LESSON 6 Flashcards
process and pathways through which excess water becomes streamflow’; excess water is that part of the total rainfall amount which runs off the land surface, and that which drains from the soil and is not consumed in evapotranspiration
Runoff
pathway in which rain falls directly into the stream channel
Channel Interception
pathway in which excess water flows
over the soil surface
Surface Runoff or Overland Water Flow
pathway in which excess water infiltratesinto the soil but then arrives at a stream channel in a short amount period of time
Subsurface Flow or Interflow
sum of channel interception, surface flow and subsurface flow
Direct Runoff or Stormflow
a tabular or graphical representation of streamflow discharge vs time
Streamflow Hydrograph
Precipitation must satisfy the demands of
evaporation, interception, infiltration,
surface storage, surface detention, and
channel detention before runoff can occur.
Runoff Process
Runoff process
- Rainfall Intensity > Infiltration Rate
- Water begins to fill depression on soil surface
- Overland flow begins as depressions are filled
- Build up water on the surface (surface detention) = P - infiltration - interception
- Flow moving into defined channels (Channel Detention)
Factors affecting runoff
- Rainfall
- Watershed
Effects of Rainfall to runoff
duration, intensity and areal distribution
- Duration influences total runoff
- Intensity influences rate and volume of runoff
- Intense storms decreases infiltration rate because of its destructive effect on soil surface
- Areal distribution of rainfall influences rate of infiltration and volume runoff
Effects of watershed to runoff
size, shape, orientation, topography, geology and surface culture
- As watershed size increases, the volume and rate of runoff increases
- As the runoff area increases, the volume and rate of runoff decreases
- Long and Narrow watersheds have low runoff rate
- Runoff is decreases for storms moving upstreams
- Flat areas have low runoff
- Geology, soil materials and vegetation affect infiltration rate, also runoff
- Structures such as dams affects runoff rate
Measurements of Streamflow
- Stage
- Rating curve
- Control Section
Height of water in a stream
Stage
Graph of stage and streamwater discharge
Rating Curve
Section of the stream where rating curve will be deployed
Control Section
Requirements for deploying Control Section
- Stream should be stable and have sufficient depth for velocity measurement
- should have a straight upstream distance of 5x width of the stream and downstream distance of 2x width of the stream
- Free of obstruction (Vegetation, boulders etc.)
Measurement of Stage
Staff Gauge
Continuous water level recorder
A graduated staff used for visual observation of water level
Staff Gauge
Measurement of Velocity
Float Method
Current Meter Method
the time required for a floating object that is tossed into the stream to travel a specified distance on the surface of the water is observed. Then surface velocity is determined by dividing distance/time. Average velocity is 80-85% of Surface velocity
Float Method
For Floating method, what is the formula for streamflow discharge?
the average velocity is multiplied by cross-sectional area of the stream to estimate streamflow discharge
Q = VA
An instrument in which a wheel is made to rotate about its axis by the force of current. The speed of rotation determines the velocity of water.
Current meter method
Formula for Water velocity for current meter method
V = a+bN
V = average velocity
a,b = constants
N = revolutions per unit time
Stream depth > 0.5m measurements are taken from where?
Stream depth < 0.5m, measurements are taken from where?
- 20% and 80% of the total depth then get the average
- 60% of the toal depth
Streamflow discharge formula for Current meter method
Q = Z(AiVi)
Z = Summation
Ai = Average area per section
Vi = Average velocity per section
barriers placed across the stream over which water flows; provides a simple and accurate means of measuring water
Weirs
height of water above the crest
Head
edge or surface over which the water flows
crest
Type of Crest
Rectangular, Trapezoidal (Cipolletti), or Triangular (V -notch)
Sharp-crested
Broad-crested
has a blade with a sharp upstream edge, so that the passing water touches only a thin edge and clear the rest of the crest
Sharp-crested
has a flat or broad surface over
which the water flows
Broad-crested
artificial open channels built to contain streamflows within designed cross-sectional areas and length
Flumes
Types of flumes
Parshall Flume
Trapezoidal Flume
