loders creek hydraulic model peer review€¦ · time step and courant number for mike flood...

10029-LodersModelReview.docx

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

Attn:

Loders Creek MIKE FLOOD Model Review

Dear

In accordance with your request, the Loders Creek 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 Loders Creek 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 Loders Creek catchment covers an area of approximately 10 km2. The creek is formed from two small streams and drains out 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 Loders MIKE FLOOD model covers an area of approximately 8.2 km2. A 2D MIKE 21 model (with a 5 m grid spacing) is used to model the floodplain. Hydraulic structures of sub-grid scale are represented in a 1D MIKE 11 model. 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 June 2005 calibration setup and the 20 year ARI 0.5 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 | [email protected] W | www.watermodellingsolutions.com.au Ref: 10029 Date: 24 July 2015

10029-LodersModelReview.docx 2

JUNE 2005 CALIBRATION MODEL SETUP

MIKE 21 MODEL

Bathymetry The selection of a 5 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 330,000 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 0.75 seconds, the average Courant number in the model domain was found to be 0.6, which is well within the recommended range. Localised higher Courant numbers of up to 1.5 were estimated for a few grid cells in the model domain 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 0.03 m have been applied. These values are within the recommended values and are entirely valid for this application.

Boundaries Two inflow boundaries and one downstream boundary 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 boundary is specified as a time varying tidal water level boundary, which is deemed appropriate.

Source Points Flows from fourteen hydrological sub-catchments have been incorporated in the MIKE 21 model. Each source point inflow has been distributed across multiple grid cells to avoid excessive velocities or ‘jetting’ at the source point locations. This is the recommended approach when modelling large flood events. The distribution of inflows across multiple cells results in a total of 52 source points included in the MIKE 21 model.

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 downstream ocean boundary has been set to -0.615 mAHD. This level matches the first time step of the water level hydrograph applied at this boundary and is a valid approach of modelling the boundary condition.

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 0.134 m2/s has been applied globally within the model. This value is at the low end of the range of eddy values recommended for a 5 m grid size and could be increased to e.g. 0.5 m2/s. This will also have a stabilising effect on the model results. If the value is not increased, it is recommended to round the number to one significant figure given the uncertainty associated with its estimation.

Resistance Seven 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.


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

Broadwater 40 0.025 0.02 – 0.04

Open pervious areas 30 0.033 0.03 – 0. 05

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 – 20 0.05 – 0.12 0.07 – 0.12

The seven zones represent roads, the Broadwater, open pervious areas, 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 of 30, 22.22 and 13.33 adopted for open pervious areas and medium and high density residential areas, respectively, 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. Manning’s M values ranging from 8.33 to 20 have been applied to areas with dense vegetation. It is recommended to use a value of 8.33 for all areas with dense vegetation for consistency and 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-two branches, all of which are coupled to the MIKE 21 model. All branches have been used to represent bridges, culverts and other hydraulic structures likely to affect flood conditions. The lengths of these branches vary from 10 m to 170 m. For structures with lengths that exceeded 10 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.

The 150-170 m long MIKE 11 branches were used to model long rectangular culverts using closed cross sections. This is in line with MIKE FLOOD modelling guidelines to account for friction losses. However, other long structures exceeding 40 m in length have been implemented as culverts in the model. It is recommended to also consider modelling these using 1D cross sections.

Manning’s ‘n’ roughness values ranging from 0.013 to 0.05 have been adopted for the culverts and are considered appropriate.

The bridge module and the energy equation calculation method have been used to model four 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 length of three of the bridges (‘MusgaveAveBridge’, ‘JohnsonSt’ and ‘GoldCoastHighway’) exceeds two MIKE 21 grid cells, meaning that the overtopping of the bridges is modelled in the 2D domain. However, the overtopping option has also been checked in the MIKE 11 model. This will duplicate the flow capacity when the modelled water levels exceed the bridge deck levels. It is recommended to deselect the overtopping option in the MIKE 11 model for those bridges to avoid duplication of flows. Apart from this, the bridge implementation is found to be appropriate.


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.

Cross sections upstream and downstream of structures extend high enough to exceed the maximum anticipated water level as well as the maximum elevation of the structure. One exception is the ‘ChirnCrescent’ structure (see Figure 1) 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 1.6 mAHD.

