Timothy Poate

Comprehensive Field Study of Swash-Zone Processes. I: Experimental Design with Examples of Hydrodynamic and Sediment Transport Measurements

AbstractA comprehensive study of swash-zone hydrodynamics and sediment transport was conducted on a macrotidal beach in Perranporth, United Kingdom. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations, water depth, near-bed velocity profiles, and high-resolution swash surface and bed-level changes on a natural beach. Data collected during the study are used to quantify the vertical profile of cross-shore and alongshore velocities and the importance of sheet flow sediment processes in the swash zone. The swash-zone boundary layer for cross-shore velocities is observed to generally occur over at least the lower 0.06 m of the water column. Alongshore velocities are often the same order of magnitude as the cross-shore velocities and are dominant near cross-shore flow reversal. Flows are often logarithmic in profile, but the instantaneous nature of the measurements renders application of the logarithmic model difficult. When valid, the logarithmic m...

Observations of the swash zone on a gravel beach during a storm using a laser-scanner (Lidar)

ABSTRACT Almeida, L.P., Masselink, G., Russell, P.E., Davidson, M.A., Poate, T.G., McCall, R.T., Blenkinsopp, C.E. and Turner, I.L. 2013. Observations of the swash zone on a gravel beach during a storm using a laser-scanner The collection of detailed field measurements from the swash zone during storms is an extremely challenging task which is difficult to execute with traditional in-situ deployments (e.g., scaffold rigs with instruments). The levels of difficulty increase for gravel beaches where the wave energy reaches the beach face with almost no loss of energy, leading to violent plunging wave breaking on the beach face that can produce large vertical morphological changes and extremely strong uprushes that can easily and rapidly damage, bury or detach instrumentation. Remote-sensing techniques emerge as the most appropriate solution to perform field measurements under such adverse conditions since they have the ability to perform measurements without being deployed in-situ. A mid-range (~ 50 m) Laser-scanner mounted on a tower (~ 7 m high) in the mid beach face of a gravel beach (Loe Bar - SW England) was used to measure bed-level changes and runup at a sampling rate of 2 Hz along one beach profile during a storm. The results from the comparison of this system with other state-of-the-art instruments (e.g., ultrasonic bed level sensors, GPS and video cameras) indicate that the quality of the measurements obtained is within the accuracy of the standard methods. The advantages of this system is the reduced logistical infrastructure required for the deployment, the capability to perform surveys with high spatial and vertical resolution, during day and night, and to reach areas of the swash zone where no other instrument can be deployed safely. Measurements performed with a laser-scanner on a gravel beach (Loe Bar) show complex and fast-changing morphology on the gravel beach, which appears to be a form of negative morphodynamic feedback to controls the hydrodynamic evolution in the swash zone.

Predicting overwash on gravel barriers

ABSTRACT McCall, R.T., Masselink, G., Poate, T.G., Bradbury, A.P., Russell, P.E. and M.A. Davidson, 2013. Predicting overwash on gravel barriers. A process-based non-hydrostatic flow model, which includes the effect of infiltration and exfiltration, but no morphology, is applied to simulate overwash events on gravel barriers. After calibration, the model is shown to produce similar predictions for overwash as the empirical Barrier Inertia Model for parameter combinations within the validity range of the empirical model. When applied to 25 historical storm impacts, the process-based model shows improvement over the empirical model in predicting overwash. The model is applied to study the sensitivity of overwash to input parameters outside the validity range of the empirical Barrier Inertia model. This analysis shows that two parameters currently missing in the Barrier Inertia Model, the depth of the gravel beach toe and the gravel beach slope, greatly affect the threshold criteria for overwash.

