31:T452,html`

3. Zambezi River Hydrology and Natural Flooding

3.1. Relief and Floodplains

Areas adjacent to the banks of the Zambezi River upstream of the proposed project are low-lying floodplains and the width of the river and extent flooded areas expand and contract depending on the flow of the river.

Figure 1 shows a satellite image of the Zambezi River and its floodplain for 12km upstream of the Ngonye Falls. The extent of the flood waters during the last 10-year-flood event (R10) which occurred in 1978 is overlaid in blue and shows how, particularly further upstream, the river has flooded over its banks onto the adjacent low laying areas.

Satellite image of the Zambezi River upstream of the Ngonye Falls (at bottom edge of picture). Light blue area is the modelled extent of flooding during the 1978 flood, the last 10-year-return flood (R10). Click to enlarge.

`32:T52f,html` The Zambezi River above Ngonye Falls is unregulated as there are no dams and so rainfall feeds directly into flow in the river downstream. However, there is a delay in the onset of the rains season leading to increasing flows in the river at Ngonye due to the attenuating effect of the vast Barotse Flood Plains. These wetlands stretch over 500km from Senanga, north of Ngonye Falls all the way to the Zambia - DRC border.

The mean flood peak flow is over ${formatFlow(calMonthly.filter(m=>m.waterMonth==7)[0].flowMean)} which is more than 10 times higher than the mean low flow of ${formatFlow(calMonthly.filter(m=>m.waterMonth==1)[0].flowMean)}. This annual range of flow acting over the relatively flat topography upstream of the Ngonye Falls causes the river to expand and contract through a seasonal flooding cycle.

The communities who live along the Zambezi River in this region have experienced these cycles for millennia and their patterns of settlement as well as practice of traditional ceremonies, fishing, agriculture and animal husbandry are all tied to these cycles.

As well as the change from flood flows to low flows through each year, there is also significant variation in the level of the flood peak from one year to the next driven by variation in total annual rainfall. `33:T452,html` There are nearly 100 years of records of daily river flow for Ngonye Falls derived from the gauge record from Victoria Falls further downstream. During that, nearly 100-year, period the highest flow measured was ${formatFlow(month_max.flowMax)} during ${month_names[month_max.month]} ${month_max.year} and intuitively this leads to the 100-year-return flood (R100) as shown in Fig 3.

Fig 3. also shows the flows for flood return periods less than 100-years and also years since these flows were last observed. These flood return flows are averages and represent the maximum flood flow that can be statistically expected to occur in the given period.

Fig 3 shows that a flow of ${formatFlow(flood_return.filter(fr=>fr.ReturnYears==10)[0].Flow)} can be expected to occur, on average, once in every ten years (R10). However, as these are average periods for the return of a given flood level then the actual number of years since a particular flood may be significantly more or less than the return period. For example, the last R10 flood was observed in 1978, which is 39 years ago. `34:T475,{ var data = [ { y: aecom_noweir_0m.map(a=>a.Elevation), x: aecom_noweir_0m.map(a=>a.Flow), text: aecom_noweir_0m.map(a=>a.FlowLabel), textposition:'top center', mode: 'lines+markers+text', hoverinfo:'x+y+text', name:'Mean', line: {shape: 'spline', color: colr_b(3), width:3}, marker: {size:7}, type: 'scatter' }]; var layout = { width: width>900?870:width-30, height: 350, margin: {t:5,b:60,l:70,r:5}, font: font, showlegend: false, legend: {x: 1,xanchor: 'right',y: 1}, xaxis: { title: 'Flow (m\u00b3/s)', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f' }, yaxis: { title: 'Elevation (m)', zeroline: false, showline: false, separatethousands:true, } } let cap=` The river surface elevation at the 'ZRA Gauge' just upstream of the Ngonye Falls for increasing river flow. The flood return periods are for the flows are also shown.` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }35:T48d,{ var data = [ { y: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>a.Area_ha), x: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>a.Return), text: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>d3.format(',.2r')(a.Area_ha) + 'ha'), mode: 'lines+markers', hoverinfo:'x+text', line: {width: 3, shape: 'spline', color: colr(1)}, marker:{size:8}, type: 'line' }]; var layout = { width: width>900?870:width-30, height: 300, margin: {t:5,b:60,l:70,r:5}, font: font, showlegend: false, legend: {x: 1,xanchor: 'right',y: 1}, xaxis: { title: 'Flood Return', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f' }, yaxis: { title: 'Area Inundated (ha)', zeroline: false, showline: false, separatethousands:true, } } let cap=`Area inundated, in hectares, by the Zambezi River for increasing natural flood returns for the first 19km upstream of Ngonye Falls.` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }36:T472,{ var data = [ { y: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>a.Houses), text: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>a.Houses), x: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>a.Return), mode: 'lines+markers', hoverinfo:'x+text', line: {width: 3, shape: 'spline', color: colr(1)}, marker:{size:8}, type: 'line' }]; var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:70,r:5}, height: 300, font: font, showlegend: false, legend: {x: 1,xanchor: 'right',y: 1}, xaxis: { title: 'Flood Return', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f' }, yaxis: { title: 'Structures Inundated', zeroline: false, showline: false, separatethousands:true, } } let cap=` Number of structures inundated by the Zambezi River for increasing natural flood returns for the first 19km upstream of Ngonye Falls.` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }37:T55f,html` The figures above show that for lower flows of Q90 to R2 the area flooded increases slowly then for floods between R2 and R10 (3,200 m3/s to 6,400 m3/s) the flooded area increases by more than 2.5 times. Beyond the R10 flood the area that is inundated again increases much more slowly. There are no structures flooded for flows Q50 and below and only 2 structures flooded for flows R2 and below. These two structures are shelters for water pumps and not dwellings.

