An investigation into river junction angles and the effect upon them by stream forces at the Ödenwinkelkees braid plain, Austria.
Group D: David Cox, Angela Cruickshanks, Anna Davies, Sarah Jolly, Andrew Law-Riding, Neil Mounsey, Alex Plumb, Andrew Welfle
Abstract
A study into bifurcations and confluences was conducted over two days in July at the Ödenwinkelkees braid plain in the Hohe Tauern region of Austria. The results obtained show clear patterns of interaction between channel morphology and flow dynamics. A model was constructed to facilitate quick analysis of the data. (Please see the excel file on CD).
Introduction
The Ödenwinkelkees Glacier (mid-point, grid reference 525500, 220400) is found in the Stubach Valley (Hohe Tauern National Park, E. Alps). It is a debris covered valley glacier in the cirque of the Ödenwinkel at the foot of a steep, 600 to 800m high rock face. The glacier covers an area of 2.1km² and terminates at an elevation of 2150- 2160m (Slupetzky, 1989).
The proglacial area is bordered by the moraine ridge of 1850 and accommodates melt water from the glacier into a proglacial outlet braided river channel to the north. The precise effect the glacier has on the hydrology of the river catchment is a function of the altitude, latitude and longitude. In Alpine Europe a special relationship also exists between glaciers and proglacial rivers. During winter snowfall the glacier accumulates mass, however during summer melt, ablation occurs and the glacier decreases in mass. The melting process of the Ödenwinkelkees influences the changing characteristics and development of the braided river channel below.
The area of the braid plain being studied is found at around 1300m from the present snout (12.63928ºN, 47.12189ºE). Field work was carried out on Friday 9th July, 2004. This area was chosen due to the discharge intensity of the river on this day, making measurements closer to the glacier snout potentially dangerous (see health and safety later).
A sketch map of the area is shown below: The study focuses on bifurcations and confluences and has three main aims:
1. To investigate river junction angles and the effect upon them by stream forces. 2. To create a digital elevation model (DEM) of the braid plain studied using Kern levelling. 3. To create an Excel model to speed up the maths involved with confluence characteristics.
Bifurcations occur where the main river divides into two channels as illustrated in the methodology.
Confluences on the other hand occur where two river channels join to form a main channel also indicted in the methodology.
The nature of glacier fed braided systems is highly variable on both spatial and temporal scales. This can make measurements hard to construct when compared to less dynamic river environments. Both bifurcating and confluence channel patterns are common in proglacial rivers of a braided nature, however previous research has tended to draw on confluences and limited knowledge is known about bifurcating channels. Confluence Dynamics
Within confluence junctions the flow is controlled by six factors. These are the area of flow stagnation, flow deflection zone, flow separation zone, maximum velocity area and flow recovery area. Shear layers are also created in the stagnation zone and the separation zone. The positions of these areas in the confluence system are illustrated in figure 2. These areas found within the confluence are largely influenced by the confluence angle and the ratio of channel discharge. (Best, p. 28) In the stagnation zone the velocities of flow are lower this is because of the deflection between the two incoming streams. As the junction angle increases the area of the stagnation zone decreases. The smaller the junction angle the greater the area of stagnation zone. The velocity inside the stagnation zone decreases as the confluence angle increases. A smaller zone of water will be affected by the surroundings, including bed, bank and other bodies of water. (Best, p. 28) The bed shear stress in the stagnation zone increases along with velocity and this scours the bed. This is because a shear stress layer is created where the two flows meet. (Best, p. 31) The flow separation zone has a low velocity (as little as 5%) and is greatly influenced by the junction angle and channel discharge ratio, where the separation zone increases in width and length along the channel wall as the angle and ratio increases. The flow separates because the tributary flow is affected by the geometry of the boundary at the junction corner downstream. The tributary flow is not able to remain attached to the wall of the channel. Characteristics of the flow separation zone are lower velocities, lower fluid pressures and the elevation of the water surfaces are less than the surrounding junction flow. The flow separation affects the amount of space the flow has to continue downstream. This is explained by a greater width of flow separation zone and means that the water will have a narrower channel to flow through. This is called the maximum velocity zone. Therefore the larger the separation zone (width and length) the greater the maximum velocity in the zone. Hence this also has an impact on the degree of sediment erosion and transport and bed morphology. (Best, p. 32) The flow recovery zone is found downstream from the confluence. Where the shear stress was created near the stagnation zone the water mixes gradually where the flow combines, and the turbulence created reduces. (Best, p. 32) Hypotheses
H1: The greatest effect upon the discharge ratio from a bifurcation will come from the influence of the hydraulic radius, i.e.; width and depth.
