Processes controlling river-mouth lagoon dynamics on high-energy mixed sand and gravel coasts

Journal Pre-proof Processes controlling river-mouth lagoon dynamics on highenergy mixed sand and gravel coasts

Richard J. Measures, Deirdre E. Hart, Thomas A. Cochrane, D. Murray Hicks PII:

S0025-3227(19)30120-3

DOI:

https://doi.org/10.1016/j.margeo.2019.106082

Reference:

MARGO 106082

To appear in:

Marine Geology

Received date:

2 April 2019

Revised date:

5 November 2019

Accepted date:

5 November 2019

Please cite this article as: R.J. Measures, D.E. Hart, T.A. Cochrane, et al., Processes controlling river-mouth lagoon dynamics on high-energy mixed sand and gravel coasts, Marine Geology (2018), https://doi.org/10.1016/j.margeo.2019.106082

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© 2018 Published by Elsevier.

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Processes controlling river-mouth lagoon dynamics on high-energy mixed sand and gravel coasts Richard J Measures1,2, Deirdre E Hart3, Thomas A Cochrane2, D Murray Hicks1 1. Sediment Processes Group, National Institute of Water and Atmospheric Research (NIWA) 2. Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand 3. Department of Geography, University of Canterbury, Christchurch, New Zealand Corresponding author: Richard Measures [email protected] NIWA, 10 Kyle Street, Riccarton, Christchurch, New Zealand

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Abstract

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The river mouth lagoons of New Zealand’s gravel-bed braided rivers are highly dynamic, responding to changes in waves and river flows at timescales of hours and days to years. Unlike other freshwater-dominated coastal wetlands and lagoons, they exist along open coasts that are typically retreating with cliffed hinterlands. These lagoons are common on New Zealand’s high energy, coarse sediment coastlines and are known locally as ‘hapua’. This paper employs field observations from the Hurunui hapua, to investigate the main processes controlling the dynamics of hapua lagoons. Based on two years of time-lapse imagery, plus concurrent wave, river flow and sea level data an improved conceptual model of lagoon dynamics was developed. The model describes how onshore bar migration of river sourced gravel and sand drives initial constriction/offsetting of the river mouth following floods. It also explains how gravel deposition in the lagoon reduces the threshold flow required to cause primary breach of the barrier opposite the point the river enters the lagoon. Three processes are identified which affect lagoon width: wave overwashing of the gravel barrier narrows the lagoon; migration of the highly dynamic lagoon outlet channel ‘resets’ the barrier position to seaward; and river floods occurring while the outlet channel is offset from the river flush sediment from the lagoon system, eroding its bed and banks. Both processes affecting lagoon widening rely on lagoon outlet channel migration, indicating that outlet channel dynamics are an important control on lagoon size.

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1 Introduction Unlike most other types of coastal lagoon (Martin and Dominguez, 1994), non-estuarine river-mouth lagoons can form on eroding coastlines with no pre-existing embayment or low-lying hinterland, including along coasts with cliff backshores. The physical processes controlling the morphology of these highly-dynamic lagoons are known to be driven by the complex interaction of wave and river driven sediment transport and hydrological influences (Hart, 2009a; Kirk, 1991). However, the way in which these processes combine to create and maintain space for the lagoons is not well described.

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In New Zealand, the indigenous Māori term ‘hāpua’ is used by the science community to refer to a type of non-estuarine, barrier beach enclosed, wave-dominated, river-mouth lagoon (Hume et al., 2016; Kirk, 1991). This term literally means ‘pool of water, lagoon or pond’, but has been applied scientifically to refer to a specific type of river-mouth lagoon. Hāpua are common in the South Island of New Zealand, particularly along the Canterbury coastline (Kirk and Lauder, 2000). Of New Zealand’s 29 identified hāpua, 21 are in the South Island including 11 in Canterbury (Hume et al., 2016).

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Hāpua form where high stream-power, braided or semi-braided gravel-bed rivers emerge onto exposed, high wave energy, micro- or meso-tidal coasts. A mixed sand and gravel barrier encloses a shore-parallel lagoon, connected to the ocean via a highly-dynamic outlet channel. Hāpua outlet channels can rapidly change position, with the lagoon end (upstream) and sea end (downstream) of the outlet channel both migrating semi-independently, causing the outlet channel to vary in length by hundreds of metres (e.g. Ashburton hāpua, Paterson et al., 2001). In some cases, hāpua outlet channels close off completely (e.g. Opihi hāpua, McSweeney et al., 2016). Hāpua are non-estuarine, with no tidal ingress. However, tidally varying sea levels cause a freshwater backwater effect whereby lagoon water levels fluctuate in the order of 1 m vertically. Although the term hāpua is only applied in New Zealand, similar related types of river-mouth lagoon exist in various high wave energy micro to meso tidal locations around the world. Documented examples of related dynamic shore-parallel river-mouth lagoons include those located on the east coast of South Africa (Cooper, 1994; Green et al., 2013; Smith et al., 2014), the east coast of Japan (Tanaka et al., 2014), and the Mediterranean coast of Israel (Lichter and Klein, 2011), although unlike hāpua most of these systems are sandy and, at least in some cases, have tidal flow reversal in their mouths. McSweeney et al. (2017) map the global distribution of intermittently closed and open coastal lakes and lagoons, including hāpua and related river mouths. In New Zealand, hāpua are associated with high recreational, ecological and cultural values (Eder et al., 2011), with their morphodynamics having a controlling influence on several of these values. For example, fish passage is important for ecology and recreational fishing, and is controlled via the outlet channel state (e.g. Waipara river, Jowett et al., 2005). Closure of the outlet channel prevents fish migration, and an elongated outlet channel restricts inward fish migration by elevating lagoon water level and sustaining high water velocities over an extended length for the full tidal cycle. Long or constricted outlet channels can also cause chronic flooding of land and communities along lagoon backshores due to raised lagoon water levels. The most significant contemporary changes affecting hāpua are likely those associated with altered river flow and sediment regimes brought on by dam impoundment and surface and ground water extractions, as well as artificial barrier breaches motivated by backshore flooding and erosion management (Hart, 2009b; Kirk and Lauder, 2000). Changes in lagoon size and outlet channel behaviour, directly tied to changes in flow regime, have been observed in several hāpua (Kirk and Shulmeister, 1994; McSweeney et al., 2016). Climate change is likely to have additionally significant

Journal Pre-proof impacts on hāpua in the future, via accelerating sea level rise and changes in wave climate and river flow regime. To date no quantitative or numerical models have been developed to predict the impact of these changes on hāpua systems. A prerequisite for such modelling is a sound understanding of the key physical processes that should be incorporated into a successful model.

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Several conceptual models have been published which attempt to explain different physical processes affecting hāpua. Early models identified a cycle of lagoon behaviour: from the outlet channel being opposite the river, through outlet channel migration until the outlet channel was offset from the river and shore parallel, to breaching of the barrier opposite the river (Kelk, 1974; Todd, 1985, 1983). Kirk (1991) linked river flow thresholds to transitions between closed, open, and breached lagoon states in order to help inform decisions about water extraction and river flow regime management. Hart (2009a) identified five different hāpua states: primary breach, narrowed and migrated outlet, extended migrated outlet, secondary breach and closure. This advanced the Kirk model by considering the importance of waves as a key driver of changes in hāpua state as well as river flow. Outlet channel dynamics have been examined in greater detail by Paterson et al. (2001) who developed a conceptual model of hāpua outlet channel migration from observation of the Ashburton hāpua.

