OTML
Sediment transport modelling review July 1999
REVIEW OF OK TEDI/FLY RIVER SEDIMENT
TRANSPORT MODELLING BY
G. PARKER AND Y.CUI
T.R.Davies
Natural Resources Engineering, Environmental Management and Design
Division, Lincoln University, Canterbury, New Zealand.
Summary
The model developed by Parker and Cui to represent the behaviour of the Ok Tedi/Fly
river system comprises a linked set of physically-based routines, using estimates and
empirical assumptions where necessary. The approach has been to use correct
physical principles and as little calibration as possible in order to make predictions.
The model embodies several concepts that were developed specifically for the Ok
Tedi/Fly situation because no other options were available. For example the complete
absence of any downstream-fining relationship for sand-bed streams from the
literature meant that such a relationship had to be developed from scratch - a
considerable task.
The model makes key assumptions regarding sand and silt transport and storage;
debris-flow runout; braided river aggradation; overbank deposition; sediment inputs;
and long-term catchment hydrology. All of these appear to be physically realistic.
Other assumptions involve model “zeroing” or initial calibration; the wash-load cutoff
diameter; the interaction of suspended material with the bed; flood frequencies premine; and the downstream boundary condition. After more detailed consideration
these were also found to be realistic.
The precision of model estimates of bed level change is about +/- 25% or +/- 0.5 m,
whichever is the greater. Predicted bed elevation changes of the order of 1 m are
thought to be close to the limit of resolution of the model.
The model in its present form produces output that agrees with field data on the
history of bed elevation change from Konkonda to Wygerin. The agreement is in most
cases closer than 1 m. This is as good as can reasonably be expected from a model of
this river system.
In its present state and with presently-available information, the model does not
reproduce the behaviour of the river from Manda to Obo to the same accuracy.
Reasons for this are understood and it appears likely that significant improvements to
the reliability of the model in this reach could be achieved if required.
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Contents
Summary............................................................................................................
1. Introduction.....................................................................................................
2. The model.......................................................................................................
3. Model assumptions.........................................................................................
4. Model performance.........................................................................................
(a) Precision of field data and model predictions....................................
(b) Comparison with field data...............................................................
(c) Model sensitivity...............................................................................
5. Assessment.....................................................................................................
(a) Overall scientific credibility..............................................................
(b) Underlying assumptions...................................................................
(c) Key model parameters and sensitivity..............................................
(d) Model uncertainties..........................................................................
(e) Alternative technology available......................................................
6. Concluding remarks.......................................................................................
7. Recommendations..........................................................................................
8. References......................................................................................................
Appendix 1 - Figures..........................................................................................
Appendix 2 - Tie Channels and ORWBs...........................................................
Appendix 3 - Terms of Reference......................................................................
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1. Introduction
The writer was engaged by Ok Tedi Mining Ltd in July 1999 to review the sediment
transport model of Cui and Parker (1999) - referred to subsequently as “the authors”.
The terms of reference (Appendix 3) were, in essence, to report on the scientific merit
of the model.
A six-day period was available for the review (17 - 22 July 1999), which was carried
out on site at Tabubil. A brief description of the model, and spreadsheets outlining the
results of two model runs (January and May 1999), were received by the writer prior
to the site visit, together with reprints of background papers. On arrival at Tabubil,
further information was made available in the form of a previous review by Dietrich
(1999) and various OTML reports. Prof. Parker was present on site at the time and
provided a detailed briefing on the rationale and development of the model. He was
also available for further discussions when required.
The writer had some prior knowledge of the Ok Tedi/Fly river system and its
problems from supervision of the Master of Engineering thesis of Arnold Moi (an
employee of OTML Environment Division), and had previously visited the site
briefly in 1996.
2. The Model
The model used to represent the behaviour of the Ok Tedi/Fly river system has been
described by Cui and Parker(1999), so further description is not provided herein. The
reader is recommended to consult that work in parallel with this review report1.
The bases of the model are relationships for sediment transport (Parker, 1990 for
gravel; Brownlie, 1981 for sand), resistance to flow (Brownlie, 1981) and mass
conservation (the well-known Exner relationship) already available in the literature; in
my opinion they are all sound and well suited to the task.