H0: The discharge ratio will not be affected by hydraulic radius.
A deeper and wider stream will intrinsically carry more water (discharge). The discharge ratio records the disparity between the confluent channels in terms of their water load. A high ratio indicates a small stream joining a large stream. The hydraulic radius is a measure of stream size calculated from width and depth. This will have a clear effect upon the discharge ratio.
H2: When the discharge ratio rises, so does the momentum ratio.
H0: When the discharge ratio rises, the momentum ratio is unaffected.
The momentum ratio is a product of width, slope, depth and angle; as is the discharge. These two factors therefore will exhibit a linear relationship.
H3: The sum of the output streams discharge will equal the amount of discharge from the input stream.
H0: The sum of the output stream discharge will be unequal to the amount of the output stream.
At a bifurcation the output streams are fed by the input stream, this means that unless outside forces act the sum of the output discharges are contiguous with the discharge of the input stream.
H4: Streams with a higher hydraulic radius will meet at a more acute junction angle.
H0: Streams with a higher hydraulic radius will meet at a more obtuse junction angle.
The larger the hydraulic radius, the greater the discharge as hydraulic radius is a function of the dimensions of the channel. If there is a higher discharge in one channel compared to the other you would expect the channel with the larger hydraulic radius to enter the main channel at a more acute angle. The channel with the smaller hydraulic radius, hence smaller discharge, will be forced to curve direction created by the larger river. Therefore the angle of the smallest channel will be larger.
H5: The higher the discharge, the more acute the junction angles.
H0: The higher the discharge, the more obtuse the junction angles.
If a stream enters the main channel with more discharge, it is more likely to flow in a more direct line and overpower the weaker channel. Therefore the angle at enters the main channel is much more acute than that of the less discharge channel. H6: As momentum ratio becomes higher, so does the junction angle.
H0: As the momentum ratio becomes higher, the junction angle becomes lower
A high momentum ratio indicates a small stream meeting a large stream. This imbalance is also seen in the radius of curvature which means a large stream has enough power to create a large junction angle.
Methodology
In order to obtain representative results, data was collected using a systematic sampling method. At the field site as many confluences and bifurcations were measured in the time allowed. The data collected was obtained in a standard way as described below:
1. Once at a bifurcation or confluence, a tape measure was placed in the direction of the main channel. This acted as a visual indicator and guidance of the main straight of the river so bearings could be taken.
2. At each site, the bearing was taken along the tape measure. In turn, the bearing along the right and the left channels were taken along the middle of each joining channel.
3. The junction angle of each joining channel was calculated by subtracting the recorded bearings from each other.
4. The slope of channel A, channel B and the main channel were measured using a clinometer. The values were all negative numbers as the measurements were always taken looking down slope.
5. The slope was measured so that the level from the clinometer was pointed at a set height, making the data suitable for calculating the slope.
6. Also for each of the 3 channels, the width was measured, from one bank to the other, using a 30metre tape measure. The location where width measurements were taken, was the most representative of the channel width.
7. Across each width measurement, depth measurements were also taken. These were measured using a 30cm tape measure, at three different locations. From this, an average depth was calculated for each of the channels. Measurements collected were taken, considering both the shallow and deep parts of the channels.
