Flood Climatology Glossary
(B) Border ice - sheet ice in the form of a long border attached to the shore.
(C) Candled ice - decaying sheet ice. Solar radiation and warm air temperatures combine to weaken the sheet ice. As the ice decays, it becomes darker in color and fails in thin vertical pieces (candles).
(D) Frazil ice - predominant form of stream ice. Only forms in areas of rapidly moving open water during periods of air temperature below about 20 degrees F. Small, spherical crystals can form at various depths and be transported downstream, where they can accumulate into a stationary ice cover. These crystals are sticky and can form different ice structures. Frazil ice formation ceases where a river freezes over since production requires that open, moving water be in contact with very cold air.
(E) Frazil pans - floating masses of frazil ice that has stuck together. Lateral dimension may range from several feet to several hundred feet.
(F) Frazil slush - loosely packed frazil ice floating on the surface of a stream.
(G) Ice effects - indicates that river ice accumulation in the vicinity of a river gage is altering the channel cross-section (in addition to increasing friction) which produces higher stages than would be expected in the absence of ice. The relationship (rating) between river flow and gage height is no longer valid. A stretch of river does not have to be ice covered to exhibit ice effects. Ice can form along the surface in slow-moving sections, along the banks and sides of the channel, and also on the channel bottom (anchor ice). Abrupt fluctuations in stage of several feet are common and should be expected during extended cold spells.
(H) Ice jam - a stationary accumulation of ice that restricts streamflow. For operational purposes, an ice jam is a mass of ice that produces significant stage fluctuations or backwater upstream of the jam. There are two types of ice jams:
Ice Jam Types
(1) Freezeup jam - generally forms in early to mid winter. Very cold temperatures in combination with steady river flows can lead to the formation of large quantities of river ice, either of the sheet or frazil type, resulting in restricted flow and possible backwater problems. The surface of a freezeup jam is smooth to moderately rough (vertical variations less than two feet). Freezeup jams rarely cause major flooding.
(2) Breakup jam - generally occurs in mid to late winter when runoff from rain and/or snowmelt produces sufficient increase in streamflow to dislodge, breakup, and move the ice cover along a reach of a stream. The dislodged ice is transported downstream and can become jammed where an island, bridge piers, stationary ice, dam, bend in the river, or a confluence serves as an obstacle to the broken ice. The surface of a breakup jam is very rough with and often visually dramatic, with large blocks of ice jutting out of the surface. Very large and rapid rises are possible behind breakup jams, which pose the primary ice-induced flood threat in the MARFC area of responsibility.
(3) Shear walls - cliff-like remnants of an ice cover or jam still attached to the shore after breakup.
(4) Sheet ice - forms on slowly flowing reaches of a stream or lake; generally smooth and clear, but may appear dark blue or black. Crystals are columnar in shape which can produce a very strong ice cover.
Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.
Minimal or no property damage, but possibly some public threat.
Some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations.
The long-term average value of a meteorological paramater (i.e., temperature, humidity, etc.) for a certain area. For example, "temperatures are normal for this time of year" means that temperatures are at or near the average climatological value for the given date. Normals are usually taken from data averaged over a 30-year period (e.g., 1971-2000 average), and are concerned with the distribution of data within limits of common occurrence.
Point (location) Floods Rank
Provides an indication of how widespread a particular flood event was within the MARFC service area. Our records show over 1,000 flood events but numerous floods have the same number of locations (forecast points) flooding. Therefore ranking range from 1 to 636. For example, presently the highest-ranked flood event (#1) caused 138 locations to reach flood stage, while the lowest-ranked flood event (#636) caused only 1 location to reach flood stage. Dozens of flood events are tied for having a rank of 636 because only one location within the MARFC service area reached flood stage during those events. Generally, the lower the total number of forecast points during a flood the more numerous the rank is.
A section of river or stream between an upstream and downstream location, for which the stage or flow measured at a point somewhere along the section (e.g., gaging station or forecast point) is representative of conditions in that section of river or stream. The forecast points designated by the weather forecast offices are used by the river forecast centers.
River Gage Datum
The arbitrary zero datum elevation which all stage measurements are made from.
The methods of predicting the attenuation of a flood wave as it moves down the course of a river.
The part of precipitation that flows toward the streams on the surface of the ground or within the ground. Runoff is composed of baseflow and surface runoff.
Seasonal floods are a 3 month total of a meteological season.
|Winter = January 1 - March 31
||Spring = April 1 - May 31
|Summer = July 1 - September 30
||Fall = October 1 - December 31
A structure over or through which excess or flood flows are discharged. If the flow is controlled by gates, it is a controlled spillway, if the elevation of the spillway crest is the only control, it is an uncontrolled spillway.
The level of the water surface of a river or stream above an established datum at a given location.
In hydologic terms, the rate at which water passes a given point. Streamflow is expressed in volume per time with units of cubic feet per second (CFS). Streamflow is often used interchangeably with discharge or flow.
Snow Water Equivalent (the amount of water content in a snowpack or snowfall).
In hydrologic terms, the runoff in inches from a rain of specified duration that causes a small stream to slightly exceed bankfull. When available, flood stage is used instead of slightly over bankfull.
U.S. Geological Survey
The Federal Agency chartered in 1879 by congress to classify public lands, and to examine the geologic structure, mineral resources, and products of the national domain. As part of its mission, the USGS provides information and data on the Nation's rivers and streams that are useful for mitigation of hazards associated with floods and droughts.
The discharge hydrograph from one inch of surface runoff distributed uniformly over the entire basin for a given time period.
The time period from October 1 through September 30.
Land area from which water drains toward a common watercourse in a natural basin.
An area that is regularly wet or flooded and has a water table that stands at or above the land surface for at least part of the year.
Year Rank by Floods
The number of flood events that occurred in the MARFC service area during a calendar year. Events are based on a period of time where one or more location (forecast points) crest at or above flood stage. Our records show several years having the same amount of events. For example, presently the highest-ranked year by events (#1) saw 24 different flood events, while the lowest-ranked flood event (#123) saw only 1 event. Generally, the lower the yearly flood events the more numerous the rank is.
Year Rank by Forecast Points
The accumulated number of locations (forecast points) that occurred in a calendar year within the MARFC service area. Our records show several years having the same amount of events. For example, presently the highest-ranked year by locations (forecast points) (#1) saw 335 locations flood, while the lowest-ranked flood event (#128) saw only 1 location flood. Generally, the lower the total number of forecast points during the year the more numerous the rank is.
A reference "zero" elevation for a stream or river gage. This "zero" can be referenced (usually within ten feet of the bottom of the channel) to mean sea level, or to any other recognized datum.
Data provided by the U.S. Geological Survey
Additional support for this project was provided by the Susquehanna River Basin Commission, National Weather Service and Penn State University (PSU) Department of Meteorology. Special thanks to Michael Kozar (PSU student) for writing numerous weather summaries in the Floods by Events section.
Questions should be directed to Charles Chillag or Alaina MacFarlane.