Copy of R B14518 003 01 Final Report€¦ · STUDY METHODOLOGY...

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INTRODUCTION

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1 INTRODUCTION

1.1 Site Setting

The township of South West Rocks is located near the mouth of the Macleay River on the mid north coast of New South Wales. It is positioned about 450 km north of Sydney and about 40 km north east of Kempsey. The Saltwater Creek catchment, which contains much of the township, drains to the ocean via Saltwater Creek and Lagoon. A locality map is presented in Figure 1-1 and Figure 1-2.

Saltwater Creek is an intermittently open creek, located behind Front Beach (Trial Bay) to the east of the township. The creek is approximately 6km long. It originates south of Gothic Street, and runs parallel to Mitchell Street, defining the western boundary of the Golf Course. The creek then passes through of the Golf Course, and flows to Saltwater Lagoon via a dredged channel.

The Saltwater Lagoon is tidal when the creek entrance is open. The lagoon wet surface area varies with tide or rainfall, and ranges between 0.25 km2 to 0.4 km2 (outside flood events). The water depth in the lagoon is relatively shallow.

Downstream of the lagoon, Saltwater Creek flows parallel to Front Beach, behind vegetated foredunes. The downstream end of the creek is subject to regular geomorphologic changes. The geometry of the downstream sand berm is the result of interactions between fluvial, tidal and wave processes, and as such, changes in response to changes in these hydraulic processes.

The Saltwater Creek entrance naturally breaks out when water levels in the creek exceed the crest level at the entrance berm. The opening process has, however, been facilitated in the past by artificial intervention. This has been done in an attempt to alleviate flood risks upstream or to restore tidal flushing. From local knowledge, the entrance berm builds to a level of about 2.2 mAHD.

1.2 The Need for Flood Study at Saltwater Creek

The Saltwater Creek catchment is under pressure by the demand for future development. Yet the exact extent of flooding within the catchment is not well defined. It is important that all future developments in the catchment take consideration of flooding.

Another cause of concern for the community is the management of the downstream sand berm. At present, the sand berm is mostly left to its natural process. But a closed entrance has proven in the past to exacerbate upstream flooding. Therefore, there is considerable pressure by local residents to “control” the entrance to minimise upstream flood impacts.

INTRODUCTION


These issues reflect the necessity for Council to understand the flood processes of Saltwater Creek. In concert with this flood study, Council is currently preparing an Estuary Management Plan for Saltwater Creek and Lagoon, which will address the future management of the catchment, and will draw from the conclusions of this flood assessment.

1.3 General Floodplain Management Approach

Floodplain management in NSW generally follows the guidelines in the Floodplain Management Manual (NSW Government, 2001). It states that implementation of the flood policy requires a floodplain management plan which ensures:

The use of flood liable land is planned and managed in a manner compatible with the assessed frequency and severity of flooding;

Flood liable lands are managed having regard to social, economic and ecological costs and benefits, to individuals as well as to the community;

Floodplain management matters are dealt with having regard to community safety, health and welfare requirements;

Information on the nature of possible future flooding is available to the public;

All reasonable measures are taken to alleviate the hazard and damage potential resulting from development on floodplains;

There is no significant growth in hazard and damage potential resulting from new development on floodplains; and

Appropriate and effective flood warning systems exist, and emergency services are available for future flooding.

1.4 Area Covered by this Study

The flood study integrates the hydrologic modelling of the whole Saltwater Creek catchment, and the hydraulic modelling of Saltwater Creek from its upstream reach along the Golf Course drain down to Saltwater Lagoon and the ocean outlet. Emphasis was put on the low-lying areas upstream of Saltwater Lagoon, where the flooding issues are higher.

The hydrological model comprises the 8.7 km2 of the Saltwater Creek catchment. Saltwater Creek is fed by several small tributaries that drain the steep catchments of Smoky Cape to the east and south east, the urban areas to the west and low-lying rural and wetland areas to the south. Catchment urbanisation is limited, with most of the area being coastal complex, moist eucalyptus forest and disturbed forest woodland vegetation. The wetland area surrounding the lagoon is protected under State Environmental Planning Policy No 14 (SEPP 14).