This methodology was used when investigating both confluences and bifurcations. At each bifurcation and confluence, the same measurements were taken. The slope angle at both the confluences and the bifurcations were negative numbers because water always flows down gradients. The most representative width and depth of the 3 channels at each junction was recorded.
Fig.1 A Generic Diagram of a Confluence
Figure 1. This diagram is a basic representation of the measurements taken in the field at the confluence sites. Channel A is the left hand channel and channel B is the right hand channel. W indicates where width was measured, D indicates where depth was measured. The direction of slope shows the general downward gradient at the confluence sites. As well as showing the direction of flow down the channel.
Fig.2 A Generic Diagram of a Bifurcation Figure 2. This diagram is a basic representation of the measurements taken in the field at the bifurcation sites. Channel A is the left hand channel and channel B is the right hand channel. W is where the width was measured, D is where depth was measured. The slope directions show the direction of flow and general downward gradient of slope.
The Devonshire Model for Confluence Behaviour (see excel file on the cd)
What is it for?
There was a need to design a simple model that is easy to use and can provide all the key characteristics of individual confluences. It was found that complex mathematical equations can be time consuming to complete and often incorrect. To facilitate the quick production of the important figures it was necessary to streamline the process into an excel sheet with a set of inputs which would run various formulae to produce the required outputs. It is believed that this model will significantly cut laborious data input and help future students.
How does it work?
The model has four main inputs that are easy to collect from the field. These include width (m), depth (m), slope in degrees and both junction angles A and B (see diagram above). These are inputted for channels one and two (the two input confluent channels). Then, the final input is an estimation of the bed roughness. This figure is Manning’s N and can be anything from 0.02 for a very smooth bed up to 0.05 for a very rough bed. When all these figures are entered into the spreadsheet, a set of formulae come into operation to calculate; 1. Hydraulic radius 2. Velocity 3. Radius of curvature 4. Discharge and discharge ratio 5. Momentum ratio 6. Angle ratio 7. Ultimately to find the most dominant channel.
Formulae
The outputs that are created from the model are products of several formulae:
1. R w.d / w 2d 2 S 0.5 .R 3 2. V N 3. r tan(90 ) 2 w.d.R 3 .S 0.5 4.Q N 2 V1 d1 w2 r2 5. M 2 . . . V2 d 2 w1 r1
6. The angle ratio is calculated by dividing junction angle 1 by junction angle 2. 7. The dominant channel is calculated by an ‘IF’ function: IF (M>1, 1, 2)
This means that if the momentum ratio (M) has a value greater than 1, then stream 1 is the dominant channel. However, if it is less than 1, stream 2 is the dominant channel.
This then tells the user which input channel is dominant in the directionality of the output channel.
Improvements
The model provides a quick answer for what happens mathematically at a confluence. It should be noted however, that it assumes no other inputs or variable outside factors. The model reduces data input time which means more time can be spent on analysis rather than completing the formulae manually.
The Devonshire Model For Confluence Behaviour
INPUTS Channel Channel 1 2 Width (m) Depth (m) Angle (Degrees) Slope (Degrees)
Manning's N 0.02 (smooth) up to 0.05 (rough) =
OUTPUTS Hydraulic Radius 0 0 Angle Ratio 0 Velocity (m/s) 0 0 Discharge (cumecs) 0 0 Discharge Ratio 0 Radius Of Curvature 0 0 Momentum Ratio 0
Dominant Channel 0
Run model, 1 = yes, 0 = no 0 Figure 3. The Devonshire Model for Confluence Behaviour. Results and Analysis-
Bifurcations and confluences were looked at separately; firstly the results for bifurcations will be set out.
Bifurcations
H1- The greatest effect upon the discharge ratio from a bifurcation will come from the influence of the hydraulic radius, i.e.: Width and depth.
