Surface Water 2 - Overland Flow

Question 1

Overland Flow

Answer 1

• Non-Channeled water flow on ground surface

Question 2

Factors that influence overland flow

Answer 2

• Precipitation intensity
• Snow melt rate (rapid or over months)
• Pre-existing soil saturation level (saturated will cause flow)
• Soil Type (hydraulic permeability
• Ground Cover
• Topography/Terrain characteristics (slopes vs. flats)

Question 3

Types of surface runoff during rainfall or snowmelt

Answer 3

• Infiltration excess overland flow (Hortonian)

- Saturation excess overland flow

Question 4

Hortonian overland flow

Answer 4

• Occurs when soil is not saturated
• Soil properties or land cover do not allow infiltration to keep up with rain/snow melt rates
• Rainfall rate > infiltration capacity = Overland flow

Question 5

Saturation excess overland flow

Answer 5

• Occurs when soil becomes saturated and there is no longer any space for water to infiltrate
• Pore spaces are filled, so water flows overland

Question 6

Interflow

Answer 6

• Faster than base flow
• Subsurface stormflow
• Aided by macropores
• Also flows along bedrock-soil interface

Question 7

Flooding

Answer 7

• Rising stage level associated with excess discharge in response to storms or melt
• Natural process that maintains river form and function
• Defined by channel cross-section and bank height

Question 8

Why is it useful to understand flooding?

Answer 8

• Design of bridges, culverts, sewers, dams, spillways (strength and height of infrastructure)
• Land-use zoning and planning
• Floodplain delineation
• Insurance

Question 9

Bankfull

Answer 9

• Threshold stage before river flows over banks

- Occurs frequently

Question 10

What is the probability of a flood happening?

Answer 10

• P = 1/Recurrence interval

Question 11

What is significant about Q1.5 (a flood recurrence interval of 1.5 years)?

Answer 11

• Most efficient at moving water and sediment
• Maintains channel form and function
• Determined from channel surveys or rating curves
• P~67%

Question 12

What is significant about Q2.33 (a recurrance interval of 2.33 years)?

Answer 12

• Most common flood discharge
• Useful for delineating floodplain
• P~43%

Question 13

How do we predict floods?

Answer 13

• Hydrographs
• Recurrence intervals
• Rational Method

Question 14

Recurrence Intervals

Answer 14

• Annual peak discharge record used to predict flood probability based on record
• R = (number of ranked observations + 1)/m (rank of the observation concerned)
• Can plot a flood frequency curve

Question 15

Rational Method

Answer 15

• Used when rivers aren’t properly gauged, there is limited historical data
• Relate the unknown area to a similar stream’s behaviour
• Estimates peak annual discharge using empirical formulae and data on rainfall and basin statistics
• Q = 0.278Ci*A
• C = Runoff coefficient
• i = rainfall intesity (mm/hr) at time of concentration t
• A = Basin area
• Hortonian flow is main flow mechanism

Question 16

Causes of coast flooding

Answer 16

• Storm surges

- Earthquakes

Question 17

What are factors that influence river floods?

Answer 17

Basin shape, slope, altitude, climate, geology, vegetation cover, infiltration, drainage network factors, channel factors

Question 18

Why are floods more hazardous in North America?

Answer 18

• Urban growth and infrastructure built in floodplains (not increased flooding)
• BC has some of the largest due to high intensity precipitation and rapid runoff in mountainous rocky terrain