Potential for forecasting inundation from flooding and storm surge: a case study of a small urban catchment during ex Tropical Cyclone Oswald.

Takehiko Nose1, Joanna M. Burston1, Daniel Gee1, and Rodger Tomlinson1

1 Griffith Centre for Coastal Management, Griffith University, Southport, Qld, Australia; t.nose@griffithuni.edu.au

Abstract Forecasting inundation resulting from the dynamics of storm tide combining with freshwater flooding in estuarine environments and the behaviour of wave set-up in tidal inlets is complex yet important for effective disaster management. As part of a Griffith University project sponsored by the Queensland (Qld) State Government, a case study of a small urban catchment with a wave-dominated tidal inlet is used to investigate the potential for numerical modelling of such processes. Ex Tropical Cyclone (TC) Oswald, January 2013, generated freshwater flooding, storm surge and extreme wave conditions along eastern Qld coastline, causing extreme water levels in the case study site of Currumbin Creek on the Gold Coast. A field survey following this event combined with measured discharge, wave, and hydrographic data allows analysis of the hydrodynamic response of the system using numerical modelling. A numerical model built using DHI’s MIKE21 is used to simulate interactions between the riverine flood, storm surge and waves within the tidal inlet during the ex-TC Oswald event. In particular, the effect of possible wave set-up at the entrance on the tailwater level is investigated. We conclude that dynamical forecasting of inundation due to a combination of storm tide, extreme waves and freshwater flooding in such environments requires detailed inputs, including accurate discharge, tailwater levels and up-to-date bathymetry.

Keywords: numerical modelling, Currumbin Creek, wave setup, storm surge, tropical cyclones, coastal estuary

1. Introduction Water levels in tidal inlets during extreme weather events exhibit complex behaviour due to the number of and dynamic interaction between the physical processes operating. These processes include storm surge, elevated riverine flow in response to extreme rainfall, astronomical tide and possible wave set-up, as well as evolution of the morphology due to channel scouring and longshore sediment. Understanding the response and sensitivity of extreme water levels for urbanised tidal inlets is important from an emergency management perspective. Much of the risk posed by storm tide along the Gold Coast, Qld, is to properties with exposure to flooding from the combination of storm tide and riverine flooding in small urban catchments. However, the dynamics of storm tide combining with freshwater flooding in such environments and the behaviour of wave set-up in tidal inlets are not well understood. The current practise of flood engineers and disaster managers when assessing the risk of inundation in real-time is to apply a tailwater level consisting of the astronomical tide combined with a static estimation of the peak storm surge from BoM warnings to a flood model (pers. comm., Gold Coast City Council (GCCC)). The present practise is to subtract any allowance for wave set-up from the tailwater level for Gold Coast catchments. This study, part of the Griffith Centre for Coastal Management (GCCM)’s “Coastal Emergency Management Decision Support System” project, an initiative of the Qld State Government in partnership with DHI Australia, Qld Cyber Infrastructure Foundation and Emergency Management Qld, aims

to investigate potential improvements to Qld’s capabilities in forecasting storm surge inundation risk for emergency planning purposes. Here, a case study has been developed to investigate the complexities involved in forecasting storm tide inundation in tidal inlet environments. The aim of this study is to investigate the contribution of various processes to the extreme water levels measured from the field survey of Currumbin Creek carried out after ex-TC Oswald through numerical modelling. The Gold Coast was chosen as the case study location given historical impacts of TCs and population exposure to flooding. The Gold Coast has been exposed to infrequent TC events including a series in 1974 including TC Pam, TC Dinah (1967), the 1954 cyclone and several events from the 1880s, to 1930s [5]. These cyclonic events have caused major coastal impacts including flooding, beach erosion, and barrier breakthrough. Since these major events, the Gold Coast has experienced intensive development including proliferation of canal estates along its urban waterways. In January 2013, the Gold Coast was impacted upon by ex-TC Oswald, with accompanying heavy rainfall, persistent east-north-east winds and associated storm surge and high wave conditions coinciding with spring tides (Section 3) causing severe beach erosion and some localised inundation. While this event was not particularly severe, it underlined the vulnerability of the Gold Coast to TC events and provided an opportunity to investigate the physical processes contributing to

oceanic inundation. GCCM staff conducted a field survey of the open beaches and of the inundation around the small urban catchment of Currumbin Creek (Figure 1). 2. Study Area Currumbin Creek (28.135 °S, 153.465 °E) is located between Currumbin Beach to the south and Palm Beach to the north on the Gold Coast of eastern Australia and has a catchment area of 48 km

