# Spatial and temporal variability of shorefaces: A morpho-hydrodynamic controlled system

Klervi Hamon-Kerivel, Derek W.T. Jackson, Emilia Guisado-Pintado, Andrew Cooper, Mouncef Sedrati

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Klervi Hamon-Kerivel, Derek W.T. Jackson, Emilia Guisado-Pintado, Andrew Cooper, Mouncef Sedrati. Spatial and temporal variability of shorefaces: A morpho-hydrodynamic controlled system. Estuarine, Coastal and Shelf Science, 2023, 281, pp.108162. 10.1016/j.ecss.2022.108162. hal-03987360

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# Spatial and temporal variability of shorefaces: A morpho-hydrodynamic controlled system

Hamon-Kerivel, K., Jackson, DWT., Guisado-Pintado, E., Cooper, A., & Sedrati, M. (2023). Spatial and temporal variability of shorefaces: A morpho-hydrodynamic controlled system. *Estuarine Coastal and Shelf Science, 281*, 1-10. Article 108162. https://doi.org/10.1016/j.ecss.2022.108162

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**Published in:**
Estuarine Coastal and Shelf Science

**Publication Status:**
Published (in print/issue): 05/02/2023

**DOI:**
10.1016/j.ecss.2022.108162

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Spatial and temporal variability of shorefaces: A
morpho-hydrodynamic controlled system

Klervi Hamon-Kerivel (1) , Derek Jackson (1,2), Emilia Guisado Pintado (3,1), Andrew
Cooper (1,2), Mouncef Sedrati (4)

(1) School of Geography & Environmental Sciences, Ulster University, Coleraine, UK

(2) Discipline of Geology, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa

(3) Geography Regional Analysis and Physical Geography, University of Seville, Seville, Spain

(4) Laboratoire Géosciences Océan UMR CNRS 6538, Université Bretagne Sud, Vannes, France

**Abstract**

Shorefaces are transitional zones between the shelf and surfzone/beach systems. They are subdivided into ‘upper’ and ‘lower’ shoreface sectors which display particular morphodynamic behaviour. The upper shoreface is morphologically active over short (annual) time scales, while the lower shoreface changes over much longer time scales (decadal and beyond).

Shoreface morphodynamics is examined using three distinct examples from the high-energy coast of Ireland. Our results show that zonation patterns and the physical extent of shorefaces display distinct spatial as well as temporal variations. The shoreface's physical limits and internal zonation are driven by the combination of local wave forcing and pre-existing shoreface morphology. Local bed shear stress distribution and energy dissipation under extreme wave conditions also operate over variable time scales (yearly up to centennial).

We show that particular seabed morphology controls the extent of as well as the bed shear
stress distribution across the shoreface. This may have significant implication for any ad-
joining beach systems as waves arriving at the surf zone acquire specific characteristics
depending on the shoreface configuration, even under similar offshore wave conditions.

Additionally, the variation of shoreface extent impacts the volume of sediment available for transport both to the nearshore zone and therefore beach systems. The influence of shoreface morphology is an important, but largely overlooked element in the study of mesoscale coastal behaviour as well as coastal response to global climate change and secular sea-level rise.

## Keywords

- Shoreface zonation

- Shoreface extent

- Depth of Closure

- Bed Shear Stress

- Nearshore morphology

## Highlights

- Full shoreface extent needed in examination of transport potential

- Variability in shoreface zonation is largely controlled by nearshore morphology

- Incoming wave energy dissipation patterns tied to shoreface morphology

- Shoreface is defined along a morphological continuum with a supply impact on beaches

# 1 Introduction

In coastal systems, the shoreface is located between the surf zone and shelf and represents an important transitional zone between the deeper offshore and the shoreline/ beach system. It modifies incident waves before they reach the surf zone and it can be a sediment source, conduit and/or sink (Hamon-Kerivel et al., 2020). Understanding shoreface behaviour in space and time is essential for comprehension of coastal system functioning. Shoreface morphologies are variable globally (Athanasiou et al., 2019), and while the general assumption is that shorefaces adopt an equilibrium profile (Dean, 1991), in nature such morphologies are only rarely developed (Hamon-Kerivel et al., 2020). Equilibrium morphologies, in fact, appear to be more an exception than the general rule, especially
under geologically-constrained conditions and/ or sediment scarcity (Pilkey et al., 1993; Thiel er et al., 1995).

In terms of dynamics, the shoreface is characterised by significant sediment transport, these fluxes usually result from wave, wind and/or tidal forcing (Héquette, 2001). The asymmetry of shoaling waves across the shoreface is the main driver of onshore directed sediment transport (Austin et al., 2009), while return flows such as rip currents and undertows are responsible for most of the offshore-directed cross shore sediment transport (Castelle et al., 2016).

