The heavier rainfall expected in the years ahead due to climate change does not threaten our drainage facilities. Blocked gully pots do, however. Such is the conclusion of dr. Marie-Claire ten Veldhuis, who defended her PhD dissertation this week.
Every year hundreds of locales in the Netherlands flood slightly because drainage facilities are incapable of draining away water from downpours quickly enough. And as if this weren’t bad enough, in 2050 a twenty percent increase in rainfall is expected. Time to improve the country’s drainage facilities, many experts say. By installing wider pipes, draining capacity can be greatly increased.
Capacity however is not really the problem, says dr. Marie-Claire ten Veldhuis, a hydrologist working at the faculty of Civil Engineering and Geosciences’ department of Water Management. But litter and sand and leaves are, says the researcher, who investigated 12,000 flood reports submitted by residents of Haarlem and Breda. She found that in seventy percent of the cases the problems were caused by blocked gully pots, and not by a lack of drainage capacity in the supporting pipes.
“When I say this to managers of drainage systems, they usually say, yes of course, we kind of knew this already”, says Ten Veldhuis. “But it was never clear exactly how big this problem is.”
Ten Veldhuis figured out that the price we pay for damage resulting from blocked gully pots over a ten-year period equals that of the damage caused by the major flooding events that, statistically speaking, occur once every hundred years. She believes she is the first person who has ever thoroughly investigated this problem in the Netherlands.
Why has this never been a topic of interest before? One reason for this, Ten Veldhuis believes, is that it is only since fairly recently (over the past ten years) that some municipalities began seriously collecting flood reports from its residents. “And”, she adds, “the focus has been primarily on modelling the water flows in a normally functioning drainage system, which is in itself a huge challenge and a fairly new research topic.”
Before starting work as a researcher in Delft, Ten Veldhuis was employed by the city of Breda, where she was involved in the management of the city’s drainage system. Consequently, she has firsthand experience of flood problems.
“At one point, while I was working in Breda, we wanted to change some parts of the drainage system; we wanted to separate the sewer systems for rain water and for household waste water. This is being done in many cities for environmental reasons. We invited residents to come have their say about our plans. Some people, who used the meeting as means of expressing their opinions on other related topics, told us about places in the city that regularly flooded. To our surprise there where seven locations unknown to us. Our hydrodynamic models had not pointed them out.”
Ten Veldhuis believes it is important that, somehow, information about blocked gully pots, and the risks of this happening, should be inserted in the existing analysis methods for urban water management: “Perhaps these happenings are too random to predict, but nevertheless it is worth further investigation. Perhaps we can discover links between the chances of this happening and the number of trees in an area or the population density.”
The researcher says it would also help if municipalities would use more sophisticated questionnaires for people who report floods, as in this way it would be easier to categorize the causes of the problems. Going trough 12,000 reports, as she did, is donkey work, especially if the reports are not all well organized.
Every year hundreds of locales in the Netherlands flood slightly because drainage facilities are incapable of draining away water from downpours quickly enough. And as if this weren’t bad enough, in 2050 a twenty percent increase in rainfall is expected. Time to improve the country’s drainage facilities, many experts say. By installing wider pipes, draining capacity can be greatly increased.
Capacity however is not really the problem, says dr. Marie-Claire ten Veldhuis, a hydrologist working at the faculty of Civil Engineering and Geosciences’ department of Water Management. But litter and sand and leaves are, says the researcher, who investigated 12,000 flood reports submitted by residents of Haarlem and Breda. She found that in seventy percent of the cases the problems were caused by blocked gully pots, and not by a lack of drainage capacity in the supporting pipes.
“When I say this to managers of drainage systems, they usually say, yes of course, we kind of knew this already”, says Ten Veldhuis. “But it was never clear exactly how big this problem is.”
Ten Veldhuis figured out that the price we pay for damage resulting from blocked gully pots over a ten-year period equals that of the damage caused by the major flooding events that, statistically speaking, occur once every hundred years. She believes she is the first person who has ever thoroughly investigated this problem in the Netherlands.
Why has this never been a topic of interest before? One reason for this, Ten Veldhuis believes, is that it is only since fairly recently (over the past ten years) that some municipalities began seriously collecting flood reports from its residents. “And”, she adds, “the focus has been primarily on modelling the water flows in a normally functioning drainage system, which is in itself a huge challenge and a fairly new research topic.”
Hugo Gastkemper, the
director of Rioned, a foundation that functions as platform
for all Dutch sewer management organizations, says he thinks Ten
Veldhuis overestimates the damage caused by blocked gully pots and he
does think that climate change is an important threat. “Three years ago
we held a poll. According to municipalities the inundations that cause
most damage are lengthy and not related to blocked gully pots. But Ten Veldhuis’ research is interessting and the problem of blocked gully pots
will probably receive more attention.”
Ten Veldhuis however also has firsthand experience of flood problems. Before starting work as a researcher in Delft, she was employed by the city of Breda, where she was involved in the management of the city’s drainage system.
“At one point, while I was working in Breda, we wanted to change some parts of the drainage system; we wanted to separate the sewer systems for rain water and for household waste water. This is being done in many cities for environmental reasons. We invited residents to come have their say about our plans. Some people, who used the meeting as means of expressing their opinions on other related topics, told us about places in the city that regularly flooded. To our surprise there where seven locations unknown to us. Our hydrodynamic models had not pointed them out.”
Ten Veldhuis believes it is important that, somehow, information about blocked gully pots, and the risks of this happening, should be inserted in the existing analysis methods for urban water management: “Perhaps these happenings are too random to predict, but nevertheless it is worth further investigation. Perhaps we can discover links between the chances of this happening and the number of trees in an area or the population density.”
The researcher says it would also help if municipalities would use more sophisticated questionnaires for people who report floods, as in this way it would be easier to categorize the causes of the problems. Going trough 12,000 reports, as she did, is donkey work, especially if the reports are not all well organized.
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