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The mixed sand and gravel barriers enclosing hāpua are similar to gravel barriers in other systems, albeit that hāpua barriers have wider range of permeability, such that it is likely that general models of gravel barrier behaviour are also somewhat applicable to hāpua barriers. Models of barrier profile response to waves are particularly relevant. Here the terms overtopping and overwashing are applied in-line with descriptions by Orford and Carter (1982). Overtopping is an accretionary process which occurs when swash just reaches the barrier crest and then infiltrates, depositing sediment on the crest. Overwashing occurs when swash flows over the barrier crest and into the lagoon, eroding the crest and creating washover lobes on the barrier backshore. Runup height relative to barrier crest elevation has been shown to provide a good predictor of the likelihood of overtopping or overwashing (Bergillos et al., 2017; Matias et al., 2012; Sallenger, 2000).

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Existing models do not fully explain processes that cause short term variations in lagoon size or those that maintain its long-term average size, particularly on retreating coasts. In one of the few studies to consider this, Todd (1992) identified that the shore-parallel deflection of river flows must play a key role in eroding coastal cliffs and shorelines behind hāpua because, despite the fact they are more sheltered from the sea, lagoon backshores experience the same rate of long term erosion as adjacent open coasts. Rates of barrier beach and backshore erosion have been little studied, with aerial photographs and beach profiles providing the best evidence to date (e.g. McHaffie, 2010). The actual processes controlling either of these erosion processes have not been studied at active timescales though, so remain relatively speculative. This paper aims first to quantitatively assess the key process mechanisms affecting hāpua at event timescales, including those controlling lagoon width and outlet channel dynamics, by studying these in detail at the hāpua of the Hurunui River on the east coast of South Island, New Zealand. Timelapse imagery with lagoon level, river flow, wave and tidal records were analysed to produce concurrent time-series of measurable hāpua attributes (lagoon width, water level, and outlet channel position and length) and relevant driver variables (river flow, sea level, wave runup height and longshore transport rate). A second aim is to consolidate the key process mechanisms into an improved conceptual model of hāpua behaviour, which will help inform hapua management and the future development of physically-based models of hāpua responses to changing drivers.

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2 Regional setting and study site The east coast of New Zealand’s South Island is very exposed, with almost unlimited fetch to the south and east across the Southern and Pacific Oceans. The offshore wave climate is dominated by swell from the southwest (Gorman et al., 2003), which drives predominantly northward littoral transport (see wave rose in Figure 1). The main rivers discharging to this coast are high-energy braided rivers with sources in the Southern Alps mountain ranges that form the backbone of the South Island, while smaller braided and meandering rivers drain the foothills and plains in between the large rivers’ fans. In all but the peninsula and headland sheltered areas, this coastline consists of mixed sand and gravel beaches (as defined by Jennings and Shulmeister, 2002). Figure 1 Hurunui hāpua location, time-lapse cameras, and wave climate. Wave rose shows

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the distribution of significant wave height (Hs) and direction at the 10 m depth contour outside

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the hāpua. Bar directions correspond to the directions the waves are approaching from, bar

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colours indicate Hs, and bar lengths indicate wave condition frequency. The black line on the wave rose indicates the shore normal direction. Background aerial imagery taken 3 April 2004,

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copyright Environment Canterbury Regional Council, released under Creative Commons

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Attribution 4.0 International.

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The Hurunui lagoon is a mid-size hāpua, stretching approximately 1,400 m longshore and up to 150 m cross-shore. The lagoon backshore is constrained by a soft limestone cliff (Amuri Limestone of Eocene age), the face of which varies in steepness between vertical and around 45°. In this regard, the hāpua is unusual compared to other similar lagoons on the South Island east coast, which typically have backshores comprising weakly consolidated sand and gravel cliffs. The Hurunui lagoon is separated from the South Pacific Ocean by a mixed sand and gravel barrier beach confined between two limestone headlands. The ocean tide is semi-diurnal with range varying from 1.0 to 2.0 m (mean range of 1.44 m), denoting a micro-tidal environment. The Hurunui River has a mean flow of 67 m3·s-1 and mean annual flood of 750 m3·s-1 (Environment Canterbury, 2019a). Draining a catchment of 2,670 km2, it flows eastward from its headwaters at the main divide of the Southern Alps. Approximately 12% of the catchment drains through Lake Sumner, a 13.9 km2 natural lake located in the Southern Alps, which has a smoothing effect on the hydrograph. The Hurunui is a moderately steep (mean slope of 0.3% across its coastal reaches), braided, gravel-bed river (surface-layer grain size median = 24 mm, mode = 38 mm). Estimates of bedload transport in the Hurunui range from 25 to 90 thousand m3·yr-1 (Measures and Hicks, 2011). The Hurunui River is in a seismically active part of New Zealand. The magnitude 7.8 ‘Kaikoura Earthquake’ took place on 14 November 2016, part way through this study’s observation period, causing vertical deformations of greater than 1 m within 40 km of the river mouth. However, the earthquake only had a small effect on ground elevations at the hāpua, lowering ground levels by 0.05 to 0.10 m (Land Information New Zealand, 2016). Similarly, the earthquake did not cause major landsliding within the Hurunui catchment, although over 10,000 landslides have been linked to this event (Massey et al., 2018). The reduction in ground level at the Hurunui Mouth is equal to approximately 5% of the normal tidal range, and whilst this change may well result in a measurable

Journal Pre-proof response over longer timescales, the earthquake was not the focus of this study and no difference in behaviour was observed over the two-year monitoring period. As such the Kaikoura earthquake is not considered to have been a significant process influence for this study.

3 Methods 3.1 Data collection

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Two time-lapse cameras, facing in opposite directions along the shoreline, were installed 34 m above sea level on the cliff backshore of the Hurunui hāpua (Figure 1). The cameras recorded 5 mega pixel colour images every 15 minutes during daylight hours. The location and height of the cameras were surveyed using RTK GPS to enable projection of the imagery. A water level recorder was installed in the centre of the lagoon below the cameras. This study analyses data from when the cameras were installed in July 2015 until the end of September 2017. No imagery was recorded for the period 18 September to 6 October 2015 due to camera failure.

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River flow data was sourced from the Hurunui at State Highway 1 (SH1) gauging station, 18.4 km upstream of the hāpua (Environment Canterbury, 2019a). As the contributing catchment to the reach downstream of the SH1 gauging station is relatively small (155 km2 or 6% total catchment area), the flow at the hāpua was assumed to be the same as at the SH1 station, but slightly delayed to account for travel time. This assumption of similar flows was confirmed by spot gauging on 30 March 2016, during a period of stable flow, which showed a 5% difference in flow from SH1 to the hāpua, which is within the uncertainty of differencing the two flow gaugings. Although varying slightly with discharge, the time delay between SH1 and the coast was assumed constant at 2.6 hours (estimation based on the observed travel time to the SH1 gauge from its nearest upstream gauge, 8-hours and 56.5 km away).

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Sea level (η) was hindcast for the hāpua by summing the astronomic tides, inverse barometer effect, and residual tidal anomaly. Astronomic tides at the Hurunui hāpua were calculated from modelled tidal constituents (Walters et al., 2001). The inverse barometer effect on sea level was calculated from air pressure recorded at the Cheviot electronic weather station, 10 km northwest of the Hurunui hāpua (NIWA, 2019a). The residual anomaly was taken from the Sumner Head sea level recorder, 85 km south of the Hurunui hāpua (NIWA, 2019b). Wave data at 10 m depth offshore of the Hurunui hāpua were hindcast using the SWAN third generation wave model (Booij et al., 1999). The model offshore boundary conditions were based on full wave spectra data from the Steep Head wave buoy, located in deep water 100 km south of the study site (Environment Canterbury, 2019b). The model also included the effect of local winds. For a detailed description of the hindcast modelling, see Hicks et al. (2018). The distribution of hindcast wave directions and significant wave heights at the Hurunui hāpua is shown in Figure 1. Significant wave height averages 1.25 m and has a mean annual maximum of 3.45 m. Waves approach from both sides of shore normal, but southerly-sourced waves prevail. The topography and bathymetry of the hāpua lagoon and barrier were surveyed on 26 August 2015 and 7 December 2016. A single beam echosounder and RTK-GPS, mounted on a remote-control boat, were used to survey the wetted areas. For the dry areas, the first survey used structure-frommotion analysis of drone imagery, similar to Guisado-Pintado et al. (2019), while the second survey used a drone mounted mobile laser scanner. In addition, three partial surveys of the lagoon water’s edge position and outlet channel position were undertaken using RTK-GPS (6 October 2015, 24 May 2017 and 26 July 2017). All water level and survey data are presented in Lyttelton Vertical Datum, LVD (a mean sea level datum).