The shoreface is often subdivided into the upper and lower shoreface zones. The upper shoreface is morphologically active at a yearly time scale (Lee et al., 1998; Ruggiero et al., 2016), while visible morphological changes are difficult to capture on the lower shoreface even if significant sediment transport is occurring (Aagaard, 2014; Patterson and Nielsen, 2016; Swift et al., 1985). Morphological change on the lower shoreface is only measurable over time periods exceeding a decade. In other words, the boundary between upper and lower shoreface is equivalent to the definition of the depth of closure (Cowell et al., 1999). This boundary, the depth of closure, has been traditionally calculated using Hallermeier (1981, 1978) Depth of Closure (DoC) formula (see methodology section). It offers a relatively straightforward method to calculate the boundary for the upper shoreface. The determination of the seaward boundary of the lower shoreface is more complicated. Historically, the lower shoreface was considered to be inert because at the time scale of observation no morphological change was usually observed (below the threshold of observation) (Hoekstra et al., 1999). However, it is now clear that sediment transport does occur across the whole shoreface and that is likely significant for the overall sediment budget and general coastal dynamics (Hamon-Kerivel et al., 2020). Several boundaries have been proposed as a viable limit for the lower shoreface. The wave base was first proposed as the seaward limit of shoaling wave by Hallermeier (1981) and used as the lower shoreface limit. However, recently, other limits have been proposed based on different variables such as Depth of Transport based on Bed shear stress (Ortiz and Ashton, 2016; Valiente et al., 2019) and Morphodynamic Depth of Closure calculated using energetics-based formulation (Ortiz and Ashton, 2016). Valiente et al. (2019)'s proposed Depth of Transport (DoT) as the seaward boundary of the shoreface where the DoT is defined as the outer boundary of the zone where sediment transport does not create a discernible morphological change but where it is volumetrically significant over time. This definition is more in line with the idea that the lower shoreface is a zone that is active, even over smaller time scales than normally detectable.

Shoreface field data (high resolution bathymetry, granulometry, sediment volume) is
scarcely available, especially consecutive datasets covering the time scale of shoreface's
observable activities. In this paper an easily implemented methodology and readily avail-
able data are used to explore shoreface behaviour on the Irish coast.

In this paper, we determine the DoC and DoT boundaries and their lateral extent, at three
shorefaces with contrasting morphologies in the west and north of Ireland. The aim is to
assess the role of shoreface morphology (width, gradient and concavity) in conditioning
shoaling waves seaward of the surf zone.

## 2 Regional Setting

<figure><img src="image_2.png" /><figcaption>Figure 1: Shoreface site locations in Ireland (1. Dingle Bay; 2. Belmullet) and Northern Ireland (3. Portstewart). Boxes represents Swan grid extent, the thickness of the outline varies according to the resolution of the grids (500, 100 and 10m). The lines represent cross-shore profiles studied.</figcaption></figure>

The west and north coast of Ireland is strongly influenced by its geological framework (Guisado-Pintado and Jackson, 2019; Jackson et al., 2005; O'Connor et al., 2011; Cooper, 2013). The coast is rocky and beaches are laterally constrained, embayed pocket beaches. Similarly, the nearshore comprises rock outcrop and sedimentary cover of varying thickness (Cooper et al., 2002). The coast was shaped by its glacial history and most of the sediments
available in the area are relict from its glacial past (Cooper, 2006). Overall, the coast has a finite sediment volume with few contemporary fluvial inputs. Beach morphology in the region is strongly influenced by geological framework, and sediment availability as well as wave conditions (Jackson et al., 2005).

The wave climate in the region is classified as a high energy, swell wave-dominated regime with a seasonal variability characterised by stormy winters. The average nearshore wave height on the west coast is 3m up to 5m in winter periods (Guisado-Pintado and Jackson, 2018). The main incoming wave direction is west to southwest (Gallop et al., 2013; Tiron et al., 2015). On the north coast the mean wave height is 1.35m, the southwest directed offshore waves are refracted towards the north facing beaches (Backstrom et al., 2015). The main wind direction is southwest to west (Gallagher et al., 2016) and the tidal regime is meso-tidal (Devoy, 2008) (Devoy, 2008).

The morphology of the Dingle Bay shoreface (Fig.1, site 1) is convex and shallow, and is backed by a wide, dissipative beach with two active dune-topped barriers separated by a tidal inlet (Orford et al., 1999). The bay is a flat, shallow synclinal basin filled with Quaternary sediments including moraines (Devoy, 2015). While the dynamics of the barriers have been previously examined (Cooper et al., 2004; Gault et al., 2011; Kandrot, 2016; O'Shea and Murphy, 2013), there is a paucity of literature on the characteristics and dynamics of the shoreface in this area.

The shoreface at Belmullet (Fig.1, site 2) is particularly steep and directly exposed to North Atlantic waves. It is backed by several pocket beaches (from north to south Annagh, Emlybeg and Belderra) separated by small headlands, themselves backed by a dune system (Crofts, 1971). The shoreface is steep, heterogeneous with extensive outcropping rock and gravel separating patchy sandy areas.