2

(Figure 1). The creek entrance can be classified as a tidal inlet rather than an estuary given the generally low freshwater discharge, and is trained with rock training walls on both the northern and southern sides. The southern training wall connects to Currumbin Rock. Along with annual maintenance dredging of the entrance, these walls serve to keep the entrance open to the sea, improving water quality within the estuary and mitigating flood impacts [7]. The entrance morphology is dynamic in response to longshore transport from the south and episodic ocean swell events [6]. The Currumbin Creek area has high ecological significance and is a popular recreational destination for surfers, boating and fishing.

Figure 1. Location of Currumbin Creek, SE Qld, Australia.

3. Event Description: TC Oswald, Jan 2013 TC Oswald developed in the Gulf of Carpentaria on 21

st January 2013, making landfall as a Category 1

system on the west coast of Cape York Peninsula [2]. The following morning, Oswald dissipated to an ex-TC and tracked southward just inland of the east coast of Qld and NSW before moving offshore near Sydney on 29

th January (Figure 2). This event

impacted the Gold Coast from 26th–28

th January,

with persistent east to east-north-east sub-100 km/h wind gusts measured at both the Gold Coast Seaway and Coolangatta (Figure 3). The maximum 1-min wind speed of 22.5 m/s (81 km/h) was

recorded around 09:00 28th January (all times given

in AEST (+1000 UTC)).

Figure 2. Synoptic chart showing the position of ex-TC Oswald, 11:00 AEST 28

th January 2013 (source: BoM).

Figure 3. Measured wind (a) speed and (b) direction at the Gold Coast Seaway for ex-TC Oswald, Jan 2013 (data source: BoM).

The peak significant wave height recorded by the Gold Coast Waverider buoy during the event was 6.27 m at 10:38 28

th January 2013 with an

associated peak wave period of 11.71 s (data source: DSITIA) (Figure 4). The peak wave direction was persistently east to east-north-east throughout the event. This would be a 5-10 year event in terms of wave height for the Gold Coast [1]. A maximum water level of 1.395 m AHD was measured at the Southport tide gauge located within the Gold Coast Broadwater (Figure 5) at 09:50 28

th January, being

0.565 m above the predicted high tide of 0.83 m (data source: Maritime Safety Qld). The tidal residual fluctuated between two peaks: 0.708 m at 15:30 27

th January and 0.707 m at 01:50 28

th

January, and a minimum of 0.446 m at 21:10 27th

January. Numerical modelling indicated that this fluctuation was attributable to a variation in offshore wind speed at this time. The tidal residual at this site is not representative of the open ocean residual given the complexity of this system with connections

to Moreton Bay, multiple freshwater inputs and the constriction of the Gold Coast Seaway.

Figure 4. Measured wave (a) heights and (b) peak period for the Gold Coast Waverider buoy for ex-TC Oswald, Jan 2013 (data: DSITIA).

Figure 5. Predicted tide, measured and residual water levels within the Gold Coast Broadwater for ex-TC Oswald, Jan 2013 (data: MSQ).

Ex-TC Oswald was associated with a heavy rainfall over the Gold Coast. While there is no official rain gauge within the Currumbin catchment, 261.8 mm fell at Tallebudgera Road and 602.8 mm fell at Tallebudgera Dam from 26

th – 28

th January. At the

river gauging station monitoring Currumbin Creek, the water level and discharge increased to a peak of 2.89 m and 155.172 m

3/s at 22:00 27

th January

(Figure 6) (data source: Qld DNRM).

Figure 6. Discharge at Nicholls Bridge on Currumbin Creek during ex-TC Oswald, January 2013 (data: DNRM).

The coastal impacts experienced along the Gold Coast as a result of ex-TC Oswald included extensive erosion of open beaches, especially those with an easterly to north-easterly exposure [3]. Overtopping of dunes and resultant oceanic inundation was observed in several locations.

Flooding was evident for Tallebudgera Creek, Currumbin Creek and the Nerang River system although no properties were inundated. The flood event was estimated to be approximately a one in 20 year event by GCCC flood engineers.

