Surface Runoff

Topics: Hydrology, Water, Surface runoff Pages: 15 (692 words) Published: September 26, 2014

1. SURFACE RUNOFF

INTRODUCTION:

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.

Direct runoff

It means surface runoff, prompt interflow and rainfall on the channel surface.

Base flow

It means delayed interflow and groundwater flow.

Stream flow

Streamflow is a combination of baseflow and runoff. These vary along the stream.

Catchment Area

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.

Precipitation-
1 Type
2 Duration
3 Amount
4 Intensity
5 Design storm event
6 Direction of movement
7 Extent
Watershed
2 Size- area of watershed
3 Topography – slope of watershed
4 Shape of watershed
5 Aspect of watershed
6 Geology
7 Soil
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
Streamflow

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)

Measuring Velocity
1 Current meters
1 Cup-type (Price and pygmy)
2 Propeller type
3 Inductance
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)

Dilution Gauging

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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