1.5 Study Objectives

The primary objective of the Saltwater Creek Flood Study was to examine and define the flood behaviour of the creek in response to different flood and sand berm erosion scenarios. The Flood

INTRODUCTION


Study results will be integrated into the concurrent estuary management study to help guide future development within the Saltwater Creek catchment.

Specifically, this study was to develop a hydrological model and a combined one & two-dimensional hydraulic model of the study area and determine maximum design flood conditions for a range of flood events (i.e. 1%, 2% and 20% AEP floods and the Probable Maximum Flood), and to assess the influence of the downstream berm height on upstream flood levels.

In addition, the model provides support for future analyses when required, including:

Determining hydraulic categories and the flood hazard;

Determining adequacy of existing flood protections;

Assessing various flood mitigation options;

Reviewing the appropriateness of Council adopted flood standard;

Establishing the effects on flood behaviour by future urban development;

Testing the impacts of specific development proposals on flooding; and

Providing Council with a flood forecasting capability.

These specific modelling objectives are normally part of a Floodplain Management Study. In the case of Saltwater Creek, these tests may be integrated in the Estuary Management Study which will be carried out subsequent to the present Flood Study.

1.6 About This Report

This report documents the Flood Study’s objectives, results and conclusions. It is divided into a main report which presents the Study in a relatively non-technical manner, and several appendices containing additional data and technical details.

Section 1 introduces the study.

Section 2 provides an overview of the approach adopted to complete the study.

Section 3 details the development of the computer models.

Section 4 details the model’s sensitivity analysis.

Section 5 presents the design flood conditions (rainfall, ocean, sand berm).

Section 6 presents the design flood results.

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2 STUDY METHODOLOGY

The general approach and methodology employed to achieve the study objectives involved:

Compilation and review of available information;

Site inspections;

Identification of historical changes to topography;

Collection of historical flood information;

Collection of additional topographic survey data;

Setup of computer models;

Calibration and/or sensitivity testing of models;

Establish design flood conditions; and

Model design floods and map results.

The above tasks are described generally in the following Sections, while results of the application of this methodology are discussed in subsequent Chapters and Appendices, as appropriate. The implemented methodology leads to accurate floodplain mapping.

2.1 Compilation and Review of Available Information

Flood investigations carried out in the past have addressed various aspects of flooding within the Saltwater Creek area. Relevant previous studies include:

Glenn S. Mounser, Consulting Engineer, “Flood Study” Nov 1981;

Glenn S. Mounser, Consulting Engineer, “Further Investigation” March 1982;

Peter A. Jelliffe, Environmental Engineering and Planning Consultants, “Buffer Zone Assessment South West Rocks STP” January 1997;

Jelliffe Environmental Pty Ltd, “Water Quality Report” October 1997;

Jelliffe Environmental Pty Ltd, “Stormwater Management Plan and Groundwater Hydrology Assessment” December 2000;

ERM Mitchell McCotter, “Assessment of Environmental Constraints to Development” March 1996;

Webb. McKeown & Associates Pty Ltd, “Lower Macleay Floodplain Management Plan” August 1999; and

DPWS, Manly Hydraulic Laboratory, “Saltwater Creek Estuary Processes Study”, Report MHL1126, November 2002.

A summary of the review of these documents is provided in Appendix A. The review concentrated on the aspects related to flood behaviour.

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Only qualitative information could be extracted from the above documents on flood behaviour. No precise hydraulic modelling has ever been undertaken over the whole creek, and subsequent topographic changes have occurred since the first Flood Study.

In addition to the report, different topographic surveys were available from Council, some on which might have been commissioned as part as the above studies.

All relevant information has been incorporated into the study, and is described, where appropriate within other sections of this report.

2.2 Site Inspections

An initial site inspection was carried out to allow study personnel to become familiar with the area and to determine additional data requirements. Additional site inspections were then carried out, on an as-required basis, during the course of the study to investigate specific details and confirm computer modelling assumptions. Site inspections were required to determine structure sizes, current vegetation cover, general ground-truthing of topographic features, current geomorphology of Saltwater Creek, and liaison with community members.

