50-YEAR FLOOD LINES FOR THE VAAL
RIVER DOWNSTREAM FROM BLOEMHOF
DAM, ON PORTION 69 OF THE FARM,
KLIPFONTEIN 334 HQ, DISTRICT BLOEMHOF,
NORTHWEST PROVINCE
Compiled by
Garfield Krige
Pr.Sci.Nat. Aquatic Sciences (Reg. No. 400068/10)
of
African Environmental Development
PO Box 1588
Rant en Dal 1751
Tel: 083 657 0560
Fax: 086 670 5102
E-mail: garfield@aed.co.za
Report Number AED-205/2012
Date of Report: 23/01/2012
Revision Number: 00
African Environmental Development
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50-YEAR FLOOD LINES FOR THE VAAL RIVER
DOWNSTREAM FROM BLOEMHOF DAM, NEAR
PORTION 69 OF THE FARM, KLIPFONTEIN 334 HQ,
DISTRICT BLOEMHOF, NORTHWEST PROVINCE
Commission and Hydrology:
African Environmental Development (AED) was commissioned by Mutual & Federal
to determine the 50-year flood lines for the Vaal River in the vicinity of the Bloemhof
Dam at Portions 47 to 78 of the farm, Klipfontein 334 HQ, Northwest Province side of
the Vaal River. The particular study area is Portion 69, the location of the River
Lodge. The Vaal River rises along the Continental Watershed in Gauteng,
Mpumalanga and Kwazulu-Natal Provinces. The catchment surface area of the river
up to the target area is 108 308 Km2 and is shown in Figure 1.
Figure 1: The catchment of the Vaal River upstream from the Bloemhof Dam.
The entire Vaal River falls in a single primary catchment, Primary Catchment C and
across several secondary, tertiary and quaternary catchments as shown in Figures 1
and 2. Each quaternary catchment has its own rainfall and run-off value. The
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quaternary catchments are shown in Figure 2 and the rainfall and surface areas are
summarised in Appendix 1.
Figure 2: The quaternary catchments of the Vaal River up to the target area (Bloemhof Dam).
The catchment of the Vaal River up to the target area is very large, spanning over
many quaternary catchments, each with its own surface area and annual rainfall. We
could not merely add the rainfall values of these quaternary catchments together and
determine the average rainfall over the entire catchment, as each of the catchments
had different surface areas. To derive a truly representative rainfall, we determined a
surface-area-weighted rainfall for each quaternary catchment, in other words, both
the surface area as well as the rainfall over that surface was used in determining the
rainfall contribution for the catchment. This is shown as an MS Excel workbook in
Appendix 1. The entire Vaal River catchment locates in Veld Zone 4 (Grasslands
and Interior Plateau) in terms of the Midgley-classification (Midgley, 1972).
In addition to the Vaal River, there is a second, much smaller, river, which impacts
the study area. This stream, a tributary of the Vaal River downstream from the
Bloemhof Dam, passes along the northern border of the study area from east to
west, before it flows into the Vaal River, shortly after passing the study area. In fact,
the series of stands, Portions 47 to 78 of the farm Klipfontein 334 HQ, of which the
study area is one, locate on a narrow “peninsula” formed between the Vaal River and
this river. Although this stream does not have an official name, we named it the
“Bloem-Stream” for the purposes of this report only. This stream rises on the farm,
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Diamanddoorns 169 HO, some 24.2 Km to the east of the town of SchweizerReneke. It falls in its entirety in Quaternary Catchment C91A, has a catchment of
781.9 Km² and a rainfall of only 464.23 mm/a (Middleton & Bailey, 2005) and locates
in Veld Zone 4 (Grasslands and Interior Plateau) in terms of the Midgley (Midgley,
1972) classification.
Methodology:
The determination of flood lines is done in two steps, 1) modelling of a “design storm”
which gives a discharge in m³/s and 2) modelling this discharge through cross
sections across representative sections of the river/stream at the study area.
