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Flood Drainage

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Urban Flood Drainage

 

      This page analyzes the possible results of tsunami flooding within a settled area and provides suggestions for an effective drainage response. To simplify the situation, we have decided to focus our efforts on creating drainage plans for a model Peruvian coastal town and a Micronesian village that can be modified to suit specific needs. The recent Hurricane Katrina disaster in New Orleans provided our group with invaluable information in the form of Army Corps of Engineering (ACE) damage reports and a step by step analysis of the drainage situation. Although Hurricane Katrina does not fall into the strict focus of Mission 2009—planning and desiging for tsunami events—the effect of KatrinaÕs flooding on this large urban area provides relevant information when planning for a worst-case urban drainage scenario. Once the water actually sits in a city, unwatering processes remain similar whether the flooding occurred due to a tsunami, hurricane, or other high-water event.

 

The following is a list of a basic unwatering process, with further explanation below:

 

Text Box: 1. Immediately following the wave impact (after the area has been declared safe and there is no danger of aftershock waves), any breaches in flood levees or seawalls should be closed using sandbags or other methods to prevent more water from inundating the city. 2. A pump team of trained volunteers from the area will begin pumping using stored portable pumps. 3. In larger towns, permanent pumps will be initially covered by floodwater but will come into view as the floodwaters drain. These pumps should be cleaned by the pump team and put into action. 4. After the bulk of the city has been drained, the portable pumps can be used to drain the last remaining water pockets around sewers or especially low-lying areas. 5. The brackish muck remaining from the flood should be cleaned and buried; this may not be possible in Micronesia or some areas of Peru. Toxicity and disease are large concerns, especially in more populated areas. 6. Now that the city is cleaned, workers can safely enter to begin structural repairs and restore normal utilities.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Text Box: These photographs illustrate some of the damage done by Hurricane Katrina in New Orleans; a large tsunami could produce similar impacts. From left to right: Pumps en route to the affected site; an engineer supervising debris removal; two large ships grounded by Hurricane Katrina; small fishing boats washed inland by the storm. All photos courtesy of http://www.mvd.usace.army.mil/hurricane/chr.php
 

 

 

 

 

 

 


1. While most at-risk locations will not present the logistical nightmare of Hurricane KatrinaÕs floodwaters, any levees or seawalls surrounding an urban area will require structural assessment and repairs after a tsunami event. These repairs must be done as quickly as possible to stop the inflx of floodwaters. By August 31—just days after Katrina hit—ACE engineers began to repair a breach in the 17th Street Canal using 3000 pound sandbags and Chinook helicopters (ÒProgresses,Ó 2005). In the event that a Micronesian or Peruvian seawall does breach (the designer obviously didnÕt consult our other group design suggestions!), smaller sandbags, crushed stone, or concrete could be placed to close the breach until permanent repair work begins. Some barriers may need to be mechanically breached in order to allow trapped floodwater to drain out naturally (ÒUpdate,Ó 2005).

 

2. After the wave passes through, drainage workers must assume that any existing sewer or pipeline systems will be clogged. Both the enormous power produced by a tsunami and muck stirred up by rising floodwaters can block or destroy existing pumps or sewers—and many of the at-risk locations we must plan for lack the money or supplies even to build a permanent sewer system. Portable pumps, such as those volunteered by Dutch Water Management Department, can expel 3,000 cubic meters of water an hour. Each pump can run for 48 hours continuously on one tank of diesel fuel. Dutch water pumps have been used with great success in several countries including Suriname, Poland, and the Netherlands (ÒHalfway,Ó 2005). Rather than rely on existing pump infrastructures within flooded New Orleans, Army Corps of Engineers coordinators employed three mobile water pumps in Plaquimines Parish, establishing moblie pump stations there to begin pumping while the permanent pump stations in that area underwent repairs (ÒHalfway,Ó 2005). The pumps should be stored in a high-elevation area away from possible tsunami dangers, perhaps with other emergency equipment.