As well as using existing relationships, the authors of the model had to develop their
own methods for predicting some important aspects of the river behaviour. In
particular, new methods were devised for estimating deposition on the floodplain due
to overbank flow of sediment-laden water, and for predicting the way in which the
grain-size distribution of sand- and silt-sized sediments changed with distance downriver. The latter was a particularly challenging task; it was accomplished in a very
short time but, as the authors acknowledge, by a preliminary methodology that would
benefit from further development.
3. Model assumptions
1
An error was found in Eq. (3b) of Cui and Parker (1999) during this review. It has been corrected in
the model code and, being a criterion for the occurrence of lower or upper regime flow, does not affect
the model results since in the reaches to which the equation applies the flow is always in the lower
regime.
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Some aspects of the river system behaviour were dealt with in a simplified and
empirical way, rather than by detailed physical descriptions. These are now outlined:
(a) It was necessary to estimate the delivery of sediment by debris-flow processes to
the upper part of Harvey Creek from the southern dump sites, and its transformation
to fluvial transport along the Ok Mani. At present there exist no published
relationships to predict this situation, and I am satisfied that the authors’ method is
both appropriate and suitably accurate for the purpose.
(b) Reduction of grain-size of the rock waste during its passage into and down Harvey
Creek, and along the Ok Tedi to Konkonda, is an important factor in the sediment
transport of the system. An empirical coefficient is used that relates abrasion to travel
distance along the channel, and this was calibrated for the sediment types present by
means of laboratory tests on site.
(c) Transport of sand- and silt-sized sediment along the Ok Tedi from Tabubil to
Konkonda is assumed to take place at the rate at which the material was supplied from
Harvey Creek, without storage in the Ok Tedi. Given the relatively minor effect on
downstream sediment behaviour that would result from such storage I am satisfied
that this assumption is appropriate.
(d) Deposition of sediment during the aggradation of gravel-bed reaches is assumed to
take place across the whole width of the river bed at a section, even though the
sediment is transported to that section by flow only in the active bed of the stream
which does not occupy the full bed width. This recognises that gravel-bed rivers in
aggradation move across the whole width of their bed over a relatively short time,
depositing sediment as they do so.
(e) The models also assume that, during degradation, the river only removes sediment
from previous deposits by direct down-cutting of its active channel without lateral
movement. By contrast with the previous assumption I feel this one is too restrictive;
in my experience a degrading river will indeed move laterally, and will thus evacuate
more sediment than the models assume to be the case. In practice, however, the
resulting adjustment to the outcome of the models might not be particularly
significant.
(f) The models are driven by water inputs derived from either recorded flow duration
curves, assumed to apply for a whole year (Ok Tedi); or from flow duration curves
derived from a simple catchment hydrology model (Ok Mani); or from synthesised
daily flows based on a randomly repeating sequence of 4 different types of rainfall
year (dry, medium, wet and El Ni§o) for the lower Ok Tedi and Fly. These methods
are appropriate.
(g) Sediment inputs to the model are derived from records of and plans for rock waste
and tailings inputs from the mine, from rock wall failures associated with dump
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operation, and from predicted channel bed erosion. The estimates appear to me to be
realistic, with the possible exception of the exhumation of sediment (both rate and
total) from Harvey Creek due to eventual channel degradation following mine closure,
noted in (g) above.
The above assumptions are clearly appropriate and satisfactory. The following
assumptions, however, required more examination:
(h) The model was “zeroed” by adjusting some parameters so that the natural water
and sediment inputs resulted in no significant change to the 1985 river bed levels over
a period of 70 years. This procedure has been criticised by Dietrich (1999) as
unrealistic because the river system (especially the middle Fly) appears not to be in
equilibrium (Dietrich et al, 1999). I feel that this criticism is overstated, because it is
certainly true that, while perhaps not in long-term equilibrium, the middle Fly has for
very much longer than 70 years past been changing very slowly indeed, if at all
(Dietrich et al, 1999). I feel that the zeroing operation as carried out is justified; it was
however not perfect because the downstream grain-size variation present in the middle
Fly in 1985 was not able to be represented by the model due to the presently limited
knowledge of downstream fining in sand-bed rivers.
