1. SURFACE RUNOFF
Runoff means the draining or flowing off of precipitation from a catchment area through a surface channel enters into a stream channel.
It is convenient to visualize three main routes of travel; overland flow (air hujan limpahanpermukaan), interflow, and
ground water flow (air dari punca bawah tanah).
1 Runoff results from rainfall occurrence in a hydrologic catchment. Rainfall-runoff relations are Interflow/subsurface and base flow/groundwater flow
very important in hydrology.
2 Most work on the prediction of runoff requires past records. 3 The problem is that some streams are not gauged. Also, non-recording gauges only gives the volume of water and not intensities 4 There is the need to get records of stream flow and recording gauge information to predict runoff from rainfall. 5 Some empirical methods are available for predicting runoff in a catchment without the stream flow and recording gauge information.
Terms of water flow
Overland flow / surface runoff
Excess rainfall moves over the land surface to reach small channels. This portion of runoff is called overland flow.
Interflow / subsurface runoff
Rainfall that infiltrates the soil surface my move laterally through upper layers of soiland returns to the surface at some location is c
Ground water flow
Infiltrated water reaching saturated zone and moves through the soil as groundwater flow. It is also called base flow and dry weather flow.
It means surface runoff, prompt interflow and rainfall on the channel surface.
It means delayed interflow and groundwater flow.
Streamﬂow is a combination of baseﬂow and runoﬀ. These vary along the stream.
1 The area of land draining in to a stream or a water course at a given location is called catchment area / drainage area / drainage basin / watershed.
5 Design storm event
6 Direction of movement
2 Size- area of watershed
3 Topography – slope of watershed
4 Shape of watershed
5 Aspect of watershed
8 Land cover
1 1mm of rain on 1km2 of watershed represents an input of 1,000 m3 of water or about 250,000 gallons of water. 2 If you prefer, 1inch of water on a 1mile2 watershed represents an input of 17 million gallons of water. 3 If a watershed, of 11 mile2 receives an annual precipitation of 40 inches, that is about 7.6 billion gallons of water each year or just over 1 billion ft3. 4 If just 15% ran off, this would be an average discharge of 4.8 cfs.
Topography and drainage density
1 Slope affects stream velocity
2 Drainage density affects travel time of precipitation to channel
1 Surface water hydrology deals with the movement of water a long earth’s surface as a result of precipitation and snow melt 2 Knowledge of quantity and quality of stream flow is a request of municipal, industrial, agricultural and other water supply projects. 3 The water flowing in stream is measured as discharge of water with a unit of volume (m3/sec, cubic of feet per second – cfs)
1 Current meters
1 Cup-type (Price and pygmy)
2 Propeller type
V = a + b´N
where V = flow velocity;
a = starting velocity to overcome mechanical friction;
b = equipment calibration constant;
N = revolutions/sec
Streamflow gauging (mecerap kadar aliran sungai)
4 Velocity-area (V-A) method
5 Mean section
6 Mid section
7 Dilution gauging method (penolokan enceran)
1 Using tracer/chemical at upstream
2 For uneven stream base and stones stream
3 Q can be determined by tracer quantity and concentration at upstream and downstream (after dilution) using mass...
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