LAST MODIFIED:

Posted: 10 June 2020

• The Role of Levees

• Levees are supposed to
• keep a river in its bed
• keep an ocean from flooding land near or below sea level
• avoid spill into floodplains
• serve as flood gates for controlled flooding


• Levees fail due to
• wave attack
• overtopping
• slumping
• piping


PROS:
• flood damage would have been greater without levees

CONS:
• building cost of levees may exceed value of structures to protect
• levees create false sense of security, encouraging more development
• should tax payer provide funds for disaster relief (people live there against better knowledge)
• higher levees eventually raise river above floodplain
  e.g. floodwater height increased 2-4m in engineered upper Mississippi basin,
  while no change in not-engineered sections

PERHAPS:
• remove some levees to restore wetland habitat and ease flood risk
• move towns
• provide disaster relief funds only for moving to higher grounds or
  raise floor level of existing houses



• Flood Frequency Curves

Flood frequency curves are used by FEMA (Federal Emergency Management Agency) to issue recommendations in disaster management and prevention. Issues addressed by flood frequency curves:

• small floods happen more often than large floods that have longer recurrence interval
• different rivers have different flood recurrence intervals, hence different flood frequency curves
• flood frequency curves help to calculate the probability of a flood of a certain size to occur within a given time span;
• the 100-year flood is a large flood (with a large discharge) that has a recurrence time of 100 years; i.e. it occurs approximately once every 100 years
• the probability of a certain-size flood is not a cumulative process, i.e. a 100-year flood has 1% probability to occur within any given year; but only 63% within 100 years.
  The physical reasoning behind this statistical number: Since the recurrence time is only an average time, the time to the next flood of the same size could be slightly shorter or longer. Therefore, there is a chance that a 100-year flood does not occur within the next 100 years but a little later. Consequently there is only a 63% probability.
• need to know the flood frequency curve of a stream, river or channel in construction projects for canals, flood channels and levees!!

• How to construct a flood frequency curve?

like earthquakes, floods can't be predicted; use statistical measures to estimate likelihood of a flood of a certain size
• determine peak discharge for each rainfall in each year
• rank all floods from largest to smallest (ignore chronologic order)
• recurrence interval for each year's maximum flood R = (N+1)/M; N=number of years of flood records; M numerical rank of the individual flood
• plot each value in diagram
• probability of a flood in any given year is the reciprocal of the recurrence interval
• the longer the records the better the flood-frequency curve is

• Runoff and Infiltration

• runoff: flowing water from precipitation above ground
• infiltration: rain water that penetrates into the ground
• natural setting: 80-100% infiltration; 0-20% runoff
• urban setting: 0-10% infiltration; 90-100% runoff
• runoff in urban setting increased due to paving and roofing, prohibiting water from penetration ground

• Hydrographs of Streams

• plots amount of rainfall and flood runoff over time during and after a storm
• runoff lags rain fall
• maximum runoff smaller than maximum rainfall (because some rain infiltrates)
• runoff curve has steep rising limb and gentle falling limb (rise of flood quicker than recovery)
• recovery prolonged because infiltrated rain may feed stream from below ground
• maximum runoff/discharge much larger in urban than natural setting
• maximum runoff/discharge and possible flooding reached much earlier in urban than natural setting

CASE STUDIES (TEST MATERIAL!)

• CASE 1: The Case San Diego

San Diego adopted the L.A. style channel system: most runoff channels are lined with concrete, even the bottom. Since San Diego practically has a desert climate (only 10.5in/year rainfall), these channels are dry most of the year, but can become a death trap during the few heavy winter storms.
Also, a growing problem in recent years - with decreased maintenance resources due to budget woes: the sedimentation in some channels can get quite heavy. The channels lose their effectiveness if these sediments and other debris are not removed. There have been reports in recent years of some cases of extensive littering and growth (reed and even trees).



Two channel projects have repeatedly made the flooding news as a result of poor planning:
• San Diego/Tijuana joint Project to redirect the Tijuana River
• 80% of river in Baja; 20% in California; last few miles in San Diego before entering the Pacific Ocean
• Agreement to adopt L.A. style channel system
• Tijuana realized its part south of the border
• San Diego part stalled due to environmental concerns
• a flood followed in Jan. 1978 that submerged farmland south of Imperial Beach
• Mission Valley Project (San Diego River)
• centuries worst flood was in Jan 1916 with a discharge of 72,000ft³/s
• U.S. Army Corps of engineers built a 245m wide channel with natural floor in 1940ies to hold maximum discharges of 115,000ft³/s
• in 1950ies, central part of Mission Valley was developed (incl. Fashion Valley at the eastern end); channel joined existing one to the west; 7.5m wide holding maximum discharge of 8000ft³/s (seems ok though not designed to hold the "100-year flood")
• in 1980ies, eastern part of Mission Valley was developed; channel joint small channel to the west; 110m wide holding maximum discharge of 49,000ft³/s
• the current situation is that the easternmost channel (upstream) can hold larger discharge than the one it is directing the water into (smaller one to the west)
• devastating floods in the central part (incl. Old Town and Hotel Circle) an inevitable!
• nearly every winter, storms cause some degree of flooding in Fashion Valley. In early 2010, the mall parking lot was completely flooded, and closed to the public. The I-8/163 connector can also be closed as a result of flooding after heavy winter storm rainfall.

