Stream measurements

____ measurements are used as an estimate of flow rate

indirect

what does the accuracy of stream measurements depends on

method and procedures used for estimation

what will the accuracy of discharge estimates be limited by if done properly

range of instrument and human error

what curve is developed when measuring streamflow

rating curve (gauge height vs Q)

what device is used to measure stream velocity

current meter

what is the speed of the current meter propertional to

flow velocity

when is it appropriate to take 1 velocity measurement? two measurements?

1 measure - for shallow streams (less than 0.70m)

2 measure - for deeper streams (over 0.70 m)

at what % of the total depth are velocity measurements taken in shallow stream? deeper stream?

shallow at 60% of depth

deeper at 20% and 80%

is velocity faster at the surface or bottom of a river

at surface

what is the most common velocity measurement instrument

Price AA current meter

what are advantages of the rod mounted instrument over the price meter? disadvantages?

ad

• can measure velocity in 2D and 3D

• measure wider velocity range

• measure in shallower water

dis

• cost $10 000 per unit

• affected by entrained air

• affected by high and low suspended particles

• takes same time as price meter to measure velocity

what instrument is used to measure velocity underneath ice

smaller meter with polymer rotor wheel

how are rating curves developed

using measurements of discharge and gauge height in a stream

what must be determined for measuring discharge

velocity and gauge height

how many verticals should be taken at any given measuring section in a stream

at least 20

what velocity measurement can be used if equipment fail

float method

what is the float method

measuring the time it takes for an object to flow down a known length of a stream

why is velocity slower at the bed of a stream

because of friction

what friction correction factor should be used for rough rocky bottoms? smooth muddy, sandy or bedrock bottoms?

rough - 0.85

smooth - 0.90

how does the acoustic doppler current profiler (ADCP) work

measures water velocity by sending sound pulse into the water and measuring the change in frequency of the sound pulse reflected back to ADCP by sediments or particulates

what is translated to water velocity when using an ADCP

change in frequency measured by ADCP

what are requirements to use an acoustic doppler current profiler

• only for rivers deeper than 0.5 m

• streams can’t have moving bed

• boats have to move slower than average river velocity

what is differential leveling a process of measuring

vertical distances from known elevation point to determine unknown elevation points

benchmark (BM)

relatively permanent feature of known elevation

backsight (BS)

sight taken to the level rod held at a point of known elevation

height of instrument (HI)

elevation of the line of sight of the telescope

foresight (FS)

sight taken on a point to determine its elevation

how do you calculate the change in elevation

change = backsight (BS) - foresight (FS)

what are common leveling mistakes

misreading the rod

field note mistakes

level rod NOT vertical

environmental (heat, rain, wind)

what is the stage

water surface elevation

what is stage recorded relative to

horizontal datum elevation (aka sea level)

what is stage a reflection of

all hydrologic processes and water transport characteristics of watershed

stream gauges continuously measure _____

stage

how is stage translated to river discharge

applying the stage discharge relation (rating)

what does the stage discharge relation depend on

shape, size, slope and roughness of channel

how are point discharge measurements and water level measurements taken

using staff gauge or pressure transducer

what are causes of rating curve instability and poor trend line fits

• continuous and progressive erosion/deposition

• sudden changes in vegetation cover

• changes from subcritical flow to supercritical flow

what is the hysteresis effect

looping effect often occurring during passage of a flood

what is a weir

a barrier across a river designed to allow the flow characteristics

what are weirs commonly used for

• prevent flooding

• measure discharge

• render rivers navigable

what are two classes of weirs

1. sharp crested

2. broad crested

how do sharp crested weirs differ from broad crested

sharp - detached water surface falling away from the downstream edge of structure (free falling nappe)

what type of weir has a weir plate installed

shar crested

what are three main types of sharp crested weirs

rectangular, V notch, Cipoletti

what shape of sharp weirs is used to control water

rectangular

what shape of sharp weirs typically has the highest discharge

rectangular

what shape of sharp weirs is used in cases of SMALL discharge

V notch

what shape of sharp weirs is best in open channel

V notch

what shape of sharp weirs has the highest accuracy

V notch

what shape of sharp weirs is a combination of two

Cipoletti

what shape of sharp weirs is common for irrigation

Cipoletti

what is a Cipletti shape used for a sharp crested weir

when discharge is too great for rectangular

____ crested weirs are typically more sturdy

broad crested weirs

what size rivers and canals are broad crested weirs used

medium to large

do broad crested weirs require the flow to be subcritical or supercritical

subcritical

what is the ideal notch size of a weir

as small as possible to minimize error association with head measurements

which weir shape has end contractions? which doesn’t?

does - V notch

doesn’t - Cipoletti

what is the minimum head at low flows for weir sizing

at least 6 cm

what should the available fall over crest (difference between crest elevation and bed elevation) compared to the maximum head

TWICE the maximum head

describe crest lengths between a rectangular weir OR Cipoletti weir

at least twice the maximum head

should instruments used to measure the head above the crest be upstream or downstream

upstream (distance of 4 times max head)

what is the bucket and stop test

temporarily dam up a small channel, put pip in middle, and time how long it takes to fill bucket (known volume)

what are two examples of a stage recorded

guide pulley (stilling well) and pressure transducers

what do pressure transducers generate a single as a function of

pressure imposed

absolute vs gauge streamflow measurement

absolute - sensor that measures the water pressure AND atmospheric pressure

gauge - sensor that measures the difference in gauge pressure at two points

what additional tech must be used with an absolute transducer and why

barometric compensation - compensate the pressure data

how do you calculate the gauge pressure

absolute pressure - atmospheric pressure

is the absolute or gauge pressure caused by the water column

gauge pressure

what is the ideal time interval for streamflow measurements

15 minutes

what should all stilling wells be able to do (5)

1. intakes deep enough to cover all ranges in stage

2. have a flushing system to remove sediment from pipes

3. means to remove sediment from well

4. operable through winter

5. both inside and outside water level reference

what flow rates may move or destroy the float and sensors in a stilling well

rapid flows

when may a stilling well NOT be feasible or economical

rough terrain with limited access and silt problem

when should a bubble gauge be used

rough terrain or when there’s a silt problem

what are the two main essential components of a bubble gauge

1. gas purge system (feed gas through to the orifice)

2. bubble gage orifice (at a fixed elevation in the stream)

where should the bubble gage orifice be installed in a bubble gauge

stream currents are NOT high

sediment accumulation won’t cover the orifice

what is an example of a non contact water level sensor

radar

what are 8 criteria for a good gauging section

1. straight channel

2. single channel at all stages

3. flow lines distributed proportionally across the stream

4. uniform slope

5. few instream obstructions

6. no downstream obstructions

7. good access

8. natural pool nearby

why are bridges beneficial

enable us to get measurements from rivers whose flows cannot be measured by wading

compare the airline and wet line components of bridge measurements

air - distance of line ABOVE the water surface

wet - distance of line UNDER the water surface

what are sounding weights and why are they used

used for measurements in streams that are too fast or deep

used to keep the meter stationary and to minimize damage as the meter is lowered into the water