The biogeochemistry of riparian alder wetlands was studied from 1995 to 1997. Nutrient and DOC chemistry was related to water level changes. The spatial and temporal patterns of nutrients (P and N) and dissolved organic carbon (DOC) were measured in the surface water flowing through a riparian alder fen and in the adjacent creek. Nutrient and DOC concentrations were extremely variable temporally but not spatially within the wetland. In the wetland and the adjacent creek concentrations of NO3-N, PO4-P and DOC were homogenous during high-flow periods and frozen conditions. After low-flow conditions water bodies were isolated from the creek. The concentration of NH4-N, PO4-P and DOC in these isolated water bodies was significantly higher than in the adjacent creeks due to low oxygen levels.
Enclosures of different sizes were installed in the wetland to study possible release rates. A large enclosure experiment in the flooded alder fen showed the same concentrations as after high-flood conditions except for DOC. The DOC concentrations were enriched in the large enclosure after decomposition from leaf litter during fall season. Small enclosures with low oxygen levels confirmed data obtained from low-flow conditions. The release rates were calculated for low-flow conditions from small enclosure experiments for 2 months a year when the alder fen is not flooded. The rates for July and August were 11.6 kg/ha NH4-N, 8.6 kg/ha PO4-P and 57.6 kg/ha DOC. The DOC concentrations for fall estimated from the large enclosure-experiments were 168.2 kg/ha for the months September and October.
This means possible output rates of N, P and DOC during the summer and DOC during fall in the adjacent river system. This can cause eutrophication and organic pollution depending on the length of the low-flow conditions and the size of the alder fen. Water level changes must be regarded as important for the management of riparian wetlands such as alder fens. The riparian alder system may vary from a nutrient sink to a nutrient source at different times of a year depending on high or low water levels. 相似文献
The highly vulnerable Pacific southwest coast of Mexico has been repeatedly affected by local, regional and remote source
tsunamis. Mexico presently has no national tsunami warning system in operation. The implementation of key elements of a National
Program on Tsunami Detection, Monitoring, Warning and Mitigation is in progress. For local and regional events detection and
monitoring, a prototype of a robust and low cost high frequency sea-level tsunami gauge, sampling every minute and equipped
with 24 hours real time transmission to the Internet, was developed and is currently in operation. Statistics allow identification
of low, medium and extreme hazard categories of arriving tsunamis. These categories are used as prototypes for computer simulations
of coastal flooding. A finite-difference numerical model with linear wave theory for the deep ocean propagation, and shallow
water nonlinear one for the near shore and interaction with the coast, and non-fixed boundaries for flooding and recession
at the coast, is used. For prevention purposes, tsunami inundation maps for several coastal communities, are being produced
in this way. The case of the heavily industrialized port of Lázaro Cárdenas, located on the sand shoals of a river delta,
is illustrated; including a detailed vulnerability assessment study. For public education on preparedness and awareness, printed
material for children and adults has been developed and published. It is intended to extend future coverage of this program
to the Mexican Caribbean and Gulf of Mexico coastal areas. 相似文献
This paper is based on statistical analysis of hourly tide measurements for some 285 equivalent full years from the stations
of Weymouth, Bournemouth, Portsmouth, Newhaven, Dover and Sheerness in the UK, and of Cherbourg, Le Havre, Dieppe, Boulogne,
Calais and Dunkirk in France. For each tidal value, surge heights have been determined and correlated with hourly or three-hourly
wind and air pressure data from nearby meteorological stations. Major surges in the area are generally produced by storms
associated with wind from north-west or south-west that tend to push oceanic water into the Channel. Recent medium-term climate
evolution does not seem to increase the flooding risk at French stations, where surge-related winds tend to decrease in frequency
and speed (Cherbourg, Dieppe and Boulogne) or show little change (Le Havre). However, the long-term risk of flooding will
increase through the loss in land elevation due to a continuation of the local relative sea-level rise, especially if this
effect will be enhanced by an acceleration in the global sea-level rise predicted by climatic models. The northern side of
the Channel (Weymouth, Bournemouth and Portsmouth) is mainly exposed to southerly winds that show variable trends. It is also
apparently affected by strong subsidence trends during the last two decades. If lasting, such trends can only increase long-term
flooding risk. The flooding risk has not increased near the eastern end of the Channel. The duration of significant cyclonic
events tends to decrease near Cherbourg but tends to increase near Weymouth, with no conclusive trends in other stations (Portsmouth,
Calais and Dunkirk), where extreme surges may occur also in relatively high-air-pressure situations. In conclusion, medium-term
coastal flooding risk seems to increase especially at Weymouth, Bournemouth and Portsmouth, and also, but less so, at Le Havre
and Sheerness. In addition, few extreme surges occurred during the last decades at the time of spring high tide, which would
seem to be a fortunate coincidence or, in some cases, an effect of tide–surge interaction. The risk of occurrence of less
favourable random events in the near future is therefore of concern, and flood potential would greatly increase if the global
sea-level rise expected in the near future is also considered. 相似文献
The Cairns region, on the north tropical coast of Queensland, forms part of the wettestarea in Australia, with mean annual rainfalls of 2,000 to 4,000 mm. During the summerand early autumn months, intense rainfalls associated with cyclones and other tropicalweather influences persist for several days, and can produce severe flooding in theBarron, Mulgrave and Russell Rivers and smaller drainage systems. There is oftensome loss of life and the damage to buildings, transport infrastructure, sugar cane andother agricultural crops can easily exceed 100 million. Very high intensity rainfallsover shorter periods, only a few hours, also present a significant urban flash flood risk,as happened at Townsville City, some 300 km to the south of Cairns, in January 1998.Despite the use of good floodplain management practices in recent decades, the combinedrisk of severe river floods and urban drainage floods is relatively high when comparedwith the other more damaging, but less frequent, natural hazards. 相似文献