HYDROLOGY Lecture 7 Measurements Assoc.Prof. dr.tarkan erdik

How can you measure an instant discharge value as an engineer?

Velocity pattern in river cross section

It is important do determine the amount of surface runoff, which is required in various problems of water resources development. Why? 1- The maximum discharge should be known in flood control projects, 2- The discharge that is available for a certain number of days in a year should be known in hydropower projects. The branch of hydrology that deals with the streamflow measurements is called hydrometry How can you measure discharge? With rare exception, stream discharge is not measured directly, but is computed indirectly from water level (stage) measurements

The water surface elevation (stage) is measured, and the corresponding discharge is read from the rating curve Datum is a reference level, usually taken as sea level. Stage is measured with respect to the average sea level.

1 .Non-recording gages: (a) Manuel gauges:These are a wooden or metal rod scaled in centimeters. More than one gage is used if the stage varies in a wide range. rod thalweg

(b) Wire weight gauges: A weight is lowered from the bridge until it reaches the water surface. Main advantages: it is not harmed by the flow and it can be reached at high flow rates because it is outside of the water.

https://www.youtube.com/watch?v=NhkFh5sUl4A

Non-recording gages are usually read once a day (at 8:00 in Turkey) or two times a day (at 8:00 and 16:00 in Turkey). During the floods, readings are repeated every 1-6 hours. So, is it possible to miss the highest level during floods? 2. Recording gages: The motion of a float on the surface of water in a stilling well connected to the stream by a pipe rotates a pulley by means of a wire attached to it

Slope of water surface measurement The slope of water surface can be determined from the stages measured simultaneously at two cross-sections, with a distance of about 5 times the stream width. The measurements must be made at both banks because the water surface slope can be different at the banks in a bend. A high precision is required, in the stage measurements because the water surface slope is generally very low.

VELOCITY MEASUREMENTS a) Cup-type current meter: It has has a vertical revolution axis and a horizontal wheel with six conical cups. It can rotate in even very small velocities. Vertical velocity components can rotate the cups causing higher velocity measurement than it is

b) Propeller type current meter: It has a propeller with a horizontal axis, and is preferred for turbulent and high velocity flows. rope is pulled across the river

The velocity at a point in a cross section is measured by a current meter, which consists of a propeller rotated by the flow around a horizontal axle, a tail piece and a weight is required to prevent the current meter to be moved by the flow. The rotation of the propeller is related to the flow velocity. The number of rotations in a minute, n, is related linearly to the flow velocity V: n: number of rotations per sec The coefficients a and b are given by the manufacturer for various ranges of n. V: velocity To measure very low velocities is not possible with that type of velocity meter.

If h<0. 5m the velocity is measured at 0 If h<0.5m the velocity is measured at 0.6 of the flow depth below the water surface. If h>0.5m the the velocities measured at points 0.2 and 0.8 of the depth below the water surface. Later, they are averaged

In very large depths, measurements are made at four points, 0 In very large depths, measurements are made at four points, 0.2 times the flow depth apart The average velocity is estimated by a weighted average, giving the four points in the middle two times weight. V=(V1+2V2+2V3+2V4+2V5+V6)/10 When the stream is covered by ice, the velocity at mid-depth is measured and multiplied by a coefficient like 0.88.

Measurement of the average flow velocity by ultrasound waves (C) In wide streams (up to 1 km) average velocity can be measured by ultrasound waves. These waves are transmitted from the points 1-2 and 2-1, respectively. The straight line at an angle of α with the flow direction intersects the two banks in both directions. V21 =C-V cos α V12=C+V cos α V=(V12-V21)/2cos α

DISCHARGE MEASUREMENTS (Direct measurements) Discharge = Σ {(u0.2d + u0.8d)/2 x StripArea} Depending on the width of the cross-section, velocity measurements are made at a sufficient (20-30) of axes.

It is very difficult to measure the velocity by current meters for the following conditions: in very turbulent flows, in very high velocities (over 7 m/s) or in very low velocities, in streams with high concentration of sediment under ice cover In such cases sudden or continuous injection methods have been used.

The tracer is introduced upstream Sudden Injection Method The tracer is introduced upstream Upstream Concentration is measured downstream Downstream

Sudden Injection Method The instantaneous injection of a slug of tracer solution produced a downstream concentration time curve

Sudden Injection Method The instantaneous injection of a slug of tracer solution produced a downstream concentration time curve

Constant Rate Injection Method QCb=Time rate of change of Mass for the background substance. qC1=Time rate of change of Mass for the substance introduced. (Q+q)C2= Time rate of change of Total Mass downstream.