As with the flooded area, there is a large increase in the number of structures subject to flooding for flows between the R2 and R10 level with 242 additional structures inundated by the R5 flood flow and a further 109 additional structures inundated by the R10 flood flow. This shows that there is a significant existing flood risk in the area.

The change in inundation area between the Q90 flow (flow than is exceeded 90% of the time) and R2 flow (flow experienced every other year) of approximately 300 hectares gives an indication of the additional area inundated by the river during an average year between the low flows in October and floods in April. These 300 hectares spread over the 19km of river length equates to a strip of land 80m wide along the river. In reality, Figure 6 shows that the inundation is greater in the north of the study area and much less in the south. `38:T4f5,{ var data = [ { x:[4000,9700], y:[42,10], mode:'lines', line:{color:'black',width:1.5}, hoverinfo:'skip', }, { x: flood_return.map(fr=>fr.Flow), y: flood_return.map(fr=>fr.MeanDays), text: flood_return.map(fr=> fr.ReturnYears + 'yrs'), textposition: 'right', showlegend:false, hoverinfo:'x+y+text', marker: {color: colr_b(3), size: 15}, type: 'scatter', mode: 'markers+text', textfont: {color:'rgb(10,10,10)', size: 12}}, ]; var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:70,r:5}, height:300, font: font, showlegend: false, legend: {x: 1, xanchor: 'right', y: 1}, xaxis: { title: 'Flow (m\u00B3/s)', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f', range:[3000,10000] }, yaxis: { title: 'Average Flood Period (days)', zeroline: false, tickformat: ',.2r', showline: false } } let cap=`Average duration of floods. The durations include the period where flow is higher than the given retrun period.` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }39:T44d,{ function trace(ret, flw, i) { return { y: noweir_backwater.map(a=>a[ret]), x: noweir_backwater.map(a=>a.Distance/1000), mode: 'lines', hoverinfo:'skip', name: ret + ' ' + formatFlow(flw), line: {shape: 'spline',color: colr(8-i), width:1.2}, type: 'scatter' } } var data=return_flows.map((f,i)=>trace(f.Return,f.Flow,i)) var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:70,r:5}, height: 330, font: font, showlegend: true, legend: {x: 1,xanchor: 'right',y: 1, orientation:'h'}, xaxis: { title: 'Distance Upstream of Ngonye(km)', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f', range:[0,10], }, yaxis: { title: 'Elevation (m)', zeroline: false, showline: false, separatethousands:true, } } let cap=`Natural backwater curves for increasing river flows.` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }3a:T722,{ function trace(ret, flw, i) { return { y: d3_backwater.map((b,i)=>d3_backwater[i][ret]-noweir_backwater[i][ret]), x: d3_backwater.map(a=>a.Distance/1000), text: d3_backwater.map(a=>ret), mode: 'lines', hovertemplate:'%{text} - %{x:.1f}km %{y:.2f}m', name: ret , showlegend:true, line: {shape: 'spline',color: colr(8-i,i/6), width:3}, type: 'scatter' } } var data=return_flows.filter(r=>r.Return!='R100' && r.Return!='R50').map((f,i)=>trace(f.Return,f.Flow,i+1)) var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:70,r:5}, height: 350, hovermode:'closest', font: font, showlegend: true, legend: {x: 1,xanchor: 'right',y: 1, orientation:'h'}, xaxis: { title: 'Distance Upstream of Ngonye(km)', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f', range:[0,10] }, yaxis: { title: 'Increase in Water Level due to
Construction of Headworks (m)', zeroline: false, showline: false, separatethousands:true } }; let cap=`Increase in water level due to the construction of the project by distance upstream of the headworks for increasing flow.` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) div.on('plotly_hover', data => { let cHover=data.points[0].curveNumber plt.restyle(div,{hoverinfo:'none',line:{shape: 'spline',color: 'rgba(200,0,0,1)', width:3}},[cHover]) }) div.on('plotly_unhover', data => { let cHover=data.points[0].curveNumber plt.restyle(div,{hoverinfo:'none',line:{shape: 'spline',color: colr(8-cHover+1,(cHover+1)/6), width:3}},[cHover]) }) return div }3b:T571,html` The figure above shows that the rise in water level caused by the headworks is greatest at lower flows and decreases for higher flows. This is because the relative size of the headworks diminishes compared to the size of the river flow during higher floods. Therefore, the additional area that is inundated post-implementation is greatest at lower flows and these areas would have been subject to flood at high flows in any case pre-implementation.

Two points where the pre and post-implementation backwater curves intersect are highlighted above. This shows that at 3.2km upstream of the project (green circle) inundation that would have occurred on average every 5 years (R5) will, post-implementation, occur every 2 years. Similarly, at 3.8km upstream (red circle) inundation that would have occurred on average every 2 years (R2) will, post-implementation, occur every year for 25% of the time. This shows that the impact of the headworks is to increase the frequency and duration of inundation for areas upstream of the project which are already at risk (albeit lower) of inundation prior to construction.