Journal Pre-proof 3.2 Image analysis To handle the large amount of high resolution time-lapse imagery, image processing was automated as much as possible using Matlab and the open source code is available an online repository (Measures, 2019). In summary, the process involved: 1. screening to remove low quality images (15% of daylight images were removed due to weather interference or low light levels); 2. assigning concurrent lagoon water level, sea level, river flow, and wave conditions to each pair of concurrent images;

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3. identifying the lagoon water’s edge using Canny edge detection (Canny, 1986) followed by

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“dilation” and “watershed erosion” to close gaps in the identified edges (e.g. Sime and

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Ferguson, 2003);

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4. manually digitising the outlet channel centreline (only 1 image per day digitised); 5. measuring the position of fixed objects in the field of view to adjust for errors in camera

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mounted on);

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orientation (required due to significant warping of the wooden utility pole the cameras were

6. projecting the lagoon water’s edge and outlet channel width into real world coordinates by

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correcting for lens distortion and projecting each image onto a horizontal plane at the elevation of the lagoon water surface (taken from the water level sensor); and 7. measuring the location of the identified outlet channel and the width of the water’s surface at discreet measurement transects located every 100 m along the length of the lagoon.

The accuracy of the image analysis process was validated by comparing against surveyed lagoon water’s edge from the two complete surveys of the lagoon and four partial surveys of the barrier backshore position (e.g. Figure 2). In addition to the quantitative image analysis, animations were created showing the raw time-lapse imagery with accompanying river flow, wave and lagoon level (Videos 1 to 10, supplementary material). Combining the data in this way provided an intuitive way to see a complete picture of the lagoon response to different drivers. Figure 2 Rectified time-lapse imagery for 6 October 2015 compared against surveyed water’s edge position for the same day. Colour imagery is from time-lapse cameras while the greyedout background image provides context of the area outside the time-lapse camera field of

Journal Pre-proof view. The measurement transects used for the image analysis are labelled by their alongshore offset from the main river channel centreline.

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Whilst the time-lapse imagery used in this study was superior to that used in previous hāpua studies (Paterson et al., 2001) in terms of resolution, temporal frequency and field of view, a number of important limitations remain. As the imagery relies on natural light, no suitable images were collected overnight. It is also notable that the accuracy of the identified lagoon edge position degrades with distance from the camera, due to reduced resolution and increased sensitivity to any inaccuracies in lens distortion correction and camera orientation. A stretch of approximately 140 m of the lagoon and barrier situated directly below the camera site was in a blind spot outside the field of view of both instruments (see Figure 2). Lagoon width data from alongshore positions between 400 and 700 m, and between 900 and 1100 m, was found to be consistently reliable, while data outside of these areas was less reliable.

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3.3 Wave metrics

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3.3.1 Runup

(1)

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𝑅ℎ𝑖𝑔ℎ = 𝜂 + 𝑅2%

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A time-series of the highest elevation of wave runup (Rhigh) was calculated as an indicator of the likely effect of storm waves on the barrier crest. Large-scale flume experiments have identified that overtopping is associated with Rhigh values that exceed barrier crest height, with overwashing becoming more common than overtopping when Rhigh exceeds elevations 0.5 m above barrier crest height (Matias et al., 2012). The highest elevation of wave runup is estimated as the sum of the stillwater level of the sea, η, including tide and surge, and the 2% exceedance runup above the still water level, R2%, that is:

As there are few runup formulations available for mixed sand and gravel beaches, two formulas were tested: the widely used formula of Stockdon et al. (2006), developed using field data from sandy beaches and shown to have some relevance to gravel beaches in wave-tank experiments (Matias et al., 2012), and a recent formula developed by Poate et al. (2016) for reflective, gravel beaches: 2

𝑅2% Stockdon = 1.1 (0.35𝛽𝑓 (𝐻0 𝐿0 )0.5 + 0.5[𝐻0 𝐿0 (0.563𝛽𝑓 2 + 0.004)] )

(2)

𝑅2% Poate = 0.49𝛽𝑓 0.5 𝑇𝑧 𝐻0

(3)

where Βf is shoreface slope, H0 is significant deep-water wave height, L0 is the deep-water wave length, and Tz is the mean wave period. As these runup equations require deep-water wave statistics and the available wave data was output from a SWAN model at 10 m depth it was necessary to reverse-shoal the data to estimate representative deep-water wave height and wave length (see supplementary material for details).

Journal Pre-proof Shoreface slope was assumed constant for application of the runup equations. Using the two complete surveys, mean shoreface slope (averaging alongshore and between the two surveys) was 11%, generally varying between 8% and 13%. On shoreface areas which were immediately adjacent to the outlet channel, or which had been recently reworked by it, there was more variability in slope (6% to 20%). Barrier crest height typically varied between 3.0 and 4.4 m above mean sea level, although crest heights were often lower than this on sections of new barrier separating the elongated outlet channel from the sea.

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Testing revealed that Eq. 2, the formula of Stockdon et al. (2006), generally under-predicted runup, as, in contrast to camera observations, it did not indicate any overtopping during the two year monitoring period. Estimations of runup using Eq. 3 (Poate et al., 2016) were higher, with the resultant overwash predictions a better match to observations, hence predictions from this formula were used for further analysis. Sensitivity to shoreface slope was investigated by calculating runup with slopes ranging from 8 to 13%. Runup was sensitive to slope but even with the steeper slope, the Stockdon et al. formula predicted too little overtopping. The mean value of shoreface slope (11%) was retained for the remainder of the analysis.

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3.3.2 Longshore transport

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Wave driven longshore sediment transport is another key driver of hāpua processes. To develop an indicative time-series of longshore transport for qualitative analysis against lagoon response, the ‘CERC formula’ (Coastal Engineering Research Center, 1984; Komar, 1971) was applied.

𝐾𝑃𝑙𝑠 Γ

(4)

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𝑄𝑠 =

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The CERC formula links volumetric longshore sediment transport rate, Qs, to the longshore component of wave power per unit length of beach, calculated at the breakpoint, Pls:

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where K is an empirical coefficient and Γ converts immersed-weight transport rate to a volumetric transport rate, as given by: Γ = (𝜌𝑠 − 𝜌)𝑔𝑎′

(5)

where ρs and ρ are the sediment and water density respectively, g is acceleration due to gravity and a’ is a factor to account for the pore spaces in the beach sediment (assumed equal to 0.6 here). Longshore wave power at the breakpoint was calculated from the 10 m depth SWAN model outputs by assuming the magnitude of wave power per unit shore length was conserved from the model output point to the breakpoint, but that the direction of wave power was influenced by refraction over assumed shore parallel contours. The approach used to shoal the wave power from 10 m depth to the breakpoint is described in the supplementary material. It is known that the K coefficient for gravel beaches is one to two orders of magnitude smaller than for sand beaches (Van Wellen et al., 2000). A K coefficient of 0.017 was selected in line with the review of experimentally derived values of K by Voulgaris et al (1999), and in lieu of site specific calibration data. In a review of transport formulas suitable for gravel beaches, the CERC formula has been shown to provide reasonable estimates of mean annual transport rate (Van Wellen et al., 2000). However there are well established shortcomings with the CERC formula and with other similar approaches to calculating longshore transport rates due to the simplifying assumptions inherent in their derivation and the high uncertainty around the empirical ‘K’ coefficient (Cooper and

Journal Pre-proof Pilkey, 2007). Due to these uncertainties the absolute transport rates calculated, and the relative magnitude of their changes in time, should be used with caution. Nonetheless, for this study we are confident that the CERC formula reliably determines the direction of longshore transport and provides a measure of its relative intensity.

