Urban Floodplain Research Articles

The Restoration of an Urban Floodplain in Rahway, New Jersey

Abstract The highly urbanized Rahway River watershed in New Jersey suffers from frequent flooding due to extensive development and destruction of riparian wetlands and floodplains. A diverse group of stakeholders worked together to restore a 1.8 ha site on the floodplain to riparian wetland with the goals of providing wildlife habitat, filtering pollutants from storm-water runoff, providing stormwater retention to minimize flooding, and providing public access for passive recreation as well as education about wetlands and watershed management. This wetland restoration site has been incorporated into the Union County Parks System as the Michael S. Bezega Wetlands Observation Area. Due to the urban setting (14 houses were removed from the site), the project design and implementation were very complex. Developing the water budgets required analyses of the tidal Rahway River as well as stormwater runoff from local drainage areas. Funding was obtained from six different funding agencies, and the project was constructed largely with city and county public works personnel. Native plants were installed by volunteers who continue to do invasive species management at the site. Key factors contributing to the success of this project include extensive involvement of a diverse partnership of stakeholders, a multi-disciplinary project team, thorough up-front design and engineering analyses, careful selection of vegetation palettes based on analyses of reference wetlands, and extensive on-site supervision of the construction crews by personnel who could make design decisions in the field.

The Impact of an Artificial Water Enhancement Scheme on Phosphorus Dynamics in an Urban Floodplain System in Vienna (Austria)

AbstractDue to human impacts, floodplains in urban regions are often simultaneously affected by eutrophication, loss of hydrological dynamics and even land use change. In order to mitigate some of these effects in a degraded floodplain within the city limits of Vienna, the “Dotation Lobau” a water enhancement scheme, was initiated in 2001. The present contribution demonstrates the effects of surface reconnection, with a low discharge (mean 0.2 m3 s–1), on the phosphorus budget, by comparing connected and isolated backwaters. The surface water supply resulted in a decrease of mean trophic levels due to a decreased hydrological retention time and the establishment of a transport phase (phosphorus output). Furthermore, the study indicated that primarily sediment‐associated processes, such as the accumulation or release of phosphorus, will influence the long term development of the floodplain. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modeling storm surge flooding of an urban area with particular reference to modeling uncertainties: A case study of Canvey Island, United Kingdom

A coupled storm surge and overland flow model is used to simulate extreme coastal flooding of Canvey Island, a dense urban area located at the mouth of the Thames Estuary, U.K. The flood model is based on a shock‐capturing numerical scheme and resolves the terrain and buildings of the study area with high‐resolution topographic data. Repeat simulation is used to propagate uncertainties in model inputs and parameters through to uncertainties in model outputs, and the main sources of uncertainties are described. The greatest uncertainties originate from the forcing inputs to the flood model, including coastal water levels and sea defense failures, rather than its internal boundaries, such as the model terrain and bottom friction. This is consistent with studies in rural and semideveloped floodplains and reflects a combination of high sensitivity to, and uncertainty in, the forcing inputs. However, other numerical and physical variables introduce significant uncertainties, of which many are specific to, or altered by, the urban floodplain. In particular, model predictions are sensitive to the spatial resolution (Δs) of the numerical grid. This includes the effects of Δs on boundary features (walls, hedges, etc.), which are described in the bed friction coefficient, and on the representation of buildings, which are described in the model terrain. Finally, model predictions are integrated across multiple combinations of breach width and location to explore the “relative” hazards (i.e., spatial and temporal variations in hazard) associated with the floodplain geography.

