GEOS 3310 Lecture Notes:Rivers and Flooding
Dr. T. Brikowski Fall 2011
ﬁle:riversFlooding.tex,v (1.24, October 14, 2011), printed October 13, 2011
• USGS general introduction to rivers • USGS real-time streamﬂow data • online Physical Geography Textbook section on rivers and ﬂuvial processes
There are a few fundamental lessons to be learned about rivers: • Natural behavior: – two types: meandering and braided – meandering streams migrate laterally, threatening nearby structures (see animation ) – rivers generally steepest at their headwaters; therefore erosion concentrated there, ﬂoods come and go quickly – rivers generally shallowest at their mouths; therefore deposition concentrated there, ﬂoods come and go slowly • Human management 2
– ﬂoodplain regulation can minimize ﬂood damage and impact at minimal cost – riverbed can be channelized when building the ﬂoodplain cannot be avoided
The Nature of Rivers
Figure 1: General features of drainage basins and streams. [Fig. 8.7, Keller, 2008]. 5
• movement of water is measured in terms of discharge, i.e. volume time passing a given point • the Discharge Equation describes this, where discharge Q equals stream cross-sectional area width W times depth D times velocity V relationship Q = W ·D·V which is eﬀectively equivalent to the saying “still waters run deep” • rivers generally end in alluvial fans when they emerge onto ﬂat plains, or deltas where they emerge into water bodies (see Mississippi or Indus river deltas) 6
... occurs when the flow of the water slows down. The amount of sediment a river is carrying generally gives color to it's ... in the Grand Canyon. Some rivers go through natural, periods of flooding and receding, at certain times of the year, or in ... with other geographical events. For the first time in thirty years, in March 1996, flood waters rages through the Grand Canyon. However ...
• stream sediment transport is made up of 3 main components (see animation ): – bed load moves along bottom by rolling, skipping. About 10% of total – suspended load silt and clay carried in the water. Usually 90% of total – dissolved load carried in chemical solution, e.g. salts
Meandering Stream Pattern
Figure 2: Idealized diagram of a meandering stream, [Fig. 9.12, Keller, 2011]. Recall animation . 8
Example Meandering Stream
Figure 3: Typical meandering stream, showing alternating pools and riﬄes, cut-bank (foreground left) and point bar (foreground right) [Fig. 9.13, Keller, 2011]. 9
Channel Cross-Section for Meandering Streams
Figure 4: Stream cross-section varies between pools and riﬄes [Fig. 5.11, Keller, 2000]. 10
Consequences of Meandering
Figure 5: Consequences of stream meandering, after a 1983 ﬂood in Tucson, AZ (after U-Ariz ).
Stream ﬂows from bottom to top of ﬁgure [similar to Fig. 5.D, Keller, 2000]. 11
Braided Stream Pattern
Figure 6: Braided stream pattern. These tend to develop where stream
gradient and sediment supply are high, e.g. in mountainous areas [Fig. 9.11b, Keller, 2011]. See also Wikipedia .
Upstream and Downstream Floods
of unknown depth!
Floods near the headwaters of a stream (upstream) tend to come and go quickly, near the mouth (downstream)
the come and go slowly [Fig. 9.18a-b, Keller, 2011]. Note half of all ﬂash-ﬂood deaths involve automobiles: don’t drive into water
Man’s Impact on Rivers
Figure 8: Natural sedimentation as indicated by archaeological artifacts. Left is steamboat Arabia site , pit bottom is 1856 river channel level; right is Temple of Khum , Esna Egypt, showing 2000 years of sedimentation. 15
Agriculture and Stream Grade
Figure 9: Changes in land-use, e.g. agriculture, can dramatically change
... and politically directed human world but on the river, among the woods, where he was never ... have reached freedom; it is the River that separates them …the River that reunites them, …” (Eliot ... both physical and both phychological- happen on the river, it has a kind of fate- determiner role ... Jim- and the road itself, the River Mississippi. “As Pascal says ‘rivers are roads that move’, […] the ...
stream bed grade (level).
These changes caused by conversion from forest to agriculture and now back to forest [Fig. 9.9, Keller, 2011].