For some areas close to the headworks inundation will become effectively permanent. This area is referred to as the headpond and has been defined as the area inundated by the Q25 flow, that is the flow which occur on average for at least 25% of the year. `3c:T629,{ var data=[ { y: noweir_backwater.map(b=>b['R2']-b['Q90']), x: noweir_backwater.map(b=>b.Distance/1000), mode: 'lines', hovertemplate:'%{x:.1f}km %{y:.2f}m', name: 'Pre-construciton' , showlegend:true, line: {shape: 'spline',color: colr(1), width:1.5}, type: 'scatter' }, { y: d3_backwater.map(b=>b['R2']-b['Q90']), x: d3_backwater.map(b=>b.Distance/1000), mode: 'lines', hovertemplate:'%{x:.1f}km %{y:.2f}m', name: 'Post-construciton' , showlegend:true, line: {shape: 'spline',color: colr(3), width:1.5}, type: 'scatter' }] var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:70,r:5}, height: 300, hovermode:'closest', font: font, showlegend: true, legend: {x: 1,xanchor: 'right',y: 1, orientation:'h'}, xaxis: { title: 'Distance Upstream of Ngonye(km)', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f', range:[0,10] }, yaxis: { title: 'Change in Water Level
Through the Course of
an Average Year (m)', zeroline: false, showline: false, separatethousands:true } } let cap=`Change in water level before and after construction of the headworks through the course of an average year (difference between R2 level and Q90 level).` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }3d:T550,html`

7. Combined Flood Level (CFL)

The water levels and flood outlines for both the natural river and for the river after construction of the headworks have been calculated using a computer hydrodynamic model. This model uses survey data for the shape of the riverbed and riverbanks, an assessment of the conditions of the riverbed and statistics on the likely flow in the river to calculate the water levels and flood outlines.

There are several sources of error and uncertainty in these computer hydrodynamic models including errors in the original surveys and errors and limitations in the setup and design of the computer models. As well as these possible errors there are other effects which may result in higher water levels than predicted by the models including the effect of high winds and boats on the water surface.

To take account of these errors, uncertainties and other effects, an algorithm to add a ‘freeboard’ to the modelled water levels has been produced which results in a set of backwater curves and flood outlines known as the Combined Flood Levels (CFL). The figure below shows the backwater curve for the R2 (2-year-flood) river flow and additional water level added to produce the CFL level. At the headworks the freeboard added for the CFL is 1 vertical meter and it reduces with distance upstream. `3e:T5b8,{ var data=[ { y: cfl_backwater.map(b=>b['R2']), x: cfl_backwater.map(b=>b.Distance/1000), mode: 'lines', hovertemplate:'%{x:.1f}km %{y:.2f}m', name: 'CFL R2 Level', showlegend:true, line: {shape: 'spline',color: colr(1), width:1.5}, type: 'scatter' }, { y: d3_backwater.map(b=>b['R2']), x: d3_backwater.map(b=>b.Distance/1000), mode: 'lines', hovertemplate:'%{x:.1f}km %{y:.2f}m', name: 'D3 R2 Level' , showlegend:true, line: {shape: 'spline',color: colr(3), width:1.5, dash:'dash'}, type: 'scatter' }] var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:70,r:5}, height: 300, hovermode:'closest', font: font, showlegend: true, legend: {x: 1,xanchor: 'right',y: 1, orientation:'h'}, xaxis: { title: 'Distance Upstream of Ngonye(km)', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f', range:[0,10] }, yaxis: { title: 'Elevation (m)', zeroline: false, showline: false, separatethousands:true } } let cap=`Backwater curves for the R2 flood flow. D3 is the raw modelled result and the CFL is the result with a freeboard added.` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }3f:T492,html` When determining the effect of the project on structures (people’s homes and other buildings) the CFL flood outlines will be used including the allowance for error and uncertainty. This gives a conservative estimate for the number of affected structures and significantly reduces the chance that people’s structures will be damaged by flood events more often than predicted.

In the case of analysing the impact, and determining compensation or other mitigation, for land rather than structures then the standard flood outlines (without the addition of the freeboard) will be used. Unexpected flooding impact of land will be temporary (figures above) and will not lead to permanent damage to property and so it will be better to compensate for this at the time when it occurs.

The figure in Section 3.2 above shows that the flow in the Zambezi always changes relatively slowly and the change in the river flow is never fast enough to be a danger to life as people always have enough time (many days) to move as the river level rises. For this reason, error and uncertainty in the modelling of the flood level does not lead to a safety risk. `40:T476,html`

8. Impact on Land and Structures

Section 2 describes the natural flooding risk experienced by areas upstream of the Ngonye Falls and how this is caused by increasing flow in the river leading to higher water levels which inundates larger areas of land and additional structures.

Section 3.1 shows how construction of the headworks for the Ngonye Falls hydropower project will affect the backwater curves and river levels upstream of the project and how this impact will vary depending on the flow of water in the river and the distance upstream of the headworks. These two effects are shown in Figure 16 and can be summarised as:

The increase of water level decreases with greater distance upstream of the hydropower project
The relative increase of water level decreases with greater flow of water in the river

The impact of construction and operation of the project will therefore be to increase, for any given structure or piece of land, the frequency of flooding (how often it happens) as well as the duration of each flood event (how long it lasts) and the depth of water during each flood. `41:T528,html`

8.1 Impact on Land

Construction and operation of the headworks will cause more land to be inundated at all river flows. Figure 21 shows the number of hectares inundated by the river for 19km upstream of Ngonye Falls under natural conditions and following construction of the power station. The figure also shows the difference in area inundated due to the construction of the power station.

Under Q25 flow conditions for example (${formatFlow(1366)}), 1,664ha (16.5 km2) are inundated pre-construction and 1,867ha post-construction which is in increase in flooded area of 204ha. This is the permanently inundated headpond area.

