Main causes of flash floods in mountains

Flood prevention in mountains : 

EU workshop INFRA 34671 Ivano-Frankivsk, Ukraine 03 February 2010 © Svetoslav P. Apostolov Flood prevention in mountains Main causes of flash floods in the European mountains

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© Svetoslav P. Apostolov, 2010 All rights reserved. Reproduction is authorised under the Use Agreement terms and conditions.

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Presentation Outlines The Basic  Flash Floods What is the problem? Areas of urgent action Conclusions

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The Basic Floods are naturally occurring events: riverine floods – slow-developing, more predictable; flash floods – extremely quickly developing, difficult to predict, can happen anywhere. However! Floods may also result from failure of hydraulic infrastructure - dams, embankments, stormwater collection system, etc.!

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The Basic In any case Floods result in huge economic losses: > €1 mlrd from a single flash flood in the Swiss Alps in 2008; floods in the summer of 2007 in the UK => £3,2bn (about €3,7 bn); floods each year in the US => $4,6 bn (data from 2007) (about €3,3 bn at today’s exchange rate). human displacement and even casualties: 2 persons died in the flash flood in the Swiss Alps in 2008; 13 people died after parts of the UK were flooded in 2007; > 100 deaths in the US per year.

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The Basic Causes: natural – heavy rainfall, intensive snowmelt, tsunami, climate change, watercourse blockage (ice blocks, runoff silt/debris, landslide), etc. man-made – changed land-use (deforestation, urbanization), accelerated climate change, hydraulic infrastructure failure, etc. Source: Rivers in Japan 1998 (http://www.adrc.asia/management/JPN/RIVERS%20IN%20JAPAN%201998.html) Source: The COMET® Program

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The Basic Runoff: the most important determinant of a flood; occurs when precipitation or/and snowmelt exceed/s the retention capacity of the soil.

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The Basic Main characteristics determining the runoff: 2. Soil saturation 1. Soil permeability Infiltration excess overland flow. Typical of: clay or compacted soil or altered surface (urbanization, fire); short and intense rainfall. Saturation excess overland flow. Typical of: saturated soil that cannot store further water; long and gentle-to-moderate rainfall or snowmelt.

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The Basic Main characteristics determining the runoff: 3. Land-use change: Macropores are formed mainly by decayed plant roots, burrowing insects and animals. Plants and plant matter on the surface slow down runoff -> more time for water to infiltrate the soil. Deforested areas are more likely to result in enhanced runoff with large sediment loading into the stream.

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The Basic Effects of urbanization: more impermeable surfaces -> less water enters the soil -> more surface runoff.

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The Basic Soil and vegetation store water and slow down the speed of runoff, thus mitigating the flood impact. Source: Upper Parramatta River Catchment Education Resource Kit, 2002 (http://www.uprct.nsw.gov.au/HTML/Info%20Sheets/Catchment/C5%20-%20Hydrology%20Overview.htm)

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The Basic Other characteristics determining the runoff: 2. Basin size 1. Contributing area 3. Basin shape Water from multiple locations more likely to arrive at outlet at same time -> greater peak flow. 6. Meanders 4. Basin slope Water from multiple locations less likely to arrive at outlet at same time. 5. Stream density

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Presentation Outlines The Basic Flash Floods  What is the problem? Areas of urgent action Conclusions

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Flash flood general characteristics: life-threatening; typical of small (less than 77 km2), fast-response basins (most likely in mountains); can occur anywhere and at any time; very difficult to predict; Flash Floods begins within 6 hours (but can take as less as few minutes) of the causative event. Flash flood in highlands and mountains Flash flood in lowlands Source: adapted from Vorhersage und Management von Sturzfluten in Urbanen Gebieten (Hatzfeld, 2006) Erosion Landslide Mudslide

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Flash Floods Major factors influencing a flash flood: precipitation duration and intensity, and/or snowmelt intensity; soil structure and saturation; stream density; topography; surface type/cover; forest fires! Steep slopes, narrow valleys or ravines, reduced stream meanders and high stream density (all typical of mountains) increase the runoff speed -> increased likelihood of flash flood.