The beach and shoreface at Portstewart strand (Fig.1, site 3) are relatively more sheltered from the direct North Atlantic wave regime. Still classed as a high-energy coastal system, the beach here is constrained by a headland on the east side and a river mouth (Bann estuary) on the west side. The river mouth provides little sediment input to the shoreface, however the 0 – 25 m depth on the shoreface appears to be accreting (Backstrom et al., 2009b) most likely due to input from deeper zones on the shoreface. The morphology of the Portstewart shoreface is steep in the 0 to 25 m zone then flattens before sloping more steeply seaward. Some sedimentary bedforms are noted at that second break in slope in which the morphology has been shown to influence the local morphodynamics (Backstrom et al., 2007, 2009a, 2015). On the east side of the system, rock and gravel are occasionally exposed (McDowell et al., 2007)).
# 3 Methods

In order to examine shoreface extent, a zonation of the shorefaces of Portstewart, Belmullet and Dingle Bay, and their morphologies were extracted from measured 3-D bathymetric grids that were also used to examine wave dynamics using wave modelling (SWAN) (Booij et al., 1997) to propagate extreme wave conditions to the shore. Input wave parameters for the SWAN model runs were derived from historic measured and modelled wave datasets. This study takes a worst case scenario approach to determine shoreface limits by propagating statistically-calculated wave conditions to the shore. In the absence of reliable field data such as multitemporal bathymetry to assess shoreface activity, the approach presented here is an attempt to evaluate the impact of shoreface morphology on incoming waves using an easily implementable methodology.

## 3.1 Extreme wave conditions

To investigate the maximum lateral extent and zonation of the shoreface, extreme wave conditions for annual, decadal and centennial scales were calculated using data from the ECMWF reanalysis dataset ERA5 (cds.climate.copernicus.eu). The parameters used were the mean wave direction (θ, degrees); mean wave period (t, seconds); significant height (m, metres) of combined wind wave and swell at a time step of 3 hours over a 39-year period spanning 1979 to 2018. The effective wave height and associated period and direction were then directly calculated.

He, the effective wave height, is the height exceeding 12h duration per year, it is calculated by extracting the wave height exceeded by 0.137% of the data. Te is the wave period most often associated with this effective wave height. The wave direction, θ, is the mean direction for the recorded waves higher or equal to He (and associated Te and θe) were calculated for each year of the time series used (annual timescale), then averaged in order to represent a 'typical' year. The effective wave height was chosen as the studied wave height as it is the wave parameter used in Hallermier's depth of closure.

The effective wave height (and associated Te and θ) was also calculated for the 39 years (He39) (height exceeding 12h in the 39 years – 0.0034% of the time) in order to calculate the multi-decadal extreme wave conditions. For longer time scales, extreme wave heights (return period of a hundred years) found in the literature (calculations made by (Clancy et al., 2015)) were used. This extreme wave height (He100) was calculated using the 'peak over threshold' method (POT) and the associated period determined using a simple relationship between height and period ($T = (4.0 \approx 4.6)\sqrt{H}$) as described by Goda (2010). The wave direction, θe100, associated with the most energetic events was used. The calcu-
lated effective wave conditions (effective wave height and associated period and direction) for annual, multi-decadal and centennial scales were used as boundary conditions for the SWAN wave model (Booij et al., 1997) in order to propagate offshore wave conditions to the nearshore and to extract hydrodynamic variables (see section 3.2.). The regional extreme wave height conditions (and associated period) for the three scenarios are recapitulated in Table 1.

## 3.2 Propagation of the wave condition

The SWAN wave model (Booij et al., 1997; Guisado-Pintado, 2020) was used to simulate nearshore wave processes using conditions described in Table 1. The wave conditions were propagated over a series of bathymetries with different resolutions (water level was set at lowest astronomical tide (LAT)). Offshore bathymetries extracted from GEBCO (www.gebco.net) at a resolution of 500m were combined with bathymetries (and LIDAR) acquired and processed by INFOMAR (www.infomar.ie) at 100m and 10m, respectively for nearshore conditions. SWAN was implemented using a nested modelling scheme, with modelling domains (Figure 1) composed of a regional 500m cell grid resolution (offshore bathymetry) encompassing the entire west and north coast of Ireland. Then for each site, using the boundaries conditions from the first run, a second nested run using a 100m bathymetry grid cell size was run. Finally, a 10m cell grid was nested within the 100m grid to refine the model around a selected cross-shore profile for each of the sites. Simulations were run in third-generation mode, using default parameters for linear wave growth and white-capping dissipation, bottom friction dissipation model (JONSWAP) following Hasselmann et al. (1973) and using JONSWAP spectral shape to represent the wave field. The wave frequency and directional space were discretized in 33 logarithmic-distributed bins from 0.03 to 1.00Hz and 36 regular distributed bins, respectively. As shown in Table 1, regional conditions for three scenarios (annual, multi-decadal and centennial) were calculated and inputted into SWAN at the first run. The second run allowed extraction of nearshore wave conditions at depth of 30m for each site in order to calculate DoC (see section 3.3).

