Stemming the Rising Floods
“A warmer climate, with its increased climate variability, will increase the risk of both floods and droughts.”
– Fourth Assessment Report: Climate Change 2007, Intergovernmental Panel on Climate Change
By Carl Atkinson, EngTech, GCGI, MInstRE
In May 2011 a DefenCell DC2 Flood Wall System was deployed in Smithland Ky., to help the town defend itself from projected record flooding. PHOTOS COURTESY J&S FRANKLING LTD.
Humanity has always battled the elements in one form or another. But the sheer amount of fresh water melting into the seas from the ice caps and thawing permafrost have serious implications to millions of people worldwide.
As worldwide temperatures rise, the likelihood of precipitation falling as rain rather than snow increases. That has a direct impact on rivers and seas causing a knock-on effect as rivers burst their banks from the pure inundation of rainwater and as sea storms and surges overwhelm low-lying coastal areas.
Whatever the direct cause of the flooding, the effects are multiplied by the vulnerability of our critical infrastructure. Lack of flood response plans to protect our critical infrastructure can cause catastrophic results that can leave thousands homeless, without power, water or access to emergency services. Lives could be lost. And the financial costs may very well be counted in the millions, perhaps billions, in repair costs, insurance and lost revenues. The most obvious form of protection would be to ensure that critical infrastructure—including power plants, fresh water supplies, emergency services and telecommunication systems—are located away from flood plains, coasts, sea ways, and rivers. This of course is not always practical. So the best protection is to use existing technology to provide flood defenses to protect and secure these vital resources from the considerable damage that flood water can cause.
All critical infrastructures cannot be protected all the time. However, measures can be put in place to protect infrastructure on a speedy basis when a threat arises.
FORGING A MARKETPLACE
In 2004, Congress tasked the U.S. Army Corps of Engineers (USACE) to “devise real-world testing procedures" for “promising alternative flood-fighting technologies.” Through the General Investigation Research and Development Program, the U.S. Army Engineer Research & Development Center (ERDC) conducted research and developed a laboratory procedure for the prototype testing of temporary barrier-type flood-fighting structures intended to increase levels of protection during floods.
Manufacturers of “Flood Fighting Technologies” are invited to bring their product to the test facility in Vicksburg, Miss., together with all equipment and supplies required to erect their product prior to testing. Either a manufacturer’s own staff or ERDC personnel (after suitable training from the manufacturer) then construct the barrier to be tested in a flood scenario.
The test facility was laid out along the perimeter wall of a reservoir with dimensions of 115-ft by 185-ft by 4-ft deep. The test facility was reconfigured specifically for innovative flood-fighting experiments by allowing levees to be constructed against two wall abutments with a 30-ft opening between the walls. A geometric testing zone footprint was laid out on the concrete floor. All levees are required to be constructed within this given footprint. One side of the footprint abuts the concrete wall at a 90-⁰angle, and the other side abuts the concrete wall at a 63-⁰ angle. The purpose for having two different angles is to simulate real-world geometric variability and demonstrate constructability and geometric flexibility of each manufacturer’s product. Additionally, the unsymmetrical geometry allows wave loading variability during hydrodynamic testing, and causes an apparent current along the 63⁰ wall. Inside the test area (leeward side of the levee), an 8-ft diameter by 8-ft deep circular pit was installed to catch any seepage or overflow water from the structure. A pair of 4-in diameter pumps were installed in the seepage pit to pump the accumulated water back into the wave basin. Two 12-in diameter pumps (12-in intake and 10-in output) also were installed to pump excess water out of the seepage pit when the capacity of the 4-in pumps was exceeded. The test area has a series of lasers to measure any movement of the flood-fighting barrier; a laser to measure changes in water surface elevation within the seepage pit; and an additional laser was placed to measure water surface elevation within the basin.
In 2010 the DefenCell DC2 Lightweight Non-metallic Barrier, passed the strict testing procedure and appeared in the list of flood protection products approved by the USACE.
DEFENSE IN ACTION
In May 2011 a DefenCell DC2 Flood Wall System was deployed in Smithland Kentucky, USA, to help the town defend itself from an imminent record flood surge. A town of just 400 people, it was estimated that they had just 75 hours to prepare for a record surge in river levels, and protect itself from the Ohio River.
“Based on (calculation) and communication with EOC (USACE Louisville District Emergency Operations Center), we determined there was insufficient time to fill and place sandbags, and that the DefenCell Wall System was the best choice to stay in front of the rising water,” geotechnical engineer Steven Shifflett said.
Within 24 hours of receiving a call, DefenCell delivered 3-mi of flood wall units that, once installed, would provide almost 4-ft of additional flood protection barrier to a key stretch of the levee in Smithland. As a testament to the system’s ease of use, within an hour of delivery, small volunteer teams were able to start placing, connecting and filling the systems. Within the first three hours, the teams had achieved an installation rate of 20-plus units per hour. In this short period they installed the equivalent of more than 22,000 sandbags.
The DefenCell barriers remained solid throughout the surge. And the town? Well simply, it was saved from flooding.
TESTING NEW TECHNOLOGY
In June 2014 the DefenCell MAC FE Flood Fighting Barrier, a geotextile lined welded mesh gabion was taken to ERDC for testing. For the purposes of this testing procedure DefenCell MAC FE consisted of a series of wire-mesh baskets (gabions) partially lined with a geotextile fabric.In the standard configuration, five baskets of 3-ft by 3-ft by 3-ft were connected together to form one unit. The wire mesh panels were joined together with wire coils. Multiple units were joined together just by dropping a metal pin through overlapping coils.
