Land-surface Elevation Changes Research Articles

Quantifying the inundation impacts of earthquake-induced surface elevation change by hydrological and hydraulic modeling

Current estimates of flood hazards are often based on the assumption that topography is static. When tectonic and/or anthropogenic processes change the land surface elevation, the spatial patterns of floods might also change. Here, we employ the hydrological and hydraulic modeling to simulate floods in the Kujukuri Plain, Japan, in the years 2004 and 2013, when two severe floods occurred. In between the two floods, land surface elevations were changed by the 2011 Tohoku-Oki earthquake. The effects of land surface elevation changes on inundation areas were quantified by changing input topographies. Our results showed that, without taking into account land surface elevation changes, around 10% of inundation areas were underestimated at the time of flood events in the year 2013. The spatial distribution of inundation locations varied with local topographical features, for example, the areas with backmarsh and valley fill deposits were sensitive to the extent of inundation by land surface elevation changes. The sub-watershed near the coastal shoreline having below-zero meter elevation areas showed that the earthquake-induced land surface elevation changes exacerbated an additional 22% inundation area. This study suggests that the inundation areas will increase in catchments suffering severe settlements, which highlights the necessity of taking into account the spatio-temporal changes of land surface elevations on the assessment of flood hazards.

Solutions for subsidence in the California Delta, USA, an extreme example of organic-soil drainage gone awry

Abstract. The Sacramento-San Joaquin Delta is at the heart of California's water supply system that provides water for irrigation and human consumption. It is also home to subsiding organic soils, decreasing native aquatic species populations, water quality degradation, vulnerable levees (levees are equivalent to dikes) and decreasing agricultural viability. There has been substantial progress in the interdisciplinary understanding and quantification of the nature and effects of subsidence and its mitigation. Because of the need for a drained rootzone, farming of crops such as vegetables, trees, vines, corn and alfalfa, results in an ongoing unsustainable cycle of continuing peat oxidation and deepening of drainage ditches to compensate for elevation loss. Despite substantial evidence for the increasing risks to the State's economy and water supply, the unsustainability of the status quo, and evidence for the benefits of alternatives, there has been limited progress in converting to land uses that can reduce, stop and reverse subsidence. Our overall approach has been to measure land-surface elevation changes; understand, quantify and model subsidence and greenhouse gas emissions from drained organic soil, and evaluate alternate land uses. Subsidence rates vary from less than 0.5 to over 2 cm yr−1, depending primarily on depth to groundwater and soil organic matter content. The primary cause of subsidence is the oxidation of organic matter, which has resulted in elevations of −3 to −9 m on about 100 000 ha. Using the results from micrometeorological measurements and modelling, we estimate that organic-matter oxidation causes an annual emission of over 2×106 t of CO2-equivalent which represents about 21 % of the State's plant-based agricultural emissions. Rewetting of the peat soils is emerging as a viable solution. Rice and wetlands stop and (in the case of wetlands) reverse the effects of subsidence and result in a net greenhouse-gas emission reduction benefit. Wetlands accrete about 3 cm of soil per year, can break the unsustainable subsidence/drainage cycle, reverse the trajectory of increasing hydraulic pressures on levees, reduce the probability of levee failure and seepage onto islands (islands are equivalent to polders), and may provide material for biofuels and animal feed. The recent implementation of a methodology for quantification of the GHG benefit is facilitating land use conversion and participation in the carbon market.

NDEI-BASED MAPPING OF LAND SURFACE CHANGES

Abstract. Topography measurements of land surface elevation changes are essential for geomorphological studies and research on natural hazards. However, conventional topography and remote sensing methods pose several challenges. Fieldwork in remote areas could be insecure or be hard to conduct due to physical barriers. Remote sensing satellite images, in turn, may lack the temporal resolution for monitoring topography changes or be covered by clouds and shadows that reduce their usefulness. Unmanned Aerial Vehicles (UAV) provide high-resolution imagery from remote areas and may give a greater insight into land surface changes. Previous studies have demonstrated the potential of the Normalized Difference Elevation Index (NDEI), derived from UAV surveys, to map changes in topography in different environments, including a quarry zone of sand mines in Colombia, and in the North Pole Ice Cap. As the NDEI is a metric capable to reveal topography disturbances, it could be exploited for rapid assessments of mass movements occurrence. For this research, a fine temporal resolution dataset of monthly NDEI values were used to explore their utility as a mass wasting identification technique. NDEI was used to map changes in the land surface in a quarry zone affected by landslides and mudflows. NDEI values were obtained from the multi-temporal analysis of digital surface models generated from UAV images and processed using Structure from motion (SfM) techniques. Main components of the proposed method are: (i)UAVbased imagery data capture, (ii)data processing using SfM techniques, (iii)NDEI calculation, and (iv)mass movement identification. Results illustrate the feasibility of using UAV-based imagery data to increase the accuracy of land changes information and produce rapid assessments of mass movements occurrence. Results show that the NDEI approach may increase the accuracy of land surface changes assessment and mapping, as well as enable the design of new methodologies to identify natural hazards and risks.

