Floodline Assessment Study Eskom Madoda 88-11kV Substation

Floodline Assessment Study – Eskom Madoda 88-11kV Substation and 2x 88Kv powerlines.

Report Prepared for

Sazi Environmental Consulting cc

Report Number SAZ008

Report Prepared by

Project No: SAZ008 Page i

S Dhaver SAZ008_Floodline Assessment Report_FINAL_15032019_001 March 2019

Floodline Assessment Study - Eskom Madoda

Sazi Environmental Consulting cc

SD Hydrological Services (Pty) Ltd Unit 124 Matumi Sands Cnr Rockery Lane & The Straight Pineslopes Extension 58, Sandton Johannesburg South Africa Website: www.sdhydrologicalservices.co.za

Tel: +27 (0) 83 304 7798

Project Number SAZ008

March 2019

Compiled by:

S Dhaver, Pr Sci Nat (400086/10) Hydrologist

E-mail: [email protected]

http://www.sdhydrologicalservices.co.za/

mailto:[email protected]

Project No: SAZ008 Page ii


Table of Contents

Acronyms and Abbreviations ...................................................................................................................... iii

1 Introduction .................................................................................................................. 1

2 Scope of Work .............................................................................................................. 1

3 Storm Rainfall Depths .................................................................................................. 3

4 Flood Hydrology ........................................................................................................... 4

4.1 Peak Flow Methodology ...................................................................................................................... 4

4.1.1 Rational Method ...................................................................................................................... 4

4.2 Summary of Catchment Hydrology and Peak Flows – Rational Method ............................................ 7

5 Floodline Modelling ...................................................................................................... 8

5.1 Introduction ......................................................................................................................................... 8

5.2 Model Development ............................................................................................................................ 8

5.2.1 Adopted Software .................................................................................................................... 8

5.2.2 Roughness Coefficients .......................................................................................................... 8

5.3 Peak Flow Estimation and Model Setup ............................................................................................. 8

5.3.1 Assumptions and Limitations ................................................................................................ 10

5.3.2 Results................................................................................................................................... 10

6 Conclusions ................................................................................................................ 12

7 References .................................................................................................................. 13

Appendices ...................................................................................................................... 14

Appendix A: HEC-RAS RESULTS ................................................................................ 15

List of Tables Table 3-1 Summary of six closest SAWS stations as per the design rainfall software ...................................... 3

Table 3-2 Summary of storm rainfall depths....................................................................................................... 3

Table 4-1 Recommended value for runoff factor (SANRAL, 2006) .................................................................... 4

Table 4-2 Adjustment factor for C (SANRAL, 2006) ........................................................................................... 5

Table 4-3 Summary of catchment hydrology for floodline modelling ................................................................. 7

Table 4-4 Summary of peak flows (m3/s) for floodline modelling ....................................................................... 7

List of Figures Figure 2-1 Locality map ...................................................................................................................................... 2

Figure 4-1 Summary of delineated catchments .................................................................................................. 6

Figure 5-1 Flood modelling setup ....................................................................................................................... 9

Figure 5-2 Summary of the 1:50 year and the 1:100 year delineated floodline ............................................... 11

Project No: SAZ008 Page iii


Acronyms and Abbreviations

Below a list of acronyms and abbreviations used in this report.

Acronyms / Abbreviations Definition

DEM Digital Elevation Model

MAMSL Meters Above Mean Sea Level

Project No: SAZ008 Page 1


1 Introduction SD Hydrological Services (Pty) Ltd has been appointed by Sazi Environmental Consulting cc, to

undertake a floodline assessment study for the proposed Madoda 88/11kV Substation and 2x 88Kv

powerlines; running from Jim Vinning and Duduza substations.

The scope of works entails amongst others the calculation of peak flows at the identified

watercourses/rivers traversing the project area together with the hydraulic modelling of the

watercourses/rivers to determine the 1:50 year and the 1:100 year floodlines.

The section to follow briefly summarises the required scope of work.

2 Scope of Work The scope of works required to undertake the floodline assessment study is listed below:

• Catchment hydrology and Peak flow calculation – Calculate the area, peak flows and

determine the catchment hydrology for respective points of interest.

• Storm rainfall depths – Estimate the storm rainfall depths applicable to the project area.

• Hydraulic modelling – Undertake floodline modelling for the identified watercourses/rivers.