Reduced complexity strategies for modelling urban floodplain inundation

Significant advances in flood inundation modelling have been made in the last decade through the use of a new generation of 2D hydraulic numerical models. These offer the potential to predict the local pattern and timing of flood depth and velocity, enabling informed flood risk zoning and improved emergency planning. With the availability of high resolution DEMs derived from airborne lidar, these models can theoretically now be routinely parameterized to represent considerable topographic complexity, even in urban areas where the potential exists to represent flows at the scale of individual buildings. Currently, however, computational constraints on conventional finite element and volume codes typically require model discretization at scales well below those achievable with lidar and are thus unable to make optimal use of this emerging data stream. In this paper we review two strategies that attempt to address this mismatch between model and data resolution in an effort to improve urban flood forecasts. The first of these strives for a solution by simplifying the mathematical formulation of the numerical model by using a computationally efficient 2D raster storage cell approach coupled to a 1D channel model. This parsimonious model structure enables simulations over large model domains offering the opportunity to employ a topographic discretization strategy which explicitly represents the built environment. The second approach seeks to further reduce the computational overhead of this raster method by employing a subgrid parameterization to represent the effect of buildings and micro-relief on flow pathways and floodplain storage. This multi-scale methodology enables highly efficient model applications at coarse spatial resolutions while retaining information about the complex geometry of the built environment. These two strategies are evaluated through numerical experiments designed to reconstruct a flood in the small town of Linton in southern England, which occurred in response to a 1 in 250 year rainfall event in October 2001. Results from both approaches are encouraging, with the spatial pattern of inundation and flood wave propagation matching observations well. Both show significant advantages over a coarse resolution model without subgrid parameterisation, particularly in terms of their ability to reproduce both hydrograph and inundation depth measurements simultaneously, without need for recalibration. The subgrid parameterization is shown to achieve this without contributing significant computational complexity and reduces model run-times by an order of magnitude.

Use of fused airborne scanning laser altimetry and digital map data for urban flood modelling

AbstractFlood modelling of urban areas is still at an early stage, partly because until recently topographic data of sufficiently high resolution and accuracy have been lacking in urban areas. However, digital surface models (DSMs) generated from airborne scanning laser altimetry (LiDAR) having sub‐metre spatial resolution have now become available, and these are able to represent the complexities of urban topography. This paper describes the development of a LiDAR post‐processor for urban flood modelling based on the fusion of LiDAR and digital map data. The map data are used in conjunction with LiDAR data to identify different object types in urban areas, though pattern recognition techniques are also employed. Post‐processing produces a digital terrain model (DTM) for use as model bathymetry, and also a friction parameter map for use in estimating spatially distributed friction coefficients. In vegetated areas, friction is estimated from LiDAR‐derived vegetation height, and (unlike most vegetation removal software) the method copes with short vegetation less than ∼1 m high, which may occupy a substantial fraction of even an urban floodplain. The DTM and friction parameter map may also be used to help to generate an unstructured mesh of a vegetated urban floodplain for use by a two‐dimensional finite element model. The mesh is decomposed to reflect floodplain features having different frictional properties to their surroundings, including urban features (such as buildings and roads) and taller vegetation features (such as trees and hedges). This allows a more accurate estimation of local friction. The method produces a substantial node density due to the small dimensions of many urban features. Copyright © 2007 John Wiley & Sons, Ltd.

The Impact of `Greater Dhaka Flood Protection Project' (GDFPP) on Local Living Environment – The Attitude of the Floodplain Residents

This paper aims to examine the impact of large-scale structuraladjustments (like the Greater Dhaka Flood Protection Project, GDFPP) on local living environment.It focuses the importance of environmental factors in flood hazard mitigation, and examines theenvironmental attitudes of the floodplain residents arising from the large-scale structural adjustments.Based on `perceived natural hazard research perspectives', this paper examines: (i) the reasons for persistentfloodplain occupation, and (ii) the importance of environmental factors in the choice, motivations and decision-makingof floodplain residents. This research used data collected from 300 households situated inthe eastern part of Dhaka. The face-to-face household survey data provided individuals' responses to a structuredquestionnaire on hazards and environment. Survey concerned urban floodplains, and looked fordata on housing, household characteristics, and residents' attitudes. Results of interview surveys wereused to: (i) explore the reasons of floodplain occupation, and (ii) residents' attitudes to tolerable levelof flood risk and willingness to accept environmental change resulting from the proposed structural embankments inthe eastern perimeter of Dhaka City, Bangladesh. Findings revealed that floodplain occupation (by theindividuals' decision-making) was a result of overall reaction to the Government's structuraladjustment policies that resulted from institutional, locational and socio-economic factors. The attitude survey results provided residents' perception to hazards and environment to be dependenton the socio-economic factors – but in a complex manner, many factors are interrelated.In addition to support for structural embankments, the study sample displayed a common concernand widespread environmental awareness. In terms of any `trade-off' between thebenefits (resources) from the embankments and costs (hazards) due to the detrimental impact on environment, the residents of Dhaka, despite some concern forsacrificing embankments for environment, tended to show a generalconsensus for embankments.