## 3.3 Depth of Closure

The depth of closure (DoC) is understood as being the limit between upper and lower shoreface which marks the seaward limit of measurable morphological change for a given time period. Depth of closure for each situation was calculated using Hallermeier (1981,
<table><caption>Table 1: Offshore and nearshore wave conditions at each site for yearly, mesoscale and centennial time scales (regional input). Nearshore wave conditions at depth of 30m for each site under the three scenarios (yearly, mesoscale and centennial) extracted from SWAN simulations.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Yearly<br>(He)</th><th colspan="2">Multi-decadal<br>(He39)</th><th colspan="2">Centennial<br>(He100)</th></tr><tr><th>Hs</th><th>T</th><th>Hs</th><th>T</th><th>Hs</th><th>T</th></tr></thead><tbody><tr><td>Regional (Deepwater Input)</td><td>10.9</td><td>13</td><td>15</td><td>14</td><td>19.2</td><td>17.5</td></tr><tr><td>Belmullet @30m</td><td>5.8</td><td>13.3</td><td>7.6</td><td>14.9</td><td>10.2</td><td>17.4</td></tr><tr><td>Portstewart @30m</td><td>2.7</td><td>14.1</td><td>3.5</td><td>14.9</td><td>4.9</td><td>17.7</td></tr><tr><td>Dingle Bay @30m</td><td>3.3</td><td>13.4</td><td>4.6</td><td>14.9</td><td>7.1</td><td>17.3</td></tr></tbody></table>

1978) 's formula: Eq 1.

$$dI = 2.28Hs - 68.5\left(\frac{Hs^2}{gTs^2}\right)$$

Where He is the effective wave height, Te the associated wave period and g the acceleration of the gravity. The DoC was also calculated using extreme wave heights with a centennial return period (He100). Previous work calculated the DoC using offshore (deep water) wave height and period and was shown that results overestimate the DoC (Kraus et al., 1998). Therefore, wave conditions at the 30 m isobath (Table 1) and obtained through the wave simulations using SWAN were used to determine the DoC for all scenarios.

The DoC value obtained was then plotted as an iso-bathymetric line using the high-resolution bathymetry from INFOMAR. While the depth of closure is spatially variable (Hinton and Nicholls, 1999; Sabatier et al., 2005), a single value for the system was deemed acceptable as only a (relatively) small area was considered (laterally constrained embayed or pocket beach no wider than 10km).

### 3.4 Depth of Transport (DoT)

For each wave condition simulated, the outputs from SWAN were used in the calculation of the lower shoreface-shelf boundary, defined as the DoT according to the methodology of Valiente et al. (2019). The DoT is therefore the threshold above which sediment transport occurs, even if no morphological changes were observable/measurable (Valiente et al., 2019).

The DoT was calculated by means of the wave Bed Shear Stress (BSS) computed according to Soulsby (1997) using the output from the SWAN wave model simulations:
Eq. 2

$$ \tau_w = \frac{1}{2} \rho f_w U_w^2 $$

Where: $U_w$ is the wave orbital velocity amplitude (a value obtained from SWAN output), $\rho$ the sea water density and $f_w$ calculated using the following formula:
Eq. 3

$$ f_w = 1.33 \left( \frac{A}{Z_0} \right)^{-0.52} $$

where: $A = U(w) \frac{T}{2\pi}$, T is the wave period and $Z_0$ is a bottom roughness parameter provided by Soulsby (1997).

The DoT threshold was then calculated using the following formula:
Eq. 4

$$ \tau_{cr} = \theta_{cr} g (\rho_s - \rho) D_{50} $$

Where: g is the acceleration due to gravity, $\rho_s$ the density of sand, $D_{50}$ the average grain size and $\theta_{cr}$ the critical value of the Shield number, here set at 1, the threshold of sheetflow transport. While adding a tidal component is likely to deepen the DoT as shown by Valiente et al. (2019), it was chosen not to include it in the BBS calculation here as we focus on the dissipation of waves over the shoreface and while the tide might deepen the DoT limit it is deemed unlikely to change the pattern of wave energy dissipation. For the calculations we selected 0.3 as a value for $Z_0$. According to the table in Soulsby (1997) this $Z_0$ value corresponds to a seabed composed of a mixed fine sand to gravel as is common in Irish offshore areas. Indeed, the sediment grabs, backscatter and resulting seabed classification conducted by INFOMAR (www.infomar.ie) shows an important heterogeneity of seabed types and grain size, even across the bathymetry grids used in SWAN (computational grid). Fine sand was ubiquitous at all sites and a single sediment size of 0.25mm was used in simulations. We consider that this sediment size is representative of the sediment fraction (out of all sediment fraction existing in heterogenous environment) most likely to be transport to and from the beach systems, making it the most influential for beach and shoreline dynamics. The third SWAN run using the high-resolution bathymetry (10 m cell size) was used to extract the output variables. This included wave height (m), total wave energy dissipation ($N.m^{-2}$) and wave energy dissipation induced by bottom friction ($N.m^{-2}$) and dissipation for each cross-shore profile at each of the sites studied and across the surf zone. The profile selected for each site is representative of the coastal behaviour in its area. In order to compare the relative distances across the different sites, and because
some of the profile do not reach the shoreline (or 0m contour), cross-shore profiles were readjusted so that -7m in depth corresponds to 0m in distance (see figure 2).

# 4 Results & Discussion

Results are presented in terms of the cross-shore distribution of the DoC, DoT and the BSS and the spatial distribution of the critical BSS for each site for the three scenarios studied (annual, multi-decadal and centennial). From this, the variation in depth and extent of the DoC, BSS and energy dissipation was analysed.

