SEDIMENT TRANSPORT IN RIVER MOUTH ESTUARY

Katsuhide YOKOYAMA, Dr.Eng.

Assistant Professor Department of Civil Engineering dredge Tokyo Metropolitan University

1-1 Minami-Osawa, Hachioji, Tokyo, Japan 192-0397 [email protected] tel;81-426-77-2786 fax;81-426-77-2772 Introduction & Study Area

0 2km N

The river mouth estuary and wetland are comprised of variety of view natural, morphologically and ecologically complex aquatic point environments. In this region, fresh water mixes with salt water, therefore the Tidal river stream runs more slowly, the suspended sediment supplied Sea from the upstream basin deposit and the shallow water area is flat created. River mouth estuary is very important area for ecosystem and 白川 fishery. On the other hand, it is necessary to dredge and enlarge the Port river channel in some cases in order to discharge the river flood into SHIRAKAWA sea safely. River The purpose of this study is to develop the rational management practices of river mouth estuarine resource. It is necessary to Flood and sediment Tidal pumping and explain the sediment transport and the topographical process. discharge sediment transport A field study was undertaken in the SHIRAKAWA river. The Sea topography change of tidal flat was surveyed and the sediment discharge by floods was measured and the annual sediment transport by tidal current was monitored. Using these results, the amount of sediment load was calculated and the influence of the sediment transport by flood and by tidal current on the topography Deposition of silt and change was discussed. sand on tidal flat Sediment Load by Flood Discharge

METHOD RESULT Self-logging Optical Measurement Result of Turbidity Monitoring during One Year Backscatterance

s) 1000 Discharge /

Sensor with Wiper 3 500 (m

800 Turbidity m)

pp 400 (

1 2 3 4 5 6 7 8 9 10 11 12 month Correlation between Turbidity and SSC Particle Size Distribution 2000 100

ss 80 a %) ( m

Measurement of /l) e e g g 60 v

Flood Discharge a t ti (m a

1000 n

l

u 06, Jul 2 ce 40 SS r m

y=0.0027x +0.5635x e 14, Jul u

2 p 21, Jul R =0.9051 C 20 29, Jul 0 0 0 500 1000 10-4 10-3 10-2 10-1 100 Turbidity (ppm) Diameter (mm) 洪水観測風景 Turbidity has high correlation with SSC. SSC time series can be calculated by these results, then Comparison of Topography Variation with sediment load will be estimated by SSC and flow Sediment Load During Flood Term rate. SS component was almost silt and clay.

) Topography variation after flood term is lager Sediment discharge was measured by combination of two 3 Silt ,Clay than sediment load during flood term. The (m method. Turbidity was monitored continuously by self e reasons are as follows; Topography variation has

m 200,000 Sand

u 3 logging optical backscatterance sensors over one year. l error of 100,000m , because the resolution of o Silt ,Clay Actual suspended sediment concentration (SSC) was V sounding survey is few centimeters. Suspended

t 100,000 determined from flood samples which were collected by n sediment load is accurate in estimation but bed e

m Sand(0) lowering the open container to the water surface with a rope. i load can not be caught by field measurement. d

At the same time, turbidity of sampled water was measured. e S Topography Change Sediment Load As a result, the quantity of fine sediment SSC time series will be estimated from turbidity monitoring transport during flood term (2001) was about data by using correlation between SSC and turbidity. 100,000-200,000m3. Topography and Deposited Materials Change on Tidal Flat

METHOD RESULT -6m -4m -2m -1m Topography of tidal flat was surveyed by echo sounder and Topography differential GPS. Bed materials was directly sampled by divers. Change These were undertaken before flood term (June 2001), just 0m after flood term (July 2001).

