10033-CoomeraModelReview.docx

City Planning City of Gold Coast PO Box 5042 Gold Coast Mail Centre QLD 9729 Australia

Attn:

Coomera River MIKE FLOOD Model Review

Dear

In accordance with your request, the Coomera River MIKE FLOOD model has been reviewed with the purpose of assessing whether the model is technically sound, physically realistic and appropriate for hydraulic modelling assessments of the Coomera River catchment for a wide range of design events. This letter summarises our findings of the model build and the models fitness for purpose with brief recommendations where appropriate.

GENERAL OVERVIEW

The Coomera River catchment covers an area of approximately 440 km2. Coomera River rises in the McPherson Ranges and flows north from the Queensland-New South Wales border through Oxenford and Coomera, splits into two branches known as the North and South Arm before discharging into the Broadwater. The land uses within the catchment predominantly consist of open space, fully urbanised and densely vegetated areas.

The 2D component of the Coomera MIKE FLOOD model covers an area of approximately 106 km2. A 2D MIKE 21 model (with a 15 m grid spacing) is used to model the floodplain. The 2D model extends from the Canungra Army camp in the upstream up to the Broadwater downstream. Hydraulic structures of sub-grid scale are represented in a 1D MIKE 11 model. The middle and upper reaches of the catchment are also modelled in MIKE 11. The MIKE 21 and MIKE 11 models are coupled via MIKE FLOOD. For this review two model setups and their corresponding results were assessed; the January 2013 calibration setup and the 20 year ARI 18 hour design storm event setup with a 100 year ARI storm surge for future (year 2100) conditions.

Water Modelling Solutions Pty Ltd PO Box 2237 Brighton Eventide QLD 4017 Australia ABN: 75 158 809 593 ACN: 158 809 593 P | +61 413 790 406 E | blake@watermodellingsolutions.com.au W | www.watermodellingsolutions.com.au Ref: 10033 Date: 27 July 2015

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JANUARY 2013 CALIBRATION MODEL SETUP

MIKE 21 MODEL

Bathymetry The selection of a 15 m grid resolution is appropriate considering the scale of features that have been resolved in the MIKE 11 model and the resulting 2D grid size of approximately 470,000 active cells. The extent of the model area is sufficient as the flood surface does not back up against ‘dry land’ cells. No obvious interpolation errors or rapidly changing/erroneous bed levels were observed in the grid data.

Time Step and Courant Number For MIKE FLOOD applications a Courant number of less than 1 generally yields stable, robust models. Based on the peak modelled water depths and velocities and a time step of 1 second, the average Courant number in the model domain was found to be 0.3, which is well within the recommended range. Localised higher Courant numbers of up to 1.0 were estimated for grid cells along the two downstream model boundaries that experience large modelled depths.

Flooding and Drying Depths Flooding and drying are enabled, which is appropriate for inland flooding applications. A flooding depth of 0.05 m and a drying depth of 0.02 m have been applied. These values are within the recommended values and are entirely valid for this application.

Boundaries One inflow boundary and two downstream ocean boundaries (for the North and South Arm) are specified in the MIKE 21 setup file and bathymetry. The bathymetry has been modified at the boundary locations to ensure smooth transition of flow into and out of the domain. The downstream ocean boundaries are specified as time varying tidal water level boundaries, which is deemed appropriate.

Source Points Flows from fifteen hydrological sub-catchments have been incorporated in the MIKE 21 model. Each source point inflow has been applied to a single grid cell; however, no excessive velocities or ‘jetting’ have been observed at the source point locations. The modelling of source point inflows is therefore considered appropriate.

Initial Surface Elevation The initial surface elevation file specified is appropriate. Boundary cells are all wet at commencement of the simulation. The initial water level at the two downstream ocean boundaries has been set to 0 mAHD. This level does not match the first time step of the water level hydrographs applied at the North Arm (-0.289 mAHD) and South Arm (-0.197 mAHD) boundaries. It is recommended to set the first time step of both water level hydrographs to 0 mAHD to avoid a surge of water out of the domain. However, as the flood peak occurs several days after the beginning of the simulation, the mismatch in levels will not affect the peak model results.