(i) Cui and Parker (1999) conventionally chose a fixed grain diameter as the cutoff
between washload and suspended load. In their January 1999 model run this figure
was set in error to the unusually low value of 20 Ê (compared with the conventional
figure of 62 Ê). The May 1999 model run corrected the figure to 43 Ê and produced
different results for this and other reasons (see (j) below). I believe this factor needs
further investigation; the model predictions of aggradation in the lower middle Fly are
somewhat sensitive to the washload cutoff diameter, and again this is a topic that has
received little attention in the literature. One would expect that whether or not a grain
of a given diameter ever settles on the bed would be a function of the turbulence and
shear velocity of the flow, rather than whether or not it exceeds a certain arbitrary
diameter. The figure of 43 Ê is clearly somewhat arbitrary but the authors had no
choice but to adopt such a figure.
(j) Exchange of fine sediment between the bed and suspension also affects
sedimentation in the middle Fly. This is a topic on which I have limited knowledge,
and do not feel well qualified to offer substantive comment. Dietrich (1999, section
5b) summarises the situation well. In the zeroing procedure Parker and Cui found that
their model for fine sediment deposition did not reproduce the degree of downstream
fining reported in the middle Fly prior to mine operation commencing; however it is
not obvious to me that this deficiency will be as important in the post-mine situation
when the natural sediment is swamped by mine-derived sediment with a different
grain-size distribution. I am less concerned than Dietrich (1999) that this weakness
might be the cause of the failure of the Parker-Cui model to predict the reported
aggradation at Bosset and Obo, because I think that this is more likely to be due to the
downstream boundary condition Parker and Cui were forced to adopt.
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(k) Pre-mine flooding frequencies were a factor in the zeroing of the model. In the
absence of adequate pre-mine data, these were chosen on the basis of expert discussion
and are thus a potential source of error in the zeroed parameters. It seems unlikely,
however, that more authoritative frequency estimates will become available except by
repeated trial-and-error model runs when more behavioural data have been accumulated
for the river system.
4. Model performance
The ability of the model to predict the behaviour of the Ok Tedi/Fly river system in
response to mine operation options can be assessed on the basis of its ability to
reproduce the known behaviour of the system in the 14 years since mine operation
commenced. This is a legitimate test because data from those 14 years have not been
used to calibrate the model; the only calibrations have been the zeroing procedure and
independent measurements of parameters such as rock abrasion coefficients.
(a) Precision of field data and model predictions
Since a major use of the model is to predict river bed aggradation in response to
sediment addition, it makes sense to test it in that mode. Bed elevation data are
available at several sites along the river system and are reported in the Draft 1997-98
Hydrology APL Report of OTML Environment Division. The river is wide and its
bed is not usually flat, being configured by water flow into bedforms such as dunes
and bars which can be several metres from trough to crest. The bed level data for a
specific section therefore need to be treated as a necessarily approximate
representation of the reach mean bed level. The scatter of sequential mean bed level
data usually occupies a range of about 1 - 2 metres (e.g. Fig. 2), so the precision of the
field data is probably of this order.
In addition, the inevitable imprecision in the model parameters and input data means
that the model predictions are also approximate; Prof. Parker (pers comm 1999), based
on his experience in developing the model, estimates that the possible error in
predicted mean bed elevation changes is probably of the order of +/- 25% for changes
greater than about 2 m, and that the lower limit of resolution is probably about +/- 0.5
m. Thus a predicted aggradation of 4.0 m probably means the value is between 3.0 and
5.0 m, while a predicted aggradation of 0.5 m is between 0 and 1.0 m. A formal
sensitivity analysis would be needed to obtain more detailed information on this
important point, but time does not permit the long series of model runs needed to
carry out such an analysis at this stage. These imprecisions stem to a significant degree
from the present state of knowledge of the processes of downstream fining of
sediment grain size in sands and silts.
Thus, for an individual data point, comparison of model and field data could, in an
extreme case, yield a difference of up to about 3 m in a situation where the match was
in fact perfect; and vice versa. Our ability to assess the model on the basis of its
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empirical performance is therefore limited. This situation is greatly improved by the
fact that there is always more than one data point available, and because errors are
unlikely to accumulate in the worst way possible. We are thus looking for field and
model data to agree reasonably closely but not exactly, and a series of model data
points matching their field equivalents to within a metre could be considered to be
very satisfactory in my opinion.
