Preliminary investigation of fine sediment dynamics in Cumbarjua Canal, Goa, India

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Preliminary investigation of fine sediment dynamics in Cumbarjua Canal, Goa, India
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UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 81/012
Mehta, Ashish J., Hayter, E. J.
Coastal and Oceanographic Program -- Department of Civil and Coastal Engineering
Coastal and Oceanographic Engineering Department, University of Florida
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Asia -- India -- Cumbarjua


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Several pages are misnumbered 5 is 4, 8 is 7,14 is 13. No text appears to be missing.
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A. J. Mehta E. J. Hayter
Coastal and Oceanographic Engineering Department
University of Florida UFL/COEL-81/012

December, 1981


Sediment management in estuaries requires an understanding of fine, cohesive sediment transport processes which typically characterize the estuarine regime. A preliminary field investigation was carried out in Cumbarjua Canal, Goa, India, where the sediment is almost entirely in the fine range and the flows are primarily tide-induced. In a 10.4 km reach of the canal, data on currents, tides, sediments and wind were obtained. The hydrodynamic and the sedimentary regimes of the canal under fair weather conditions is distinct from the regimes in monsoon. In fair weather the flow is vertically mixed with a small longitudinal salinity gradient. Under the typically moderate tides, the suspended sediment concentrations are low, the shearing rates in the flow are low to moderate, and aggregation of the flocculated kaolinitic sediment occurs, but the order of aggregation is low, and small diameter aggregates with low settling velocities are formed in suspension. Consequently the waters do not clarify at slack. There appears to be a net flux of sediment from Zuari River towards the Tonca-Surlafonda region where consolidated shoals have formed. During monsoon the flow is stratified, and under increased freshwater flow the salinity drops to near-zero levels. Under these conditions, sediment load in the lower layers of the flow is probably enhanced, and it is likely that there is a net transport of the sediment towards Zuari River. Wind-induced waves appear to play a role in contributing to the suspended sediment load. The overall sediment balance is determined by the cumulative contributions to the transport during fair weather and during monsoon. The canal appears to be well suited for further work in elucidating the mechanisms characterizing suspended cohesive sediment transport in tidal waterways.
The field measurement program was carried out with support from the National Institute of Oceanography, Goa, India, while the first author was a Visiting Scientist at the Institute, in the Ocean Engineering Division. Encouragement given by Dr. S. Z. Qasim, Director and Dr. B. U. Nayak, Head of the Ocean Engineering Division, made the field investigation possible. The study was completed at the University of Florida during the period when related cohesive sediment transport studies were supported by the U.S. Environmental Protection Agency (Grant No. R806684010) and the U.S. Geological Survey.


ABSTRACT........................................................... ii
ACKNOWLEDGEMENT....................................................... ii
LIST OF TABLES........................................................ v
LIST OF FIGURES....................................................... vi
I. INTRODUCTION.................................................... 1
1.1 Estuarine Fine Sediment Dynamics.......................... 1
1.2 Scope of the Present Investigation........................ 6
II. FIELD INVESTIGATION............................................. 9
2.1 Cumbarjua Canal............................................ 9
2.2 Field Measurements......................................... 14
2.2.1 Bathymetry........................................ 14
2.2.2 Tides............................................... 21
2.2.3 Discharge........................................ ... 21
2.2.4 Time-Velocity Records........... ................. 21
2.2.5 Wind................................................ 25
2.2.6 Sediment............................................ 25
3.1 Scope...................................................... 33
3.2 Bottom Sediment Analysis.................................. 33
3.2.1 Grain Size.......................................... 33
3.2.2 Minerals............................................ 35
3.2.3 Organic Matter..................................... 35
3.2.4 Cation Exchange Capacity........................... 36
3.2.5 Fluid Composition.................................. 36
3.3 Bed Roughness and Time-discharge Relationship ........... ..37
3.4 Mechanisms Controlling the Rate of Aggregation.......... ..40
3.5 Order of Aggregation and Transport........................ 43
3.6 Shearing Rates in the Canal............................... 49
3.7 Settling Velocity.......................................... 52


TABLE OF CONTENTS (Continued) Page
3.8 Sediment Transport Rate................................... 56
3.9 Wind Effect................................................ 57
3.10 Mode of Transport in the Canal............................ 59
IV. RECOMMENDATIONS FOR FURTHER WORK............................... 62
V. REFERENCES...................................................... 65


Table Title Page 2-1 Canal End Widths and Depths.................................. 9
2-2 Estimated Monthly Fresh Water Outflows....................... 11
2-3 Maximum Currents and Salinity................................ 12
2-4 Suspended Sediment Loads in the Canal (after
Rao, et al., 1976)........................................... 14
2-5 Measurement Stations......................................... 17
2-6 Dimensions of the Four Cross-Sections........................ 20
2-7 Current Profiles for Discharge Measurement................... 20
2-8 Measured Discharge........................................... 20
3-1 Properties of Brunswick Harbor Sediment (after Krone, 1963).. 44 3-2 Computation of u* using Eq. 3-17............................. 53
3-3 Parameters for Eq. 3-16 and Computed Values of w............. 53
3-4 Sediment Transport Rates at Station I........................ 57


1.1 Schematic Representation of Transport and Shoaling
Processes in the Mixing Zone of the Estuary, including
Ebb Predominance Factors..................................... 4
1.2 Longitudinal Salinity and Suspended Sediment
Concentrations in the Hooghly River Estuary (India).......... 7
2.1 Cumbarjua Canal Connecting the Mandovi River and
the Zuari River, Goa......................................... 10
2.2 Monthly Salinity Distributions in the Canal; Ebb,
---- Flood (after Rao, et al., 1976)......................... 13
2.3 Selected Stations for Field Investigation.................... 15
2.4 Centerline Depth Profile in the Canal........................ 16
2.5 Canal Cross-sections at Stations 1, 2, 3 and 4............... 18
2.6 View of the Canal near Station 2 ............................ 19
2.7 Tidal Measurements at Stations 1, 2, 3 and 4 on
Feb. 27, 1980................................................ 22
2.8 Vertical Velocity Profiles at Station 1 for
Discharge Determination...................................... 23
2.9 Vertical Profiles at Station 4 for Discharge
Determination................................................ 23
2.10 Time-Velocity Records at Stations 1, 2, 3 and 4.............. 24
2.11 Wind Record at Station 1, Feb. 27, 1980....................... 24
2.12 Vertical Velocity Profiles at a Fixed Lateral
Position at Station 1, 0845-1200 on Feb. 27, 1980............ 26
2.13 Vertical Velocity Profiles at a Fixed Lateral
Position at Station 1, 1230-1530 on Feb. 27, 1980............ 26
2.14 Vertical Velocity Profiles at a Fixed Lateral
Position at Station 1, 1600-1700 on Feb. 27, 1980............ 27
2.15 Time-Suspended Sediment Concentration Profiles
over Depth at Station 1, Feb. 27, 1980....................... 29
2.16 Surficial Time-Suspended Sediment Concentration
Profiles at Stations 1, 2, 3 and 4........................... 30
2.17 Typical Canoe used in the Field Program...................... 32
3.1 Grain Size Distribution of Bottom Sediment from
Station 3.................................................... 34


LIST OF FIGURES (Continued) Page
3.2 Computed Time-Discharge Relationships for
Feb. 27, 1980: a) Station 1, b) Station 4.................... 41
3.3 Typical Variation of the Depth-mean Velocity Over
a Tidal Cycle in an Estuary.................................. 47
3.4 Shearing Rate as a Function of Elevation above
the Bed Over a Tidal Cycle An Illustrative Example......... 47
3.5 Bed Shear Stress and Aggregate Shear Strength
An Illustrative Example...................................... 47
3.6 a) Laterally Averaged Bed Shear Stress Variation with
Time in the Canal, Station 1, Feb. 27, 1980 b) Shearing Rates in the Canal at Elevations
z/h = 0.1, 0.5 and 0.9, Station 1............................ 50
3.7 a) Laterally Averaged Bed Shear Stress Variation with
Time in the Canal, Station 4, Feb. 27, 1980 b) Shearing Rates in the Canal at Elevations
z/h = 0.1, 0.5 and 0.9, Station 4............................ 51
3.8 Normalized Suspended Sediment Concentration Profiles
in the Canal (based on data obtained on Feb. 27, 1980)....... 54
3.9 Transport of Bank-derived Suspended Sediment in the
Presence of Wind-generated Waves............................. 54
3.10 Normalized Suspended Sediment Concentration in the
Presence of Wind-............................................. 58
3.11 Sediment Entrainment near East Bank.......................... 58


1.1 Estuarine Fine Sediment Dynamics
Cohesive sediments are comprised largely of terrigenous clay-sized particles plus fine silts. The remainder includes biogenic detritus, algae, organic matter, waste materials and sometimes small quantities of very fine sand. Although in water with very low salt concentrations (less than 1-2 parts per thousand) the sediment particles can be found in a dispersed state, small amounts of salts are sufficient to cause the electrochemical surface forces on these particles to become attractive, with the result that the particles aggregate to form flocculated units which possess settling velocities that are much larger than those of the individual particles. The transport properties of the aggregates of a given sediment are affected both by the hydraulic conditions and by the chemical composition of the fluid. Most estuaries contain abundant quantities of cohesive sediments which usually occur in the flocculated form in various degrees of aggregation. Therefore, an understanding of the transport properties of cohesive sediments in estuaries requires a knowledge of the manner in which the aggregates are transported in these waters. Sediment movement in estuaries is an integral component of the natural phenomena which are characteristic of these water bodies. The necessity of improving the current level of understanding of this phenomenon is evident upon examination of the effects of these sediments on the following two factors involved in estuarial management.
The first pertains to water quality for aquatic biota. The effects of sediments on water quality for aquatic biota include limitation of the penetration of sunlight and the sorption of toxic compounds from solution. The concentrations of nutrients for algae in some estuaries are often sufficient to cause excessive algae blooms. The rate of multiplication of algae in such estuarial waters is limited by a reduced light supply resulting from high turbidity caused by suspended sediment particles. Estuarial waters are often used by industries as convenient dump sites for waste products. Pollutants such as heavy metals, pesticides, herbicides, and organics are often found sorbed on sediment materials with equilibrium between dissolved and sorbed materials frequently favoring the sorbed phase (Ariathurai, MacArthur and Krone, 1977). Due to their property of cohesion, these sediments appear to