2.3 Survey Data

The computer models developed for the Flood Study require the input of the project area topography. The overall topography was obtained by compiling seven different sets of topographic survey data, which includes bathymetric survey of the creek and lagoon, floodplain spot heights and topographic contours.

In addition to the creek and catchment survey data, the downstream sand berm was also surveyed on five different occasions.

2.4 Computer Models

2.4.1 Hydrologic Model

A hydrologic model, discussed in Section 3.3, was developed to simulate the rate of storm runoff from the catchment. The model predicts the amount of runoff from rainfall and the attenuation of the flood wave as it travels down the catchment. This process is dependent on:

Catchment slope, area and vegetation;

Variation in distribution, intensity and amount of rainfall; and

Antecedent conditions of the catchment.

The output from the hydrologic model is a series of flow hydrographs at selected locations such as at the boundaries of the hydraulic model. These hydrographs are used by the hydraulic model to simulate the passage of a flood through Saltwater Creek.

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2.4.2 Digital Elevation Model (DEM)

A digital elevation model, discussed in Section 3.4, was developed to provide a topographic base surface for the hydraulic model. The DEM interpolates linearly between the existing topographic survey points.

2.4.3 Hydraulic Model

The hydraulic model, discussed in Section 3.5, developed for this study includes:

A high resolution two-dimensional model of the Saltwater Creek lagoon and floodplain, and the downstream sand berm; and

A low resolution one-dimensional model representation of Saltwater Creek, between the lagoon and the sand berm.

2.4.4 Geomorphologic Model

The geomorphologic model, discussed in Section 3.6, developed for the study is integrated with the hydraulic model. The geomorphologic model recalculates the hydraulic model bed levels at each time step in order to represent the erosion and deposition of the sand berm at the downstream end of Saltwater Creek.

2.5 Calibration and Sensitivity Analysis of Models

There is insufficient historical hydrographic information for the Saltwater Creek catchment to support a model calibration process. The lack of rainfall, streamflow, and historical flood level data as well as sand berm topographic survey prior to the flood event, means that no historical flood event can be described accurately.

Given the lack of calibration data, sensitivity analysis of the different model input parameters was undertaken in order to assess the dependence of model results to technical assumptions. The sensitivity analysis provides a technical basis for providing confidence in the flood study results.

2.6 Establish Design Flood Conditions

The validated hydrologic and hydraulic models were modified as necessary to represent present day design flood conditions. The Saltwater Creek entrance sand berm was based on the average of five (5) historical berm geometries, as surveyed by Council.

Design floods are statistical-based events which have a particular probability of occurrence. For example, the 1% Annual Exceedance Probability (AEP) event, which was formerly known as the 1 in 100 year ARI flood, is the best estimate of a flood that has a 1% (i.e. 1 in 100) chance of occurring in any one year (on average). In the case of Saltwater Creek, no stream flow records allow an analysis of long-term historical records of floods in the creek or lagoon. Design floods were therefore based on design rainfall estimates according to Australian Rainfall and Runoff (IEAust 1987).

A series of sensitivity tests were carried out on the predicted modelling results. These tests were conducted to determine the relative importance of different hydrologic and hydraulic factors, such as

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flow resistance coefficients and boundary conditions. The tests assist in establishing the relative accuracy of the modelling results.

The design flood conditions are presented in Section 5.

2.7 Design Flood Mapping

Design flood mapping is undertaken using output from the hydraulic model. Maps are produced showing water level, water depth and velocity vectors for each of the design events. The maps present the peak value of each parameter. Provisional flood hazard categories and hydraulic categories are derived from the hydraulic model results and are also mapped. These maps are explained and presented in Section 6.2 and/or in the attached CD-Rom.

In addition, longitudinal profiles of the maximum water levels along Saltwater Creek, from the golf course to the entrance sand berm are presented in Section 6.2.

COMPUTER MODELS


3 COMPUTER MODELS

3.1 Introduction

Computer models are the most accurate, cost-effective and efficient tools to model the flood behaviour of a catchment. For this study, four types of models were used:

A hydrologic model, covering all the sub-catchments of the project area;

A digital elevation model of the whole catchment;

A two-dimensional (2D) hydraulic model extending from downstream of the South West Rocks golf course to downstream of Saltwater Creek lagoon and another area located at the entrance sand berm of the creek. The two areas are joined together with one-dimensional (1D) storage-based elements; and

A geomorphologic model of the entrance sand berm.