The first part of the process comprises the modelling of a series of “design storms”
with statistical return periods of 50-years and durations ranging from 1 to 100 hours
in the case of the Vaal River and 1 to 24 hours in the case of the “Bloem-Stream”,
falling over the catchment of the study area.
When a catchment is larger that approximately 50 Km² (such as the Vaal River and
“Bloem-stream”), the standard statistical Hydrological Research Unit model, as
described by Bauer & Midgley (1974), is used. Storms with durations of 1, 2, 3, 4, 5,
6, 8, 10, 12, 14, 16, 18, 20, 22 and 24 hours were synthesised for the Bloem-Stream,
while storms ranging in duration between 1 and 100-hours were synthesized for the
Vaal River catchment, using modelling techniques described in Report No 1/72
“Design Flood Determination in South Africa” (Midgley, 1972) and Report No 1/74 “A
Simple Procedure for Synthesizing Direct Runoff Hydrographs” (Bauer & Midgley,
1974), both documents produced by a joint venture of the Hydrological Research
Unit, a division of the Department of Civil Engineering of the University of the
Witwatersrand and the CSIR. Using the modelling techniques described by the
documents above, direct run-off hydrographs were derived from these storms. The
hydrograph that produced the highest discharge was selected. A graphical summary
of the hydrographs is presented in Figures 3 and 4.
Figure 3 Vaal River: A graphical representation of the hydrographs showing the 50-year
flood discharges produced by storms ranging from 1 to 100 hours in duration. A storm with an
84-hour duration produced the highest discharge, i.e. 4 606.92 m3/s.
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Flood Discharge Hydrograph
600
500
Discharge (m³/s)
400
300
200
100
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Storm Duration in Hours
Figure 4 “Bloem-Stream”: A graphical representation of the hydrographs showing the 50year flood discharges produced by storms ranging from 1 to 24 hours in duration. A storm
with an 8-hour duration produced the highest discharge, i.e. 585.0 m3/s.
In the second part of the process, the discharge in m3/s, produced by the storm with
a statistical return period of 50 years which produced the greatest discharge, is
routed through cross sections across the river within the study area, as shown in
Figure 5, using Mannings formulae for Open Channel Flow. Mannings roughness
coefficients for the streambeds were assessed using satellite photography dated
12 October 2008 (Google Earth, 2008). A total of 3 cross sections were plotted
across representative sections of the Vaal River and 3 across the “Bloem-Stream” at
right angles to the direction of stream flow.
For this particular exercise we used a combination of 5-meter contour lines
obtainable from the Chief Surveyor General (SG), and 0.5-meter contour lines
surveyed particularly for this project. This survey was commissioned, as the SG
contour lines had 5-meter intervals and were not of a sufficiently high resolution. For
this reason we contracted a land survey company, African Land Surveys, to survey
the area in the vicinity of the two streams. During the survey, however, the landowner
on the southern side of the Vaal River, denied the surveyors access to his land,
located roughly opposite the study area, i.e. on Portion 1 and the Remaining Extent
of the farm Goede Hoop 809 on the Free State Province side of the Vaal River. For
this reason, we used the SG 5-m contour lines to fill in the missing spaces in the
survey. The flood lines in this area are referred to as “Inferred 50-year flood lines”, as
opposed to the other flood lines, drawn on the high-resolution contour lines, referred
to a “50-year flood lines”.
Please note that the 5-m contour lines from the SG are accurate to that particular
resolution (5-m), but for this particular project it was necessary to obtain a higherresolution contour line set, hence the survey by African Land Surveys. There is,
however, nothing wrong with the SG contour lines, the only difference being the
resolution to 5-m instead of the 0.5-m resolution of our survey.
The 5-m contour lines were also used to determine the gradient of the Vaal River and
to obtain the general 50-year flood lines for a larger length of the Vaal River, while
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the 1-m contours were used to plot the flood lines in the vicinity of the study area to a
higher vertical accuracy. For this reason, we only certify the 50-year flood lines
between cross sections 5 and 8 as shown in Figure 5.