4 Results

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The Hurunui hāpua exhibited a wide range of morphological states and behaviours over the twoyear monitoring period (Table 1 and time-lapse video 1 in supplementary material), with frequent and complex changes in planform of the outlet channel and lagoon (see snapshots in Error! Reference source not found.). Periods which provide clear examples of different hāpua behaviours have been highlighted and are labelled A to I in Error! Reference source not found.. Time-lapse videos of each of these periods are included in the supplementary material. Wherever possible, the identified periods show examples of individual drivers of hāpua behaviour in isolation, as opposed to periods where multiple drivers influence the hāpua simultaneously.

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Table 1 Summary timeline of Hurunui hāpua behaviour from 29 July 2015 through 29

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September 2017. Letters A to F in the right-most column correspond to the conceptual model

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state in Figure 8. The terms ‘weak barrier’ and ‘established barrier’ are used to differentiate between narrow, low, newly formed barriers and higher, wider barriers which are more

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resistant to wave overtopping and breaching.

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Figure 3 Snapshots of Hurunui hāpua planform. Hāpua barrier and outlet channel position

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were mapped from projected time-lapse imagery, while backshore position moves with a much longer timeframe so was mapped from August 2015 ortho-imagery and is the same in all snapshots.

Figure 4 Time-series of Hurunui hāpua dynamics and process observations from July 2015 to September 2017. Time-series show (top to bottom): outlet channel position(s); lagoon width (at transect locations offset northward along the length of the lagoon); river flow; longshore transport rate; wave runup height; lagoon water level; and sea level. Multiple concurrent data points for outlet position indicate times when the channel(s) through the barrier had multiple openings (lagoon and sea openings of the barrier channel are indicated by black and grey dots, respectively). Multiple concurrent data points for lagoon width indicate the presence of lagoon

Journal Pre-proof islands. Key time periods mentioned in the text are indicated with pink vertical bands labelled by the letters A to I.

4.1 Outlet channel migration

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Over the monitoring period, the outlet channel moved intermittently northward alongshore, starting 300 m north of the river centreline and reaching 1200 m north of the river centreline before the barrier breached 400 m north of the river centreline on 22 July 2017 during a river flood (Error! Reference source not found., period I). Movement of the lagoonward end of the outlet channel was dominated by intermittent ‘jumps’ in position. Apart from the July 2017 river flood breach, which created a completely new channel (Error! Reference source not found., I), these jumps in position generally involved breach/erosion of the narrow gravel ridge separating the lagoon and the shoreparallel outlet channel. Erosion of this ridge creates a new entry to the outlet channel from the lagoon, effectively shortening the length of the outlet channel and leaving islands of barrier sediment in the lagoon (e.g. 19 January 2017 in Figure 3, and the multiple ‘lagoon end of channel’ measurements in the top plot of Figure 4 at the same time). At times, bank erosion causes slow migration of the lagoonward end of the outlet channel, but at rates less than 1 m per day, except during significant river floods.

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In contrast to the lagoonward end of the outlet channel, the position of the seaward end changed much more frequently and by greater amounts. The alongshore offset between the seaward and lagoonward ends varied from 400 m south to 800 m north (with individual offsets being up to 800 m in channel length), with the most elongated channels occurring during extended periods of stable river flows and wave conditions (e.g. the period November 2015 to April 2016 in Error! Reference source not found., B to C).

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Like the lagoonward end of the channel, the seaward end experienced intermittent jumps in position, shortening the channel length when new openings formed between the outlet channel and ocean. Some of these truncations occurred during very minor wave or river flow events, for example on 29 November 2015 when a small 80 m3·s-1 fresh breached the seaward side of the outlet channel, shortening it from 680 to 380 m in length (see the end of period B in Error! Reference source not found.). Other truncations were the result of more significant river floods (e.g. the 213 m3·s-1 flood on 15 July 2016, see Error! Reference source not found. and Error! Reference source not found., period D) or wave events (e.g. 4.2 m runup height on 15 June 2017, Error! Reference source not found. period H). Continuous migration was much more significant at the seaward end of the channel, with rates up to 50 m per day common. Migration was most often northward, although some periods of southward migration did occur (e.g. 28 February 2017, Error! Reference source not found. and Error! Reference source not found., period G). The direction of migration aligns with the direction of longshore transport, confirming that longshore transport plays a dominant role in driving migration of the seaward end of the outlet channel. Interestingly, there were some periods when northward migration continued in the face of southward longshore transport, for example in November 2016, before and after period E in Error! Reference source not found.. It appears that once angled in a particular direction the seaward end of the outlet channel is likely to continue migrating in that direction, and it takes a substantial reversal of longshore transport to change the direction of migration.

Journal Pre-proof 4.2 River flood effects on morphology Three floods larger than 500 m3·s-1 were observed during the monitoring period, all of which caused major changes in the lagoon. All three floods resulted in a short and wide outlet channel, but only the 22 July 2017 548 m3·s-1 flood (Error! Reference source not found., period I) caused a ‘primary breach’ of the barrier, whereby a new outlet channel was created close to where the river enters the lagoon (Hart, 2009a). Neither of the other two large floods caused a primary breach, even though the 19 January 2017 flood had an almost identical peak flow (551 m3·s-1) and the 19 September 2017 flood was larger (733 m3·s-1).

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The river floods transported large amounts of sediment, as evident from the deposits of gravelly bedload which were observed both inside and outside the lagoon following the floods. It was notable that after the two large floods that did not cause a primary breach, a large gravel fan was visible in the time-lapse imagery, located at the point where the river enters the lagoon. During the July 2017 primary breach most of this gravel fan was eroded. Another striking example of this process of gravel fan development followed by erosion and breach was photographed after completion of the time-series monitoring described in this paper (Figure 5).

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Figure 5 Gravel fans deposited and eroded by river floods in the south end of Hurunui hāpua. A) 24 May 2018, Gravel fan visible in lagoon during low flows. B) 15 June 2018, Erosion of a

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gravel fan during a primary breach. The flood peaked at 490 m3·s-1 two days prior to this

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photo.

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Outside the lagoon a submerged delta of sand and gravel was observed in front of the outlet channel following floods. It was not possible to survey this feature (due to the high-hazard exposed coast), but its effect on breaking waves was visible in the imagery and there was a notable bulge in the low tide shoreline (e.g. Video 7 supplementary material). Following the floods, waves rapidly re-worked the submerged deposits onshore, forming a narrow shore-parallel bar across the outlet channel, constricting the outlet, and deflecting it alongshore across the front of the barrier. In sections of the lagoon between the river entry point and the lagoon outlet channel, scour was observed on both banks and on the lagoon bed following the floods (e.g. Error! Reference source not found. Period F, Video 7 supplementary material). Along the barrier backshore a straight, eroded bank was visible in the time-lapse imagery following floods where beforehand there had been uneven wave washover lobes protruding into the lagoon. Deposits of mud and gravel along the cliff toe at the lagoon backshore were removed by floods, and once water clarity improved following a flood, patches of exposed bedrock were visible on the lagoon bed where previously there had been gravel (observed during site visit). Beyond the outlet channel in the ‘dead arm’ of the lagoon, extensive deposits of fine sediment were left after floods, forming a soft silty drape.