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Flash Floods Major impacts: Death or serious injury; Immediate property damage; Long-term property damage; Loss of cultural heritage; Loss of critical infrastructure; Groundwater contamination; Sediment and silt deposition; Missed opportunities. Source: Vorsorgender Hochwasserschutz (UBA, 2006) Source: Flood Safety: What Have Historic Flash Floods Taught Us? (Kelsch ?) Effects of the 2002 flash flood in Wesenstein.

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Flash Floods Source: Hydrothemen Nr. 16 / Mai 2009 (http://www2.hydrotec.de/unternehmen/hydrothemen/hydrothemen1609/urbas) Flash floods in Germany in the period 1980 – 2007: 422 flood events registered between 1980 and 01.09.2007; 298 (or ~72% of all) events defined as flash floods; costs of the reported damages: all flash floods – about € 2,85 mlrd; average cost of a single event – € 10 mln; average cost per year – € 160 mln.

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Source: YouTube user resvol2 Flash flood in the Swiss Alps in 2008 Flash Floods From one minute to the other a torrent raged through the town.   Nothing can withstand the impact of such a body of water.   Within seconds, each car that was parked in it’s way was turned into a wreck.   Ground floors of houses were gutted.   The people who fled onto roofs and trees had to be rescued with helicopters.   Thanks to the courage of the rescue teams, only 2 persons were lost to the catastrophe.   As the flood was over, about 800 car wrecks were piled up in the street with several thousandtons of silt and debris.   The water came down the mountain for about 12 hours. The removal of the damages took several months and left a financial loss of over a billion euro.   The disastrous body of water which plunged this Swiss mountain town into chaos came from the Mediterranean Sea.

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Presentation Outlines The Basic Flash Floods What is the problem?  Areas of urgent action Conclusions

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What is the problem? Floods pose significant risk not only to the economy, human health and life but also to the environment and mankind’s cultural heritage. The constantly increasing probability of a flood event and impact of such an event result in an increasing flood risk. Why are the probability and impact constantly increasing? Natural drive: changing climate: => changing precipitation patterns; => sea-level rise. Anthropogenic drives: 1) changing land use: => changing run-off patterns; => soil erosion. 2) riverbed engineering; 3) wetland drainage and filling-up; 4) ageing hydraulic infrastructure.

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Source: The Rhine - A river and its relations (ICPR, 2008) What is the problem? The more intensive and the less suitable the use of the area, the greater the flood risk and, consecutively, the damage when the flood actually occurs. Flash floods are much more dangerous than riverine floods.

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What is the problem? The main anthropogenic drive for flash floods in the European mountains is believed to be... … logging (clear-cutting). “In general, studies indicate an increasing water yield following clearcutting of forest areas. […] In addition, more intense snowmelts are generally reported…” (Brandt, M., Bergström, S., Gardelin, M. (1988) Modelling the effects of clearcutting on runoff - Examples from Central Sweden. Ambio, 17, 5: 307-313.) However! Important role: size of cut area; size of the basin (water catchment); soil structure and saturation.

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Presentation Outlines The Basic Flash Floods What is the problem? Areas of urgent action  Conclusions

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Areas of Urgent Action Man “invades” areas naturally prone to floods or creates such ones: => impossible to stop these natural disasters from happening; => possible to prevent them from turning into a catastrophe: How? Adapt uses of the area to the existing hazard: Restrictions in the use of high-risk areas. 2) Develop appropriate measures to reduce the risk of flood damage: Inventory of the areas at risk; Flood forecasting and early warning system/s; Mitigation and non-technical control (preventive) measures; Technical control (defensive) measures; Contingency relief programmes (private insurance, state funds, etc.). + prevent the increase of the risk for the endangered areas; + limit the damages.

