Timely Low Resolution SAR Imagery To Support Floodplain Modelling: a Case Study Review

It is widely recognised that remote sensing can support flood monitoring, modelling and management. In particular, satellites carrying Synthetic Aperture Radar (SAR) sensors are valuable as radar wavelengths can penetrate cloud cover and are insensitive to daylight. However, given the strong inverse relationship between spatial resolution and revisit time, monitoring floods from space in near real time is currently only possible through low resolution (about 100 m pixel size) SAR imagery. For instance, ENVISAT-ASAR (Advanced Synthetic Aperture Radar) in WSM (wide swath mode) revisit times are of the order of 3 days and the data can be obtained within 24 h at no (or low) cost. Hence, this type of space-borne data can be used for monitoring major floods on medium-to-large rivers. This paper aims to discuss the potential for, and uncertainties of, coarse resolution SAR imagery to monitor floods and support hydraulic modelling. The paper first describes the potential of globally and freely available space-borne data to support flood inundation modelling in near real time. Then, the uncertainty of SAR-derived flood extent maps is discussed and the need to move from deterministic binary maps (wet/dry) of flood extent to uncertain flood inundation maps is highlighted.     

Freshwater inflow and estuarine variability in the Gamtoos Estuary, South Africa

The Gamtoos is a shallow flood-tidal estuary located on the south coast of South Africa. Even though it has an extensive catchment area, dams limit runoff and mean freshwater inflow is estimated at less than 1 m³ s^?1, and the flood tidal deltas constrict and at times even close the mouth. The results presented here derive from an intensive measurement program carried out over a 3-wk period at the end of 1992, immediately after good rains in the Gamtoos catchment region. Freshwater inflow increased to more than 10 m³ s^?1, driving the salt wedge downstream and resulting in intense haloclines in the mid-estuary region. The program monitored the return to more average estuarine structures, and even though tidal exchange was restricted, marked differences occurred in stratification at neap and spring tides; tidal exchanges provided the dominant mixing forces. It is found that the shallower upper reaches of the estuary are flushed with relatively small increases in freshwater inflow, though a balance exists with the tidal exchanges through the constricted mouth. The variation in the position of the salt wedge and in the salinity stratification can have substantial implications for biota.     

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Morphology of Red Creek,Wyoming, an arid-region anastomosing channel system

Red Creek, in the Red Desert area of the Great Divide Basin, Wyoming, is an arid-region anastomosing stream. The narrow, deep, and sinuous main channel is flanked by anastomosing flood channels, or anabranches. Most anabranches are initiated at meander bends. The primary mechanism of anabranch initiation is avulsion during overbank floods. Anabranch enlargement occurs by headward erosion. Anabranches act as distributary channels during floods, when water and sediment from overbank flows are transported to and deposited on the floodplain via the anabranches. During periods of low discharges, the anabranches act as tributaries to the main channel, transporting runoff from the floodplain and surrounding hillslopes to the main channel of Red Creek. Aggradation is occurring in the main channel and on the floodplain throughout the study reach. Infilling of the main channel occurs primarily by lateral accretion, while the floodplain accretes vertically through deposition of overbank sediment from the main channel and anabranches. Infilling of the main channel may cause avulsion of the main channel into an anabranch. The abandoned main channel segment may then fill completely or act as an anabranch. Because lateral migration of channels is inhibited by the high cohesion of the silt and clay channel sediment, periodic avulsion is the primary form of lateral mobility in the system.     

Response of douglas fir (Pseudotsuga menziesii) to uraniferous groundwater in a small glaciated drainage, Northeastern Washington State

Douglas fir trees and associated soils were sampled from the slopes of a small ( 4 km²) drainage basin in northeastern Washington to investigate the biogeochemical response to locally uraniferous groundwater. Uranium is preferentially incorporated in needles and twigs compared to larger branches or the trunk. The U concentration in needle ash ranges from 0.2 to 5.8 μg g⁻¹ (ppm) and shows no correlation with the U concentration in associated soils. Rather, the distribution of anomalously uraniferous douglas fir (>1.0μg g⁻¹ U in needle ash) appears to be controlled by observed or readily inferred pathways of near-surface groundwater movement in the drainage. These pathways include: (1) general downslope movement of subsurface runoff; (2) increased flux of near-surface groundwater near the toe of an alluvial fan; and (3) emergence of uraniferous (100–150 ng ml⁻¹ [ppb] groundwater in the vicinity of a slope spring. The data also indicate the presence of near-surface uraniferous groundwater along a structurally controlled zone that parallels the north-south strike of the valley, and that includes the slope spring. The results suggest that biogeochemical sampling may be used to supplement more direct, but more limited, measurements of groundwater quality and flow regime in areas of near-surface contaminated groundwater.     

Returning home after Superstorm Sandy: phases in the return-entry process

Natural Hazards - Despite current advances in research related to return-entry process following disasters, the need to understand this process from the perspective of the returnees remains. This...     