provide a large assimilative capacity as well as the transporting mechanism for such toxic compounds. Storage of river waters upstream and their diversion for agricultural, urban and industrial uses will sharply reduce sediment inflows as water resources become scarce. Therefore, it will be necessary to predict the effects of reduced sediment inflows to ascertain the minimum waste management needed to achieve and maintain desirable water quality. Several aspects of water quality problems related to sediment contamination have been discussed in a series of papers edited by Baker (1980).
The second factor concerns the maintenance of navigable
waterways. Under low flow velocities, sometimes coupled with hydraulic conditions which favor the formation of large aggregates, cohesive sediments have a tendency to deposit in areas such as dredged cuts or navigations channels, basins such as harbors and marinas, and behind pilings placed in water. In addition, as described later, the mixing zone between upland freshwater and seawater in estuaries is a favorable site for bottom sediment accumulation. Inasmuch as estuaries are often utilized as commerce routes to the sea, it is desirable to be able to accurately estimate the amount of dredging required to maintain navigable depths in these water bodies, and also to predict the effect of new estuarial development projects such as the construction of a port facility or dredging of additional navigation channels.
Indian estuaries are uniquely characterized by two distinct regimes
- one during the months of monsoon and the other in fair weather. Muddy sediments predominate these coastal features (Ahmad, 1972). During monsoon the suspended loads typically are high, under comparatively large freshwater outflows. In fair weather the loads are lower and the flows primarily tide-induced. For example, in the Hooghly River, the average suspended sediment concentration upstream of Naihati (500 km upstream of Garden Reach) increases from a value which is less than 0.2 gm/liter in dry season to a value in excess of 1 gm/liter in the freshet season, which is a five-fold or more increase in magnitude (Hydraulic Study Department, 1973). As a result of the large amounts of sediments which deposit in the docks, turning basins and navigation channels at many Indian ports on both the coasts, prediction of the rate of shoaling under existing as well as under altered physical conditions is an important consideration in port design.


The transport of fine sediments in estuaries is a complex process involving a strong coupling between the baroclinic flow field and the aggregated sediment. This process has been described extensively elsewhere (Postma, 1967; Partheniades, 1971; Krone, 1972, Kranck, 1980). In Fig. 1.1, a schematic description is given. The case considered is one in which the estuary is stratified, and a stationary saline wedge is as shown. Various phases of suspended fine sediment transport are shown, assuming a quasi-steady state, i.e. a tidallyaveraged situation. In the case of a partially mixed estuary, the description will be modified, but since relatively steep vertical salinity gradients are usually present even in this case, the sediment transport processes will generally remain the same as depicted in Fig.
The vertical variation of the horizontal flows on a tidallyaveraged basis can be conveniently described by computing the ebb predominance factor, EPF, defined as
f0 u(z,t)dt
EPF = (1-1) T TF
foEu(zt)dt + f0 u(z,t)dt
Where u(z,t) = instantaneous longitudinal current velocity at an elevation z above the bed, TE = ebb period and TF = flood period, noting that T = TE + TF, where T = tidal period. If the strengths of flood and ebb were the same throughout the water column, EPF would be equal to 0.5 over the entire depth of flow. This is almost never the case, and usually EPF < 0.5 near the bottom, particularly in the wedge, and EPF > 0.5 in the upper layers. The net upstream bottom current is due to the characteristic nature of flow circulation induced by the presence of the wedge, which means that the strength of this current will decrease as the limit of seawater intrusion is approached, and is theoretically zero at the limit (node) itself (Keulegan, 1966). Distributions of EPF at three locations at the mouth, in the wedge and at the node, will qualitatively appear as shown in Fig. 1.1. When interpreted in terms of the tidal flows, these distributions correspond to the general observation that in the mixing zone of the estuary (i.e. the region


where seawater mixes with fresh water) flood flows landward at the bottom and ebb flows seaward at the surface.
The trends indicated by the EPF distributions suggest the
dominating influence of flow hydrodynamics on sediment movement. As noted in Fig. 1.1, riverborne sediments from upstream sources arrive in the mixing zone of the estuary. The comparatively high degree of turbulence and associated shearing rates will cause the aggregates to grow in size as a result of frequent interparticle collisions, and the large aggregates will settle out because of their high fall velocities. This material will eventually be carried upstream near the bottom to a point where the bed shear stresses at the peak flow velocity are unable to resuspend the material deposited during slack. The sediment will consolidate here and shoals will be formed. Some fine material will be re-entrained through-out most of the length of the mixing zone to levels above the salt water-fresh water interface and will be transported downstream to form larger aggregates once again, and these will settle to the lower portion of the water column as before. At the seaward end some material may be transported out of the system, a portion or all of which could return ultimately with the net upstream current. The strength of this upstream current is often enhanced by the inequality between the flood and the ebb flows induced by the usually observed distortion of the tidal wave. Inasmuch as the low water depth is often significantly less than the depth at high water, the speed of the propagating tidal wave, being proportional to the square root of the depth, is higher at high water than at low water. This typically results in a higher peak flood velocity than peak ebb velocity and a shorter flood period than ebb period. Such a situation tends to enhance the strength of the upstream bottom current, and the sediment is sometimes transported to regions upstream of the limit of seawater intrusion.
The shoals formed in the mixing zone may be periodically scoured by high freshwater discharges (e.g. during the monsoon period in India), and the material will deposit near the estuarine mouth or in the sea. During periods of low freshwater discharge, the sediment will slowly return to the shoal area with the net upstream current. In a typical estuary the sediment residence time in the mixing zone is large, and the


transport rates often are an order of magnitude greater than the rate of inflow of "new" sediment derived from upland sources. The estuarine sedimentary regime is characterized by several periodic (or quasiperiodic) time-scales. These are:
a) The tidal period (diurnal, semi-diurnal, or mixed),
b) One-half the lunar cycle (spring to spring or neap to neap),
c) Yearly cycle,
d) Periods greater than a year, e.g. the 19 year metonic cycle.
Of these, the first is of course the most important since it is the fundamental period which characterizes the "micro-scale" picture of the sediment transport phenomenon in the estuary. The second is important from the point of view of determining net shoaling rates in many cases of engineering interest, and by the same token the third and sometimes the fourth time-scales are involved in considerations of long-term stability and shoaling in estuaries.
As an example of the yearly cycle, Fig. 1.2 shows the depth-mean distributions of salinity and suspended sediment concentrations in the Hooghly River estuary (inset). It is observed that following the freshet season, as the freshwater outflow is reduced, the salinity progresses upstream at a rate of approximately 30 km/month. Thus in November, the penetration is 40-50 km upstream of Gasper Shoals, and in the following April the penetration distance is in excess of 180 km. The mean of these two profiles gives an average for the dry season, as shown. It is observed that the turbidity maximum for the same period occurs at a distance of 90-110 km which is where the dry season salinity (normalized with respect to seawater salinity) is reduced to approximately one-tenth the seawater value. Overall, the Hooghly has several troublesome zones of shoaling, including those in the navigation channel. Two in particular (Auckland and Jellingham) are problematic for the navigation route from the sea (Bay of Bengal) to the port of Haldia (Hydraulic Study Department, 1973).
1.2 Scope of the Present Investigation Collection and analysis of data on suspended sediment transport in a well-defined tidal waterway can be a first step towards improving our understanding of the phenomena of aggregation, settling, consolidation, resuspension, dispersion and advective transport of estuarine fine



sediments. It was felt that the development of a major field measurement program at the National Institute of Oceanography, Goa, for the purpose of obtaining parameters relevant to estuarine suspended sediment transport necessitated an initial effort at a chosen site in the commutable proximity of the Institute. Cumbarjua Canal offers three advantages, namely: 1) it has a reasonably well-defined, "twodimensional" geometry, 2) the bottom sediment is predominantly in the fine size range, and 3) it is at a commutable distance from the Institute. Provided appropriate instruments for measurement are available, this canal appears to be a suitably located body of water for carrying out extensive field investigations. To that end, a preliminary effort was carried out during February, 1980, and the results are reported here. The main objective was to characterize the sediment transport regime in a 10.4 km reach of the canal, so as to facilitate the design of future, more comprehensive data collection experiments.



2.1 Cumbarjua Canal
Cumbarjua Canal (Figure 2.1) connects the Mandovi River estuary with the Zuari River estuary at upstream distances of 14 km and 11 km from their mouths in the Arabian Sea, respectively. The canal is 17 km long, and is wider and deeper at the Zuari end than at the Mandovi end. The dimensions are as followsi: Table 2-1
Canal End Widths and Depths
Location Width at Mean Mean Depth Below Tide Level (m) Mean Tide Level (m)
Zuari end 210 7.5 Mandovi end 25 3.5
At distances of 1.3 km and 4.0 km from the Mandovi end, the canal bifurcates; consequently, the flow pattern near these two junction is somewhat more complicated than in the remainder of the canal (Rao, et al., 1976). The navigable route is utilized for the transport of iron and manganese ore which is carried on 500 DWT barges between the mines in the Bicholim area and Mormugao harbor. The traffic is comparatively heavy during the monsoon period, when the build-up of the Aguada Bar blocks the flow connection between Mandovi River and the Arabian Sea, causing a complete diversion of the barge traffic through the canal. A major segment of the canal has been dredged recently to accommodate larger (1,000 DWT) barges.
The tidal range measured at the Zuari end during three days in
1969-70 varied from 0.54 m to 1.72 m, and the corresponding variation at the Mandovi end was 0.36 m to 2.00 m, with a dominant semi-diurnal constituent (Das, et al., 1972). Considering the tide at Marmagao
1Data for the Mandovi end of the canal are based on survey in the early seventies (Rao, et al., 1976). Data for the Zuari end are based on a 1980 survey, carried out as a part of the present investigation.