Figure 5: The cross sections used in determining the final 50-year return period flood lines for
the Vaal River and “Bloem-Stream”. Also shown are the contour lines and the area where the
surveyors were refused access by the landowner.
The gradient of the river at this particular site was determined using the difference
between the elevation at the crest of the DWA Weir and the elevation at its
downstream foot. The gradient (or fall) of the river is particularly low in this section of
the river, with a fall of only 0.000 352 9 m/m. the water level in the Vaal River for all
intents and purposes was the same all the way from the weir up to the Bloemhof
Dam wall.
In spite of the surveyors not covering the area they were refused access, we were
still able to produce 3 cross sections across the Vaal River, one upstream and two
downstream from the area. It turned out that, due to the relatively narrow area where
the weir was constructed, the cross section immediately upstream from this weir
produced the highest elevation for the 50-year flood. This elevation was
subsequently used for the entire distance from the weir to the Bloemhof Dam wall for
the 50-year flood line.
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Results:
The elevation containing the maximum discharge, at each cross section along the
two rivers, were plotted on either side of the rivers’ centre-line and transferred, in
plan, to the drawing, to demarcate the 50-year flood lines. The resulting flood lines
are supplied as a separate CAD file together with this report. For convenience, we
have also converted the AutoCAD files to a Google Earth file. Please note that the
latter file is meant as a guide for easy reference only, but it should be noted that the
Google Earth resolution is not nearly as accurate as the AutoCAD files from which all
the graphics used in this report were produced. The flood lines for the study area are
shown in Figures 5 and 6.
Figure 6: The 50-Year flood lines for the Vaal River and “Bloem-Stream”, shown on a colourcoded elevation map of the area covered by the high-resolution survey. As can be seen, the
flood lines of the “Bloem-Stream” is at a lower elevation than the flood lines for the Vaal River
and therefore the Vaal River flood lines will “flood” the lower “Bloem-Stream” flood lines.
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Photo 1: The DWA Weir in the foreground, some of the buildings neighbouring the River
Lodge to the left of the photo and the Bloemhof Dam wall in the distance. It can be observed
that most of the buildings are built on stilts, presumably in an attempt to clear the floodwaters.
Photo 2: The same buildings shown in Photo 1, but this time during the January 2011 floods!
(Both Photos 1 and 2 courtesy Panoramio, http://www.panoramio.com)
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Comments:
Flood Line Elevations:
Due to the presence of the DWA weir shown in Photo 1, the 50-year flood line along
the entire length of the Vaal River within the study area is at a single elevation. This
elevation is 1 220.36 mamsl (mamsl = “metres above mean sea level”). The “BloemStream”, however, show a more traditional trend in the flood line, i.e. it follows the fall
of the river. The 50-year flood line begins at an elevation of 1 217.04 mamsl at Cross
Section 1, remains at that elevation up to Cross Section 2 as a result of the damming
effect of the bridge (access road to the study area) over the stream and then falls to
1 216.65 mamsl at Cross Section 3. It is therefore clear that the Vaal River flood
lines are considerably higher than those of the “Bloem-Stream” and therefore the
only flood lines that would be effective will be the flood lines of the Vaal River.
The flood lines of the Vaal River are significantly higher that the highest elevation on
the “peninsula” between the Vaal River and the “Bloem-Stream”, where the River
Lodge locates. The highest elevation of the entire strip of land was actually recorded
at the River Lodge itself. This elevation was at the residence of the landowner (on the
front porch of the homestead) and was recorded as 1 219.48 mamsl, while the 50year flood line was at an elevation of 1 220.36 mamsl. The highest part of the
“peninsula” will therefore be covered with water to a depth of almost 900 mm during a
flood with a return period of 50-years. The “peninsula” will also be cut-off form the
town of Bloemhof as the lowest point of the bridge over the “Bloem-Stream” is only
located at an elevation of 1 216.5 mamsl. This means that this road would be
covered by 3.86 m of water during a 50-year flood.
