic and hydraulic modelling specifications for departmental projects
Hydrologic and hydraulic models must be developed to a high standard and be fit for the intended purpose. It is desirable to have high quality models already developed in the early project phases, however at the latest, hydrologic and hydraulic models need to be suitable to be utilised during the Business Case and Detailed Design phases with minimal change to the Base Case models.
• Hydraulic modelling and calculations must be undertaken to demonstrate that the general requirements, performance requirements, and drainage design requirements will be met.
• An appropriately qualified hydraulics RPEQ must undertake appropriate flood modelling and certify the Hydrologic and Hydraulics Report.
3 Minimum modelling requirements 3.1 Survey requirements
Survey is a critical component of any hydraulic model and may be collected specifically for a particular project. In general, up to date Airborne Laser Scanning (ALS) data of the floodplain and a detailed Ground and Feature Model (GFM) derived from ground survey in the vicinity of the area of interest are sufficient to undertake the hydraulic analysis. Mobile Laser Scanning (MLS) can also be used to enhance the road surface data in areas where no ground survey is available. In some cases, it may be necessary to obtain additional creek survey data, especially if there are areas that have not been picked up by the ALS data (below water terrain). In areas where no ALS data is available, ALS should be obtained and only a preliminary hydraulic assessment in the early planning stages should be undertaken. All new survey data specifically collected for departmental projects shall be undertaken as prescribed in the current TMR Surveying Standards which provide instruction for the delivery of Ground and Feature Model information at bridge sites.
3.2 Hydrology
A hydrologic assessment is required to determine inflows and boundary conditions for the hydraulic model. The following guidelines must be followed when undertaking a hydrologic assessment for the department:
• Utilise Australian Rainfall and Runoff (ARR19) in general, with some clarifications as below. Note – full reference is Australian Rainfall and Runoff: A guide to flood estimation, Commonwealth of Australia (Geoscience Australia), 2019.

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• As climate change has the potential to alter the prevalence and severity of rainfall extremes, storm surge and floods, it needs to be considered when designing roads and bridges for the department. For design rainfall events, the relevant increase in rainfall intensity and sea level rise must be determined in accordance with the latest relevant State policies and/or the recommendations in ARR19 and be approved by the department prior to commencement of any project.
• At locations with a stream gauge in close upstream or downstream proximity, hydrologic and hydraulic models should be calibrated to recorded stream gauge data. Design discharges should be calibrated to a Flood Frequency Analysis (FFA) at the gauge location if sufficient recorded data exists. The stream gauge rating curve should be checked and improved if needed (e.g. by using a hydraulic model) before undertaking the FFA analysis. Any limitations with the rating curve should be clearly identified.
• Use runoff-routing modelling (preferably XP-RAFTS, but URBS, RORB or WBNM are also acceptable) with model parameters determined either by calibration OR regional parameters as per ARR19 for uncalibrated models. The Rational Method (as per the department’s Road Drainage Manual (RDM)) with ARR19 IFD and/or other techniques such as QRT should be used as a check on peak flows.
• Validation of the appropriateness of the Rational Method (as per the department's RDM) with ARR19 IFD data is currently being undertaken by the department. Until further information is available, the Rational Method as a primary flow estimation method is only supported for longitudinal drainage design or for small rural catchments with catchment areas of less than 100 ha and/or corresponding times of concentration of less than 30 minutes.
• The use of ARR19 Regional Flood Frequency Estimation (RFFE) as a primary flow estimation method is not supported at this time. RFFE can be reported as a check of peak flow only.
• Losses obtained from the ARR19 Datahub cannot be relied on and must be reviewed and adjusted if appropriate based on either calibration or local knowledge.
• Initial and continuous loss rates must be applied to each catchment in accordance with ARR19. It is acceptable to apply loss rates that have been derived from global pervious and impervious loss rates based on the fraction of imperviousness of each catchment.
• All temporal patterns need to be checked for “embedded bursts” in accordance with Section 5.9.4 (Book 2) of ARR19 and consideration be given to filtering out any embedded bursts.
• A range of design flood events up to the 1% Annual Exceedance Probability (AEP) needs to be assessed. This range should include the standard AEPs of 50, 20, 10, 5, 2 and 1% for most projects. In addition, the 0.05% AEP design flood and the overtopping event is required for the structural design of a bridge under Ultimate Limit State (ULS) conditions and to determine hydraulic impacts under an extreme event.