20 20 15 5
73 0 45 50 55' T4!
Fig. 2.1 Cumbarjua Canal Connecting the Mandovi River and the Zuari River, Goa. C


73 E



Harbor to be the representative sea tide at the mouths of the two estuaries, the narrower width of the Mandovi and the longer travel distance to the canal through Mandovi in comparison with the Zuari causes the arrival of the tidal wave at the Mandovi end of the canal to lag the arrival at the Zuari end. The corresponding time lags with respect to Marmagao Harbor are 1 hr and 0.5 hr (Rao et al., 1976). In order words, the time of arrival of high or low water at the Mandovi end lags that at the Zuari end by 0.5 hr, which provides a driving force for the tidal motions in the canal.
Inasmuch as the Mandovi has a larger tributary system than the Zuari, the salinity at the Mandovi end of the canal is consistently lower than at the Zuari end. This condition becomes more pronounced during the monsoon period (June-September), when a portion of the Mandovi River freshwater outflow which flows through the canal has a marked influence on the salinity distribution in the canal.
Magnitudes of fresh water flow through the canal are not
available. However, Table 2-2 gives estimates of the outflows through the Zuari and the Mandovi on a monthly basis (Mehta, 1981). The rates are observed to be substantial during July-August, and it is not surprising that the canal becomes a complete, or near-complete fresh water body, since it may be expected that a significant amount of the flow is diverted through the canal. Recorded variations in the currents and salinity at the two ends of the canal are given in Table 2-3 (Rao, et al., 1976).
Table 2-2
Estimated Monthly Fresh Water Outflows
Month Outflow(m3/sec)
Zuari River Mandovi River
June 20 40 July 170 340 August 250 500 September 80 160 October 30 60
November-May Negligible Negligible


Table 2-3
Maximum Currents and Salinity

Location Max. Current Salinity (m/sec) (ppt)
Flood Ebb
Zuari end 0.60 0.60 34.0-35.4 Mandovi end 0.60 0.15 29.0-35.0
Zuari end 0.90 1.10 16-29.6
Mandovi end 0.50 0.75 0-8.5
The data in Table 2-2 indicate a high degree of correlation between ebb dominated currents due to freshwater outflow in the monsoon period and the corresponding reduction in canal salinity. The canal is essentially "flushed out" by the fresh water from Mandovi River. The vertical and the spatial distributions of salinity are shown in Fig. 2.2 on a monthly basis (Rao, et al., 1976). These data illustrate the rather substantial variations in salinity which typically occur over a year. The measurements, which were obtained in 1972, show two significant trends. First, there appears to be a measurable longitudinal movement of the vertical salinity gradients with tide. The length scale of this movement appears to be on the order to 1-2 km. Second, there is also a significant seasonal variation of salinity. Peak, spatiallyaveraged salinity occurs in May, which is the driest month, whereas during July-August much of the canal water has negligible salinity. Following August, as the freshwater outflows from the Mandovi and the Zuari begin to decrease, the salinity begins to rise, and continues to increase until it attains another maximum during the following May. The vertical structure of the flow is correspondingly affected. Thus, during monsoon it may be expected that the flow would be stratified due to the contribution from fresh water flows. During pre-monsoon months, the flow is vertically mixed (Rao, et al., 1976). In Fig. 2.2, some stratification is observed in the October distribution, whereas during the March-June period, except for the region near the Mandovi, the vertical variation in canal salinity does not appear to be significant.


Table 2-4
Suspended Sediment Loads in the Canal (after Rao, et al., 1976)
Month Suspended Sediment Load (mg/liter)
During ebb During flood
January 10-50 10-40 February 10-50 20-70 March 20-80 20-100 May 20-100 40-120 June 30-65 ---July 50-80 20-60 August 70-120 60-90 October 10-60 10-50
Measured suspended sediment loads (Rao, et al., 1976) during the year are given in Table 2-4. noted that the load in May (and possibly in June, during flood), is greater than the load in October and January (and possibly in November and December). The observed magnitudes in February (10-50 mg/liter during ebb and 20-70 mg/liter) are comparable to those reported in this study.
Coupled with salinity changes, water temperature variations can also have a marked effect on cohesive sediment transport rates since increasing the fluid temperature tends to weaken the interparticle bonding forces. At Cumbarjua Canal, however, the yearly variation is comparatively small, ranging between 270C and 320C. The influence of temperature is thus likely to be of secondary importance only.
2.2 Field Measurements
Measurements were carried out during February of 1980. Four
stations were selected as shown in Fig. 2.3. These are identified in Table 2-5.
2.2.1 Bathymetry
Fig. 2.4 shows the centerline depth profile of the 10.4 km reach of the canal between stations 1 and 4. It is noteworthy that the stretch between stations 3 and 4 is characterized by the presence of significant shoals. At low tide, and in the presence of wind-generated waves, these shoals are likely to contribute measurably to the suspended sediment load in the canal. It should be noted that these shoals do not in


GandaulimE Orgao G O ombarjua Oro
Banastarim N O Corlim (D Adcalna
3.8 km
0 Boma
DongrimE 39krfbCundaim
2.7 km (D Tonca 0 1 2 3 4 5km G Agacaim
Fig. 2.3 Selected Stations for Field Investigation.


(2) Distance from Station I (km)
0 1 2 3 4 5 6 7 8 9 10
-3 -

Fig. 2.4 Centerline Depth Profile in the Canal.

Table 2-5
Measurement Stations

Station Location Distance from Station 1
1 Tonca 0
2 Surlafonda 2.7 3 Cundaim 6.6 4 Banastarim 10.4
general extend laterally along the entire width of the canal. Thus the observed depths over the shoals should not be confused with the controlling depths in the navigable channel which does not necessarily run along the canal centerline everywhere. The presence of these shoals near the Zuari end of the canal is an indication that the primary source of sediment in the reach of the canal under consideration is the Zuari River.
Cross-sections at 1, 2, 3 and 4 are shown in Fig. 2.5. Of
particular interest is the comparatively wide section at 1. Here, the western bank is shallow and stretches over a distance of approximately 150 m. A consequence is that as the tide rises above the low water level during flood, i.e. when the flow is towards the Mandovi, the waterline travels this distance of 150 m within minutes, giving the appearance of a much wider canal when it is "bankfull" than the width of its deeper section, which is comparatively narrow. Fig. 2.6 shows the canal near station 2. At low tide when the muddy bottom is exposed, small holes made by various types of burrowing animals are observed everywhere (see Fig. 3.11). Apart from the fact that these biota actively participate in the reworking of the benthic sediments, the perforated bed surface resulting from the presence of these organisms would be expected to influence the bottom roughness, tending to enhance the form drag and hence the energy dissipation at the bed.
Water level datum indicated in Fig. 2.5 is the mean tidal elevation derived from water surface profiles obtained on February 27, 1980. This datum should not be confused with the hydrographic datum, which is not considered here. Dimensions of the four cross-sections are given in Table 2-6.


Datum Instantaneous w.s. during Discharge Measurement

Profile ISETO I
Float Datum
_w-Float 3Datum
Datum Float
, -Instantaneous w. s. during Discharge Measurement SECTION 4
S10 203040 50 km SImn Scales

Fig. 2.5 Canal Cross-sections at Stations 1, 2, 3 and 4.

East Bank




View of the Canal near station 2

Fig. 2.6

Dimensions of

Table 2-6 the Four Cross-Sections*

Section Mean Depth Maximum Depth Width Area
(m) (m) (m) (m2)
1 2.54 6.1 315 800 2 3.23 7.1 153 494 3 2.68 4.3 194 520 4 3.63 6.1 103 374
*Relative to selected mean tidal datum
Table 2-7
Current Profiles for Discharge Measurement
Station Date Stage Time Number of Period Profiles
1 Feb. 14, 1980 ebb 1130-1157 6 4 Feb. 26, 1980 ebb* 1210-1245 5
*close to slack
Table 2-8
Measured Discharge
Station Date Stage Time Discharge (m3/sec)
1 Feb. 14, 1980 ebb 1144 247 4 Feb. 26, 1980 ebb 1228 61


2.2.2 Tides
Tidal measurements at the four stations obtained on February 27, 1980, are shown in Fig. 2.7. At each station, the water level was recorded on a graduated pole installed near the east bank and leveled with reference to a temporary bench mark. The selected datum for each station is the mean tide level applicable only to the corresponding record shown. Its relationship to the local hydrographic station, or
the relationship between the four selected datums, are not known The tidal range is observed to be approximately 1.3 m, and the time between high and low waters is approximately 9 hrs, indicating that the tide was probably of a mixed type on this day. Low water at station 4 is observed to lag the low water at station 1 by approximately 1.2 hr.
2.2.3 Discharge
For discharge determination, vertical velocity profiles were
obtained at stations 1 and 4, using a small Savonius rotor-type current meter designed at the National Institute of Oceanography. Characteristics of the measurements are given in Table 2-7.
Because of the comparatively short duration over which the velocity profiles were obtained at each section, they may be construed to yield the instantaneous discharges. The profile positions, and the position of the instantaneous water surface are shown in Fig. 2.5. Profiles themselves are given in Figs. 2.8 and 2.9. At station 1, no flow was recorded at the position of profile 6. At station 4, although surface currents were ebbing, a current reversal is observed to have had occurred near the bottom. Discharges computed on the basis of these profiles are given in Table 2-8.
2.2.4 Time-Velocity Records
Time-velocity records were obtained at the four stations on
February 27, 1980 and are shown in Fig. 2.10. Whereas surface floats (float sphere diameter was 30 cm) were employed at stations 2, 3 and 4, a Savonius rotor current meter was used at station 1. Lateral position of the float at each station is shown in Fig. 2.5. A metal cross-piece
2Attempts to tie the mean tide datum with the hydrographic datum recorded on two benchmarks, one on a road bridge at Banastarim and the other in the Tonca vicinity yielded apparently spurious results.


Tide at Stations 1 2 3 and 4 Feb.27- 1980

w CL)

Fig. 2.7 Tidal Measurements at Stations 1, 2, 3 and 4 on Feb. 27, 1980.


04 1-




12 15 16 17 18 19 TIME (hrs)

, ,


10 11


-04 -










Velocity Profiles 6t I for Discharge, Feb. 14,1980
Position Time h(m)



1140 1145 1152 1157

4.25 6.25
425 1.5G


0.4 0.6'
u (m/sec)



Fig. 2.8 Vertical Velocity Profiles at Station 1 for Discharge Determination.

Velocity Profiles at4 for Discharge, Feb.26,1980

S2 o 3 4 D 5

Time 1010
1220 1230 1235

320 5.20
2.20 5.20 5.20







u ( m/sec)



Fig. 2.9 Vertical Profiles at Station 4 for Discharge Determination.













a a
z ,

Wind at Station I, Feb.27,l Elev. : 1.5 m above w.s. Direction : along Canal Axis



/ I I I
/ %



TIME (hrs)


Wind Record at Station 1, Feb. 27, 1980.


