The Hydrologic (Water) Cycle

Surface Water Oceans Rivers and streams Lakes and ponds Springs – groundwater becomes surface water

Problems and concerns with surface water Too little = drought Too much = flood Contamination

How do we measure the volume of water in a river? Since the water is flowing in an irregular channel that changes with location, measurement is difficult. The volume of water a stream carries at a particular point is called the discharge.

How do we measure discharge? Discharge (Q) is a combination of the volume of the channel and the rate of flow. Q depends on size of channel and rate of flow. Q = w*d*v W = width D = depth V = velocity Discharge changes all the time!

Changes in discharge Seasonal changes Input from groundwater Rainfall Snow and ice melt Factors that affect runoff vs. infiltration Earth materials: permeable or not Land use/vegetation Slope Degree of saturation Duration of precipitation Intensity of precipitation

What is a flood? Floods are natural events, not disasters Flood - an event in which Q is greater than the capacity of the channel When a river overflows, the excess water moves onto the floodplain. Larger floods inundate larger areas of the floodplain Floods are predictable!

How can we predict floods? Historical records – data on Q Floods come in different sizes Size is related to Recurrence interval How often is a Q of a particular size likely to occur? Recurrence interval is related to probability P = 1/RI

Recurrence interval, Probability and sizes of floods Small floods have small recurrence intervals Therefore, high probability Large floods have large recurrence intervals Therefore, low probability What recurrence interval or probability should we use for safety?

What is an acceptable risk? For most planning purposes, an RI = 100 years is used. For RI = 100 years, the probability in any given year is 1/100 or 1% This is called the 100 year flood.

How often do 100 year floods occur? On average, once every 100 years. If a 100 year flood occurs in 2005, what is the probability that another will occur in 2006? Can a 100 year flood occur more than once in a year?

How do we cope with floods? Hard solutions Levees Dams Channelization Soft solutions Land use management Zoning Buy outs