Location of Sounding Survey 0 2km N

before flood after flood N.32° sedimentation area 47.22‘

Center diameter ・・・・・ Bed Materials 0.4 Change 0.2 46.00‘ 白川 (mm) Port SHIRAKAWA 100 Mud content 80 River 60 40 E.130°33.45‘ 36.17‘ 20 (%) 4 3 2 1 0 Location of Bed Materials Sampling Distance from river mouth (km) -6m -4m -2m -1m The elevation became high in front of the river 0m Sedimentation Quantities mouth after flood term. In the sedimentation area, for Each Particle size particle size became small and mud content ratio which is under 0.074mm increased. Type Variation(m3) These results shows that a lot of silt and clay Silt and Clay 239,682 were discharged by floods and were deposited on Fine Sand 175,018 the tidal flat. Sand 9,430 Total amount 424,131 The sedimentation quantities are calculated for each particle size, silt and clay were deposited about 240,000m3 during flood term. Field Experiments on Sediment Transport by Tidal Current

DEPLOYMENT 0 2km N Location 36hr measurement on SSC, Temperature and Salinity Three field measurements were carried out to investigate sediment transport by tidal current. Water current, SSC and salinity were Tidal monitored over one year. Acoustic Doppler Sea flat A profiler (ADP) was deployed on the deepest bottom and water current profile was recorded B every five minute. Self-logging OBS and C 白川 salinity-temperature sensors were attached at Port SHIRAKAWA near bottom, 1m above bottom and surface on River the side of H beam and they recorded at 5 minute intervals. Monitoring Station Detailed vertical profile of SSC, salinity and flood temperature were measured for 36 hours at 10 minute intervals. OBS 4m Salinity meter At the same time, turbid water was sampled 5m and suspended particles were soon observed ebb using digital microscope. Measurement stations were maintained at ADP tidal flat (A), river mouth (B), river cannel (C).

100～300m Particle Observation Acoustic Doppler profiler (Self-logging type)

OBS with wiper and salinity- temperature sensor Movement of Turbidity Maximum

OVERWIEW OF TIDAL CYCLE (Feb 2001) FLOCCULATION OF SUSPENDED SEDIMENT 4 Tide level (Spring tide, Jul 2001) (m) e 2 ud 0 Velocity 80(cm/s) m) 4 ( ltit -2 e A d 0 u 8 Wind velocity t lti ity s) c / A -80 o

l 4

e 0 (m

V 200(mg/L) 0 4 SSC m) (

SSC e d

L) 300 100 /

Tidal Flat u t g SS 150 lti (m

A 0 0 SSC 4 Salinity 30(psu) m)

L) 300 / River Mouth ( e g SS 150 d 15 u (m t lti

A 0 SSC 0

L) 300 / River Channel g 10:00 7, Jun 22:00 8, Jun SS 150 time (m Particle 1, Feb 2001 date 17, Feb 2001 Photograph by High SSC appeared at the spring tide and the latter period on tidal flat and in Micro-scope river mouth. When there were strong wind, it appeared too. On the other hand, it was rare occurrence in river channel and it appeared at after the spring tide. Floc size=0.1mm This shows that the surface mud on tidal flat may be eroded and rolled up by Particle size the turbulence or shear by tidal current and wind. D10=0.002mm D50=0.009mm In river channel, the current distribution (red color is following current and blue is D90=0.030mm countercurrent) shows vertically uniform. The water current was fast at both ebb and flood tidal currents, although SSC was asymmetry between ebb and flood, the peak appeared only at flood tidal current, especially at the start of saline water intrusion. Sketch of Sediment Transport Processes