Eddy Viscosity Various empirical relationships exist for estimating appropriate values of eddy viscosity in the absence of observed eddy behaviour. High eddy values will normally smooth out the flow variability by transferring the high energy flow from one grid cell to the neighbouring cells with lower energies. A velocity based eddy viscosity of 1 m2/s has been applied globally within the model and increased to 10 m2/s at coupled locations to promote stability. These values are within the recommended guidelines for a 15 m grid size.

Resistance Eight zones of resistance have been defined. The adopted Manning’s ‘M’ values and the AR&R recommended range of roughness values for each land use type (Smith and Wasko, 2012) are listed in Table 1.

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Table 1 Adopted hydraulic roughness values

Land Use Manning’s ‘M’ Manning’s ‘n’ AR&R Recommended Range

(Manning’s ‘n’)

Roads 50 0.02 0.02 – 0.03

Waterways 33.33 –40 0.025 – 0.03 0.02 – 0.04

Open pervious areas 30 0.033 0.03 – 0. 05

Residential areas - low density 20 – 25 0.04 – 0.05 0.10 – 0.20

Residential areas - medium density 22.22 0.045 NA

Mangroves 16.67 0.06 0.05 – 0.08

Residential areas - high density 13.33 0.075 0.20 – 0.50

Dense vegetation 8.33 – 10 0.10 – 0.12 0.07 – 0.12

The eight zones represent roads, waterways, open pervious areas, low, medium and high density residential areas, mangroves and dense vegetation. Based on visual inspection of aerial photography the number of regions and Manning’s M values defined for these regions are generally appropriate, although the regions could be more accurately defined in some areas. The Manning’s M values adopted for open pervious areas and low, medium and high density residential areas, could be decreased slightly. However, due to the coarse delineation of residential areas where the same roughness value has been applied to buildings as well as some open pervious areas, it is not recommended to reduce the Manning’s M values for residential areas to fit within the AR&R recommended range of values. As hydraulic roughness is the main calibration parameter, these changes could be undertaken as part of the model calibration phase of the next model update.

MIKE 11 MODEL

Network and Structures The MIKE 11 model consists of twenty-seven branches, sixteen of which are coupled to the MIKE 21 model. Fourteen branches have been used to model the Coomera River and its tributaries upstream of the MIKE 21/MIKE 11 coupling location. The remaining branches have been used to represent bridges, culverts and other hydraulic structures likely to affect flood conditions. These branches are all 30 m long. For structures with lengths close to or exceeding 30 m (two grid cells) only a culvert is modelled in MIKE 11. The overland flow on top of the culvert is modelled in the 2D domain. This approach is in line with MIKE FLOOD modelling guidelines to avoid duplication of flow capacity over the structure.

Manning’s ‘n’ roughness values ranging from 0.013 to 0.03 have been adopted for the culverts and are considered appropriate.

The bridge module and the energy equation calculation method have been used to model seven bridges. The bridge method offers more flexibility for incorporating bridges into the model. The energy equation is often the most stable method for calculating flow through bridges. The ‘submergence’, ‘overflow’ and ‘piers’ options have been checked for all bridges, which is appropriate as all bridges have been coupled using structure links, where both submergence and overtopping is modelled in MIKE 11. At some of the bridges the left and right levee banks (markers 1 and 3) have been set to the bridge bottom level. It is recommended to adjust these to exceed the bridge deck level and the maximum anticipated modelled water level at the bridge locations to account for bridge overflow properly. An example of a bridge plot before and after correcting the bank markers is shown in Figure 1. Apart from this, the bridge implementation is found to be appropriate.

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Figure 1 ‘HopeIsRd_GracemereGardenCanal_Bridge’ plotted before (top) and after (bottom) the extension of left and right levee markers to the top level of the upstream cross section

Cross Sections The natural shape of cross sections upstream and downstream of structures has been maintained where possible. Cross sections were enlarged slightly in order to fit the structure if the cross sections were smaller than the structure dimensions in their unmodified form. This is required by the modelling software. Some cross sections upstream and downstream of culverts have a rectangular or trapezoidal shape. Using simplified cross sections is justified when modelling small structures.

