The Influence of Macrophyte Cutting on the Hydraulic Resistance of Lowland Rivers

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J. Aquat. Plant Manage.
47: 2009. 65
studies were funded in part by the Florida Department of En-
vironmental Protection and the Aquatic Ecosystem Restora-
tion Foundation.
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The Influence of Macrophyte Cutting on the
Hydraulic Resistance of Lowland Rivers
KRIS D. BAL AND P. MEIRE
1
INTRODUCTION
Macrophyte growth has an important effect on river flow
velocity patterns during summer when high vegetation biom-
ass (up to 1100 g.m
-2
) is present (Champion and Tanner
2000). Within vegetation patches, velocity measurements
have shown a decline in stream velocity (Watson and Rose
1982, Sand-Jensen and Mebus 1996). Due to this decreased
velocity, flow is deflected around the vegetation patches
(Sand-Jensen and Pedersen 1999) resulting in highly vari-
able stream velocities within natural cross sections (Marshall
and Westlake 1990, Sukhodolova et al. 2004). In general, the
average flow is obstructed and channel resistance increases,
leading to greater water depths (Pitlo and Dawson 1990).
The increase in channel resistance can be an order of magni-
tude greater than the minimal channel resistance (Bakry et
al. 1992), but under exceptional conditions this parameter
can increase even further (Green 2003).
The natural variation of macrophytes in space and time is
highly variable (Barrat-Segretain 1996, Feijoó et al. 1996).
When a patch is formed, flow conditions change and coloni-
zation by other species is possible. This gradual appearance
and decline of species alters the dynamic characteristics of
the resistance associated with vegetation; therefore, a better
understanding of seasonal vegetation development is need-
ed.To reduce flow resistance and ensure drainage of sur-
rounding arable land and prevent flooding at high precipita-
tion events, macrophytes are mechanically cut and removed
from the system. River management objectives are to ensure
hydraulic efficiency, minimize the impact on river ecosys-
tems, and monitor the effect of vegetation regrowth on flow
resistance. For example, high vegetation cutting regimes re-
duce hydraulic resistance but are detrimental to the ecosys-
tem and increase maintenance costs (Dawson 1989). Also,
the effectiveness of this technique is dependent on the vege-
tation regrowth capacity. The regrowth capacity from cut
stems is high, and within three to five weeks (Rawls 1975,
Cooke et al. 1990, Crowell et al. 1994, Bal et al. 2006) biom-
ass can reach pre-harvested values, resulting in a second and
even third cutting regime. With this frequent vegetation re-
moval the travel time of water is reduced (Hamill 1983), re-
1
Department of Biology, Ecosystem Management Research Group, Uni-
versity of Antwerp, Universiteitsplein 1C, B-2610 Wilrijk, Belgium. Received
for publication March 4, 2008 and in revised form September 25, 2008.
J. Aquat. Plant Manage.
47: 65-68

66
J. Aquat. Plant Manage.
47: 2009.
sulting in increased flooding risks downstream of the
maintained river stretch (Trepel et al. 2003). Therefore, a
better understanding of the seasonal vegetation variation
and the impact of vegetation cutting on channel flow resis-
tance is necessary to increase the efficiency of vegetation re-
moval.
We investigated the impact of vegetation removal on the
hydraulic flow resistance of three lowland rivers to (1) in-
crease the knowledge on the seasonal variation of macro-
phyte abundance in combination with their effect on river
flow resistance, and (2) quantify the temporal impact of river
management on the hydraulic efficiency of lowland rivers.
MATERIALS AND METHODS
We calculated resistance on three lowland river stretches
(Desselse Nete, Wamp, and Grote Caliebeek) in the Nete
catchment (Belgium) during the growing season of 2005.
These rivers were chosen because they have well-developed
aquatic vegetation populations that were cut once a year,
with the exception of the Wamp where the vegetation was
not removed. The macrophyte community in these river
stretches was dominated by a single species,
Potamogeton na-
tans
L. (>75% of the macrophyte coverage). Other species,
such as
Sagittaria sagittifolia
L.,
Callitriche platycarpa
Kütz,
Stuckenia pectinatus
L., and
Ranunculus penicillatus
were
present but in densities reaching a maximum of 5%. Because
P. natans
concentrates its leaves on/or nearby the water sur-
face, and due to its relatively high stem diameter (up to 0.6
cm), a high impact on the resistance is expected.