(b) Comparison with field data
The predictions of the May model run for the increase of bed elevation from 1985 to
1999 at Konkonda, Ok Tedi 13 km above D’Albertis, Kuambit and Wygerin were
compared to recorded bed elevation increases at those sites. At these sites the match
between model and field data throughout the period is good (Fig. 1); it is always close
to, and in the majority of cases well within, the one metre criterion for satisfactory
agreement. Dietrich (1999) reached the same conclusion on the basis only of bed
elevations in 1996/96. On this basis I conclude that the model is reliable from
Konkonda to Wygerin. The results of the January model run are less satisfactory, as
would be expected.
At Manda and Obo the model predicts no significant aggradation prior to the end of
the simulation period in 2055. This does not agree with field data (Draft 1997/98 APL
Report) that show about a metre of aggradation at Manda, about 2 m at Bosset and
about 2 m at Obo at the present time. This difference between the model and field data
is sufficiently large to be significant, and was the main reservation expressed by
Dietrich (1999) about the reliability of the model.
Further scrutiny of the Draft 1997/98 APL Report, however, reveals that between
1988 and 1990 the mean bed level at Ogwa (downstream of the Strickland confluence
at Everill Junction) increased by about 4 m (Fig. 2). Because the flow and sediment
loading in the Strickland are both significantly greater than those in the Fly, it is
unlikely that this aggradation was caused by mine sediment. The same APL Report
shows about 2 m of aggradation at Obo between 1993 and 1996, and somewhat more
at Bosset between 1990 and 1993. The aggradation at Ogwa must have imposed a very
significant backwater condition on the lower middle Fly, causing water surface slope
and bed shear stress to reduce upstream of Everill Junction and allowing sediment to
deposit that would otherwise have been moved past Obo in suspension. In other
words, the aggradation reported at Obo, Bosset and Manda probably would not have
occurred without the aggradation at Manda.
Since the data used to synthesise water levels at Obo are for the years 1993, 1994,
1996 and 1997, which include the backwater effect from Ogwa, it might seem that the
model simulations should indeed be capable of reflecting the backwater-induced
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aggradation at Bosset and Manda. Because Obo water levels are preset in the model,
however, any aggradation at Obo cannot cause the model water level there to increase,
and thus the ability of the model to predict aggradation at Obo is severely restricted
compared to that which occurs in reality.
If it is necessary for the model to have better capability to predict aggradation at Obo,
and thus at Bosset and Manda, then its lower boundary must be set farther
downstream. It would seem sensible to set it at Ogwa (or even farther downstream if
possible); this was the original intention, but unfortunately the daily flow series at
Ogwa is incomplete and unsuitable for this purpose. A feasible alternative would be to
develop a means of generating daily flows at Ogwa from the incomplete data there and
using these as the downstream boundary condition.
It appears likely to me that, given the success of the model as far as Wygerin, it would
probably also be reliable in its present form as far as Obo if the lower boundary were
set at least as far downstream as Ogwa. Whether this extension is worthwhile
depends on the need for reliable prediction of aggradation downstream of Wygerin.
(c) Model sensitivity
The difference between the predictions of the January and May model runs was large in January a sediment front about 4 m high was predicted in the middle Fly, whereas
in the May run the front was rather greater than 1 m high. This suggests that the
model is sensitive to the differences in input conditions between these two runs.
The differences in input parameters between the two model runs were
(i) an increase in the threshold diameter for washload from 20 Ê to 43 Ê;
(ii) a change in the fine sediment transfer conditions between the bed and the sediment
load.
As noted above, the 20 Ê value was set in error; the intention in the January model
was to use 43 Ê (G.Parker, pers comm 1999). Since a significant proportion of the
TSS lies within the range 20 - 43 Ê, it is not surprising that the change caused
significantly greater sediment transport in the May model run, and allowed less
sediment to deposit in the lower middle Fly, reducing the height of the aggradation
wave by rather more than a metre compared with the January model.