## 4.1 Spatial variability

### 4.1.1 DoC and inner shoreface width

At the multi-decadal (He39), results show that the DoC was 7.5m at Portstewart, nearly 10m in Dingle Bay and 15.6m at Belmullet (Table 2). Because of the differing gradients, however, these are located at markedly different distances offshore. The Portstewart inner shoreface is <60 m wide, while that of Dingle Bay exceeds 5 km and consequently experiences much more wave attenuation seaward of the surf zone. The depth of transport was

<table><caption>Table 2: Depth and Distance of Closure and Transport for all three sites at multi-decadal (He39). The Distance of the DoC indicates also the limit between the upper and lower shoreface (LS).</caption><thead><tr><th rowspan="2">He39</th><th colspan="2">DOC</th><th colspan="2">DOT</th><th rowspan="2">LS width (m)</th></tr><tr><th>Depth (m)</th><th>Distance (m)</th><th>Depth (m)</th><th>Distance (m)</th></tr></thead><tbody><tr><td>Belmullet</td><td>15.6</td><td>1314</td><td>62.4</td><td>11130</td><td>9817</td></tr><tr><td>Portstewart</td><td>7.5</td><td>57</td><td>16.3</td><td>10361</td><td>10304</td></tr><tr><td>Dingle Bay</td><td>9.8</td><td>5119</td><td>33.9</td><td>31277</td><td>26158</td></tr></tbody></table>

around 16m at Portstewart, 34 m at Dingle Bay and > 60 m at Belmullet and the width of the lower shoreface was around 10 km for both Portstewart and Belmullet, and 26 km at Dingle Bay.

### 4.1.2 DoT and outer shoreface limit and width

Distinct variability was noted in the distribution of the bed shear stress at each site as shown in Figure 2 and 3. For all three sites most of the energy dissipation occurs near the shore as a result of surf breaking. Selecting only the energy dissipation due to bottom
friction, it is noticeable that Dingle Bay has a more dissipative morphology than the other two sites, with a higher rate of dissipation over the whole profile. At Belmullet, energy dissipation increases with decreasing depth nearer to the shore, suggesting a more "reflective" state (as described by Wright and Short (1984) for beaches). At Portstewart, the variations in dissipation of the energy due to bottom friction seem to be linked to the presence of sandy features on the shoreface.

$N.m^{-2}$ after the critical DoT value of 3.985 $N.m^{-2}$ is reached. A zone of intense BSS stretches over 5 km in width, encompassing both the upper shoreface and nearly half of the lower shoreface.

At Portstewart, the width of the shoreface is similar to Belmullet (about 10 km), however,

<figure><img src="image_3.png" /><figcaption>Figure 2: Cross-shore bathymetric profiles (A.) showing the BSS (B.), Wave heights (m) (C.), Energy wave Dissipation ($N.m^{-2}$) (C. and D.) for each site over multi-decadal wave conditions.</figcaption></figure>

the distribution of the bed shear stress over the shoreface is markedly different, with values of BSS near the DoT threshold for most of the surface of the shoreface. Further, it displays a limited area where the BSS falls under the DoT threshold. More intense BSS (up to $7 N.m^{-2}$) occurs only nearer the shoreline.

The width of the shoreface in Dingle Bay is more than 30km. This wide, shallow shoreface has relatively low shear stress (Figure 3 site 1). The BSS is nearly constant (ca. $5N.m^{-2}$) over the DoT threshold with no obvious peak apparent. Belmullet has a steeper, narrower shoreface with a peak of BSS located between 0 and 5 km offshore. Its more variable
morphology influences the distribution of BSS. Similarly, at Portstewart, its more complex
morphology was reflected in the distribution of BSS (Figure 3 site 3). Here the presence
of bedforms influences the DoT and resulting in a more complex distribution of the energy
dissipation. The BSS peaked at the location of sand ridge and declined under the DoT
threshold at the location of a topographic low, most likely due to tidal currents in the area.
In these circumstances, it is more difficult to determine a continuous shoreface.

These alongshore and cross-shore variations in calculated BSS and wave energy dissipation (Figures2 and 3) are further accentuated if the grain size heterogeneity across the shoreface is taken also into account.

<figure><img src="image_4.png" /></figure>

Figure 3: Map of the distribution of He 39 Bed Shear Stress for 1. Dingle Bay, 2. Belmullet and 3. Portstewart. Grey line represent the profiles studied for each site at 10 m resolution.

## 4. 2 Multi-temporal

4. 2. 1 Variation in depth and extent

Over the three sites, the DoC increased with the time period used (annual, multi-decadal and centennial). This was also associated with an increase in inner shoreface width. For the shallower shoreface of Dingle Bay (Figure 4), an increase of 2.7 m between the annual and the multi-decadal DoC, increased the distance of closure by slightly more than 5 km.
By contrast, for the steeper shorefaces at Belmullet and Portstewart (Figure 5 and 6), increases of 3.7 and 1.7 m of the DoC between the annual and multi-decadal, resulted in only about an 800 and 200 m increase, respectively in inner shoreface width. Between the multi-decadal extreme and the 100-year extreme, the increase in DoC is the same (5.2 m) at Belmullet (≈21 m) and Dingle Bay (15 m), but the increase in distance is only about 1km at Belmullet whereas, in contrast, it is nearly 9 km at Dingle Bay.