• Surface water report – Provide a floodline assessment report which details all the finding

whilst providing the necessary conclusions.

A locality map indicating the project location is shown in Figure 2-1 below.



Figure 2-1 Locality map



3 Storm Rainfall Depths

The design storm rainfall depths were obtained from the design rainfall software (Smithers and

Schulze, 2002). The programme is able to extract the storm rainfall depths for various recurrence

intervals for the six closest rainfall stations as shown below in Table 3-1.

Table 3-1 Summary of six closest SAWS stations as per the design rainfall software

Station Name SAWS

Number Distance

(Km)

Record length (Years)

Mean Annual Precipitation

(mm)

Altitude (mamsl)

GLEN ROY(STATION) 0476563 W 7.4 43 661 1590

SUB-NIGEL (GM) 0476802 W 7.4 65 701 1577

SPRINGS (RWB) 0476736 W 12.1 96 709 1610

NIGEL-1 0476835 W 12.1 94 703 1555

HEIDELBERG-PS 0476660 W 12.7 61 712 1560

SPRINGS-OLYMPIA PARK 0476766 W 13.7 80 711 1622

The summary of the storm rainfall depths are shown below in Table 3-2, and will be used in the

calculation of peak flows for all applicable catchments.

Table 3-2 Summary of storm rainfall depths

Duration Rainfall Depth (mm)

(m/h/d) 1:2 year 1:5 year 1:10 year 1:20 year 1:50 year 1:100 year 1:200 year

5 m 8.8 12.2 14.8 17.5 21.4 24.7 28.4

10 m 12.7 17.5 21.2 25.1 30.8 35.6 40.8

15 m 15.7 21.7 26.2 31.1 38.1 44 50.5

30 m 20.2 27.8 33.7 39.8 48.9 56.4 64.7

45 m 23.3 32.2 38.9 46.1 56.5 65.3 74.9

1 h 25.9 35.7 43.2 51.1 62.7 72.4 83

1.5 h 29.9 41.3 50 59.1 72.5 83.7 96.1

2 h 33.2 45.8 55.4 65.6 80.4 92.8 106.5

4 h 39.8 55 66.5 78.7 96.5 111.4 127.8

6 h 44.3 61.2 73.9 87.5 107.3 123.9 142.2

8 h 47.8 66 79.8 94.4 115.8 133.7 153.4

10 h 50.7 70 84.6 100.1 122.8 141.8 162.7

12 h 53.2 73.4 88.7 105 128.8 148.7 170.7

16 h 57.3 79.2 95.7 113.3 138.9 160.4 184.1

20 h 60.8 83.9 101.5 120.1 147.3 170.1 195.2

24 h 63.8 88.1 106.5 126 154.5 178.5 204.8

1 d 55.3 76.3 92.3 109.2 133.9 154.6 177.4



4 Flood Hydrology

4.1 Peak Flow Methodology

Due to the size of the onsite project catchments (Figure 4-1), the Rational method was adopted, Peak

flows for the 1:50 year and the 1:100 year event was calculated for the delineated catchment area.

4.1.1 Rational Method

The Rational Method is a hydrological method used to predict peak runoff with the equation being

shown below.

𝑄𝑇 =𝐶 𝐼 𝐴

3.6

Where:

QT = Peak Flow (m3/s for specific return period);

C = Runoff Coefficient (%);

I = Rainfall Intensity (mm/hr); and

A = Area (km2).

The runoff coefficient C is based on a number of different physical characteristics of the site. These

include the vegetation type and the slope drainage properties of the oil. The percentage of land used

for residential or industrial development or under paved roads is also taken into account. The Rational

Method is suitable for small catchments and is a method used extensively around the world.

A spreadsheet calculation using the Rational Method (as presented in the SANRAL Drainage Manual)

was used to estimate peak flows to be used in undertaking the floodline delineation. The runoff

coefficients for each catchment were estimated using the SANRAL Drainage Manual, summarised in

Table 4-1 and Table 4-2.

Table 4-1 Recommended value for runoff factor (SANRAL, 2006)



Table 4-2 Adjustment factor for C (SANRAL, 2006)

The time of concentration was estimated for channel flow using the equations below:

𝑇𝑐 𝑐ℎ𝑎𝑛𝑛𝑒𝑙 = (0.87𝐿2

1000 (𝐻0.85𝐿−𝐻0.10𝐿

(1000)(0.75𝐿))

)

0.385

Where:

Tc channel = time of concentration for channel flow (hours);

L = hydraulic length of catchment (km);

H0.10L = elevation height at 10% of the length of the watercourse (m);

H0.85L = elevation height at 85% of the length of the watercourse (m);

The worst-case rainfall event for each catchment (i.e. duration = time of concentration) was taken from

the storm rainfall depth estimates presented in Table 3-2.