4.3 Changes in lagoon width The area of the lagoon water-surface controls the relationship between water level and volume, thus influencing how quickly the water level changes in response to changes in river flow, outlet geometry or tide level. Given that the lagoon length remains reasonably stable, lagoon width is a useful measure of lagoon area and is also useful to understand how hāpua lagoons maintain their size in the long term. Lagoon width is shown in the second plot of Error! Reference source not

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found. for transects 400 m to 1100 m north of the river centreline (refer to Figure 2 and Error! Reference source not found. for transect locations). The lagoon width dataset is somewhat noisy due to the effect of lagoon water level on width, but it does show some clear patterns of variation in time and space. Widths at the seven measurement transects shown range from 30 m to 150 m. Sometimes the plot shows multiple widths for a given transect, representing periods when the image analysis has identified the banks either side of an island or the outlet channel. At any given transect, width gradually reduces over time, except for infrequent (once every 1 to 2 years) sudden increases in width. All locations along the length of the lagoon showed similar patterns of variation in lagoon width, but the patterns were out of phase, with some parts being wide when other parts were narrow. For example, on 14 May 2016 (Error! Reference source not found., period C) the lagoon width at the 400 m and 500 m transects increased during a 300 m3·s-1 flood; while over the next few months the widths at the 600 and 700 m transects widened; and on 19 November 2016 (Error! Reference source not found., period E) the width at the 900, 1000 and 1100 m transects widened during a 140 m3·s-1 flood, although major islands remained until 24 January 2017.

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Inspection of the time-lapse videos (supplementary material) at the times of width changes observed in Error! Reference source not found. reveals three key process affecting lagoon width: wave overwashing of the barrier; river flooding coinciding with an offset outlet channel; and migration of the lagoon outlet channel. These processes, their frequencies, and examples from the time-lapse imagery are shown in Figure 6.

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Figure 6 Summary of key processes affecting lagoon width via time-lapse imagery taken

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before, during and after a typical example of each process. Notable features labelled in the images are: (A) wave overwashing sluicing gravel into the lagoon; (B) erosion of an island and

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the lagoon side of the barrier visible after a flood recession; and (C) narrowed barrier left behind after the lagoon end of the outlet channel has migrated past this area, with the

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migration direction indicated by an arrow.

Wave overwashing is the main process driving lagoon narrowing. Major wave overwashing events occurred approximately six times per year on average during the monitoring period, although overtopping occurred more frequently. Wave overwashing rolls the barrier backwards and fills in the seaward edge of the lagoon. For example, during 20-22 May 2017 wave overwashing occurred in the absence of river flooding and resulted in a 20 m narrowing at the 1100 m transect, with between 2 m and 10 m narrowing occurring along the rest of the lagoon (Error! Reference source not found. period H, Figure 6, and Video 9 in supplementary material). Wave overwashing was observed to form distinct overwash lobes where gravel had been washed into the lagoon. A particularly welldefined overwash throat and fan were visible on 26 August 2015 at a point 750 m north of the river centreline (Figure 7). By October 2015 this fan was observed to be larger, following a series of overwash events in late September 2015. By 8 August 2016 the same fan had grown right across the lagoon, with infilling likely occurring during a major overwash event on 24 May 2016. Unfortunately, this overwash fan was almost entirely in the camera’s blind spot (Figure 2) such that changes were only documented through field-visit observations. Between the infilling event and the site visit on 8 August 2016, flows to and from the disconnected northern arm of the lagoon, occurring as a result of varying lagoon water levels, had eroded a narrow and shallow channel through the deposit. At low tide the northern end of the lagoon was still perched slightly above the rest of the lagoon.

Journal Pre-proof Figure 7 Wave overwash throat (OT) and associated gravel lobe developing in the Hurunui hāpua. Orthophoto from drone imagery collected 26 August 2015. By May 2016 the gravel lobe extended right across the lagoon.

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River floods occurring while the lagoon outlet channel was offset were the only process which was observed to remove significant amounts of sediment from the bed and backshore of the lagoon. Error! Reference source not found.F documents a significant river flood (peak flow 550 m3·s-1) that caused erosion of both the barrier and lagoon backshores, with photos in the middle image column of Figure 6 illustrating this event. Only a slight increase in average lagoon width occurred as a result of this event, but erosion was clearly visible during field inspection and in the imagery in the form of barrier backshore straightening, with wave washover lobes trimmed-off, and lagoon backshore scouring, with talus ramps, debris and rocks removed, as well as partial erosion of the lagoon islands.

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4.4 Lagoon level

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Migration of the lagoon outlet channel was observed to have a major effect on the position and width of the barrier. As the outlet channel migrated alongshore past a section of barrier, the existing barrier was largely eroded (apart from any islands left behind by step changes in the position of the lagoonward end of the outlet channel) and replaced by a new barrier section formed by wave deposition. The new barrier section was generally much narrower than the older pre-existing barrier (which had been widened by numerous wave overwash events), resulting in lagoon widening. An example of the widening caused by outlet channel migration can be seen 400 m north of the river centreline between August 2015 and July 2016 in Error! Reference source not found. and Error! Reference source not found..

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As explained earlier, hāpua lagoons rarely experience tidal ingress but they are subject to distinct cycles of water level variation due to the backwater effect of sea level on lagoon outflows. The minimum or base level that the lagoon fell to, and the magnitude of the daily range in level, showed three distinctive patterns (Error! Reference source not found., bottom plot). Firstly, there are ‘wellconnected’ periods when the lagoon level is strongly influenced by the ocean tide, with the water level range greater than 0.5 m, often around 1.0 m (e.g. period A in Error! Reference source not found.). This behaviour is typically observed when the outlet channel is short (< 200 m) and/or wide. Secondly, there are ‘perched’ periods when the lagoon level is perched continuously above high tide level, with little variation in lagoon level (e.g. period B in Error! Reference source not found.). These periods are associated with a long, constricted outlet channel (generally >200 m long, e.g. 18 Nov 2015 Error! Reference source not found.). Thirdly, there are distinct spikes in lagoon level: these ‘lagoon flood’ events are associated with wave overwash events, river floods, or a combination of both. Even small river floods or wave overwash events can cause lagoon floods if the lagoon is already in a perched condition (i.e. event D in Error! Reference source not found.). Following lagoon floods the lagoon water level often reverts to a well-connected state, as water draining out of the lagoon following the lagoon flood truncates and/or widens the outlet channel.

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5 Discussion 5.1 Improved conceptual model of hāpua dynamics

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Figure 8 Conceptual model of hāpua behaviour.

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Having a comprehensive understanding of hāpua dynamics, how they are driven by different external forcings, and the feedbacks resulting from the role of antecedent hāpua state is important to be able to make assessments of how hāpua will respond to changing external drivers. Based on the observations of the Hurunui hāpua and the earlier models introduced earlier, a more detailed conceptual model of hāpua behaviour has been developed (Figure 8). This new conceptual model builds on the cycle of (1) breach of the barrier opposite the river mouth, (2) mouth migration and (3) development of a shore parallel extended/migrated outlet channel described by Kelk (1974) and Todd (1992, 1983) which was subsequently expanded by Hart (2009a) to include (4) secondary breach and (5) closure of the lagoon. The new model (Figure 8) incorporates and expands on the mouth migration processes observed at the Ashburton hāpua (Paterson et al., 2001) as well as including additional detail about barrier formation, bar bypassing and the role of bedload delivered during floods. Key new aspects which extend the previous models are discussed further in the following sections.

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5.1.1 Barrier formation

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The barrier formation processes observed at the Hurunui hāpua and captured in the conceptual model (Figure 8B) include a new process that has not been described previously at hapua but closely resembles the formation and reworking of ephemeral flood derived deltas observed at sandy river mouths (for example, those on the wave-dominated Natal coast of southeast Africa as described by Cooper, 1990). The submerged delta of flood-derived sand and gravel deposited outside the lagoon is instrumental in constricting and deflecting the outlet channel alongshore when it is reworked onshore by waves during and following the flood recession. This process highlights the important role of river bedload supply in influencing mouth processes. Wave direction during initial barrier formation controls the initial outlet channel migration. This initial offset direction is significant because of it was observed that once offset, the outlet tends to preferentially migrate in the direction of offset, resisting migration in the opposite direction. The initiation process is important because the constricted mouth prevents the lagoon from becoming estuarine.