Figure 4-
Stepwise Regression: Discharge Ratio versus D2/D3, W2/W3, ...
Alpha-to-Enter: 0.15 Alpha-to-Remove: 0.15
Response is Discharg on 4 predictors, with N = 21
Step 1 2 3 Constant -0.2827 -1.2871 -2.3401
W2/W3 1.37 1.32 1.22 T-Value 4.61 5.88 6.64 P-Value 0.000 0.000 0.000
D2/D3 1.12 1.34 T-Value 3.92 5.52 P-Value 0.001 0.000
Slope Ra 0.93 T-Value 3.21 P-Value 0.005
S 0.877 0.662 0.537 R-Sq 52.81 74.54 84.16 R-Sq(adj) 50.33 71.71 81.37 C-p 33.1 12.0 3.8
The stepwise regression above was performed in minitab using four variables as predictors of the discharge ratio. The predictors used were the ratios of width, depth, slope and junction angle of the two channels. The ratios are derived by dividing the value of channel one by the value of channel two. The stepwise regression above shows that the width ratio is the most accurate predictor of discharge ratio, providing 50% of the R-squared value. The next most important factor is the depth ratio followed by the slope ratio, bringing the overall R-squared value up to just over 81%. The predictor of angle ratio is shown not to have a significant enough effect upon the discharge ratio in order to raise the R-squared value. The final p-value of 0.005 and R- squared value of 81.37% show that this test is both significant and reliable.
The graphs below show the visual relationship between width ratio (fig. 5),depth ratio(fig. 6) and discharge ratio. Width ratio vs. Discharge ratio
6
) 5 s o i s t e a l
r 4
n e o i g
r s 3 a n h e
c 2 m s i i d d ( 1 0 0 0.5 1 1.5 2 2.5 3 3.5 width ratio (dimensionless)
Fig. 5 – Width ratio vs. discharge ratio graph
Depth ratio vs Discharge Ratio
6 5 o i
t 4 a R 3 h t d i 2 W 1 0 0 0.5 1 1.5 2 2.5 3 Depth ratio
Fig.6 – Depth ratio vs. Discharge ratio graph
H2- When the discharge ratio rises, so does the momentum ratio.
By using a simple correlation (Fig. 7) this relationship was shown conclusively to be very significant.
Correlations: Discharge Ratio, Momentum Ratio
Pearson correlation of sum of predictors and Discharge Ratio= 0.963 P-Value = 0.000
Fig. 7 – Correlation of discharge ratio and momentum ratio from minitab
This P-value shows that we can be virtually 100% certain that these two factors have a significant relationship. As a corollary of momentum ratio it was decided that the following formula should be used: 0.5 Q1 cos(ang1) S1 W1 D1 0.5 Q2 cos(ang2) S 2 W2 D2
The relationship should be an almost perfect correlation if the field results collected were correct. The P-Value above and the graph below (Fig. 8) show this to be the case. Discharge ratio vs. sum of predictors ) s
s 6 e l n o
i 5 s n e 4 m i d ( 3 o i t a r
2 e g r
a 1 h c s i 0 d 0 1 2 3 4 5 sum of predictors (dimensionless)
Fig. 8 – Sum of predictors (momentum ratio) vs. Discharge ratio
H3- The sum of the output streams discharge will equal the amount of discharge from the input stream.
To calculate this the discharges of the two output streams were summed and then subtracted from the input stream’s discharge. A percentage error was then calculated to see how close the discharge measurements were (see fig. 9 below).
