Maximum Flood Level Research Articles

Optimum design of large flood relief culverts under the A89 motorway in the Dordogne?Isle confluence plain

The planned A89 motorway will go through the Dordogne — Isle confluence plain, which is regularly flooded under the effect of both river discharges and ocean tides. Hydraulic transparency of the motorway embankment was one of the prerequisites imposed by the French government. In order to optimize the cost/efficiency ratio of large culverts located under the motorway, their position and size had to be determined with great accuracy. This task has been fulfilled thanks to complementary sophisticated physical scale models and 2-D numerical models. The schematization principles adopted in the 2-D numerical model concerning bridge pier grouping were validated using the physical scale model. The finite element TELEMAC-2D code was used to draw up the optimum design of this river engineering scheme. TELEMAC-2D solves the 2-D Shallow Water equations on non-structured grids in the presence of alternately dry and wet beds. The 2-D numerical modelling based on high quality topographic data allows considerable improvements to be made in the computation of flood flows in a flat valley, as compared with traditional one-dimensional techniques. Near flood relief culverts in particular, the various physical effects contributing to the overall head loss can be distinguished: bottom roughness, medium size bed variations, strong curvature of the streamlines and vortices behind abutments. The hydraulic impact of the optimum solution has been studied under different aspects (rise of maximum flood levels, increase in maximum velocity, changes in the water flow patterns, submersion time of flooded land, modification of flood routing characteristics) for a wide range of flood hydrological events. Copyright © 2000 John Wiley & Sons, Ltd.

LATE HOLOCENE SEDIMENTATION RATES AND GEOMORPHOLOGICAL SIGNIFICANCE OF THE NCAMASERE VALLEY, OKAVANGO DELTA, BOTSWANA

ABSTRACT This paper presents the preliminary results of geomorphological and sedimentological investigations in the Ncamasere Valley, a seasonally back-flooded tributary of the Okavango River in northwestern Botswana. Geomorphological studies indicate the importance of the Okavango River flood regime for determining the past and present hydrology of the Ncamasere, and suggest that water levels in the Okavango have been up to 1.5 m higher at times during the Holocene compared to maximum historical flood levels. A series of cores supported by four radiocarbon dates, taken in a transect along the valley, indicate the chronology of mid- to late-Holocene sedimentation since c. 3850 yrs BP. Analyses reveal that much of the sedimentary fill within the valley is derived from Okavango back-flooding as opposed to down-valley transportation. Studies of organic horizons within cores indicate the differences in organic material generated within different ecological habitats within the Okavango system and confirm the highly dynamic nature of organic cycling within this wetland environment.

Great Lakes Flood Thresholds and Impacts

Using the location, data, and water levels from flood events along the Canadian shore of the Great Lakes, flood damage thresholds were determined to identify and compare water levels at which static and storm-induced high water impact shoreline interests on several shore reaches of Lakes Erie, Huron, Ontario, and St. Clair. Spatial differences identified may be related to several factors, including: 1) nearshore bathymetries; 2) extent of residential development along low-lying shorelines; 3) degree of riparian adjustment to flooding; and 4) location relative to dominant wind or storm directions. Correlation analyses found that flood damage levels are more closely correlated to fluctuations in static levels on Lakes Ontario, Huron, and St. Clair, while flood damage levels are more closely correlated to maximum instantaneous water levels on Lake Erie. Correlation analyses of individual gauge data identified locations possibly more susceptible to storm surges. A conservative approach to determining flood damage thresholds is suggested, being based on a standard deviation below the mean of maximum instantaneous flood levels for a given gauge. The standard deviation threshold, while lower than current “critical levels” used in management, is more representative of the majority of flood damage levels than thresholds based on lowest maximum instantaneous lake levels. However, caution is urged in applying any critical level solely based on water level gauge information as Great Lakes flooding is a highly site-specific phenomenon influenced by meteorologic factors.

Flood risk assessment using an integrated hydrological and hydraulic modelling approach: a case study

This paper describes an integrated hydrological and hydraulic modelling approach for the risk assessment of a flood-prone area and its application to analysing the effects of extreme flood events on the Montalto di Castro thermoelectric power plant. The approach is based on four major steps. The first step entails a detailed analysis of available critical events as well the collection of hydro-meteorological and cartographic data to perform a statistical evaluation of extreme rainfall events and an estimation of the probable maximum precipitation (PMP). The second step involves the calibration of a rainfall—runoff model for the upper catchment area based on the data observed during a recent flood event. The third step involves the calibration of a two-dimensional hydraulic model for simulating the flood plain inundation using the previously reconstructed runoff and a comparison of the results with the maximum flood levels observed during the same event. The fourth and final step concerns the simulation by the two-dimensional hydraulic model of the flood wave obtained via the rainfall—runoff model using the extreme and PMP values of rain defined in the first step. The results of this approach appear to be extremely useful and easily transferable to other areas.