The photograph of suspended particles sampled at SSC peak shows the existence Flocculate of both floc and mineral particle. River Sea channel Therefore the suspended particles eroded in tidal flat are transported by the saline Accumulate water front and they intrude into river channel. On the way to the upstream, they become floc and accumulate on the border of the saline water as sedimentation velocity increase. Erosion and Roll up New Technique for Sediment Transport Monitoring 通過土砂量の算定 ESTIMATION METHOD OF SSC PROFILE USING ECHO INTENSITY Narrow-band Acoustic Doppler Profiler Acoustic Doppler profiler (ADP) measures the water velocity using a physical process known as Doppler shift when the transmitted sound is reflected off particles suspended in the water. The reflected sound intensity (signal amplitude) in itself may be expected to associate with suspended Echo Intensity Profile sediment concentration. Velocity Profile When SSC profile is estimated by echo intensity profile, flux of sediment load should be calculated from velocity and estimated SSC profile which are SSC Profile the output of one ADP instrument. 1st cell （r=0.25m） 5th cell （r=1.0m） 103 103 Flux of Sediment Load

2 2 ） 10 10 /l) /l g g m m ( RESULT OF SSC ESTIMATION （ 1 1

10 SS 10 SS Time Series of SSC Distribution 100 100 SSC 0 25 50 75 100 0 25 50 75 100 4 measurement 200(mg/L) m)

Echo Intensity (dB) Echo Intensity (dB) ( e d

u 100 Comparison between SSC and echo intensity at each layers shows that t lti these echo intensity is highly related to SSC and echo magnitude decay as the A 0 pulse travel further away from the transducer. Thus, It will possible to 0 estimate detailed SSC distribution from ADP echo intensity. High correlation 4 calculation is exhibited by narrow-band type ADP. m) ( e

Propagation loss of sound wave is generally expressed by the effect of d u geometric spreading and the absorption of acoustic energy by the water, t lti A (1) Tl  20logr  2r 0 where Tl is propagation loss, r is distance from sensor, α is absorption 10:00 07, Jun time 22:00 08, Jun coefficient. The correlation between SSC and echo intensity shown in the figures can be written Predicted time series of SSC distribution agree well with measured SSC. This new technique is useful monitoring equipment for sediment transport, (2) 20logSSC  I  B  Tl since time-spacial distribution of suspended sediment concentration can be where I is echo intensity, B is base sound magnitude. obtained only one equipment deployed either on the bottom or near the surface. Sediment Transport by Tidal Current and Flood Discharge

SEDIMENT LOAD BY TIDAL CYCLE CONCLUSION

September: Low flow rate 3 2,500m Annual Balance on Suspended Sediment Transport, 2001 river mouth channel

-5,000 -2,500 1) Sediment discharge during → 3 3,000 1,500 3) Sediment transport by flood tem 105,000m tidal pumping→45,000m3 total -2,000 -900 Sedimentation in river sedimentation: 1,100 channel→16,400m3 November: High flow rate 2,500m3 Sea river mouth channel

-3,500 -600 4,300 3,300 2)Deposition on tidal flat total 800 2,700 →240,000m3 sedimentation: 1,900 Total amount during one year, except flood term 15,000m3 In this study, suspended sediment transport in river mouth estuary river mouth channel was investigated being divided into three processes. One is flood discharge from river basin, one is sediment deposition on tidal flat -46,100 -16,000 after flood term and one is suspended sediment transport by daily tidal pumping. 40,700 27,400 During flood term, discharged and deposited fine sediment were 3 3 -5,400m 11,000m 100,000m3 to 200,000m3. The flood of this year occurs in a frequency sedimentation: 16,400 of once in three years.

Suspended sediment flux through the channel and river mouth are Annual sediment load by tidal pumping was about 45,000m and estimated from velocity and SSC profile which are both obtained by sedimentation volume in river channel was about 16,000m. These ADP measurement. Horizontal velocity profile is assumed to be volume correspond to middle crass flood which occur a frequency of uniform and corrected by several approach so that the water flux at 2 to 3 times in one year. each stations are coordinate. In Japan, the river water is very clear usually, but daily sediment Two calculation for one month shows that suspended sediment transport by tidal current would have a influence on the estuary were deposited in river channel in any condition. Total accumulation ecosystem and topography. of net flux during one year was 16,400m3.