Distribution of bottom stress and tidal energy dissipation in a well-mixed estuary

Estimates of area-averaged tidal bottom stress are made for four channel segments of the Great Bay Estuary, N.H. Current and sealevel measurements are used to estimate acceleration and pressure gradient terms in the equation of motion, while the equation of motion itself is used to infer the remaining stress term. Dynamic terms, bottom stress values, friction coefficients and energy dissipation rates are estimated for each site. The analysis shows that while throughout the estuary the principal force balance is between the frictional stress and the pressure gradient forcing, RMS values of total bottom stress range from 2·67 to 10·38 Nm^?2 and friction coefficients vary from 0·015 to 0·054. Both stress and energy dissipation are largest in the seaward portion of the estuary with an order of magnitude decrease in dissipation at the most inland site.These distributions of stress and energy dissipation are consistent with cotidal charts of the principal semi-diurnal tidal constituent (M₂) which indicate that the estuary is composed of a highly dissipative more progressive tidal wave regime seaward and a less dissipative standing wave regime landward.     

数字水准仪误差分析与检定

分析了数字水准仪系统的误差源，阐述了仪器i角误差和综合精度的检定方法。     

环境影响二级模糊综合评价法的研究——以宋芳屯油田为例

本文在多级模糊评价的基础上，结合油田开发建设工程所涉及的具体情况，提出了与之相适合的环境影响综合评价的二级模糊综合评价法，并以宋芳屯油田为例，研究了二级评价中参评要素、各要素参评因子的划分及其隶属度函数的建立等一些基本问题。     

Inundation Potentials Analysis for Tsao-Ling Landslide Lake Formed by Chi-Chi Earthquake in Taiwan

Without any omen, massive landslides induced by the Chi-Chi earthquake blocked up gorges of Ching-Shui creek, and produced a new landslide lake.Although emergency spillways have been constructed to prevent dam failures,overtopping and possible breaching may still occur due to excessive inflows in raining seasons. As a result, the downstream valleys will have serious inundation and the safety of people and properties will be in immediate danger. The purpose of this work is to simulate and to analyze the inundation potentials downstream of Tsao-Ling landslide lake using a hydrologic/hydraulic approach and GIS (Geographic Information System) technology. Hydrologic analysis is employed to describe regional rainfall-runoff characteristics andto design rainfall/runoff scenarios. One-dimensional dam break flood routings are performed with different return periods of rainfall events and dam failure durations for downstream creeks. The depletion hydrographs of dam break routings are applied into two-dimensional overland flow simulations for downstream lowlands. The results of hydraulic computations are evaluated with GIS maps for inundation potentials analysis, which can be usedto assist the planning of emergency response measures.     

Statistical analysis of modern seed assemblages from the San Francisco Bay: applications for the reconstruction of paleo-salinity and paleo-tidal inundation

The feasibility of developing a seed calibration data set to infer environmental constraints was examined by studying modern seed and vegetation assemblages from 53 stations at three distinct tidal marshes within the San Francisco Bay (salt, brackish and freshwater). Pearson's correlation indicates autochthonous derivation of seeds at the marsh sites. Multivariate statistical analysis of the vegetation and seed data indicates discrete assemblages, which can be used as a proxy for modern salinity and tidal inundation within the estuary. Canonical discriminant analysis indicates that the vegetation and seed assemblages of the three marsh types are statistically distinct. Cluster analysis suggests that marsh vegetation is zoned at the mean higher high water mark at all the sites. The calibration set was then applied to stratigraphic seed data from the estuary to infer changes in paleo-salinity and -tidal inundation during the Holocene. The results of the calibration are compared to earlier reconstructions within the estuary. While the calibrated reconstruction broadly compares to previous reconstructions, a period of inferred lower salinity within the estuary between 3800–2000 cal yr B.P. is calibrated as brackish and not fresh by the calibration model.     