Estimation of expected maximum possible water level along the Meghna Estuary using a tide and surge interaction model

In this paper an estimate of expected maximum flood levels associated with tide and surge interaction is done along the Meghna River estuary in Bangladesh. For this purpose, a vertically integrated model is developed using shallow water equations. In fact, a fine mesh estuarine model is nested into a coarse mesh bigger model covering up to 15° N of the Bay of Bengal. The fine mesh is considered in order to incorporate the islands and bending cost line properly in the numerical scheme. Appropriate tidal condition is generated in the estuary by applying tidal forcing through the southern boundary of the bigger model. For tide and surge interaction purpose a circulatory system, representative of a tropical storm, is introduced in the previously generated tidal oscillation. Thus, the model is verified for a few previous tropical storms. Then hypothetical storms of highest possible intensity are allowed to move along different tracks favorable for high surges so that they (storms) reach the coast during high tide in the estuary. It is seen that the surge level is highly sensitive to the path of the storm. Expected maximum possible water level at each location is estimated based on highest possible intense storm and the path whichever contributes to the highest surge at that location.

Variations of the solar activity and irradiance, and their influence on the flooding of the river Nile

Autocorrelation and power spectra analysis are carried out for different lengths of time series Of the following data:(l) Solar activity (sunspot, faculae and radio flux on 2800 MHz);(2) Irradiance (solar Constant measured at earth's surface and by the artificial satellite (Nimbus- 7); (3) River Nile flood (old water level, maximum flood level, and the difference between the both levels) measured at Cairo.
 
 The results showing remarkable similarity between the power spectra of solar activity, irradiance (solar constant) and river Nile flood.
 
 We conclude that any short or long-term variations in the solar activity lead to similar variations in the solar constant. Also, annual and secular variations of solar activity yield information’s on the suspected annual and secular variations of the river Nile flood.

HURRICANE ALICIA: STORM SURGES AND SHORE PROCESSES

Hurricane Alicia moved inland over the Texas coast during the night of August 17, 1983 creating waves and surges in the Gulf of Mexico and adjacent bays. Waves eroded beaches and dunes and surges overtopped low-lying areas of barrier islands and inland areas adjacent to the bays behind the barriers. A three-day survey of field evidence of water levels and flow directions was carried out one week after the storm. Physical evidence, such as the elevation of debris lines, water marks in buildings and debris caught on fences was used along with additional data from tide gages operating in the area to estimate the maximum flood levels and flow directions associated with the storm. Before and after aerial photography was used to obtain data on beach recession, retreat of the vegetation line behind the beach and extent of oyerwash deposits. The evidence gathered shows that the barrier islands were overtopped from front-to- back in some areas and from back-to-front in other areas with quite different results. There was little or no beach erosion to the left of the storm as it came ashore; however, serious beach erosion occurred for 18 miles (29.0 km) to the right of the storm and there was significant erosion for 55 miles (88.5 km) to the right of the storm. Maximum water levels in the Gulf, including the effects of normal tides and storm effects, were 9 to 11 feet (2.74 to 3.35 m) and maximum water levels along the nothern portion of Galveston Bay were 11 to 14 feet (3.35 to 4.27 m).

Tennessee Valley Extreme Wind Speed Climatology

The hydrologic design of dams and determining flood levels for nuclear plant siting requires postulating windwave occurrence concurrent with maximum flood levels. Practical postulations of windwave magnitudes require knowledge of wind probabilities by direction and time of year. Probability estimates of daily extreme 30‐minute and 60‐minute wind speeds are presented for each direction for the months of March, April, and June through September for the Tennessee Valley. Some broad guidelines concerning generalized regions of homogeneous wind speeds are also presented. The methods used are applicable to a wide variety of applied problems requiring analyses of wind extremes and comparisons between frequency distributions.

Mathematical model for flood and embankment prediction in a tidal river

A mathematical model is developed for a study of the interaction of an incoming tide from the downstream end with an abnormal flood discharge from landward end in a tidal river. It is shown that the study determines the maximum flood level and in turn the design height and spacing of embankments in the river. The problem is solved numerically by using the explicite finite difference scheme combined with the leap-frog operator. The computational results of the model are compared with the observed data in the river Rupnarain.

Changes in sea level in the German Bight

Summary. Tidal data recorded in the German Bight over the last 200 yr show that MHW (mean high water) has risen by about 20 to 30 cm per century. Corresponding data on maximum flood levels over the last 400 yr show a similar increase of approximately 25 cm per century. At Cuxhaven, the graph of maximum annual water levels drawn from 150 yr of continuous recordings reveals a long term oscillation superimposed on the progressive linear increase in water levels. This oscillation has a period of roughly 80 yr and extrapolation suggests the next peak should occur around 1985. These rates of increase in sea level over recent centuries are compared against the perspective of much longer term trends.

Flood Protection of Crystal River Unit 3 Nuclear Plant

To satisfy U.S. Atomic Energy Commission (AEC) safety criteria, a required evaluation of the worst site-related flood is performed for the Crystal River Plant, located on the Gulf Coast of Florida, the probable maximum stillwater flood levels are likely to be a result of the probable maximum hurricane. Flood protection requirements for the Crystal River Plant are determined by considering the most severe combination of probable maximum hurricane parameters for the Gulf Coast Region. These parameters are used as input to a model of hurricane surge generation and attendant wave activity in order to determine the maximum flood levels at the Crystal River Plant.