Flooding

What are some of the key factors in the hydrologic cycle?

• Evaporation

• condensation

• precipitation

• runoff

• infiltration

• percolation

Define stream

a body of flowing water confined to a channel, regardless of size

Define drainage basin

The region from which a stream draws its water supply

How is discharge calculated?

Water cross section (width x depth) x velocity (distance/time)

measured in m3 per second

define stream capacity

the total sediment load carried by a stream

closely tied to discharge (faster stream=more material moved)

Define competence

the streams ability to carry material given it’s size

How does sediment sorting change with stream velocity?

slow moving water only has the energy to carry fine grained sediment

fast moving waters has more energy and can carry larger grain sizes

What happens to a stream over time?

It will level its highs and lows with erosion and sedimentation- becomes graded

however grading will change if conditions of the stream change (ex. discharge rate, climate, tectonics, human intervention, etc.)

What controls a streams velocity?

its gradient- how steep the change in between the highest point of the stream and the base level (wherever the stream flows into)

What is typically the base level for a stream?

Sea level, if it runs into the sea

can be higher or lower depending on final destination

What are the two major stream types?

1. Meandering stream - north sask river

2. braided stream- Athabasca river in jasper

Define meandering stream

occurs on gentle gradients, carry fine sediment, erode unconsolidated sediments and weak bedrock - therefore path shifts with changes to bank

Define braided stream

many channels which diverge and merge - typical of steep gradients and high sediment loads, and frequent variations in discharge (looks more braided in during high discharge period)

How are point bars formed?

the outside edge of a stream is eroded at faster (cut bank) and lost sediment is deposited on the inside bank, forming point bars

How are natural levees formed?

Floodwater spreads out over floodplain, losing speed and dropping sediment- most of the heavier sediment drops quickly, right as the flood leaves the stream margins, which builds up into levees over several flood events

Why are hydrographs important?

• record fluxuations in discharge ors tream height over time

• useful for monitoring stream behavior remotely

What factors govern flooding?

• excessive rainfall

• snowmelt off in the mountains

• severe storms

• hazardous blockages of stream channel (trees or rock fall)

Define stage

the elevation of the water (flood stage is when water exceeds stream bank height)

Define crest

maximum stage is reached

Define upstream flood

occurs in small, localized, upper part of basin

Typically caused by a local event- i.e. dam burst, intense rainstorm

Define downstream flood

occurs in larger, lower part of basin

Typically a result of longer term build up- i.e. heavy rains, snow melt

Often longer in duration

Define Flash flood

type of upstream flood, characterized by rapid rise of stream stage

What are the 4 things that influence surface runoff?

1. ground cover- infiltration rates based on soil, rock, pavement, etc.

2. Topography- steepness determines amount of runoff vs infiltration

3. vegetation- plants provide physical barrier and increase soil infiltration/absorption capacity

4. Climate- determines precipitation intensity and frequency, also soil absorption (frozen vs thawed)

How do we calculate flood frequency curve?

R= (N+1)/M

R- recurrence interval 

N- number of years

M - ranking of annual maximum discharge 

Why are flood frequency curves helpful and harmful?

Helpful- useful tool for evaluating frequency of flood events, and creating long-term records

Harmful- can only provide a probability, and is based on historical information (may be less accurate as climate shifts)

What are the effects of development on a floodplain?

• concrete/pavement- reduces infiltration

• buildings- replaces water volume, raises height of flood

• storm drains- rapid delivery of stormwater to streams increases stream height

• vegetation loss- exposes soil for erosion, silts up stream (reduces stream competence)

modifications around the floodplain will increase water height during flood, increasing range of damage caused by flood

How has urbanization changed precipitation response times?

Stream response is much more intense and rapid- water reaches stream faster, with less interception/infiltration, so peak discharge is also higher

List some flood hazard reduction strategies

• restrictive zoning

• retention pond- traps excess water to allow absorption or evaoporation

• diversion channel

• channelization

• levees

• flood control dams and reservoirs

• not building in a flood plain

How are levees beneficial and detrimental?

Allows water to reach a higher stage without spilling onto the floodplain

However, the higher stage allows stream to increase in velocity, which may increase downstream flooding 

also, when they fail, more significant damage and loss of life occurs than if they hadn’t been there in the first place

How are flood control dams beneficial and detrimental?

Beneficial- retain excess runoff, and release it slowly into streams, can be used for agriculture and hydroelectric power

Detrimental- hard for aquatic life to navigate, extra weight of water retained can cause earthquakes, sediment loaded waters deposit at dam and increase stage, sediment-free waters released from dam can increase erosion downstream