STREAMFLOW and

HYDROGRAPH ANALYSIS

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.

Stream Flow Measurements

• Serves as the basis for many water resources engineering designs

• Two approaches

– Measurement of water stage;

– Measurement of flow velocity

• Measurement of Water Stage

– Water stage: the elevation above some arbitrary datum of water surface at a station

– Types of Gages Measuring River Stage:

• Staff gage – vertical or inclined

• Suspended – weight gage

• Recording gage

• Crest – stage gage ( used to indicate high water mark)

• Misc (Table 1).

Figures of Stream Gauges

List of StreamFlow Measurement Methods

Stage-Discharge Relation

• When water stages are measured, we need additional information to estimate the flow rates (or discharges)

Q

HH Q

t t

Stage Hydrograph

Stage-Discharge Curveor Rating Curve

Discharge Hydrograph

Stage-Discharge Relation

• Typical relationship: Q = a(H +b)c

• The function relationship between H & Q has to be calibrated locally for different stations

Storage Hysterisis

• In natural rivers, the H-Q relationship in general appears to be a loop, rather than single-valued.

Devices for Flow Velocity Measurement

• Current Meters

– Cups

– Propellers

V = a + bN

where V = flow velocity; a = starting velocity to overcome mechanical friction; b = equipment calibration constant; N = revolutions/sec.

• Pitot Tubes: Suitable only for clean water

• Floats: Suitable for straight channel, V = L/T

Current Meters

Mean Flow Velocity Estimation

• Velocity Profile

D eph < 0 .6m 0.6d

VV ; 0 .6 w ater dep th from the w ater su rface

2mDepth0.6m 2

0.8dV

0.2dV

V

2mDepth 4

0.8dV

0.6d2V

0.2dV

V

Measurement of Stream Flow Discharge - 1

• Stage – Discharge (Rating) Curve

• Mid-Section Method

iA

iV

ii

QQ

iA

iV

iQ

ibd

iA

Measurement of Stream Flow Discharge - 2

ii

QQi

Ai

Vi

Q

)Vi

V(2

1i

V

)1i

di

(d2

bi

A

1i

(c) Mean-Section Method

Measurement of Stream Flow Discharge - 3

(d) Area-Slope Method

By Manning formula:

where

K1 K2 if flow cross-section is not uniform, minor loss should be subtracted from hL.

21

L

h K2

1

fS K Q

L

21,i ,32

iR

iA

in

1i

K

21

)2

K 1

(KK

Measurement of Stream Flow Discharge - 4

( e ) B y h y d r a u l i c S t r u c t u r e : W e i r s , P a r s h a l l F l u m e , e t c .

W e i r e q u a t i o n : H LCQ ; 1 . 5 . C h e m i c a l D i l u t i o n M e t h o d

C o Q o = C 1 Q 1

W h e r e C o = c h e m i c a l c o n c e n t r a t i o n a t t h e s o u r c e ; Q o = r a t e o f c h e m i c a l b e i n g d i s c h a r g e d ; C 1 = c h e m i c a l c o n c e n t r a t i o n a t d o w n s t r e a m m e a s u r e m e n t p o i n t ;

Q 1 = f l o w d i s c h a r g e i n t h e r i v e r .

Hydraulic Structures for Discharge Measurement

Regime Theory

- W = aQb ; D = cQf ; V = kQm

Q = VWD ack = 1.0 subject to b+f+m = 1.0    Generally applicable up to mean annual discharge. For flows larger than the mean annual discharge, different relationships exist.

Extension of Rating Curve During the event of large flood, it is impossible or impractical to

measure discharge directly. More often than not, the flood stage goes beyond the range of the data range used to define the rating curve. Therefore, extrapolation of the ration curve is needed when water level is recorded below the lowest or above the highest level.

 Large errors can result if the functional form of rating curve, Q = a(H+b)c, is extrapolated beyond the recorded gauge discharges without consideration of the cross-section geometry and controls

 Graphical extension or by the fitted Q-H relationship is adequate only for small extension

 For large extrapolation beyond the active channel cross-section, hydraulic formula can be used to estimate the stage-discharge relation.

Steven Extension DA

Q K 1

DA

Based on Chezy formula,

with A = flow cross-section area; C = Chezy Coefficient; R = hydraulic radius, A/P; and S = channel slope. For a given section, = constant whereas for a wide channel (W>10D) RD. Therefore,

RSACQ

SC

DA K Q

Example (Extension of Rating Curve)