Supervisors info:
Ευθύμης Λέκκας, Καθηγητής ΕΚΠΑ
Εμμανουήλ Σκούρτος, Ε.ΔΙ.Π. ΕΚΠΑ
Βαρβάρα Αντωνίου, Ε.ΔΙ.Π. ΕΚΠΑ
Ανδρέας Βασιλόπουλος, Καθηγητής Γεωπονικού Πανεπιστημίου
Summary:
In the present study we are going to analyze the anti-flood protection of Great Rafina Stream and suggest a system of early warning for the prevention of consequences arising due to flooding. Floods have been more severe in the last decades mainly due to urbanization and climate change. At the same time there is an increase in the effects of this phenomenon in the natural as well as in the urban environment. Early warning systems are the most widespread means of direct updates for public entities and citizens regarding the possibility of a flood.
Great Rafina Stream is situated in Mesogeia of Eastern Attika and more specifically in the south of mount Penteli and Northeast of mount Ymittos. The total expanse of the drainage basin is 140 km2 and crosses the jurisdiction borders of the municipalities of Rafina – Pikermi, Spaton – Artemidas, Pallinis and Paiania. These urban areas are expected to expand and at the same time increase their population, therefore the immediate anti-flood protection of the area is necessary. The hydrographic network of the drainage basin is composed of several small streams, which drain to a main axis. It crosses fields and its average decline is 1.5%. The average altitude of the drainage basin is over 170m and is mostly classed as flatland. Quaternary deposits, neogenic formations, mesozoic - paleozoic formations and explosive occurrences occur geologically. Climatically, heavy rainfall is shown during the period from October to March. The stream runs a high risk of flooding especially for reference period greater than 20 years (T>=20). In the past (1977, 1988 and 2013) there have been floods which resulted in the loss of human lives and the destruction of infrastructures. The main matters that need to be resolved or prevented, concern the causes of the flood and are as follows:
• The high floodwaters and the inability of the riverbed to drain these floodwaters
• The morphology as well as the narrowing of the riverbed in individual parts due to vegetation, accumulation of loads and cross-sectional technical works
• Restricting or even covering the stream bed to serve various anthropogenic uses
An important flood management plan for the region has been prepared under Community Directive 2007/60/EC. Based on the flood scenarios studied for a T=50 years recovery period, the peak flow rate is 552 m3/s, for T=100 years is 701.3 m3/s and for T=1000 years at 1,568.7 m3/s. In the optimistic scenario, small floods will affect cultivated areas in the south of Pikermi for T = 50 years. For T=1000, the areas of Petreza and Iremos Pefkos are more affected in the south of Pikermi and to a lesser extent Rafina (estuaries). For T=1000 it mainly affects the city of Rafina, especially in the last 1.5 km of estuaries. Actions are proposed to address the floods, but few are implemented. Targeted flood protection projects are proposed by the Environmental Impact Study for settlement – delimitation of Rafina stream. Proposals concern the construction of cross-sections of various types along the stream (Phase A) and the construction of a dam in the next 10 years (Phase B) and in particular:
• Downstream section of Rafina stream (from estuary -4+400) 4.4 km long
• Upstream section of Rafina stream (4+400-14+997) 10.6 km long
• Flood Restraint Dam
Based on the estimates for T=50 (reference value for flood estimation) the benefits for the different scenarios are as follows:
• Current status: 484.9 (Peak m3/s)
• Future status: 519.9 (Peak m3/s)
• Future with projects: 541.9 (Peak m3/s)
• Reflection in the plain: 520.2 (Peak m3/s)
• Dam with overflow correction +61.5: 458.4 (Peak m3/s)
• Mountain Hydraulic projects: 517.4 (Peak m3/s)
From the data of the Directive and the Environmental Impact Study the significant degree of risk of flooding is recognized. For this reason, setting up and operating an early warning system is proposed to fill the prevention gap until the flood protection projects are implemented. Even after completion of projects the system will be an important additional precautionary measure.
