River Discharge Flashcards
What is river discharge
Rivers response to precipitation moderated by the catchment characteristic
What are the two types of hydrograph
Flashy or damped
What regulate the hydrograph
Regime of rainfall and catchment characteristics
What are the 7 catchment characteristics
Basin shape Basin area Drainage density Basin/channel slope Vegetation type Infiltration Catchment storage
How does basin shape affect the hydrograph
A long catchment means bits are close to outlet but others further away so damped. Circular everything is close
How does basin area affect the hydrograph
Increases how much is going down river
How does drainage density affect the hydrograph
A lot of channels means water doesn’t travel far on the surface before joining with a river
How does channel slope affect the hydrograph
Steep catchment means water will be moving faster compared to a low relief catchment so it’s more flashy
How does vegetation type affect the hydrograph
If when it rains a significant proportion never make it to the ground because it’s grabbed by conifers then it will never go into channel so more subdued
How does infiltration affect the hydrograph
Roads built to remove logs - compacted surface and tracks provide channels for water to flow down
How does catchment storage affect the hydrograph
If there’s an aquifer underground that’s permeable rock like chalk, dampened
What is stormflow percent
From hydrograph separation it’s the part above the base flow.
What is the original concept of stormflow vs now
Overland flow towards rivers but now it’s all hydrograph (mostly subsurface flow)
What are the three ways to separate out a hydrograph
Straight line separation
Hursh and Brater method
Hibbert method
What is hursh and brater method
Slope is dependant on catchment area. Groundwater recession to peak time. Then rise at a rate n= 0.8A^0.2
N is days and A is drainage area
What is the hibbert method
Slope with a rate of increase of 0.0131m^3/skm^-2/d. Increase line according to this amount everyday with reference of how bit catchment is
What does Hibberts method say about stormflow in Danum
It’s 50%
What does hydrograph seperations suggest
Precipitation regime is most important
How to measure discharge
Volumetric gauging Velocity area methods Mean section methods Dilution gauging Structural methods Slope area methods
What is volumetric gauging
Simplest.
Collect flow or river over time period.
Accurate.
Can’t get under most rivers though.
What are velocity area methods
Float gauging - measure velocity of water x cross section of area. Use bouyant object.
current metering - arms low friction bearing so turn will small flow. Got to count each turn and record the turns over time. Can set at 0.6 of depth
Why is float gauging not great
Velocity distraction down a river changes, closer to Riverbed is slower because it’s slowing it down. There is a logarithmic decline with that so point of average flow is about 0.6 down from surface
How to correct velocity distribution
Reducing it with a curve. Reducing speed of flow to the average
Where is the point of average flow
0.6 down from surface of 0.4 up from the bed
What happens in a meandering stream
There is a zone of really high flow the move from one side to the other. Huge distribution of velocity o. Would zoom across the fast flowing over estimate the speed of water. No way to correct
What are mean section methods
Average velocity over whole channel cross section. Divide channel into series of equally spaced verticals. Areas between vertices known as segments
What happens at each sediment during the mean section method
Two rods. Need to be 0.2 and 0.8 measurements to get average. Get velocity at that location with rod, do it to the other V and get average velocity between the two. The area is depth of rod and width. Give us velocity times area for one little area of channel. (Qseg)
What are the ways of dilution gauging
Constant injection method
Integration (gulp) method
When is constant injection method good
Steep mountain environment as it’s difficult to get to if river floods or you don’t want to stand on t
What is dilution gauging
Flux of water particles is equal to discharge of river multiplied by concentration of particles naturally in the river.
Add into river some tracer and the rate at which the tracer is going in is little q.
Could rearrange the equation for bit Q and discharge of river.
What is the dilution gauging equation
(Q+q)C2=qC1+QCo
Q=q (C1-C2/C2-C0)
What is the constantly injection method
Concentration starts to go up and then hits a plateau which is the c2 value (conductivity meter). Bottle is sealed with outlet and speed at which water comes out through pipe.
