What is the term for the lowest level to which a stream can erode?

GEO-113: ENVIRONMENTAL GEOLOGY

STREAMS AND FLOODING

LECTURE NOTES

Previous geological processes discussed were internal processes. Tend to build land up, affect restricted areas, are located primarily along plate boundaries, and are driven by the earth shedding its internal heat.

Surface processes driven by suns external energy or by gravity. They tend to erode or tear the land down, occur over broad areas, affect many more people and cost more money in an average year, are usually not as deadly because they are more predictable, and may involve, water, ice, or wind.

Floods have been a problem since manπs earliest settlement. Rivers are "people magnets." They are good sources of fresh water and food, were the first avenues of transportation, and their floodplains often provided the best soil for agriculture. All civilizations started along rivers.

Floods are both natural occurrences and human induced (or at least made worse). Generally restricted to low-lying areas so we know were they will occur. In the U.S., on average floods kill ~100 people/year, cause $2+ billion damage and affect 25+ million people. 1993 Mississippi River cost $12+ billion. Worldwide thousands killed each year with $5-10 billion in damage. Most are predictable!

I. Stream and Flood General Terms

1) Stream - Flowing body of water conferred to a channel; no size implied; includes creek to river.

2) Drainage basin - Area drained by a particular stream; separated by divides;

3) Gradient - Change in elevation per mile downstream; typically decreases downstream. Higher the gradient, the higher the velocity.

4) Discharge - Volume of water per unit time flowing past a given point; typically given in cubic feet per minute; varies from 1 to 10 million cubic feet per second; equal to stream velocity x stream cross sectional area.

5) Base level - The lowest elevation to which a stream can erode down to. May be sea level (if stream flows into ocean) or elevation of a lake (reservoir).

6) Competence - Ability of stream to erode; largest particle stream can move; increases with velocity. It is a function of gradient and channel shape. Sediment generally becomes better sorted and smaller downstream.

7) (Total) Load - Combined total of material carried by a stream. Done in four ways.

            1) Bed load - sediment pushed and rolled along bottom of stream.

2) Suspended load - lighter material floating in water; gives stream a muddy appearance.

3) Saltated load - particles that ≥bounce≤ along bed; sometimes suspended, sometimes suspended.

4) Dissolved load - compounds dissolved in water. Includes things like Cu, Fe, Mg, Na, Cl, SO3, CO3, etc.

8) Capacity - Volume of water (discharge) and total load stream can hold without flooding.

9) Floodplain - Area where water collects when stream overflows its banks; flat area on either side of channel. They are separated from channel by natural levees (ridges). Produced by migration of meanders (bends in stream channel that change position due to erosion on outside of curve and deposition on inside of curve). Made of alluvium and they are extremely fertile. Meanders may be cut off (naturally or by man) forming oxbow lakes. They act as natural water storage areas.

10) Crest - Maximum elevation of flood waters. Larger the flood, the higher the crest.

II. Causes of Flooding: usually caused by excess precipitation. Also caused by snow and glacier melts, ice dams, dam failures and landslides.

In all cases water discharge exceeds stream capacity

1)    Human causes:

Urbanization - general term for development; increases runoff and decreases infiltration.

Channelization - artificial channels; usually increase gradient by wall off flood plain.

            Dams - when they fail they release huge volumes of water very rapidly.

Destruction of vegetation - allows soil to be removed and fill up channel; less volume to hold water.

Agriculture - often results in previous problem and leaves fields unprotected.

2)    Flood types: two extremes. Most floods are a combination of the two, although one type may be dominant.

Upstream (upland) - affects a small drainage basin along a tributary stream. Often caused by local intense rainfall or dam failure.  Usually they are brief in duration (hours to days) but severe in intensity. Often referred to as a flash flood.

Downstream (lowland) - affects large drainage basins along a main or trunk stream. Prolonged in duration (weeks or months) and regional in extent. Usually caused by prolonged precipitation and/or snowmelt.

III. Probability of Floods: usually based on historical records. The longer the record the better the accuracy at being predictive.

Probability determined by flood frequency curves, which are plots flood discharge versus how often it occurs. Gives recurrence interval as a percentage probability.

Can be used to predict very large floods that occur rarely. Give probability that a flood of a certain size will occur in a given year. Does not mean it will, or does it preclude the possibility of two floods of the same size occurring in the in same year. For example, the 50-year flood has a 2% chance of occurring in any given year and a .04% (probability of .0004) of occurring two years in a row.

Can be used to make plans for land use of a particular area.

Only valid if stream characteristics have not changed with time; i.e. no development or climate changes - a lot of uncertainty here.

How do you know if a particular flood is a 50-year flood or a 200-year flood? May have only 50 years of records. Can determine by the following equation:

                                R=(N+1)/M

Where N = # of years of records, R = recurrence interval, and M = rank of flood in time N. Example N=25 years, M= 7:

R= (25+1)/ 7= 3.7. Probability is 27% of that flood occurring in a given year.

IV. Human Development and Flooding

Flooding is a natural stream process; it should be expected. Floodplain development aggravates problem.

We live in floodplains because of:

1)    ignorance

2)    flat land good for building

3)    may be only land available for settlement

4)    excellent soils

5)    may be only area where there is water

6)    streams are used for transport; cities form along them

7)    good scenery and recreation

Effects of human development along streams:

1)    Increased runoff - areas get paved and soil gets covered by impermeable materials. Water can't soak into the ground. Water has no place else to go other than into the local streams.

2)    Increase discharge from storm servers and drained farm fields

3)    Fill in channel and flood plain - floods become more severe.

4)    Destruction of vegetation - more sediment load - less water capacity

5)    More property to protect - now need to protect ourselves from the floods we helped create or have allowed ourselves to become affected by.

V. Flood Control Mechanisms

1)    Zoning - restrict land use - avoidance of hazard. Not an attempt to control flood process, only reduce risk. Works best prior to development.

2)    Relocation - Move structures out of flood plain.

3)    Economic incentives - provide grants or tax relief if you move out of flood prone areas.

4)    Education - make people aware of process and risks.

Items 1-4 all involve avoiding the hazard, not controlling it.

5)    Retention ponds (basins) - hold water during times of high inflow and allow it to infiltrate into ground. Good for controlling upstream floods. May be used for other things when dry.

6)    Reforestation ≠ try to decrease runoff and sediment load.

7)    Good farming practices - control soil erosion.

Items 5-7 all try to reduce inflow. Best for upstream floods.

8)    Channelization - channel improvements intended to increase stream discharge. Includes widening, deepening, and straightening of channel. All increase gradient. In some instances a completely artificial channel is created. Usually no access to flood plain.

9)    Construction of floodwalls or levees - increase height of channel in order to increase stream capacity. Can increase flood severity and duration. Floodwaters can not be stored in flood plain so now more water to deal with. Also keeps water on flood plain once it is flooded. They give a false sense of security. Leads to greater development. Now have more property to protect so more structures are built.  Get into a vicious cycle where problem is never solved and may only get worse!

10) River diversion ≠ floodwaters diverted to a planned area. Recreate the flood plain that was there originally.

11) Dams and reservoirs ≠ large scale retention basins. Can silt up and become useless. Reservoirs drown large areas underwater displacing people and destroying habitats. Often result in incompatible priorities among need for irrigation water, recreation, flood control, and hydroelectric power generation. Water below damn is sediment free and erodes more effectively downstream of dam. Finally, earthquakes may be produced.

There are no easy answers to flood control. Often the cure is worse than the disease. May wind up spending more than the value of the property you are trying to protect. Have begun to rethink all this.