GEOL105 - Rivers and Flooding

What is a stream?

• A body of water that flows downslope through a defined channel

How do we determine the volume of precipitation that reaches the stream?

• Runoff - infiltration - interception - evaporation

What is a drainage basin?

• Area drained by a single stream

• Often called a watershed, river basin, or catchment 

What happens in zone 1?

• Steep channel gradients where erosion happens 

• Narrow V-shaped channels, no floodplain

What happens in zone 2?

• Erosion and deposition occur

• Meandering and braided rivers, broad floodplains created

What happens in zone 3?

• The gradient shallows, and the river loses energy bc the sediment load had to be deposited

• Ex. river deltas

What are the main causes of floods?

• Heavy rainfall due to hurricanes and tropical storms, warmer than average temps, sudden or heavy snow melt

• Human causes: dam or levee failure

What is the deadliest natural disaster in the world?

• Floods have killed the most people worldwide since 1900

What does discharge measure?

• The volume of water per time 

• Q = V x A

What is the first step for calculating flood discharge?

• Measure the stage of the river (height of the water level in the channel, average depth)

What is the second step for calculating discharge?

• Measure the width of the river

• Then multiply the width by the average stage

What is the third step for calculating discharge?

• Measure the stream velocity 

• Then multiply that by the area 

What forces contribute to discharge?

• Precpitatoin (lots = high discharge)

• Runoff: sheeting action across the landscape

• Also, infiltration (high in areas with forest or grass), interception (plants, dams) and evaporation

What are characterizations of a flood?

• Stage: height of water in a river

• Bankfull stage: height of a river, above which water will spill over its banks

• Flood stage: height at which the river is already overflowing its banks and impacting floodplain

• Discharge: volume of water per unit of time passing through a channel that causes the height of the water to reach flood stage

• Peak discharge: maximum recorded discharge

What is lag time?

• The time discrepancy between peak precipitation and peak discharge

• Peak discharge usually occurs after the heaviest rain has already stopped 

What does it mean when theres a short lag time?

• Flash flood

• Rate of precipitation is greater then rate of infiltration = lots of runoff that sheets straight to river

• Poor infiltratoin: hard bed rock, paved surgces

• Poor interception: deforestization/urbanization

What does it mean when theres a long lag time?

• Slow and steady rain

• Rate of precipitation is less then rate of infiltration

• Higher infiltration: porous thrisryy soil

• Higher interception: woodland, grassland

What happens to streams during flash flooding and where does this occur?

• Zone 1: upstream, small capacity, steep channel walls so water rises quickly

• Causes = high precipitation, low infiltration

• Common in: higher elevations in a small drainage basin

• Arid environments with steep slopes or little vegetation

What happens during downstream flooding and where does this occur?

• Zone 2 and 3: collects water from upstream = long lag time 

• Slow increase in discharge but flooding remains for a long time 

• “Regional floods” 

What are levees?

• Manmade structures meant to decrease flooding

• How tall do we make it?

• It can be eroded or overtopped

• Narrower channels and cause more flooding upstream

What happens with downstreams in zone 3?

• Deltas

• They are unpredictable because water levels are aalread high 

• Theres low elevation with little relief → uncertain adn changing flow paths