What so integration (gulp) method
Background concentration, mixed concentration of tracer and in this case we mix with a known volume of water that we throw straight into channel and it mixes with flow in channel and you get a breakthrough curve. Arrival of tracer downsreqmZ
How to work out integration method
Volume of tracer in x concentration divides by area of curve. Trapezium rule.
Critical characteristic of a good water tracer
Chemically conservation (won’t bind on sediments) Non reactive High water solubility Relatively non toxic Measured in field Cheap
How does salt tracer get measured
By electrical conductivity
What happens to salt in large rivers
Dilutes and becomes undetectable so need a better trace r
What could we use that’s better than salt as a tracer
Radioactive tracer
Fluorescent tracer
What are structural methods
Continuously measure levels in river but combine with spot discharge measurements to get rating curve
What is the rating curve
Relation between river level (stage) and spot discharge
What do we need when doing structural methods
One energy state. Usuallt there’s two which is a problem for a head discharge rating - prevents shooting flow at stage measurement location -> add fixed hydraulic drop so place we measure can never flip.
What is the point of critical flow
Point at which a river flips between the two regimes - hydraulic drop and hydraulic jump. Difficult to measure - usuallt just measure upstream. Characterised with froude number
What is flow less than critical called
Subcritical
What is flow more than critical called
Super critical
What are the creations of a large hydraulic drop
Weir from fall. Rectangle thin plate.
Flume from constriction
Problem with weirs
Sediments accumulate and structure won’t work properly. Could use a sediment trap upstream of weir .
Why are flumes good
They use a construction in channel but allow sediment to pass. Width construction generates hydraulic drop
What do we need with flumes or weirs
Water level recording device - could be a laser one. stilling well reduces chart noise. Stilling pool stops waves moving through the weir.
What are slope area methods
Sometimes we have floods that destroy monitoring stations. Under those circumstances we can get an estimate of peak discharge using slope area methods but this is very approx rule
Examples of slope area methods
Trash line after flood (max height floor trash lifted) surveyed channel cross section up to line and slope of trash running in a down stream relation. Slope of trash line = slope or water-surface during the flood.
What is discharge equal to in slope area methods
Proportionate to the slope on the water surface (S)
What is river discharge proportionate to in turbulent flow
Slope on the water surface strictly to the slope on the energy line raised to the power of 0.5. Can use this to get manning equation and estimate peak flow
What is A in manning equation
Cross sectional area of flow in channel
What is R in mannings equation
Cross section area divided by wetted perimeter
What is n in manning equation
Mannings roughness coefficient. Calculated during normal flow. The rough bed slopes down the flow and effect relationship between water level and slope perimeters and discharge. Get rougher estimate of peak flow.
What are the three stream classifications
Ephemeral stream
Intermittent stream
Perennial steam
What is the ephemeral stream
Flows only during storms
What is intermittent stream
Seasonally dry
What is a perennial stream
Flows all year
What is the flow duration curve
Method used for characterising the regime of the river.
How to derive the flow duration curve
Comparing the logged river to adjacent catchment that’s never been disturbed to see if there’s a difference that’s apparent in flashiness of river
How to plot the flow duration curve
Number of days that have flows within that range to cumulate the frequency from high flow to low flow.
Present the cumulative frequency as a percentage where we have percentage time of the here the flow is equal to or exceeded the first column on the left hand side
What scale do we need to use in the flow duration curve
Log-log otherwise it hugs the x-axis - becomes more like a straight line that can be used to compared with another catchment
What do relatively flat flow duration curves have
Relatively uniform flows indicating high amounts of storage like deep groundwater
What do highly variable flow duration curves suggest
Steep catchments of impermeable geologise
Flow duration curve timeline for Baru catchment (disturbed) and W8S5 (undistburbed)
FDC slope (range 30-70%) less steep and less flashy.
Why does a disturbed catchment not make the hydrograph more flashy
You have a lot more flow left in river because of less evaporation during times of low flow. Low flows are brought up but it doesn’t really affect high flows so you end up with a more uniform flow regime post logging
