Production of flood hazard maps and flood hazard class maps for two areas of significant flood impact in the Republic of Slovenia

The scope of the task is set out in the Framework Programme for the Implementation of the Directive on the Assessment and Management of Flood Risks for the period 2009–2015 (Ministry of the Environment and Spatial Planning of the Republic of Slovenia, May 2009). The legal framework for the task is set out in Article 6(5) of Directive 2007/60/EC (OJ EU, No L 288), which has been transposed into Slovenian law by Articles 11, 12 and 13 of the Regulation on the content and method of preparation of a detailed flood risk reduction plan (Official Gazette of the Republic of Slovenia, No. 7/2010), adopted pursuant to Article 60a of the Act Amending the Water Act (Official Gazette of the Republic of Slovenia, No. 57/2008). The primary purpose of the task is to provide a technical basis for setting objectives and measures within the framework of flood risk reduction planning in 2014 and 2015 for 61 areas of significant flood impact. Articles 11, 12 and 13 of the Regulation on the National Flood Risk Assessment (NZPO) set out the method, content and timetable for the preparation of flood hazard and risk maps. With regard to the method of preparing the maps, it is stipulated that the Ministry must ensure that the maps are prepared at the most appropriate scale for areas of significant flood risk, in accordance with the regulation on the methodology for determining flood areas at the level of river basin districts under the Water Act. Flood hazard maps cover geographical areas where the following types of flooding may occur:

floods with a low probability of occurrence, or floods resulting from exceptional events,
floods with a medium probability of occurrence, i.e. a 1 per cent probability of occurrence,
floods with a high probability of occurrence, where appropriate.

For each type of flood referred to in the previous paragraph, the following information must be provided:

the extent of the flooding,
water depth or water level, as appropriate,
flow velocity or the corresponding discharge, where appropriate.

Flood hazard maps and flood hazard class maps have been produced for the areas of significant flood impact in Izola and Piran (analysis of the risk of coastal flooding). For the first time, wave conditions have been calculated for the entire territorial sea of the Republic of Slovenia, and water levels have been assessed for the simultaneous occurrence of probable high tide and waves in accordance with the Regulations for the entire coastline. The internationally recognised and validated DHI MIKE 21 NSW numerical model was used to calculate wave propagation. Wave calibration was only possible using measurements at a single location (off Piran Point), namely at the Vida buoy. In 2014, two further buoys were installed; however, the measurements taken over a period of several months do not provide sufficient data for further calibration. For proper calibration, particularly to achieve a more accurate calibration of wave propagation along the coast, wave measurements should also be carried out at several points along the coast. It should also be emphasised that the calibration of a wave model is not as straightforward as, for example, the calibration of hydraulic models of watercourses, as, for a given wind strength and wind direction, there may be significant variation in the measured wave height data; this is because, due to its quasi-stationary nature, the model cannot account for previous waves and variable wind conditions. Calibration was therefore carried out in such a way that the specified wave height at the open edge of the model, for the selected wind conditions, does not decrease across the computational grid, whilst at the Vida buoy location it remains within the range of the measured data. For more accurate calculations of wave propagation, we recommend establishing periodic or continuous measurements of wave height near the coasts of Koper, Izola, Piran and Portorož.

All surface level combinations (G10, G100 and G500) include the probable simultaneous occurrence of maximum tide and waves, based on the probability of the event occurring in accordance with the Regulations. The Regulations do not specify the probability of waves occurring (height) in combination with the probability of high tide occurring (height). Nor do they specify which wave height is to be used for calculating water levels, e.g. mean height, typical height or maximum height. Furthermore, the Regulations do not allow for the mitigation (or even exclusion) of less likely scenarios, e.g. the simultaneous occurrence of a high tide and winds from the bora or tramontana directions, as, under such wind conditions, air pressure is not usually very low, and the wind does not ‘push’ the sea into the Gulf of Trieste as it does when the wind is from the south. High tides are, in fact, almost always accompanied by a southerly wind. In any case, it should not be assumed that the G100 water level, as defined in the Regulations, corresponds to an event with a probability of exceedance of 0.01 in a given year, or an event with a 100-year return period. In fact, it is an event with a considerably lower probability of occurrence. The same applies to the G10 and G500 levels. The risk of coastal flooding, the determination of which (methodology) is defined by the Regulations, is therefore perhaps exaggerated, and in light of the study’s findings, it might be sensible to somewhat relax the provisions of the Regulations.

The characteristic wave height, which is usually provided as an output by numerical models, was used to calculate the water levels. The characteristic wave height is greater than the mean wave height and less than the maximum wave height; it may therefore be more suitable for calculating the range of sea waves along the coast than the maximum wave height or the mean wave height, as waves lose energy upon contact with coastal infrastructure (e.g. walls, breakwaters, buildings, etc.), and their range depends on the amount of energy lost relative to the specific type of coastal infrastructure. Energy dissipation and wave overtopping of coastal infrastructure is a hydraulic phenomenon that is typically highly three-dimensional and requires consideration on a case-by-case basis. A reduction in wave height due to the influence of coastal infrastructure is inevitable but uncertain, partly because of the interaction between different types of infrastructure, as waves do not always propagate in a straight line towards the coast, and oblique waves may increase in height due to the influence of coastal infrastructure. An empirical reduction in wave height based on individual types of coastal infrastructure could lead to an increased risk, as it would not account for the hydraulic phenomena resulting from the simultaneous operation of individual infrastructure elements along the coast. Whilst using the maximum wave height to calculate wave run-up provides the greatest certainty, it also imposes the greatest spatial constraint. It is assumed that the energy of the highest waves is reduced upon contact with coastal infrastructure to such an extent that the wave height at the coastal infrastructure is equal to the characteristic wave height. For more accurate calculations of wave propagation in the coastal zone (on land), it would be necessary to use other numerical models that can also take into account the influence of coastal infrastructure.

Although the areas under consideration in Piran and Izola cover only a small part of the coastline, wave modelling was carried out for the entire territorial sea, and the envelope of maximum wave values was calculated for the entire coastline and projected onto 100-metre sections of the coastline. This data (wave height, directions, sea level and mean sea level) can be further utilised by spatial planning authorities to assess the risk of coastal flooding in other sections as well. Such a need has already become apparent, as previous approaches to wave calculation were not only flawed but also always of an empirical nature. The results of wave propagation modelling can also be applied more broadly, for example:

as a boundary condition for more accurate modelling of wave propagation within ports, marinas and moorings;
to calculate wave-induced loads when designing coastal infrastructure, as the numerical model also provides the energy of the wave spectrum as a result;

to identify areas of Slovenian waters where vessels can take refuge during periods of high waves.

Link to further content:

Production of flood hazard maps and flood hazard class maps for two areas of significant flood impact in the Republic of Slovenia. Annual report on work carried out under task I/2/1, task leader: Blažo Đurović. Water Institute of the Republic of Slovenia, September 2014. Client: Ministry of the Environment and Spatial Planning, IzVRS work programme for 2014.