Flooding and Flood Control in the Sacramento Region
I. The Historic Flood scape of the Sacramento Valley
- flooding common in winter / spring; causes are:
- heavy, continuous rain on saturated ground (affects local creeks)
- warm rain on Sierra snow pack (affects rivers)
- annual spring snowmelt (affects rivers)
- extensive floodplain: This term refers to the low area along a river which is covered with water during a flood. Much of the Sacramento area and the Central Valley are in a floodplain.
- natural levees along rivers: Elevated bank of sediment where river spills out of its channel as it begins to flood. Created because stream velocity (energy) suddenly decreases when stream spills over its banks, resulting in very high sediment deposition along the banks…
- alluvial deposits on floodplain (make excellent soils): During a flood, velocity of water slows. Massive amounts of sediment (called alluvial deposits or alluvium) are deposited in floodplains. These sediments are nutrient rich and make excellent agricultural soils.
- extensive wetlands (called back swamps in text) and riparian forest: Standing water in the low-lying floodplain creates conditions for marsh vegetation (such as tulles, cattails, and rushes). Also, riparian (streamside) forest grows on the well-drained natural levees (the most common trees are willow and cottonwood). Once extremely common in the Sacramento Valley, only about 5% of this original vegetation remains. The rest has been lost to development.
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II. Flooding and Human Settlement
- Native Americans
- Permanent Settlements (Why did we build where we did?)
- Rivers served as excellent transportation corridors
- Soils were perfect for agriculture
- Impact of Hydraulic Mining (late 1800s)
- Late 1800s, hydraulic mining caused massive erosion in mountains
- The resulting sediment was dumped into mountain rivers, which transported it to the valley. Once these rivers reached the valley, the sediment deposited in the river beds.
- Clogged with so much sediment, the rivers carried much less water and flooded more often.
III. Structural Flood Control Solutions: Structural solutions are structures which hold back and/or divert flood water.
- Most Common Examples
- Dams
- They are very effective flood control structures
- They are often ALSO used to generate hydroelectric power
- They are also used to store water
- These different uses are not always compatible (e.g. flood control is best with reservoir empty, hydro power is best with reservoir full)
- Requires careful compromise – leave it empty during riskiest months!
- Levees (walls along the river)
- predictable river channel maintained (most of the time)
- all land up to river bank is protected for use and development
- Weirs and Bypasses (discussed below)
- Other: emergency pumps, movable barriers, storm drains…
- Criticisms
- Dams
- natural flow patterns are significantly altered
- significant habitat is lost beneath reservoir
- fish migration is blocked
- dams trap sediment – lifetime of dams is limited
- breakage is rare but disastrous
- Levees
- “narrow levee” design eliminates most wetland habitat: Levees built right next to river rather than farther back. Natural wetlands no longer flood. Only 5-10% of original Central Valley wetlands remain.
- “narrow levee” design doesn’t provide enough channel capacity to prevent flooding: Original designers of levees estimated that flows would be fast enough to scour out sediment and maintain channel depth. They were wrong, but it’s too late in many cases.
- “narrow levee” design sends problem downstream: flood prevention upstream sends more flood water downstream.
- levees block flood water from draining back to river: levees are great at holding water on flooded land; flood is prolonged
- Dams
IV. Non-Structural Flood Control Solutions: With non-structural solutions, it is assumed that flooding will occur. These are strategies which will minimize damage if flooding does occur.
- Compatible Land Use Choices:
- NOT residential!!!!!
- Instead, choose agricultural, recreational, wildlife preserve, possibly industrial
- American River Parkway is a good example
- Flood-Friendly Building Designs:
- Delta style houses (where first story starts half story up; common in older areas of Sac)
- Houses on stilts
- Elevated roads and highways
- Industries equipped to raise machinery to ceiling
V. The Sacramento Valley Bypass System – The Key to Flood Control is “Controlled Flooding” (Colusa, Sutter, and Yolo Bypasses)
- Developed in early 1900’s, conceived decades earlier: Landowners were paid for allowing their land to be occasionally flooded.
- Bypass System becomes an emergency, “alternate” river
- Weirs (diversion gates) opened at flood stage
- Water diverted to wide (1 – 5 miles) channels with set back levees – these are called bypasses
- Bypasses are inter-connected and flow roughly parallel to Sacramento Riv.
- Bypass system has huge capacity (600,000+ cfs; equal to the average discharge of the Mississippi)
- Bypass system works in conjunction with dams and other flood control measures
- Major interstates elevated (I-80 between Sac and Davis and I-5 north of airport)
- Benefits of the Bypass System
- open space preserved
- some agriculture still feasible: farmers grow at own risk, flooding possible
- wetland habitats restored and expanded: Water is purposely diverted to lands designated for restoration. Wetland plants reintroduced; wildlife population rebounding. Only 5-10% of original wetland area remains.
- hunting and recreation opportunities: Hunting organizations donate the largest amount of money to wildlife and habitat conservation.
- ground water recharge: flooding allows water to soak back into ground
VI. Future options to improve flood protection (and decrease risk)
- improve and strengthen existing levee system: cheapest of all options; nonetheless, still costs over $1million per mile to improve a levee
- Folsom Dam remodel: It was raised 3.5 feet to provide significantly more flood storage capacity. Also, a new outlet was constructed to allow it to be emptied faster before a flood.
- Auburn Dam (terrible idea – imho): This is a proposed project on the North Fork of the American River…. It is incredibly costly, environmentally damaging; the current proposed site sits on an active earthquake fault line.
- Continue to improve storm forecasting
- Quit building in flood plain!!!
Sample Questions: Questions similar to these will be on your exam. As you study you should anticipate how I might use these questions to create new questions on the same concepts.
| 1 | Why does water occasionally flow fast and heavy in dry, desert areas? |
| 2 | What sediments and/or substances will commonly be found in playas (dry lakes)? Why? |
| 3 | What are the primary natural causes of flooding in the greater Sacramento Area? |
| 4 | How do bypasses work to reduce the risk of flooding in the greater Sacramento Area? |
| 5 | Although dams are effective flood control structures, what can be their negative impacts? |
| 6 | Which of the following is a non-structural flood control strategy? |
| 7 | What are the problems associated with building levees right along the river banks, instead of set back? |
| 8 | What types of land uses make the most sense in flood prone areas? |
GOOD NEWS!!! The practice questions above – plus many, many more – can be found in the online practice quizzes discussed in your syllabus and in class. You can take each quiz multiple times, and each time you will get some new questions. Once submitted, the quizzes are graded automatically, with the correct answers provided immediately. This is a great way to prepare for the exams!!!
SERIOUSLY, TAKE THE ONLINE PRACTICE QUIZZES!!!