0.4 0.2

Currents at Stations 1,2,3 and4, Feb. 27, 1980
--o--- I 5.Om above bed(current meter)
2 0.5m below instantaneous w.s.(float)
---- 3 0.5m below instantaneous w.s.(float) o 4 1 .Om below instantaneous w.s. (float)
810 12 14.--

Time-Velocity Records at Stations 1, 2, 3 and 4.





- A



Fig. 2.11






was attached to the float at station 4 (each of the four fins of the cross-piece was 28 cm long, 20.5 cm wide and 1 cm thick). The effective depth at which this float may be considered to have recorded the current velocity was 1 m. At station 1, the current meter was used to obtain "instantaneous" velocity profiles at a position shown in Fig. 2.5. The profiles themselves are plotted in Figs. 2.12, 2.13 and 2.14.
Fig. 2.10 indicates that the maximum ebb velocities were on the
order of 0.4 to 0.5 m/sec. Low water slack at station 4 lagged the low water at station 1 by 0.75 hr.
2.2.5 Wind
Wind was recorded at station 1 using a hand-held anemometer (made by OTA Keiki Seisakusho (OTH), Japan) on February 27, 1980 (Fig.
2.11). Wind direction was approximately along the axis of the canal (and therefore normal to the cross-section), and into the canal. Although no wind speed was recorded at stations 2, 3 and 4, the general observation was that the wind speed decreased from station 1 to 4, and
inf act at 4, there was very little wind. It is noted that the speed at station 1 reached a maximum of 4.5 m/sec at 1430 hr. The observed temporal distribution is characteristic of the onshore wind during this part of the year.
During those times when the wind speed was appreciable, a wave
generation, growth and breaking phenomenon was observed, particularly at stations 1 and 2, and especially after the flow reversed in the late afternoon, at the onset of flood flow. The waves reached heights of the order of 0.15 m at breaking, causing a noticeable degree of sediment resuspension along the eastern bank, where the breaking was most pronounced.
2.2.6 Sediment
One of the main objectives of the field investigation was the measurement of the suspended sediment load in the canal. Suspended sediment concentrations were obtained at the four stations on February 27, 1980, simultaneously with measurements of tides, currents and wind. At station 1, the measurements were obtained with a van Dorn bottle (of 2 liter capacity) at three elevations "surface," "middepth" and "bottom." The samples were stored in 2 liter plastic bottles. Each sample was dried, filtered through 0.45 micron Millipore






zjh2 Ebb




Vertical Velocity Profiles at a Fixed Lateral

rI'1u11 COL










r L

(Y71 nor% 0N45 6.1 0940 7.1 1040 7.1 1100 7.1 1130 6.1 1200 6.1

mi rmu. r/


Vertical Velocity Profiles at a Fixed Lateral Position at Station 1, 1230-1530 on Feb. 27, 1980.

0 E a E 0 E SE SE


1230 1330
1400 1430 1500 1530

6.1 6.1



+ 'f~ '



Fig. 2.12


E bb ,

Fig. 2.13




Stage Time h(m)

0.4 +


- -I I ~Food






Time h(m)
1600 6.1 1630 6 1 17005.1

I Ebb



u (m/sec)
Fig. 2.14 Vertical Velocity Profiles at a Fixed Lateral Position at Station 1,
1600-1700 on Feb. 27, 1980.




paper using the standard vacuum filtration procedure and weighed in an electronic balance accurate up to four decimal places (made by Dhona, model HD/100). The time-concentration profiles are shown in Fig.
2.15. They indicate a qualitative trend which is in agreement with the current profiles shown in Fig. 2.10. Peak concentrations on the order of 30-40 mg/liter occurred between 1330 hr and 1430 hr. During this period, the ebb flow was past its peak value at station 1, and was on
the order of 0.3-0.4 m/sec. This was however coupled with a significant wind, which varied between 3.5 and 4.5 m/sec (Fig. 2.11). Since the wind direction was opposite to that of the flow, the drag on the surface would have tended to reduce the surficial current speed. However, the breaking of the wind generated waves on the east bank was observed to have resuspended a considerable amount of muddy sediment. This material was transported towards the centerline of the canal by the lateral secondary currents, where it was kept in suspension at the surface by the comparatively high degree of surface turbulence. Figure 2.15 shows that this infact resulted in a higher concentration of the sediment at the surface than at mid-depth or at the bottom. In order words, the "inverse" vertical gradient of sediment concentration, for instance at 1430 hr, can be explained by the observation that the primary source of the surficial sediment was not the channel bed but the bank.
Figure 2.16 shows surficial time-concentration profiles for the four stations (record for station 1 corresponds to the surface measurement shown in Fig. 2.15). Comparing these profiles with the current records (Fig. 2.10) and wind record (Fig. 2.11), it is recognized that the effect of the wind is strongest at station 1, and weakest at station 4, where the concentration variation seems to correlate primarily with the current.
Bottom sediment samples were collected at stations 1 and 4, using a grab sampler. It was noted that the sediment at station 1 was coarser than at station 43. This observation appears to be in agreement with the time-concentration profiles of Fig. 2.16. Thus it is noted that even though the flow velocities in the reach of the canal under
3This was a qualitative observation as no grain size analysis was performed on samples obtained at stations 1, 2 and 4.


Suspended Sediment at Station ', Feb. 27, 1980
Surf acez
S8 10 12 14 16 18
TIME (hrs)
Fig. 2.15 Time-Suspended Sediment Concentration Profiles. oVer Depth at Station 1 Feb. 27, 1980.

Suspended Sediment at Stations 1,2,3 and 4, Feb. 27,1980 I- 0.25m depth below instantaneous w.s.

m depth below instantaneous w.s N3


TIME (hrs)


Fig. 2.16 SUrficiaV Time-Suspended Sediment Concentration Profiles at Stations 1, 2, 3 and 4.















I ,






3 0.5



investigation were of the same order of magnitude (Fig. 2.10), the concentration in suspension at station 1 was generally lower than that at station 4 until about 1330 hr. This may be attributed to the larger grain size of the bed material at station 1, causing it to be resuspended with greater difficulty than that at station 4. The rather rapid increase in the concentration after 1330 hr is likely to be due to the transport of comparatively finer material derived from the banks, as noted previously4.
Locally available canoes were utilized in the field program, for current and sediment measurements. The small draft makes such a vessel useful in canals such as Cumbarjua, where the small depths in the shallower portion of the flow cross-section limit the use of boats with

outboard engines.

Fig. 2.17 shows one such canoe.

4Caution is warranted when interpreting the degree of resuspension in terms of grain size alone. When the bed material is cohesionless, it is a reasonable expectation that the larger the grain size, the lower the amount of material in suspension. When the material is a mixture of cohesionless and cohesive sediments, the same interpretation is applicable to the cohesionless portion of the sediment, when the latter is the predominant constituent by weight. With increasing fraction of the cohesive component, the mixture has a tendency to behave as a composite cohesive unit, and increasing consideration must be given to the flocculation characteristics of the sediment, and therefore to the size and shear strengths of the aggregates composing the bed. Individual grain size will be of even lesser significance when the material is primarily in the clay range. Aggregation is discussed in Chapter III, briefly.



3.1 Scope
The limited data collected in this study enable a cursory appraisal of the sedimentary processes in the canal. A detailed description must await a comprehensive data collection program which covers periods of spring, mean and neap tides, at least once in fair weather and once during monsoon. Such a measurement program will be of particular importance in elucidating the mechanism for the observed long-term shoaling in the canal. Some of the important features relevant to fine sediment transport are highlighted in the sequel. In Section 3.2 certain physical and physico-chemical properties of the bottom sediment collected at station 3 are described. In Section 3.3 bottom roughness and time-discharge relationships are derived for stations 1 and 4 based on, 1) instantaneous discharge measurements at the two stations (see also Section 2.2.3) and 2) time-velocity data obtained at these stations (see also Section 2.2.4). Following this a brief description of the mechanisms which influence the rate of particle aggregation is given in Section 3.4. In Section 3.5, the role of aggregation in characterizing the transport of fine sediments is illustrated by a hypothetical (typical) example which is further highlighted by computations based on the canal data in Section 3.6. In Section 3.7 an attempt has been made to obtain a few representative values of the settling velocity of the suspended aggregates, and in Section 3.8 the role of wind in influencing the vertical distribution of the suspended sediment concentration (and therefore the settling rates) is briefly discussed. Finally in Section
3.9 a tentative description of the overall transport process is attempted.
3.2 Bottom Sediment Analysis
3.2.1 Grain Size
Fig. 3.1 shows the grain size distribution of the dispersed
sediment. It is noted that the percentage of particles greater than 0.06 mm, i.e. in the sand range, is no more than about 5, indicating that at station 3, where the sample was collected, the material is almost entirely in the fine range. Fifty-seven percent of the material is clayey and the remaining is in the silt size range. This type of material is generally very cohesive.


i-r I I I I -i l I I I I 1Ji l --I +- Sand -I Silt +|- Clay

Bottom S

ediment Station 3 1I I 1 1 1 1 1 1 1 1 1

2 0.1 0.05 0.02 0.01 0.005

0.002 0.001 0.0005 0.0002

Fig. 3.1 Grain Size Distribution of Bottom Sediment from Station 3.






