We are available day or night to assist you. Open Accessibility Menu. Who is responsible for measuring floods? What are stream gauges? How often are water levels monitored? What happens when an area is at risk for flooding? What can I do if my area receives a flood warning? Flood Watch vs Warning: What's the Difference? Family emergency plan , safe shelter, preparedness kit — you want to This gage datum level usually is located slightly below the lowest point of the stream bottom such that the stage is greater than the maximum depth of water.
Since gages are sparse in Alaska, the stage measured at each gage is used as an index of water level characteristics upstream and downstream of the gage in addition to the status at the gage. In many cases, there is only one gage on a stream system and thus the gage represents the water level characteristics in the entire stream basin. The gage is also often used to indicate the water level status of other streams or lakes in the general area that have similar characteristics to the gaged stream.
People living or recreating near any stream or lake should identify the closest gage that can be used as an index for the expected water level changes at their location. High water terms used by the National Weather Service include bankfull stage , action stage , and flood stage as defined below. In Alaska, the sparse gage network requires that these terms be defined with a broader definition that reflects characteristics of the gaged waterbody and well as nearby waterbodies.
Thus the definitions consider both the specific impacts that can be documented in the vicinity of the gage as well as the expected impacts that could result on any waterbody in the general area during an event of that magnitude. The assignment of these stages thus includes the combined assessment of specific impacts and the frequency of occurrence of the event. When high water stages are determined from a flood frequency analysis, the impacts listed for the applicable stages will be the recurrence interval associated with that level and a qualifier to assess the quality of the recurrence interval estimate.
Bankfull Stage - an established gage height at a given location along a river or stream, above which a rise in water surface will cause the river or stream to overflow the lowest natural stream bank somewhere in the corresponding reach. These levels indicate the estimated height of the floodwater. Flooding which causes inconvenience such as closing of minor roads and the submergence of low-level bridges. Main traffic routes may be flooded. Flood Warnings may contain observed, peak or predicted river heights, these are often referenced from a flood gauge at a certain point along a river.
The height in metres to which the river is predicted to rise at the river gauge referred to in the warning. The actual depth of flood water will vary across the floodplain. Knowledge of past flood events, as well as estimates of flood levels from flood studies, are used by local Councils, emergency services and landowners to determine which areas are likely to be flooded from the predicted river height.
However, even more damaging than rainfall-based flooding can be flash flooding caused by non-rainfall factors, such as the flooding of New Orleans in , caused by levee breakage, and the Indian Ocean tsunami, a deadly wave caused by an earthquake underneath the sea. Sudden melting of ice on mountains can cause rivers to swell and overflow their banks as well. Whether it's major flooding triggered by a slow-moving thunderstorm or flash flooding sparked by a hurricane storm surge, it's possible for expert hydrologists to measure a flood's height, water velocity and other properties that reveal its severity.
The USGS has thousands of sites around the country that monitor stream stage, river height and stream flow -- the amount of water flowing at a point in time. Gages at those sites measure "gage-height," a term that refers to the height of water in a stream. These gages enable the agency monitor waterways and warn people about dangerous flooding that can occur. After a flood occurs, they also help the flood investigators determine a flood's peak height.
When they log flood data, they can better plan development around a waterway and maintain historical records of stream stages that occur over time. Several types of equipment exist that gather important hydrological data. They include the float-tape gage -- often placed inside a stilling well to measure water levels as they rise and fall. A stilling well protects equipment and also reduces fluctuations in a river, stream or other land feature though which water flows. Pressure transducers measure pressure that a column of water above a measuring device produces.
Other devices include the manometer, float sensor gage, staff gage and water-stage recorder.
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