Hydrological Investigation of the Catastrophic Flood Event in Mandra, Western European Geosciences Union General Assembly 2018, 8 – 13 April, 2018 Vienna, Austria NH1.3/HS11.27 – Flood Risk and Uncertainty (co-organized) Ch. Ntigkakis (1), G. Markopoulos-Sarikas (1), P. Dimitriadis (1), Th. Iliopoulou (1), A. Efstratiadis (1), A. Koukouvinos (1), A. D. Koussis (2), K. Mazi (2), D. Katsanos (2), and D. Koutsoyiannis (1) (1) National Technical University of , Water Resources & Environmental Engineering, ; (2) Institute for Environmental Research & Sustainable Development, National Observatory of Athens, Greece

1. The mysterious storm Observed 30-min Rainfall 3. Rainfall estimation through inverse hydrological modelling INTRODUCTION 12 On 14-15/11/2017, a flash flood occurred in Mandra is a small industrial city, located 40 km west of Athens, 10 Flood arriving • Problem statement: Estimation of rainfall from Nov. 14 10:00 am to Nov. 15 10:00 am, resolved in 30-min intervals (48 values), at a 8 in Mandra Western Attica (west of Athens, Greece) causing that has significantly grown during the last years. The city is Vilia hypothetical X-station, located in the part of Sarantapotamos basin that has been considerably affected by the storm event. 24 fatalities and substantial damages in the city crossed by two small ephemeral streams (Soures, Agia Aikaterini) 6 Mandra •

Rain (mm) Rain Key assumption: The point rainfall at X-station controls 80% of the runoff of Sarantapotamos basin, upstream of Gyra Stefanis; the remaining of Mandra. The storm causing the flooding was draining an area of 75 km2. 4 Elefsina runoff is controlled by the point rainfall at Vilia station, thus the areal rainfall is 0.8*Xrain + 0.2*ViliaRain. 2 intense, but its spatio-temporal characteristics At around 7:00 am on November 15th, a large and fast-moving 0 • Methodology: Inverse calibration against the observed hydrograph at Gyra Stefanis station and a h(t) = 0.2*Vilia + 0.8*Xstation are unknown. A debate ensued on whether the flood wave, carrying heavy sediment loads, arrived in Mandra. At simple event-based hydrological model, comprising two components: devastating results were due to an extreme that time, and for several hours earlier, the weather around the rainfall or due to poor flood protection works. Parameters city area was quite clear. Taking this into account, as well as the • the SCS-CN relationship for extracting the effective, he(t), from the total rainfall, h(t), using two CN and I

a SCS-CN procedure

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14/11/2017 16:00 14/11/2017 18:00 14/11/2017 22:00 14/11/2017 12:00 15/11/2017 15/11/2017 10:00 15/11/2017 14:00 15/11/2017 The study reported here contributes to resolving small extent of the catchment upstream of Mandra, it becomes 20:00 14/11/2017 parameters, the curve number, CN, and the initial abstraction, Ia. this question (1) by presenting information apparent that the flood event was due to an unusual storm, of • a lag-and-route model for propagating the generated runoff, r(t), to the basin outlet, q(t), with gathered from several sources, including extreme intensity yet very local scale. parameters a recession coefficient k and a lag time τ. r(t) = h (t) – h (t – 1) hydrometric data from the neighboring basin of XPOL-NOA accumulated rainfall (mm) e e This assumption is supported by the observed rainfall at the • Hydrological scenarios: Model runs with two parameter sets; for both scenarios we set CN = 60, as Sarantapotamos, point rainfall data from the meteorological stations in Mandra, Elefsina and Vilia, all located in estimated from flood-event analyses during 2012-2014. 250 s(t) = s(t – 1) + r(t) – q(t) wider area of interest, estimates of areal rainfall the wider area around Mandra, but outside of the two catchments • Calibration assumptions: Fitting to measured flow data (until Nov. 15 9:00 am, Q = 50 m3/s), based on satellite images and a meteorological of interest, Soures and Agia Aikaterini. Nevertheless, the volume max radar, and audiovisual material, and (2) by 200 generation of flows quite larger than the capacity of the culvert (~100 m3/s) during the morning hours of the observed rainfall in those stations is not significant enough Parameters attempting to unravel the flood event via of Nov. 15 (thus taking advantage of the known overflow of the bridge, which is key information). q(t) = k s(t – τ) to explain such a severe flooding. k and τ reverse rainfall-runoff modelling; further, it 150 • Probabilistic analysis: Use of a given idf relationship (ombrian curve) from the station in Elefsina, to The mysterious rainfall event can be better understood with the analyzes the available data to approximately estimate the return period, T, of the two point rainfall scenarios at the X-station, for temporal scales Hydrological scenario A B help of the (approximate yet indicative) rainfall information estimate the return period of the storm event. (durations, d) from 0.5 to 24 hours; its analytical expression is: Initial abstraction, I (mm) 20 10 recorded by an X-band weather radar, which shows an elongated 100 a -1 and narrow core of the storm passing outside the area covered by i = 213.4 (Τ0.125 - 0.641) / (1 + d/0.124)0.622 Recession coefficient, k (h ) 0.15 0.05 Lag time, τ (h) 3.0 2.0 the three stations. 50 where i is the rainfall intensity (mm/h). The rainfall pattern estimated based on the radar data agrees with Sketch of flood simulation procedure and 0 parameter values considered for the two reports by residents in the catchment upstream of Mandra. 14-Nov-2017 13:49 to 15-Nov-2017 12:00 UTC 4. Simulation results and return period estimations According to these reports, an intense storm started in the early hydrological scenarios (table) hours of November 15th and continued during the night. The soil Observed rainfall at three meteorological stations (up) and X-station simulated 30-min rainfall (scenario A) Streamflow (scenario A) image from the weather radar (down) 30 180 Discharge Peak flow got likely saturated, resulting in a significant flash flood. 160 3 25 capacity of 153 m /s 140