Input=output Q2 Q3 Input Q ? % error 0.179473 0.398967 0.570152 -0.008287794 -0.0000472530 0.028904 0.005681 0.060665 0.02607997 0.0000158213 0.250731 1.149434 1.254411 -0.145753609 -0.0018283498 0.437877 0.984452 1.261855 -0.160474419 -0.0020249539 0.223759 0.092066 0.10278 -0.213045379 -0.0002189670 0.257531 33.76124 59.42856 25.4097919 15.1006744986 5.18346 20.59741 31.13512 5.354248133 1.6670513864 1.198771 10.41538 4.75702 -6.857130754 -0.3261950473 0.06435 0.05965 0.483675 0.359674672 0.0017396565 6.653804 4.349643 11.13203 0.128578756 0.0143134205 0.085142 0.207295 0.434342 0.141904994 0.0006163532 0.142781 0.101705 0.109029 -0.135456982 -0.0001476872 0.220866 3.216379 4.713871 1.276625497 0.0601784813 0.172828 0.829071 0.646547 -0.355352339 -0.0022975207 0.165521 0.36559 2.576193 2.045081446 0.0526852456 1.05325 0.33566 0.207564 -1.181345288 -0.0024520487 0.522778 2.657485 3.136064 -0.044198609 -0.0013860969 6.189928 6.356177 7.578891 -4.967214371 -0.3764597805 0.071656 0.29298 0.3248 -0.039836525 -0.0001293891 0.074222 0.077244 0.18181 0.030343795 0.0000551680 0.077244 0.182354 0.072544 -0.18705358 -0.0001356965
Fig. 9 – Excel table showing relative discharges and the error term.
From this table it is evident that only 2 out of 21 sites have an error term over 1%, most of the other sites have errors lower than 3 decimal places below 0. This indicates that the sampling was highly accurate and the individual bifurcations were (on the whole) closed systems. The only two that exhibited a significant error had an input higher than the sum of the outputs. This may be due to an additional un-sampled output stream (i.e.: a complicated stream junction) or simply human error.
Confluences
H4- Streams with a higher hydraulic radius will meet at more acute junction angles.
Hydraulic radius vs. Junction angle
80 70 e
l 60 g
n 50 a
n
o 40 i t
c 30 n u
J 20 10 0 0 0.05 0.1 0.15 0.2 0.25 Average H.Radius
Fig. 10 – Hydraulic radius vs. junction angle Contrary to the hypothesis it was found that as hydraulic radius increases, so does the junction angle. This was not as expected, so the relationship between radius of curvature and hydraulic radius was then interrogated. This also proved to be positive, with higher hydraulic radii equating with higher radii of curvature. This means that deeper and wider streams have more turning power to affect the confluence system.
Radius Of Curvature vs. H. Radius
0.25
0.2 . e v A 0.15 s u i d
a 0.1 R
. H 0.05
0 0 1 2 3 4 5 6 7 Radius of curvature
Fig. 11 – Hydraulic radius vs. radius of curvature The radius of curvature was then related to junction angle to show another positive relationship as would be expected through the greater turning force of the deeper and wider channels.
Radius Of Curvature vs. Junction angle
80 70
e 60 l g
n 50 a
n 40 o i t
c 30 n u
J 20 10 0 0 1 2 3 4 5 6 7 Radius of curvature
Fig. 12 – Radius of curvature vs. junction angle
From this chain of graphs it has been made clear that deeper and wider streams (higher hydraulic radius) have greater force behind them to turn and create larger junction angles. It is obvious that the hypothesis has been proved wrong but there is a sensible explanation.
H5- The higher the discharge, the more acute the junction angle.
Input discharge vs. junction angle
80 70 e l 60 g n
a 50
n 40 o i t
c 30 n
u 20 J 10 0 0 2 4 6 8 10 Total input discharge
Fig. 13 – Total input discharge vs. junction angle
The graph above (Fig. 13) shows that junction angle increases with higher discharge. This is in disagreement with the hypothesis. This may be because streams with high discharge meet more infrequently from different aspects of the catchment. This would mean that the junction angle is higher as the larger streams drain into each other only occasionally. Smaller discharge ‘meetings’ occur more regularly and at lower angles as they tend to be draining the same way to begin with.
H6- As momentum ratio becomes higher, so does the junction angle.
