Run-Off-Analysis Flashcards
means the draining or flowing off of precipitation from a catchment area through a surface channel.
Runoff
This portion of the runoff
Overland Flow
where it travels all the time over the surface as overland flow and through the channels as open-channel flow and reaches the catchment outlet
Surface Runoff
It is the part of the runoff which enters the stream immediately after the rainfall.
Direct Run off
The delayed flow that reaches a stream essentially as groundwater
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
is therefore stream flow in its natural condition. i.e., without human intervention.
True Runoff
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, curves, and tables are based on different regions.
Empirical Formulas
are graphical representations that depict the relationship between two or more hydrological parameters.
Empirical Curves
are widely used in hydrology to provide quick reference information for estimating various hydrological parameters.
Empirical Tables
Empirical Formulas Based on Different Regions
I. Johnstone and Cross
II. Snyder’s Method
○ 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
2 Dominant Methods of flood estimation in the Philippines
- Hydrometeorological
- Flood frequency method
Also known as “Precipitation Characteristics”.
Storm Characteristics
7 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 movement
7 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 movement
Factors Affecting Runoff
Storm
Meteorological
Basin
Storage
4 Meteorological Characteristics
a. Temperature
b. Humidity
c. Wind Velocity
d. Pressure Variation
Two Types of Catchments
- Fan Shaped Catchment
- Fern Leaf Catchment
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
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
The capacity decreases with time and ultimately reaches a constant rate, caused by filling of soil pores with water, which reduces capillary suction.
Horton’s Infiltration Capacity curve
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 is the Rational Method. 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
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 Hydrograph
- Direct Runoff Hydrograph
- Unit Hydrograph
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