Dam Eﬀect on Streams
Figure 10: Installation of dams or other artiﬁcial changes in stream gradient redistribute erosion and deposition zones [Fig. 9.10, Keller, 2011]. 17
Development and Flooding
Figure 11: Increasing urbanization leads to increased overbank ﬂows (ﬂoods) [Fig. 9.19, Keller, 2011]. 18
Stream Discharge Increases
Figure 12: Increasing urbanization leads to larger ﬂoods for a given recurrence interval [Fig. 9.21, Keller, 2011]. 19
More Runoﬀ (Rowlett Ck)
Rowlett Creek near Sachse, TX (USGS gauge 8061540) 450
1988 Water Year
Figure 13: Increasing urbanization leads to more runoﬀ as well. E.g. Rowlett Creek near Sachse (Firewheel).
Extended records of streamﬂow are used to construct
of discharge-frequency curves that allow prediction of return period (frequency) of a given magnitude ﬂood [Fig. 9.E, Keller, 2011].
Approaches toward managing rivers can be summarized in three main categories: • barriers: levees, ﬂoodwalls, storage basins, riprap • adjustment: ﬂoodplain regulation (i.e. zoning) • redesign: channelization (e.g. Trinity River)
Natural Floodplain Features
Figure 15: A natural ﬂoodplain in a mountain valley [Fig. 9.3, Keller, 2011]. 24
Legal Floodplain Features
Figure 16: Zoning features of a regulated ﬂoodplain [Fig. 9.30, Keller,
2011]. The ﬂood hazard area is generally deﬁned at the 100-year ﬂoodplain. See 2011 example of why its unwise to build in a ﬂoodplain.
Figure 17: Riprap is any erosion-resistant material added to a streambank, [Fig. 9.27a, Keller, 2011]. 26
... the forms and the page 1 Figure 2. value stream map example causes of waste and ... Drawing a value stream map happens by using universal symbols like the example in figure two. The ... interesting comparison where the results of value stream mapping and the difficulties for every environment are ... lean manufacturing. Typical results after making value stream maps are reduced inventory levels which will ...
Figure 18: Placement of riprap (e.g. rock baskets) to minimize erosion from meandering [Fig. 9.27a, Keller, 2011] 27
Bank Protection – Gabions
Figure 19: Gabion is the technical name for rock baskets. These are the most common slope and streambank protection method used in the U.S. 28
Stormwater retention basins (like along the creek east of the library) are used to decrease the speed and
severity of local ﬂooding [Fig. 9.24, Keller, 2011]. Also used for groundwater recharge in areas like Los Angeles . See UTD example and Wichita project .
Figure 21: Channelized portion of the Los Angeles River [Fig. 9.28a, Keller, 2011]. Flood eﬀects are minimized, but riparian environment is totally gone. 30
Consequences of Channelization
Figure 22: Comparison of natural vs. channelized streams, [Fig. 9.25, Keller, 2011]. 31
The textbook puts it well [p. 123, Keller, 2000]: “The main lesson learned from the 1993 ﬂoods is that construction of levees leads to a false sense of security. It is diﬃcult to design levees to withstand extremely highmagnitude ﬂoods for a long period of time. Furthermore, because of loss of wetlands, there is less ﬂoodplain space to soak up the ﬂoodwaters.” • along the central Mississippi several ﬂooded-out communities were bought out by FEMA and relocated out of the ﬂoodplain • similar issues for towns ﬂooded out in the Northeast by Hurricane Irene 2011 32
This is intended to be an ever-evolving list of useful links on the general topic of this note set. • USGS Real Time Streamﬂow Data • USGS White Rock Creek streamgauge • USGS Mississippi at Baton Rouge streamgauge • NASA Flooding Hazard webpage • Debris ﬂow example from California • sediment transport movies 34
• Dallas Region interactive ﬂoodplain map .
E. A. Keller. Environmental Geology. Prentice Hall, Upper Saddle River, NJ, 8th edition, 2000. ISBN 0-13-022466-9.
E. A. Keller. Introduction to Environmental Geology. Prentice Hall, 4th edition, 2008. ISBN 9780132251501. URL //www.pearsonhighered.com/educator/academic/ product/0,3110,0132251507,00.html.
... ensure survival and a future for the planet. References: Keller, D. R. (2010). Environmental Ethics: The Big Question. New York: John Wiley ... . and Lo, Y. (2009). “Environmental Ethics”, The Stanford Encyclopedia of Philosophy (Winter 2009 Edition), Edward N. Zalta (ed. ), derived on ...
E. A. Keller. Introduction to Environmental Geology. Prentice Hall, 5th edition, 2011. ISBN 9780321727510. URL //www.pearsonhighered.com/educator/product/ Introduction-to-Environmental-Geology-5E/9780321727510.page.