3.1. Field Survey Description GCCM staff undertook a field survey of the Currumbin Creek catchment and surrounding open beaches on 1

st February 2013 to measure the

spatial position and elevation of the highest debris line position, used to infer the peak water level, using a Leica GS15 GPS with a positional accuracy of 0.02 m for open locations. The field survey results generally show an increase in the inferred peak water level from within the creek to the open ocean (Figure 7). A debris line position of 1.7 m AHD was measured beneath the Gold Coast Hwy Bridge. At Thrower Drive Bridge, debris line positions of 1.46 m and 1.42 m AHD were measured on the western and eastern sides of the creek, respectively. Further upstream, the debris line position on the bank opposite the Currumbin Waters canal estate was 1.44 m AHD. East of the Gold Coast Hwy Bridge, the debris line position was generally higher than at the bridge. The debris line position in the north-western corner of the estuary mouth varied between 1.75-2.16 m AHD. Along the northern bank of the estuary mouth, the debris line position varied between 2.03-3.23 m AHD. There was no evidence of wave over-topping of the dune on the northern side although the breakwater was overtopped by waves as evidenced by debris. On the southern bank of the estuary mouth, the debris line position was 1.88 m AHD. A sand delta extends into the creek mouth from its southern bank and the peak debris line positions here were 2.28-2.66 m AHD on the seaward-exposed northerly side and between 1.62-1.91 m AHD on the non-exposed southerly bank. At the pocket beach just within the training walls on the southern side of the inlet, the debris line position varied from 2.25-3.24 m AHD. The higher levels at this location indicate that this beach was subject to some wave energy during the event. On the easterly-exposed northern end of Currumbin Beach, the maximum run-up level was 5.08 m AHD. On Palm Beach, the maximum run-up level was 3.95 m AHD, which did not overtop the dune. The southern end of Palm Beach has north-north-east exposure and would have been subject to slightly lower wave energy than Currumbin Beach.

Figure 7. The maximum extent of inundation around the Currumbin Creek entrance (m AHD) due to ex-TC Oswald, January 2013 (imagery from OpenSeaMap.com).

3.2. Field Survey Interpretation

The general increasing trend in peak debris line position from Thrower Dr Bridge to the Gold Coast Hwy Bridge then towards the creek entrance could be due to several different dynamic processes. These include the penetration of wave set-up for a limited distance into the estuary, the dynamics of the incoming tide and storm surge against the riverine flow and a change in bed level during the event. In particular, the difference in inferred peak water level on either side of the delta feature may indicate that elevated water levels due to wave run-up were experienced on its seaward side, being a minimum of (2.28–1.91 m=) 0.37 m. The contributing processes to the peak water levels within the Currumbin Creek tidal inlet during this event are subsequently explored. 4. Numerical Modelling of Currumbin Creek

4.1. Model Set-Up and Calibration In order to simulate the inundation associated with the ex-TC Oswald event, a hydrodynamic model of Currumbin Creek was built in the finite volume 2D depth-integrated hydrodynamic model MIKE21 FM by DHI (Figure 8). Bathymetry for the creek was obtained from a hydrographic survey conducted for GCCC in September 2012. The offshore bathymetry was interpolated from a combination of beach survey data (GCCC) and RAN chart data. The model was calibrated using water level measurements collected by DHI for the period 14

th

April - 30th May 2011 for two sites: upstream

(28.16151°S, 153.46324°E) and downstream (28.13584°S, 153.47737°E) (Figure 8).

Figure 8. Model mesh and bathymetry for Currumbin Creek showing up- and downstream measurement points.

The model forced with discharge measured at Nicolls Bridge as the upstream boundary condition and astronomical tidal forcing on the open ocean boundary for the calibration period shows good agreement to the measured water levels at both locations (Figure 9). To consider the model’s performance in a period of high river discharge, the results of the TC Oswald simulation to follow were compared with output from GCCC’s in-house flood model and found to be comparable in timing and water level (pers. comm., Dr H. Mirfenderesk, 2013) (not shown). This calibrated model was determined to be suitable for further use.

Figure 9. Sample comparison of modelled and measured water levels for (a) upstream and (b) downstream locations in Currumbin Creek, April – May 2011.