w 13-




The size distribution was determined by the standard hydrometer test (Bauer and Thornburn, 1958) with the modification that the sample was not dried initially for obtaining the total dry weight of the material used in the test. This was done in accordance with the observation made by Krone (1962), and confirmed in the present investigation, that if the sample is dried, it is likely that it will not redisperse completely even when a sufficient amount of the dispersing agent (sodium hexa-metaphosphate) is added. This in turn means that the subsequently measured size distribution will indicate larger particle sizes in comparison with those obtained by using the
original wet sample. For this reason, the total dry weight of the sample was obtained from a separate sub-sample, and the corresponding value for the test sample was calculated by assuming that both samples had the same water content.
3.2.2 Minerals
X-ray diffraction analysis of 1) the bulk sample, 2) less than 2
micron sample and 3) glycolated, less than 2 micron sample indicated the presence of clay minerals kaolinite (as predominant constituent), illite and montmorillonite. Among non-clay minerals, quartz was identified. Traces of other clay and non-clay minerals appear to be present as well, but their identification requires further confirmatory tests.
It may be expected that although the percentage of coarse material varies spatially in the stretch of the canal under investigation (see Section 2.2.6), the clay mineral constituents are likely to be wellmixed inasmuch as the canal is relatively short and their relative amounts are probably invariant throughout. Reworked fine sediments in estuaries tend to exhibit such a uniformity, as in San Francisco Bay (Krone, 1962).
3.2.3 Organic Matter
The sediment sample was found to contain 6.9% organic matter by
weight using the Walkley-Black procedure (Allison, 1965). It is likely that the organic content shows a spatial as well as seasonal variation. A comprehensive sediment sampling program is required to confirm (or reject) this hypothesis. Sorption of organic molecules on a clay surface has a considerable influence on the clay behavior in suspension. The subject matter is vastly complicated by the variability


in the type of clay and in the composition of the organic matter. An additional time-dependent factor arises from biodegradation which markedly influences the stability of dilute, fine sediment suspensions (Luh and Baker, 1970).
3.2.4 Cation Exchange Capacity
The cation exchange capacity, CEC, is a useful property of fine sediment, clay minerals in particular, and is defined as the number of (exchangeable) cations from the pore fluid that are attracted to the negatively charged surfaces of clay particles per unit surface area or per unit weight of the sediment. The CEC value as well as the kind of exchangeable cations present have an important influence on the sediment behavior. It is usually measured in terms of milliequivalents per 100 gm. The sediment sample analyzed was determined to have a CEC value of 94 meq/100 gm using the procedure described in the USDA Soil Survey Investigations Report No. 1, 1972. This high value, which falls within the range of CEC values typically found for montmorillonitic clay minerals, indicates that the sediment sample analyzed has a high level of activity. Since it was found that the sediment contains a larger quantity of kaolinite (which has a reported range of 3-15 meq/100 gm) than montmorillonite, it is believed that the high CEC value obtained is at least partially attributable to the organic matter present in the sediment, as CEC values ranging from 150 to 500 meq/100 gm have been reported for the organic fraction of some soils (Grim, 1968).
3.2.5 Fluid Composition
A sample (9 ml) of the supernatant fluid associated with the
sediment was collected with the help of a pipette and subjected to analysis for pH, Na+, K+, Ca+, Mg+, Mn", Fe and Cl~, with the following results: pH = 7.8, Na+ = 2,800 ppm, K 115.0 ppm, Ca = 56.0 ppm, Mg 189.0 ppm, Mn = 2.8 ppm, Fe = 0.2 ppm, and Cl = 1,200 ppm. The total salt concentration was measured to be 30,500 ppm. The relative abundance of the cations Na+, Ca and Mg which typically (and in the present case as well with the exception of K+) are dominant in the fluids associated with soils, may be characterized by the Sodium Adsorption Ratio, SAR, defined as:
SAR Na (3-1) (Ca" + MgH-)]1/2


where the concentrations are in milliequivalents per liter (Arunlanandan, 1975). The above values of the concentrations of Na+, Ca++ and Mg++ yield SAR = 28. The pH indicates that the fluid was slightly basic.
The magnitude of SAR in the pore fluid signifies the degree of flocculation of the sediment. As SAR increases, soil flocculation decreases, with inter-particle bonds weakening and surface soil particles detaching more easily. It must however be assumed that in Cumbarjua Canal, the surficial sediment deposit is periodically resuspended, and since during fair weather the salinity does not vary significantly with time, it may be expected that the pore and the eroding fluids have similar ionic compositions. Experimental evidence using distilled water as the eroding fluid (Arulanandan et al., 1973) indicates that for a given pore fluid SAR in the neighborhood of the value measured in this study (i.e. 28), increasing the salinity (NaCl) in the eroding fluid suppresses the erodibility of the soil. Therefore, whereas based on the pore fluid SAR = 28 one may be tempted to conclude that at this relatively high value of SAR the soil may possess a low degree of flocculation and therefore relatively high erodibility, it is essential to refrain from arriving at such a conclusion inasmuch as the influence of the eroding fluid composition on erodibility must also be taken into consideration. At the present time there appears to be no realistic substitute for testing the sediment in a laboratory flume in order to measure the critical shear stress and the rates of erosion under applied bed shear stresses, for characterizing the sediment erosion potential.
In the present study the eroding fluid composition was not measured directly. However, it is reasonable to assume that the sediment in the surficial bed layers is in equilibrium, or at least quasi-equilibrium, with the eroding fluid. The measured pore fluid salt concentration of 30,500 ppm is likely to be close to the salt concentration in the eroding fluid. With reference to Fig. 2.2, the measured value appears to be in agreement with previous measurements during the month of February.
3.3 Bed Roughness and Time-discharge Relationship
Computation of bed roughness and the determination of the timedischarge relationship from velocity measurements are part of a general


procedure which was reported by Mehta, Hayter and Christensen (1977) previously. The following steps are involved:
a. At the selected cross-section, the instantaneous discharge, Qm
is measured.
b. Qm is equated to the discharge Q computed analytically from the
following expressions:
Q = ) AQ, (3-2) i= 1
2.5 VgS(d ) 3/2AW I
A~i -m ii i
=1-Q (3-2a) K' (1-D )
k 1-2Ki
1 29.7(d (I + 29.7(dK /
1 M n +11 [9 7( i~ m 3/2 (32b
I = en I k +1] +1 1 (1-o) dw (3-2b)
where the cross-section has been divided into m sub-sections
and i refers to the i-th sub-section. Here, Ki is defined as the degree of fullness of the sub-section (K = 1 if the subsection bottom is horizontal), AW = width of the sub-section, AQ = discharge through the sub-section, dmi, ami-I = the two
end depths of the sub-section, Di = dmi-1/dmi, w = dummy
variable, S = slope of the energy grade line (assumed to be
invariant across the section) and k5 = Nikuradse bed roughness
of the cross-section. Two basic assumptions inherent in the
derivation of Eqs. 3-2,a,b are: 1) the time-mean value of the
bed shear stess is proportional to the local depth, and 2) the velocity profiles are logarithmic in the vertical. The flow is
considered to be in the hydraulically fully rough range. To
the extent that these assumptions are constrained in any given situation, the method considers the system to be an equivalent idealized open channel. The unknown in Eqs. 3-2a,b, when Q is
matched with Qm, is ks, whose value can thus be computed
through a numerical iterative procedure.


c. At some suitable position in the cross-section, a current meter
is installed for recording the variation of the velocity, uc,
with time, t, together with a tide gage for obtaining the corresponding record of the water surface elevation, n(t).
d. Knowing uc(t), n(t) and ks, the following equations are
utilized to yield Q(t):
Q(t) = E(t).Uc(t) (3-3) where
i (d 3/2
) (dm i WIi
E = (3-3a) 1/2 29.7pd (-+
d n k c+ 1)
1 -K
K K i(3-3b)
(-f) (1-Di
I i
where dc(t) is the water depth at the site of the meter, p = rc/dc and rc = elevation of the meter above the bed. Application of the method to shallow waterways similar to Cumbarjua Canal has yielded
reasonably good results which have been verified through independent
measurements of discharge (Mehta and Sheppard, 1979).
As noted in Section 2.2.3, Qm = 247 m3/sec and 61 m3/sec were
obtained at stations 1 and 4, respectively. These yield corresponding values of ks = 0.172 m and 0.01 m. The comparatively high value at station 1 appears to be consistent with the presence of shoals in the vicinity, which appear to have altered the flow boundary layer in such a way as to indicate an effectively higher degree of bed resistance to the flow at this station. Next, with these values of ks, and uc(t) and n(t) derived from data given in Figs. 2.10 and 2.7, respectively, Q(t) is computed as per the described method. It should be noted that while uc(t) should correspond to current measured at a fixed elevation above the bed, the data given in Fig. 2.10 for station 4 were derived from float observations. Since the float elevation above the bed varied with


the water surface elevation, it became necessary to obtain a corresponding record of flow velocity at a fixed elevation above the bed, as required. Assuming a logarithmic velocity distribution in the vertical, the measured current um(t) can be converted to uc(t) utilizing the following relationship:
Zn(29.7 k+ 1)
u (t) = s u (t) (3-4)
cz (t) m
Xn(29.7 mk + 1)
where zm(t) is the float elevation above the bed. A complete description of the programming effort and examples have been given elsewhere (Hayter, 1979). The time-discharge plots for stations 1 and 4 are shown in Fig. 3.2a, b. The peak ebb discharge was 253 m3/sec at station 1 and 151 m3/sec at station 4 on February 27, 1980. The flow reduction over the 10.4 km distance appears to be significant and must be attributed to energy dissipation at the bed.
3.4 Mechanisms Controlling the Rate of Aggregation
Under estuarine conditions, suspended particles in the clay size range, and to a lesser degree in the silt range, become cohesive as a result of the mutually attractive electro-chemical surface forces on the particles. When subjected to repeated collisions the particles combine to form comparatively large aggregates, each consisting of perhaps thousands or even millions of individual particles. The size, the strength and the density of the resultant aggregates play an important role in characterizing the transport of cohesive sediments under tidal conditions.
There are three principal mechanisms of inter-particle collision in suspension, and these influence the rate at which particle aggregation occurs (Ariathurai, MacArthur and Krone, 1977; Hunt, 1980). The first is due to Brownian motion resulting from thermal motions of molecules of the suspending ambient medium. The frequency of collision, I, on a given particle by other particles has been given by Whytlaw-Gray and Patterson (1932) as:
4kTn (3-5) 3V


IO 100 50

ks =0.172m

- 0.010 m




TIME (Hours)



TIME (Hours)

Fig. 3.2 Computed Time-Discharge Relationships for Feb. 27, 1980:
a) Station 1, b) Station 4.