The hydrologic model simulates the catchment rainfall-runoff processes, producing the catchment inflows that are used in the hydraulic model.

The digital elevation model interpolates the creek, floodplain and catchment bed and ground levels between the existing topographic survey points.

The hydraulic model simulates the flow behaviour of the waterway and associated floodplains, producing flood levels, discharges and velocities.

The geomorphologic model updates the hydraulic model bed geometry depending on sand erosion and deposition.

Information on the topography and characteristics of the catchment, creek, sand berm and floodplains were built into the models. The models produce as output, flood levels, flows (discharges), velocities (current speed), hydraulic categories and provisional hazards.

Development of a computer model for a flood study follows a relatively standard procedure as shown below:

1 Discretisation of the catchment, creek, floodplain, etc (see Section 3.1.1);

2 Incorporation of physical characteristics (catchment areas, creek cross-sections, etc);

3 Setting up of hydrographic databases (rainfall, ocean levels, flood levels) for historic events; and

4 Calibration and/or sensitivity analysis of parameters to measure the dependence of the results to the model assumptions.

Once the model development is complete it may then be used for:

establishing design flood conditions;

providing flood information for development control plans; and

modelling “what-if” management options to assess hydraulic impacts and benefit-cost ratios.

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Only the first dot point above has been carried out as part of this Flood Study. The other two dot points would generally be addressed as part of a subsequent Floodplain Risk Management Study.

3.1.1 Models Discretisation

Models discretisation is necessary to simplify the real-world into one that can be represented by discrete elements. The computer then solves equations at every discrete element to simulate the hydrologic, hydraulic and geomorphologic processes.

Within the model, the smaller the elements become, the closer the model approaches the real-world situation. However, as the number of elements increases, the computational resources required to run the model becomes more demanding, while the model also becomes more difficult to set up and manipulate. Also, there is a point where increasing the number of elements in a model may not provide any significant improvement in model predictions and accuracy. Therefore a suitable balance needs to be found between the number of elements used to represent the study area and the practicalities of using the model for future management purposes.

In constructing the model, the number, size and location of elements needs to take into account:

location of available data (eg. Creek/lagoon section surveys);

location of recorded data (eg. gauging sites);

location of controlling features (eg. dams, embankments, bridges, weirs, flow constrictions);

desired accuracy to meet the study’s objectives;

limitations of the computer software (ie. the number of elements the software can handle, and more importantly, to keep within the constraints of the mathematical solution); and

limitations of the computer hardware.

3.2 Data Sources

A variety of data was collated and used to develop the different model databases or used to develop model parameters. The main sources of data were:

Topographic maps (1:25,000);

Historical aerial photographs;

Five historical topographic surveys of the entrance sand berm (26/09/2000, 24/10/2000, 04/10/2001, 30/01/2002 and 17/01/2003);

Bathymetric survey of Saltwater Creek and Lagoon;

Existing ground levels of Lots 19 and 52 (data provided by the property owner at the time of the study);

Three (3) other sets of topographic survey of Saltwater Creek floodplain; and

Historic flood anecdotes collected through community consultation.

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3.3 Hydrologic (Catchment Runoff) Model

The hydrologic model simulates the rate at which rainfall runs off the catchment. The amount of rainfall runoff and the attenuation of the flood wave as it travels down the catchment is dependent on:

the catchment slope, area, vegetation and other characteristics;

variations in the distribution, intensity and amount of rainfall; and

the antecedent conditions (dryness/wetness) of the catchment.

These factors are represented in the model by:

Sub-dividing (discretising) the catchment into a network of subcatchments inter-connected by channel reaches representing the creeks. The subcatchments are delineated, where practical, so that they each have a general uniformity in their slope, landuse, vegetation density, etc; and

The amount and intensity of rainfall is varied across the catchment based on available information.

The antecedent conditions are modelled by varying the amount of rainfall which is “lost” into the ground and “absorbed” by storages. For very dry antecedent conditions, there is typically a higher initial rainfall loss.

