Lecture 5+6: Runoff + Dynamic modelling Flashcards
Explain the fate of precipitation (5 steps)
Precipitation –> loss (evapotranspiration, interception, depression, detention, retention, infiltration) –> Net (rainfall excess) –> Surface runoff –> Drainage flow.
What is the definition of effective rainfall?
The actual precipitation minus the evapotranspiration loss minus the initial loss minus the infiltration.
Which factors does evapotranspiration depends at?
- Paved or unpaved surface
- Type of plants
- Temperature of soil or pavements.
What is the definition of initial loss and how can it be estimated?
The Initial Loss is known to be the amount of rainfall that occurs before the start of surface runoff.
If the rainfall is plotted on the x-axis and the runoff is plotted at the y-axis then the intercept with the x-axis of the regression line will be the initial loss.
What are the initial loss and reduction factor in this formula: y = 0,3081x - 0,0461
Initial loss = 0,0461/0,3081 = 0,1496
Reduction factor = 0,3081
Mention different types of losses included in the initial loss.
- Interception: due to the vegetation some of the water stays there…and never runs off.
- Detention: short term storage depleted by flow away from it (It connects to drainage line)
- Depression: Water accumulate in hollows over ground surface
- Retention: Storage held by a long period of time and depleted by evaporation
What is infiltration and what does it depends on?
- Infiltration is the flow into the ground.
- The infiltration is depending on soil type condition, land cover and the antecedent wetness condition.
What is the overall effect of urbanization to runoff?
We have more and more surface cover and denser population. Surface the 50 yeas ago could infiltrate water cannot work like that today.
The urbanization is creating a faster and more intense runoff.
We are not only covering the surface with housing and pavement, but we are also using water and generating more wastewater, reducing the vegetation in areas. Finally we are developing drainage systems for the waste water and may due to that move it by force from the catchment to the sea – infusing the water balance in the area.
What is the rational methods used for and how is it expressed?
It is a simple method used for hand calculations of maximum runoff flow and also used for simple pipe flow calculations.
It is expressed as: Q=CIA
Q is the calculated flow (m3/s). C is a runoff coefficient, I is the rainfall intensity, A is the area size (m2).
What are the pros and cons of the rational method?
Pros:
- Fast & easy
- Gives absolute max flow
- Widely used for design of new pipes.
Cons:
- Not good for big catchments.
What can be done for making the rational method slightly more advanced?
Combine it with the kinematic wave model, that describes the speed of the water flow depending on the surface roughness. The model has no hydrological memory – which means it is cannot take effects of the water flow itself into account
What are the pros and cons of combining the kinematic wave model approach with the rational method?
Pros:
- Fast & Easy
- Widely applied
- Applied for simple river modelling
- Also applicable for 2-D surface runoff modelling
Cons:
- No hydrological memory
- No Back water –i.e. no flooding….
How is the kinematic wave model expressed?
V = M * R^2/3 * sqrt(I)
V= flow velocity (m/s)
M= The Manning Number (m^1/3/s)
R= Hydraulic Radius (m)
I = Slope
R= Flow area/wetted perimeter ≈ Water depth
What is the manning number?
The Manning number (M) is a constant which expresses the roughness of the surface and therebye the effect of the surface resistance to flow.
How is the combined kinematic wave model and rational method expressed?
Q = M * W * Y^5/3 * sqrt(I)
Q= (m3/s)
M= The Manning Number (m^1/3/s)
Y= Water depth
W = The flow width = the width catchment
I = Slope