Figure 2 ‘MontereyKeys_Sw_Ck’ bridge plotted with the upstream (top) and downstream (bottom) cross sections as background

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Cross sections upstream and downstream of structures generally extend high enough to exceed the maximum anticipated water level as well as the maximum elevation of the structure. However, there are some exceptions. An example is the ‘MontereyKeys_Sw_Ck’ structure (see Figure 2) where the cross sections upstream and downstream have not been extended above the structure obvert. It is recommended to extend the maximum level at both cross sections to at least 7.5 mAHD (bridge deck level is at 7.12 mAHD).

Most of the cross sections in the model were found to have monotonically increasing conveyance curves. Left and right low flow bank markers have been introduced at some cross sections to remove any inflections in the conveyance curves, which is considered good modelling practice. At the few cross sections where this approach still does not resolve the issue, it is recommended to recalculate the levels in the conveyance curve using the ‘equidistant’ level selection method, see Figure 3.

Figure 3 Conveyance curve for cross section ‘UC_Dummy’ Ch 13141 when using the default ‘automatic’ level selection method (top) and the ‘equidistant’ level selection method with 50 levels (bottom)

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The invert levels of the cross sections match the level ‘z’ values in the MIKE 21 bathymetry to which the cross sections are coupled. A match between the levels improves model stability and is considered good modelling practice.

Boundary Conditions Fifty-seven boundary conditions have been assigned in the boundary file. Water level boundaries have been defined at both ends of the branches used to model structures. This is the necessary and accepted approach when coupling 1D branches to a MIKE 21 grid. In addition to the dummy water level boundaries, fifteen source point/distributed inflows have been applied to the ‘UPPERCOOMERA’ branch and its tributaries used to represent the upper reaches of the Coomera River.

Hydrodynamic Parameters A global Manning’s ‘n’ roughness value of 0.05 has been applied. Local Manning’s ‘n’ values ranging from 0.025 to 0.12 have been applied for some of the Coomera River tributaries. The roughness values (especially the Manning’s ‘n’ value of 0.12) are very rough and usually used to represent areas of dense vegetation. However, some of the upstream reaches in the Coomera River catchment are quite densely vegetated, so the use of higher than usual roughness values can be justified. The delta value on the Default Parameters tab of the HD11 file is used to control the time centring of the solution scheme. The solution scheme is fully centred in time when delta is equal to 0.5. A delta value greater than 0.5 will have a dissipative effect on the wave front, but can also improve model stability. A value of 0.7 was found to have been applied. Care should be taken when using such high delta values, especially in areas affected by changing tidal conditions. It is recommended to reduce the delta value to 0.6 in future model runs; however, for this application, the effect is expected to be minor as MIKE11 has only been utilised for 1D structures in the model area affected by tide.

MIKE FLOOD MODEL

Structure and Standard Links Twenty-nine standard and structure links have been defined in the model and depth adjustment has been activated. A momentum factor of one has been applied to all standard links, which is appropriate. An exponential smoothing factor of 0.1 has been applied to the links. The exponential smoothing factor introduces smoothing of the water level values transferred between the models. A value of one means no smoothing will be applied whereas a value closer to zero creates strong smoothing in the model and may aid stability. The adopted exponential smoothing factors are considered appropriate in this application.

MODEL RESULTS

The MIKE 21 model has a 15 minute save interval and produces a result file of approximately 5.3 GB. The outputs saved in the 2D result file are water depth, P and Q fluxes as well as surface elevation. Some of the MIKE visualisation and data post-processing tools as e.g. the ‘TxStat’ statistics tool in MIKE Zero toolbox will only work on standard HPQ MIKE 21 result files, i.e. files containing only the water depth and P and Q flux items. It is therefore recommended to remove the surface elevation item from the list of outputs in any future model runs.

No instabilities were found in the MIKE 21 or MIKE 11 result files. The MIKE 21 and MIKE 11 water balance errors are both reported by the model engine as 0%. An animation of the overland water movement did not show water experiencing sharp changes in flow direction at any locations. The overland peak flow velocity ranges from 0 to 7.5 m/s with an average peak velocity of 0.4 m/s. Cells with velocities exceeding 4 m/s are located along the downstream model boundaries and are likely a result of high bed level gradients.

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The calibration dataset for the January 2013 event consists of continuous water level gauge records at the Oxenford Weir, Coomera Shores and Monterey Keys Alert stations. A comparison of the modelled and recorded peak water levels is shown in Table 2. There is a good match between the recorded and modelled water levels at the Oxenford Weir and Coomera Shores Alert stations. The model overestimates the recorded flood level by 0.39 m at the Monterey Keys Alert station; however, the gauge failed around the flood peak, so the difference between the modelled and recorded peak levels at this location could be smaller.