“Greatest lake period” and its palaeo-environment on the Tibetan Plateau

The Tibetan Plateau is the biggest lake distribution area in China, in which lake evolution is one of the important compositions for plateau environmental changes. It is also very significant in the study of regional environmental evolution. To answer the questions that how and when the paleo lakes evolved, the indicators and their dating data of the important events must be identified. As the key items for detecting the past environment, they are also the most concerned problems in geo-scien…     

Preventing flooding or regulating flood levels?: Case studies on perception of flood alleviation in Bangladesh

Based on a systematic sample survey among the residents of two contrasting floodplain environments - the Tista floodplain (a shallow flood area) and the Ganges-Brahmaputra floodplain (a deep flood area) - this study tests a central hypothesis that the floodplain residents of Bangladesh preferred regulation of flood levels as the main flood alleviation measure. The study found that, despite significant differences in property setting, flood hazard experience and the nature of indigenous adjustments to floods between the two sample areas, an overwhelming 95% of the respondents from each area preferred regulation of flood levels within a range of 0.3 to 2.0 m. The study attempts to provide an explanation for the popularity of regulated flood levels in the context of indigenous adjustments of floodplain crops and infrastructures tonormal flood levels in Bangladesh, which coincided with the respondents' preferred range of flood levels. The study points out that the objective of the UNDP/World Bank-sponsored embankment compartmentalization projects also coincides with this goal of regulating flood levels. However, to test the physical limitations of regulating water levels between neighbouring compartments and to assess the environmental impacts of the proposed projects, the study stresses the need for initiating environmental baseline surveys on the pilot compartmentalization project, which is located on the left bank of the Brahmaputra.     

Is anthropogenic land subsidence a possible driver of riverine flood-hazard dynamics? A case study in Ravenna,Italy

ABSTRACT

We investigate possible changes in flood hazard over a 77-km² area around the city of Ravenna. The subsidence rate in the area, naturally a few mm year^?1, increased dramatically after World War II because of groundwater and natural gas extraction, exceeding 110?mm year^?1 and resulting in cumulative drops larger than 1.5?m. The Montone–Ronco river system flows in the southern portion of the area, which is protected against frequent flooding by levees. We performed two-dimensional simulations of inundation events associated with levee breaching by considering four different terrain configurations: current topography and a reconstruction of ground elevations before anthropogenic land subsidence, both neglecting and representing the main linear infrastructures (e.g. roads, artificial channels). Results show that flood-hazard changes due to anthropogenic land subsidence (e.g. significant changes in computed water depth and velocity) are observed over less than 10% of the study area and are definitely less important than those resulting from construction of the linear infrastructures.     

Development and application of an analytical model of stream/aquifer interaction

A mathematical model to simulate stream/aquifer interactions in an unconfined aquifer subjected to time varying river stage was developed from the linearized Boussinesq equation using the principle of superposition and the concept of semigroups. The mathematical model requires an estimate of three parameters to simulate ground-water elevations; transmissivity, specific yield, and recharge. The solution has physical significance and includes terms for the steady-state water level, the steady-state water level as influenced by a change in river stage, a transient redistribution of water levels in the aquifer from the previous day, and a transient change in water level caused by a change in river stage. The mathematical model was tested using observed water table elevations at three locations across a 2-km-wide alluvial valley aquifer. The average absolute deviation between observed and simulated daily water levels was 0.09 m. The difference in river stage over the test year was 4.9 m.     

The relationship between saline groundwater within the Arava Rift Valley in Israel and the present and ancient base levels as detected by deep geoelectromagnetic soundings

The time domain electromagnetic (TDEM) geophysical method was employed to detect saline groundwater bodies within and in the close margins of the Arava Rift Valley. The Arava Valley aquifers are known to occupy fresh to saline groundwater. The lateral subsurface inflow to the Arava from west and east is characterized by fresh to brackish waters. The results of the present study indicate that salination of groundwater is controlled by both present day and ancient base levels, namely by the Dead Sea in the north and by the Gulf of Elat in the south. The configuration obtained by the TDEM survey exhibits interfaces and palaeo-interfaces between fresh to brackish waters and underlying seawater or diluted seawater intruded inland from both base levels as well as brines intruded from the northern base level. The central Arava structural and hydrological divide seems to escape seawater or brine encroachment at least to the considerable depth of the TDEM measurements.