3.3.1 Model Setup

The RAFTS-XP software was used to develop the hydrologic model. Details of the software are presented in Appendix B.

The Saltwater Creek catchment, with a drainage area of about 8.7 km2 in Section 1.4, has been discretised into 15 subcatchments, which feed into Saltwater Creek and the associated floodplains at 7 different locations. The average sub-catchment surface area is 59.2 ha, with a standard deviation of 71 ha. The biggest sub-catchment has an area of 258 ha. The surface of the lagoon itself accounts for approximately 20 ha of the total catchment area. Smaller subcatchments are more likely to receive uniform rainfall distributions.

The sub-catchment limits have been defined so that the descriptive parameters are generally uniform within the boundaries (which are slope, landuse, permeability, roughness). The topographic maps allowed for calculation of the subcatchments’ slopes and to determine landuse characteristics. The slopes vary from 0.3% (around the lagoon) to 21% (average 7.7%, standard deviation 6.4%). The vegetation cover is forested for most of the catchment (south, east and creek channel). The area west of the lagoon is currently cleared. Only the west end of the catchment is classified as urbanised (South West Rocks).

The rainfall intensity for each of the Saltwater Creek subcatchments was investigated. Due to the small size of the Saltwater Creek catchment, the Intensity Duration Frequency (IFD) coefficient remains the same throughout the sub-catchments. The spatial rainfall variability for design events is insignificant at the scale of the model.

The schematic of the model network is shown in Figure 3-1.

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3.4 Digital Elevation Model (DEM)

A Digital Elevation Model (DEM) is a three-dimensional (3D) representation of the ground surface. A DEM is used to define the ground surface levels of the hydraulic model. Given that ground levels are required for over 370,000 individual nodes within the hydraulic model, a DEM represents the most effective way for these levels to be determined automatically.

A DEM of the Saltwater Creek model was created from five different survey data sources:

DEM Data 1 (Saltwater Creek bathymetry) surveyed by: Graeme Sayer of East Coast GPS P/L:Plans, August 2002;

DEM Data 2 (private property, Lots 19 & 52) surveyed by: Dallas Laundry Surveyor, 1999;

DEM Data 3 (Upstream of Saltwater Lagoon) surveyed by R Kemsley and A Castle, June 2004;

DEM Data 4 (_684A2 File) surveyed by Bob Moore (Kempsey Shire Council);

DEM Data 5 (_02076T File) surveyed by: Graeme Sayer of East Coast GPS P/L:Plans, August 2002;

Land & Information Centre (LIC) 2m contour data, as shown on topographic maps, to define remainder of the Saltwater Lagoon floodplain; and

Land & Information Centre (LIC) 10m contour data, as shown on topographic maps, to define higher ground in the upstream sub-catchments.

Of the above data sources, the DEM Data 1 to 5 are expected to be of the usual ground survey accuracy (+/- 0.1m), which is considered reasonable for the project models.

The LIC data is the least accurate data source, with a usual tolerance of +/- 1-2 metres. Since the floodplain is relatively flat, the accuracy of the LIC data is increased. Quality checks on the 2m-contour against available ground spot levels (Dallas Laundry Surveyor, 1999 showed that a vertical error of +/- 0.20m could be expected by the LIC data. The LIC data source was only used to define the overbank part of the creek channel (modelled in 1D) and the lagoon eastern floodplain. Given the flat water level gradient, the LIC data inaccuracy would have negligible impact on the predicated flood levels.

The DEM was constructed as a Triangular Irregular Network (TIN), which is simply a mass of interconnected triangles. For each triangle, the ground level is defined at each of the three vertices, thereby defining a plane surface over the area of the triangle. Where the datasets indicate that there are significant variations in the ground surface slope and elevation, the density of triangles was increased.

Figure 3-2 shows the extent of the different sources of survey data used for the creation of the DEM.

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3.5 Hydraulic Model

3.5.1 Model Setup

The hydraulic model simulates the dynamic flooding behaviour in Saltwater Creek, including the interactions between the creek, the lagoon, the entrance sand berm and the floodplains.