A distribution law for relative humidity in the upper troposphere and lower stratosphere derived from three years of MOZAIC measurements

Data from three years of MOZAIC measurements made it possible to determine a distribution law for the relative humidity in the upper troposphere and lower stratosphere. Data amounting to 13.5% of the total were obtained in regions with ice supersaturation. Troposphere and stratosphere are distinguished by an ozone concentration of 130 ppbv as threshold. The probability of measuring a certain amount of ice supersaturation in the troposphere decreases exponentially with the degree of ice supersaturation. The probability of measuring a certain relative humidity in the stratosphere (both with respect to water and ice) decreases exponentially with the relative humidity. A stochastic model that naturally leads to the exponential distribution is provided. Mean supersaturation in the troposphere is about 15%, whereas ice nucleation requires 30% supersaturation on the average. This explains the frequency of regions in which aircraft induce persistent contrails but which are otherwise free of clouds. Ice supersaturated regions are 3-4 K colder and contain more than 50% more vapour than other regions in the upper troposphere. The stratospheric air masses sampled are dry, as expected, having mean relative humidity over water of 12% and over ice of 23%, respectively. However, 2% of the stratospheric data indicate ice supersaturation. As the MOZAIC measurements have been obtained on commercial flights mainly between Europe and North America, the data do not provide a complete global picture, but the exponential character of the distribution laws found is probably valid globally. Since water vapour is the most important greenhouse gas and since it might enhance the anthropogenic greenhouse effects via positive feedback mechanisms, it is important to represent its distribution correctly in climate models. The discovery of the distribution law of the relative humidity makes possible simple tests to show whether the hydrological cycle in climate models is represented in an adequate way or not.     

Estimates of the Climate Response to Aircraft CO2 and NO x Emissions Scenarios

A combination of linear response models is used to estimate the transient changes in the global means of carbon dioxide (CO₂) concentration, surface temperature, and sea level due to aviation. Apart from CO₂, the forcing caused by ozone (O₃) changes due to nitrogen oxide (NO_x) emissions from aircraft is also considered. The model is applied to aviation using several CO₂ emissions scenarios, based on reported fuel consumption in the past and scenarios for the future, and corresponding NO_x emissions. Aviation CO₂ emissions from the past until 1995 enlarged the atmospheric CO₂ concentration by 1.4 ppmv (1.7% of the anthropogenic CO₂ increase since 1800). By 1995, the global mean surface temperature had increased by about 0.004 K, and the sea level had risen by 0.045 cm. In one scenario (Fa1), which assumes a threefold increase in aviation fuel consumption until 2050 and an annual increase rate of 1% thereafter until 2100, the model predicts a CO₂ concentration change of 13 ppmv by 2100, causing temperature increases of 0.01, 0.025, 0.05 K and sea level increases of 0.1, 0.3, and 0.5 cm in the years 2015, 2050, and 2100, respectively. For other recently published scenarios, the results range from 5 to 17 ppmv for CO₂ concentration increase in the year 2050, and 0.02 to 0.05 K for temperature increase. Under the assumption that present-day aircraft-induced O₃ changes cause an equilibrium surface warming of 0.05 K, the transient responses amount to 0.03 K in surface temperature for scenario Fa1 in 1995. The radiative forcing due to an aircraft-induced O₃ increase causes a larger temperature change than aircraft CO₂ forcing. Also, climate reacts more promptly to changes in O₃ than to changes in CO₂ emissions from aviation. Finally, even under the assumption of a rather small equilibrium temperature change from aircraft-induced O₃ (0.01 K for the 1992 NO_x emissions), a proposed new combustor technology which reduces specific NO_x emissions will cause a smaller temperature change during the next century than the standard technology does, despite a slightly enhanced fuel consumption. Regional effects are not considered here, but may be larger than the global mean responses.     

Flood-plain mapping: a critical discussion of deterministic and probabilistic approaches

Abstract

Different methodologies for flood-plain mapping are analysed and discussed by comparing deterministic and probabilistic approaches using hydrodynamic numerical solutions. In order to facilitate the critical discussion, state-of-art techniques in the field of flood inundation modelling are applied to a specific test site (River Dee, UK). Specifically, different flood-plain maps are derived for this test site. A first map is built by applying an advanced deterministic approach: use of a fully two-dimensional finite element model (TELEMAC-2D), calibrated against a historical flood extent, to derive a 1-in-100 year flood inundation map. A second map is derived by using a probabilistic approach: use of a simple raster-based inundation model (LISFLOOD-FP) to derive an uncertain flood extent map predicting the 1-in-100 year event conditioned on the extent of the 2006 flood. The flood-plain maps are then compared and the advantages and disadvantages of the two different approaches are critically discussed.

Citation Di Baldassarre, G., Schumann, G., Bates, P. D., Freer, J. E. & Beven, K. J. (2010) Flood-plain mapping: a critical discussion of deterministic and probabilistic approaches. Hydrol. Sci. J. 55(3), 364–376.     

Sea-level history of the past two interglacial periods: new evidence from U-series dating of reef corals from south Florida

As a future warm-climate analog, much attention has been directed to studies of the Last Interglacial period or marine isotope substage (MIS) 5.5, which occurred ～120,000 years ago. Nevertheless, there are still uncertainties with respect to its duration, warmth and magnitude of sea-level rise. Here we present new data from tectonically stable peninsular Florida and the Florida Keys that provide estimates of the timing and magnitude of sea-level rise during the Last Interglacial period. The Last Interglacial high sea stand in south Florida is recorded by the Key Largo Limestone, a fossil reef complex, and the Miami Limestone, an oolitic marine sediment. Thirty-five new, high-precision, uranium-series ages of fossil corals from the Key Largo Limestone indicate that sea level was significantly above present for at least 9000 years during the Last Interglacial period, and possibly longer. Ooids from the Miami Limestone show open-system histories with respect to U-series dating, but show a clear linear trend toward an age of ～120 ka, correlating this unit with the Last Interglacial corals of the Key Largo Limestone. Older fossil reefs at three localities in the Florida Keys have ages of ～200 ka and probably correlate to MIS 7. These reefs imply sea level near or slightly above present during the penultimate interglacial period. Elevation measurements of both the Key Largo Limestone and the Miami Limestone indicate that local (relative) sea level was at least 6.6 m, and possibly as much as 8.3 m higher than present during the Last Interglacial period.