To determine the anticipated peak flows at the respective catchment outlets using the Rational

method, the catchment hydrology of the project area will have to be assessed, this involves:

• Delineation of catchment areas for identified outlets at the identified rivers/watercourses.

• Determining the appropriate runoff coefficient (C-Factor) which best represents the specific

catchment, and is based on site visit observations and/or areal imagery and topography data.

• Determining the length of longest flow path, which is the identified flow path within the specific

catchment from the upstream catchment boundary down to the outlet.

• Calculate the time of concentration (Tc). This is the time taken for a single drop of water to

flow from the furthest point in a specific catchment to the outlet.



Figure 4-1 Summary of delineated catchments



4.2 Summary of Catchment Hydrology and Peak Flows – Rational Method

The Rational method was used to estimate the peak flows for catchment 1. The summary of the

catchment hydrology and peak flows are shown below in Table 4-3 and Table 4-4 respectively.

Table 4-3 Summary of catchment hydrology for floodline modelling

Name Area (km2)

Length of longest

watercourse (m)

Height Difference

(m)

Rainfall Intensity

(Q50) Tc (hours) C-Factor

Catchment 1 15.96 7228 78.1 44.5 1.74 0.42

Table 4-4 Summary of peak flows (m3/s) for floodline modelling

Name Peak flows for various recurrence intervals (years)

2 year 5 year 10 year 20 year 50 year 100 year

Catchment 1 25.1 36.6 46.5 58.9 82.8 108.7



5 Floodline Modelling

5.1 Introduction

Floodline modelling was undertaken for a total of 1 drainage section. The main objective of the floodline

model assessment is to delineate the 1:50 year and the 1:100 year floodline for the section of the

mentioned drainage located within the project area.

5.2 Model Development

5.2.1 Adopted Software

HEC-RAS 5.0 was used for the purposes of routing the peak flows resulting from the 1:50 year and

1:100 year storm event through the identified watercourses/rivers. HEC-RAS is a hydraulic programme

used to perform one-dimensional hydraulic calculations for a range of applications, from a single

watercourse to a full network of natural or constructed channels. The software is used worldwide and

has consequently been thoroughly tested through numerous case studies.

HEC-GeoRAS is an extension of HEC-RAS which utilises the ArcGIS environment. The HEC-GeoRAS

extension is used to extract the cross sections and river profiles from a Digital Elevation Model (DEM)

for export into HEC-RAS for modelling, and is used again to project the modelled flood levels back

onto the DEM to generate the extent of flooding.

5.2.2 Roughness Coefficients

The Manning’s roughness factor n is used to describe the frictional characteristics of a specific surface.

Selection of the Manning’s roughness factor is based on the surface characterisation of the river

section being modelled. The surface characteristics investigated includes vegetation cover and also

the degree of meandering of the river. According to (Chow, 1959), meandering rivers can increase the

Manning’s roughness factor by as much as 30 percent.

Therefore based on the vegetation cover identified from areal imagery and site visit photography, an

average Manning’s n factor ranging between of 0.045 and 0.06 was selected to best represent the

frictional characteristics of the surface of the watercourse which includes the main channel and

floodplain.

5.3 Peak Flow Estimation and Model Setup

The peak flows calculated in section 4 which includes the 1:50 year and 1:100 year peak flows was

routed using the HEC-RAS program, through the section of the drainage located within the project

area.

The summary of the model setup which includes amongst others, the digitised river sections and cross

sections are shown in Figure 5-1 below.



Figure 5-1 Flood modelling setup



5.3.1 Assumptions and Limitations

The following assumptions are made:

• The topographic data provided was of a sufficient accuracy to enable hydraulic modelling at a

suitable level of detail.

• The DEM used to model the section of the drainage was obtained from the Japan Aerospace

Exploration Agency Earth Observation Research Centre (/www.eorc.jaxa.jp), as the level of

accuracy was observed to be much higher than the DEM derived from the 5 m contour dataset.