4.2. Simulation of ex-TC Oswald The calibrated model was used to simulate the ex-TC Oswald event for the period 09:00 25

th January -

09:00 30th January 2013. The upstream boundary

discharge was that measured at Nicolls Bridge (Figure 6). The offshore water level boundary condition was a combination of the predicted

astronomical tide and storm surge generated using GCCM’s calibrated Coral Sea tidal model forced with global tidal model boundary conditions and BoM’s modelled ACCESS-R wind and pressure fields [4]. The modelled winds had good agreement with the measured winds at Gold Coast Seaway in both speed and direction (Figure 10) as well as other sites along the Qld coastline (not shown). They also produced waves with good agreement in height, period and direction to those measured at DSITIA’s Gold Coast waverider buoy when used to force GCCM’s Coral Sea spectral wave model (not shown). These results show that the modelled wind data have sufficient accuracy and resolution to replicate the peak wind in the event.

Figure 10. Comparison of ACCESS-R modelled with measured wind speeds (both converted to 3s gusts) at Gold Coast Seaway (data source: BoM).

The modelled storm surge offshore of Currumbin peaked at 0.19 m, much lower than the 0.708 m residual measured within the Broadwater (Figure 5). This difference was investigated, and found to be attributable to the fact that the Broadwater residual was associated with the storm surge generated within the complex Moreton Bay system due to the persistent east-north-east winds as ex-TC Oswald moved southwards, setting up a gradient in water level from north to south. In the regional Coral Sea model results, the direction of flow through the Gold Coast Seaway throughout the Oswald event was seaward regardless of the phase of the tide (not shown). Therefore, the measured water levels within the Broadwater were not representative of the open ocean conditions at this time.

4.3. Model Results The model simulations were run by incrementally adding the processes considered in order to understand the contribution and interaction of each to the resultant water level. Model output was compared to the inferred peak water line data collected in the field survey, mainly the 1.7 m level close to Gold Coast Hwy Bridge and higher water

levels seaward of that location (Table 1). The peak water level at this bridge was not replicated by forcing the model with the river flow, the tide and the tidal residual (either that modeled or measured in the Gold Coast Broadwater), although the ~1.4 m water level at the Thrower Drive Bridge further inland was replicated in run (4). No model simulations resulted in a slope in the peak water levels towards the entrance as observed in the field survey, even when a change to the entrance morphology was made to simulate sand build-up. Table 1. Hydrodynamic model results for Currumbin Creek for ex-TC Oswald, Jan 2013. Model Run Peak Water Level at Gold

Coast Hwy Bridge (m AHD) Measured 1.70 m (1) Upstream river flow 0.56 m (2) River flow + tide 1.07 m (3) River flow + tide + modelled storm surge (peak 0.19 m)

1.17 m

(4) River flow + tide + measured tidal residual at GC Seaway (peak 0.704 m)

1.42 m

River flow + tide + measured tidal residual at GC Seaway + (Entrance constricted with 0.5 m AHD sand bar)

1.54 m (change in timing of peak to previous high tide – not in agreement with observations of residents)

A preliminary spectral wave model was set up to look at the penetration of waves into Currumbin Creek. The wave boundary conditions were supplied from the regional Coral Sea wave model forced with BoM’s ACCESS-R wind fields. Waves up to 0.25 m were found to propagate to the sand delta feature but not beyond for a simulation including the water level variations and also local wind forcing. This would account for the difference in the maximum measured water level on either side of the sand delta feature, but not those on the northern bank. 5. Discussion The field survey of the extreme water line resulting from ex-TC Oswald for Currumbin Creek showed a distinct gradient in water level towards the entrance. We were unable to replicate this gradient by numerical modelling of the river flow and storm tide, even considering different entrance bathymetries. We postulate that wave set-up and run-up caused elevation of the water level in estuary mouth during this extreme event, grading over a distance of approximately 500 m inland. From preliminary wave modelling exercises, the role of wave set-up in this event was not resolved. We conclude that accurate estimation of run-up levels would require more detailed modelling of wave propagation.

