River discharge (fact sheets 9,10,11) Flashcards
Storm hydrograph
A river’s response to precipitation moderated by catchment characteristics
Stages of a storm hydrograph
4
rising stage, peak, recession limb, falling stage
‘Time to peak’
the time from the centroid of the rainfall event to the hydrograph peak
Catchment characteristics that affect the shape of the river hydrograph
7
basin shape, basin area, drainage density, basin or channel slope, vegetation type, infiltration, catchment storage
Basin shape (Catchment characteristics that affect the shape of the river hydrograph)
elongated catchments have a more damped hydrograph
Basin area (Catchment characteristics that affect the shape of the river hydrograph)
double the area, double the hydrograph peak
Drainage density (Catchment characteristics that affect the shape of the river hydrograph)
length of river channel per unit catchment area,
an increase in drainage density gives an increase in the channel intercepting subsurface water, giving increased flashiness
Basin or channel slope
Catchment characteristics that affect the shape of the river hydrograph
increase the slope, increase the rate at which water moves through the catchment
Vegetation type (Catchment characteristics that affect the shape of the river hydrograph)
conifer forest evapotranspires greater volumes of water than does grassland, and therefore may damp hydrographs
Infiltration
Catchment characteristics that affect the shape of the river hydrograph
low infiltration surfaces increases river flashiness
Catchment storage (Catchment characteristics that affect the shape of the river hydrograph)
eg: chalk aquifers have a high storage and, therefore, damp hydrograph response
how can the flashiness of a river be characterised numerically?
hydrograph separation
methods of hydrograph separation
3
- straight-line separation
- the Hursh and Brater method
- Hibbert method (most common)
The Hibbert method for separating the hydrograph
a separation line with a slope increasing at a rate… (?)
What is river discharge measured in
cumecs
Spot discharge
river discharge at one point in time
Ways to measure river discharge
3
- volumetric gauging
- float gauging
- current metering
Volumetric gauging, in order to measure spot discharge
only possible with small river flows or indirectly with the velocity-area methods float gauging and current metering
Float gauging, in order to measure spot discharge
involves measuring the velocity of a neutral buoyancy object (eg: orange) and multiplying this by the average cross-sectional area of the river
Why should float gauging be reduced by a coefficient?
in order to measure spot discharge
because the objects float close to the river surface, which is faster than the average velocity of the water profile
Current metering (in order to measure spot discharge)
the rotation of a current meter’s impeller gives the local water velocity following application of a calibration equation, measurements should be made at difference depths to cope with vertical velocity
How can the transverse distribution of water velocity be characterised?
dividing the channel cross section into segments
What are the edges of segments called when dividing the channel into cross sections?
and their use?
verticals,
the locations at which the measurements are to calculate profile-average velocity should be made
Two methods to calculate discharge for each segment?
‘mean section method’ or ‘mid section method’
When is current metering not an accurate method of river gauging?
when the river is very shallow
When should dilution gauging be used?
when the river water is very shallow
What method is dilution gauging based on?
the two component mixing method
Two approaches to dilution gauging?
- constant injection method
2. integration/ gulp method
Constant injection method
a known tracer concentration is trickled into a river at a fixed rate
Integration/ gulp method
Involves the addition of a known volume and conc. of a tracer into the river as a single ‘slug’
Instead of measuring spot discharge, how can continuous traces of river discharge be measured?
the structural method
The height of water in the river using the structural method
stage
Rating curve (structural method)
the relationship between a stage and several spot discharge measurements
Hydraulic drop
eg: river moves over a boulder and goes from a subcritical flow to a supercritical flow
Hydraulic jump
supercritical to subcritical flow
How is an accurate trace discharge calculated?
building an artificial hydraulic drop
Example of an artificial hydraulic drop
2
weirs and f,lumes
How do weirs generate a hydraulic drop?
the water falls from a stllling pool
How do flumes generate a hydraulic drop?
use a constriction
How to measure peak discharge during a flood when only max. height is known/ river gauges are flooded
slope area methods
Most common slope area method
Manning equation
Three types of stream classification
Ephemeral rivers
Intermittent channels
Perennial channels
Ephemeral rivers
Only flow during flood events
Example of an ephemeral river
desert wadi
Intermittent channels
Will be dry for one or more seasons
Example of an intermittent channel
headwater channels
Perrenial channels
Flow all year
How is the return period of extreme events calculated?
using an annual maximum series,
i.e. a data series of the max. daily discharge recorded during each year
Flow duration curve (FDC)
a graphical method to show whether a river has a variable flow or relatively uniform flow
