Modeling Great Britain s Flood Defenses AIRCurrents Editor s note: AIR launched its Inland Flood Model for Great Britain in December 2008. The hazard module captures the physical processes of rainfall-runoff generation, overland surface flow, river routing and flood 01.2009 inundation. In Great Britain, the flood defense system plays an important mitigating role. When defenses fail, however, the consequences can be catastrophic. In this article, AIR Research Hydrologist Dr. Yizhong Qu describes typical defenses found in Britain, how they are treated in the AIR model, and the potential impact of their failure on insured losses. Flood Defense in Great Britain For centuries, as populations have become more concentrated and therefore more vulnerable, man-made flood defenses have been built to supplement natural ones. In Great Britain, there are about 30,000 kilometers of coastal and inland flood defenses and more than 50,000 point structures. Natural defenses typically refer to the banks that form naturally along the course of alluvial rivers as a result of erosion and sediment deposition when the flow overtops the bank. Over time, natural waterways adjust their shape and conveyance to the precipitation regime over the river basin upstream. Geomorphologic research indicates the bankfull flow of a river channel that is, the maximum flow that natural river banks can accommodate without overtopping has a return period of about 2 years. Figure 1a shows an example of natural river banks. To enhance the protection offered by natural river banks, other man-made flood defenses are built. In Great Britain, these are dominated by four types: embankments, flood walls, storage areas and point structures. Embankments (an illustration is provided in Figure 1b) are widely used. They are typically made of local materials with erosion protection in the form of rock armor or revetment on the sides and top. The fill materials may be soil or rock, depending on local availability. As of 2003, there were more than 15,000 km of embankments in Environmental Agency s National Flood and Coastal Defense Database, which represents more than 90% of the total length of all inland defenses. Flood walls, as their name suggests, are vertical structures that protect properties from floods of a defined return period a design level referred to as the Standard of Protection (SoP). These are generally made of concrete or steel, and achieve stability by their gravity or a supporting structure. They are designed and constructed according to rigorous structural and geotechnical codes. Figure 1c provides an illustration of a flood wall. Storage areas are a very important component in flood mitigation and may be natural or man-made. They may take the form of lakes, reservoirs, wetlands, ponds or roadside
swales. Flood storage areas temporarily accommodate storm water and thereby reduce peak river discharge. The effectiveness, ecological function and low maintenance costs of storage areas make them one of the preferred solutions for storm water management. Figure 1d shows a road-side swale along the site of a new housing development. Finally, hydraulic structures are built at locations where they are needed to change flow conditions or even alter the flow course for example, to divert, lift or stop the flow altogether. Pump, sluice and gates are examples of such structures. Figure 1e shows an example of a sluice. In reality, flood defenses can be comprised of a combination of several of the types discussed above. For example, Figure 1f shows an example of an embankment combined with a vertical flood wall. Modeling Flood Defenses For the purpose of modeling flood risk, it is important to capture the impact of flood defenses, both when they fail and when they don t. The AIR model employs large databases of flood defenses that include such detailed characteristics as type, Standard of Protection and grade. Flood storage areas are extracted from detailed topographic maps and high resolution digital terrain models. In the case of storage areas such as lakes and reservoirs, these mitigate flood risk to the extent that they divert water from the stream or river link into which a catchment flows. In essence, they exert an attenuating effect by increasing the time before the peak river flow is reached and decreasing the peak flow itself. The digital terrain model is critical in quantifying the attenuating effect for each storage area, which among other things is a function of the surface area of the lake or reservoir, the contributing area of the catchment that the lake or reservoir lies within, and the total number of lakes and reservoirs within the catchment. The digital terrain model used by the AIR model is from the Ordnance Survey and has a 10-meter horizontal resolution and a 0.1-meter vertical resolution. Figure 2 shows the spatial distribution of reservoirs and lakes across Great Britain. Figure 1. Types of flood defenses: (a) natural river banks; (b) embankment; (c) flood wall; (d) roadside swale; (e) sluice; (f) combined embankment and flood wall. Source: National Audit Office (UK) and AIR Figure 2. Reservoirs and Lakes in Great Britain with Diameter of 100 meters 2
