Development of the suspended-sediment concentration measuring system withdifferential pressure transmitter in rivers and reservoirs
T. Sumi,1 S. Morita,2 T. Ochi2 and H. Komiya2
1
Department of Civil Engineering, Kyoto University, Sakyo, Kyoto, 606-8501, JAPAN, PH+81-75-753-4751; FAX +81-75-753-4794; email: sumi@basewall.kuciv.kyoto-u.ac.jp
2
SMD Engineering, Co., Ltd., Honmachi 3-33-11, Nakano, Tokyo, 164-0012, JAPAN,
PH & FAX +81-3-3372-0217; email: saichirou_morita@nifty.com
Abstract
It is important to know quantity of wash load for the integrated sediment managementfrom upstream through downstream river basins. Generally, both continuous turbidity
measuring and bottle sampling are used for suspended-sediment concentration measurement
in rivers or reservoirs. Here, we have developed a new suspended sediment concentration
measuring system with differential pressure transmitter (hereafter, we call SMDP).
The SMDP has advantages in long-term and high turbidity measurement because
differential pressure transmitter is measuring density of water directly. Based on measuringconditions, a submersible type and a water circulating type SMDP are available, and both ofthem have shown good performances through laboratory tests and field experiments at theMiwa dam, the Tenryu river and the Kurobe river.
Introduction
Reservoir sedimentation is a complex process that varies with watershed sedimentproduction, rate of transportation, and mode of deposition. Sedimentation reduces reservoir
storage capacity for flow regulation and with it all water supply and flood control benefits,
plus hydropower, navigation, recreation, and environmental benefit that depend on released
from storage. Besides of storage loss, many types of sediment related problems can also occur
both upstream and downstream of dams. The combination of sediment trapping and flow
regulation also has dramatic impacts on the ecology, water transparency, sediment balance,
nutrient budgets, and river morphology. Even coastal process can be affected; accelerated
coastal erosion can be occurred because of sediment trapping in far upstream reservoirs andlacking of sufficient sediment supply to downstream. Thus necessities for sedimentmanagement of reservoirs are not only for a reservoir itself but also for the integrated

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sediment management in the total river system.
In order to realize the integrated sediment management, it is necessary to know the total
sediment load consisting of bed load, suspended load and wash load that is mainly produced
and transported from upstream through downstream. If there are some storage reservoirs in
the river basin, outline of the annual the sediment production can be estimated from timeseries of sedimentation records in those reservoirs. It is, however, relatively difficult to know
total incoming wash load, since some amount of total load is easily passing thorough in caseof small reservoirs.
It is general to combine continuous turbidity measurement with the turbidity meter andbottle sampling for calibration to know time variation of flowing wash load in rivers orreservoirs. However, there is a subject in stability for long time since the turbidity is measured
by an optical method and, for bottle sampling, it is very difficult to collect samples without
missing at the time of discharge peaks in case for small catchment areas having very shortrun-off times. Although the long time stability has been improved by the technology
improvement in recent years, there is some limitation for extremely high turbidity
measurements in case of high floods that may transport almost all sediments in the whole year
or sediment flushing operations to prevent reservoir sedimentation.
This research is to develop a new suspended sediment concentration measuring systemwith differential pressure transmitter (hereafter, we call SMDP). This method measuressuspended sediment concentration by the water density directly with a differential pressuretransmitter, without depending on a conventional optical method for the purpose of thesolution of such a problem.
This method can be applicable for the continuous measurement and the high turbidity
measurement. There are two types of SMDP; one is the submersible type and another is thewater circulation type. In order to make clear the basic performance of this system, bothlaboratory and field experiments at the Tenryu river, Miwa dam, and also the Kurobe river
have been executed individually.
Measuring method
Measurement object
Objects of this measuring system are mainly wash load of 0.1〜0.2 mm or less indiameter. Below, suspended sediment and SS (mg/l), suspended sediment concentration, mean
this wash load and its concentration respectively. In the river management, it is required tomeasure SS from very low ranges of 100 mg/l or less to high ranges of 100,000 mg/l over. In2

other words, the maximum SS of 1,000-100,000 may occur at the flood peak in mountainous
rivers or during sediment flushing operations. On the other hand, measuring SS of 50 mg/l orless is required to operate water intakes or settling basins of the power station. Measuringsystem should be applicable for such a wide concentration range.
Measurement principle of SMDP
Densities of water with and without suspended sediments are different. There are many
types of concentration measurement method of liquid (Wren at al. 2000). Here, we havedeveloped a new method to measure density of liquid by differential pressure between two
standard points, PH: the high-pressure side and PL: the low-pressure side, that distance H is
kept constant. Figure 1 shows the concept of this measuring system.
From the Bernoulli’s theorem, eq.(1) is derived where average density ρ, gravitational
acceleration g, velocity at the high pressure and low pressure points VH, VL respectively.
PH/ρｇ+VH2/2g＝PL/ρｇ+H+VL2/2g

(1)

hPL

E lectrical
S ignalPressure difference ∆ P is calculated by eq.(2).