The differential transport of different-sized sands was computed in the January run
by a preliminary routine outlined by Cui and Parker (1999). An attempt was made in
the May model to increase the sophistication of this routine, with some success; a
degree of downstream fining was reproduced in the “zero” model run as a result, but
this was not sufficient to reproduce the measured pre-mine degree of fining. This
change also caused the quantity of sediment predicted by the model to be in transport
to increase, and the degree of aggradation to decrease accordingly. This effect also
reduced the height of the aggradation front by a metre or more.
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The result of these adjustments was to increase substantially the sediment transport
capability of the river in the sand-silt range in the May model, giving rise to the lower
front to the aggradation wave. The justification for the change in washload cutoff size
seems to me to be entirely sound and I agree with it. As mentioned above I am less
well qualified to comment on the change in sediment mixture behaviour; this is work at
the very leading edge of sediment transport knowledge, and a more specialised expert
would be needed to comment authoritatively. Both changes, however, appear to me to
be exactly the sort of improvements that are to be expected during the development of
a model of this sort.
5. Assessment
(a) Overall scientific credibility
The procedures developed by Prof. Parker and Dr Cui are in my view at the very
leading edge of world river modelling technology. The river system modelled is very
large and complex; and involves a huge range of processes from debris flows, through
alluvial fans and braided gravel-bed rivers to meandering sand-bed rivers. Many models
are available worldwide that attempt to represent the behaviour of individual
components of this system, often with limited success, so it is not immediately
apparent that a modelling framework as complex as that attempted by Parker and Cui
would be successful. That it is indeed successful in representing the measured
aggradation through all but the very downstream part of the system is a considerable
accomplishment. This particularly true when it is recalled that there is very little
calibration involved apart from the initial zeroing procedure; this suggests very
strongly that the underlying physical concepts are sound.
Undoubtedly parts of the modelling framework can be criticised, and suggestions could
be made for improving it. In the context of the information required, and of the nature
of the system being modelled, however, I have no doubt that leading researchers
worldwide would find the framework scientifically credible.
(b) Underlying assumptions
These have been dealt with in detail earlier. I am satisfied that they are reasonable, and
that similar assumptions would have been made by other competent researchers
engaged on this task. Some of them are of a preliminary nature; those that would
benefit most from reassessment seem to me to be the downstream boundary condition
and the sediment sorting routine.
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(c) Key model parameters and sensitivity
Again these are dealt with in detail above. I believe that the input parameters of the
model have been estimated to the best degree possible at this time, and where
uncertainty exists honest attempts have been made to select a reasonable figure (as, for
example, with pre-mine flood frequencies). Some parameters are admittedly
preliminary in nature at this time and, if warranted, could be improved with further
work.
The sensitivity of model predictions to variations in parameters is always a matter of
concern in a predictive model such as this. A formal sensitivity analysis has not yet
been performed. The sensitivity of the output to some parameters has been indicated
above, for example the sensitivity of aggradation wave front height to washload cutoff
diameter and to sediment sorting. It appears that there is some sensitivity of predicted
aggradation to the degree of sediment sorting performed by the model, and this is the
most tentative component of the model; in the opinion of Prof. Parker, this makes up
a significant proportion of the uncertainty in the model predictions.
It is clear to me that the May version of the model is an improvement on the January
version, because it matches field data significantly better from Konkonda to Wygerin.
In this I disagree with Dietrich (1999), who places more trust in the January model.
(d) Model uncertainties
As with any prediction, the degree of uncertainty increases as one moves away from
known points in time and/or space. The model predicts present aggradation from
Konkonda to Wygerin to within a metre. Farther down the Fly the present aggradation
at Manda, Bosset and Obo is less well matched, due probably to the approach to an
unrealistic downstream boundary condition as well as to the increasing distance from
known upstream conditions. It is therefore likely that the predictions of the model
over the near future will continue to be good to Wygerin, and will match the present
degree of agreement with field data. Beyond, say, ten years from now, and below
Wygerin, it is to be expected that the predictions could drift, especially if the least
satisfactory components and inputs remain unimproved. Generally I expect the
predictions to continue to match the field data to the degree outlined in section 4(a)
above.
