Faculty Engineering Technology Chapter 4 Hydrology and hydraulics

| Faculty Engineering Technology Chapter 4 Hydrology and hydraulics Streamflow and runoff analysis Najihah Abdul Rashid

Streamflow and runoff analysis To analyse streamflow measurement and identify runoff characteristics of stream, catchment characteristics and storage analysis

Streamflow Stream flow is one of the most important topics in engineering hydrology because it directly relate to water supply, flood control, reservoir design, navigation, irrigation, drainage, water quality, and others

Streamflow measurement Define as a flow channel into which the surface runoff from a specified basin drains. It is measured in units of discharge (m 3/s) occurring at a specified time.

Streamflow Measurement • • • Floodplain management Flood forecasting & analysis Reservoir operations Low flows – water quality concerns Design structures – culverts, bridges, stormwater systems • Evaluate changes in land use on watersheds and/or changes in climatic regimes

Measurement of stage Water surface elevation measured above datum. This datum can be the Mean Sea Level (MSL) or any arbitrary datum connected independently to the MSL

Manual Gauges • • • Staff Gauge Fixed rigidly to a structure (abutment, pier, wall, etc) Vertical or inclined with clearly and accurately graduated permanent markings Wire gauge Measure the water surface elevation from above the surface such as from a bridge or similar structure. A weight is lowered by a reel to touch the water surface A mechanical counter measures the rotation of the wheel which is proportional to the length of the wire

Manual gauges (Cont) • Gauges should not be located in rivers with scouring characteristics. • The locations should steer clear of river bends because the water surface is inclined and there is turbulence making the stage measurement inconsistent. • The upstream of a natural control eg. a rapid should be used, not downstream.

(cont) • A uniform channel helps good stage measurement. Irregular cross sections should be avoided. O. K. Avoid this irregular section

Automatic Stage recorders Float gauge recorder • Float movement fluctuates with change in stage and this is recorded by a chart. In hydrologic measurements, both the big and low flows are measured within the chart.

Fig 2. 1: Stream gauges

• • Bubble gauge Compressed air or gas is made to bleed out at a very small rate through an outlet placed at the bottom of the river Advantages of bubble gauge Cheap (compared to stilling wells) Large change in stage (up to 30 m) can be measured Due to constant bleeding action there is less likelihood of the inlet getting blocked or choked

Fig 2. 2: Bubble gauge

Stage data • Presented in the form of a plot of stage against chronological time known as stage hydrograph. • It is crucial for designing hydraulics structures, flood warning and flood protection works.

Fig 2. 3: Stage hydrograph

Describe the hydrograph that you have been given in detail

Streamflow measurements Measurement of velocity • • Current meter Floats: Suitable for straight channel, V = L/T Dilution Manning Equation

Streamflow measurements Measurement of velocity Current meters The most common instrument in hydrometry. It has a propeller which is rotated when water hits it and is connected to magnets which actuates recorders when the propeller rotates. The velocity of water increases the propeller rotation.

Fig 2. 4: Current meters

Current meters Fig 2. 5: Vertical-axis meters

Current meters Fig 2. 6: Horizontal-axis meters

Current meters • The number of rotations are measured and correlated to velocity using the formula: • v = a + b. N where N is the rotation of the propeller (revolutions per sec {rps}) • a and b are coefficients determined by calibration in an experimental flume.

Current meters Velocity b 1 a Propeller Rotation, N v = a + b. N

Measuring streamflow in small streams with a current meter

Large rivers – from bridges or boats

For streamflow measurement, we need: • • • Wading Bridges Boat Cablecar Cable way

Velocity area method • Mostly/frequently used • River cross-section determined • Velocity measured using • Float (for linear channel) • Current meter • Vertical velocity measured at 0. 2 d and 0. 8 d if depth, d >0. 6 m. If d<0. 6 m, velocity measured at 0. 6 dm.

Velocity area method (Cont) • Q = [Velocity x Area] • Need to know width of channel (w), Depth of channel (d), and Velocity of flow (V) (ft/s or m/s) • Area = w x d • Because depth & velocity vary across a channel: • Important to divide the channel into manageable segments (slices); Typically use 10 -20 segments • For each segment measure depth, width and velocity

Velocity area method Measuring streamflow discharge • Procedure: at each segment measure depth then velocity – If Depth < 0. 6 m, take one reading @ 60% depth – If Depth > 0. 6 m take 2 measurements and compute the average • One @ 20% depth • One @80 % depth • Average the two readings

Measuring streamflow discharge

Velocity measurement by floats • Any floatable substance eg. a tennis ball is placed at a point and the time(t) it takes it to move a known distance is noted • d/t gives the average surface velocity of the water Where, d = distance travelled in time t

Area velocity method

Area velocity method Consist of measuring the area of cross section of the river at a selected section called the gauging site and measuring the velocity of flow through the crosssectional area Gauging site must be selected with care to assure that the stage-discharge curve is reasonably constant over a long period of about a few years

Area velocity method (cont) • The stream should have: – A well defined cross section which does not change in various seasons – It should be easily accessible all through the year – The site should be in a straight, stable reach – The gauging site should be free from backwater effects in the channel • At the selected site, the section line is marked off by permanent survey markings and the cross section determined

Fig 2. 7: Stream section for area velocity method

Calculation of discharge • Two method of measurement • Mean section method • Mid section method