Planning as a Tool to Deal with Extreme Events

This paper describes the recent changes in the Water Resources Management System of Brazil and its proposed approach on how to be more effective when dealing with extreme events. The geographic extent of the country, its regional differences, and the difficult economic conditions lead to a comprehensive reform in the water resources sector in order to increase efficiency of the water uses, to reduce costs, and to promote development. The major change occurred in 1997, with the promulgation of the National Water Resources Management Act, which established the National Water Resources Policy and the National Water Resources Management System. A primary strength of the new system is its decentralization. Different geographical regions of the country must cope with different extreme events. The developed South suffers with frequent flood events due to large urbanized areas and impervious floodplain areas and with water scarcity due to excessive use. The rural Northeast is frequently devastated by severe drought periods with consequences such as migration and poverty. In both cases, the decentralized decision process is a powerful tool to better manage such problematic areas. In fact, to help controlling extreme events is one of the major objectives of the new system, as stated in the law. In intermittent rivers of the Brazilian Northeast, participation of users associations can help finding the best water allocation scheme. When it comes to flood control, community participation in the selection of solutions is essential. Flood protection is a community rather than individual benefit. Decision on the land use will only be accomplished with the participation of different government levels together with the community.

Modelling Uncertainty in Flood Studies

Estimates of average annual damage (AAD) are a key central component of the hydrologic, hydraulic, and economic information developed in the evaluation of flood damage reduction plans. AAD or the expected value of annual damage, in dollars or other monetary units, is a probability-weighted average of the economic losses associated with the full range of possible flood magnitudes. Economic benefits are assessed as the reduction in AAD which would result from implementation of a particular plan. Traditional methodologies for performing hydrologic, hydraulic, and economic studies and estimating AAD have recently been expanded to explicitly incorporate analysis of the uncertainties inherent in the data collection and modelling effort. This paper presents a strategy for modelling uncertainties in the estimation of AAD for ungauged urban floodplains.

Sediment Problems in Korea

The Korean peninsula has been subject to hydrological extremes since historic times. This forum article shares some insights into the sediment problems Korea presently faces, and it briefly introduces several investigations that have been, or are being, carried out in Korea. In particular, the article focuses on recent problems associated with sediment erosion and accumulation in streams, rivers, and reservoirs, and around several hydraulic structures in Korea. Problems include riverbed degradation, bridge scour, urban floodplain deposits, sedimentation at municipal water intakes and water intakes for nuclear power plants, and accelerated erosion at construction sites. The most important single issue concerning sediment in Korea is variation in alluvial-riverbed elevation. Riverbed changes cause problems in many areas, including flood control, navigation, and performance of various hydraulic structures. Hydraulic engineering and research activities concerning sediment have risen in interest and diversity over the past decade in Korea.

The role of phosphorus and heavy metals in the spread of weeds in urban bushlands: an example from the Lane Cove Valley, NSW, Australia

Weed invasion of bushland in the Lane Cove Catchment, to the north west of central Sydney, is related to contamination of the floodplain by heavy metals and nutrients, increased frequency of flooding resulting from the urban development that surrounds the bushland, and site disturbance. Weed species are significantly different between the floodplain areas and the disturbed hillslope areas, probably as a result of differences in soil pollution, microclimate, and the nature of the disturbance. Seed availability does not seem to be a critical factor in determining weed invasion and composition. Weed invasion of disturbed upland areas could be reduced by replanting native plants. For contaminated floodplains there is little that can be done to restore the original vegetation unless the source of pollution is removed, and even then some considerable time may be required before pollution levels in the floodplain sediments decline. Use of native mesic vegetation, which is more adapted to higher nutrient levels, may be necessary for the management of contaminated floodplains. Whether the introduction of stormwater control systems into new subdivisions can avoid the contamination of urban floodplains is uncertain. There is an argument for accepting the inevitable contamination of urban floodplains and deliberately managing landscape ecology for this eventuality.

Choice and constraints in floodplain occupation: the influence of structural factors on residential location in peninsular Malaysia.