Table 2 Comparison of recorded and modelled peak water levels – January 2013 event

ID Location Recorded (mAHD)

Modelled (mAHD)

Difference (m)

Difference in Timing

of Peak (min)

1 Oxenford Weir (Coomera River) 6.02 5.97 -0.05 Model peaks 15 min too early

2 Coomera Shores (Coomera River) 1.47 1.66 0.19 Model peak matches the record

3 Monterey Keys (Saltwater Creek) 1.53 1.92 0.39 Model peaks an hour too late* *Gauge record is not continuous around the flood peak, so the water level could have peaked a bit later

SUMMARY

Overall the model has been developed within accepted guidelines and is fit for purpose. While a few changes are recommended, none of the below recommendations are believed to significantly affect the modelling results.

Key recommendations:

Set the first time step of both water level hydrographs to 0 mAHD to avoid a surge of water out of the model domain;

Review the MIKE 21 spatial resistance values applied for open pervious areas and low, medium and high density residential areas in any future model revision projects;

Adjust the left and right levee banks for bridge structures to ensure they exceed the bridge deck level and the maximum anticipated modelled water level at the bridge locations;

Extend the maximum level at MIKE 11 cross sections to ensure they exceed the structure obverts; Recalculate the levels in the conveyance curve using the ‘equidistant’ level selection method for cross

sections with non-monotonically increasing conveyance curves; and Reduce the delta value to 0.6 in the HD parameter file. 20 YEAR ARI 18 HOUR DESIGN STORM EVENT SETUP WITH A 100 YEAR ARI STORM SURGE

The storm surge model setup is almost identical to the January 2013 calibration setup. Only files that are different to those used in the calibration setup have been reviewed. The findings are summarised below.

MIKE 21 MODEL

Time Step and Courant Number For MIKE FLOOD applications a Courant number of less than 1 generally yields stable, robust models. Based on the peak modelled water depths and velocities and a time step of 1 second, the average and maximum Courant numbers in the model domain were found to be 0.4 and 1.0, respectively.

Boundaries The downstream ocean boundaries are specified as time varying storm surge water level boundaries, which is considered appropriate.

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Source Points Flows from fifteen hydrological sub-catchments have been incorporated in the MIKE 21 model. Each source point inflow has been applied to a single grid cell. No excessive velocities or ‘jetting’ is observed in the result file.

Initial Surface Elevation The initial surface elevation file specified is appropriate. Boundary cells are all wet at commencement of the simulation. The initial water level at the two downstream ocean boundaries has been set to 0.56 mAHD. This level matches the first time step of the water level hydrographs applied at the North and South Arm boundaries.

MODEL RESULTS

The MIKE 21 model has a 15 minute save interval and produces a result file of approximately 1.9 GB. No instabilities were found in the MIKE 21 or MIKE 11 result files. An animation of the overland water movement did not show water experiencing sharp changes in flow direction at any locations. The overland peak flow velocity ranges from 0 to 8.0 m/s with an average peak velocity of 0.4 m/s. Cells with very high velocities are located along the downstream model boundaries and are likely a result of high bed level gradients. This is not believed to have an adverse impact of the modelling results, but smoothing out the bathymetry adjacent to the boundaries could be considered in the next model update.

SUMMARY

Overall the model has been developed within accepted guidelines and is suitable for assessing the potential for flooding within the Coomera River catchment. During the next model update, it is recommended to:

Smooth out the bathymetry along the downstream model boundaries.

Please do not hesitate to contact me if you require further clarification. Yours sincerely,

References

DHI (2014). MIKE FLOOD 1D-2D Modelling User Manual. DHI, Hørsholm, Denmark. DHI (2014). MIKE FLOOD Modelling of River Flooding Step-by-Step Training Guide. DHI Hørsholm, Denmark. Smith, G. and Wasko, C. (2012) Australian Rainfall and Runoff, Revision Project 15: Two Dimensional Simulations in Urban Areas - Representation of Buildings in 2D Numerical Flood Models. Engineers Australia, Barton, ACT, February 2012.