run off analysis Flashcards
means the draining or flowing off of precipitation from a catchment area through a surface channel. It thus represents the output from the catchment in a given unit of time.
runoff
It is the part of the runoff which enters the stream immediately after the rainfall. It includes surface runoff, prompt interflow and rainfall on the surface of the stream.
direct runoff
The delayed flow that reaches a stream essentially as groundwater flow
baseflow
Runoff representing the response of a catchment to precipitation reflects the integrated effects of a wide range of catchment, climate and rainfall characteristics.
natural flow
are mathematical equations whose free parameters we have to specify by a given set of experimental data about a particular state to obtain formulas for predicting other data about this state
empirical formulas
are widely used in hydrology to provide quick reference information for estimating various hydrological parameters.
empirical tables
are graphical representations that depict the relationship between two or more hydrological parameters.
empirical curves
proposed one of the first relationships for t, the time of concentration in hours
Johnstone and Cross (US, 1949)
was the first to develop a synthetic UH based on a study of watersheds in the Appalachian Highlands. In basins ranging from 10 to 10,000 mi2
Snyder’s Method
Dominant Methods of flood estimation in the Philippines
- Hydrometeorological
- Flood frequency method
Factors Affecting Runoff
I. Storm Characteristics
II. Meteorological Characteristics
III. Basin Characteristics
IV. Storage Characteristics
Also known as “Precipitation Characteristics”.
Storm Characteristics
Storm Characteristics
a. Type or nature of storm and season
b. Intensity
c. Duration
d. Areal extent
e. Frequency
f. Antecedent Precipitation
g. Direction of storm movementStorm Characteristics
II. Meteorological Characteristics
a. Temperature
b. Humidity
c. Wind Velocity
d. Pressure Variation
III. Basin Characteristics
a. Size
b. Shape
c. Slope
d. Altitude or Elevation
e. Topography
f. Geology (Type of Soil)
g. Land use/Vegetation
h. Orientation
i. Type of Drainage net
j. Proximity to Ocean and Mountain
k. Ranges
Two Types of Catchments
- Fan Shaped Catchment
- Fern Leaf Catchment
IV. Storage Characteristics
a. Depressions
b. Pools and Ponds/Lakes
c. Stream
d. Channels
e. Check Dams
f. Upstream Reservoir/ Tanks
g. Food plains, Swamps
h. Groundwater storage in previous deposits (Aquifers)
Runoff Estimation Methods
a. Empirical Formula, Curves and Table
b. Infiltration Method
c. Rational Method
d. Overland Flow Hydrograph
e. Unit Hydrograph Method
is that balance of rain water, which flows or runs over the natural ground surface after losses by evaporation, interception and infiltration.
Runoff
The _______ of a catchment (usually means yield) is the net quantity of water available for storage, after all losses, for the purposes of water resources utilization and planning, like irrigation, water supply, etc.
yield
is the discharge in times of flooding of the catchment area, when the intensity of rainfall is greatest and the condition of the catchment regarding humidity is also favorable for an appreciable runoff.
Maximum flood discharge
is the movement of water from the soil surface into the soil. As long as the rate of water addition to the soil is slower than the rate of infiltration, water will move into the soil, and there will be no runoff.
Infiltration
Depending on how saturated the ground is, the water can continue downwards to replenish water tables and aquifers.
percolation
showed that when the rainfall rate i exceeds the infiltration rate f, water infiltrates the surface soils at a rate that generally decreases with time
Horton (1933)
One of the most commonly used procedures for calculating peak flows from small drainages less than 200 acres. This method is most accurate for runoff estimates from small drainages with large amounts of impervious area. Examples are housing developments, industrial areas, parking lots, etc.
rational method
Limitations and assumptions in the Rational Method
• The drainage area should not be larger than 200 acres.
• The peak flow is assumed to occur when the entire watershed is contributing to runoff.
is that water which travels over the ground surface to a channel. occurs as a thin sheet of water over the ground surface (soon after a storm starts), joins a stream channel, and then flows in the channel to the concentration point. is essentially a uniform flow over the surface as developed by C.F. Izzard (1948).
Overland Flow or Surface Runoff
clay is hard to pick up due to cohesion
true
sand is hard to pick up due to size
true
• Water begins to collect near the top of the slope
Formation of Rills
is a geographical feature that includes a shallow channel that is common on sloping grounds, like hillslopes. Rills are typically around 0.3 m deep and wide and are typically curvy in shape.
Gully Formations
is a landform sunken or depressed below the surrounding area. form by various mechanisms.
nick or depression forms
The headwall of a glacial cirque is its highest cliff. The term has been more broadly used to describe similar geomorphic features of non-glacial origin consisting of a concave depression with convergent slopes typically of 65 percent or greater forming the upper end of a drainage valley.
true
referred to as Re, is used to determine whether the fluid flow is laminar or turbulent. The dimensionless Reynolds number plays a prominent role in foreseeing the patterns in a fluid’s behavior. It is one of the main controlling parameters in all viscous flows where a numerical model is selected according to pre-calculated Reynolds number.
reynolds number
Basin outflow resulting from one unit of direct runoff generated uniformly over the drainage area ay a uniform rainfall rate during a specified period of rainfall duration
unit hydrograph method
Three Types of Hydrographs:
flood
direct-runoff
unit
graphs that show how a drainage basin responds to a period of rainfall
- Flood Hydrograph
obstructions and losses are deducted but the results of excess precipitation do not equal to 1 inch.
- Direct Runoff Hydrograph.
results in 1 inch of excess precipitation.
- Unit Hydrograph
one method to develop hydrograph.
Linear Superposition Method
Three Obstructions
- Interception from trees or vegetation
- Infiltration
- Detention storage
precipitation that does not reach the soil, but is instead intercepted by the leaves, branches of plants and the forest floor
- Interception from trees or vegetation
the flow of water from aboveground into the subsurface
- Infiltration
the volume of water temporarily lying upon a watershed surface
- Detention storage
Time parameters of a Unit Hydrograph
Duration, (D)
Lag Time, (Tp)
Time of Rise, (Tr)
Time of Concentration, (Tc)
Time Base, (Tb)
the time from start to finish of rainfall excess.
Duration, (D)
0the time from the center of mass rainfall excess to the peak of the hydrograph.
lag time, Tp
the time from start to finish of rainfall excess to the peak of hydrograph
time of rise, Tr
the time from the end of net rainfall to the inflection point of the hydrograph.
time of concentration, Tc
– the total duration of the Direct Runoff hydrograph
time base, Tb
2% to 10%
rolling
greater than 10%
hilly
Cf
10 yrs or less (1.0)
25 yrs (1.1)
50 yrs (1.2)
100 yrs (1.25)
re < 1000
laminar flow
re > 1000
turbulent flow
re = 1000
transition region flow