The area of land that is flooded increases as the flow water in the river increases but Figure 21 also shows that the increase in flooded area due to the project generally decreases with increasing flow. The reason for this is clear from Figure 16 which shows how the rise in water level due to construction of the headworks is lower for higher river flows with the impact of the weir becoming almost imperceptible by the R20 flow.

Figure 16 also shows that the increase in water level due to the impact of the headworks decreases with distance upstream. For this reason, the additional area of land flooded also decreases with distance upstream. `42:T48a,{ var data = [ { y: flood_impacts.filter(i=>i.Scheme=='D3.03 B').map(a=>Math.round(a.Area_ha)), x: flood_impacts.filter(i=>i.Scheme=='D3.03 B').map(a=>a.Return), mode: 'lines+markers', name:'Post construction', line: {color: colr(1)}, type: 'line' }, { y: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>Math.round(a.Area_ha)), x: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>a.Return), mode: 'lines+markers', name:'Pre construction', line: {color: colr(4),dash:'dash'}, type: 'line' }]; var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:90,r:5}, height: 300, font: font, showlegend: true, legend: {x: 1,xanchor: 'right',y: 0}, xaxis: { showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f' }, yaxis: { title: 'Area
Inundated (ha)', zeroline: false, showline: false, separatethousands:true, } }; let cap=`` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }43:T4d0,{ var data = [ { y: flood_impacts.filter(i=>i.Scheme=='D3.03 B').map((a,l)=>Math.round(a.Area_ha-flood_impacts.filter(i=>i.Scheme=='No Weir B')[l].Area_ha)), x: flood_impacts.filter(i=>i.Scheme=='D3.03 B').map(a=>a.Return), mode: 'lines+markers', hovertemplate: '%{y:.0f} ha', name:'Difference', line: {color: colr(5)}, type: 'line' }]; var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:90,r:5}, height: 250, font: font, showlegend: true, legend: {x: 1,xanchor: 'right',y: 1}, xaxis: { title: 'Flood Return', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f' }, yaxis: { title: 'Increase Area
Inundated (ha)', zeroline: false, showline: false, separatethousands:true, } } let cap=` Top: Area of land inundated by the Zambezi River before and after construction on the project for increasing flow. Bottom: Increase in area of land inundated due to construction of the project for increasing flow.` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }44:T410,html`

8.2 Impact on Structures

The impact of the project on inundation of structures follows a similar pattern to that of the flooding of additional land area. The figure below shows the number of structures affected by flooding for pre and post-construction depending on the overall river flow as well as the increase in affected structures due to the project. The impact due to the project permanent inundation (the headpond) is 20 structures.