Figure 1 ‘ChirnCrescent’ culverts plotted with the upstream (top) and downstream (bottom) cross sections as background

Some of the cross sections are significantly wider than the modelled culverts and the number of MIKE 21 grid cells they are coupled to. This can result in unrealistically high head losses across the structures. One example is the ‘WardooSt’ structure, see Figure 2. The branch used to model the structure has been coupled to one MIKE 21 grid cell at both upstream and downstream ends. It is recommended to reduce the width of those cross sections to approximately match the width of the MIKE FLOOD links.

All but one cross sections in the model were found to have monotonically increasing conveyance curves, see Figure 3. Non-monotonically increasing conveyance curves can result in model instabilities. It is recommended to introduce left and right low flow bank markers to remove any inflections in the conveyance curve at the upstream ‘FalconSt’ cross section, see Figure 3.


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

Figure 3 ‘FalconerSt’ cross sections and corresponding conveyance curves without (top) and with (bottom) left and right low flow bank markers


The invert levels of the cross sections match the level ‘z’ values in the MIKE 21 bathymetry to which the cross sections are coupled with the exception of the ‘SouthportNerangRd’ branch at chainage 0, where the difference between the MIKE 21 bathymetry and the MIKE 11 cross section invert level is 0.1 m. A match between the levels improves model stability and is considered good modelling practice. However, a 0.1 m difference is not believed to have an adverse impact on the modelling results as no instabilities were found in the result file at this structure.

Boundary Conditions Forty four boundary conditions have been assigned in the boundary file. Water level boundaries have been defined at both ends of the branches used to model the structures. This is the necessary and accepted approach when coupling 1D branches to a MIKE 21 grid.

Hydrodynamic Parameters A global Manning’s ‘M’ roughness value of 30 has been applied and is considered appropriate. 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 0.8 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 and not entire waterway reaches.

MIKE FLOOD MODEL

Structure and Standard Links Forty-four 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. Exponential smoothing factors of 0.1, 0.2 and 0.3 have 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 7.5 minute save interval and produces a result file of approximately 10.6 GB. No instabilities were found in the MIKE 21 result file. Some instabilities in modelled water levels and discharges were found in the MIKE 11 result file, an example is shown in Figure 4. However, the instabilities occur prior to and/or past the flood peaks and will not have an adverse impact on the peak flood results. The small instabilities are most likely a result of very small head losses across the structures. It is recommended to smooth out the bathymetry adjacent to the coupled cells to ensure a smooth transition of water into and out of the couples. In addition, the eddy viscosity value and Manning’s M values could be enlarged to promote stability.

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 4.8 m/s with an average peak velocity of 0.4 m/s. Cells with velocities exceeding 4 m/s are located in the vicinity of the Loders Creek Dam wall and are likely a result of a high bed level gradient.


Figure 4 Model results at the ‘ChirnParkCarpark’ structure (black - water level upstream, blue - water level downstream, red - discharge)

The calibration dataset for the June 2005 event consists of continuous water level gauge records at the Loders Creek Alert and Loders Dam Alert stations and seven surveyed debris marks. According to the Loders Creek Catchment Hydraulic Study report (CoGC, 2015) the fit between recorded and modelled levels is reasonably good, but could be improved by adjusting hydrologic inflows for this event.

A review of the hydrological inputs is not a part of the scope of this model review. A much better fit between recorded and modelled water levels at the two Alert stations has been achieved for the two most recent flood events in January 2008 and January 2013 for which the hydrological inputs are believed to be more reliable. This supports the finding that the hydraulic model setup has generally been set up within the recommended guidelines and the poorer calibration to this event is relate to the hydrologic inputs.

SUMMARY

Overall the model has been developed within accepted guidelines and is fit for purpose. However, a few changes are recommended, some of which could affect the modelling results to some extent.

Key recommendations:

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

Use a Manning’s ‘M’ value of 8.33 for all areas with dense vegetation for consistency and to fit within the AR&R recommended range of values.