/s) Scenario A Scenario B 20 74.3 mm 3 120 the culvert 100 1000 15

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2. The neighboring catchment and its valuable information T Rain (mm) Rain 10 60

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REFERENCES X-station simulated 30-min rainfall (scenario B) Streamflow (scenario B) Peak flow Scenario A Scenario B 180 30 127.2 mm 3 60 Apostolidis, I., V. Perleros, V. Tsatiris, and G. Vassilopoulos, Report 160 160 m /s

th 25 (mm/h) 50

on the flood of November 15 2017 at Mandra and Nea 140 i /s) Peramos, Attica, December 2017; http://www.geotee-anste.gr 20 3 120 40 100 15 Kalogiros, J., A. Retalis, D. Katsanos, M. Anagnostou, and E. 80 30

Nikolopoulos, National Observatory of Athens press release on (mm) Rain 10 23.1 mm 60 20 20-11-2017; http://www.noa.gr (m Streamflow 40 5 20 10

Lekkas, E., N. Voulgaris, and S. Lozios, Flash Flood in 0 0 0

station rainfall intensity, intensity, rainfall station -

(Mandra, Nea Peramos) November 15, 2017; Newsletter of X 0 3 6 9 12 15 18 21 24 Environmental, Disaster and Crisis Management Strategies, Issue X-station rainfall duration, d (h) No 5

Scatter plots of simulated rainfall intensities

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Michailidi, E., Investigation of flood simulation for design in 6:00 15/11/2017

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14/11/2017 10:00 14/11/2017 12:00 14/11/2017 14:00 14/11/2017 16:00 14/11/2017 18:00 14/11/2017 20:00 14/11/2017 22:00 14/11/2017 15/11/2017 10:00 15/11/2017 (up) and estimated return periods (down) vs. ephemeral basins – Application to Sarantapotamos basin, Simulated Observed Diploma thesis, Department of Water Resources and time scale (duration), for the two hydrological Environmental Engineering – National Technical University of Simulated rainfall at X-station (left) and simulated vs. observed flows (right) for hydrological scenarios A (up) and B (down) scenarios Athens, December 2013 DEUCALION – Assessment of flood flows in Greece under 5. Conclusions conditions of hydroclimatic variability: Development of physically- established conceptual-probabilistic framework and We study the catchment of Sarantapotamos, the largest stream in the region. It is a narrow basin north of Mandra, • computational tools; http://deucalionproject.itia.ntua.gr/ Hydrological scenario B provides systematically larger rainfall intensities at the hypothetical X-station; at all temporal scales up to four hours, stretching from East to West with slopes ranging from 10% to 30%, and in some areas up to 100% or even higher; the the maximum intensities that are estimated by scenario B are 2.0 to 2.5 times larger than those estimated by scenario A. basin is also characterized by great water permeability, due to the karst nature of the limestone in the region. CONTACT • Despite their significant differences in rainfall estimations, in terms of intensity and temporal pattern, both scenarios ensure perfect fitting to Charalampos Ntigkakis We gathered data from a hydrometric station installed in a culvert near Gyra Stefanis and point rainfall data in Vilia. the observed flows of Sarantapotamos until 9:00 am, and they also result in almost identical estimations of the peak flow, some hours later. e-mail: [email protected] However, due to the extreme flows, the water rose rapidly to ~0.5 m from the stage-gauging sensor (the sensor’s • measuring limit), and then overtopped the bridge above the culvert, also destroying the instrument’s assembly; thus, the Scenario B, resulting in significant intensities over short durations seems to be closer to reality, given that due to the small size of the two The presentation is available at: catchments upstream of the city of Mandra, their response time is quite short, thus the maximum rainfall at such duration is the most critical. http://www.itia.ntua.gr/1785/ collected data do not cover the entire flood range. Due to the fact that such a flash flood could not be justified by the rainfall observed in Vilia, it prompted us to investigate it further. • Ongoing research aims at improving rainfall estimations, by taking advantage of quantitative estimations by satellite and radar data.