For structures, the project impact is calculated using the CFL flood lines which include an additional vertical distance to take account of any error and uncertainty in the modelling process (see Section 6). The structures used in this analysis were digitised from aerial imagery and there has not been any attempt to differentiate between dwellings, commercial buildings or any other type of structure. A detailed ground survey will be required to determine the type of structures that are impacted and therefore the best mechanism for mitigating the increased flood risk. `45:T47a,{ var data = [ { y: flood_impacts.filter(i=>i.Scheme=='CFL B').map(a=>Math.round(a.Houses)), x: flood_impacts.filter(i=>i.Scheme=='CFL B').map(a=>a.Return), mode: 'lines+markers', name:'Post construction', line: {color: colr(1)}, type: 'line' }, { y: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>Math.round(a.Houses)), x: flood_impacts.filter(i=>i.Scheme=='No Weir B').map(a=>a.Return), mode: 'lines+markers', name:'Pre construction', line: {color: colr(4),dash:'dash'}, type: 'line' }]; var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:90,r:5}, height: 300, font: font, showlegend: true, legend: {x: 1,xanchor: 'right',y: 0}, xaxis: { showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f' }, yaxis: { title: 'No. of Structures', zeroline: false, showline: false, separatethousands:true, } }; let cap=`` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }46:T4cc,{ var data = [ { y: flood_impacts.filter(i=>i.Scheme=='CFL B').map((a,l)=>Math.round(a.Houses-flood_impacts.filter(i=>i.Scheme=='No Weir B')[l].Houses)), x: flood_impacts.filter(i=>i.Scheme=='CFL B').map(a=>a.Return), mode: 'lines+markers', hovertemplate: '%{y:.0f}', name:'Difference', line: {color: colr(5)}, type: 'line' }]; var layout = { width: width>900?870:width-30, margin: {t:5,b:60,l:90,r:5}, height: 250, font: font, showlegend: true, legend: {x: 1,xanchor: 'right',y: 0}, xaxis: { title: 'Flood Return', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f' }, yaxis: { title: 'No of Structures', zeroline: false, showline: false, separatethousands:true, } } let cap=` Top: Number of structures inundated by the Zambezi River before and after construction on the project for increasing flow. Bottom: Increase in number of structures inundated due to construction of the project for increasing flow.` const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) return div }47:T9fe,function backwaters(xrange,yrange,addMarks, cap){ function trace(backwater_ref,ret, flw, i) { var backwaters=backwater_ref=='noweir'?noweir_backwater:d3_backwater; return { y: backwaters.map(a=>a[ret]), x: backwaters.map(a=>a.Distance/1000), mode: 'lines', hoverinfo:'none', name: ret , showlegend:true, line: backwater_ref=='d3'?{shape: 'spline',color: colr(9-i,i/7), width:3}:{shape: 'spline',color: colr(9-i,i/7), width:2,dash:'dash'}, type: 'scatter' } } var data=return_flows.filter(r=>r.Return!='Q90' && r.Return!='R100').map((f,i)=>trace('noweir',f.Return,f.Flow,i+1)) data=data.concat(return_flows.filter(r=>r.Return!='Q90' && r.Return!='R100').map((f,i)=>trace('d3',f.Return,f.Flow,i+1))) if (addMarks){ data=data.concat([{ x:[3.9, 3.05], y:[990.13, 990.35], text:['R2->Q25','R5->R2'], textposition:'top right', textfont:{color:'black'}, marker:{size:20,color:['rgba(150,0,0,1)','rgba(0,150,0,1)']}, showlegend:false, mode:'markers+text' }]) } var layout = { width: width>900?870:width-30, margin: {t:addMarks?10:30,b:60,l:70,r:5}, height: 400, hovermode:'closest', font: font, showlegend: true, legend: {x: 1,xanchor: 'right',y: 1, orientation:'h'}, xaxis: { title: 'Distance Upstream of Ngonye(km)', showgrid: false, zeroline: false, ticks:'outside', tickformat:',0f', range:xrange, }, yaxis: { title: 'Elevation (m)', zeroline: false, showline: false, separatethousands:true, range:yrange } }; const div=container(cap, width>900?890:width-10) plt.newPlot(div, data, layout,{displayModeBar: false}) div.on('plotly_hover', data => { let cHover=data.points[0].curveNumber let c1=cHover>6?cHover-7:cHover let c2=cHover>6?cHover:cHover+7 plt.restyle(div,{hoverinfo:'none',line:{shape: 'spline',color: 'rgba(200,0,0,1)', width:3.5,dash:'dash'}},[c1]) plt.restyle(div,{hoverinfo:'none',line:{shape: 'spline',color: 'rgba(200,0,0,1)', width:3.5}},[c2]) }) div.on('plotly_unhover', data => { let cHover=data.points[0].curveNumber let c1=cHover>6?cHover-7:cHover let c2=cHover>6?cHover:cHover+7 plt.restyle(div,{hoverinfo:'none',line:{shape: 'spline',color: colr(8-c1,c1/6), width:3,dash:'dash'}},[c1]) plt.restyle(div,{hoverinfo:'none',line:{shape: 'spline',color: colr(8-c1,c1/6), width:3}},[c2]) }) return div }48:T47f,function structs(side) { var flows=['s[flow + '_' + side]), y: noweir_struc.map(s=>s.Distance), marker: {color: colr(i)}, type: 'bar', name: flow, orientation:'h', } } var data = flows.map((f,i)=>series(f,i)) var layout = { barmode: 'stack', height: 450, width:230, margin:side=='East'?{t:0,b:40,l:5,r:60}:{t:0,b:40,l:60,r:5}, font: font, legend: {x: 0, xanchor: 'left', y: 1}, showlegend: side=='West'?true:false, xaxis: { //title: 'Number', showgrid: false, zeroline: true, ticks:'outside', range: side=='East'?[0,30]:[30,0] }, yaxis: { //visible: side=='East'?false:true, side: side=='East'?'right':'left', title: side=='West'?'Distance Upstream (km)':'', zeroline: false, showline: false, showgrid:true, separatethousands:true, } }; 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Upstream Impact

Ngonye Falls Hydroelectric Project

\n\n${book_index('upstream-impact')}\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":768,"value":"html`\n

\n \n\n

Interactive map of the upstream impact models for the Ngonye Falls project. Click here for a fullscreen version.

\n\n\n`\n","pinned":false,"hidden":false,"mode":"ojs"},{"id":774,"value":"html`\n
\n

\n1. Introduction
\n2. Sources
\n3. Zambezi River Hydrology and Natural Flooding
\n\n4. Headworks Structures
\n5. Operation of the Headworks
\n6. Headpond and Permanent Inundation
\n7. Combined Flood Level (CFL)
\n8. Impact on Land and Structures
\n\n
\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":17,"value":"html`\n

1. Introduction

\nThe proposed Ngonye Falls hydroelectric will include the construction of headworks infrastructure, comprising a series of adjustable weirs and embankments across the Zambezi River. The headworks are designed to control the elevation of the river and to direct water into the power station for electricity generation.\nThe operation of the headworks will have an impact on the natural pattern of flooding upstream of the power station.\n

\n

2. Sources

\nThe analysis of the natural, pre-construction and post-construction flood risk has been completed by Mott MacDonald, the project’s Owner’s Engineer as part of their work for the project Technical Feasibility Study. The surveys, methods and detailed results of this work are given in their report: MMD-XXXX-FS-RE-G-0830-A Headpond level weir and flood impact assessment report.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":21,"value":"$31","pinned":false,"hidden":false,"mode":"ojs"},{"id":29,"value":"html`\n

3.2 Seasonal Flow Cycles and Year-to-Year Variation

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":74,"value":"html`\nSee also: Catchment Hydrology\n

\nThe figure below shows that the flow in the Zambezi is naturally highly seasonal with low flows July through to January and then increasing flow up to a flood peak generally in April. This pattern is driven by the seasonal nature of rainfall in the Zambezi river basin where there is generally no rainfall at all between April and October and all the annual rain – of around 1,000mm – falls in the rains season from November to March. \n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":32,"value":"meanFlow","pinned":false,"hidden":false,"mode":"ojs"},{"id":82,"value":"$32","pinned":false,"hidden":false,"mode":"ojs"},{"id":89,"value":"floodReturns","pinned":false,"hidden":false,"mode":"ojs"},{"id":94,"value":"$33","pinned":false,"hidden":false,"mode":"ojs"},{"id":122,"value":"html`\nUsing standard statistical methods for river flood it is possible to extend the prediction for return periods of flood flows beyond the observed record of 100 years. The figure above shows the extended flood flows up to return periods of 10,000 years. According to this analysis, a flow of at least 12,000 m\\u00b3/s can be expected to occur once every 1,000 years.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":128,"value":"dailyChange","pinned":false,"hidden":false,"mode":"ojs"},{"id":136,"value":"html`\nAlthough the flow in the Zambezi varies significantly through the year, the rate of change is always slow, and the river does not undergo ‘flash flooding’ where flows and river levels rise and fall significantly over a short period of time.\n