In Peninsular Malaysia 'structural' factors are found to influence strongly people's persistent occupation of floodplains. Thus, despite a high level of flood hazard awareness, a high level of pessimism and a high level of expectation of future floods, poorer individuals seldom attempt to leave for more advantageous locations but are instead trapped in their present locations by structural factors such as poverty, low residential and occupational mobility, low educational attainment, traditional land inheritance, government aid, and government disaster preparedness, relief and rehabilitation programmes. These forces exert a strong influence upon individuals and largely control their choice of residential location in response to flood hazards, thereby reinforcing the persistent occupation of floodplains. Structural factors such as landlessness, rural-urban migration, floodplain encroachment and squatting are also highly influential in leading people to move. Even for those who move, structural factors have largely confined their choice of residential location to urban floodplains.

Two‐Dimensional Water Flood and Mudflow Simulation

FLO‐2D is a two‐dimensional finite difference model that simulates clear‐water flood hazards, mudflows, and debris flows on alluvial fans and urban floodplains. Interactive flood or mudflow routing between channel, street, and floodplain flow is performed using a uniform grid system to describe complex floodplain topography. A quadratic rheological model, developed from field and laboratory mudflow data, enables appropriate simulations of flooding conditions ranging from clear water to hyperconcentrated sediment flows. Computer‐aided design (CAD) graphics of predicted time‐sequenced flood depths automates the delineation of flood hazards. Replication of the 1983 Rudd Creek mudflow in Utah demonstrates the capability of the model.

Poldering vs compartmentalization: The choice of flood control techniques in Bangladesh

The choice of flood control techniques in Bangladesh is reviewed in the context of a case study on the impact of a small-scale polder, the Dhaka-Narayanganj-Demra (DND) Project. The results of a questionnaire survey among the urban floodplain residents, who have settled inside the polder and a control population from outside the polder, indicate that the project has achieved significant flood alleviation but has also induced considerable environmental degradation due to stagnation of water within the enclosed embankment. The project also experienced numerous problems of structural instability of embankments during major flood events. An attempt is made to extrapolate some of these adverse environmental impacts of the polder to the compartmentalization scheme, which has been proposed by a UNDP-sponsored Flood Policy Study as an innovative technique for floodplain management in Bangladesh. The findings of the study, however, do not provide an adequate basis for assessing viability of the proposed compartmentalization scheme. Nonetheless, the study illustrates the usefulness of questionnaire surveys among floodplain residents for obtaining data for project evaluation.

The Urban Floodplain as a Focus for Geographical Education

The Urban Floodplain as a Focus for Geographical Education

Flood Hazard in Kansas City since 1880

Occupance and management of the Blue River floodplain in Kansas City illustrate the intensity of development despite increasing flood hazards. They have led to adoption of structural controls and floodplain regulations, although flood-control measures receive a low priority from the city. Evidence suggests that continued development may limit their effectiveness. T HE purpose of this article is to improve understanding of the relationships among settlement, flood hazards, and flood-control policies in urban areas. The approach here is to explain how a certain portion of the Blue River floodplain in Kansas City, Missouri, was urbanized, to examine responses of decision makers to local flood hazards, and to evaluate the effectiveness of structural controls and managerial practices. We selected the lower Blue River floodplain, because it is intensely developed and is presumably representative of other urban floodplains in the United States (Fig. 1).1 We were fortunate to find archival materials, many of them private, that dated back more than a century to the period when urbanization was initiated on this floodplain. Our analysis begins with a brief description of the physical parameters to facilitate comparisons with other urban floodplains. This type of information often appears in legal documents prepared in the late 1960s and early 1970s, when many American communities mapped their floodplains and assessed flood hazards in order to participate in the National Flood Insurance Program (NFIP). The Blue River, a tributary of the Missouri River, drains an area of approximately 272 square miles. The shape of this drainage area is almost rectangular with a maximum length of 30 miles and a maximum width of 12 miles. Surface elevations range from 1,135 feet in the headwaters region * We express our gratitude to Donald Hurlbert, Alan McCoy, and Earl Kane for comments on an early draft of the text. We thank the federal, state, and local officials as well as the personnel at Armco who facilitated our research. ' Gilbert F. White and others, Changes in Urban Occupance of Floodplains in the United States, University of Chicago, Department of Geography Research Paper 57, Chicago, 1958. e DR. DRIEVER is an associate professor of geography at the University of Missouri-Kansas City, Kansas City, Missouri 64110-2499. DR. VAUGHN is an assistant professor of geography at Jacksonville State University, Jacksonville, Alabama 36265. Copyright ? 1988 by the American Geographical Society of New York This content downloaded from 157.55.39.123 on Mon, 18 Jul 2016 05:46:51 UTC All use subject to http://about.jstor.org/terms THE GEOGRAPHICAL REVIEW