• A sub critical flow regime, steady state hydraulic modelling was selected for the running of the

model.

• No storage facilities where modelled.

• No flood protection infrastructure was modelled.

• The floodlines produced should only be used for indicative and environmental purposes, and

not for detailed engineering design, unless signed off by a registered engineer.

5.3.2 Results

Summary of the key results are listed below:

• Summary of the delineated 1:50 year and 1: 100 year floodlines are indicated in Figure 5-2

below.

• The HEC-RAS output results are shown in Appendix A.



Figure 5-2 Summary of the 1:50 year and the 1:100 year delineated floodline



6 Conclusions The following is concluded:

• All infrastructures including poles (wherever applicable) should be placed outside of the 1 in

100 year floodline.

• The first option is to ensure all poles fall outside of the 1 in 100 year floodline. However if they

are placed within the mentioned areas, they are required to be fixed securely as per Eskom

design standards, so that when inundated with floodwaters the risk of being washed away or

collapsing is minimal.

Prepared by

Sivan Dhaver, (Pr Sci Nat)

Hydrologist



7 References HEC-RAS 5.0, US Army Corps of Engineers, Hydrologic Engineering Centre, 2016. Introduction to Flood Hydrology, J HAARHOFF and AM CASSA, 2009. SANRAL, “Drainage Manual-Sixth Edition”, The South African National Roads Agency Limited, Pretoria, 2013. WR2005, “Water Resources of South Africa, 2005 Study (WR2005)”, Water Research Commission, Pretoria, 2009.



Appendices



Appendix A: HEC-RAS RESULTS



HEC-RAS Output

Cross Section Profile Peak Flow (m3/s) Water Stage Elevation (m) Velocity (m/s) Froude Number

2391 50 year 82.8 1582.61 1.03 0.39

2391 100 year 108.7 1582.74 1.13 0.4

2270 50 year 82.8 1581.71 1.71 0.84

2270 100 year 108.7 1581.79 1.89 0.87

2115 50 year 82.8 1580.27 1.13 0.49

2115 100 year 108.7 1580.39 1.2 0.49

2016 50 year 82.8 1579.46 1.28 0.65

2016 100 year 108.7 1579.5 1.52 0.74

1923 50 year 82.8 1578.13 1.44 0.78

1923 100 year 108.7 1578.24 1.45 0.7

1814 50 year 82.8 1577.76 0.68 0.27

1814 100 year 108.7 1577.86 0.76 0.29

1654 50 year 82.8 1577.26 0.95 0.48

1654 100 year 108.7 1577.34 1.02 0.48

1530 50 year 82.8 1576.61 0.73 0.37

1530 100 year 108.7 1576.72 0.81 0.38

1411 50 year 82.8 1575.53 1.89 0.99

1411 100 year 108.7 1575.62 1.96 1

1330 50 year 82.8 1574.37 1.26 0.54

1330 100 year 108.7 1574.5 1.33 0.55

1227 50 year 82.8 1573.91 0.84 0.37

1227 100 year 108.7 1574.02 0.91 0.39

1074 50 year 82.8 1573.34 0.9 0.4

1074 100 year 108.7 1573.47 0.95 0.39

810 50 year 82.8 1571.92 1.31 0.54

810 100 year 108.7 1572.06 1.37 0.58

716 50 year 82.8 1571.21 1.16 0.58

716 100 year 108.7 1571.29 1.25 0.58

610 50 year 82.8 1571.21 0.28 0.08

610 100 year 108.7 1571.28 0.34 0.1



481 50 year 82.8 1571.2 0.17 0.05

481 100 year 108.7 1571.27 0.21 0.06

415 50 year 82.8 1571.2 0.16 0.04

415 100 year 108.7 1571.27 0.2 0.05

392 Culvert

352 50 year 82.8 1568.92 1.95 0.92

352 100 year 108.7 1569.1 1.7 0.82

254 50 year 82.8 1568.37 1.03 0.38

254 100 year 108.7 1568.5 1.14 0.41

207 50 year 82.8 1568.32 0.57 0.2

207 100 year 108.7 1568.45 0.65 0.22

152 50 year 82.8 1568.2 0.96 0.39

152 100 year 108.7 1568.32 1.05 0.4

104 50 year 82.8 1567.87 1.45 0.64

104 100 year 108.7 1567.98 1.57 0.65

Documents

Floodline Assessment Study Eskom Madoda 88-11kV Substation