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For each location under consideration a critical duration analysis must be undertaken for each AEP event in accordance with ARR19. The full range of storm durations with associated temporal pattern ensembles must be assessed. Critical durations, critical flow rates and critical temporal pattern must be documented. The following definitions apply:
− at any location under consideration the critical duration of each AEP design event is defined as the duration that results in the highest mean peak flow rate of the associated temporal pattern ensembles
− the critical flow rate at the location under consideration is defined as the mean peak flow rate of the critical duration temporal pattern ensemble
− the critical temporal pattern at each location under consideration is defined as the temporal pattern that results in a peak flow rate that is closest to the mean flow rate with a bias of 2 to those exceeding the mean.
• To reduce the amount of critical temporal patterns that need to be assessed in the hydraulic model, it is acceptable to consolidate critical temporal patterns if a suitable common temporal pattern can be determined. It needs to be demonstrated that the adoption of a suitable common temporal pattern has negligible impacts on the hydraulic results.
3.3 Hydraulics
The modelling process must be capable of accurately determining flows in the main channels and across the floodplain. In particular, the base case model must allow for the effects of addition / removal of bridges, culverts and other infrastructure within the floodplain so that their influence can be understood.
• Two-dimensional hydraulic models are required in all but the simplest applications (preferably TUFLOW, but MIKEFLOOD, HEC-RAS 2D are also acceptable).
• Rain on Grid models are discouraged.
• The model must include sufficient detail (grid point spacing, structures, etc) to represent the features that are significant for the road corridor being modelled.
• If critical temporal patterns change between Base Case and Design Case, both cases need to be assessed with both critical temporal patterns.
• Hydraulic boundary conditions for the hydraulic model (inflows) for a specific design event must be based on a single, model-wide applied temporal pattern. To be clear, it is not acceptable to apply inflows at different model locations that have been derived from different temporal patterns. It may therefore be required to undertake multiple hydraulic model simulations for a specific critical duration at all locations under consideration if:
− a suitable common temporal pattern cannot be determined
− the critical temporal pattern changes between Base Case and Design Case.
• For each relevant AEP event, peak water levels of all critical durations (in either Base Case or Design Case) must be enveloped into a single composite peak water level (peak of peaks) for that specific AEP event. Afflux for each AEP event is determined by the difference in composite peak water levels between the Design Case and the Base Case.

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• Design blockage for new transverse drainage structures must be assessed using the approach detailed in ARR19. Compliance of flood immunity must be demonstrated assuming design blockage at all new cross-drainage structures. Impact assessments (afflux) must be undertaken assuming a no blockage scenario at all cross-drainage structures. Note: for new infrastructure in greenfield sites it may be appropriate to also assess impacts under a blocked scenario.

• Any bridges that are relevant for the hydraulic assessment of the project works must be modelled in the hydraulic model as 2d structures if the bridge spans three or more grid cells and must have their head loss estimates validated using an alternative independent method (for example: HEC-RAS, Austroads Guide to Bridge Technology Part 8: Hydraulic Design of Waterway Structures or Computational Fluid Dynamics Modelling).

• Safety barriers must be represented in the hydraulic model as 100% fully blocked for concrete barriers. Guardrail must include an appropriate blockage factor from pavement to the underside of the first cross-member, and then 100% blocked above the underside of the cross member to the top of the barrier. However, all safety barriers that include motorcyclist injury countermeasures must be assumed as 100% blocked and modelled as a solid feature for the entire height of the barrier.

• Noise attenuation structures must be represented in the hydraulic model as 100% fully blocked.

3.4 Bridge design considerations

Detailed hydraulic analysis needs to be undertaken to inform the structural bridge design. Flood and debris forces need to be considered in accordance with the current AS 5100.2 and the department’s Design Criteria for Bridges and Other Structures. Important bridge design inputs are peak water levels and peak velocities under Serviceability Limit State (SLS) and the Ultimate Limit State (ULS) conditions of the bridge design.

Hydraulic modelling to undertake a scour assessment at bridge piers and abutments needs to be conducted in accordance with the department’s current Bridge Scour Manual.

3.5 Recommended initial TUFLOW form loss values

3.5.1 Bridge losses

In case a bridge is modelled in TUFLOW as a ‘Layered Flow Constriction’ (LFC), the bridge should be modelled using the PORTION approach and the following initial form loss values for bridge deck and pier losses are recommended for departmental bridges. These values are only to be applied if no calibration data is available and does not replace the requirement that all bridges must have their head loss estimates validated using an alternative independent method (see above).

Table 3.5.1(a) - Recommended LFC loss coefficients for departmental bridge sub-structures (piers and headstocks)

LFC input variable Layer 1 obvert
Layer 1 blocked ratio
Layer 1 form loss coefficient

PSC 550 mm octagonal piles
Top of headstock
Ratio of waterway area blocked by the bridge substructure
In accordance with Figure 3.5.1 and pier configuration number 5 or 6.

Cast in place piles
Top of headstock
Ratio of waterway area blocked by the bridge substructure
In accordance with Figure 3.5.1 and pier configuration number 1.

