FLOOD06873

• County: Statewide
• Basin: Statewide
• Title: Hydraulic Engineering volume 1
• Date: 1/1/1994
• Prepared By: American Society of Civil Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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EVAUJAnNG FLOW RESISTANCE USING Ht:I<:Iffot' ROIJGIINESS PluyraUSIONs

Mardova Ivanal

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On the basis of field measurements on the ~umava mountain rivers heights of
roughness protrusions have been used for calculation of flow resistance. The resull
resistance equations, compared with those using a grain size, are statistically
insignificant Also a change oflhe Manning roughness coefficient was investigated.

Introduction

In previous years there has increased an interest in mountain and submounlalfl
rivers from the point of view of water management, ecology, forest science, fishery,
recreation and from the point of view of fe-naturalization of river systems as well
Dealing with re-naturalization of river systems, the logical procedure is in the
direction "down waler". The number of moun lain and submountain rivers, not only in
the Czech Republic. is also interfered with industrial emissions, that causes a
degradation of vegetation. Therefore in these regions the changes of run-oft' relallons
are expectable.

At present an often used equalion for calculation of Ilow resistances is the
equation

U (aR)
-=5,6210g --
VIIo mks

(I)

where U = mean velocity, VIIo ,.. mean shear velocity .JgRS~. R =:. hydrauhc radius.
kJ:; hydraulic roughness, paramclt.:1 a expresses the etTecl orthe cross seclional shallc.
In :..: parameter expressing the etlecl of lhe grain size distribution l;urve. BRAY ( 11)7111
has staled lhal Ih~ parameter nt depends only on a grain used from the grain SII'.l'
dislribution cup,e (for instance m = ].5 for 1)K4)'.

IScnior LCClurcr. C"J:Ch Tct:hnical University iu Pr.tguc. Dcpnrllllcnl olll)"draullcs and UydroloID.
TMkurova 7, 16629 Prague 6. C.lJ:ch Republic

112

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EVALUATING FLOW RESISTANCE

713

Field measurements

. For verii)'ing the validity of.the. semilo~thmic law experimental river sites were
Installed on the Sumava mountam overs Blamce, Mal!e, Volyi\ka and Spulka. During
4 years 74 measurements were collected from 7 river reaches. Within these
measurements in every experimental river reach the basic geometric, hydraulic and
roughness characteristics were investigated. Namely: currents Q. shapes and
dimensions of cross sections (cross sectional area A. wetted perimeter P, channel
~idt~ ~). longitudinal water level slopes S and friction slopes Sf. grain size
dlstnbuuon of the bed surface material and also heights and shapes of roughness
protrusions. their vertical projected area and their cumulative frequency function as
well.
The resistance function was calculaled from the equation

where U
equation

Q/A.

U Q
U. - AJgRSr
The Manning roughness

coefficient D was calculated from the

(2)

AR 2/3S 1/2
n- r
Q
The range of main measured characteristics is staled in Tab. I.

Tab. I. RaDge of main measured characteristics

(3)

-0 S S. A R U U/U. n
m3s.1 - - m2 m ms.) - sm9)n
min 0.30 0.0020 0.0021 0.85 0.15 0.29 2.39 0.0321
ma. 15.70 0.0115 0.0118 10.9] 0.81 1.68 9.38 0.1019

. (\ s~bstitution ~f hydraulic roughness ks by a cenain grain from the grain size
dlstnbutton curve IS a certain siml'lJ)'ing accepted due to practical reasons. For
mountain rivers this procedure is possible only by compiling grain size distribution
curves of the number of slones [WOLMAN's (1954) grid by sample method]. where
a Size of every element is being measured by hand.

Of course. lhere exists a difference between the real height of hydraulic
roughness k, and a grain Di used from a grain size distribution curve. That was a
reason for using also another procedure: Cross sections were measured with a fair
de~sity of points (distances from O.05m to O.lm apart) so that the bed elements'
helghls (and the cross sectional area as well) would be described properly in every
cro~ Section and in greater details. Hydraulic roughness II., was expressed using the
"height of roughness protrusions ki distribution curve". Values of 50 and 16
percentage from this distribution curve have been used, where "so is such a height of
roughness protrusion, for which 500.10 of protrusions are !a.!:&u than the protrusion
~ (similarly for k..l.

A scheme defining the height of protrusions is represented by a figure I.

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