Adding to the toolbox for tidal-inundation mapping in estuarine areas.

In estuaries, land-surface and tidal elevation conspire to influence the amount of salt-water inundation in a specific location, ultimately affecting the distribution of estuary vegetation. Plants vary in their tolerances to salinity and inundation. Understanding even small changes in land-surface elevation at a site scale provides relevant information to managers seeking to design effective long-term restoration projects. Restoration of estuary habitats has been identified as a tool to mediate some anticipated effects of climate change, including flooding from sea-level rise, precipitation regimes, and storminess. Further, habitat restoration that is effective in the face of climate uncertainty is critical to the sustainable production of seafood and maintenance of ecosystem functions. We offer a simple method that links tidal elevations to upslope topography, allowing managers to determine where tidal inundation of upslope areas may occur. This method does not require complex modeling, rather we combine existing high- accuracy tide-gage information with LiDAR imagery. However, we found that if LiDAR is not flown at low tide, or at consistent tidal heights, it poses significant challenges in the interpretation of tidal elevations. Where LiDAR is consistently collected at low tide, this method of linking the tidal datum to upslope topography is not data-intensive, and does not require long-term data collection. Along with locally specific information, the types of map products that can be developed using this method should identify places that may be potentially vulnerable to salt-water inundation, along with places that may be effective migration corridors for marshes and other habitats.

Automatic mapping of land surface elevation changes from UAV-based imagery

ABSTRACTThis article proposes an automated method for detection and mapping of land surface elevation changes from unmanned aerial vehicle (UAV)-based imagery. In contrast to conventional methods, which require a significant human effort to succeed, the novel approach, based on multi-temporal digital terrain model analysis, detects and outlines zones of mass deposition or mass remotion with no manual input. Main components of the proposed method are: (i) a normalized difference elevation index and (ii) an automated edge-detection procedure. Performance of method is evaluated for topographic change detection in a quarry zone using a multi-temporal UAV-based imagery dataset. It is demonstrated that the proposed method allows for a rapid and automatic elevation change mapping and that results are as accurate as conventional methods based on time-consuming thresholding and visual interpretation.

Present-day oxidative subsidence of organic soils and mitigation in the Sacramento-San Joaquin Delta, California, USA.

Subsidence of organic soils in the Sacramento-San Joaquin Delta threatens sustainability of the California (USA) water supply system and agriculture. Land-surface elevation data were collected to assess present-day subsidence rates and evaluate rice as a land use for subsidence mitigation. To depict Delta-wide present-day rates of subsidence, the previously developed SUBCALC model was refined and calibrated using recent data for CO2 emissions and land-surface elevation changes measured at extensometers. Land-surface elevation change data were evaluated relative to indirect estimates of subsidence and accretion using carbon and nitrogen flux data for rice cultivation. Extensometer and leveling data demonstrate seasonal variations in land-surface elevations associated with groundwater-level fluctuations and inelastic subsidence rates of 0.5–0.8 cm yr–1. Calibration of the SUBCALC model indicated accuracy of ±0.10 cm yr–1 where depth to groundwater, soil organic matter content and temperature are known. Regional estimates of subsidence range from <0.3 to >1.8 cm yr–1. The primary uncertainty is the distribution of soil organic matter content which results in spatial averaging in the mapping of subsidence rates. Analysis of leveling and extensometer data in rice fields resulted in an estimated accretion rate of 0.02–0.8 cm yr–1. These values generally agreed with indirect estimates based on carbon fluxes and nitrogen mineralization, thus preliminarily demonstrating that rice will stop or greatly reduce subsidence. Areas below elevations of –2 m are candidate areas for implementation of mitigation measures such as rice because there is active subsidence occurring at rates greater than 0.4 cm yr–1.

Historic, Recent, and Future Subsidence, Sacramento-San Joaquin Delta, California, USA

To estimate and understand recent subsidence, we collected elevation and soils data on Bacon and Sherman islands in 2006 at locations of previous elevation measurements. Measured subsidence rates on Sherman Island from 1988 to 2006 averaged 1.23 cm/year (0.5 in/yr) and ranged from 0.7 to 1.7 cm/year (0.3 to 0.7 in/year). Subsidence rates on Bacon Island from 1978 to 2006 averaged 2.2 cm/year (0.9 in/yr) and ranged from 1.5 to 3.7 cm/year (0.6 to 1.5 in/yr). Changing land-management practices and decreasing soil organic matter content have resulted in decreasing subsidence rates. On Sherman Island, rates from 1988 to 2006 were about 35% of 1910 to 1988 rates. For Bacon Island, rates from 1978 to 2006 were about 40% less than the 1926-1958 rates. To help understand causes and estimate future subsidence, we developed a subsidence model, SUBCALC, that simulates oxidation and carbon losses, consolidation, wind erosion, and burning and changing soil organic matter content. SUBCALC results agreed well with measured land-surface elevation changes. We predicted elevation decreases from 2007 to 2050 will range from a few centimeters to over 1.3 m (4.3 ft). The largest elevation declines will occur in the central Sacramento-San Joaquin Delta. From 2007 to 2050, the most probable estimated increase in volume below sea level is 349,956,000 million cubic meters (281,300 acre-feet). Consequences of this continuing subsidence include increased drainage loads of water quality constituents of concern, seepage onto islands, and decreased arability.

