Flood Defence Plan Sigurd MELIN Terra Firma , Box 98, Näsviken, 820 64, Sweden Tel: +4670-3330140, Email: [email protected]1. Introduction In recent years mapping the threat of floods has become a big task in many countries. Investigating the threat is of course a very important first step, but we must not stop there. The natural next step is to make a strategy for the defense against the water. The problem so far is that there has been very few adequate kinds of weapons. Permanent earthen embankments and concrete walls is of course an option, and we have been building such for a very long time. However, restoring old structures and building new ones along all watercourses that may flood will require enormous investments, and will take very long time to complete. Meanwhile citizens living in communities along those same rivers will stay unprotected for many decades. Permanent walls of sufficient height will also mean that the very much appreciated water contact is cut off, both practically and visually. Temporary barriers of sandbags is another possibility, but as they require much labour force, such a solution can only be considered for very concentrated efforts to save crucial installations. However, in recent years new technology has been developed within this field - mobile flood barriers. Those are both light and easily transported, and may therefore allow a limited working crew to build a defense line wherever a flood threat arises. Today there are already about 25 different mobile flood barrier products only on the European market. Still they are rather unknown. The ideal solution seems to be a combination of old and new - low permanent embankments for moderate water levels, completed with mobile barriers should the water levels rise further. To make this strategy work reliably in a critical situation one should have decided in advance on appropriate defense lines along which mobile barriers shall be deployed, have checked that the water cannot find other routes through pipes and gullies etc. There is a need for a plan. 2. Objectives In a joint R&D project the Swedish flood protection development company Terra Firma and Future Position X have now developed a model for such a plan, a Flood Defense Plan (FDP) for the Ekeby Water Treatment Plant in Eskilstuna in mid Sweden, located close to a river.
Transcript
Flood Defence PlanSigurd MELIN
Terra Firma , Box 98, Näsviken, 820 64, SwedenTel: +4670-3330140, Email: [email protected]
1. IntroductionIn recent years mapping the threat of floods has become a big task in many countries. Investigating the threat is of course a very important first step, but we must not stop there. The natural next step is to make a strategy for the defense against the water. The problem so far is that there has been very few adequate kinds of weapons.
Permanent earthen embankments and concrete walls is of course an option, and we have been building such for a very long time. However, restoring old structures and building new ones along all watercourses that may flood will require enormous investments, and will take very long time to complete. Meanwhile citizens living in communities along those same rivers will stay unprotected for many decades. Permanent walls of sufficient height will also mean that the very much appreciated water contact is cut off, both practically and visually.
Temporary barriers of sandbags is another possibility, but as they require much labour force, such a solution can only be considered for very concentrated efforts to save crucial installations.
However, in recent years new technology has been developed within this field - mobile flood barriers. Those are both light and easily transported, and may therefore allow a limited working crew to build a defense line wherever a flood threat arises. Today there are already about 25 different mobile flood barrier products only on the European market. Still they are rather unknown.
The ideal solution seems to be a combination of old and new - low permanent embankments for moderate water levels, completed with mobile barriers should the water levels rise further.
To make this strategy work reliably in a critical situation one should have decided in advance on appropriate defense lines along which mobile barriers shall be deployed, have checked that the water cannot find other routes through pipes and gullies etc. There is a need for a plan.
2. ObjectivesIn a joint R&D project the Swedish flood protection development company Terra Firma and Future Position X have now developed a model for such a plan, a Flood Defense Plan (FDP) for the Ekeby Water Treatment Plant in Eskilstuna in mid Sweden, located close to a river.
Previously the entire plant area had been classified as flood prone, although earthen embankments along the river were assumed to protect the area from water levels with a return period of 100 years. However, as these embankments hadn’t been maintained for many decades, the risk of failure was obvious.
3. MethodologyFirst the entire area was investigated, and all objects that might be in danger (like electrical installations, doorsteps, window frames, vents etc) were listed in a table. The exact (crucial) height of each was also measured.
Based on meteorological data and models the return periods of different flows were calculated. As were the corresponding water levels. All these data were brought into a ”Threat Picture”, a diagram illustrating the threat graphically. This Threat Picture served as a tool when deciding about the flood protection ambitions for the area. Together with the plant staff two protection levels were chosen, one lower level (corresponding to a 90 years return period) to be protected with permanent measures and one higher level (corresponding to a 1,000 years return period) for the mobile barriers. These levels were brought into the diagram, making it the first step of the plan.
The next step was to create a very accurate terrain model, using data from airborne radar scanning. The model was then confronted with the different water levels, to get a map showing flood water depths.
This map helped us make a preliminary design of the flood protection arrangements, Finally, the investigation was completed with a notification of the underground conditions, to avoid unpleasant surprises from below.
4. ResultsThe final plan showed, somewhat surprisingly, that only two buildings were in fact threatened. Previously it was believed that the whole area was in danger. Further, these buildings could be easily protected by a 150 meter long and 0.5 meter high mobile barrier, deployed along a paved street running between the houses and the river. Finally, a back flow valve had to be installed in the local rainwater drainage system.
The plan can be downloaded in its entirety from www.terrafirma.se/oversvamning_eng.htm
5. Business BenefitsRecently a second plan was carried out for another place, Gysinge bruk, a cultural heritage
site that was struck by a flood two years ago. Illustrating how the area could be protected, the Flood Defense Plan plan made it possible for the Local Community to engage private tourism entrepreneurs to invest in the development of the area.
1.Threat Picture of Ekeby, completed with flood protection ambitions