Consider modelling long culverts (exceeding 40 m in length) using closed cross sections to account for friction losses;

Deselect the overtopping option in the MIKE 11 model for the ‘MusgaveAveBridge’, ‘JohnsonSt’ and ‘GoldCoastHighway’ bridges to avoid duplication of flows;

Extend the maximum level at the ‘ChirnCrescent’ cross sections to ensure it exceeds the structure obvert; Reduce the width of the MIKE 11 cross sections to approximately match the width of the MIKE FLOOD links; Introduce left and right low flow bank markers to remove any inflections in the conveyance curve at the

‘FalconSt’ cross section at chainage 0; Reduce the delta value to 0.6 in the HD parameter file;


Review the bathymetry in areas with high modelled velocities and around structures with minimal head loss to smooth the transition of bed elevations where possible;

Increase the eddy viscosity value to 0.5 m2/s; and Adjust the invert level of the upstream ‘SouthportNerangRd’ cross section to match the level ‘z’ values in

the MIKE 21 bathymetry to which the cross section is coupled. 20 YEAR ARI 0.5 HOUR DESIGN STORM EVENT SETUP WITH A 100 YEAR ARI STORM SURGE

The storm surge model setup is almost identical to the June 2005 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 1.5 seconds (the time step was increased in this setup), the average Courant number in the model domain was found to be 1.1. Courant numbers of up to 2.4 were estimated within the Loders Creek channel, ponding areas and the Broadwater where the modelled water depths are large. If the model produces unstable results using a time step of 1.5 seconds, it is recommended to reduce the time step to ensure the Courant numbers below 1 are maintained.

Boundaries The downstream ocean boundary is specified using a time varying storm surge water level time series, which is considered appropriate.

Source Points Flows from fourteen hydrological sub-catchments have been incorporated in the MIKE 21 model. The distribution of inflows across multiple cells results in a total of 52 source points included in the MIKE 21 model. 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 downstream ocean boundary has been set to 0.4 mAHD. The first time step of the water level hydrograph applied at this boundary is 0.42 mAHD. This small discrepancy is not believed to adversely impact the model results. However in future model runs, it is recommended to ensure the levels at the first time step match to avoid a surge of water in or out of the model at the commencement of the simulation.

Eddy Viscosity A velocity based eddy viscosity of 0.333 m2/s has been applied globally within the model. Eddy viscosity is not generally a calibration parameter (unless data on observed eddy behaviour has been collected). It is estimated based on empirical relationships for a range of model grid sizes and time steps. Given the uncertainty associated with the estimation of eddy viscosity, it is recommended to use one value for all model runs and round the value to one significant figure.


MIKE 11 MODEL

Hydrodynamic Parameters A global Manning’s ‘M’ roughness value of 22 has been applied, which is lower than the value of 30 applied in the June 2005 setup. Both values are within the recommended range of roughness values for waterways depending on the degree of vegetation in the channels. The reduction of the Manning’s ‘M’ roughness is not expected to have a major impact on the results as MIKE11 has only been used to represent 1D structures.

A delta value of 0.85 has been applied in this setup. 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.

MODEL RESULTS

The MIKE 21 model has a 3 minute save interval and produces a result file of approximately 0.6 GB. No instabilities were found in the MIKE 21 result file. 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 3.7 m/s with an average peak velocity of 0.3 m/s. Some instabilities in the discharge time series were found in the MIKE 11 result file, see example at the ‘GoldCoastHighway’ structure in Figure 5. No instabilities in water levels were observed and the instabilities in discharge occur prior to the peak in modelled water levels. The instabilities in discharge are therefore not believed to have an adverse impact on the model results. However, as part of the next model update, it is recommended to check if the bathymetry adjacent to the structure location can be adjusted to smooth the transition of water into and out of the couples.

Figure 5 Model results at the ‘GoldCoastHighway’’ structure (black - water level upstream, blue - water level downstream, red - discharge)


SUMMARY

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

Smooth out the bathymetry adjacent to the structures with minor instabilities in discharges; Reduce the time step if the model produces unstable results to ensure Courant numbers of less than 1 are

maintained; and Reduce the delta value to 0.6.

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. CoGC (2015). Loders Creek Catchment Hydraulic Study. Council of the City of Gold Coast, Australia. 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.

loders creek hydraulic model peer review€¦ · time step and courant number for mike flood...

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