Vegetation cutting times differed between the beginning
of May for Grote Caliebeek and end of June for Desselse
Nete. Biomass was collected with a lightweight sampler ac-
cording to the design of Marshall and Lee (1994). This 360º
rotating device cut the stems just above the sediment surface
on an area of approximately 0.22 m
2
. For each sampling site,
10 biomass samples were collected randomly along a length
of approximately 100 m. Dry mass of the samples was deter-
mined by drying them for 48 hr at a temperature of 75 °C.
The most widely used equation to calculate the hydraulic
resistance, especially in vegetated streams in which flow resis-
tance is mainly generated due to skin drag (Lee and Fergu-
son 2002), is the Manning equation:
Q A
-1
= k R
2/3
S
1/2
n
-1
where R = hydraulic radius (m); S = energy gradient (dimen-
sionless); k = factor to keep the equation dimensionally cor-
rect (m
1/3
s
-1
); n = Manning coefficient (dimensionless); Q =
discharge (m
3
s
-1
); and A = cross-sectional area (m
2
).
The downside of using this equation is that the resistance
factor (n) in the equation assumes that the objects, in this
case macrophytes, are rigid. However, due to their flexibility
and their marginal positive buoyancy, macrophytes tend to
lay over with increasing velocity, reducing the Manning coef-
ficient in this manner.
The energy grade line (S) of the different lowland rivers
was measured by determining elevation differences between
up- and downstream points. The hydraulic radius of a river
channel is defined as the ratio of its cross sectional area (A)
versus the wetted perimeter (part of the river bank in contact
Figure 1. Seasonal variation of the Manning’s n (a) and macrophyte biomass
(b) on three slow flowing lowland rivers during 2005. Error bars (n = 10) are
shown.
Figure 2. The effect of biomass (a) and discharge (b) on the Manning’s n
coefficient.

J. Aquat. Plant Manage.
47: 2009. 67
with the water). By measuring the bottom profile and the wa-
ter level on a fixed location, the wetted perimeter and cross
sectional area of the three river stretches could be deter-
mined and thus the hydraulic radius calculated.
No river bends were present in the study stretches. Veloci-
ty measurements were taken at fixed cross sections on a
bridge on stretches not disrupted by aquatic vegetation, re-
sulting in more accurate discharge calculations. Five or six
velocity measurement points were performed with an elec-
tromagnetic flow meter (Valeport, type 801) for 30 sec each
along the width of each cross section (Table 1). At each
point, a vertical velocity profile was measured with intervals
of 20 cm. When water depth was below 40 cm, water veloci-
ties were measured each 10 cm. By integrating the velocities,
cross section discharges were calculated. Velocity and water
depth measurements started March 2005, when new vegeta-
tion appeared, and continued until December 2005.
RESULTS AND DISCUSSION
The hydraulic resistance on all three lowland rivers, as in-
dicated by the computed Manning’s n, increased in spring
until June, when maximum values were reached of 0.34,
0.66, and 0.71 for the Desselse Nete, Grote Caliebeek, and
Wamp, respectively (Figure 1a). On other river systems, resis-
tance values ranged between 0.25 and 2.25 (Dawson 1978,
Watson 1987, Hearne and Armitage 1993, Green et al. 2006),
which is within the range of our experiments. Resistance in-
creased for Wamp and Grote Caliebeek by a factor of 7.6 and
2.2, respectively, in 1 mo, comparable with the three- to five-
fold increase found in other river systems (Bakry et al. 1992,
Champion and Tanner 2000, Sellin and van Beesten 2004).
This sharp increase in resistance until June was not seen on
Desselse Nete because biomass, consisting entirely of leafless
Potamogeton natans
L. stems, was still high in winter (260 g m
-
2
; data not shown), resulting in increased resistance. These
stems were not divided evenly along the river, but followed
the cutting pattern used in 2004. Points with lower stem den-
sity were those where vegetation was mechanically cut. Next
winter, after a total removal of the vegetation during sum-
mer, much lower and more evenly spread
P. natans
infesta-
tions were observed. Further research is needed to explain
this variation.
