New Orleans and Hurricane Katrina. III: The 17th Street Drainage Canal

The failure of the levee and floodwall section on the east bank of the 17th Street drainage canal was one of the most catastrophic breaches that occurred during Hurricane Katrina. It produced a breach that rapidly scoured a flow pathway below sea level, so that after the storm surge had largely subsided, floodwaters still continued to stream in through this breach for the next two and a half days. This particular failure contributed massively to the overall flooding of the Metropolitan Orleans East Bank protected basin. Slightly more than half of the loss of life, and a similar fraction of the overall damages, occurred in this heavily populated basin. There are a number of important geotechnical and geoforensic lessons associated with this failure. Accordingly, this paper is dedicated solely to investigating this single failure. Geological and geotechnical details, such as a thin layer of sensitive clay that was laid down by a previous hurricane, proper strength characterization of soils at and beyond the toe of the levee, and recognition of a water-filled gap on the inboard side of the sheet pile cutoff wall are judged to be among the most critical factors in understanding this failure. The lessons learned from this study are of importance for similar flood protection systems throughout other regions of the United States and the world.     

Neural Networks for Estimation of Scour Downstream of a Ski-Jump Bucket

The estimation of scour downstream of a ski-jump bucket has remained inconclusive, despite analysis of numerous prototypes as well as hydraulic model studies in the past. It is partly due to the complexity of the phenomenon involved and partly because of limitations of the traditional analytical tool of statistical regression. This paper addresses the latter part and presents an alternative to the regression in the form of neural networks. The depth of the scour hole developed along with its width and length is predicted using neural network models. A network architecture complete with trained values of connection weight and bias and requiring input of grouped parameters pertaining to discharge head, tail water channel depth, bucket radius, lip angle, and median sediment size is recommended in order to predict the depth, the location of maximum scour, as well as the width of scour hole. The neural network predictions have been compared with traditional statistical schemes. Although the common and simple feed forward back propagation network took a very long time to train as compared to some advanced schemes, it was found to impart equally reliable training as the latter. Use of causative variables in grouped forms was found to be more rewarding than that of their raw forms probably due to lesser scaling effect.     

Hydrodynamic Modeling Approaches for Agricultural Storm Water Impoundments

Hydrologic modeling of storm water impoundments is an effective tool in evaluating different water management options for addressing regional water issues in Florida. However, modeling impoundment water dynamics could be challenging because of the difference in scale between canals and the entire impoundment. Water pumped into the impoundments is first discharged into canals inside the impoundments, which distributes the water. The canal eventually overflows and water floods all the impoundment. Two modeling approaches to simulate this process were tested on two impoundments using the integrated MIKE-SHE and MIKE 11 model. The first approach simulates the one-dimensional flow in the canal in a link-node model; and once water floods, it is modeled as two-dimensional flow. The second approach simulates the entire impoundment as a canal. In both impoundments, Modeling Approach 1 resulted in overestimation of peaks and poor results. Modeling Approach 2 showed considerable improvements in the results and a satisfactory match between observed and simulated water levels. The difference is attributed to the difficulty in representing the canal flooding process in hydrodynamic models.     

Flood Damage to Historic Buildings and Structures

This paper presents typical examples of damage to immovable cultural heritage due to flooding. Flooding can damage architectural heritage, historic infrastructure consisting of individual structures, buildings, and objects, as well as objects of art standing alone or firmly attached as an integral part of buildings. All these objects are subjected to various forces and actions during flood situations. These forces can be categorized according to the types of damage that they can cause: horizontal static pressure of raised water; upward hydrostatic pressure; dynamic low velocity streams; dynamic high-velocity streams; dynamic impact of waves; dynamic impact of floating objects; compacting of soils or infill; changes in subsoil conditions; saturation of materials with water; contamination of materials with chemical and biological agents; formation of barriers; ice floes; and postflood effects. These typical actions may occur in combinations and work in synergy. Typical illustrative examples of damage caused by individual actions are presented in the paper, which goes on to present some general lessons applicable for the most vulnerable categories of immovable heritage objects.     

Channel change and flood events since 1783 on the regulated river tay,Scotland: Implications for flood hazard management

An analysis of old maps and documentary sources reveals that major changes in river channel planform have occurred over the last 200 years on the River Tay system, Scotland, UK. Reaches showing natural river channel planform change, however, are relatively small and a stable planform is characteristic of many sections of the river. River planform instability appears to be controlled by channel bed slope, sediment load and the enhanced vulnerability of former river channel courses to erosion. Flood protection embankments built in the 19th and 20th centuries modified unstable multichannel wandering gravel bed river sections to narrower single-channel reaches, with limited lateral migration. On the River Tummel, 20th century impoundment has caused further geomorphological change in response to clearwater erosion close to the dam and aggradation processes within the regulated river downstream, but isolation of the effects of impoundment from those of channelization are problematic. An examination of the geomorphic effects of a high magnitude flood event in 1990 and historical accounts of earlier large floods reveal that the 1990 flood was the third largest since 1800 in the study area. Despite river regulation and bank protection the zones naturally characterized by instability are still susceptible to planform changes causing flood embankments to be breached, channel shifts and development of gravel bars.     

Integrated flood plain management strategy for the Vaal

Owing to development pressure for land bordering the Vaal river, in South Africa. Rand Water are revising their development policy. Restrictions were in place to avoid flooding and obstructing the flood flow of the river. Relaxation of the regulations will permit controlled development along the river. A flood hazard-risk index was developed to indicate where development could be permitted. An economic comparison of costs and benefits supports the relaxation.     

Modeling Landslide Dambreak Flood Magnitudes: Case Study

Landslide dams typically comprise unconsolidated and poorly sorted material and are vulnerable to rapid failure and breaching, resulting in significant and sudden flood risk downstream. Hence they constitute a serious natural hazard, and rapid assessment of the likely peak flow rate is required to enable preparation of adequate mitigation strategies. To determine the relative utility and accuracy of dambreak flood forecasts, field estimates of peak outflow rates from the failure of the Poerua landslide dam in October 1999 were compared with estimates from physical laboratory modeling, empirical methods, and computer modeling. There was reasonable agreement among the field estimates, laboratory modeling, and computer modeling. Some empirical estimates were less reliable. Reasonably reliable estimates of peak outflow can be obtained from computer model routines sufficiently rapidly to be of use in an emergency management situation. The laboratory modeling demonstrated the effect of dam batter slopes and valley bed slope on peak outflow; this information could be used to refine empirical or numerical estimates of peak outflow.