Subsidence Reversal in a Re-Established Wetland in the Sacramento-San Joaquin Delta, California, USA

The stability of levees in the Sacramento-San Joaquin Delta is threatened by continued subsidence of Delta peat islands. Up to 6 meters of land-surface elevation has been lost in the 150 years since Delta marshes were leveed and drained, primarily from oxidation of peat soils. Flooding subsided peat islands halts peat oxidation by creating anoxic soils, but net accumulation of new material in restored wetlands is required to recover land-surface elevations. We investigated the subsidence reversal potential of two 3 hectare, permanently flooded, impounded wetlands re-established on a deeply subsided field on Twitchell Island. The shallower wetland (design water depth 25 cm) was almost completely colonized by dense emergent marsh vegetation within two years; whereas, the deeper wetland (design water depth 55 cm) which developed spatially variable depths as a result of heterogeneous colonization by emergent vegetation, still had some areas remaining as open water after nine years. Changes in land-surface elevation were quantified using repeated sedimentation-erosion table measurements. New material accumulating in the wetlands was sampled by coring. Land-surface elevations increased by an average of 4 cm/yr in both wetlands from 1997 to 2006; however, the rates at different sites in the wetlands ranged from -0.5 to +9.2 cm/yr. Open water areas of the deeper wetland without emergent vegetation had the lowest rates of land-surface elevation gain. The greatest rates occurred in areas of the deeper wetland most isolated from the river water inlets, with dense stands of emergent marsh vegetation (tules and cattails). Vegetated areas of the deeper wetland in the transition zones between open water and mature emergent stands had intermediate rates of land-surface gain, as did the entire shallower wetland. These results suggest that the dominant component contributing to land-surface elevation gain in these wetlands was accumulation of organic matter, rather than mineral sediment, and that accumulation of organic matter in emergent marshes is strongly affected by hydrologic factors. Re-established, non-tidal wetlands with managed hydrology can produce significant increases in land-surface elevations, which can help to improve levee stability and protect subsided islands from future flooding.

Stability of creeping soil and implications for hillslope evolution

The geomorphic behavior of a soil‐mantled hillslope undergoing diffusive creep involves a coupling between changes in land surface elevation, soil transport rates, soil production, and soil thickness. A linear stability analysis suggests that the coupled response of the soil mantle to small perturbations in soil thickness or surface topography is influenced by two factors. The diffusive‐like behavior of soil creep has a stabilizing effect wherein perturbations in land surface elevation are damped. The relation between the soil production rate and soil thickness may be either stabilizing or destabilizing. A monotonically decreasing production rate with soil thickness reinforces the stabilizing effect of diffusive land surface smoothing. An increasing production rate with soil thickness has a destabilizing effect wherein perturbations in soil thickness or the soil‐bedrock interface are amplified, despite the presence of diffusive land surface smoothing. This coupled behavior is insensitive to the transport relation, whether the soil flux is proportional to the land surface gradient or to the product of the soil thickness and land surface gradient. The latter type of relation, nonetheless, could lead to a more complex hillslope form than might otherwise be expected for purely diffusive transport. Moreover, the response to periodic (sinusoidal) variations in the rate of stream downcutting at the lower hillslope boundary involves upslope propagation of coupled (damped) waveforms in the land surface and the soil‐bedrock interface. The distance of upslope propagation goes with the square root of the product of the transport diffusion‐like coefficient and the period of the downcutting rate. The upper part of the hillslope is therefore insensitive to relatively high‐frequency variations in stream downcutting, so together with a stable behavior of the coupled soil‐mantle‐bedrock system, this part of the hillslope may exhibit a tendency toward uniform lowering, while the lower part behaves transiently. Conversely, in the presence of low‐frequency variations in stream downcutting, hillslope morphology and soil thickness variations are more likely to reflect unsteady conditions over the entirety of the hillslope.

Subsidence of Agricultural Lands in the Sacramento‐San Joaquin Delta, California: Role of Aqueous and Gaseous Carbon Fluxes

To examine the causes of land subsidence on marshes drained for agriculture, carbon fluxes and changes in land‐surface elevation were determined on three islands in the Sacramento‐San Joaquin Delta, California. Over the time period of March 1990 to May 1992, gaseous CO2 fluxes were determined approximately monthly using closed chambers, and dissolved carbon fluxes were determined from the dissolved carbon loads of drainage ditches adjacent to each field site. Surface elevation changes were measured continuously by measuring the distance between the land surface and an elevated structure anchored beneath the organic soil layer. Gaseous CO2 fluxes accounted for most of the permanent subsidence measured over the monitoring period. Gaseous CO2 fluxes are strongly affected by soil temperature. Net subsidence rates for the three islands, which have different depths of organic soils and water‐management practices, range from 0.46 to 1.06 cm/yr. Estimates of dissolved organic carbon fluxes for all three islands were small relative to gaseous CO2 losses and represent &lt;1% of the measured subsidence.