\nThe figure above shows the average daily change in flow in the river by month. The highest changes are seen from February to June, around the annual flood peak. The highest average rate of change is only ${formatFlow(-calMonthly[4].dailyFlowChangeMean)} per day in February. This change is ${d3.format(',.0%')(-calMonthly[4].dailyFlowChangePercentMean)} of the total river flow per day showing that the river flow changes very slowly over time.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":142,"value":"html`\n

3.3 Natural Flood Inundation

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":172,"value":"html`\nFigure 5 shows how the elevation of the river surface increases as the flow of water increases. The area that is inundated by the river is determined by the elevation of the water surface and so this inundation area increases with increasing flow.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":149,"value":"$34","pinned":false,"hidden":false,"mode":"ojs"},{"id":175,"value":"html`\n\nFigure 6 shows how floods with greater return periods – and therefore higher flows – inundate increasingly large areas. Figure 7 and Figure 8 shows the number of hectares and number of structures respectively flooded by these flows for the 19km upstream of Ngonye Falls - the distance that has been modelled. \n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":177,"value":"html`\n

Current natural flood zones upstream of Ngonye Falls. Blues – permanently flooded, Greens - R2 to R15 and Reds - R20 to R100 floods. Click to enlarge.

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":182,"value":"$35","pinned":false,"hidden":false,"mode":"ojs"},{"id":198,"value":"$36","pinned":false,"hidden":false,"mode":"ojs"},{"id":200,"value":"$37","pinned":false,"hidden":false,"mode":"ojs"},{"id":246,"value":"html`\n

\n ${structs('West')}\n

\n ${structs('East')}\n

\n\n

Number of structures flooded under natural conditions by distance upstream of Ngonye Falls. Left chart, west bank structures. Right chart, east bank structures.

\n\n`\n","pinned":false,"hidden":false,"mode":"ojs"},{"id":285,"value":"html`\nFigure 9 shows the distribution upstream of the Ngonye Falls of structures affected by natural flooding risk. The figure shows that within the first 4.5km of the Ngonye Falls there are only 2 structures affected by floods up to the R20 flow, both on the east bank of the river. Further upstream, more structures are affected by natural flooding risk. \n

\nThis pattern occurs because closer to the Ngonye Falls the banks of the river are higher and better defined, so people have built their structures above the natural flood banks. \nFigure 9 also shows that the majority of structures currently subject to flood risk are on the east bank of the river.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":313,"value":"$38","pinned":false,"hidden":false,"mode":"ojs"},{"id":337,"value":"html`\nThe period of time that a flood lasts depends on the flood flow with higher floods generally lasting for a shorter time than lower floods. Figure 10 shows the average number of days that floods of different return periods have lasted. The figure includes the trend-line showing that, as is expected, larger floods generally last for a shorter number of days than smaller floods.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":339,"value":"html`\n

3.4 Backwater Curves

\nAcross distances of up to a few 100’s of meters the surface of a slow-moving river can be assumed to completely flat. Over longer distances the gradient of the surface of the water, known as its backwater curve, becomes more apparent. The slope of the backwater curve and any changes to it caused by building infrastructure in the river is important when considering flooding impacts.\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":354,"value":"$39","pinned":false,"hidden":false,"mode":"ojs"},{"id":374,"value":"html`\nThe figure above shows the backwater curves of the Zambezi River for the 10km upstream of the Ngonye Falls with increasing flows of water in the river. As flow increases, the elevation of the water surface increases – leading to inundation of additional land area – but also the shape and gradient of the backwater curve changes. \n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":386,"value":"html`\n

4. Headworks Structures

\nSee also: Design and Operation of the Power Plant\n
\nThe headworks structures (Figure below), which will be constructed as part of the Ngonye Falls hydropower project, comprise a series of weirs, embankments, gates and barrage which span across the Zambezi River above the Ngonye Falls. The headworks, which are around 3m high, are designed to direct flow into the power canal and to raise the water level upstream of the project and maintain it, to the extent possible, at a constant elevation of 990m. The area upstream of the headworks where water levels are raised is known as the headpond.\n

\nThe headworks include a number of adjustable crest-height weirs (flap-gates) which can be raised and lowered to try to maintain the level of the water behind the headworks at the constant elevation of 990m. \n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":392,"value":"html`\n

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":407,"value":"html`\n

Map of the proposed headworks structures for the Ngonye Falls hydropower project (top) and (bottom) visualisation of the headworks structures in the western channel of the river showing e-flow flap-gates (flowing in foreground) and flood control flap-gates (raised in background).

\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":409,"value":"html`\n

Cross-section through a pneumatically controlled flap-gate. Water flows left to right.