The proposed system consists of 4 interconnected parts. The first concerns risk knowledge where it is considered satisfactory as recent flood events in Attica (e.g. Mandra) have increased the knowledge and experience of citizens and government. The second part concerns monitoring and warning services and in particular:
• Current level measurement sensors at various high risk locations: Measuring the level combined with rainfall data will extract data for early warning of flood events. For example, if the sensors detect streamflow at specific points and with preceding particular millimeters of rainfall, it may, in the future, be able to warn about the danger early when it receives that particular rainfall value. On the basis of the projected flood plans and the forecasts for the current flow, the main areas to be sensed are the downstream (estuary) and the upstream south of Pikermi as it is estimated that flooding periods greater than 20 years flood phenomena will be manifested. The key features that the sensors need to have are security and accuracy in collecting and transmitting data, low maintenance requirements and self-sufficiency as well as endurance against extreme weather conditions
• Remote sensing data: The collection of remote sensing data such as aerial photography, satellite imagery etc. after the occurrence of a flood helps to identify the signs of overflow. Remote sensing to estimate soil moisture, using solar radiation with wavelengths between 0.4 and 2.5 µm, measures the reflected sunlight from the Earth's surface. The most appropriate method of measurement is that of thermal inertia which combines simplicity and ease of application while achieving high accuracy in estimating soil moisture. Three satellites that could provide such data and have been called in to monitor flood events are ERS-2, ERS-1 and Sentinel-2. For accurate results, the use of image collapse technique is required, where color and multi-spectral images must be recorded at close dates
• UAV data: They provide the ability to collect airborne data, overview of existing coverages and changes that occur in surface area. They provide high-precision aerial photographs, with greater directness (as opposed to satellite) and with territorial coverage even in inaccessible areas. Aerial photography is processed by specialized software (e.g. Postflight Terra 3d) and produces orthophotos for better recording, flood management and impact assessment in Rafina stream.
• Wireless data transmission over mobile (3G or 4G)
• Data processing by specialized software and database use for estimating flood events: The meters will produce stream level data at various points around the clock. The level, above which a flood is caused, will be set in advance for each meter (point of interest). In addition, data from satellite images and orthophotos will be imported. Finally, the meteorological data on rainfall are introduced. Combining all this data will constantly increase the accuracy of the results and reduce the chance of error. The General Secretariat for Civil Protection is the body that operates and monitors the system and is responsible for sending warnings or alerts to the competent public bodies (Attica Region, Municipalities) and citizens
The third part deals with the dissemination of information and early warning. As soon as there is a risk of flooding, it is important that the information reach competent authorities and citizens immediately. It is proposed that we install a civil protection siren in the settlements near the stream and especially in the city of Rafina for the immediate warning of the citizens in case of extreme storm. This system will also have to be connected to emergency number 112 to send immediate alerts to citizens and municipalities. Forecasting based on meteorological data in most cases will take place one to two days in advance and thus the General Secretariat for Civil Protection will be able to update in a timely manner (mainly via 112). Finally, the fourth part deals with the responding ability of government, security forces and citizens, as well as training and information on operational parties and how to react in case of alarm. The ability to respond is just as important as the dissemination of information. The government and especially the services and employees of the Attica Region and Municipalities must be adequately trained to deal with the effects of floods. The same applies to citizens who need to be informed locally about the risk and the potential impact it may have on their health and property. It is important that a plan for evacuation of settlements be drawn up and especially for the city of Rafina as well as periodic training for evacuation of buildings and the road network. Equally important is the role of the security forces (army, police and fire brigade) and hospitals. All of this should be coordinated by the General Secretariat for Civil Protection and there should be training programs, scripts and coordination exercises for the above forces and public entities.
In conclusion, Rafina stream has a significant degree of flood risk and flood protection projects are costly (e.g. dam construction) and will be implemented over a period of time (approximately 10 years). Thus, it is imperative that the proposed flood warning system be immediately installed in order to mitigate the expected impacts.
Keywords:
Early Warning System, Great Rafina Stream, floods, anti-flood protection, prevention of natural disasters