Figure 3 shows a topographic map of the River Thames near the London borough of Kensington and Chelsea. Note that the vertical flood walls along both banks can be clearly seen. Again, because these structures are in the digital terrain model, they are recognized in the cross-sectional geometry data used for hydraulic modeling. Thus in the AIR Inland Flood Model for Great Britain, the flood defenses are accounted for physically. Modeling Flood Defense Failure Flood defenses fail for a variety of reasons and by various mechanisms. As simple earthworks, embankments are extremely vulnerable to erosion, particularly when they are overtopped. When that happens, a localized breach may quickly spread if no remedy is taken. Even when the hydraulic loading is less than the design level and there is no overtopping, embankments can fail from breach due to external factors or piping. For example, rodent infestation may have weakened the internal structure, which can lead to excessive seepage. When the seepage velocity is high, a form of erosion known as piping can occur due to the frictional forces on the soil particles, ultimately resulting in the failure of embankments. Cracks caused by earthquake or landslide may also cause breaches even when the hydraulic loading is low. Figure 3. Topographic map of area near Chelsea. The vertical flood walls are clearly seen along the River Thames. When the peak river discharge is above the design level, or SoP, of defense that is, when the river flow overtops the embankment or flood wall, the model allows flood water to spread beyond the defense and flood elevation is calculated over the floodplain. Figure 5. Breached embankment. Source: Performance and Reliability of Flood and Coastal Defences When the peak discharge is less than the SoP and the flood defense does not fail, there will be no overtopping and therefore no flooding on the floodplain, as illustrated in Figure 4a. If the defense fails, however, the hydraulic model calculates flood extent and depth as illustrated in Figure 4b. Flooding walls and point structures (including those that regulate storage areas) are generally less vulnerable, as they are constructed according to rigorous code. Still, they are susceptible to toe erosion and corrosion of structural members when the loading is low. As the hydraulic loading increases beyond the design level, the probability of failure is much higher and ultimately approaches 100%. In all cases, periodic inspection and proper maintenance can help reduce the risk of failure significantly when the hydraulic loading is less than the design level. Figure 4. Modeled flood extent when (a) defense holds and (b) when defense fails. 3
While the presence of flood defenses is accounted for physically in the AIR Inland Flood Model for Great Britain through the digital terrain model, the failure of those defenses is modeled probabilistically. In addition to information from the Environmental Agency s National Flood and Coastal Defense, the AIR model leverages information on more than 4,000 dams and reservoirs. Nevertheless, nationwide information on the dimensions, specifications and conditions of flood defenses is generally not available. Therefore, the failure of flood defenses is modeled using fragility curves such as the one illustrated in Figure 5. The fragility of a flood defense is defined as the probability of failure given an intensity of loading, where intensity in this case is peak discharge. The gray fragility curve shown in Figure 6 is a simplified one commonly used in hydraulic modeling. It indicates that that the probability of failure is zero until the design level is exceeded, when the probability of failure becomes 100%. In reality, however, although the defense is designed for a specific SoP, there remains a non-zero chance that it will fail at levels below the design SoP. Failure here may be a result of piping, as discussed above, or structural failure due to corrosion of construction materials. Similarly, there is a nonzero chance that the defense will hold for levels of loading greater than the SoP, since design codes are typically conservative. This more realistic fragility curve is the one employed by the AIR model. Ultimately, however, for severe events, the probability of failure reaches 100%. In the AIR model, fragility curves are developed for every stream link in the river network, and their shape varies based on the type and grade of the defense. The failure condition (fail or not fail) is sampled probabilistically based on the fragility curve, by link, by event. Once failed, the modeled flood elevation is used to calculate losses. Otherwise, the defenses are assumed to have played their role and no loss is calculated within the protected area. The Importance of Flood Defense The use of fragility curves to model the probability of flood defense failure makes it possible to perform what if scenarios, such as exploring the mitigative effect of increased governmental spending to improve the state of repair of existing flood defenses, or the consequences of doing nothing. Or what would insured losses be were there no inland defenses at all (or, put another way, if they all failed)? The importance of flood defenses is highlighted by answering the last question. AIR estimates that were there no inland flood defenses in place during the devastating floods in Great Britain of June and July 2007, insured losses would have been in excess of GBP 6.5 billion more than double the actual reported loss of GBP 3 billion. The debate over whether and how much to spend on the improvement of flood defenses in Great Britain continues and is a particularly challenging one, given the current economic downturn. There is little doubt, however, about their mitigating impact particularly as the number of new homes and businesses built on floodplains continues to rise. Figure 6. Flood Defense Fragility Curve 4
About AIR Worldwide Corporation AIR Worldwide Corporation (AIR) is the scientific leader and most respected provider of risk modeling software and consulting services. AIR founded the catastrophe modeling industry in 1987 and today models the risk from natural catastrophes and terrorism in more than 50 countries. More than 400 insurance, reinsurance, financial, corporate and government clients rely on AIR software and services for catastrophe risk management, insurance-linked securities, site-specific seismic engineering analysis, and property replacement cost valuation. AIR is a member of the ISO family of companies and is headquartered in Boston with additional offices in North America, Europe and Asia. For more information, please visit www. air-worldwide.com. 2008 AIR Worldwide Corporation. All rights reserved. 7