Class Notes: Geo 1330: Dr DupreĽ

Surface Water

 

Global Reservoirs of H2O include: (see also fig. 10.2)

Ocean

____%

Ice

____%

Ground Water

____%

Lakes

____%

Soil Moisture

____%

Atmosphere

____%

Rivers

____%

Biosphere

____%

 

Why Study Rivers?

BENEFITS

- Landscapes

- Water Supply

- Hydro-Electric Power

- Transportation

- Waste Disposal

- Recreation

- Aesthetics

- Nature Habitats

- Geo-Political Boundaries, etc, etc

HAZARDS

- Pollution

- Flooding

- Erosion

 

Flooding

Flooding along a river occurs when the volume of floodwaters exceeds the volume of the channel to contain those waters.

 

 

What determines the volume of floodwaters at a specific location?

1)

2)

3)

 

Runoff =                              -                             

 

Drainage Basin: the area drained by a river at a particular point. Also referred to as the "Watershed".

 

Drainage Divide: separates adjacent drainage basins (fig. 13.21)

 e.g. Colorado River Drainage Basin (fig. 13.22) and Mississippi R. Basin.

 

Factors influencing Infiltration include:

- Ground slope, soil type, rainfall intensity, soil saturation, and vegetation/land use.

-

Calculate the volume of runoff that results from a 1" rain over the Brays Bayou Drainage Area.

 

 

 

 

 

 

 

 

 

 

Itís not the volume of flood waters that determines the flood risk, it is rate at which the water flows past a specific pointÖ.i.e. DISCHARGE

 

Discharge = rate of flow = volume / unit time; e.g. cubic feet/second (cfs) or cubic meters/second/

 

 Discharge (Q) is measured using a simplified form of the Continuity Equation:

Q = VA

Q = discharge (L3/s)

V = average flow velocity (L/s)

A = cross sectional area of water (L2)

 

Example at low discharge (fig. 12.14a)

 

 

Example at high discharge (fig. 13.14b)

 

 

A                               plots discharge (or stage) at a gaging station over time.

 

                       is the elevation of the floodwaters.

 

 Sketch and label a typical flood hydrograph:

 

 

 

Flood and Stage Hydrographs at Brays Bayou for April 3, 2000

 

 

 

 What is a Rating Curve and how is it used?

 

 

 

 

 

 

What is a Floodplain Zonation Map and how do we make one?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Quotes from the Houston Chronicle after major flooding in October, 1998

"If theyíll let me, Iíll rebuilt right here."

"Iíve lived here 45 years and itís never flooded, except in the streets."

"They say this is a 100 year flood. I donít guess Iíll see the next one anyway."

 

How Bad is Too Bad?

It depends on the frequency and magnitude of the event, and how and what you (or society) consider an acceptable level of risk!

 

Flood Frequency is measured by:

1) Recurrence Interval

2) Exceedence Probability

 

The Hundred Year Floodplain is the area flooded by the hundred year flood.

 

The Hundred Year Flood is a flood with an annual recurrence interval of 100 years.

 

 The Annual Recurrence Interval ( R I ) of an event of a particular magnitude is the average number of years between events of similar or greater magnitude.

 

Thus, the 100 year flood is one which occurs, on the average, once every 100 years.

 

The annual recurrence interval ( R I ) of an event of a particular magnitude can be determined using the following equation:

R I = (N + 1)/m

Where N = number of years of record and m = rank of the event

 

Exceedence Probability is the probability (P) of an event a particular magnitude being equaled or exceeded in any given year. It is the reciprocal of RI.

P = 1/RI

 

Therefore, a 100 year flood has a 1 in 100 ( i.e. 1%) chance of being equaled or exceeded in any given year.