Observation of Wave Transformation on Macro-tidal Rocky Platforms

ABSTRACT Poate, T.G.; Masselink, G; Austin, M.; Dickson, M.E., and Kench, P., 2016. Observations of Wave Transformation on Macro-Tidal Rocky Platforms. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 602–606. Coconut Creek (Florida), ISSN 0749-0208. Correctly predicting the transformation of ocean waves across rocky platforms has direct implications for cliff stability modelling, coastal defences and long-term coastal evolution. Wave transformation across rocky intertidal platforms is dependent on the morphological characteristics of the platform, including platform width, slope and roughness, and forcing characteristics, including wave and tide conditions. In this paper we present early observations from four field studies providing detailed measurements of wave processes across contrasting rocky platform sites with wave conditions between Hs = 0.5 m and Hs = 1.9 m, water depths between h = 0.5 m and h = 6.8 m and variable platform morphology. Results show that the relative wave height in the surf zone H/h is generally larger than in previous studies (H/h is c. 0.6, instead of 0.3–0.5) and wave dissipation greater for sites with considerable roughness.

Storm-driven cusp behaviour on a high energy gravel beach

ABSTRACT Poate, T.G., Masselink, G., McCall, R.T., Russell, P.E., Davidson, M.A., 2014. Storm-driven cusp behaviour on a high energy gravel beach. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 645–650, ISSN 0749-0208. Gravel and mixed sand-gravel beaches are characterised by steep reflective profiles which provide effective forms of wave absorption and therefore coastal defence to many mid-latitude regions, including northwestern Europe and North America. In the UK the combination of energetic wave conditions and large tides creates very dynamic and responsive morphology often dominated by cuspate features. Recent storm-responsive field campaigns at Loe Bar, Cornwall, UK, have captured highly energetic wave conditions (Hs = 2.5–5.8 m) using temporary video camera installations, low tide 3D topographic surveys with real time kinematic GPS, local tide level measurements and inshore directional wave data. Characterised by fine gravel (D50 = 3 mm) and a steep reflective profile (tanβ = 0.118), the barrier at Loe Bar is exposed to an annual 10% exceedence significant wave height Hs10% of 2.4 m arriving predominantly from the southwest (Atlantic Ocean) and shore-normal to the beach. Under medium-wave conditions (Hs = 2–3 m), contrasting cusp behaviour was recorded with accretion and erosion, principally, through horn growth and decay (bed-level change Δz = c. 1 m). During more energetic conditions (Hs = 5.8 m), the morphological response is more consistent and the waves drive erosion of the lower profile causing bed-level changes over a tide in excess of 1.5 m. Very rapid recovery to pre-storm bed levels is observed with defined cusp evolution occurring within 12 hours during the falling limb of the storm as incident wave energy decreases. The unique gravel cusp dataset suggests free behavior due to cusp morphodynamic feedback, rather than hydrodynamic forcing, plays an dominant role in cusp evolution.

The Role of Bed Roughness in Wave Transformation Across Sloping Rock Shore Platforms

We present for the first time observations and model simulations of wave transformation across sloping (Type A) rock shore platforms. Pressure measurements of the water surface elevation using up to 15 sensors across five rock platforms with contrasting roughness, gradient, and wave climate represent the most extensive collected, both in terms of the range of environmental conditions, and the temporal and spatial resolution. Platforms are shown to dissipate both incident and infragravity wave energy as skewness and asymmetry develop and, in line with previous studies, surf zone wave heights are saturated and strongly tidally modulated. Overall, the observed properties of the waves and formulations derived from sandy beaches do not highlight any systematic interplatform variation, in spite of significant differences in platform roughness, suggesting that friction can be neglected when studying short wave transformation. Optimization of a numerical wave transformation model shows that the wave breaker criterion falls between the range of values reported for flat sandy beaches and those of steep coral fore reefs. However, the optimized drag coefficient shows significant scatter for the roughest sites and an alternative empirical drag model, based on the platform roughness, does not improve model performance. Thus, model results indicate that the parameterization of frictional drag using the bottom roughness length-scale may be inappropriate for the roughest platforms. Based on these results, we examine the balance of wave breaking to frictional dissipation for rock platforms and find that friction is only significant for very rough, flat platforms during small wave conditions outside the surf zone.