100 50

= 0'

0 C',






where k = Boltzmann constant, T = absolute temperature, n = number concentration of suspended particles and p = dynamic viscosity of the fluid (water). Under typical conditions at 20*C, I = 5 x 10-12n collisions per second. Generally, aggregation rates by this mechanism are too slow to be significant in estuaries unless the suspended sediment concentration exceeds 10 gm/liter. Aggregates formed by this mechanism are weak, with a lace-like structure, and are easily dispersed by shearing in the flow or are crushed easily when deposited (Ariathurai, MacArthur and Krone, 1977).
The second mechanism of inter-particle collision is that due to internal shearing produced by the local velocity gradients in the fluid. Collision will occur if the paths of the particle centers in the velocity gradient are displaced by a distance which is less than the sum of their radii which is referred to as the collision radius, Ri, between i-size and J-size particles. The frequency of collision, J, on a suspended spherical particle was derived by Smoluchowski (1917) as
4 3
J n R G (3-6)
where G is the local velocity gradient. Aggregates produced by this mechanism tend to be spherical, and are relatively dense and strong because only those bonds that are strong enough to resist the internal shearing due to local velocity gradients can survive. The
product n R is large when aggregates are mixed with a large number of dispersed particles, as in the case of an estuarial mixing region.
The third mechanism of inter-particle collision results from the fact that particles of different sizes have different settling velocities. Thus a larger particle, due to its higher settling velocity, will collide with smaller, more slowly settling particles along its path and will have the tendency to "pick up" these particles on its way down. The frequency of collision, H, due to this mechanism has been obtained by Fuchs (1964) as
H = rER n AW (3-7)


where E = a capture coefficient and AW' = relative velocity between particles. This mechanism produces relatively weak aggregates and contributes to the often observed rapid clarification of estuarial waters at slack.
All three mechanisms operate in an estuary, with J and H generally being dominant in the water column excluding perhaps the high density near-bed layer, where Brownian motion is likely to contribute significantly as a collision mechanism. Then again, J is probably more important than H during times excluding those near slack, when collision and coherence due to differential settling would be expected to be the main mechanism controlling the rate of aggregation.
3.5 Order of Aggregation and Transport
Given the mechanisms which influence the rate of particle
aggregation in the estuary the order of aggregation, which characterizes the packing arrangement, density and shear strength of the aggregates, is determined by the following factors: 1) sediment type, 2) fluid composition, 3) local shear field, and 4) concentration of particles available for aggregation.
Primary or 0-order aggregates consist of highly packed arrangements of primary particles, with each aggregate consisting of perhaps as many as a million particles. Typical values of the void ratio (volume of pore water divided by volume of "solids") have been estimated to be on the order of 1.2. This is equivalent to a porosity of 0.55, which is a more "open" structure than commonly occurs in cohesionless sediments (Krone, 1963). Continued aggregation under favorable shear gradients can result in the formation of loosely packed arrays of 0-order aggregates. Each succeeding order consists of aggregates of lower density and lower shear strength. Experimental observations (Krone, 1963; 1978) tend to indicate the following approximate relationship between the shear strength, Ts, and floc density, Pf, for many (although not all) sediments
Ts = a(P -1) (3-8)
where a and a are coefficients which must be determined experimentally for each sediment. As a result of the fact that the shear field in the


estuary exhibits significant spatial and temporal variations, a range of aggregates of different shear strengths and densities are formed, and the highest order is determined by the prevailing shearing rate, provided that the sediment and the fluid composition remain invariant, and given that sufficient number of suspended particles are available for promoting aggregation.
The determination of Ts and pf corresponding to each sediment-fluid mixture can be carried out through rheological diagrams of applied shear stress against the shearing rate, du/dz. Such plots were developed by Krone (1963; 1978) with the help of a specially designed annular viscometer. Each order of aggregation corresponds to a given volume fraction of the aggregates (volume occupied by the aggregates divided by the total volume of the suspension) which in turn can be shown to be related to the relative differential viscosity, i.e. the viscosity of the suspension divided by the viscosity of the suspending medium (water). Given the viscosity of the suspending medium, the relative differential viscosity is determined from the slope of the rheological diagram, and hence the volume fraction can be calculated. pf is then computed from the volume fraction. The intercept on the applied shear stress axis of the rheological diagram corresponds to Ts'
Table 3-1 gives the order of aggregations, shear strength and density of Brunswick Harbor, Georgia sediment aggregates. The mineralogical composition of this sediment is similar to the one from Cumbarjua Canal.
Table 3-1
Properties of Brunswick Harbor Sediment (after Krone, 1963)
Order of Density Shear Strength
Aggregation pf(gm/cm3) Ts(N/m2)
0 1.164 3.40 1 1.090 0.41 2 1.067 0.12 3 1.056 0.062


The average internal shearing in a fluid, G, is obtained from
G I = dz (39) 11 11
where P = energy dissipated per unit volume of the fluid, V = dynamic viscosity and T = shear at any elevation z above the bed. This relationship is obtained by considering the balance of forces and conservation of energy for a differential fluid element (Streeter and Wylie, 1975). In the laminar case, G = du/dz, whereas in the turbulent case Eq. 3-9 is an approximation (Friedlander, 1977). In the rheological experiments of Krone (1963), shearing was produced at relatively low speeds at which viscous forces were dominant. Eq. 3-9 can be utilized to calculate the shearing rate which can be withstood by an aggregate of a given order, by equating the aggregate shear strength with the viscous shear stress which is equal to the dynamic viscosity of the fluid multiplied by the shearing rate. For example, the shearing rate which can be withstood by 0-order aggregates in Table 3-1 is 3,370 sec~1, whereas 3-order aggregates will be severed when the shearing rate exceeds 61 sec-1.
Assuming the existence of a Prandtl-von Karman logarithmic vertical velocity distribution, T is related to z according to
T =pu (1 ) (3-10)
du _* u(3-11)
where u* = friction velocity, p = fluid density, h = depth of flow and K = Karman constant. Substitution of Eqs. 3-10 and 3-11 into Eq. 3-9 yields
G U / -1 )1/2 (3-12)
()1/2 h1/2 z/h


Fig. 3.3 exemplifies the time-variation of the depth-mean velocity, u, as would occur in an estuary. As a result of the distortion that a progressive tidal wave typically experiences, the peak flood velocity is shown to be higher than the peak ebb velocity, and the flood period is shorter than the ebb period. Fresh water discharge is assumed to be negligible. Since
S 1/2
U2 u (3-13)
where f = Darcy-Weisbach friction factor, Eq. 3-12 can be written as
G = (f/8) u ( 1 -_) 1/2 (3-14) 1/2 1/2 z/h
(Ky) h
For the purpose of illustration,the time-variation of G is plotted in Fig. 3.4 for values of z/h = 0.1 (near-bed), 0.5 (mid-depth) and 0.9 (near-surface), given assumed values of f = 0.025, h = 4.6 m, v =
1.06x10-6 m2/sec and K = 0.3 (for sediment-laden flows). Also given in the figure are typical shearing rates which can be withstood by 3-, 2and 1-order aggregates. The following observations can be made:
1. The magnitude of the shearing rate, G, varies both temporally
and spatially quite significantly. The increase in G with depth means that, once formed, aggregates of a given "base"
order will survive near the surface in preference to the
bottom layers where they will be broken up more easily. There
will therefore be a tendency for the comparatively large
aggregates to settle downward (due to their high settling velocities), and for smaller aggregates to move upward by
diffusion, thus setting up a vertical sediment circulation
cell. The strength of this circulation will, in general, vary
temporally as well.
2. During flood only 0- and 1-order aggregates will be able to
withstand the level of shearing at the strength of flow
(assuming the shearing rate at z/h = 0.1 to be representative
of the near-bed shearing regime), whereas during ebb 0-, 1-


02 4 6 8 TIME (Hours)
Fig. 3.3 Typical Variation of the Depth-mean
Cycle in an Estuary.




TIME (Hours)


10 12 Velocity Over a Tidal

10 12

Fig. 3.4 Shearing Rate as a Function of Elevation above the Bed
Over a Tidal Cycle An Illustrative Example.









8 10 12

TIME (Hours)
Fig. 3.5 Bed Shear Stress and Aggregate Shear Strength An
Illustrative Example.



6 r I I I I I

-Q3 -n I I I I I




w Z

0 I I I I I I I if or e
0 --22-_ rrde
_____3- order--10 ~ ~~2~-order- __ 0I
-ord er

. 0-order
I order
2- order 3 order
-~ 3-order~
2- order
- 1I order-Oorder





and 2-order aggregates can occur, as the shearing rates are observed to be insufficient to break up 2- order aggregates.
The implication is that the aggregates deposited at slack
after flood will tend to be an order lower than those
deposited at slack after ebb.
It has been found that freshly deposited mud will consist of
aggregates which can be one order higher than the order of aggregates forming the bed by deposition (Krone, 1972). However, after a layer of 2-3 cm thickness is formed, the aggregate volume fraction underneath is reduced due to consolidation by overburden, and aggregates one order lower are formed. Consequently the shear strength of the deposit will increase with depth up to a limiting value corresponding to the lowest order aggregates which occur in the lower layers of the deposit.
The critical shear stress and the rate of resuspension of the
deposited sediment are dependent on the shear strength of the aggregates in the deposit and on the applied shear stress. In Fig. 3.5, the bed shear stress, To, for the same illustrative example is plotted as a function of time. Also given are typical magnitudes of the shear strength, Ts, of the aggregates, as would be determined experimentally. It is observed that whereas during flood aggregates of all orders are resuspended, the ebb shear stresses are too weak to allow the resuspension of 0-order aggregates. Therefore, the amount of the material resuspended will be less during ebb than during flood. Given a greater inequality between flood and ebb flows, a situation can arise whereby the bed material is resuspended during flood only, with the result that a predominantly upstream sediment transport will occur. Such a "rectification" of the transport has been observed for instance during neap tides in Savannah Harbor (Krone, 1972). Flood dominance near the bottom is typically enhanced in estuaries as a result of vertical salinity gradients which will augment the rectified transport. Indeed rectification of sediment transport is the mechanism by which shoaling in estuaries occurs. A full description of this phenomenon at Cumbarjua Canal remains to be verified, pending future investigations in which data during fair weather as well as during monsoon must be collected.


