Hydrographs are visual representations of the variation in stream or river depth over time. These graphs can also be used to represent water discharge, which is a measure of flowing water volume per unit time, such as cubic feet per second.
Baseline Flow
The flow of a stream or river under normal weather conditions and patterns is called the base flow or baseline flow, and this water originates primarily from sub-surface groundwater flows. On a hydrograph, this is usually represented as a relatively straight horizontal line along the bottom of the graph. There is, however, natural variation in the base flow, and the line may appear higher or lower as needed.
Rain Events
The addition of water to a stream or river from rain events will cause the discharge to increase. These are often represented as peaks on hydrographs as they show large increases in a relatively short time period.
Stage Hydrograph
Stage hydrographs represent the depth of a stream over time. With floods, the depth naturally increases, causing a rise in the slope of the hydrograph line.
Floods
Flooding occurs when the depth (stage) of the stream exceeds the depth from the stream bank to the stream bottom. This depth is indicated on a hydrograph with a horizontal line, and when the depth line rises above the flood line, flooding occurs.
Uses
Flood hydrographs can provide researchers with estimates of flood frequencies and magnitudes. Understanding hydrological fluctuations and dynamics can help in designing bridges, culverts and flood protection structures, and assist in the management of flood plains.
Tags: stream river, base flow, horizontal line, over time, when depth
