Paolo Ciavola

Flow Resistance and Associated Sedimentary Processes in a Spartina maritima Salt-Marsh

Abstract The vertical accretion of salt marshes is mainly due to flow reduction and wave damping by vegetation. However, the details of the hydrodynamics are only partially understood, and have been studied mainly in the laboratory. This study presents detailed field investigations of the water flow in a Spartina maritima salt-marsh in the Ria Formosa, a shallow, meso-tidal lagoon in Southern Portugal. Detailed velocity profiles were obtained within and above the 30 cm high canopy using a high-precision velocimeter. Results show that the influence of the bottom becomes negligible a few centimetres above the bed, and that the flow depends on the vegetation density at each level of the canopy. When the canopy is partially emergent or is only slightly submerged, the upward increase of horizontal velocity is roughly linear. A more drastic flow reduction exists when the canopy is well submerged, with a slow, nearly constant velocity in the denser part of the canopy and a faster, logarithmic shaped velocity profile above. This dampening effect of the vegetation is expected to promote sedimentation. However, the short-term sedimentation rate obtained with sediment traps during fair-weather conditions is usually lower in the Spartina marsh than in the surrounding areas. Therefore, the effect of the Spartina canopy for sediment accumulation seems to be more that of erosion protection during storms than of sedimentation enhancement during normal conditions. Using these results, a simple conceptual model is proposed for the sedimentary processes taking place in the intertidal areas of the studied lagoon.

Field observations of sand-mixing depths on steep beaches

Abstract A series of field studies were carried out on three Portuguese beaches (Garra˜o, Faro and Culatra) to assess sand-mixing depths under a range of wave heights (0.34–0.80 m), wave periods (5.1–7.0 s) and mean grain sizes (0.26 and 0.38 mm). All the studied beaches had a reflective profile with a steep upper foreshore (tanβ of 0.10–0.14) and a more gentle low-tide terrace. In all experiments, plunging waves were breaking on the beach face. The study has identified a linear correlation between significant wave height at breaking ( H b ), and average sand-mixing depth ( Z m ), whereby Z m = 0.27H b . The empirical relationship is ten times larger than a previous one proposed by other authors working on gentle-slope dissipative beaches, and confirms previous findings on similar reflective beaches carried out in the USA. It was also confirmed that Z m is related to wave period, but does not seem to be a function of mean sand size.

Title: MICORE: DUNE EROSION AND OVERWASH MODEL VALIDATION WITH DATA FROM NINE EUROPEAN FIELD SITES

In this paper we present the first results of beach profile hindcasting with XBeach using recently measured coastal data acquired under storm conditions at eight European sites, including a comparison to model results obtained with off-the- shelf models. The results show consistently that the XBeach has skill in predicting the coastal profile, albeit that in most cases the erosion around the mean water line is overpredicted and the depositions at the lower beach face are overpredicted. The causes for this model effect are under active investigation but not resolved yet. Likely candidates are the modeling of onshore (asymmetry) transports which reduces the offshore transports due to undertow (currents) or the modeling of sediment motion in the swash zone.

Relation between river dynamics and coastal changes in Albania: an assessment integrating satellite imagery with historical data

The geomorphology of three major river deltas in Albania (Shkumbini, Semani and Vjose) was studied using Landsat multi-spectral imagery. Palaeohydrological studies using meander geometric properties concluded that all rivers decreased their discharges in the first half of the present century due to land reclamation and creation of irrigation networks. The comparison between paleohydrological assessments and recent measurements at gauging stations confirmed that simple formulae overestimate mean water discharge. The deltas of the three rivers have a rapidly changing morphology, so that the traditional tripartite delta classification has a limited applicability. A comparison between the sediment yields of these rivers with worldwide examples points out that they are extremely active hydrological systems.