H +h
ΔP＝PH−PL＝ρｇＨ＋ρ(VL2- VH2)/2(2)H
PH

If VH＝VL is assumed, ∆ P is calculated by eq.(3).
ΔP＝PH−PL＝ρｇＨ(3)

ρFigure 1.

Measuring principleHere, average density ρ and SS can be calculated by measuring differential pressure ∆ P,
since g, H are known. VH≠VL in general and ρ(VL2- VH2)/2 may become an error if
assuming ΔP=ρg H. In order to raise accuracy, either designing the system to be VH≒VL ormeasuring VH, VL simultaneously is considered, and the former is realistic.
Next, we can estimate the differential pressure that may occur actually. Ifρ0(g/cm3) isdefined as the density of water of SS=0 mg/l, the change of the differential pressure from
SS=0 mg/l (Ⅰ) to SS=10,000 mg/l (Ⅱ) can be calculated by eq.(4) from eq.(3).
ΔＰⅡ−ΔＰⅠ＝((ρ0+0.01)−ρ0) g H(4)3

E le c t r ic a l s ig n a l
If assuming H = 1,000 mm, the small pressure change
P o w e r so u rc eof about 10mmH2O becomes needed to be detected. Here,Low er
we are using the differential pressure transmitter of thep re ssu e r
d e te c to r
silicon vibration type in this system to detect such a smallpressure change with high accuracy. The silicon
D if f e r e n t ia l
p re ssu ret r a n s m it t e r
semiconductor process technology has enabled to form
two oscillators on the single crystal diaphragm of silicon
H ig h e r
p re ssu e r
de te c to rthat are placed in the vacuum chamber. Original
resonance frequencies of the two oscillators will slightly
change because of those elastic strain caused by Figure 2. Schematic diagram ofdifferential pressure between the top and bottom face ofthe measuring equipment
the silicon diaphragm. By measuring these frequency
1m×1m×0. 8 mchanges, differential pressure can be detected.
Float
W. L.

Measuring systems of SMDP

ca.2m

Center
The schematic diagram of SMDP is shown in Figure 2.
Measuringelevationequipment
Two pressure detectors are placed vertically with the1. 6 m
distance H=1,000mm and connected by small pipes filled
up with water to the differential pressure transmitter
φ0. 5 5 m
mounted at the center of the system.
Figure 3. Schematic diagram ofNext, we have developed two different unites tothe submergible type SMDP
measure water with suspended sediments based on
different measuring situations. One is thesubmersible type and the other is the water
circulation type. The submersible type is shown in
Figure 3. The main measuring equipment issuspended from a float and submerged in themeasuring water directly. Advantages of this typeare simple structure and easy maintenance. On theother hand, it is necessary to secure enough water

depth over about 2.5〜3.0 m for measuring, to keep
almost equal velocities between the high and low
pressure detectors and to protect the measuring

4Figure 4. General view of thesubmergible type SMDPequipment by floating debris such as gravels ordriftwoods. We have solved this subject by shielding
the equipment with the shell type cylinder. This
submergible type is applicable for measuring such asin reservoirs. Figure 4 shows the measuring
equipment for the field experiment in the Miwa dam

Measuringequipment
W. L.

reservoir, the Tenryu River, explained later．
POn the other hand, adopting the submergible type
Submergible pump
SMDP becomes difficult in the case that the water
depth changes largely depend on time like a natural Figure 5. Schematic diagram of theriver. Then, we have also developed the water
water circulation type SMDP
circulation type SMDP as shown in Figure 5.
Here, the main measuring equipment is installed
in a water tank that is placed in land and two
pipes are connected. One is to intake measuring
water from a river and another is to drain fromthe tank. An intake pipe is connected to a
submergible pump in river water.
This type can be applicable to anywhere, if
the minimum water depth for intake is secured,and is more safety than the submersible type atthe time of the flood. On the other hand, pump
Figure 6. General view of the water
operation is needed for continuous measurement
circulation type SMDP
and constant discharge ofaccumulated sediment from thebottom of the water tank. The
system has the electromagnetic
flow meter to measure quantityof water, and the accumulated
sediment can be discharged
periodically from the valve
installed at the bottom of the tank.
This water circulation type isFigure 7. General view of the Miwa dam reservoir
applicable for even at theand the bypass tunnel
natural rivers and we are doing field experiment in the Kurobe River, as shown in Figure 6.