The increase in resistance followed almost the same pat-
tern as the biomass growth (Figure 1b). When resistance was
plotted against biomass, significant linear relations were
found for Desselse Nete (R
2
= 0.70, p < 0.01, F = 13.9) and
Wamp (R
2
= 0.56, p = 0.05, F = 6.45; Figure 2a). This relation
was not significant for Grote Caliebeek (R
2
= 0.06, p = 0.61, F
= 0.3) due to high resistance values in June and November,
T
ABLE
1. S
UMMARY
OF
CALCULATED
DISCHARGES
,
MEASURED
WATER
DEPTHS
,
ENERGY
GRADIENTS
(S)
AND
HYDRAULIC
RADII
(R)
OF
THREE
LOWLAND
RIVERS
DURING
2005.
Grote Caliebeek Desselse Nete Wamp
May Discharge (m
3
s
-1
) 0.36 0.34 0.63
Depth (m) 0.84 1.24 0.52
S (m m
-1
) 0.0014 0.0012 0.0005
R (m) 5.18 5.48 8.84
June Discharge (m
3
s
-1
) 0.02 0.22 0.23
Depth (m) 0.28 1.08 0.83
S (m m
-1
) 0.0007 0.0010 0.0011
R (m) 4.06 5.16 9.46
July Discharge (m
3
s
-1
) 0.30 0.54 0.42
Depth (m) 0.50 0.81 1.06
S (m m
-1
) 0.0005 0.0010 0.0003
R (m) 4.50 4.62 10.02
August Discharge (m
3
s
-1
) 0.08 0.29 0.37
Depth (m) 0.51 1.04 1.06
S (m m
-1
) 0.0005 0.0010 0.0002
R (m) 4.52 5.08 9.92
September Discharge (m
3
s
-1
) 0.05 0.33 0.34
Depth (m) 0.41 1.11 0.77
S (m m
-1
) 0.0007 0.0012 0.0001
R (m) 4.32 5.22 9.34
November Discharge (m
3
s
-1
) 0.07 0.38 0.26
Depth (m) 0.49 0.79 0.40
S (m m
-1
) 0.0028 0.0009 0.0002
R (m) 4.48 4.58 8.60
December Discharge (m
3
s
-1
) 0.29 0.44 0.68
Depth (m) 0.38 0.77 0.36
S (m m
-1
) 0.0028 0.0008 0.0010
R (m) 4.26 4.54 8.52

68
J. Aquat. Plant Manage.
47: 2009.
despite the low biomass, probably a result of cutting activities
and natural senescence. In June, these high resistance values
were the result of very low discharges, around 0.015 m
3
s
-1
(Figure 2b). The high resistance values in November were
the result of the high-energy gradient (0.0028 m m
-1
) and low
discharges (0.07 m
3
s
-1
; Table 1).
Together with the growth of aquatic vegetation, the water
depth increased on all three lowland rivers, as shown by Daw-
son and Robinson (1984). This linear relation between water
depth and biomass was significant (all p values <0.03) for all
three rivers.
On both managed river stretches (Desselse Nete and
Grote Caliebeek) the resistance was strongly reduced after
vegetation removal to values around 0.09 and 0.07
but in-
creased again due to fast regrowth (Rawls 1975, Cooke et al.
1990, Crowell et al. 1994, Bal et al. 2006), resulting in a sec-
ond resistance peak later in the season. This regrowth indi-
cates that vegetation cutting did not happen when biomass
reached a peak, or belowground reserves would be depleted
(Linde et al. 1976) resulting in low regrowth. In the unman-
aged stretch of Wamp, only one resistance peak was seen ear-
ly in the vegetation season (June). This peak did not occur
when biomass was highest (July), but one month earlier due
to low discharges and a high energy gradient (Table 1).
From this moment on, a decrease in resistance occurred to-
gether with a decline in biomass.
Vegetation cutting in combination with harvesting is a
useful tool to increase drainage when flooding threatens
human activities, but only for a short period. Within one
month, biomass will increase again resulting in increased
water levels. Over time an integrated approach is necessary,
including the reduction of nutrients to prevent unnaturally
high macrophyte biomass. With strategic timing and fre-
quency of cutting practices, river management can result in
less but more efficient cutting activities causing fewer hy-
draulic problems.
ACKNOWLEDGMENTS
Special thanks goes towards IWT (Institute for Promotion
of Innovation through Science and Technology, Flanders)
and FWO (Fund for Scientific Research, Flanders) for fund-
ing this research (project number G 0306 04).
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