 

"Texas has $36 billion worth of properties in 100-year floodplains" (based on data from the National Flood Insurance Program database)

 

One house in Houston, Texas (valued at $114,480) flooded 16 times between 1989 and 1995. The owners collected $806,590 in federally subsidized insurance claims! (National Wildlife Federation, 1998)

 

How do we plot RI and exceedence probability to estimate the 100 year flood from actual flood records (see fig. 13.1)?

 

 

 

 

 

Floodplain Zonation Map

 

 

 How and why do hydrographs before and after urbanization differ?

 

 

 

 

 

Hydrographs from Similar Sized Storms on Brays Bayou, 1946 and 1976

 

 

 

 

 

 How has Brays Bayou changed over the past 60 years?

 

 

 

 

Question: How can you reduce flooding without reducing discharge?
(hint: remember the continuity equation)

 

 

How can you increase the velocity of water in a channel?

1)

2)

3)

 

What is "Mannings Equation"?

 

 

 

 

 

V = Average Water Velocity

n = Mannings Roughness Coefficient ( a measure of the resistance to flow)

R = Hydraulic Radius ( a measure of the efficiency of the channel cross section)

S = Slope of the Energy Gradient (approximated by the slope of the channel)

 

 

Examples of different roughness coefficients:

 

 

Channel Efficiencies is measured by hydraulic radius:

Hydraulic Radius R = A/P where:

A = area
P = wetted perimeter

Compare the efficiency of a wide, shallow channel with one semi-circular in cross section.

 

 

 

 

The                            of the channel (S) between 2 points is the ratio of the vertical change (Y) in stream elevation divided by the horizontal distance (X) between the 2 points: i.e. S = ∆Y / X (see fig. 13.16)

 

 

The                            of a stream is a measure of how much it is meandering. It is defined as the channel distance / linear (or valley) distance, i.e. distance as the fish swims / as the crow flies.

 

How has the sinuosity of Brays Bayou changed from 1916-1978?

 

 

 

How did the change affect the slope of the bayou?

Why was this done?

          

 

 

ButÖisnít reduced flooding good?

- Concrete-lining reduces natural habitats.

- Concrete-lined channels look bad and reduce groundwater recharge.

- Channelization increases flooding downstream!

-       It may increase development of floodplains and result in increased losses

 

Stream Classification

All natural streams can be classified as either:

                       Streams

or

                       Streams

 

                                 : Streams cut within hard "bedrock". The pattern of such streams reflects the structure of the underlying bedrock in the drainage basin.

 Examples:

 

The drainage pattern of bedrock streams often affects the nature of the underlying bedrock (fig. 13.23)

.

                                  streams are those that became incised when subsequent deformation occurred. (fig. 13.24)

 

                                  streams are those that became incised and exhumed pre-existing structures.(fig. 13.25)

 

                                 : Streams cut within reworked stream-deposited material.

 

Most alluvial streams can be subdivided on the basis of the pattern in map view into:

                                  Streams: characterized by multiple channels, several of which may be active at one time. (fig. 13.12)

 

 

                                  Streams: characterized by a single meandering (sinuous) channel. (fig. 13.11)

 

Q. Why do rivers sometime make poor boundaries?

A.

 

 

 

The three processes by which streams change their location are:

1)

2)

3)

 

Evidence for lateral migration includes:

1) point bars

2) cut banks

3) meander scrolls

 

 

 

 

 

Evidence for Meander Cutoffs: (see fig. 13.10)

 

 

 

What are natural levees and how do they form? (fig. 13.1)

 

 

 Case Study: The Red River and the Texas-Oklahoma state boundary:

 

 

 

 

 

 Comparison of the Flathead and Red Rivers

 

 

 

Case Study: Pipeline Ruture on San Jacinto River, 1974

 

 

 

 

 

Case Study: Oyster Creek and the Brazos River: a case of river avulsion.

 

 

 

 

Case Study: the "Shifty Mississippi" (fig. 13.26, 13.28))

 

 

 

 

 

Case Study: "Simonton has a Problem"

What is Simontonís Problem and how bad is it? (do extra credit assignment to find out!)