Eresos basin (Lesvos) was selected for detailed study, as it is the only pilot beach for which there is a sufficient hydrographic network of constant flow and available data. The basis was investigated using 3 different approaches: (1) Focusing on 2 ephemeral streams (Karasaris and Chalandra) and study the hydraulics of riverbeds; (2) Studying the river basin as a whole through continuous hydrologic modeling and sediment transport assessments on watershed scale; and (3) Focusing on the artificial water reservoir (dam) inside the watershed and studying its siltation rate.
Hydrographic network including the location of cross-sections used in HEC-RAS modeling and Eresos dam
The first phase of the study involved the collection/analysis of the available meteorological data, using the Hydrognomon software (developed by the National Technical University of Athens (NTUA)) and calculating the meteorological parameters which were then used as inputs into the modeling software. A soil map of Lesvos was created through combination of data from the European Soil Database (ESDB) of JRC, the geological maps of Institute of Geological and Mineral Exploration (IGME) and soil maps from the Forestry Department of Ministry of Agriculture. In addition, a topographical survey of Karasaris and Chalandra riverbeds, using (RTK-DGPS) took place together with collection of soil samples from the riverbed for granulometric analysis. Finally a bathymetric survey of the reservoir was conducted for the estimation of siltation rate of the dam.
The hydraulic study involved an Intensity-Duration Frequency (IDF) analysis, to generate the flood magnitudes for return periods of 20, 100 and 500 years, using the rational formula. The hydraulic simulation was performed using the Hydraulic Engineering Center – River Analysis System (HEC-RAS), developed by the U.S. Army Corps of Engineers. HEC-RAS is an up-to-date software for hydraulic analysis, with which the user interacts through a figure simulating the river. It has the ability to simulate the water free surface under conditions of steady, unsteady gradually varied flow, the subcritical, critical and supercritical or mixed state of flow, the sediment transport and mobile bed transport, and water temperature. It requires as inputs, geometrical, steady and unsteady flow data, and sediment data for the sediment transport simulation. For this study, the data used were the calculated flows for the different return periods, the topographic survey cross-sections and the granulometric analysis, from which resulted an estimation of the riverbed erosion behavior during extreme events, the major process of downstream sediment transport in ephemeral rivers, and the supply with sediment to the beach.
Hydrologic modeling of the river basin was carried out using the Soil and Water Assessment Tool (SWAT) of Agricultural Research Service of USDA (ARS-USDA). SWAT is a semi-distributed, semi-empirical watershed management model that can simulate the hydrology, sediment transport, transport of nutrients (nitrogen and phosphorus), pesticides and microorganisms and evaluate hypothetical scenarios on a watershed scale level. It requires as inputs, time-series of daily temperature and precipitation, or of a smaller time-step, the digital terrain model (DTM), the soil and land use map of the area, all of them are available by the National Meteorological Service (N.M.T.) and European Union databases. It is calibrated depending on the process, by using daily streamflow measures for the hydrologic component, daily or weekly measures for the sediment transport component, and weekly or of a larger timestep measures for the rest. The primary and most important part of the modeling procedure is the hydrologic part due to the fact that water movement is the major factor of sediment, nutrient and pollute transport. However, usually in the case of small river basins of intermittent or ephemeral streams there are no streamflow records, and there is complete lack of continuous sediment transport records, forcing either the collection of new measurements, or using alternative calibration schemes (like the importation of calibrated parameters from neighboring watersheds with available records) or making assumptions that reduce the accuracy of the results. After calibration/validation, the model can study different scenarios to evaluate the results of management practices.
The study of the dam in the upstream section of Chalandra stream included the estimation of the trapped sediment volume in the dam from the commencement of operation until 2016 and the derivation of a yearly average siltation rates. The initial topographic survey (prior to construction) and the bathymetric information acquired in within the framework of the ERA-beach project were used. In addition, the siltation rate was also estimated using the result of 210Pb isotope dating analysis of a sediment sample from the bed of the dam.