Calculation of discharge mid-section method Δ Qi = yi x (Wi/2 + Wi+1/2) x Vi Δ Qi = discharge in the ith segment For the first and last sections, the segments are taken to have triangular areas and area calculated as: W = (W 1 + W 2/2)2/ 2 W 1 (For the first and last section) W = (Wi/2 + Wi+1/2) (For the rest of the segments)

Calculation of discharge

Example 4. 1 • The data pertaining to a stream-gauging operation at a gauging site are given below. The rating equation of the current meter is v= 0. 51 Ns + 0. 03 m/s • Where, Ns = revolution per second. Calculate the discharge in the stream Distance from left water edge (m) 0 1. 0 3. 0 5. 0 7. 0 9. 0 11. 0 12. 0 Depth (m) 0 1. 1 2. 0 2. 5 2. 0 1. 7 1. 0 0 Revolution of a current meter kept at 0. 6 depth 0 39 58 112 90 45 30 0 Duration of observation (s) 0 100 150 100 0

Example 4. 2 The following data were collected during a stream-gauging operation in a river. Compute the discharge Distance from left water edge (m) Depth (m) Velocity (m/s) At 0. 2 d At 0. 8 d 0. 0 1. 5 1. 3 0. 6 0. 4 3. 0 2. 5 0. 9 0. 6 4. 5 1. 7 0. 5 6. 0 1. 0 0. 6 0. 4 7. 5 0. 4 0. 3 9. 0 0. 0

Dilution technique of streamflow measurement The dilution method of flow measurement, also known as the chemical method , depends upon the continuity principle applied to a tracer which is allowed to mix completely with the flow

Dilution gauging • Using tracer/chemical at upstream • For uneven stream base and stones stream • Q can be determined by tracer quantity and concentration at upstream and downstream (after dilution) using mass transfer equation. • Example of tracer: – Chemical: Sodium cloride, sodium dicromat, manganese sulphate – Dye: sodium fluoroscein, Rhodamine-WT – Radioactive: Bromine-82, Sodium-24, Iodine-132 • 2 method – Sudden injection – Constant rate injection

Fig 2. 8: Sudden injection method

Fig 2. 9: Constant rate injection method

Sudden injection method Where, Q = discharge of the stream V 1 = the volume of the tracer solution injected into the stream C 1 = concentration of the tracer solution injected into the stream C = is the measured tracer concentration at a given time at the downstream sampling site Cb = is the background concentration of the stream T = time

Constant rate injection method Where, q = is the rate of flow of the injected tracer solution Q = discharge of the stream Cb = is the background concentration of the stream C 1= concentration of the tracer solution injected into the stream C 2= is the measured tracer concentration of the plateu of the concentration – time curve (fig 2. 9)

Example 4. 3 A 25 g/L solution of a fluorescent tracer was discharged into a stream at a constant rate of 10 cm 3/s. The background concentration of the dye in the stream water was found to be zero. At a downstream section sufficiently far away, the dye was found to reach an equilibrium concentration of 5 ppb. Estimate the stream discharge

Hydraulic Structures • Used for small watersheds – such as experimental watersheds – where need accurate, continuous flow measurements. Two types: – Weirs – Flumes

Weirs • Obstruct flow and force • it through a notch • Stage-Q relationship established mathematically for different types of notches

Trapezoidal Weir

Trapezoidal Weir

Rectangular Weir

90 degree V-notch Weir

900 V-notch Weir Q = 2. 5 H 2/3

Flumes • An artificial open channel built to contain flow within a designed crosssection and length • No impoundment • Water height in flume measured with a stilling well

Flumes • Used to measure flow in: – – – water and wastewater treatment plants irrigation channels agricultural runoff plots – research applications small watersheds

Large Crest Flumes

Parshall Flume

Runoff • Runoff results from rainfall occurrence in a hydrologic catchment. Rainfall-runoff relations are very important in hydrology. • Most work on the prediction of runoff requires past records. • The problem is that some streams are not gauged. • Also, non-recording gauges only gives the volume of water and not intensities.

Runoff (cont) • There is the need to get records of stream flow and recording gauge information to predict runoff from rainfall. • Some empirical methods are available for predicting runoff in a catchment without the stream flow and recording gauge information.

Runoff (cont) • Based on the time delay between the precipitation and the runoff, the runoff is classified into two categories: – Direct runoff – Base flow

Fig 2. 9: Different routes of runoff

Runoff • Direct runoff – Runoff which enters the stream immediately after the rainfall • Base flow – Delayed flow that reaches a stream essentially as groundwater flow is called based flow. – Many times delayed interflow is also included under this category

Runoff Natural flow • True runoff, stream flow in its natural condition (without human intervention). • When there exists storage or diversion works on a stream, the flow on the downstream channel is affected by the operational and hydraulic characteristics of these structures and does not represent the true runoff, unless corrected for the diversion of flow and return flow

The natural flow volume in time Δt at the terminal point of a catchment is expressed by water balance equation as RN = (R 0 – Vr) + Vd + Ex + ΔS Where, RN = natural flow volume in time R 0 = observed flow volume in time at the terminal site Vr =volume of return flow from irrigation, domestic water supply and industrial use Vd = volume diverted out of the stream for irrigation, domestic water supply and industrial use E = net evaporation losses from reservoirs on the stream Ex = net export of water from the basin ΔS = change in the storage volumes of water storage bodies on the stream