Time of Concentration
Encyclopedia
Time of concentration is a concept used in hydrology
to measure the response of a watershed
to a rain event. It is defined as the time needed for water to flow from the most remote point in a watershed to the watershed outlet. It is a function of the topography, geology, and land use within the watershed.
Time of concentration is useful in predicting flow rates that would result from hypothetical storms, which are based on statistically-derived return periods
. For many (often economic) reasons, it is important for engineers and hydrologists to be able to accurately predict the response of a watershed to a given rain event. This can be important for infrastructure development (design of bridges
, culverts
, etc.) and management, as well as to assess flood risk
.
showing lines of equal elevation, a map with isolines
can be constructed to show locations with the same travel time to the watershed outlet. In this simplified example, the watershed outlet is located at the bottom of the picture with a stream flowing through it. Moving up the map, we can say that rainfall which lands on all of the places along the first yellow line will reach the watershed outlet at exactly the same time. This is true for every yellow line, with each line further away from the outlet corresponding to a greater travel time for runoff traveling to the outlet.
Furthermore, as this image shows, the spatial representation of travel times can be transformed into a cumulative distribution plot detailing how travel times are distributed throughout the area of the watershed.
Hydrology
Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...
to measure the response of a watershed
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
to a rain event. It is defined as the time needed for water to flow from the most remote point in a watershed to the watershed outlet. It is a function of the topography, geology, and land use within the watershed.
Time of concentration is useful in predicting flow rates that would result from hypothetical storms, which are based on statistically-derived return periods
Return period
A return period also known as a recurrence interval is an estimate of the interval of time between events like an earthquake, flood or river discharge flow of a certain intensity or size. It is a statistical measurement denoting the average recurrence interval over an extended period of time, and...
. For many (often economic) reasons, it is important for engineers and hydrologists to be able to accurately predict the response of a watershed to a given rain event. This can be important for infrastructure development (design of bridges
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...
, culverts
Culvert
A culvert is a device used to channel water. It may be used to allow water to pass underneath a road, railway, or embankment. Culverts can be made of many different materials; steel, polyvinyl chloride and concrete are the most common...
, etc.) and management, as well as to assess flood risk
Flood risk assessment
A flood risk assessment is an assessment of the risk of flooding, particularly in relation to residential, commercial and industrial land use.-England and Wales:...
.
Example
This image shows the basic principle which leads to determination of the time of concentration. Much like a topographic mapTopographic map
A topographic map is a type of map characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods. Traditional definitions require a topographic map to show both natural and man-made features...
showing lines of equal elevation, a map with isolines
Contour line
A contour line of a function of two variables is a curve along which the function has a constant value. In cartography, a contour line joins points of equal elevation above a given level, such as mean sea level...
can be constructed to show locations with the same travel time to the watershed outlet. In this simplified example, the watershed outlet is located at the bottom of the picture with a stream flowing through it. Moving up the map, we can say that rainfall which lands on all of the places along the first yellow line will reach the watershed outlet at exactly the same time. This is true for every yellow line, with each line further away from the outlet corresponding to a greater travel time for runoff traveling to the outlet.
Furthermore, as this image shows, the spatial representation of travel times can be transformed into a cumulative distribution plot detailing how travel times are distributed throughout the area of the watershed.