The modelling software, TUFLOW, was used to develop a 2D/1D hydraulic model of the study area. The model is a mixture of one-dimensional (1D) and two-dimensional (2D) domains with the 2D domains covering the key areas of existing and future management interest. 2D domains produce a significantly higher order of resolution in terms of hydraulic computations. 1D domains are suited to modelling areas away from the areas of interest, where flow is essentially unidirectional, or where the 2D resolution does not adequately depict the shape of a key flowpath (eg. when the width of a creek is only covered by one or two cells).

2D domains were established in the following areas:

Saltwater Lagoon and upstream floodplains; and

The downstream end of Saltwater Creek, at the location of the entrance sand berm.

The hydraulic model network and its relevant branches is provided in Figure 3-3.

As TUFLOW is a finite difference model , the Saltwater Creek Flood Model has been constructed using elements with a regular grid of size 10m x 10m in the upstream 2D domain (Lagoon and floodplains) and 4m x 4m in the entrance sand berm area. This means that hydraulic parameters are calculated separately for every 10m square of the 2.0km2 upstream study area represented in 2D, and for every 4m square of the 0.4km2 downstream study area represented in 2D. Over 47,000 individual elements make up the flood model, each with individual levels, roughness, boundary conditions, flow constrictions and flow structure details where appropriate.

The two-dimensional TUFLOW model domains are also dynamically linked to a one-dimensional model, representing the channelised section of Saltwater Creek downstream of the lagoon that separates the specific areas being modelled in 2D. The hydraulic behaviour of the channelised sections of the lake is unidirectional and can be accurately represented by 1D elements. The extent of the 2D model domains and the locations of the one-dimensional elements are provided in Figure 3-3.

The computational timestep used for the TUFLOW model is two (2) seconds. This means that the hydraulics within each of the 47,000 model elements is recalculated for each two second time step throughout the flood event. For a 24 hour flood simulation, this equates to nearly 2 billion calculations.

Figure 3-4 presents the different development stages of the model.

Information on the TUFLOW software is presented further in Appendix B.

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k:\B14881.pev\docs\d003.vsd

Site Inspections

Historic FloodInformation Survey

Review StudiesCollate Data

Additional TopographicSurveys

DataCollection

&Collation

GISDatabase

Historic Flood LevelsDigital Terrain Model

HydraulicModel

Catchment Runoff(Hydrologic) Model

River, Floodplain &Hydraulic Structure

Topography

Flood Control StructuresRoughness Coefficients

HistoricFlood Levels

RainfallCatchment DetailsMeasured Flows

HistoricFlood Patterns

AR&R DesignStorms

Flood Mapping

Design Floods

HydraulicModel

Calibration

Flood FrequencyAnalysis

HydrologicModel

DesignFloodFlows

CalibratedHydraulic

Model

Figure 3-4 Flood Study Model Development Process

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3.5.2 Model Input

Inputs to a hydraulic model include:

Topography of the creek and the floodplain based on the DEM and creek cross-sections. The DEM is discussed in Section 3.4;

Hydraulic roughness of the channel bed and the floodplain land. The final set of hydraulic roughness values is determined during the sensitivity analysis;

Hydraulic structures along the creek (bridges);

Inflows: the rainfall runoff calculated by the hydrologic model at nodes adjacent to the creek (see Section 3.3); and

Boundary water levels: depending on the sand berm geometry, the ocean levels can influence water levels in the lake. The downstream ocean condition takes into account all dominant ocean level parameters (astronomical tide, storm surge and wave setup).

A range of sensitivity tests and checks on the hydraulic model were carried out during the course of the model calibration. The checks carried out confirm the correct interpretation of the input data and sensitivity tests were carried out to develop an understanding for the most influential hydraulic parameters. Examples of tests and checks carried out are:

Checks for any irregularities in the conveyance values;

Influence of the downstream sand berm; and

Mass balance checks.

3.5.3 Model Outputs

Model outputs are flood levels, flows, and velocities describing the flood behaviour over time for a given flood event. Based on these outputs, flood categories and hazards describing the risks associated with flood flows can also be determined.