5.1.2 Differentiating ‘weak’ and ‘established’ barriers and their migration processes The observations of mouth migration behaviour in ‘established’ barriers at the Hurunui hāpua largely match with previous observations in the Ashburton hāpua (Paterson et al., 2001). However, at the Hurunui a different behaviour was observed in the ‘weak barriers’ formed by onshore bar migration after floods. For this reason, the conceptual model differentiates between weak and established barriers (Figure 8, C and D respectively). Weak barriers rapidly elongate due to longshore transport and continuing onshore movement of sediment, causing the seaward end of the outlet channel to migrate and extending the length of the outlet channel (Figure 8C). Weak barriers were observed to be very vulnerable to breaching by even slightly elevated waves or river flows. Cycles of elongation and breaching of these barriers were observed to take anywhere from hours to a few weeks in the Hurunui imagery, with this cycle continuing for weeks to months until waves had built up the height and width of the new barrier to a size that remained resilient to breaching (i.e. it became ‘established’).

Journal Pre-proof 5.1.3 Bar-bypassing at the seaward end of the outlet channel

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In the Hurunui hāpua a defined shallow bar was visible across the seaward end of the outlet channel at low river flows. This is consistent with anecdotal observations from the Rakaia hāpua where local residents walked across the river mouth bar during a period of low flows (Southward, 2012). The presence of a bar and associated bar-bypassing of sediment differs from previous assumptions that at hāpua “most sediment travelling as beach drift will bypass in the process of the spit growth-breach sequence (‘spit bypassing’)” (Kirk, 1991, p277). The proportion of longshore transport which passes a river mouth via bar-bypassing has been shown to be an important control of river mouth behaviour in a recent modelling study by Nienhuis et al. (2016). Interestingly that study showed that “the rate of river mouth migration is maximized for the intermediate discharge scenario, while the alongshore sediment bypassing fraction continuously decreases for increasing discharge” (Nienhuis et al., 2016, p671). This linkage between river flow and migration rate could have significant implications for hāpua where flow regimes which have been artificially modified. Bar bypassing is an important process at hāpua which would need to be included in any numerical model of outlet migration, meaning that simple models of river mouth migration behaviour (e.g. Tanaka et al., 1995) may not be appropriate.

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5.1.4 Increased likelihood of primary breaching with sediment build-up in lagoon

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Primary breaching occurred only once during the two-year monitoring period, but several floods were observed that did not cause primary breaches, including one of very similar magnitude to, and one with peak flow 30% greater than, the flood which caused the primary breach. This is inconsistent with previous models where breaching due to river floods was considered to be primarily hydraulically driven, with breaching associated with floods exceeding a threshold peak flow that raised the lagoon level sufficiently to initiate a breach (Kirk, 1991). Although Kirk does note that breaching is more likely after “long periods of spit extension (low river flow)”. The time-lapse imagery collected for this study provides an explanation for at least part of the apparent variability in the threshold flow required to initiate breaching: sand and gravel deposited at the point where the river enters the lagoon raises bed levels, reducing the flow required to cause primary breaching. This process means that river bedload supplied during small floods (which do not cause a primary breach) progressively reduces the threshold flow required to initiate a future breach. This linkage between antecedent in-lagoon sedimentation and the flow required to cause breaching means that the frequency and magnitude of small bedload-competent floods is likely to have an impact on the frequency with which primary breaching occurs. This role of bed level increases due to deposition causing primary breach during river floods is analogous to river avulsion in delta systems. Channel aggradation is well established as the primary driver of avulsions in river systems (e.g. Slingerland and Smith, 2004). Physical and numerical modelling of deltaic systems shows focussed deposition in the backwater zone causes avulsions to occur repeatedly at a consistent distance from the coast (Chadwick et al., 2019; Ganti et al., 2016; Ratliff et al., 2018). In hāpua, focussed deposition occurs at the point where the relatively steep braided river enters the lagoon, driving increased likelihood of breach at that location. Other drivers for breaching to occur in that location include bank erosion and superelevation of water levels against the barrier backshore opposite the river as, prior to a breach, flood flows are forced to bend round a sharp corner as it enters the lagoon.

5.1.5 Processes affecting lagoon size in the short- and long-term Basic hydraulic principles (e.g. water volume continuity) suggest that lagoon size has an important feedback on hāpua behaviour, with the water level of smaller lagoons responding more quickly to

Journal Pre-proof changes in river flow rate or outlet channel geometry. Example implications of this increased responsiveness are that (for a given river flow) a smaller lagoon is less likely to experience closure (as the lagoon level will rise rapidly as the outlet is constricted), and a larger lagoon is more likely to experience tidal flow reversals in the outlet channel. The effect of lagoon size is clearly illustrated by the differences between hāpua and waituna type river mouth lagoons in New Zealand. Both exist on similar high energy coastlines but waituna are formed on low-lying coastlines with pre-existing embayments so have much larger lagoons. As a result the mouths of waituna are closed for extended periods, even during high river flows, and when open experience tidal flow reversals (Hume et al., 2016; Kirk and Lauder, 2000).

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The backshores cliffs of hāpua are generally set back (further landward) relative to the cliff face line of the adjacent coasts, allowing space for the lagoon. This setback occurs on stable coasts (such as the Hurunui hāpua) but also on rapidly eroding coasts. Examples of this are shown in Figure 9A and 9B, which show the Ashburton and Rangitata hāpua, where the coast is eroding at rates of 0.5 to 1.0 m/yr (Gibb and Adams, 1982; Hicks and Enright, 2010). It is important to understand how lagoons maintain their size in the long term, and how their size is influenced by different processes.

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Lagoon size relates to both lagoon width and length. Three key processes affecting lagoon width were identified in the Hurunui hāpua: wave overwashing of the barrier; river flooding occurring concurrent with an offset outlet channel; and migration of the lagoon outlet channel. Each of these processes produce characteristic features that can be identified in aerial imagery. These same characteristic features are visible in aerial imagery of other hāpua, confirming that they are not unique to the Hurunui (Figure 9). Whilst the processes affecting lagoon width in the Hurunui hāpua all occur over relatively short timescales, their interaction controls the width of the lagoon in the long term. Figure 9 Aerial photos of other examples of New Zealand hāpua, showing evidence of the

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three key processes identified as controlling lagoon size. (A) Ashburton hāpua, South

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Canterbury, 44° 3.2'S 171° 48.3'E; (B) Rangitata hāpua, South Canterbury, 44° 11.2'S 171° 30.7'E; (C) Waiau Lagoon, Southland, 46° 11.8'S 167° 37.0'E. Key features marked are: (1) wave overwash gravel lobes; (2) lagoon backshore cliffs set-back relative to open coast cliffs, with historic flood erosion visible; and (3) differences in lagoon width as a result of barrier re-setting caused by mouth migration.

In regard to lagoon length, both identified lagoon widening processes at the Hurunui hāpua are linked to outlet channel migration, so it can be inferred that the maximum distance the outlet channel migrates between primary breach events controls lagoon length. If the maximum distance the outlet channel migrates reduces, the lagoon would shorten because the end of the lagoon would fill with sediment due to wave overwashing. The distinct wave overwash lobes observed in the Hurunui hāpua (e.g. Figure 7) are a clear example of how overwashing can infill the lagoon and reduce its area. Whilst these overwash fans have not been described previously in hāpua, the overwash observed in the Hurunui included examples similar to both the ‘sluicing overwash’ and ‘throat confined overwashing’ observed at Hurst Spit in Southern England (Bradbury et al., 2005). Indeed, the similarities between processes at hāpua and at other gravel barrier systems suggests

Journal Pre-proof that recent research into gravel barrier morphology, including field studies (e.g. Almeida et al., 2017), flume modelling (e.g. Matias et al., 2014) and numerical modelling studies (e.g. McCall et al., 2014), all have potential application for hāpua.