RISC-KIT: Resilience-Increasing Strategies for Coasts - toolKIT

ABSTRACT Van Dongeren, A., Ciavola, P., Viavattene, C., De Kleermaeker, S., Martinez, G., Ferreira, O., Costa C., R. McCall, 2014. RISC-KIT: Resilience-Increasing Strategies for Coasts–toolkit. 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. 366–371, ISSN 0749-0208. Recent and historic high-impact events have demonstrated the flood risks faced by exposed coastal areas. These risks will increase due to climate change and economic development. This requires a re-evaluation of coastal disaster risk reduction DRR strategies and prevention, mitigation and preparedness PMP measures. To this end, the UN Office for Disaster Risk Reduction formulated the Hyogo Framework for Action, and the EU has issued the Floods Directive. By their nature, neither is specific about the methods to be used to assess coastal risks, particularly those risks resulting from dune and structure overtopping, the non-stationarity of surge and flash flood events, and coastal morphodynamic response. This paper describes a set of open-source and open-access methods, tools and management approaches to fill this gap. A Coastal Risk Assessment Framework will assess coastal risk at a regional scale. Thus critical hotspots can be identified for which an impact-oriented Early Warning System/Decision Support System is developed. This can be applied in dual mode: as a forecast and warning system and as an ex-ante planning tool to evaluate the vulnerability. The tools are demonstrated on case study sites on a range of EU coasts with diverse geomorphic settings, land use, forcing, hazard types and socio-economic, cultural and environmental characteristics. Specific DRR plans will be developed for all sites. A management guide of PMP measures and management approaches is to be developed. The toolkit will benefit forecasting and civil protection agencies, coastal managers, local government, community members, NGOs, the general public and scientists.

On the displacement of marked pebbles on two coarse-clastic beaches during short fair-weather periods (Marina di Pisa and Portonovo, Italy)

The aim of the investigation was to define the mechanisms of sediment transport in the swash zone of microtidal coarse-clastic beaches in the very short term by evaluating the displacement rates of marked pebbles under low-energy wave conditions. Tests were performed at two sites (Marina di Pisa, Ligurian Sea, and Portonovo, central Adriatic Sea) to check the consistency of the data over a range of different grain sizes. Two recovery campaigns were carried out at both sites, one 6 h and the other 24 h after the injection. During the experiments wave action was at a minimum (wave heights never exceeded 0.3 m). The results show that 20% of pebbles ranging in diameter from 30–90 mm moved significantly (more than 0.5 m) already 6 h after the injection, with some tracers being lost (3%). After 24 h, 40% of the pebbles were significantly displaced and 10% were lost. The preferential downslope movement of tracers, which suggests that coarse sediment movement under low-energy conditions is mainly controlled by gravity processes enhanced by steep beachface slopes, represents the novelty of the results reported here. It would appear that swash processes on low-energy beaches cause a significant rate of pebble displacement through the destabilization induced by wave uprush and backwash. Despite the microtidal range, the position of the mean water level plays a major role in changing the beach level at which swash processes can actually trigger pebble movement. The results of this study show that considerable, and mostly seaward-directed, coarse sediment transport takes place even during short fair-weather periods.

Sedimentation Processes on Intertidal Areas of the Lagoon of Venice: Identification of Exceptional Flood Events (Acqua Alta) Using Radionuclides

ABSTRACT Particular atmospheric conditions produce frequent storm surges in the Lagoon of Venice, locally called “acqua alta”: the highest event of this kind was registered in 1966. The process became of some importance in the last 100 years, when man-made subsidence caused a gradual sinking of the town and lagoons bed. Four cores were collected in a range of intertidal environments of the Northern lagoon to undertake radionuclide studies using profiles of natural 210Pb (using the Constant Rate of Supply and Constant Initial Concentration models) and anthropogenic 137Cs. The best agreement between the three dating methods was found at San Giacomo, an eroding saltmarsh at the edge of a navigation channel, with a sedimentation rate ranging from 0.22 to 0.29 cm yr−1. This site recorded the flood of 1966 as a characteristic break in the 210Pbex profile, as dated by the CRS model. For the Cona tidal flat, both the CRS model and the position of the Cs peak-marker gave similar accretion rates, 0.16 and 0.18 cm yr−1. However, two different CIC accumulation rates were calculated, 0.29 cm yr−1 for the deepest section of the core and 0.17 cm yr−1 for the uppermost part. The break in the 210Pbex profile, again corresponds to the flood of 1966. The effects of subsidence were recorded as an increase in accumulation rate between 1910 and 1931, when there were up to 15 floods per year (1926). Higher sedimentation took place during the period 1958–1973, when years with over than 50 flood events were frequent. The maximum deposition rate (0.43 cm yr−1) occurred again around 1967, consistent with the record of the exceptional flood, if the accuracy of the dating is taken into account. The sedimentation rates calculated for the two other mudflats, Rosa and Saline, were more problematic to interpret because of downcore mixing and/or the occurrence of reducing conditions.