I believe that all the significant uncertainties in the model have been identified and
taken into account, and that the remaining ones are relatively minor; I would be very
surprised if further significant uncertainties arise during the future use of the model but
the possibility of changes in input parameters (for example, the friability of the rock
waste input to Harvey Creek) must be borne in mind.
(e) Alternative technology available
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To the best of my knowledge there is no other model available worldwide able to
accomplish what Cui and Parker’s model does. There is certainly no off-the-shelf
model that will even approach the functionality of the present model. I know of no
other research group that has attempted such an ambitious task.
There are other detailed models available for individual components of river systems,
but these are not designed to integrate with each other in the way essential to the
present problems. The Ok model series is capable of predicting the effect on the Fly
river at Wygerin of variations in rock waste output from the mine hundreds of
kilometres upstream, over the next fifty years; no other model I know can do that.
6. Concluding remarks
This review has necessarily been brief, and has not gone deeply into the details of the
model. Rather it has examined the framework of the model to establish whether the
basis is sound, then looked at the degree to which comparison with field data confirms
the predictions of the model. The approach has also been to be aware of whether the
behaviour of the model corresponds to what would be expected from a qualitative
geomorphological point of view.
In general terms the model performs extremely well on all counts. It is hard to
overemphasise the difficulty involved in modelling a 100 km long gravel-bed river
experiencing severe aggradation due to sediment loading; or of modelling the passage of
an aggradation wave through 400 km of low-gradient sand- and silt-bed river. When the
two have to operate in series, and are fed with both coarse rock waste and tailings
slurry, the former of which abrades substantially to generate more sand and silt, the
magnitude of the task is apparent. Nevertheless, the model succeeds in predicting
aggradation (of up to six metres) to within a metre where it has been possible to test it,
based only the application of sound physical principles and an initial zeroing
calibration.
In my opinion the model is a reliable predictive tool with which to generate future
aggradation scenarios from a variety of mine operation options. I believe the model can
be used with confidence in its present state.
The limitations of the model comprise, firstly, the unsatisfactory downstream
boundary condition; this is fairly easily rectifiable. The second limitation is the
present fairly crude (but state-of-the-art) procedure for estimating sorting in the sandbed reach. This would need a concentrated research effort to generate a substantive
improvement.
7. Recommendations
(a) The present form of the Parker-Cui model is suitable for use as a basis for
planning, bearing in mind its limitation of precision on predicted bed elevation changes
(+/- 25% or +/- 0.5 m, whichever is the greater).
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(b) If more precise predictions are required in the lower middle Fly, confidence in
model predictions there could be increased by:
- improving the lower boundary condition by moving it downstream at least as
far as Ogwa;
- improving the sediment sorting procedure.
8. References
Brownlie, W.R. (1981). Prediction of depth and sediment discharge in open channels.
Report No. KH-R-43A, W.M.Keck Laboratory of Hydraulics and Water
Resources, California Institute of Technology, Pasadena, California, USA,
232 p.
Cui, Y. and Parker, G. (1999). Sediment transport and deposition in the Ok Tedi-Fly
river system, Papua New Guinea: the modelling of 1998-1999. Report to
OTML, 16 p.
Dietrich, W.E. (1999). Review of the sediment transport predictions reported by
Professor Gary Parker and Dr Yantao Cui. Report to OTML, 20 June 1999,
18 p.
Dietrich, W.E., Day, G. and Parker, G. (1999). The Fly River, Papua New Guinea:
inferences about river dynamics, floodplain sedimentation and fate of sediment.
In: Varieties of Fluvial Form, Ed. A.J.Miller and A.Gupta, J.Wiley and Sons
Ltd, 345-376.
Parker, G. (1990). Surface-based bedload sediment transport relation for gravel rivers.
Journal of Hydraulic Research, 28, 4, 417-435.
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APPENDIX 1 - FIGURES
Fig. 1 - Comparison of model predictions (points) with field data (shaded areas)
Fig. 2 - Ogwa bed levels 1986 - 1998 (Draft OTML Hydrology APL Report, 199798)
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APPENDIX 2 - TIE CHANNELS AND ORWBS
I was requested by the OTML Board to report also on the matter of “tie channels”,
following discussion with Prof. Dietrich . Unfortunately his arrival in Tabubil was
delayed and we had time for only a very brief discussion the evening before my
departure. The following represents my thoughts, having had very little opportunity
to consider the subject.