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It is of interest that most hāpua occur on retreating shores, whereon the hāpua backshore cliffs appear, in the long term, to be retreating in pace with their adjacent coasts thereby maintaining a stable time-averaged width (Kelk, 1974; Kirk and Lauder, 2000). This occurs despite somewhat different processes that are not phase-locked. That is, while the hāpua backshore cliff erosion appears to be ultimately controlled by the capacity of river floods to remove the buttressing talus at the toe of the hāpua backshore (and likely augmenting erosion by direct scour after the talus is removed), the erosion on the adjacent open coast is controlled by the rate at which coastal storm waves can scour the talus that accumulates at the toe of the coastal cliffs (and then directly undercut the cliffs with the talus removed) (Sunamara, 1983). Since the hāpua shoreline is ‘pinned’ by that of the adjacent shore, there must be a feedback involving lagoon width that regulates the rate of back-lagoon erosion to keep pace with the adjacent shore – the logical mechanism being lagoon narrowing concentrating flood flows. This is consistent with anecdotal observations reported by Todd (1992) of historic river floods that caused backshore erosion on the Ashburton hāpua.

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5.1.6 Outlet closure

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The new conceptual model is designed to be general to all hāpua and as such it includes the closure process, as although this was not observed during this study, it is a key process observed in other locations. Extended closures of weeks or months have been described at many hāpua including the Opihi hāpua (McSweeney et al., 2016), Ashburton hāpua (Hart, 2009a; Kirk, 1991) and in many smaller hāpua (Creed, 2014). For extended closures to occur lagoon inflows need to be sufficiently low that they can be discharged entirely via barrier seepage. If inflows exceed what can seep through the barrier, lagoon levels rapidly increase following closure and initiate the breach of a new outlet channel (or reopening of a previous channel). Closure has also been observed on lagoons with inflows significantly greater than barrier seepage. For these lagoons the closures are only transient, lasting only for the duration (typically hours) it takes the lagoon to fill then breach a new outlet. Transient closure lasting several hours has been observed at the Rakaia hāpua on 2 and 3 May 2012. Based on an analysis of lagoon water level data it appears each closure lasted less than 2 hours, during which time the lagoon level increased to the point where the barrier overtopped. Inspection of the barrier following the closures revealed a new outlet which had formed at a low point in the barrier 600-700 m south of the original outlet (Hicks, 2012).

5.2 Implications for management Human activities affect hāpua in a range of ways, including direct engineering interventions such as artificial barrier breaches or groynes limiting outlet channel migration, or changed environmental drivers, such as modifications to river flow regime and coastal or river sediment supply. Understanding/predicting the impacts of changes is important to enable informed management. The improved conceptual model developed by this study highlights several ways in which human activities may influence hāpua which were not previously well documented. Outlet channel migration is identified by this study as key to the maintenance of lagoon size. This suggests that any changes affecting the migration or breach process have the potential to modify lagoon size in the long term. For example, repeated artificial breaches would be likely to reduce hāpua length in the long term. Artificial breaches are used to mitigate lagoon flood risk or closure risk (e.g. Opihi River: McSweeney et al., 2016), or have been recommended as mitigation for increased risk due to flow regime change (e.g. Rakaia River: Salmon et al., 2012).

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The delivery of river gravel and sand bedload to hāpua during floods are identified by this study as having an important role in increasing the likelihood of primary breaches and promoting the initial barrier formation which constricts/offsets the outlet channel. This highlights that changes in river sediment supply have the potential to influence hāpua behaviour. Dams, gravel extraction, and water extraction all influence bedload supply and delivery in the gravel-bed braided rivers discharging into hāpua. A mechanism clearly identified from this study is how small, frequent bedload-competent floods can reduce lagoon volume and lower thresholds for primary breaching, which means that flood-harvesting, for example, would reduce the incidence of primary breaching. Also, changes in river sediment delivery that affect the initial barrier formation after a primary breach could be significant in controlling the behaviour of “intermittent hāpua”, which can switch states over years to decades between a hāpua condition with a freshwater lagoon and a constricted outlet channel to an estuarine condition with a wider mouth, tidal ingress and a saline lagoon (Hume et al., 2016). For intermittent hāpua vulnerable to switching in this way, river sediment supply could be of considerable importance in determining whether the outlet become sufficiently constricted following a flood event to elevate water levels and promote a hāpua condition.

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As well as the role of floods bringing sediment to the lagoon, this study also highlights that floods occurring while the mouth is offset are likely a key process for maintaining lagoon size on eroding coastlines by scouring the lagoon backshore. This is an important consideration when considering the effect of changed river flow regimes due to dams or flood-harvesting type water extraction.

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As a final point, monitoring hāpua is increasingly becoming a requirement for river managers (e.g. Eder et al., 2011) but is challenging due to their highly dynamic nature. This study demonstrates that time-lapse imagery can provide an excellent way to monitor lagoon physical behaviour, however the analysis/processing of the derived data is time-consuming. Collection of lagoon water level data is more straightforward and when combined with river flow and wave records can be interpreted to infer aspects of lagoon behaviour, for example by quantifying the duration and frequency of wellconnected vs perched (or closed) lagoon conditions and identifying lagoon floods and their causes.

5.3 General applicability to other hapua

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One limitation of this study is that detailed data have only been analysed for a single hāpua, meaning the analyses focus on the processes which dominate at that location. Notably, outlet channel closure was not observed at the Hurunui hāpua and is a key process documented at other locations. Also, the Hurunui hāpua exists on a relatively stable coast backed by limestone cliffs, whereas many other hāpua are located on retreating coastlines with unconsolidated gravel and sand hinterlands. Despite these limitations the conceptual model has been designed to be applicable to other hāpua, and as such the closure process has been included.

6 Conclusions An improved conceptual model of hāpua dynamics has been developed which is summarised graphically in Figure 8. The model builds on previous understanding of hāpua by better explaining several key processes. One area captured by the new model which was not identified in previous studies is the role of river gravel/sand delivery to the lagoon in driving two hāpua processes: initial barrier formation and primary breaching. Initial barrier formation occurs following river floods when the wide post-flood lagoon outlet channel is rapidly constricted and deflected alongshore by wave driven onshore movement of gravel/sand bars formed from sediment deposited outside the lagoon during the flood. The barrier formed by onshore bar migration is generally low and easily breached, resulting in a rapid cycle of

Journal Pre-proof channel elongation and truncation until waves build up the barrier to the point where the initial ‘weak’ barrier becomes ‘established’ and more resistant to breaching. Gravel accumulation in the lagoon influences the likelihood of floods causing a primary breach; that is, when a significant river flood breaches the barrier near the point where the river enters the lagoon. Floods which do not cause a primary breach deposit their gravelly bedload in the lagoon, raising bed levels and increasing the likelihood of a breach during the next event. These links between river bedload transport and hāpua processes are important as they indicate ways that changes in sediment delivery (for example due to gravel extraction or dam construction) are likely to impact hāpua behaviour.

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Another area where this study has expanded on existing knowledge of hāpua behaviour is identifying and documenting the processes influencing lagoon size, particularly width. By measuring lagoon width from time-lapse imagery three key processes influencing lagoon width were identified: (1) wave overwashing of the gravel barrier rolls back the barrier, pushing lobes of gravel into the lagoon causing it to narrow; (2) river floods occurring while the outlet channel is offset erode the bed and banks of the lagoon; and (3) alongshore migration of the lagoon outlet channel resets the barrier width and position, widening the lagoon and narrowing the barrier. These processes all occur over relatively short timescales but their interaction controls the size of the lagoon in the long term. Both identified lagoon widening processes are linked to outlet channel migration, highlighting the importance of the migration processes for maintaining lagoon size.

