Lecture 9 - Surface Water

Earth’s Water is

97.5% Salt Water, 2.5% Fresh water, 0.4% Surface water.

2L bottle analogy:

• 1940 ml in ocean

• 43 ml (~5 Tbs) in glaciers

• 12 ml (2.5 Tsp) in groundwater

• 5 ml in lakes, atmosphere, and streams

Hydrologic Cycle

The cyclical movement of water between the oceans, atmosphere, Earth’s surface, under the ground, and back to the oceans.

A reservoir

Any place that water exists in the water cycle.

• Atmosphere

• Ocean

• Groundwater

• River

The range of temperatures on Earth allow water to exist in 3 states…

• Liquid (water)

• Gas (Vapor)

• Solid (Ice)

Evaporation

Changing liquid water into gaseous water vapor.

Precipitation

Condensing water vapor into liquid water (rain) or solid Ice (snow).

Infiltration

Water enters rock and soil through cracks or between small pores.

Runoff

The sum of all water that flows over the Earth’s surface, including streams and soil saturation.

• Runoff = Precipitation – Infiltration

What determines how much H2O infiltrates?

• Intensity & duration of rainfall

• Prior moisture content

• Slope

• Vegetation / Permeable Ground

Meandering River

Low-velocity stream that curves and bends over shallow sloped land or lowlands.

• Velocity is greater on outsides of curves = greater erosion

• Velocity is slower on insides of curves = deposition

Centuries of erosion and deposition…

Reshape rivers.

Point Bars

Deposited sediments on the inside bank where the current is weaker.

Cut Banks

Eroded sediments on the outside bank where the current is strongest.

Oxbow Lake

An abandoned curve in a river that stays filled with water from the stream.

Floodplain

The flat area at the level of the top of the channel that lies on either side of the channel.

• Precipitation ↑ + Infiltration↓

• Runoff↑

What is the role and benefit of the floodplain?

Floodplains work if only we didn’t develop in them.

Floods

An overflowing of a large amount of water beyond its normal confines, especially over what is normally dry land.

• Stream can no longer carry water

• Ground is too saturated to absorb water

Recurrence interval

The average time between events like earthquakes, floods, landslides, or river discharge flows.

• 500 year flood = 1/500 = 0.2% chance

• Purely Statistical

Recurrence Interval

(n+1)/m

• n= number of events

• m=rank

• (26 +1)/6 = 4.5 yr flood

What factors affect flooding?

• Intensity of rainfall

• Duration of rainfall

• Ground Absorbance

• Vegetation

Upstream Flood

A flood that occurs when a small area receives a large amount of rain in a short period of time.

• Intense Rainfall

• Short Time

• Small Area

• Short Lag Time

Downstream Flood

Occurs when the water level in a river or stream rises above its normal level in the downstream portion of the river system.

• Larger Areas

• Long Durations

• Long Lag Time

• No Localized Flooding Possible

Flash Floods

A sudden, localized flood that occurs when heavy rainfall or other causes fill normally dry areas in a short period of time.

• Intense Rainfall

• Short Time

• Small Area

• Very Short Lag Time

• Can Happen Anywhere

Lag time

The time between precipitation and the river cresting.

Downstream Flood example

Mississippi & Missouri Rivers Flood 1993:

• Wet Fall

• Heavy Snows

• 750% more rain in Spring

• Rivers rose 20ft

• Flooding lasted 200+ days

• 100,000 homes destroyed

• 2 towns permanently relocated

• $15 Billion in Damages

Hydrographs

A way of displaying water level information over time. A hydrograph plot may display stage, streamflow, and sometimes both.

Urbanization Flooding

Urbanization increases the magnitude and frequency of flooding.

• Sewers move water to streams faster

• Removal of vegetation

• Destruction of wetlands

• Loss of permeable ground

Why does peak flooding occur more rapidly after urbanization?

• Increased speeds of water to streams

• Increased intensity

• Increased flooding

• Increased amounts of water to streams

Wetland

A place where the land is covered by water; marshes, ponds, edges of lakes & oceans, deltas, low-lying areas that frequently flood.

Why have we destroyed ~50% of the Earth’s wetlands?

• Fertile soil, good for agriculture and development

• Removes mosquito breeding grounds

What could go wrong with wetlands?

• Loss of wildlife habitat

• Reduced infiltration

• Increased flow to streams

• Increased frequency of flooding

• Increased intensity of flooding

How can we prevent wetland problems?

We engineer our way out!

• Control Runoff

• Control Flood Stage Height

Levees

A ridge of sediment deposited alongside a river during floods.

• Compact Earth Embankments

• Contain Flooding

• Divert Flow

DFW Levees (2007)

Levees Pro’s

• Best for people along a river

• Reduces probability of flooding

Levee’s Con’s

• Constricts area, increasing velocity

• Floodplain loses nutrients

• Creates an upstream bottleneck

• US levees average a D rating

Channels

Artificial structures that use gravity to convey water for flood protection or drainage.

• Range in size from a street gutter to large river

Los Angeles River

Channels Pros:

• Immediate area control

• Controls flooding and erosion

• Navigable

Channel’s Cons:

• Destroys wetlands

• Destroys natural flow patterns

• Loss of infiltration

• Mostly visually unappealing

Retention Ponds

Water-filled pools that change in reaction to rainfall and runoff.

• Provides runoff control in urbanized areas

• Can provide natural areas

Retention Ponds Pros:

• Great for controlling runoff

Retention Ponds Con’s:

• Reduces land use

• Can create anaerobic conditions

• Limited size

• Welcomes invasive species

Dams

A barrier that prevents the flow of water or other materials, such as snow or soil.

• Can provide regulated flow downstream

Three Gorges Dam, China

Dams Pros:

• Flood Control

• Water Storage

• Electricity Generation

Dam’s Con’s:

• Trapped Sediments

• Increased downstream erosion

• Impacts fish migration

• Changes groundwater flows

The US Infrastructure: Dam’s

• 91,000 in US

• Average age is 57 years

• ~20% are considered High Hazard!

• Repairs cost ~$66 Billion

Texas Infrastructure

• Texas is not doing much better!

Lewisville Lake Dam, TX

The dam called trouble, needs millions of dollars to repair this dam, one of the nation’s most dangerous, a breach could put 431,000 people in harm’s way.

Examples of surface water

• Oceans

• Glaciers

• Groundwater

• Lakes

• Streams, wetlands, and swamps

• Rivers

US Streams

All flowing down hill to the Mississippi river, then into the ocean.