 

 

3-4. This is a general template for drainage activities based on a worst-case scenario. Each individual situation should be assessed by the inhabitants of the area. A trained group of civilians who know how to operate and repair the pumps could also assess where their services are most needed. Some locations may require the portable pumps to lowerwater levels until permanent pump stations might be repaired; most locations will not possess a permanent pump station. An early pump assessment by a ACE engineer shows the need for an adaptable pumping plan which allows for flexibility in the face of haphazard damages:

Text Box: On the morning of Sept. 7, approximately 60 percent of the city is still under water. Three pumps are now operating at the 17th Street Canal and are discharging water at around 2,250 cubic feet per second, or cfs. Pump station 19 at the Industrial Canal, just north of Florida Avenue, is currently pumping 1,300 cfs. An additional generator is to arrive today that will allow the Corps to activate another pump at this location and remove an additional 1,000 cfs. Pump station 8, located in St. Bernard Parish in the vicinity of St. Mary, is running at full capacity at 837 cfs (ÒUpdate,Ó 2005)
 

 


 

 

 

 

 

 

 

 

 

 

5. Floodwater is never simply flood Òwater;Ó both small basement floods and disaster-scale events result in a nasty, brackish mix of sewage, oil, miscellaneous chemicals, and mold. A massive urban flood, as is possible with a large tsunami, produces what one Time reporter describes as

 

Text Box: A solution of oil, feces, battery acid, human and animal rot, burst containers of bug spray and paint thinner and nail polish and antifreeze. The primary sensory experience of New Orleans now is the smell, a gagging foulness of the charnel, of the hundreds of bloated fish pooled in the 17th Street Canal and a million other nasty things floating everywhere. The masterless dogs are so hungry and delirious in the 92¡ heat that they drink this mix, at least a lap or two, and then stagger away. The city smells dead, and although the French Quarter and a few other areas were blessedly spared, whatever exists in many neighborhoods here a year from now will be vastly differentÓ (Cloud, 2005).
 

 

 

 

 

 

 

 

 

 


Thankfully, most at-risk locations within our class mission are smaller than New Orleans and do not exist simply at the mercy of civil engineersÕ wizardry. Despite the smaller scale of debris and cleanup activities, all emergency workers must receive tetanus and hepatitus shots, as ACE workers were required to do before entering affected areas.

         While the New Orleans drainage team used Lake Ponchartain as a reservoir for the flood muck, freshwater reservoirs in the Peruvian coastal desert and Micronesia are too precious to be used like this. The most practical solution is to empty floodwaters into the ocean; this should be monitored and kept clear of any existing fisheries or water supplies. In areas where the floodwater is not likely to disperse easily (such as a barrier island/lagoon structure in Micronesia) or could harm coral reef structures, care should be taken to disperse water where it will cause the least damage. Although this is not a great solution, diverting floodwaters to an inland reservoir is simply not an option. Cheap, simple filters such as those used to purify water in developing nations could be used to strain the worst muck and toxicity out of the effluent.

 

 

 

6. While some repairs can be done while the drainage is taking place—such as reviving damaged pump stations or repairing seawalls—the bulk of structural repairs should be done after the water is gone and the area is reasonably free of disease agents. Advice for post-tsunami repair work and structural improvements is not covered here but can be found on our other group pages or Mission 2009 sites.

 

References

 

U.S. Army Corps of Engineers. (2005, August 31). The U.S. Army Corps of Engineers progresses in hurricane recovery efforts. Archived News Releases. Retrieved October 24, 2005 from the World Wide Web: http://www.mvd.usace.army.mil/hurricane/news/midnight_31_aug_corps_response_to_katrina_recovery_efforts.pdf

 

U.S. Army Corps of Engineers. (2005, September 5). Up-date on the New Orleans flood fight. Archived News Releases. Retrieved October 24, 2005 from the World Wide Web: http://www.mvd.usace.army.mil/hurricane/news/up_date_on_the_new_orleans_flood_fight090505.pdf

 

U.S. Army Corps of Engineers. (2005, September 15). Corps marks halfway point in unwatering mission. Archived News Releases. Retrieved October 2, 2005 from the World Wide Web: http://www.mvd.usace.army.mil/hurricane/news/14_sept_corps_marks_halfway_point_.pdf

 

Cloud, John. (2005, September 19). Mopping New Orleans. Time Magazine.

 

Page by Elizabeth Finn