Large-scale morphodynamic experiments on a beach drainage system

This research describes a unique set of experiments undertaken at the Large Wave Channel (Hannover) aimed to study the morphodynamic behaviour of a beach face containing a buried drainage system. The advantage of using a large wave flume is that scale effects that affected previous laboratory experiments are minimized. This compares the response of the undrained beach under controlled wave forcing with the response of the drained one with the same wave action. Low, medium and high levels of wave energy were used for the experiments, with significant wave heights of 0.39–0.83 m and wave steepnesses between 0.004 and 0.013. Any positive effect of the drains on the beach face was confined by the position of the cone of depression in the aquifer surface. The best performance of the system was observed with two drains operating at the same time under low to moderate wave energy.

Performance of a Beach Dewatering System—Chiaiolella Beach, Procida Island, Italy

Abstract Chiaiolella Beach is a 1.5-km-long pocket beach located on Procida Island (Italy). The beach is formed by medium sand generated by erosion of vulcanoclastic rocks that form cliffs around the island and at the backshore of the study site. A beach dewatering system was installed in April 2002 to provide a wider beach for users and to prevent cliff undercutting during storms. It has not been operative since December 2004, as local authorities decided to undertake cliff stabilization measures. Surveys carried out immediately before (October 2001) and 1 year after installation (October 2002) showed negligible volumetric change on the drained beach in Ciraccio (−0.52 m3/m), the NNE sector of Chiaiolella Beach, whereas on the undrained control section there was an accumulation of 1.41 m3/m. Since no long-term monitoring of beach profiles was available at the site previous to installation, to obtain an indication of the nearshore hydrodynamics and morphodynamics in absence of the drain, historical shoreline from high-resolution georeferenced aerial photography and numerical model were used. Overall, no evident positive effects of the dewatering system were confirmed, at least for mild wave energy levels. For higher wave energies, e.g., during a storm that occurred in December 2002, part of the beach (SSW sector, Ciracciello) had consistent volume losses and the dewatering system was not able to compensate them, as at some stage it was even damaged. This study points out the inadequacy of the dewatering system as coastal protection under high wave conditions.

Beach sediment mixing under drained and undrained conditions

ABSTRACT Ciavola, P., Contestabile, P., Aristodemo, F., and Vicinanza, D., 2013. Beach sediment mixing under drained and undrained conditions. The paper describes a set of experiments undertaken at the Grosser Wellen Kanal (GWK) in Hannover on sediment mixing on a beachface exposed to the action of a buried drainage system. Experiments were performed using a set of drains (up to three working simultaneously), located at variable distance from the shoreline. Wave energy during the tests ranged from medium (0.59 m < Hs < 0.61 m) to high (0.76 m < Hs < 0.83 m). Beach profiles were measured at the end of each test. Further validation came from measurements using a grid of rods that were monitored at the same time as the beach profiles were surveyed. While some beach stabilization was observed for low energy conditions, for medium and high energy conditions the benefit of having the drains operative was not so clear. The comparison between beach profiler and rods showed good agreement regarding relative vertical accretion and erosion events. In any case, it was evident that the sedimentary effect on the beachface was confined to a narrow area above the drains. The drained area showed slightly lower values of sediment mixing under drained conditions, but evidently this factor was not important enough to trigger a widespread beach stabilization. Values of sediment mixing averaged across the whole beachface ranged between 2–4 % and 4–6 % of the breaking wave height respectively for high and medium energy wave conditions. Grain size, beach slope and drain configurations used here are fully comparable to field installations present in the literature.