Erosion Control June 2012 : Page 46
Roman says. “They were diked, dredged, and filled for land development and other human needs. But now we realize that salt marshes can act as a barrier for storm surges and floods in addition to providing habitat for fish, birds, and other wildlife. “There were productive salt marshes all over New England during the Colonial era, but over time roads and other manmade structures such as earthen dikes were built across the marshes that restrict the amount of water that enters the marsh at high tide,” Roman says. “Tidal restrictions can lead to decreased transport of sediment and nutrients to upstream marsh areas. Consequently, marsh areas upstream of tidal restrictions can be impacted by severe subsidence.” For example, Roman has seen marsh beds upstream of tidal restrictions that are a full meter lower than on the unre-stricted ocean side. “That can be a real www.erosioncontrol.com/shoreline-protection For related articles: issue for flooding in the future,” he points out. “Subsidence increases upstream flood potential in the event of sea level rise or if a restriction such as a road is breached dur-ing a storm.” Passive self-regulating tide gates (SRTGs) and controlled RTGs have been developed for salt marsh restoration and flood control, and although they are com-mercially available and effective in some areas, they are not universally suitable for all site locations, Roman and Quigley maintain. “They can be complicated and expensive to install, maintain, and oper-ate,” says Roman. “These traditional tide-gate systems typically cannot be installed without heavy machinery, which can be cost-prohibitive or not feasible for remote or inaccessible sites. “Traditional tide gates may not be an option for sites without readily available power or sites where there is a need for flood control to protect nearby infrastruc-ture,” he adds. In contrast with traditional gates, the RTG system can be installed with minimal labor and no heavy equipment, has low capital and expected maintenance costs, 46 EROSION CONTROL WWW.EROSIONCONTROL.COM and can be used in new installations and predictable, and precise synthetic inunda-retrofit situations without major infra-tion patterns and frequencies.” For example, the RTG system can be structure modifications, says Roman. deployed in restoration mode by leaving The RTG includes three major compo-the valve open as the tide comes in. When nents: a valve or gate subsystem, a com-the tide begins to ebb, the valve is closed, pressor subassembly, and the real-time detaining water in the upstream system. control platform. The bidirectional controlled valve sys-When the tide turns again and begins its tem consists of a high-density polyethyl-flood stage, the valve is opened to allow ad-ene (HDPE) pipe fit with a rectangular, ditional water into the system. As a result, full-circumference industrial rubber blad-the water level of the degraded upstream der. The bladder serves as an inflatable salt marsh can be incrementally increased pinch valve and can inflate or deflate over multiple tidal cycles, allowing nutrient according to flow needs. A standard me-rich ocean water to inundate the system chanical compressor sub-assembly is used and foster restoration. Another advantage is that the system is to inflate and deflate the bladder. It oper-ates on a 24-V DC power source with a monitored remotely, Roman adds. “We can 20-gallon tank capacity and is compatible monitor any number of variables such as with off-the-shelf solar power compo-valve status, current and past water levels in the system, and whether any storms are nents for independent power. The RTG system uses a real-time con-forecast,” Roman says. Data are streamed to any Internet-trol platform developed by Geosyntec called OptiRTC that interfaces with in-the-enabled device such as a computer or cell field measurement devices and Internet phone. “Real-time data streaming enables data feeds, such as weather forecasts. In end-users to assess and react to informa-particular, real-time data are collected tion in the field on a continuous basis,” from pressure transducers installed on the Roman says. “As a result, the system can upstream and downstream sides of the even be adapted to serve as a flood-alert valve. These data are logged to Internet-system based on measured and predicted connected servers. An algorithm deter-water levels.” Roman says any measure must work mines when to activate the compressor subassembly and inflate or deflate the valve in concert with the concerns of environ-to achieve intended water surface elevation mental groups regarding the welfare of goals. The controller is equipped with fail-upstream resources. “I think the real issue safe mechanisms for control when power is that if a landowner were to try to imple-is lost or to override the system in an ment a standard flood protection measure such as a flap gate to protect upstream in-emergency. The benefit of an RTG over a pas-frastructure from sea level rise, it would be sive flood protection measure such as a difficult to get the environmental groups self-regulating tide gate or floodwall is or local and state permitting authorities that it enables real-time monitoring and to sign on for that because it would end can be programmed to meet changing or up degrading upstream resource areas. unanticipated environmental conditions, With the RTG system, we can enable both groups to meet in the middle by providing Roman says. “Let’s say you install a flood barrier, but flood protection in concert with resource protection and restoration.” 20 years down the road, sea The RTG system will level rise is higher than antici-have more flexibility in ar-pated during the design phase, People eas with large tide ranges, or storms become more in-didn’t Roman says. While the tense. The flood barrier might Northeast has been the fo-not work as it was designed,” realize that cus of Geosyntec’s efforts, he he says. “If the RTG system is salt marshes envisions the RTG system used instead, relevant variables can be used in other coastal can be changed on the fly to were among scenarios. To date, the RTG adapt to unforeseen changes. the most system has not been used on Cycling between inundation productive a wide scale. goals, low-level goals, and “We’d like to test it out at flood control goals, one can ecosystems achieve a variety of complex, in the world. low-risk locations prior to
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