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":389,"value":"html`\n

5. Operation of the Headworks

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":428,"value":"html`\nThe figure below shows that even under natural conditions, before construction of the weir, the river level will rise above, 990m around the weir location, under high flood conditions of approximately ${formatFlow(8500)} or the R50 flood. \n

\nOnce the headworks have been constructed then, even with all of the weir gates fully lowered, then the level of the river behind the weir will begin to rise above 990m at lower flows, above approximately ${formatFlow(5700)} or the R8 flood. For river flows below ${formatFlow(5700)} the river elevation around the location of the weir will be held at 990m.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":479,"value":"backwaters([0,10], null, false ,'Backwater curves for increasing flow rates for the natural state before construction (dash line) and after construction of the headworks (solid line).')","pinned":false,"hidden":false,"mode":"ojs"},{"id":482,"value":"backwaters([0,5],[989,992],true ,'Closeup of a portion of figure above with two crossing points of pre and post-construction backwater curves marked. Red circle: pre-construction R2 and post-construction Q25 flows at 3.8km. Green circle: pre-construction R5 and post-construction R2 flows at 3.05km.')\n","pinned":false,"hidden":false,"mode":"ojs"},{"id":550,"value":"html`\nThe figures above show the backwater curves for the river in its natural, pre-construction state and following construction of the headworks are shown. The lower figure also shows the difference in elevation of the two backwater curves, pre and post-construction for different flow rates and distance upstream. This is equal to the difference in elevation between the two sets of backwater curves in the top figure. \n

\nThe figures show that close to the headworks (within around 2.5km) the water level is higher post-implementation for all flows but is very much less variable than under natural conditions. Further upstream the increase in water level is much more limited and the natural variation in level is maintained. This means that close to the headworks there will be areas which were naturally only inundated under high flood flows which become permanently inundated once the station is constructed.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":554,"value":"$3a","pinned":false,"hidden":false,"mode":"ojs"},{"id":582,"value":"$3b","pinned":false,"hidden":false,"mode":"ojs"},{"id":586,"value":"html`\n

6. Headpond and Permanent Inundation

\nConstruction of the headworks will create an additional area upstream of the project which is effectively permanently inundated which is called the headpond. In this case we define permanent as the water level produced by the Q25 flow of ${formatFlow(1366)}. The water level is at or below the Q25 flood level for 75% of an average year in all locations. \n

\nThe regulating effect of the headworks on the water level upstream of the weirs also means that the change in water level in an average year between the lowest flows in November and highest flows in March and April is reduced. \n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":597,"value":"$3c","pinned":false,"hidden":false,"mode":"ojs"},{"id":608,"value":"html`\nThe figure above shows the effect of this regulation in water level. At 2km upstream of the headworks the water level will remain constant throughout an average year after construction whereas under the current, natural, conditions the variation of water level through the year is around 2m. By around 14km upstream of the headworks this regulating effect has reduced so \nthat 90% of the variation which occurred before construction is present after construction. \n

\nThe figure below shows a map of the permanent inundation (the Q25 flood level) for both pre and post construction. The map also shows the Q25 flood level for the Combined Flood Level (CFL) which is described in more detail in section 6. The map shows that the permanent inundation stretches approximately 3km upstream of the headworks.\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":611,"value":"html`\n

Permanent inundation pre-construction (light blue), post-construction (dark blue) and post-construction including freeboard - CFL (dashed line). Click to enlarge.

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":615,"value":"html`\n

Flood outlines for pre and post-construction for an area on the west bank just upstream of the headworks weir. Click to enlarge.

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":628,"value":"html`\nThe figure above shows maps for pre and post-construction of flood outlines for an area of the west bank around 1km upstream of the headworks. The headpond area (blue shading) is expanded in the post-construction case and the overall variation in water level is reduced as shown by the width of the green zones.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":630,"value":"$3d","pinned":false,"hidden":false,"mode":"ojs"},{"id":810,"value":"The CFL level at any point upstream of the weir is calculated according to Section 6.11.1 of the project Feasibility Study (Mott MacDonald 2018) as the larger of the following two values:
\n

991m (being the headpond level of 990m at the weir, plus a fixed freeboard of 1m) and;
for the point in the river, the pre-construction water level plus 50% of the difference between the pre and post-construction water levels.

\n","pinned":false,"hidden":false,"mode":"html"},{"id":635,"value":"$3e","pinned":false,"hidden":false,"mode":"ojs"},{"id":646,"value":"$3f","pinned":false,"hidden":false,"mode":"ojs"},{"id":648,"value":"$40","pinned":false,"hidden":false,"mode":"ojs"},{"id":668,"value":"$41","pinned":false,"hidden":false,"mode":"ojs"},{"id":676,"value":"$42","pinned":false,"hidden":false,"mode":"ojs"},{"id":691,"value":"$43","pinned":false,"hidden":false,"mode":"ojs"},{"id":720,"value":"$44","pinned":false,"hidden":false,"mode":"ojs"},{"id":722,"value":"$45","pinned":false,"hidden":false,"mode":"ojs"},{"id":726,"value":"$46","pinned":false,"hidden":false,"mode":"ojs"},{"id":1113,"value":"html`\n

9 Flood Maps

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":742,"value":"html`\n

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":736,"value":"html`\n

Flood Return Outlines Post-Construction.

\n`\n","pinned":false,"hidden":false,"mode":"ojs"},{"id":745,"value":"html`\n

Flood Returns Post-Construction Including Freeboard (CFL).