3.6 Shearing Rates in the Canal
Fig. 3.6a shows the time-variation of the bed shear stress at
station 1 on February 27, 1980. The shear stress is based on the crosssectional mean flow velocity and is essentially a laterally averaged quantity. The corresponding shearing rate, G, at relative elevations z/h = 0.1, 0.5 and 0.9 are given in Fig. 3.6b. The rates are comparatively low throughout most of the water column, although at elevations of the order of a few aggregate diameters, above the bed, G values exceeding 1,000 sec~1 can occur. If the depth-averaged velocity in the deepest part of the cross-section is used instead of the crosssectional average velocity, a peak value of G = 14 sec~1 at z/h = 0.1 can be estimated, as compared with G = 9.4 from Fig. 3.6b.
Figs. 3.7a,b correspond to the same information as in Fig. 3.6a,b, but for station 4. Here the G values are higher, and it can be shown that a peak value of G = 34 sec~1, as opposed to G = 16 sec~1 at z/h =
0.1 can be estimated in the deepest part of the cross-section. In general, it may be concluded that on the day of the observations, the shearing rates were low to moderate. Higher shearing rates often occur in larger estuaries such as Savannah Harbor (Krone, 1972).
Krone (1963) has shown that the maximum shear on a floc surface due to drag from rotation can be calculated by considering the viscous drag on the edge of a thin disc at the equator. This yields
1 du (3-15)
where T = maximum shear stress on a suspended floc, p = dynamic viscosity of the fluid and du/dz = shearing rate. As an example, considering the shearing rate of relative elevation z/h = 0.1, the maximum shearing rate of 34 sec~I at station 4 would correspond to T=
0.0046 N/M2. Assuming the canal sediment to have properties similar to those given in Table 3-1, it is seen that aggregates of all three orders can easily exist over most of the water column. This of course will be the case provided that a sufficient amount of suspended sediment is available for the rate of aggregation to be high. Inasmuch as the suspended sediment load in the canal is very low, aggregation is likely to proceed at a slow rate, and the order of aggregation is probably low




Station I
7 I
z/h= '.I







TIME (Hours)




1800 20,00

TIME (Hours)
Fig. 3.6 a) Laterally Averaged Bed Shear Stress Variation with Time
in the Canal, Station 1, Feb. 27, 1980
b) Shearing Rates in the Canal at Elevations z/h = 0.1, 0.5
and 0.9, Station 1.


Station' I


0.33 0.67

I I~1i~

6.67 0.00

c: C.,















TIME (Hours)







TIME (Hours)

Fig. 3.7 a) Laterally Averaged Bed Shear Stress Variation with Time
in the Canal, Station 4, Feb. 27, 1980
b) Shearing Rates in the Canal at Elevations z/h = 0.1,
0.5 and 0.9, Station 4.


En U,
cc I

1 -1 1 1 1 1 1 1 1 1
Station 4
- -

I r~r)I



Station 4
0 -.
7 z /h =0.1
3 -


U) I
c: wd Uf)




as well. This hypothesis appears to be corroborated by the settling velocity estimates.
3.7 Settling Velocity
The assumption of steady state conditions for transport under a fully developed turbulent flow results in the following expression for the vertical distribution of suspended sediment concentration, C (Vanoni, 1975):
C(z) = [a(h z) *(
C z(h a)] (3-16)
where C a = concentration at a reference elevation, a, above the bed, h = depth of flow, z = elevation above the bed, w = settling velocity, K = Karman constant and u* = friction velocity. Application of Eq. 3-16 to the present case can be justified on the grounds that the temporal variations induced by the tide are of a comparatively low frequency, and that the suspended sediment concentration is too low for hindered settling to occur. Concentration profiles are significantly influenced by the hindered settling of the settling aggregate "network" when concentrations exceed 10-15 mg/liter (Krone, 1962).
Suspended sediment concentration profiles given in Fig. 2.16 at
station 1 can be utilized for the application of Eq. 3-16. Profiles at 0845, 1230 and 1330 are of a nature where C(z) increases monotonically from surface to bottom, and are suitable for the determination of w. The surface values are considered to correspond to C at a depth of 0.3 mbelow the instantaneous water surface, and the bottom values 0.3 m above the bed.
The friction velocity, u*, can be computed from
Ih 1/2
U- {_}I u (3-17) C 2R
where g = acceleration due to gravity, h = instantaneous depth at the site of the measurement Cz = Chezy coefficient, R = instantaneous hydraulic radius of the cross-section and u= = cross-sectional mean instantaneous flow velocity. The data and the computed values of u* are given in Table 3-2.


Table 3-2
Computation of u, using Eq. 3-17

Time u C R h U*
(m/sec) (ml/2/sec) (m) (m) (m/sec)
0845 0.025 42.8 3.00 6.7 0.018 1230 0.267 40.3 2.19 6.2 0.222 1330 0.153 39.9 2.06 5.8 0.127
Suspended sediment concentration profiles at 0845, 1230, and 1330 are plotted in Fig. 3.8. These profiles are normalized according to Eq. 3-16, using the values of u*, h, a and Ca, given in Tables 3-2 and 3-3. The reference elevation, a, is considered to be 0.3 m, which means that the corresponding concentration Ca is taken to be the measured bottom concentration. The ratio a/h is noted to be 0.045, 0.048 and
0.052 which is reasonably close to the usually recommended value of 0.05 (Vanoni, 1975).
Table 3-3
Parameters for Eq. 3-16 and Computed Values of w
Time h a Ca w
(m) (m) (mg/liter) (m/sec)
0845 6.7 0.3 42.5 0.00101 1230 6.2 0.3 18.5 0.00430 1330 5.8 0.3 33.0 0.00557
The slopes of the lines of Fig. 3.8 yield values of w which are reported in Table 3-3. When analyzing data from Savannah Harbor in a similar manner, Krone (1972) obtained values of w ranging from 0.0037 to
0.046 m/sec which indicates that the presently obtained values are somewhat lower. The mean settling velocity is 0.0036 m/sec. The corresponding Stokes' diameter, d, is obtained from


d = {181w 1/2




- 0845 x 1230 1330
o~oi -- -



0.5 1.0

Fig. 3.8 Normalized Suspended Sediment Concentration Profiles in the Canal (based
on data obtained on Feb. 27, 1980).

Main Flow
Sediment Transport
~t \
Bank Erosion
by Waves + -Secondary
Wave- induced Surficial Turbulence and Mixing
Sediment Transpor t
Fig. 3.9 Transport of Bank-derived Suspended
Sediment in the Presence of Windgenerated Waves.

.~ -~

I . I

where V = dynamic viscosity of the suspension, Y. = unit weight of the aggregates and y = unit weight of the fluid. Eq. 3-18 yields a value of 0.29 mm corresponding to w = 0.0036 m/sec. This is nearly one-half the value of 0.6 mm (first order aggregate) determined for Savannah Harbor, and indicates the presence of small and probably lower order aggregates at Cumbarjua in comparison with those at Savannah Harbor. This conclusion appears to be in agreement with the nature of the suspended sediment profiles of Fig. 2.16. There it is observed that during periods close to slack (0910 hr and 1515 hr) at station 1, clarification of the fluid resulting from a rapid deposition of the suspended sediment is not nearly as pronounced as at Savannah Harbor, where the majority of the suspended load deposited at slack. The same observation is apparent from the data from the three other stations shown in Fig. 2.15. In fact, the data collected in this study indicate comparatively minor temporal changes produced by deposition and resuspension, and it is felt that another investigation carried out when the tides are likely to be much stronger would be expected to yield more information concerning the transport process in the canal than is possible to derive from the available information. Furthermore, the present information is limited by the unavailability of data during a flood flow.
An interesting observation concerns the role of salinity in
influencing the rate and the order of aggregation of the sediment. At very low salt concentrations, the electro-chemical surface forces become repulsive, and the material will disperse. When the salt concentration is less than 1-2 parts per thousand, flocculation is generally incomplete, and weak aggregates, which can be easily broken up, are formed. If this indeed occurs under the high freshwater flows during monsoon (see Fig. 2.2), the transport rates would be strongly influenced by the salinity, under a given hydrodynamic regime in the canal. The influence of salinity on aggregate structure has been clearly demonstrated previously (Krone, 1963). If the material disperses, the turbidity level could increase significantly, and the sediment will behave essentially as wash load. A device such as the in situ settling velocity measuring tube developed by Owen (1971) can be a useful apparatus for ascertaining the degree of flocculation of the sediment in monsoon.


3.8 Sediment Transport Rate
The instantaneous sediment transport rate per unit width over the depth of flow, qs, at any given vertical in a channel cross section is given by
q = f C(z)u(z)dz (3-19)
where 6 = interface elevation above the bed between bed-load and suspended load, and u(z) = time mean flow velocity at an elevation z about the bed. The analytic expression used for C(z) is that given in Eq. 3-16. The expression utilized for u(z) was that of Christensen (1972). Substituting these in Eq. 3-19 yields
h C a u* a(-)KUt
q = aK h-z *n(29.7 -+1)dz (3-20)
6 5 with 0.05h being the usually recommended value for 6.
For illustrative purposes, values of qs were determined by
numerical integration of Eq. 3-20 for Station 1 verticals at 0845, 1230 and 1330 hr on February 27, 1980. The values given in Tables 3-2 and 3-3 for h, u*, a, Ca and w at the three times were used in evaluating Eq. 3-20, with K = 0.30 and k = 0.172 m and 6 = 0.05h as given previously. For 0845, 1230 and 1330 hr, the following values were determined: qs = 8.9, 69.4 and 54.3 gm/m/sec respectively. As expected, the lowest value was obtained at the time closest to slack (0845 hr). An estimate of the total transport rate, Qs, through Station
1 can be made using the following approximation: q Bh
Q _~ (3-21) s h
where B = instantaneous canal width and h =-instantaneous crosssectional average depth. Values for B and h, determined using Fig. 2.5, qs and Qs are given in Table 3-4.