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The conceptual process-understanding developed in this study is currently being used to help inform the development of a physically-based numerical model of hāpua morphodynamics. This model aims to provide a means of testing the generality of the identified processes and enable predictive modelling to explore the effect of changing drivers – such as river flows - on hāpua form and behaviour.

7 Acknowledgements

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This paper relies on river and wave monitoring datasets collected by Environment Canterbury and NIWA. Jeremy Walsh and Kathy Walter assisted by helping access this data. Marty Flanagan, Peter McGuigan, Kevin Wines, Paul Bealing, Justin Harrison, Nick Key, Jeremy Rutherford, Jo Hoyle, Jochen Bind, Dave Plew, Brendon Smith, Mark Crump, Graham Elley, Greg Kelly and Gu Stecca all assisted with field data collection and instrumentation at the Hurunui hāpua. Richard Gorman supplied wave data from SWAN modelling of the Canterbury coast. Richard Measures and Murray Hicks were funded by the NIWA Freshwater and Estuaries Research Program 2: Sustainable Water Allocation (2017/18 SCI).

8 References Almeida, L.P., Masselink, G., McCall, R., Russell, P., 2017. Storm overwash of a gravel barrier: Field measurements and XBeach-G modelling. Coast. Eng. 120, 22–35. doi:10.1016/j.coastaleng.2016.11.009 Bergillos, R.J., Masselink, G., Ortega-Sánchez, M., 2017. Coupling cross-shore and longshore sediment transport to model storm response along a mixed sand-gravel coast under varying wave directions. Coast. Eng. 129, 93–104. doi:10.1016/j.coastaleng.2017.09.009

Journal Pre-proof Booij, N., Ris, R.C., Holthuijsen, L.H., 1999. A third-generation wave model for coastal regions. IModel description and validation. J. Geophys. Res. 104, 7649–7666. doi:10.1029/98jc02622 Bradbury, A.P., Cope, S.N., Prouty, D.B., 2005. Predicting the response of shingle beach barriers under extreme wave and water level conditions in Southern England, in: Sanchez-Arcilla, A. (Ed.), Coastal Dynamics 2005. American Society of Civil Engineers, Barcelona, Spain, pp. 1–14. doi:10.1061/40855(214)94 Canny, J., 1986. A Computational Approach to Edge Detection. IEEE Trans. Pattern Anal. Mach. Intell. PAMI-8, 679–698. Chadwick, A.J., Lamb, M.P., Moodie, A.J., Parker, G., Nittrouer, J.A., 2019. Origin of a Preferential Avulsion Node on Lowland River Deltas. Geophys. Res. Lett. 46, 4267–4277. doi:10.1029/2019GL082491

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Coastal Engineering Research Center, 1984. Shore protection manual. US Army Corps of Engineers, Vicksburg, Mississippi. doi:10.5962/bhl.title.47830

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Cooper, J.A.G., 1994. Sedimentary processes in the river-dominated Mvoti estuary, South Africa. Geomorphology 9, 271–300. doi:10.1016/0169-555X(94)90050-7

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Cooper, J.A.G., 1990. Ephemeral stream-mouth bars at flood-breach river mouths on a wavedominated coast: Comparison with ebb-tidal deltas at barrier inlets. Mar. Geol. 95, 57–70. doi:10.1016/0025-3227(90)90021-B

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Cooper, J.A.G., Pilkey, O.H., 2007. Longshore Drift: Trapped in an Expected Universe. J. Sediment. Res. 74, 599–606. doi:10.1306/022204740599

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Creed, S., 2014. A classification of small hapua-type river mouths on the mixed sand and gravel coasts of Canterbury, New Zealand (Dissertation). University of Canterbury.

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Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof

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28Jan - 12Feb 13Feb - 26Feb 27Feb - 4Apr 5Apr - 15Apr 16Apr - 20Apr 21Apr - 23May 24May - 21Jul 22Jul - 22Jul 23Jul - 26Jul 27Jul - 13Aug 14Aug - 20Aug 21Aug - 17Sep 18Sep - 29Sep

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Outlet remains wide until wave event builds barrier, constricting outlet channel. Unusual southward deflection of outlet due to longshore transport - weak barrier. Weak barrier migration and truncation during period of low river flow & waves. 3 -1 Two river floods (395 and 327 m ·s ) straighten and widen the outlet despite accompanying large waves which cause overwashing and narrowing of lagoon. Waves rapidly constrict outlet. Ongoing wave overtopping and overwashing widen barrier switching outlet to more established behaviour and narrowing lagoon. Cycles of outlet channel elongation and truncation. Lagoon perched above 2 m for extended period with little tidal influence. 3 -1 Large river flood (548 m ·s ) initially widens existing outlet then causes primary breach of barrier approximately 400 m north of river centreline. Waves constrict both outlet channels and northern channel eventually closes. Migration and truncation of seaward end of outlet channel (weak barrier). 3 -1 Two flood events (227 and 290 m ·s ) straighten and widen outlet channel Waves rapidly constrict outlet which then migrates northwards - system quickly moves from weak to established barrier behaviour. 3 -1 Large river flood (733 m ·s ) straightens/widens outlet (without causing primary

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Hapua behaviour Weak (narrow, low) barrier with elongated outlet channel and perched lagoon. 3 -1 245 m ·s river flood straightens and widens outlet channel. Wave overtopping fills in remains of elongated outlet. Outlet constricted by big wave event: barrier formation. Weak barrier showing elongation and truncation cycles. Camera failure Weak barrier showing elongation and truncation cycles, perched lagoon. Wave overtopping widens barrier shifting system to established barrier behaviour. Cycles of outlet channel extension and truncation. 3 -1 303 m ·s river flood straightens and widens outlet channel, part of lagoon is widened as a result of outlet channel migration. 3 -1 River flow remains high (100 to 150 m ·s ) but large wave runup event constricts outlet and develops barrier. Wave overtopping and associated rapid constriction/elongation of outlet raise lagoon to flood levels (>3 m) causing breach/truncation of seaward end of outlet. 3 -1 194 m ·s river flood widens outlet, but waves immediately constrict it again. Rapid outlet channel elongation including cycles of truncation and elongation of seaward end of channel, constricts outlet causing lagoon level to become perched. 3 -1 213 m ·s river flood causes lagoon flooding followed by truncation of seaward end of outlet channel. Wave events build barrier constricting outlet channel. Rapid elongation of outlet channel (weak barrier). 3 -1 240 m ·s river flood widens outlet but channel remains angled allowing rapid return to elongation/migration of seaward end. Cycles of outlet channel elongation and truncation, moving from weak to established barrier. Outlet channel length up to ~800 m. Camera blind spot makes interpretation difficult at times. Wave event constricts outlet causing lagoon level to rise breaching/truncating the lagoon end of the outlet and leaving an island of sediment in the lagoon. A 3 -1 subsequent river flood (228 m ·s ) widened the channel but did not break the cycle of mouth migration behaviour. Migration and truncation cycles affect seaward end of outlet channel through established barrier. 3 -1 Large river flood (551 m ·s ) straightened and widened the outlet channel.

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Journal Pre-proof breach). Flood causes significant erosion of barrier backshore.

Highlights Non-estuarine river mouth lagoon dynamics are explained via a new conceptual model

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Onshore bar migration of flood bedload drives initial outlet channel constriction

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Bedload deposition in the lagoon increases likelihood of barrier breach by river

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Wave overwash, outlet channel migration and river flood erosion control lagoon width

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