5Miwa dam
Reservoir

Fieldexperimentofthesubmersible type SMDP at theMiwa dam
Test outline

JindenbridgeDiversionweir

Sediment bypass tunnel'under construction'Dam height
Water depth
69.1m
5～6m

Measuringpoint

Sediment
trap weir[℃][mg/l]

6[EL. m][m3/s][mm]

We are doing the field
Accumulated
sediment
experiment to measure inflowing
suspended sediment concentration
Figure 8. Field experiment of the submersible
to the Miwa dam reservoir, completedintype SMDP at Miwa dam1959, 69m high and 29.95
0million m3 of the total storage
5Average precipitation at thecapacity and 311km2 of the10Miwa dam catchment [mm]15
total catchment area. This dam
120812
Inflow discharge [m3/ｓ]
is very famous for severe100
Outflow discharge [m3/ｓ] 810
reservoir sedimentation. At theWater level [EL .m]
80808
60806
flood time, high suspended
40804
sediments flow into the20802
reservoir and the Miwa dam
0800
252000has been already silted up toＳＳ(Jinden bridge） ［ｍｇ/ｌ］ 23
60 % of total storage. Now, a1500
Water temperature ［℃］
new sediment bypass tunnel,
211000
24,300 m length, 47m in cross
19500
section and 300m3/s of the17discharge capacity is under
015
1
713 191713 191713 19
constructionforreservoir
sedimentation
management.
Aug. 21
Aug. 22
Aug. 23
Figure 7 shows general view ofFigure 9. Field experiment at the Miwa dam
the Miwa dam reservoir and the’Typhoon No.11, 2001’
bypass tunnel.
The measuring equipment is moored at the bridge pier, located at the middle of thereservoir with about 5-6 m water depth as shown in Figure 8. Figure 4 shows the equipmentthat connected by an electric cable and a signal line to the recording unit placed on the bridge.

19.0

2000

18.0

1500

17.0

1000

ＳＳ(min.mg/l）

500

16.0

ＳＳ(max.mg/l）
Water temperature（℃）

15.0

21:00
21:10
21:20
21:30
21:40
21:50
22:00
22:10
22:20
22:30
22:40
22:50
23:00
23:10
23:20
23:30
23:40
23:50
0:00
0:10
0:20
0:30
0:40
0:50
1:00

0[℃]

2500

Figure 10. SS and water temperature change inevery minute ’Typhoon No.11’
05
10
1520

Average precipitation at theMiwa dam catchment [mm]300[mg/l][m3/s]

also dropped from 21℃ to808
20017 ℃at the peak
806
100804hydrograph. Figure 100802
shows the maximum andminimum value in every
25
4000
3500
ＳＳ（Jindenbridge）［ｍｇ/ｌ］
minute data during SS is
Water temperature ［℃］300020rising up. It became clear
2500
2000
that
high-suspended
1500
15
sediment
flow
have
1000
500
entered to the reservoir
010
upstream and passed
1713
19
17131917
through the measuring
Sep. 10
Sep. 11
Sep.12point in a very shortperiod from 21:40 toFigure 11. Field experiment at the Miwa dam
21:50 P.M. SMDP has’ Typhoon No.15, 2001’
important advantages insuch a real-time and high-suspended sediment concentration measurement.
SS was also measured as shown in Figure 11 at the typhoon No. 15. This flood, peakdischarge exceeded 300m3/s, was larger than the typhoon No.11 and SS also rose up to the7[EL.m]

Inflow discharge [m3/ｓ]812
Outflow discharge [m3/ｓ]
810
Water level [EL .m]

400[℃][mm]In 2001, there weretwo major floods causedby the typhoon No.11,
August 21 to 22, andNo.15, September 10 to
11. Figure 9 shows
measured records by
SMDP at the typhoon
No.11. Sampling data iscollected per second andrecorded per minute as theaverage, maximum andminimum value.
Water temperature have

[mg/ｌ]