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":849,"value":"

Appendix

","pinned":false,"hidden":false,"mode":"html"},{"id":909,"value":"import {meanFlow} from \"@westernpower/catchment-hydrology\"","pinned":false,"hidden":false,"mode":"ojs"},{"id":914,"value":"import {floodReturns} from \"@westernpower/catchment-hydrology\"","pinned":false,"hidden":false,"mode":"ojs"},{"id":921,"value":"import {dailyChange} from \"@westernpower/catchment-hydrology\"","pinned":false,"hidden":false,"mode":"ojs"},{"id":440,"value":"$47","pinned":true,"hidden":false,"mode":"ojs"},{"id":216,"value":"$48","pinned":false,"hidden":false,"mode":"ojs"},{"id":640,"value":"cfl_backwater=(await d3.csv(upstreamFolder + \"/cfl_backwater.csv?\" + Math.floor(Math.random() * 1000),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":438,"value":"d3_backwater=(await d3.csv(upstreamFolder + \"/d3_backwater.csv?\" + Math.floor(Math.random() * 1000),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":345,"value":"noweir_backwater=(await d3.csv(upstreamFolder + \"/noweir_backwater.csv?\" + Math.floor(Math.random() * 1000),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":343,"value":"return_flows=(await d3.csv(upstreamFolder + \"/return_flows.csv?\" + Math.floor(Math.random() * 1000),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":211,"value":"noweir_struc=(await d3.csv(upstreamFolder+\"/noweir_structures.csv?\" + Math.floor(Math.random() * 1000),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":183,"value":"flood_impacts=(await d3.csv(upstreamFolder + \"/flood_impacts.csv?\" + Math.floor(Math.random() * 1000),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":144,"value":"\naecom_noweir_0m = (await d3.csv(upstreamFolder + \"/aecom_noweir_0m.csv?\" + Math.floor(Math.random() * 1000),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":101,"value":"month_max=monthly.sort((m1,m2)=>m2.flowMax-m1.flowMax)[0]","pinned":false,"hidden":false,"mode":"ojs"},{"id":96,"value":"monthly = (await d3.csv(hydrologyFolder + 'monthly.csv?' + Math.floor(Math.random() * 500),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":34,"value":"calMonthly = (await d3.csv(hydrologyFolder + 'calMonthly.csv?' + Math.floor(Math.random() * 500),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":90,"value":"flood_return = (await d3.csv(otherFolder + \"ngonye_floods.csv?\" + Math.floor(Math.random() * 1000),d3.autoType))","pinned":false,"hidden":false,"mode":"ojs"},{"id":883,"value":"

Sources

","pinned":false,"hidden":false,"mode":"html"},{"id":925,"value":"upstreamFolder = server + '/upstream'","pinned":false,"hidden":false,"mode":"ojs"},{"id":930,"value":"otherFolder = server + '/other/'","pinned":true,"hidden":false,"mode":"ojs"},{"id":898,"value":"hydrologyFolder = `${server}/syntheticFlowSeries/${hydrologySet}/processed/`","pinned":false,"hidden":false,"mode":"ojs"},{"id":896,"value":"peFolder = `${server}/ngonyePlantModels/models/${peModelName}/`","pinned":false,"hidden":false,"mode":"ojs"},{"id":894,"value":"hydrologySet='2024'","pinned":false,"hidden":false,"mode":"ojs"},{"id":892,"value":"peModelName='sh_2024_7.8mhead'","pinned":false,"hidden":false,"mode":"ojs"},{"id":890,"value":"server='https://data.westernpower.org'","pinned":false,"hidden":false,"mode":"ojs"},{"id":864,"value":"

Style

","pinned":false,"hidden":false,"mode":"html"},{"id":881,"value":"function formatFlow(flow) {\n return d3.format(',.2r')(flow) + ' m\\u00b3/s'\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":856,"value":"makeTrans=function(col,opacity) {\n let c=d3.color(col)\n c.opacity=opacity\n return c.formatRgb()\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":859,"value":"style=html`css `","pinned":false,"hidden":false,"mode":"ojs"},{"id":44,"value":"import {font} from \"@westernpower/commonlibrary\"","pinned":false,"hidden":false,"mode":"ojs"},{"id":875,"value":"container = function(captionText, width_over) {\n const div=DOM.element('div')\n div.setAttribute(\"class\",\"border m-2 p-2\")\n div.setAttribute(\"style\",`width: ${width_over?width_over:(width>800?800:width)}px`)\n if (captionText) {\n const caption=DOM.element('figcaption')\n caption.setAttribute(\"style\",\"max-width:100%\")\n caption.append(captionText)\n div.append(caption)\n }\n return div\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":879,"value":"function colr_b(i = 1, a = 1) {\n return d3hsv.hsv(215, 1, ((6 - i + 1.5) * 15)/120, a).toString();\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":877,"value":"function colr(i = 1, a = 1) {\n return d3hsv.hsv(26, 0.85, ((6 - i + 1.5) * 15)/100, a).toString();\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":906,"value":"html`\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":13,"value":"

Libraries

","pinned":false,"hidden":false,"mode":"html"},{"id":5,"value":"import {book_index} from \"@westernpower/commonlibrary\"","pinned":false,"hidden":false,"mode":"ojs"},{"id":110,"value":"import {month_names} from \"@westernpower/commonlibrary\"","pinned":false,"hidden":false,"mode":"ojs"},{"id":36,"value":"plt = require(\"https://cdn.plot.ly/plotly-latest.min.js\")","pinned":false,"hidden":false,"mode":"ojs"},{"id":38,"value":"d3 = require(\"d3@7\",\"d3-hsv@0.1\")","pinned":false,"hidden":false,"mode":"ojs"},{"id":852,"value":"d3hsv = require(\"d3-hsv@0.1\")","pinned":false,"hidden":false,"mode":"ojs"}],"files":"$W49","stdlib":"1","titling":"auto"}