Individual model outputs are provided for every 2D element within the model. This means that for the Saltwater Creek model, results at over 47,000 different locations (every grid cell) are provided at every timestep. Model outputs are also available at 1D elements. Given this vast amount of output data, a Geographic Information System (GIS) was used to assist in presentation of the spatially-dependent results.

3.6 Geomorphologic Model

3.6.1 Introduction

The Saltwater Creek outlet is located at the western end of Trial Bay (Front) Beach. Marine sand is transported along the beach in a net westerly direction, with deposition inside the creek entrance.

The downstream reach of Saltwater Creek is influenced by two primary hydrodynamic processes. These are the regular tidal movement of seawater and the episodic freshwater runoff from the catchment draining to the sea. The hydrodynamics constantly rework marine sands around the

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entrance, particularly during times of flood. The ability to model morphological changes to the entrance during a flood is therefore critical to the flood study.

A geomorphologic module was added to the hydrodynamic software, in order to integrate sediment transport into the flood study process. The geomorphologic module is based on the Van Rijn (1990) theory.

The Van Rijn formulation of sand transport is generally accepted as being the latest and most accurate method for estimating sand transport. It does, however, contain approximations related to the little understood complexity of sand transport. Although these approximations are unavoidable, the Van Rijn method is still appropriate to combine with the 2D (depth-averaged) TUFLOW hydraulic routines, as it determines the bed shear stress from the near-bed shear velocity, calculated using conventional theory.

3.6.2 Sand Transport Formulation

Quantification of sand transport rates is achieved by the use of two unifying and fundamental concepts:

(i) The combined action of currents and wave orbital motion mobilises the bottom sands and sets them into motion, and

(ii) The bottom sediment, once mobilised, is moved in the direction of the prevailing net current. The net current can be the result of factors such as river flow, tides, wind, wave radiation stresses or asymmetry in the oscillatory wave motion, or a combination of these.

At present, when undertaking flood induced entrance scour in TUFLOW, the wave orbital motions are not being simulated. However, the dominant scour process in a flood event is related primarily to the current-induced breakout that occurs when flood levels overtop a sand berm. The wave orbital motions are therefore no being simulated. A separate wave model would need to be established and integrated with the hydrodynamic model to achieve this. The model allows for subsequent deposition of sediment in the nearshore zone downstream of the sand berm.

3.6.3 Wave/Current Relationships

Comprehensive formulae that incorporate the independent influences of currents, waves and sand properties have been developed and verified by a range of researchers. Methods such as Van Rijn (1990) are the most comprehensive as they incorporate the fundamental processes of:

Bed shear stress due to the action of the currents at the bed, providing for both the shear velocity at the near-bed boundary layer for both waves and current, together with the influence of bed form (ripples and dunes) on the shear stress;

A critical bed shear stress below which no sediment motion occurs;

The near-bed sediment concentration (at the designated reference height), transported at the near-bed shear velocity as bed load; and

The vertical distribution of both the current speed and the suspended sediment concentration, combining to provide the vertical distribution of the suspended load sediment transport in the water column.

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The Van Rijn formulation is presented in Appendix B.

3.6.4 Geomorphologic Modelling Extent

The combined TUFLOW/Van Rijn equations can be implemented over the entire 2D model domains. However, the long modelling time, as well as the increased computing instabilities, imposes a practical limit for the sediment transport modelling area. While some sediment transport can be expected throughout Saltwater Creek during flood events, preliminary calculations show that most transport occurs at the downstream end in the immediate vicinity of the entrance sand berm.

The geomorphologic model is laid over a section of the TUFLOW hydrodynamic mesh. The area selected to incorporate sediment transport calculation and the variable bed feature is centred on the entrance sand berm. It covers an area of 0.4 km2 for the design flood events. The geomorphologic modelling area ensures that it extends laterally into permanently dry land, downstream into deep water (20m depth) to allow for deposition and upstream, where the flow acceleration due to the entrance channel constriction is not yet developing, and where the upstream bed erosion does not impact on adjacent flood levels.

3.6.5 Geomorphologic Model Imput

Inputs to the geomorphologic model include:

D50: 0.25 mm;

D90: 0.5 m;

Fall Velocity: 0.015 m/s;

Sand Grain Density: 2650 kg/m3; and

Water Density: 1035 kg/m3.

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