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Hydrologic and Hydraulic Modelling Figure 3.5.1 - Incremental backwater coefficients for piers (amended from Austroads 2018)

Table 3.5.1(b) - Recommended LFC loss coefficients for departmental bridge super-structures (deck and bridge rail)

LFC input variable

Standard departmental deck units

T‐Girders

Layer 2 depth

Deck unit only (excluding kerb)

Deck only (excluding kerb)
Note: Additional validation may be required for bridges with T-girders and the standard approach may not be appropriate due to the large depth of Layer 2.

Layer 2 blocked ratio

100%

100%

Layer 2 form loss coefficient

1.6

1.6

Layer 3 depth

1.1 m (guardrail + kerb)

1.1 m (guardrail + kerb)

Layer 3 blocked ratio*

30%

0%

Layer 3 form loss coefficient

0

0

* Note: The applied blockage ratio only represents structure blockage and does not include an allowance for debris blockage. The blocked area ratio will need to be increased to allow for additional debris blockage.

3.5.2 Guardrail losses
It is recommended to model guardrail in TUFLOW as a ‘Layered Flow Constriction (LFC)’. The form loss values detailed in Table 3.5.2 (based on limited CFD modelling) for standard departmental guardrail (w-beam, thrie beam, and box beam barriers) are recommended if no other calibration data is available. However, all safety barriers that include motorcyclist injury countermeasures must be assumed as 100% blocked and modelled as a solid feature for the entire height of the barrier instead of using a LFC (for example, z-line instead of lfcsh).

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Table 3.5.2 - Recommended LFC loss coefficients for standard departmental guardrail

LFC input variable

Standard departmental guardrail

Layer 1 obvert

Underside of the first cross-member

Layer 1 blocked ratio*

5.5%

Layer 1 form loss coefficient

0.05

Layer 2 depth

Depth of cross-member

Layer 2 blocked ratio*

100%

Layer 2 form loss coefficient

0.7

*Note: The applied blockage ratio only represents structure blockage and does not include an allowance for debris blockage. The blocked area ratio will need to be increased to allow for additional debris blockage.

4 Delivery specification 4.1 Reporting requirements
Any assumptions underlying a modelling parameter shall be clearly stated and explained, especially the assigned roughness coefficients and if applicable, eddy viscosity.
Following acceptance of the draft document, the Consultant shall present a final report. Whilst the structure / format of the report is not rigid, the report shall incorporate the methodology and findings of the study in sufficient detail to support the validity of the conclusions. Therefore, the report must state clearly any assumptions, parameters used, their justification and methods adopted in the hydraulic design. The report shall include the following as a minimum:
• Summarise previous work and outline relevant literature such as the hydrologic investigation. • Mapping (at an appropriate scale) of flood extents, levels, velocities and flow distributions for
all requested design events. This includes indicating where relevant:
− flow rates upstream and downstream of constrictions
− maximum flood depth and afflux values at representative locations
− flow velocity and depths at locations near to infrastructure
− flood mapping scenario differences (levels, velocities), and
− volume passing within drainage infrastructure.
• Flood peak duration (Time of closure). • If applicable, details of properties flooded over floor, floor level, change in flood depths, times
of closure / submergence and calculation of Average Annual Time of Closure.
• If applicable, all raw flood damage data including the location, floor level, size, age and nature of all buildings likely to be affected by flooding.
• If applicable, include all relevant design information (i.e. design layouts or design strings) on flood maps
• If applicable, identify property which may be inaccessible by conventional vehicles during flood events.
• Identify undersized road infrastructure crossing(s) or inlets which may constrict flows or are susceptible to blockage (i.e. box or pipe culverts with respect to the 1% AEP flows).

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• Include a model log. The model log should clearly indicate where model data has been changed for particular option scenarios including future development and major infrastructure scenarios.
• The following afflux legend (contour intervals and colours) is preferred by the department for all projects.
Figure 4.1 – Afflux legend

4.2 Supply of hydraulic model and result files
Hydraulic reports including all relevant models and results must be provided to the department. Sufficient data is to be provided to facilitate a peer review of the hydraulic modelling and the recreation of model results. Electronic results of all design and scenario model runs are to be provided for interrogation as part of model documentation. A file fully defining and explaining the relevant files provided and their inter-relationship and purpose is also to be provided.
The spatial reference for the model is to be in the department's adopted datum and projection. The height datum is metres AHD.
• Model data files and accompanying results and background are to be arranged in a file structure shall be clear and logical (Project > Model / Background > Design / Calibration > Runs).
• Log data of model runs, i.e. the input files used to make up each model run, to enable recreation of results in future.
4.3 Model GIS data
The results of the modelling shall be mapped. The data shall be a contiguous digital data set and arranged within a workspace, i.e. not as separate files for each individual map sheet. The files should include flood attributes such as depth, level and velocity. All modelled design and calibration events shall be included in the dataset.

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