Similar patterns are noted for the lower shoreface's outer limit (DoT) and width (distance between DoC and DoT). Between annual and multi-decadal conditions, the DoT (depth) increased by 20 m, 4.5 m and 18 m at Belmullet, Portstewart and Dingle Bay, respectively. The associated increases in shoreface limit were 3.8km, 9.5km, 11.8km (Figures 4, 5 and 6).

<figure><img src="image_5.png" /></figure>

<table><thead><tr><th rowspan="2"></th><th colspan="2">DOC</th><th colspan="2">DOT</th><th rowspan="2">LS Extent (m)</th></tr><tr><th>Depth (m)</th><th>Distance (m)</th><th>Depth (m)</th><th>Distance (m)</th></tr></thead><tbody><tr><td>He</td><td>7.1</td><td>10</td><td>15.7</td><td>14359</td><td>14349</td></tr><tr><td>He39</td><td>9.8</td><td>5119</td><td>33.9</td><td>31277</td><td>26158</td></tr><tr><td>He100</td><td>15</td><td>13899</td><td>108.9</td><td>85155</td><td>71256</td></tr></tbody></table>

Figure 4: Cross-shore profile (A.) at Dingle Bay showing multi-temporal BSS (B.), wave heights (C.) and energy dissipation (C. and D.). The table represents results from multi-temporal depth and distance of closure and transport, and extent of the lower shoreface(LS)
<figure><img src="image_6.png" /></figure>

<table><thead><tr><th rowspan="2">Belmullet</th><th colspan="2">DOC</th><th colspan="2">DOT</th><th rowspan="2">LS Extent (m)</th></tr><tr><th>Depth (m)</th><th>Distance (m)</th><th>Depth (m)</th><th>Distance (m)</th></tr></thead><tbody><tr><td>He</td><td>11.9</td><td>517</td><td>42.3</td><td>6579</td><td>6062</td></tr><tr><td>He39</td><td>15.6</td><td>1314</td><td>62.4</td><td>11130</td><td>9817</td></tr><tr><td>He100</td><td>20.8</td><td>2286</td><td>105.6</td><td>27991</td><td>25705</td></tr></tbody></table>

Figure 5: Cross-shore profile (A.) at Belmullet showing multi-temporal BSS (B.), wave heights (C.) and energy dissipation (C. and D.). The table represents results from multi-temporal depth and distance of closure and transport, and extent of the lower shoreface (LS).
<figure><img src="image_7.png" /></figure>

<table><thead><tr><th rowspan="2">Portstewart</th><th colspan="2">DOC</th><th colspan="2">DOT</th><th rowspan="2">LSe Extent (m)</th></tr><tr><th>Depth (m)</th><th>Distance (m)</th><th>Depth (m)</th><th>Distance (m)</th></tr></thead><tbody><tr><td>He</td><td>5.8</td><td>-180</td><td>11.8</td><td>673</td><td>853</td></tr><tr><td>He39</td><td>7.5</td><td>57</td><td>16.3</td><td>10361</td><td>10304</td></tr><tr><td>He100</td><td>10.6</td><td>465</td><td>37.3</td><td>14209</td><td>13744</td></tr></tbody></table>

Figure 6: Cross-shore profile (A.) at Portstewart showing multi-temporal BSS (B.), wave heights (C.) and energy dissipation (C. and D.). The table represents results from multi-temporal depth and distance of closure and transport, and extent of the lower shoreface (LS).
4. 2. 2 Evolution of BSS and energy dissipation distribution

In Portstewart (Figure 6), the nearshore peak in BSS migrated offshore under more extreme
wave conditions. For centennial conditions this nearshore peak in BSS (8.6 N.m⁻²) was
slightly exceeded by the peak of 8.8 N.m⁻² (at 9.2 Km) seemingly created by a sand ridge
present on the shoreface. Further, the increase in BSS at the seabed feature was also present
for other wave conditions (annual and multi-decadal), but did not reach the nearshore as
the BSS lowers again under the DoT threshold landward of the seabed feature. A similar,
albeit opposite, effect was observable in shallower bathymetries 2km offshore, where the
BSS dipped before the nearshore peak because of a bathymetric low point. This effect
seems to be accentuated under more extreme wave conditions. Under 100-year conditions
(centennial He100) the somewhat shallow morphology of the lower shoreface appears to
increase BSS and surf breaking energy dissipation throughout the lower shoreface, which in
turn attenuates the intensity of the bed shear stress in the nearshore. Under multi-decadal
wave conditions, the BSS values border the threshold for most of the shoreface then peaks
across the nearshore as described previously. For annual wave conditions, the BSS only
reached the 3.985 N.m⁻², the threshold, at the nearshore.

At Belmullet (Figure 5), like Portstewart, the nearshore bed shear stress peaked further offshore under more energetic wave conditions. On the one hand, for annual conditions, the peak (of ≈10 N.m⁻²) occurred close to the zero distance mark (approximately -7m depth). On the other hand, for the centennial conditions the peak (nearly 14 N.m⁻²) occurred at around 5 km offshore (about -30m depth). The energy dissipation followed a similar pattern to that in Portstewart, displaying a nearshore peak of dissipation due to surf breaking that migrates further offshore when the conditions are more energetic, this is also associated with an increase in bottom friction-related dissipation.