The Potential Impact from the Bloemhof Dam:
The Bloemhof Dam in the Vaal River upstream from the study area (refer Photo 3)
has a total of 20 radial sluice gates. Theoretically each of these sluice gates can
discharge a maximum of 500 m³/s for a short while when the dam is at 100 % full
supply level. This means that theoretically this dam can produce a man-made flood
of 10 000 m³/s, significantly more than the 50-year flood of 4 606.92 m³/s or the 100year flood of approximately 5 220.59 m³/s. This, flow, can, however, only be
sustained for a short period, i.e. until the dam’s level begins to drop. Unlike the Vaal
Dam, which was constructed with an additional 26 % spare capacity above its 100%
mark, which is intended to be used for flood attenuation purposes, under normal
circumstances, the Bloemhof Dam can only let out what flows into it.
The Potential Impact from the Road Bridge Downstream from the DWA Weir:
There is a bridge immediately to the west of the study area crossing the Vaal River.
This bridge connects the R59 road with the town of Bloemhof. The impact from this
bridge was not assessed during this study. The survey we commissioned did not
cover the extent of this bridge. However, this bridge was constructed taking into
account the floods likely to occur in the Vaal River, it has a wide span and would
probably not cause a significant backup of water during a 50-year flood.
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Photo 3: The 20 radial sluice gates at the Bloemhof Dam spillway
CAD Drawings:
The GIS program, ArcView/ArcGIS, was used to produce the drawings used in this
document. Please note this programs swaps the + and – signs of the X and Y
coordinates in the drawing when compared to AutoCAD files.
General:
It is likely that even the highest point of the ground surface of the entire “peninsula”
on which Portions 47 to 78 of the farm Klipfontein 334 HQ are located will be
underwater during a flood with a return period of 50 years in the Vaal River.
River Lodge
(Portion 69)
Figure 7: Portions 47-77, Klipfontein 334 HQ. The River Lodge is located on Portion 69.
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100-Year Flood Lines: Vaal River at Bloemhof
Certification of Methods Used
23/01/2012
TO WHOM IT MAY CONCERN,
This is to certify that the 50-year flood discharge values for the Vaal River and the
smaller river to the north of the Vaal River, between the town of Bloemhof and the
Vaal River were derived using methods described in the Report No 1/74 “A Simple
Procedure for Synthesizing Direct Runoff Hydrographs” of the Hydrological Research
Unit, a division of the Department of Civil Engineering of the University of the
Witwatersrand.
The 50-year return period storms were synthesised from direct run-off hydrographs
developed using methods described in Report No. 1/72 “Design Flood Determination
in South Africa” of the same unit.
Both the above reports were developed as a joint venture between the CSIR and the
University of the Witwatersrand and are considered to be the only accurate methods
for use under rainfall conditions in South Africa.
The flood discharges determined in terms of paragraph 1 were routed through crosssections plotted across the two rivers at regular intervals and using Mannings theory
for Open Channel Flow. These design floods through these cross sections produced
the flood elevations for the 50-year return period storms, as indicated in the
accompanying CAD files.
P. R. Bond
Pr. Eng. (Reg. No. 740216)
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References:
Midgley, D. C.: (1972) CSIR/University of the Witwatersrand Report No. 1/72, ‘Design
Flood Determination in South Africa‘
Bauer, S. W., Midgley, D. C.: (1974) CSIR/University of the Witwatersrand Report
No. 1/74, ‘A Simple Procedure for Synthesizing Direct Runoff Hydrographs‘
Midgley, D. C.; Pitman, W, V.; Middleton, B. J. (1994). ‘Surface Water Resources of
South Africa’, Water Research Commission Report No. 298/94
Middleton, B. J.; Bailey, A. K. (2005): Water Resources of South Africa, 2005
(WR2005) Version 1. Water Research Commission Project No. K5/1491
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Appendix 1:
African Environmental Development
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E-mail: - garfield@aed.co.za
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AutoCAD and Google Earth Files
Double-click on the above icon to open the zipped folder
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Appendix 2: Catchment Surface Areas and AreaWeighted Rainfall Values
Double-click on the above icon to open the MS Excel Workbook
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