The units are shipped folded on a pallet, and a single pallet held all of the units used to construct the 84-ft 5-in long barrier, plus some spare parts. The units are shipped with the geotextile lining pre-attached, so construction consists largely of just unfolding the units, pinning together adjacent units, and filling with sand or other locally available material. The geotextile fabric lines the sides of each basket, with a 6-in wide flap extending into the bottom of the basket. The bottom is otherwise open. The weight of the sand resting on the 6-in flap provides stability to the units.
Within the first three hours, the teams had achieved an installation rate of 20-plus units per hour. In this short period they installed the equivalent of more than 22,000 sandbags.
One pallet containing the units coils and connecting pins was shipped to ERDC and together with the tools and supplies needed to install the test barrier, brought by the construction crew fitted in one pickup truck. Installation of the 84- ft 5-in barrier was completed by a three-person team from the manufacturer, plus one individual from ERDC to operate a skid steer loader, in 16.1 man-hours.
The basin was then filled to depth of 1-ft. Water depth is measured by a laser reflecting off a float at the bottom of a standpipe, to measure seepage. Water depth in the basin had to reach 0.2-ft before the float was lifted off the basin bottom. Average seepage rate during the first two hours after the set depth was reached was 0.037-gal/min per foot of wall, with the wall length measured along the mid-point of the barrier. Seepage rate gradually decreased as the fill settled so that the average seepage rate during the last two hours of the 22 hour test was 0.038-gal/min per foot of wall.. Static water seepage rates at a basin depth of 1-ft, 2 ft, and 2.85-ft were rounded up to 0.04-, 0.08- and 0.13-gal/min per foot of wall, respectively.
To measure movement of the barrier, a distance-measuring laser was aimed at each of the three walls of the barrier at the approximate midpoint (vertically and horizontally) of the wall, which was laid out in a modified “U” shape with a left wall, center wall, and right wall.
Minor apparent movement of a sand-filled barrier is normal. As the sand settles and/or becomes saturated, the walls of the baskets may bulge out slightly without any movement of the structure base. Also, the water pressure may cause the barrier to lean inward slightly, again without any movement of the base. Recorded “movement” of the three walls of the barrier are between 3 and 4 thousandths of a foot, with a maximum movement on the center wall of about 6 thousandths of a foot (a little over 1/16-in), and can be ignored.
Hydrodynamic tests included tests with waves and an overtopping test. The wave tests included small (2-in), medium (6- to 8-in) and large (10- to 12-in) wave heights, all with a 2-sec wave period. All wave heights were run at low water (67 percent of structure design height) and repeated at high water (80 percent of structure design height). Design height of the DefenCell MAC FE units tested was 3-ft. The wave tests were therefore conducted at depths of 2-ft and 2.4-ft. The higher water level was intended to insure that some of the waves would overtop the structure. During all of the tests with waves, the barrier appeared stable and solid.
An overtopping test determines any damage to the structure from waves breaking over the top of the structure. This was conducted following the wave tests. There was no apparent damage to the structure from the overtopping. Minor washout of the sand was observed later when the plastic cover was removed from the barrier. However, fastening the plastic sheeting to a lower wire on the inside panels should eliminate the washout.
A debris impact test examines flood fighting structures for their ability to withstand impact from debris carried by the current in an actual flood. This test involves towing two logs into the barrier with a winch located inside the test area. The logs were towed in at a 20-⁰ angle at a speed of 5-mph (7-ft/sec). Power to the winch was cut just prior to impact with the structure. Both logs were 10-ft long and cut from a creosote-coated telephone pole. The smaller log was 12-in diameter and weighed 610-lbs dry; the larger log was 16.5-in diameter and weighed 790-lbs dry. Both logs had been soaking in water for 1-1/2 weeks prior to testing and undoubtedly had increased in weight. A wooden platform was placed on top of the barrier to protect the barrier from being cut by the cable
The only damage during the debris impact test was two wires on the gabion slightly bent by the log impacts. There was no damage to the functionality or reliability of the barrier.
DefenceCell barriers in position.
Disassembly and removal took four individuals 1 hour and 2 minutes to completely remove the units and stack them on a pallet in the back of a truck. All that was left was a pile of sand and the plastic sheeting left on the floor under the sand. From here the rest of the disassembly required only the Bobcat operator plus one person to remove scraps of plastic from the sand as the Bobcat dug out the sand. Total removal time was 5.87-man-hours, or approximately 0.07-man-hours/ft.
Using baseline data collected in 2004, following the same protocol, the DefenCell MAC FE was compared to a sandbag barrier of similar height and length. The barrier took less than one-tenth as long to construct, two-thirds as long to remove, had lower seepage rates at all water levels, withstood the waves and overtopping, had less damage, and required no repairs.
READY TO RESPOND
Although many of our critical infrastructures do have permanent protection, many still do not. In the United Kingdom, for instance, while the National Grid has a 1.7-km mobile flood defense system in place that can be deployed to substations anywhere in the country within 12 hours to 24 hours, and all new substations and gas compressor stations are designed to be resilient to a one-in-1,000-year-flood event, some 57 substations are still identified as potentially vulnerable to flooding.
With emergency flooding scenarios likely to increase around the world, having the right systems, equipment and procedures already pre-tested and available to use will be vital in mitigating the effects.