Thursday, August 2, 2012
Not long after our last flood here in Kampot a couple years back, the PM suggested the river be dredged to prevent a further swamping. (Never mind that that flood was at least partly caused by a breach in Kamchey dam - which the authorities vehemently denied - which was then under construction). Unfortunately, the PM’s grasp of geography leaves a bit to be desired. The river, which is officially called a bay (though I’ve never seen a bay that was a hundred meters wide and more than 10 kilometers long) is in fact an estuary. Dredging it can make no difference to the water level which is based almost entirely on ocean tides with the addition of occasional storm surges from the ocean and heavy rainfall coming down from the mountains. When high tide is combined with the latter two there’s a possibility of flooding. For sure, conditions were ripe for the big flood even without the breach in the dam.
Just the opposite is true when the river is narrowed through filling. Constricting the channel cannot help but exacerbate the flooding problem and that’s exactly what’s been happening. A couple years back a large area of several hectares, just across the river from old town, was filled in. Somebody important must have big plans for the area, though the land sits idle. The river is obviously public space, so filling in that area was clearly a valuable gift to a mighty mucky-muck.
Dredging did commence last dry season and quite a lot of sand was removed. As long as the sand is taken from the center of the river, it shouldn’t cause serious erosion, but it clearly looked like some was dredged very close to the bank, which is almost certain to cause problems later on. Much of the sand went to Singapore, some of it was used to fill in the riverbank. One use is that riverside park is being extended south about 100 meters... a great idea, but not in terms of flooding. On both sides of the river there are now strips of very high piles of sand with no clear purpose that I’m aware of. Dredging and sale of sand was stopped a while back on the PM’s orders, but the sand remains. Property owners along the river also are prone to build retaining walls which extend out into the river a bit.
All of which point to serious problems down the line. I live about 200 meters from the water. I was fortunate during the last flood to be out of town, since the water would’ve been up to the windows in my Camry and essentially trashed it. My landlady was around to raise up everything in the house that could’ve been damaged. The flood previous to that one happened before the dam construction began so flooding is a regular occurrence here. I’m a plant collector and I now have maybe three times as many plants as I did back then - more than 300 potted plants. In case of flooding where the water remains for more than a day most will not make it. As a result I’m mired in the conundrum everyone living in a floodplain faces. I’m very comfortable in an otherwise ideal situation so I have no desire or intention of moving, but man, will I be sad when the torrents rush in… but, it can’t happen to me, can it?
We had another serious flood event in February 2010 that had nothing to do with the river. The authorities were baffled, The river is fine so how could there be a flood? A simple rain gauge would’ve provided the answer since we had 4 1/2 inches - 12 cm - in less than 2 hours. That is a lot of rain to fall in such a short time and was probably a 5 year, maybe even a 10 year event. (Actually, considering that happened in February in the middle of dry season, it was probably a 50 or 100 year event… but that’s not pertinent to the drainage issue.) It’s extremely expensive to build a system designed to handle an event that might happen only once every ten years. Nonetheless, since Kampot is somewhat of a showcase city for tourism the government has lots of money to spend here, so not long after the flood some streets were being torn up in order for much larger drainage pipes to be installed. So now we’re more prepared for extreme rainfalls, but there’ll probably still be considerable flooding in ten year events.
Also, while global warming skeptics will insist and climate scientists will admit that single events cannot be attributed to anthropogenic - human caused - climate change, extreme rainfalls are consistent with climate change models. Warmer air holds more moisture so when it does come down, it’ll come in greater amounts, so we are likely to see ever heavier rainfalls. Just recently in early June an unprecedented 80cm of rain - one year’s worth - fell in a few days on parts of Kyushu, Japan’s southern island. A good indicator for what the future will hold climatewise.
Moving over to Phnom Penh, a very heavy rainfall recently caused severe flooding and prompted many complaints about the ineffectiveness of the $30 million flood control project for the riverside area that was funded by the Japanese. (That was the second of three phases, the first happened around 2004, the third began last January.) That was reported to be a 92mm - 3 1/2 inch - deluge. That amount of rain in a short period of time probably happens once every year or two on average. Once again, as in Kampot, it would be very expensive to design a system capable of handling that amount of water without flooding, though in fact the flooding situation was much improved over the past. Previous to the project, floodwaters would remain very high many hours after the rain had stopped. I remember one time wading across putrid water above my knees at Streets 13 and 154 at midnight, four hours after the rain had stopped about 8 pm. It still floods today, but drains much faster. I wasn’t there for the above event, but I did witness a heavy rainfall in that area that drained within about 30 minutes.