Changes in American urban floodplain occupancy since 1958: the experiences of nine cities

In 1958, researchers from the University of Chicago documented increases in the number of structures in the floodplains of 17 American cities, a phenomenon attributed to the prevalence of flood control structures. Because federal policies have shifted to managing floodplains, this paper updates the 1958 study for nine of the cities. Several trends are apparent. Development pressures determine flood-plain encroachment; floodplain management regulations have been implemented where it was easiest to do so. Further, structural measures continue to dominate. Thus, while many local officials are becoming more aware of advantages of flood-plain regulation, implementation and enforcement are inconsistent and uneven.

Urban Flood Damage Estimating Curves

Because of increased flood damages resulting from development in flood plain areas many local and state politicians in Georgia have expressed interest in adopting state legislation which would encourage or mandate the development of local stormwater management programs. Before such legislation is drafted or programs developed for specific areas, the legislature wanted some documentation to estimate the magnitude of present and future flood damages for urban areas throughout the state. Thus, one of the objectives of a recently completed research project was to develop general flood damage curves to roughly estimate flood damages for urban flood plain areas. The development of the flood damage curves was accomplished by using the results from an economic computer model and its application to generalized data in Georgia. The results of applying curves to five case study areas in Georgia are given. Uses and limitations of the curves are outlined.

Knowledge, Warning, and Stress

A field study investigated the role of knowledge, experience, and warning in medi-ating the stress associated with urban flooding. Among residents of an urban floodplain, knowledge about flooding was associated with less trauma during the last flood and reluctance to support indiscriminate government intervention. Increased prior warning of a flood, however, was related to intensified residual fear and feelings of desperation, to more frequent somatic manifestations of stress, and to greater support for all government intervention programs. Previous flood experience (especially recurring experience) was associated with increased fear, depression, and health-stress outcomes. Commitment to adaptive community intervention programs was related to recency of one's flood experience. Having flood insurance was unrelated to stress measures.

Preliminary model of an urban floodplain under changing land use

The hydrologic response of Brays Bayou, a rapidly developing watershed located in southwest Houston, was modeled using the U.S. Corps of Engineers HEC-1 flood hydrograph package. Six subwatersheds were delineated using existing drainage specifications and topographic maps, while aerial photography revealed the current land-use patterns. Unit responses of the individual subcatchments were generated as a function of various basin parameters and the extent of urbanization. Calibration was achieved using several historical storm events at four separate gauges within Brays watershed. The detention storage capacities in the less developed subwatersheds were simulated by placing a reservoir at the outlet of each subwatershed. The additional storage necessary to reduce the peak discharge downstream to a level below the capacity of the channel was considered to be detention storage. A variety of rainfall patterns with different durations and intensities were used to determine the design detention storage for each subwatershed. Land uses were simulated for present conditions and the predicted ultimate scenario.

Threat, Knowledge, and Support for a Collective Response to Urban Flooding1

The most realistic solution to recurring urban flooding in most American cities is some form of land use planning (zoning), i.e., an integrated, collective response to this type of threat to the community. Yet there is frequently little public support for such action. The present study surveyed persons living in and around a notorious urban floodplain to assess support for such land use planning as a function of (a) threat, i.e., the motivational effect of proximity to the floodplain, ownership of property at risk, personal flooding history, perceived likelihood of one's home being flooded in the next 5 years, and the presence of small children in the home; and (b) amount of knowledge about variables affecting flooding, i.e., the cognitive element. Although proximity was related strongly to expressions of concern, none of the measures of threat was related to support for land use planning. Knowledge of the variables involved, however, was positively related to support for a planning solution. The findings are discussed in terms of theoretical implications of the cognitive element in collective responses to environmental hazards.