This is to be expected as a high momentum ratio indicates a small stream meeting a large one. This imbalance in stream power will create a large junction angle as the radius of curvature force from the large stream is high. This relationship is shown by the graph below (Fig. 14). Momentum ratio vs. junction angle
80 70 60 e l g
n 50 a
n 40 o i t c
n 30 u J 20 10 0 0 1 2 3 4 5 6 7 8 Momentum ratio (dimensionless)
Fig. 14 – Momentum ratio vs. junction angle
This is a significant relationship as proved by a correlation test below.
Correlations: Angle tot, Momentum Ratio_1
Pearson correlation of Angle tot and Momentum Ratio_1 = 0.499 P-Value = 0.025 Fig. 15 – a minitab correlation of momentum and junction angle
Limitations
There are many things that limit the study carried out. The majority of these were due to human error, be it in preparation or in taking the measurements.
In future studies it would be advantageous to take global positioning satellite (GPS) readings at each sample site, as this would not only allow analysis of the entire system, but in a constantly changing environment would stop any possibility of measuring the same site twice.
Depths
Measuring the depth of the channels was limited, especially in some of the larger channels. The depth of each channel was measured using a 3m tape measure but in some of the more powerful channels a measuring rod had to be used as the current was too great for the tape to withstand. In some instances the measuring rod was carried downstream, producing a slanted and hence false depth reading. Multiple measurements could not always be taken as the widths of some channels was too great to measure the depths in the centre. Many of the larger channels were too deep and fast flowing to safely stand in; hence the depth recordings from these were limited to the reach of the measurer. Meaning that these measurements were likely to be unrepresentative of the channel depth.
Additional water sources
Water input from other sources could prove problematic in some cases: for instance if one of the bifurcations had a confluence occurring almost immediately after, perhaps too close to measure a depth and width readings. In this scenario, measurements would have to be taken at the other side of the confluence, perhaps resulting in higher values than expected.
Diurnal variations
It can also be said that the changing discharge throughout the day would have an effect on the study. The data will be relative to a specific time of day that it was collected. Also some braid plains, ‘dry’ up through the day, due to water taking other paths. This may mean channels dry out, creating problems when measuring depth etc.
Reading of equipment
Human error can occur when reading the measurements from the tape measure, clinometer and compass. The location at which these instruments are used is all important. A problem of accessibility can be a problem. The optimum position to obtain the best representative data, can sometimes be hard to get too.
New Science
Measuring bifurcations is a relatively new science. Meaning that the best techniques for measuring bifurcations may not be the same as for measuring confluences. We used the same method for data collection for each junction type, as this is the only way known. Although the data collected may not be of best accuracy.
Conclusions
The results that were collected contained many possible avenues for investigation. It was felt that an equal weighting of hypotheses relating to confluences and bifurcations was required to do the data set justice. It was found that with a large amount of numerical data it is time consuming to input all the data and then perform statistical transformations on it. This is why it was decided that a model should be constructed to help produce the ‘useful’ statistics as quickly as possible.
Hypotheses 1,2,3 and 6 were proved to be correct whereas 4 and 5 were incorrect. These two however were shown to have legitimate explanations for their failure and therefore cannot be considered a problem.
The data set that was worked with was probably too large for the amount of time available and therefore too much was taken on. It is felt however that we made significant findings in this area of cutting edge geographical science and therefore all deserve a nobel prize. A first will be settled for and a price shall be agreed for the use of the “Devonshire model for confluence behaviour”. We will be in touch…
References
Acta, A.R. (1989) Das gletschhervorfeld des Odelwinkelkees (Hohe Tauern) als geographisch-glaziologishes exkursionsziel (mit einer karte es vorfelds 1:5000) Universitat Salzburg.
Best, J.L. (1987) Flow dynamics at river channel confluences: implications for sediment transport and bed morphology, The society of economic paleontologists and mineralogists.