This means that for less energetic conditions, the BSS and therefore maximal reworking
of the shoreface, is closer to the shore, whilst for the most energetic events, the maximal
BSS was further offshore.

In Dingle Bay (Figure 4), the BSS distribution did not show any peak, and had a rather
flat distribution with slight increasing towards the shore. For annual conditions, bed shear
stress values reached the threshold (4 N.m⁻²) at 16 km from the shoreline and the shore-
ward values of BSS border the threshold never exceeding (4.5 N.m⁻²) with several passages
below the critical BSS. Under multi-decadal conditions, the BSS is nearer 3 N.m⁻² for most
of the profile, exceeding the 3.985 N.m⁻² critical threshold at 31Km and increased to al-
most 7 N.m⁻² at around 13km from the shoreline. Similarly, the BSS over centennial
periods was quite high throughout the profile, with the threshold reached at 85 km off-
shore and values of BSS around 5 N m⁻² for most of the profile that eventually increase to 8 N·m⁻² at 15km, before decreasing towards the shore. Similarly, energy dissipation due to bottom friction occurs over large areas offshore while never exceeding 4.10⁻⁴ m²s⁻¹ and does not peak nearshore, however, it exceeds 1.10⁻⁴ m²s⁻¹ for most of the profile. For the more energetic conditions (centennial), dissipation due to breaking also occurs offshore.

# 5 Discussion

In coastal environments, the shoreface is an important transitional zone operating between the surf zone and the shelf, that not only modifies the incident waves but also plays a crucial role as an area of sediment source, transportation and distribution (Hamon-Kerivel et al., 2020). The characterisation of the upper and lower shoreface have traditionally been linked to formulations of wave base and depth of closure (Hallermeier, 1981), but recently the study of depth of transport (Valiente et al., 2019) has been used to determine the lower shoreface-shelf boundary. However, relatively little attention has been given to the definition and analysis of shoreface boundaries, its overall extent and its behaviour of the within contrasting coastal areas.

Our results show that shoreface morphology (width, gradient and concavity) of the three sites has a strong influence in forcing shoaling of waves and the resulting energy dissipation patterns, ultimately conditioning wave characteristics before they reach the surf zone. As expected the DoC, DoT, and their distance offshore at all three sites increased over the time scale being considered. However, the variations in distance (of closure and of transport) are all modulated by the initial morphology of the shoreface; flat shallow morphologies (e.g. Dingle Bay have wider shorefaces than steeper shorefaces (e.g. at Bellmullet).

At all three sites, BSS peaked close to the shore, with the peak migrating seaward during more energetic conditions, coinciding with enhanced change (sediment transport and corresponding morphological variation) took place further (and deeper) offshore. This corresponded to a variation in the depth of observed morphological change over the time scale of observations, i.e. the typical variations in DoC mentioned above. The influence of seabed morphology is also clear in the distribution of the BSS over the profiles at each of the three sites. At Dingle Bay, due to its flat and shallow morphology, the thresholds of critical BSS were reached relatively far offshore and the shoreface area was particularly extensive. This implies that while reworking might not be very intense (mild BSS, low energy dissipation), it occurs over an extended area across a large sedimentary zone thereby representing a significant potential sediment source area for overall sediment budgets. This
pattern of energy dissipation i.e. relatively low dissipation over a large area, also results
in a marked reduction in incident energy arriving to the surf zone.

Conversely, at Belmullet, the shoreface has a relatively steep morphology and the thresh-
olds of critical bed shear stress occurred relatively close to the shore, with little prior
dissipation of energy. This resulted in more intense reworking (high BSS and energy dissi-
pation) over a narrower shoreface. This particular shoreface is therefore likely to be more
morphologically active over shorter time scales (annual to decadal), which in turn would
facilitate dynamic exchanges between shoreface and surf zone at shorter time scales and
during high energy wave events.

Additionally, as an example at centennial scale conditions, the Belmullet shoreface ex-
tended 28 km offshore which implies that theoretically, the surface sediments over that 28
km width were available for transport and may therefore contribute to the overall coastal
budget. However, rock outcrops between 14 km and 8 km offshore, could impede sediment
transport across this zone (whether onshore- or offshore-directed). If this is the case and
no over topping is assumed then only sediments situated shoreward of these outcrops con-
tribute to the coastal sediment budget.

At Portstewart, depending on wave conditions, partial reworking of the shoreface occurs only around bedforms where the critical BSS is exceeded. It subsequently falls below the threshold towards the shore. Consecutive surveys (Backstrom et al., 2009b) of the nearshore area (over a two-year period) showed that the upper part shoreface of Portstewart Strand has no typical response to energetic events. Indeed, Backstrom et al. (2009b) have not only documented both accretion and erosion but also no change as a response to high energy events. The lack of uniformity in the response may also be related to the complexity of the local offshore morphology producing an area susceptible to reworking from certain wave conditions.