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Areas of Urgent Action Restrictions in the use of in areas at risk of flooding: industrial, agricultural, tourist or private purposes; building development (or modify it to use porous or pervious paving materials). Information about restrictions should be easily accessible! Source: Environment Waikato web site (http://www.ew.govt.nz/Environmental-information/Regional-hazards-and-emergency-management/River-flooding/Preventing-floods-on-your-property/)

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Areas of Urgent Action Develop appropriate measures to reduce the risk of flood damage: Inventory of the areas at risk: identification and mapping – based on geological, meteorological and hydrological data, as well as past events (frequency, severity).

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Areas of Urgent Action Flood forecasting and early warning system/s: earliest possible warning (problematic for flash floods); real-time automatic meteorological and hydrological information collection system for the entire basin; fully automated data communication system (official warnings in the media, state and private broadcasting services, satellite-based communication system, alarm calls on the radio, mobile telephones, the Internet and Teletext, volunteers, etc.).

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Areas of Urgent Action Flood forecasting and early warning in Germany: http://www.hochwasserzentralen.de

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Areas of Urgent Action © Svetoslav P. Apostolov, 2009 Main non-technical (preventive) and mitigation measures: flood preparedness and response plans (for flash floods the local level is most appropriate); conservation, where necessary – restoration, of vegetation and forests in mountainous areas; restoration of river meanders, floodplains and riparian wetlands or the hydraulic link/s to these; afforestation; public information and awareness raising; training of the population, volunteers, crisis managers and rescue teams.

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Areas of Urgent Action However! Prevention and mitigation measures are more efficient and more sustainable in the long term, especially in mountains. Flood prevention is never absolute. Residual risk can be reduced by means of technical measures. Nevertheless…

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Areas of Urgent Action Main technical control (defensive) measures: embankment; dam; dyke; by-passing channel; flood protection wall; terrace; barrier, threshold; sandbags; reservoir; riverbed maintenance. Source: Blobel Umwelttechnik GmbH web site (http://www.blobel.com/e_html/02-02e-flood-protection.html/) © Svetoslav P. Apostolov, 2009

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Areas of Urgent Action However! Flood protection is never absolute. Take residual risk into consideration. Therefore…

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Areas of Urgent Action Contingency relief programmes (private insurance, state funds, etc.): compensation for economic disadvantages or losses suffered as a result of measures against flooding; insurance system to provide for speedy recovery after a flood. Source: The ODNR Division of Soil and Water Resources' web site (http://www.ohiodnr.com/Water/tabid/18985/Default.aspx) Legend: “Pre-FIRM” means the structure was built prior to the community's initial Flood Insurance Rate Map (i.e., prior to the identification of the community's flood risk). Subsidized flood insurance rates. “Post-FIRM” means the structure was built after the community's initial Flood Insurance Rate Map. Risk-based insurance rating.

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Areas of Urgent Action River basins have no respect for political or administrative borders -> coordinated measures to reduce the flood risk required. Germany: long history of international co-operation at the river-basin scale: Since 1950: nine states – one river basin. Overall objective: improve the chemical and ecological state of the Rhine. Total surface: 197.000 km2 Germany: ca. 100.000 km2 Switzerland, France, Netherland: each 25-35.000 km2 Italy, Austria, Liechtenstein, Luxemburg, Belgium: ca. 6.000 km2 EU: Directive 2007/60/EC on the assessment and management of flood risks (OJ of the European Union L 288, 06.11.2007). Source: The Rhine - A river and its relations (ICPR, 2008)

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Presentation Outlines The Basic Flash Floods What is the problem? Areas of urgent action Conclusions 

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Further research on the processes leading to flash floods, as well as building capacities for improved monitoring and early warning are needed. Flood management requires a multi-disciplinary and cross-sectoral approach at the basin scale. Forecasting and early warning system/s should be put into operation. Flood risk should be assessed for the whole basin. Flood-risk assessment should be integrated into land-use and spatial planning strategies. Flash floods are best managed at the local level. However, this does not preclude the need for a higher-level flood management policy/ies. Best use should be made of non-technical and technical flood control measures. Residual risk should always be taken into consideration. Flood control measures must be designed to take into account most likely climate development scenario Financial mechanisms to mitigate effects of flash floods should be put into operation. Conclusions

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