III. Streams and Floods (chapters 11 and 12)
Stream: Any body of flowing water confined to a channel.
Stream system: A set of associated streams that drain a common area. Includes the main stream (river) and its tributaries.
Drainage basin: The total area drained by a stream above a certain point.
Drainage divide: Elevated land (ridge) that separates drainage basins.
Streams carry water and erode the landscape. Thus, flowing water and the sediments it caries are important to understanding streams.
Discharge (Q): The amount of water flowing past a point on a stream in a given period of time. Discharge is given by the equation Q = vA where; Q = discharge, v = velocity of water, and A = the cross sectional area of the stream (p. 289).
Sediments in streams (p. 272-277):
Sediments include gravel, sand, silt and clay.
Load: The total amount of sediment transported by a stream; Consists of
Bed load: sediment pushed along the stream bed Usually sand and gravel too heavy to remain suspended in the stream.
Suspended load: sediment suspended in the stream. Generally silt and clay to small to settle out of the water.
Capacity: the total amount of sediment that the stream can potentially carry, if the sediment were available. The greater a streams discharge, and the higher it's energy, the greater it's capacity.
Types of streams (p. 77-281)
Braided streams: streams with numerous interweaving channels. These streams form when there is insufficient flow (discharge) to carry the available sediment (the streams capacity is exceeded).
Bedrock streams: Streams with rough, sediment poor channels that cut into bedrock. Most common in the upper reaches of a stream system, usually in mountainous areas.
Meandering streams: Sinuous streams, usually contained in a channel within a flood plain.
Major parts of a meandering stream (p. 277-280):
Channel
Flood plain
Valley walls
Meanders consist of
Cut bank: outside curve of a meander characterized by fast moving water and erosion along the bank.
Point bar: inside curve of a meander characterized by slowly moving water and deposition (generally deposits of sand and gravel).
Riffle: Straight, shallow portion of a channel between meanders.
Meanders migrate through erosion along the cut bank and deposition at the point bar. This way, channels constantly migrate and the river constantly changes course.
One of the most dramatic ways a river changes course is through the formation of oxbow lakes. Here, two meanders migrate towards each other. Then the two meanders merge, they cut off the old river channel, which remains as an oxbow lake.
In all streams, floods occur when discharge is very high, and water over tops the channel walls. In meandering this occurs when water flows over the levees. Levees form when sand is deposits along the channel walls as the water suddenly looses velocity as it flows onto the flood planes.
Hydrographs:
Hydrographs are plots of a streams discharge over time. These are used to record periods of normal stream flow as well as periods of flooding. During a flood, the streams discharge increases (rising limb) until the flood crests at a flood peak (the maximum discharge). The time difference between the rain event that causes the flood, and the flood peak is called lagtime.
Types of floods:
Flash (upstream) floods: Short duration floods that usually occur in the upper reaches of a stream system. Associated with short term, high intensity periods of rainfall, often with a thunder storm. Characterized by short lag times (period between rain event and flood peak). Occur over small area since larger streams downstream from the flooded tributary can absorb the excess water without flooding.
Regional (down stream) floods: Floods that occur over a broad regions and occur over a long period of time. These floods generally occur in the downstream portions of a stream system. Usually associated with long period rainfall occurring over a broad region. Characterized by long lag times. Occur over large areas since excess water enters main streams from numerous tributaries.
A classic example of a regional flood is the great Mississippi River basin flood of 1993. With 100,000 square kilometers inundated, this ranks as the largest flood disaster in U.S. History. This flood exceeded the 100 year flood level. The most sevier flooding occurred upstream of St. Louis and affected the Mississippi, Missouri, and Illinois river basins over a wide area. The states of North Dakota, South Dakota, Kansas, Nebraska, Minnesota, Iowa, Missouri, Wisconsin, and Illinois were affected.
The widespread 1993 flooding resulted from the polar jet stream parking itself over the mid west. This spawned numerous frontal systems and associated thunderstorms. This is not unusual weather for the mid-west, but in 1993, a strong high pressure system developed in the southeast. This had two effects; 1) moist, southerly winds were enhanced, and 2) the high blocked the eastward movement normal for thunderstorms. This resulted in a longterm period of heavy rainfall in the upper Mississippi river basin.
Recurrence interval: The average number of years between flood events of a given intensity or discharge. Given by:
T= Recurrence interval (R in the figure below)
n = Number of years of data
m= rank
The 100 year flood level is the discharge that occurs, on average, every 100 years. This does not meant that such a flood will occur every 100 years, but that this id the average time between such a flood event. It is possible to have a 100 year flood in any given year. Put another way, there is a 1% chance ((1/T)*100) of a 100 year event in any given year. Even with a few years of data, the 100 yr flood level can be found graphically.
Humans have long attempted to control streams and flooding. Controls include;
Levees
Channelization
Dams
All of these means effect water flow and sediment transport. Any method used to control water flow will effect flooding and sedimentation in other parts of the stream system.