On October 25, 2011, the Union Tribune had an article about the decision made by the city council to press forward with the clearance of the flood control channels, even in environmentally sensitive cases. check out newsclip

• CASE 2: The Mississippi River

• Geography
• world's third largest drainage basin
• length: 2,340 mi
• length of Missouri-Mississippi River system: 3,710mi
• world's 4th longest river system
• one of world's largest river deltas
• delta arms change on 100-1000 year time-scales
• large accumulations of oil-bearing sediments (up to 10km)
• one of world's largest sedimentary basins
• The fight of Human vs. Nature:
• building levees shortens but steepens path
• increase of flow speed
• potential floods in bottleneck downstream
• Mississippi flooded several times in the last 100 years
• flood severity seems to increase
• costs of floods increase
• Example 1: The fight against the Atchafalaya River
• 1831: dig channel to by-pass one of Mississippi's "irritating" meanders
• Mississippi accepted the new channel but Old River arm of meander remained active (Red River upstream!)
• 1839: removed 30mi log jam down Atchafalaya River to ease flow (of Red River/Old River)
• Mississippi now drained into Old River, ultimately into Atchafalaya River
• took away too much water downstream (Baton Rouge/industry; New Orleans/tourism)
• 1939: Atchafalaya took 30% of Mississippi's flow
• 1963: decision to keep Atchafalaya from draining more of Mississippi's water; ACE built old River control structure
• 1973: barely withstood flood; greatly damaged!
• 1980: auxiliary structure that wasn't ready for 1983 flood; first structure barely survived
• Example 2: The 1927 flood
The 1927 flood of the Mississippi River has probably been the most troublesome flood of this River, with the most controversial measures taken to protect a certain area, on the expense of another.
• according to Abbott: flood breached levees in 225 places; inundating 19,300mi ² and drowning 183 people
• according to Wikipedia: flood breached levees in 145 places and flooded 27,000mi ², drowning 246 people
• according to Earthshock by Robinson (and many other sources): the flood threatened New Orleans; therefore, levees downstream were dynamited to allow the water to flood other areas, including plantations; this flooding created a sea covering 26,000 mi² in 7 states; it was 18ft deep and 80mi across; this flood cost 246 lives, maybe 500 and made 650,000 people homeless, half of them African-American; according to Wikipedia, many Black people were detained and forced to labor at gunpoint during flood relief efforts.
• A flood gage in Vicksburg, MS measured a flood level of 17.1m (56.2ft) above normal. Had the levees held, the Mississippi River would have risen to a staggering level of 19.0m (62.2ft).
• according to Rising Tide by Barry: the flood profoundly changed race relations, government, and society in the Mississippi River valley; many Black people left the area and migrated to northern cities. The flood also had profound consequences for plantations in the Deep South.
• check out Great Flood of 1927 at Wikipedia
• the causes and consequences of the 2005 Hurricane Katrina flood has been likened in its impact to the 1927 flood
• Example 3: The 1993 flood
• at $ 12 billion 4th costliest natural disaster in U.S. history
• upper Mississippi flooded for 160 days
• greatest inundation flood in 140 years
• Mississippi alone flooded 12,500 mi²
• 50 fatalities
• adverse weather situation: wet winter, spring and summer due to south-shifting jet stream
• largest discharge of any Mississippi flood
• check out Great Flood of 1993 at Wikipedia
• Example 4: New Orleans and Hurricane Katrina 2005
• Katrina was a category 3 hurricane when she roared ashore August 29, 2005
• New Orleans levees broke at dozens of locations, flooding large portions of city, especially the poor 9th Ward
• three key locations where levees broke: London Ave (canal to Lake Pontchartrain); 17th Street Canal (Lake Pontchartrain); Industrial Canal
• investigations revealed that levees were not overtopped but failed due to shifting soil beneath
• the Inner Harbor Navigational Canal meant to direct and protect shipping traffic into Lake Pontchartrain acted as funnel to enhance effects of storm surge into Industrial Canal
• New Orleans is below sea level and subsiding and protection requires update
• canal system prevents flooding of wetlands thereby contribution to subsidence
• according to Tidwell in an interview on CSPAN-2 in October 2005: restoration of wetlands would help preventing such floods; estimated cost: $18 Billion
• Example 5: The 2011 Floods
to be augmented

• Recommended Reading

• Natural Disasters: Earthshock by Andrew Robinson, Norton and Comp., 2002, ISBN: 0-500-28304-4
• The 1927 Mississippi Flood: Rising Tide by John M. Barry, Simon&Schuster Publ., 1998, ISBN: 0-684-84002-2
• The loss of the Mississippi delta wetlands: Bayou Farewell by Mike Tidwell, Vintage Publ., 2004, ISBN: 0-375-72517-2

1. because it occurs every 10 years
2. because it is only a statistical average
3. because its recurrence time decreases over time

1. to estimate the average flood stage of a river
2. to estimate the highest flood likely in a 100-year interval
3. to predict the average rainfall over the river
4. all of the above
5. a) and b)

1. flood stage precedes rainfall maximum
2. flood stage lags rainfall maximum
3. recovery from flood stage takes longer relative to reaching flood stage
4. all of the above
5. b) and c)

1. urban
2. rural
3. both the same

1. urban
2. rural
3. both the same