DEM construction for the demonstration of proglacial bifurcation in a braided river channel environment.
Aims.
To complete a levelling survey of a section of channel containing a proglacial bifurcation using a kern leveller and measuring staff.
T complete some trigonometry based calculations to construct the appropriate vectors and distances needed for the construction of a DEM.
To input this data into an appropriate program for DEM construction, interpolated from the field measurements.
Predictions.
If a sufficient sampling strategy is maintained (such as the 1m³ sampling strategy covering the area under investigation), then the DEM should illustrate the bifurcation effectively in a visual manner. This can then potentially be used in conjunction with the calculations from the previous investigation. Error will most likely be present, yet not sufficient to warp the resulting DEM beyond use.
Methodology.
A small area of the braided river system (as shown on the map following) was surveyed. This was completed using a kern leveller, a measuring staff and 30m measuring tape. The kern leveller was positioned at a fixed location on the braid bar overlooking the bifurcation, and remained fixed for the duration of the surveying (to ensure an even and fair observation of the remaining points. One control point was used at the start and end of the survey. This was to ensure that the kern leveller stayed in a fixed position, providing a benchmark to ensure that the leveller remained at the same correlation. The control point was approximately 9metres away from the kern leveller, on the far side of one of the channels that formed the bifurcation further down the stream.
At 1m intervals measurements were taken comprising of elevation and bearing (using the kern leveller) and distance (measured using a 30m tape measure from the fixed point of the leveller tripod). Starting from an initial point downstream of the bifurcation, approximately 30m from the kern leveller (see diagram). In total 155 elevation, distance and bearing points were measured. This included increased sampling at locations of swift topographical variations, and formed a mesh of sample sites covering the observed area (an effective spread of points relative to both the area and the topographical variation will hopefully ensure that a good DEM will be produced).
The data collected was entered into a spreadsheet. This data is then in a condition to be manipulated with use of trigonometry to form a dataset that can be inserted into a GIS program capable of the construction of DEMs. – (With assistance from Prof. Lane) - The point data, as it was when it was compiled, was interpolated in order to form the DEM you see, rather than isolated points that would have resulted. The interpolation does have an error margin associated with it, yet this is negligible when compared to the point density used.
Above: The finished DEM compiled from the interpolated survey data. Conclusions.
As you can see from the DEM, our surveying and sampling strategy have worked effectively to produce an accurate picture of the small scale relief variations for the bifurcation concerned. The bifurcation is clear (white arrows on the left of the image), as well as several other smaller bifurcations in the braided region. The increased sampling at locations of variable topography proved to be a useful addition also, as these are the regions where the majority of the errors occur in normal larger DEMs this was a useful exercise.
This should be sufficient to map the bifurcation mathematically, providing you can obtain field measurements of the depths of the channels concerned. Therefore, a larger scale DEM of this nature for other braided channel systems would be advantageous for an investigation such as the one seen previous to this; enabling results in the field to be collected swiftly and therefore more comprehensively (more results are generally better in terms of general science).
There are several issues that this brings out however:- Bifurcations in braided systems are notoriously variable and change rapidly so a fixed DEM may not be appropriate unless it could be easily updated and adapted to compensate for these short term changes. In addition, manual capture of DEM points using surveying equipment is a very time consuming process, which may not be appropriate for all field sites. Increased manpower or more efficient capture methods may be an improvement worth considering. Equally, taking measurements by hand has numerous errors associated with it, including human error from miscalculations or general errors, and even boredom factors with large numbers of survey points (similar to the errors associating with the manual digitization process in GIS. Formulation of a program that can take the measurements of the variables directly in the field (via laptop) would be very advantageous. It could take the distances, bearing and elevation directly, and complete the trigonometry itself, and display the DEM being constructed. This way, a picture could be seen immediately and errors identified in the field, enabling repeats under the same conditions to be completed, an important factor in such a variable environment.