Measuring results

9:0
10 0:0
11 0:012 0:013 0:014 0:0
15 0:0
16 0:017 0:018 0:019 0:0
20 0:0
21 0:022 0:023 0:00
0:00[℃][mg/l]

highest 3,700 mg/l. Figure
5000
ＳＳ ｍｉｎ.［mg/ｌ］2412 also shows the every
4500
ＳＳ ｍａｘ.［mg/ｌ］
4000
minute data, and it is clear
Water temperature ［℃］ 22
3500
203000
thattheinstantaneous
2500
18maximum SS was up to2000
1615004,600 mg/l. Figure 12 also
1000
14500
shows the maximum and0
12
minimum value in a minute,and fluctuation of SS isadmitted after the flood
Figure 12. SS and water temperature change in every
peak around 19 PM. As forminute ‘Typhoon No.15’
this fluctuation, the destruction of the thermal stratification in the reservoir by this large floodis estimated as the main cause.
Field measurements in the Miwa dam showed that the submersible type SMDP can beused for SS monitoring in reservoirs with advantages of real-time and high-suspended
sediment measurement.
Field experiment of the water circulation type SMDP at the Kurobe river
Test outline
In the Kurobe river, there are twodams, the Unazuki and Dashidairadams that are periodically emptied toflush out accumulated sediments fromreservoirs. Figure 13 and 14 shows theschematic diagram of the Kurobe river,and the Unazuki and Dashidaira dams.
During those flushing operations,monitoring of SS in the downstream
river is needed for the environmental
assessment, and the field experimentof the water circulation type measuringsystem is planned as shown in Figure 5and 6.
Figure 13. Kurobe river system

8Unazuki dam

Dashidaira dam

Figure 14. Schematic diagram of the Unazuki dam(left) and the Dashidaira dam(right)[mm]

0
Average precipitation at theUnazuki dam catchment [mm]5230
2202102500

SMDP（ＳＳ） ［ｍｇ/ｌ］
Unazuki dam 濁度［度］
Aimoto weir ＳＳ[mg/l]
Water temperature ［℃］

2000
1500

19171513119751000
500

2001.June.19

June.20

June.21172159131721
159131721
159131721
1591301SS[mg/l]，[NTU]

貯水位 [EL. m]

Inflow discharge [m3/ｓ] 250
Outflow discharge [m3/ｓ]
240
Water level [EL .m]

600
500
400
300
200
1000June.22

Figure 15. Field experiment at the Kurobe river

9[℃][m3/s]

10

Measuring result

From June 19 to 22, 2001, high-suspended sediment concentration has been recorded at
the measuring point downstream from these dams during the flushing operation. Figure 15
shows the record of SS by SMDP compared with the conventional turbidity measurement andthe bottle sampling. The maximum of SS, 900 mg/l, was recorded at 8 A.M. June 21
immediately after the start of free flow flushing in the Unazuki dam. It is clear that the SSchange by SMDP coincides with other data, but SS values are slightly different from them bybottle samplings. The main cause of this gap may come from the limitation of the submersible
pump capacity. It became clear that the SS measurement by the water circulation type SMDP
is applicable for the monitoring of the natural rivers effectively.
Conclusions
We have developed the system, SMDP, that continuously measures suspended sedimentconcentration by using the differential pressure transmitter, and carried out the field test about
the submersible type and also the water circulation type corresponding to measurement
environment. The result that was obtained can be summarized as follows.
(1) At the field experiment in the Miwa dam, SS during natural flood flows passing throughthe reservoir have been measured stably, and the submersible type SMDP can be used for
SS monitoring in reservoirs with advantages of real-time and high-suspended sedimentmeasurement.
(2) At the field experiment in the Kurobe river, SS during sediment flushing operation fromreservoirs have been measured, and the water circulation type SMDP can be used for SS
monitoring in natural rivers where enough water depth is hardly secured constantly.
References
Wren,D.G., Barkdoll, B.D., Kuhnle, R.A., and Derrow, R.W. (2000). “Field techniques for
suspended-sediment measurement.” J. Hydr. Engrg., ASCE, 126 (2), 97-104.
Sumi, T., Morita, S., Ochi, T., and Komiya H. (2001). “Development of the suspended
sediment concentration measuring system with differential pressure transmitter.”, DamEngineering, 11(3)，4-12 (in Japanese).
Sumi, T. (2000). “Future perspective of dam reservoir sediment management, Proc. ofInternational workshop on reservoir sedimentation management, Toyama, 145-156.

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