runoff & flood (3)

flooding generating mechanisms

• rainfall

• spring-summer snowmelt

• spring-summer snowmelt w/ rain

• glacier melt

• glacier melt w/ rain

• winter rain-on-snow

• glacier outburst floods

• “geomorphic” floods

• etc.

rain-dominated regime: atumn-winter floods graph

• rain-dominated regime: streamflow controlled by rain

• atumn-winter floods: heavy rainfall & already saturated soils → increased runoff → higher stream discharge → flooding

snow-dominated regime: spring summer floods graph

• snow dominated: streamflow controlled by snowmelt

• spring-summer floods: rising temps melt accumulated winter snow → increased runoff → higher stream discharge → flooding

• peak flows occur may-jun

hybrid regime: spring-summer and atumn floods

• hybrid regime: streamflow controlled by both snow&rainfall

• spring-summer floods: snowmelt causes increased runoff

• atumn floods: heavy rainfall and saturated soils increase streamflow

• peak flows occur may-jun and sept-nov

what is flood frequency analysis

• used to relate magnitude of extreme runoff or river flow events to their frequency of occurrence through the use of probability distribution functions

• historical records provide essential info

  • predict reurrence interval of hydrological extremes

    • predict future floods → management & mitigation

flood frequency analysis: exceedance probability definition

probability in any year that a given annual maximum flow (x) will be equaled or exceeded

flood frequency analysis: return period definition

mean length of time between events that equal or exceed a given value

define flow duration analysis

conducted by ranking event discharge values and estimating the % of time the discharge amount of exceeded

flow duration analysis graph

flow duration curve shows % of time a given discharge is equaled or exceeded

• high discharge = low exceedance fraction

  • exceedamce fraction is the fraction/% of time that river discharge is greater than a specific flow value

• low discharge = high exceedance fraction

• describes how frequently different flow magnitude occur NOT timing of floods

extreme value analysis

• low flows

• peak flows

  • partial duration series

    • analyze all peak flows above some threshold (e.g., flow that will scour fish habitat or overflow the channel) annual extreme series

  • annual extreme series

    • analyze only the highest flow each year

assumptions made during flood frequency analysis

• all flow events are independent (defined next)

• all flow events are due to same mechanism

• flood probabilities do not change through time

event independence

when conducting an extreme value analysis, only “independent” events should be included (an assumption made to conduct a flood frequency analysis)

• independent = magnitude of one event has no influence on the magnitude of another event

good graph

comparison of annual extreme and partial duration peak flow series

flood frequency analysis: approaches

fitting a probability distribution to annual peak flow series → gives annual measurements

• graphical (visual, uses graph paper)

• numerical (more obejctive)

  • methods of moments (mean, variants)

  • maximum likelihood (finds parameters that max liklihood of oberving data)

  • L-moments (combination of…)

flood frequency analysis:

graphical approach

flood frequency analysis:

methods of moments approach

probability of distribution

graph method example

risk analysis

what is the probability that a T-year event (event w/ certain magnitude) will occur at least once in a period of “n” years?

• P = probability that flood occurs at least once during n yrs

• T = return period (e.g., 10-yr flood)

• n = number of yrs considered

• 1/T = probability of that flood occuring in any single yr

• 1-1/T = probability of the flood NOT occuring in a single yr

• (1-1/T)^n = probability of the flood NOT occuring over n yrs

• 1 - (1 - 1/T)^n = probability of the flood occuring at least once over n yrs

in-class risk analysis example equation

answer: ~0.096

flood routing

• tracks how a flood wave moves through a watershed from its source to the downstream outlet / POI

flood routing temporal lag

temporal lag accounting for time it takes for transition of:

• (source) active melting snow → runoff → streamflow → downstream to outlet/POI (leaving system)

  • along the way could also add to GW, evaporate, etc (slowed down as it finally leaves channel storage)

• can be also be temporarily delayed by bank storage

flood routing (temporal lag) bank storage

water from the river enters riverbanks when river level rise, then slowly returns to the channel as levels fall

→ impacts flood routing !

lillooet river case study (geomorphic flood event)

landslide (debris flow from some amount of water input causing instability) → river blockage (as it material flows into meager creek → dam) → water accumulation → dam failure → outburst flood → erosion + sediment deposition → channel change

geomorphic hazards associated with glacier retreat

• sagging of over-steepended, de-buttressed slopes, → Affliction glacier

• exposure of unstable sediments → capricorn creek

How does ppt become streamflow? → how did it get there from the hillslope ? 

stream runoff generation mechanisms (3)

stream runoff generation mechanisms (3)

1. infiltration excess overland flow

2. saturation excess overland flow

3. shallow subsurface stormflow / groundwater flow / baseflow

explain stream runoff generation mechanism:

1. infiltration excess overland flow

• ‘Hortonian’ overflow 

• Ppt arrives but can't infiltrate fast enough, flows over ground surface, flows into streamflow 

• Occurs when rainfall intensity exceeds the infiltration capacity 

• ‘Saturation from above’

• Rapid!!!!

• Common in: 

  • Places with lots of impervious cover 

    • Like an urban environment with concrete covering ground/ concrete buildings 

  • Semi-arid environments 

  • Infiltration capacity reduced due to anthropogenic activities 

    • Compaction 

    • Freezing of ground surface (covers ground) 

    • After fire (hydrophobic soil) 

explain stream runoff generation mechanism:

2. Saturation excess overland flow

• ‘Dunne overflow’ 

• Water infiltrates through unsaturated zone → encounters GW table (subsurface saturates) → causes water table to rise → rises to point where it intersects ground surface → water exfiltrates and will flow over surface as surface overland flow into channel 

• ‘Saturation from below’ 

• Common in

  • Humid environments 

  • Places covered in grasslands/forest

  • Convergent parts of landscape 

    • Ridgetop down to a hollow (downslope areas) 

    • Above channel head 

• Pretty fast

  • Flashy flow ???

  • Moves quicker aboveground than below ofc 

explain stream runoff generation mechanism:

3. Shallow subsurface stormflow /  groundwater flow / baseflow 

• Made up of all the subsurface pathways that water can take to the channel  

• Groundwater table rises (in response to infiltrating water from unsaturated zone) → creates an increased head gradient to deliver more water to the channel 

• Could also… have a layer of decrease hydraulic conductivity (like the soil bedrock boundary → may get perched water table on top of the boundary (still in the subsurface, still flowing as subsurface flow down to the channel) 

• Slower than overland flow but can still be quick 

• One example that triggers runoff generation rapidly: may have (above the water table) significant capillary fringe → tension saturated, needs little more water from vertical unsaturated flow to cause positive pressure → rapidly become GW flow 

• Thicker zone will steepen head gradient towards the channel and increases thickness of hillslope aquifer system → transmissivity increases as you move towards the channel