Furthermore, the same study found an overall accretion of the nearshore, upper shoreface
area that was attributed to a supply from the lower part of the shoreface. The sand ridge
present on the shoreface acts as an energy dissipation zone, and while it may have a shel-
tering effect for the shoreward part of the shoreface, its reworking may as well be the one
of the potential sources of sediments feeding the accretion documented by Backstrom et al.
(2009b)

Comparison of the three sites on this high-energy coast, suggests potential shoreface in-
fluences on beach and surf zone morphodynamics. First, because the depth (and associated
extent) of the ‘active’ shoreface is often linked to the upper shoreface and a morphologi-
cally active part of the shoreface, which may result in the lower part of the shoreface being
overlooked while in fact, non-negligible amounts of sediment transport take place, as was
shown by Valiente et al. (2019) at the embayed coast of Cornwall, UK. For rocky coasts,
phenomena such as headland by-passing, can play an important role in the redistribution
of sediment, highlighting the importance of a better understanding of the shoreface zona-
tion and its extent.

Furthermore, our results suggest a possible range of shoreface behaviour under a variety of
settings from steep, dissipative, rock-dominated shorefaces (e.g. Dingle Bay) to a shallow
sand-rich, reflective shorefaces (e.g. Belmullet) scenario. The presence of seabed features
also seem to play an important part, both as a dissipative function and as a sediment source
(e.g. Portstewart). We postulate that each individual shoreface configurations might even-
tually be shown to fall into this range. And while this assertion might seem instinctive,
there is a lack of literature on the potential importance of shoreface morphology on the
overall coastal behaviour both at short and meso timescale (Anthony and Aagaard, 2020;
Hamon-Kerivel et al., 2020). Furthermore, shoreface morphology is rarely referred to as
falling into a reflective/dissipative spectrum, and while this is common for beach systems
and provides a simple way of assessing their behaviour, a similar terminology would be
appropriate to describe the shoreface's impact on wave energy dissipation.

The different behaviours are closely linked to the morphology of the shoreface which in
turn seems to be related to the sediment volume on the shoreface. Typically sand-rich
shorefaces seem to have a more dissipative morphology, potentially providing a slow but
steady supply of sediment for the adjacent beaches and surfzones, assuming no longshore
transport, it would then be expected to have a more stable adjoining beach and shoreline
system in the longer term. Similarly, shorefaces with sediment-rich bed features seem to
have more dissipative properties which might contribute to enhanced beach resilience in a
similar way. On the other hand, a steep, rocky shoreface can be expected to have more
exchanges with the surf zone and beach over a shorter time scale, while also acting as a
sediment sink during energetic events, potentially resulting in more unstable beach and
shoreline at a shorter time scale. It is reasonable to assume that shorefaces that have
sediment starved features and therefore a paucity of sediment for transport, such as sorted
bedforms, should behave in a similar way.

In short, shoreface extent and zonation is controlled by both local wave climate and an-
tecedent seabed morphology. This means that shorefaces can assume various configurations
on a continuum ranging from dissipative to reflective, that factors in surf zone and beach
dynamics as shorefaces both conditions incoming waves and act as a source and/or sink
for coastal sediment. Furthermore, while it is customary to express coastal boundaries
according to their depth, adding relatively easily obtainable information about their as-
sociated distance greatly improves the amount of information provided about the studied
area by adding information about shoreface morphology and therefore giving a first im-
pression about the “type” of shoreface, thereby providing valuable information about the
reflective/dissipative nature of the shoreface in question.

6 Conclusion

This study examined the variability in shoreface zonation and its extent over three separate
sites on the West and North-western seaboard of Ireland. It investigates the influence of
each sites' contrasting morphologies on the dissipation of incoming wave energy, providing
insights into the shoreface zonation and morphology and the resulting energy delivery to
nearshore (beach and shoreline) coastal areas. Overall, our study concludes that:

* Knowing the full shoreface extent (and therefore depth of closure or depth of transport) provides important information in examining all transport potential.

* Shoreface zonation at the sites examined is controlled largely by nearshore morphology, with different shoreface morphologies producing differences in depth and extent of the shoreface.

* Incoming wave energy dissipation patterns respond closely to shoreface morphology, with sediment-rich shorefaces or large sediment bodies dissipating more of the incoming energy offshore.

* For rocky coasts, shoreface morphology is largely dictated by the local geological framework present leading to limited sediment availability for coastal beach systems.

* Shoreface morphology can be classified along a continuum ranging from dissipative to reflective with each morphological 'state' having a distinct impact on adjacent beach/shoreline behaviour, especially in laterally-constrained regions such as Ireland where there is little longshore transport.

The characterisation of shoreface limits and extent is a measure that can provide valu-
able information about the role of shoreface morphology in its hydrodynamic environment
by placing each morphology within a reflective-dissipative spectrum giving a preliminary
assessment of shoreface behaviour and potential impact on the shore and beach systems.
Because it is calculated from widely available data i.e. reanalysis wave data and 500m to
100m resolution bathymetry it can provide an entry point for the better understanding
of shorefaces and their role in coastal systems in various setting worldwide where reliable
field observations are still rare.

Acknowledgements

This work was supported through funding from a DfE International research Award to
Klervi Hamon-Kerivel.
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