Table 3-4
Sediment Transport Rates at Station 1

Time B h qs Qs
(M) (M) (gm/m/sec) (gm/sec)
0845 330 2.62 8.9 1150 1230 315 2.55 69.4 8990 1330 305 2.50 54.3 7140
3.9 Wind Effect
The role of wind appears to be dual, and is particularly
significant near station 1, where onshore wind from the sea is diverted into the canal during afternoons in fair weather. First, wind affects the flow distribution by generating a surface current (whose magnitude will approximately be 3 percent of the wind speed at 10 m elevation) plus relatively high frequency gravity as well as surface tension controlled waves. Second, when the gravity controlled waves break on the banks, additional sediment is brought into suspension. Since a tidal current of sufficient strength is generally present, this bankderived material is transported both longitudinally with the main current and laterally with the secondary currents towards the center of the canal. Settling of the finer portion of this material could be hindered by the relatively high degree of surficial turbulence and mixing due to the presence of the wind-generated waves. If during this period resuspension of the bottom sediment is limited by the magnitude of the prevailing bed shear stress, a situation could arise wherein the surficial suspended sediment concentration is greater than that near the bottom. Fig. 3.9 schematically depicts the contribution to the suspended sediment load made by sediment derived from bank erosion. The resulting situation is highlighted by the suspended sediment concentration data at station 1, between 1330 and 1500 (Fig. 2.16). In Fig. 3.10, the normalized concentration, C/Ca, is plotted against normalized elevation, z/h. Here Ca refers to "bottom" concentration. The wind velocity (in m/sec) corresponding to the time at which each profile was measured is also given. Two noteworthy observations can be made. First, it is noted that the surficial concentration relative to















Fig. 3.10

Normalized Suspended Sediment Concentration in the Presence of Wind.


ig 3.11 Sediment Entrainment near East Banks.


3.6m/s 4.1 m/s / 4.3m/s 4.6m/s
- 1330 x 1400
- 1430 A 1500


that at the bottom increases with increasing wind speed. Second, as the wind speed increases, the shape of the profile deviates from what normally occurs in open channels under steady flows, namely a monotomic decrease in sediment load with elevation above the bed. Thus it is observed that a situation exists wherein the surficial sediment load is greater than that at the bottom, or mid-depth of both. It is postulated that bank erosion due to waves is the causative factor. As noted in Chapter IT, erosion of the eastern bank due to 0.15 m breaking waves was recorded. Fig. 3.11 shows the bank with entrained sediment.
At low tide, the shoals in the canal between stations 1 and 2 are no more than 0.5 to 1 m below the water surface. Hence, particularly when wind is present, it would be expected that the contribution to the suspended load from this source may not be insignificant during times close to low water.
3.10 Mode of Transport in the Canal
The following description of the sediment transport in the 10.4 km reach of the canal from Tonca to Banastarim is tentative, pending further investigations.
The sediment is predominantly in the fine size range (silt plus
clay). The fine sand portion, which constitutes less than 5% by weight of the sediment near the upstream end of the study segment (i.e. Cundaim-Banastarim), increases somewhat in the direction from Banastarim to the Zuari end of the canal. Near the Zuari end (Surlafonda-Tonca) shoals are present, and the navigation channel is somewhat tortuous. The primary source of sediment in the shoals appears to be the Zuari River. The sediment, which is composed of kaolinte plus some illite, montmorillonite and quartz, contains varying amounts of organic matter (probably 5-15%). Burrowing marine creatures abound in the bottom sediment. The fluid is slightly basic and the salinity varies from near-zero to as much as 36 parts per thousand, depending on the season.
Sediment transport in the canal is almost entirely as suspended load rather than as bed-load, which occurs only when cohesionless sediment is present. The sediment in suspension is primarily bed material with the possible exception of a portion of the organic material, but the transport regime is such that a portion of the bed material load exhibits apparent characteristics of wash load. Suspended


sediment concentration varies from 10 mg/liter to 120 mg/liter or more depending on the season. Fair weather concentrations are lower than those during monsoon.
The transport is primarily tide-induced, but with measurable
contribution from wind-induced effects, and from high freshwater flow effects during monsoon. In general, the transport regime can be divided into two categories normal or fair weather regime and monsoon regime. These regimes, taken together, essentially characterize the long-term processes in the canal.
The normal hydrodynamic regime is characterized by moderate tides (~1 m), low freshwater flow and low to moderate winds which result in a wind-induced surface current plus waves and associated turbulent mixing in the surficial water layer. The flow is vertically mixed, but a small longitudinal gradient of salinity (which decreases by 2 to 4 ppt over the 10.4 km distance) exists. The total salt concentration (23-36 ppt) is however well above the minimum (1-2 ppt) required to complete flocculation and above the limit (~10 ppt) below which aggregation is influenced by the amount of salt present. Under the bed shear stresses induced by the prevailing moderate tidal conditions, the bed sediment appears to be resistant to erosion. This is suggested by the rather low suspended sediment concentrations measured in the canal (~10-120 mg/liter). Except during slack, it is likely that the available sediment rapidly aggregates in the near-bed layer of thickness ranging from a few aggregate diameters to perhaps 1-2 cm where high shearing rates prevail. This near-bed layer will be saturated with aggregates of the base, i.e. lowest, order that can be formed in the system (probably 0-order in the present case), and a high density suspension layer with a density of the order of 1.6 gms/cm3 will exist most of the time5. During the period of increasing bed shear stress when resuspension of the deposit will occur, this high density suspension will gain sediment from the bed deposit by erosion of the bed, and will lose sediment to the fluid column above, where the shearing rates are low to moderate.
5Such a suspension was not noted in the canal because special sensors are required for their detection (Parker and Kirby, 1977). The transport rates given in Table 3-4 do not include contribution from the near-bed layer.


Under a quasi-steady state condition, the rates of gain and loss of the sediment will be nearly equal, but since the rate of gain, being equal to the rate of erosion of the bed deposit, appears to be limited, it can be postulated that the rate at which sediment escapes the high density suspension to the upper layers is correspondingly small. This in turn means that inspite of the mechanisms available for collision to occur, the base order aggregates do not aggregate further (e.g. to second or third order) to any significant extent due to the presence of insufficient number of aggregates in suspension. The canal thus contains low order aggregates of relatively small diameters (-0.3 mm) and low settling velocities (-0.0036 m/sec). Consequently the difference between the maximum and minimum sediment concentration during a tidal cycle remains moderate (20-30 mg/liter), and the waters are not clarified at slack. The aggregates with low settling velocities plus the organic matter contribute to the observed turbidity of the canal even during times close to slack. The existence of a longitudinal salinity gradient however implies that there is likely to be a small net upsteam transport of sediment which enters from Zuari and proceeds towards Banastarim. It is possible that some of this sediment is transported by the near-bed high density suspension.
The monsoon regime is more severe as a result of high freshwater flow, wind and direct precipitation. The flow becomes vertically stratified, and in July-August the salinity drops to near-zero values. It is likely that suspended sediment is "trapped" in the relatively more saline lower layer, and that sediment entrained in the upper layers is carried by the freshwater flow to the Zuari River. There is likely to be a net depletion of the bottom sediment from the canal, but the total amount depleted is probably equal to or less than the net amount transported into the canal during fair weather, as the lower consolidated layers of the Tonca-Surlafonda shoals are believed to be resistant to erosion. Under such a mechanism, and particularly when dredging is carried out, there will be a tendency for the canal to "fill up" to some equilibrium depth when the net influx of sediment balances the net outflux. Evidence corroborating this effect can be obtained from a series of accurate post-dredging surveys of the canal.



Cumbarjua Canal does not appear to currently pose any engineering problems which cannot be solved by available technology. The purpose of the investigation reported herein was to identify important aspects of the fine sediment transport regime for future research-related needs, rather than the proposal of a solution for a particular sedimentation problem. Along these lines, the following relevant points may be noted:
1. It is emphasized that the canal is well suited for a more in-depth
investigation of the basic aspects of fine sediment transport in
tidal waterways.
2. As a prerequisite for such a comprehensive study, it will be
necessary to characterize the canal hydrodynamics in greater
detail. This can be achieved in the following manner:
a. An accurate bathymetric survey of the 10.4 km reach of the
canal between station 1 and 4 should be performed during the fair
weather season (in monsoon this is desirable as well, but may be
b. Either a tide gage (preferably) or a tide staff should be
installed and leveled into the local tidal datum at each of the
four stations. During both fair weather and monsoon, water surface
elevation, current, wind velocity, electrical conductivity (or
salinity) and temperature profiles should be measured at each of the four stations at least once, preferably during a spring tide.
The data collection period should be at least 14 hours. Both
vertical (over the local water depth) and lateral (spanning the
canal width) profiles of the current, conductivity and temperature
should be obtained. Where the flow depth allows, these
measurements should be made at a minimum of three depths over each
vertical: one-half meter below the water surface, mid-depth and
one-half meter above the bottom.
c. This data set will be utilized for the calibration of a
numerical hydrodynamic model, which will then be used to predict
the temporal and spatial distributions of flow and salinity under the range of tides and freshwater discharges which occur during a
"normal" year.


3. The above set of measurements must be repeated during the following
year, with three sets of measurements under spring, mean and neap
tides collected during each season. In order to properly
characterize the sediment regime, suspended sediment concentrations
must as well be measured at each of the four stations simultaneously
with the other parameters. It is recommended that fluid-suspended
sediment samples be collected at the points over the vertical and
lateral station dimensions used previously by pumping in small
volumes of the fluid-sediment mixture via a small battery operated
pump with an intake tube which can be lowered to any desired
depth. In addition, in order to adequately characterize both the existing (consolidated) bed and any new (unconsolidated) deposits,
at least six undisturbed five inch diameter sediment cores (at least
one meter in length) should be obtained at each of the four
stations. The six cores should be taken at approximately equally
spaced locations across the width of the canal at each station.
4. The purpose for obtaining these sediment cores is to determine the
following sediment-related properties: modes of sediment
aggregation, settling velocity, minimum applied shear stress at
which deposition will occur, the shear strength, bulk density, rate
of erosion, and thickness of each stratum of both the existing bed and new deposit, and the consolidation characteristics of the new
deposit. In addition, the average CEC value as well as the chemical
composition of the pore fluid in each of the sediment cores should
be determined.
5. Beyond this point, investigations can proceed along a number of
different lines depending upon research priorities. It is however
recommended that the data collected in the second series of
experiments, i.e. those including sediment measurements, be utilized
for the purpose of calibrating a numerical fine sediment transport model such as the one developed by Ariathurai, MacArthur and Krone
(1977), which will enable a reasonably accurate estimation of the suspended sediment transport rates through the canal under various
hydrodynamic conditions. In turn this will allow for the prediction of reshoaling rates in the canal after any potential dredging. Much
greater utility of the application of such a comprehensive field


program/modeling effort lies in other, larger potential problem areas involving port construction and management, in terms of cost/benefit computations when heavy shoaling or sediment redistribution of the sediment in channels is likely to occur.



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