Tie channels (Dietrich et al, 1999) are the narrow channels that connect the main stem
of the river to the remnant meanders and other lakes that comprise many of the “off
river water bodies” (ORWBs) on the floodplain. The ORWBs are thought to be very
important as floodplain habitat, and the tie channels are the means by which the
ecology of the ORWBs connects with the main channel. It has been found that the tie
channels are prevented from silting up under natural (pre-mine) flow conditions by
rapid flow in either direction due to level differences between the river and the
ORWBs. This process is particularly active when occasional unusually low river
water levels cause outflow from the ORWBs to erode the tie channel beds by
headward retreat of small waterfalls (“nick-points”) that start at the river bank and eat
their way back up the tie-channel.
The effect of mine-induced changes to the middle Fly river on the ORWBs is likely to
be:
1. Increased suspended sediment in the river causing tie channels to silt up and
perhaps even to become blocked, with serious impacts on movement of fauna
between tie channels and river.
2. Aggradation of the river bed increasing the low-flow water level and reducing the
ability of outflow from the ORWBs to scour the tie channels
The first effect has already been present since 1985, prior to aggradation, and will
decline very rapidly following mine closure. The second effect will increase slowly for
some years, depending on location down the river system, and will decrease very
slowly starting some time after mine closure.
In geomorphic terms one would expect that the ORWBs would respond to the
increase in their base level (river level) caused by aggradation by increasing their own
mean water levels; this could occur either by additional inflow of river water carrying
sediment, or if the direct rainfall input to the floodplain is sufficient, from that local
rainwater input also.
Given that the plan area of the ORWBs will increase very substantially with surface
elevation, however, it is by no means certain that the ORWB mean water level will rise
to the same extent as mean river water level. A change in floodplain hydrology could
therefore occur, whereby instead of being dominated by local rainwater it becomes
dominated by sediment-rich river water until mine closure. This seems likely to
increase the rate of siltation of ORWBs and, presumably, impact on their ecology. The
floodplain will also aggrade but probably less than the beds of the ORWBs, so the
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Sediment transport modelling review July 1999
ORWBs seem likely to become shallower. The impact of bed aggradation and high
sediment load on the ORWBs therefore seems likely to be negative until mine closure.
After mine closure the high river sediment loads will decline rapidly, and river bed
levels will eventually decline much more slowly. So the ORWBs will experience a
period of high water levels and natural sediment loads, before their base level slowly
declines again; but they will remain shallower than prior to mining. Following the
decline in river level the ORWBs will be higher than they are now relative to river
level, because the floodplain will have aggraded; thus the ORWB volume in the long
term will be less than it was pre-mine. The floodplain sedimentation rate will also be
less than it was pre-mine, due to the higher floodplain elevation..
The hydrologic and geomorphic processes relating the ORWBs to the river are clearly
fairly complicated. What happens to the ORWBs and their ecology depends on
• the extent and degree of ORWB bed sedimentation
• the extent and degree of floodplain sedimentation
• the extent to which the tie-channel flows reduce, due to siltation and decrease in
head difference between river and ORWBs
• the increase in penetration of river sedimentation onto the floodplain following
aggradation
• the timing of peak aggradation and the subsequent rate of degradation
Of these only the second is addressed by the Cui and Parker (1999) model. If better
assessment of the impact of mine sediment on tie channels and ORWBs is required, a
better understanding of the physical processes linking them to river behaviour is
needed than presently exists.
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OTML
Sediment transport modelling review July 1999
APPENDIX 3 - TERMS OF REFERENCE
General.
The reviewer will be required to provide an independent scientific review of the
Sediment Transport Model by Prof. Gary Parker. The review outcome will be in the
form of a review document.
Specific.
Specifically the reviewer will be required to:
1. Comment on the overall scientific credibility of the model results
2. Comment on the underlying assumptions that form the basis of the model
3. Comment on key model parameters and their sensitivity
4. Comment on model uncertainties
5. Comment on model with respect to alternative technology available worldwide.
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