The Japanese project had two parts. One part involved building much larger drains and laying larger drainage pipes under the street. Previously there were single drains with a small reservoir underneath. Today in many locations you’ll see three drainage ports lined up, and if you could look underneath you’d see a relatively large reservoir connecting them. Norodom and Street 154 is a good example.
Most of the $30m was spent on very large underground reservoirs - one is about 260 cubic meters - designed to store water when the river is very high. When the Tonle Sap reaches above the sewer outfalls that drain into it they have to be closed off else high river water would push the sewer water back into the streets. That was by far the biggest expense, but has no effect whatever when the river is low.
One change in the Psar Kandal area that decidedly exacerbated the flooding problem was the paving over of most of the several hectares of the former T-3 prison site at Streets 13 and 154. When the area was a prison it consisted of a lot of small buildings in a campus setting where most of the rainwater that fell there was absorbed into the ground. Now, 90% of the area is impervious surfaces and precipitation goes directly into the drainage system. In Portland, Oregon (and probably most of America) that would not be allowed. Anybody who seeks to pave over a large area there has to include dry wells to absorb rainwater. The well consists of a stack of three 1-meter-diameter concrete pipes (actually since it’s America they’re probably 1 yard diameter, which is almost a meter wide). The lowest one is perforated so that in addition to the well being open at the bottom, collected rainwater can also seep into the ground through the perforations. The entire paved area is sculpted for the rainwater to drain into the dry wells.
Portland has a unitary system where toilet water, gray water (wash water) and rain water are combined and sent to a sewage treatment plant. Needless to say there’s no need for clean rainwater to go through an industrial cleansing process, but separating the systems, in an American context, is a lot more expensive so combined they are. Still, that being the case, the city is keen to reduce the amount of rainwater going into the system as much as possible. Thus the dry well requirement. In addition property owners are given incentives to disconnect their downspouts from the sewers and process rainwater on site and where natural creeks remain, rainwater is diverted to them. I’m not sure if dry wells would work in PP since ground water in rainy season is very close to the surface and rain falls much faster than it does in Oregon.
Cambodia has a dual system where toilet water goes to septic tanks connected to every building and the rest - rain and graywater - goes directly into the drainage system and then dumped without any processing into the rivers. If you are skeptical of the toilet water/septic tank connection, think about those ugly old tank trucks you see around town with their big suction hoses; they’re emptying out full septic tanks. Graywater isn’t toxic, but it is smelly and ugly, so it really should be processed before being dumped into the river. Wetlands work well for that purpose, but there aren’t many (aren’t any) of those around the city anymore. A swamp was drained to make way for Psar Thmei but I doubt if we can go back to the former wetland just to process graywater. Too bad some form of graywater processing wasn’t included in the big Japanese flood control project.
Phnom Penh’s leaders seem to be doing everything they can to exacerbate the flooding problem. Not intentionally, of course, though the end result is the same. In the latest onslaught against the drainage system, the 7 hectares of wetlands on the edge of Olympic Stadium, which were designed into the complex by famed Cambodian architect Vann Molyvann to absorb all of the rain falling there, are currently being filled in. Thus a very large amount of precipitation is being added to the drainage system and a lot of money will be required to minimize flooding from that latest crime against the city’s greenery.
The politicians insist that won’t be a problem, but they’re paid to, prone to, say any inane, ridiculous thing to justify their latest scheme. And I’m not singling out Cambodia’s leaders, it’s a failing of politicians everywhere.
In the North of the city in Russei Keo district there was serious flooding after the lake where the unfinished Camko City project was filled in. Public schools had to be closed for months, people lived with water in their houses for that time. Friends who worked at Lim Kok Weng U. had to drive through foot deep - 30cm - standing water for months. That problem was partly mitigated by spending millions on a pump station and pipes to carry the water further north to a lake that hasn’t yet been filled in. The problem there is that at the height of rainy season all bodies of water are full, so transporting the water north can only cause additional flooding there.
The third phase of the Japanese flood control project, costing close to $50 million, involves laying more than 20 kilometers of large pipes from as far north as O’ressey market and includes most of the rest of the southern part of the central city - the area within the map we are all so familiar with - to Boeng Trabek in the south. The government website now refers to it a retention pond rather than a lake. Nonetheless, similar to the situation in the north, that lake will naturally fill during the height of rainy season just from its immediate area so transporting large volumes of water there will only move flooding from one part of town to another.
Important people are being given large areas of filled in lakes and wetlands to play with, but the city is left with the consequences of the flooding that’s the inevitable result. Fortunately for Cambodia, Japan is coming up with the funds to pay for mitigation efforts, which have been, in the end result, directly caused by over zealous development. Those efforts may help but they still won’t stop flooding in heavy rainfall events.