LAB 9: ANALYSIS OF HYDROLOGICAL DATA
OBJECTIVES:
-To undertake basic hydrological analysis of stream discharge data.
-To calculate probabilities and return periods of extreme events.
In Lab 8, you learned how stream discharge is measured at a cross-section at one point in time. These instantaneous measurements are of limited use by themselves. More often, they are combined to show variation in stream flow over time. In this Lab, you will learn two common ways of analyzing discharge variation through time. The first, a hydrograph, is used to show the rise and fall of discharge during a single event such as a flood. The second "flood frequency analysis" is a means of estimating the likelihood of floods of different sizes over the long term.
I. RATING CURVES
The discharge of most large streams in the U.S. is measured at permanent gauging stations established by the US Geological Survey (USGS) and various local agencies. Permanent gauging stations are usually sited at smooth, stable cross-sections that are unlikely to change shape drastically even during floods. Bedrock channels and bridge abutments are favorite gauging sites.
Such sites vary from simple staff gauges (essentially long vertical rulers, often mounted on bridge supports) to elaborate installations with stilling wells (in which turbulence is eliminated for more accurate measurements) and housed in concrete buildings elevated well above flood level and continuously recorded. You can see an example of a gauge on Eagle Creek at the downstream side of Lynnhurst Drive Bridge near West 10th street.
At these permanent stations, the relationship between discharge and water level (called stage) is known from past measurements at many different discharges. Thus the gauges actually record stream stage, which is converted to discharge by a rating curve. A rating curve is a graph that relates stage to discharge. This relationship is not usually linear, but it often appears linear when scales of both gauge height and discharge are made logarithmic. Thus rating curves are usually plotted on log-log paper. With a rating curve one can estimate the discharge of a stream by reading gauge height and using the rating curve to convert it to discharge. After the rating curve has been developed there is no need to get into the water.
II. STORM HYDROGRAPHS
A graph of the rate of discharge (runoff on the watershed) plotted against time for a point on a channel or a hillside is called a hydrograph. The graph below shows the general form of a hydrograph for a single event, composed of a rising limb (increasing discharge), a peak discharge, and a falling limb.
Between such events, the stream returns to a more constant low level called base flow. When the precipitation, which has caused the discharge to increase, is superimposed on the same graph, it is also possible to plot the lag time between precipitation input (beginning of precipitation) and stream response. This information is of obvious importance for people living downstream who must prepare for a flood.
III. FLOOD FREQUENCY
A statement of the probability of floods greater than certain limits (or their average frequency of occurrence) is the basis for much planning that concerns river channels and valley floors. Such information is required for engineering design, planning, flood-insurance plans and land use zoning. It is also important in understanding the development of landforms. In geomorphology large floods are responsible for catastrophic processes, but generally the amount of work done (i.e., erosion) in extreme floods is exceeded by the amount accomplished by more frequent floods. The geomorphologist therefore needs to know the magnitude (size) and the frequency (how often they occur) of a complete spectrum of floods.
The frequency of hydrological events can be discussed in terms of either:
1. Probability (P): sometimes called exceedance probability, is the chance an event will equal, or exceed, some given discharge value.
2. Recurrence Interval (RI): sometimes called the return period, is the average interval in years between events equaling or exceeding a given magnitude.
The magnitude of an extreme event is inversely related to its frequency, i.e., very severe events occur less frequently than more moderate ones. The objective of frequency analysis is to relate the magnitude of extreme events to their frequency of occurrence through probability distributions.
The techniques you will use in this lab concerning frequencies of events are appropriate not only to flood events. They can also be used to assess the probability/return period of intense rainstorms, drought, snowfall, high winds, etc.