The maximum measured run-up level of 5.08 m AHD on Currumbin Beach implies a wave run-up of a minimum of 3.9 m given the maximum (tide + surge) ocean water level of ~1.2 m in the event. Using empirical equations for the maximum offshore Hrms and Tp and an estimated beach slope of 0.11, the R2% wave run-up is calculated to be 4.02 m [10], which is inclusive of the wave set-up component of 1.68 m [8], which fit well with the measurements on the open beach. However, it is obvious that wave set-up of this magnitude was not acting across the creek entrance, although some intermediate level of wave set-up may have been contributing to the tailwater level (in addition to the modeled storm surge) given the improved model water level in the estuary when using the measured tidal residual from the Gold Coast Broadwater. A case study of the trained Brunswick River entrance by Hanslow et al. (2001) [9] investigated the effects of wave height and riverine flooding on wave set-up, and identified a water level anomaly over 0.5 m associated with TC Roger (1993) not attributed to storm tide or riverine flow. They found that when compared with the wave set-up on a natural beach, wave set-up in a similar river entrance is typically smaller under similar wave heights at the same depth, possibly due to the river current causing local set-down as it spreads out of the entrance, counteracting the effect of the local wave-driven set-up. The behaviour of this process under extreme wave conditions is not clear, and some proportion of the wave set-up could still be acting as an increased tailwater level. They also raise the possibility that the anomaly could have been due to coastally trapped waves. In summary, we have not been able to identify the process leading to the peak water levels measured in the Currumbin Creek from numerical modelling of riverine flow and storm tide alone. 6. Implications of Findings The findings of this case study have implications for both emergency managers and the overarching storm tide forecasting project. It underlines the complexity of extreme water level behavior in tidal inlets, in particular its dynamic and interrelated response to the riverine flow, oceanic water levels, wave penetration and changing bed morphology. Forecasting inundation at high spatial resolution in such environments would require complex coupled models with detailed bathymetric, flow and oceanic water level inputs.

Some other findings of this study include:

• The difficulty of validating model output against debris line data and hence;

• The importance of monitoring of open ocean water level conditions for understanding storm tide;

• The knowledge gap in the behaviour of wave set-up across river entrances in extreme events affects inundation forecasting;

• The complex behaviour of water levels within individual estuaries precludes the use of design levels derived from measurements at such sites to be applied as open ocean tailwater levels.

7. Acknowledgements We acknowledge the kind contributions of data and advice from Saeed Shaeri (PhD Candidate, GCCM), GCCC, DHI, MSQ, BoM, DSITIA and DNRM. 8. References [1] Allen, M. and Callaghan, J. (1999) Extreme wave conditions for the south Queensland coastal region. In: 14th Australasian Coastal and Ocean Engineering Conference, 14-16 April 1999, Perth WA.

[2] BoM (2013) Special Climate State 44 – extreme rainfall and flooding in coastal Queensland and New South Wales. 5th February 2013.

[3] Burston, J.M., Gee, D., De Boer, J, (2013) Coastal impacts of ex-Tropical Cyclone Oswald 2013 on the Gold Coast, Queensland. Griffith Centre for Coastal Management Technical Report. May 2013.

[4] Burston, J.M. and Symonds, A.M. (2013) Coral Sea Hydrodynamanic Model Set-up and Calibration. Griffith Centre for Coastal Management Report. May 2013.

[5] Callaghan, J. (2012) Known Impacts of Tropical Cyclones, East Coast, 1958-2008. Archive for HardenUp Queensland. http://hardenup.org/

[6] Castelle, B., Bourget, J., Molnar, N., Strauss, D., Deschamps, S., and Tomlinson, R. (2007) Dynamics of a wave-dominated tidal inlet and influence on adjacent beaches, Currumbin Creek, Gold Coast, Australia, Coastal Engineering. 54(1): 77-90.

[7] GCCC (2012) Currumbin Creek Dredging. Gold Coast City Council Fact Sheet. http://www.goldcoast.qld.gov.au/ currumbin-creek-dredging-4520.html.

[8] Hanslow, D. & Nielsen, P. (1993) Shoreline set-up on natural beaches, Journal of Coastal Research SI 1-10.

[9] Hanslow, D., Nielson, P., and Hibbert, K. (2001) Wave Setup at River Entrances. Coastal Engineering Proceedings, 1(25). doi:10.9753/icce.v25.

[10] Nielsen, P. and Hanslow, D.J. (1991) Wave runup distributions on natural beaches. Journal of Coastal Research 7: 1139-1152.