Garaidh Ghualaich Hydro Scheme. Hydrology Report

Transcription

1 Garaidh Ghualaich Hydro Scheme Hydrology Report Report No: P730-1 / Garaidh Ghualaich River Hydrology Report r0 Hydroplan UK Unit 12, Riverside Park Station Road Wimborne Dorset BH21 1QU Tel: Fax: info@hydroplan. co.uk Web: www. hydroplan. co.uk

2 1 Introduction An accurate assessment of the flow regime in the burn is essential to enable the design of the optimum hydropower scheme on the Garaidh Ghualaich. The optimum scheme will achieve its maximum potential whilst keeping the impact upon the biodiversity of the river and the wider environment to an acceptably low level. This hydrology report explains the methods used to derive the long-term flow duration curves ( FDC) for the site of interest. The FDCs will be used by the Scottish Environment Protection Agency ( SEPA), to determine the required compensation flows and the acceptable maximum abstraction rates. They will also be used to predict the average energy output of the proposed hydro scheme. 2 Method 2.1 Method Steps SEPA have created a set of templates and these have been used as the basis of the analysis. The basic procedure is outlined in the following steps: Setting up a gauging station ( Section 4). Carrying out manual flow gaugings and collecting 15-minute water level data at the gauging station ( Section 6). Establishing a stage / discharge relationship using the SEPA template ( Section 6). Selecting a representative analogue flow measurement station ( FMS) ( Section 7). Deriving the Gauging Station flow duration curve ( FDC) using the SEPA template and comparing it to the Low Flows Enterprise estimate ( Section 8). Assessing the flow regime at the points of abstraction by scaling the gauging station FDC ( Section 9). 3 Catchment Characteristics from Low Flows Enterprise The software package Low Flows Enterprise ( LFE) provides river flow estimations for ungauged catchments. It calculates theoretical flow statistics using catchment area, soil types and prevailing meteorological conditions. LFE was used to estimate the catchment characteristics and to predict the naturalised flow statistics for the Garaidh Ghualaich Gauging Station, which will be used for comparison with the synthesised statistics. Below the catchment characteristics by LFE are shown. for the Garaidh Ghualaich Gauging Station computed Table 1 Characteristic Gauging Station Catchment Characteristics Unit Garaidh Ghualaich Catchment Area km² Rainfall mm 2392 Potential Evaporation mm 427 Runoff mm 2039 Base Flow Index P730-1 Garaidh Ghualaich Hydrology Report R0 Page 2

3 4 Gauging Station A water level sensor with data logger was installed on the Garaidh Ghualaich at National Grid Reference NN and is recording water levels at 15-minute intervals since 02/12/2015. Figure 1 Gauging Station Location The data logger was downloaded at regular intervals and manual current meter gauging was carried out at the same location. The results of the current meter gauging ( discharge measurements) were compared with the corresponding recorded water levels ( stage), to produce the stage discharge relationship, shown in Section 6. P730-1 Garaidh Ghualaich Hydrology Report R0 Page 3

4 5 Stage Discharge Relationship The results of the flow gauging ( discharge) are shown in Table 2 along with the corresponding recorded water levels ( stage). Table 2 Table 3 Table 4 Measured Discharge and Recorded Stage at the Garaidh Ghualaich Gauging Station No. Date Time Start - Stage ( m) Finish - stage ( m) Average stage ( m) Variation Discharge m3/s) 1 02/12/ :15: Steady /01/ :08: Steady /01/ :35: Steady /02/ :33: Steady /02/ :05: Steady /03/ :18: Steady /03/ :40: Steady /04/ :00: Steady /04/ :25: Steady /05/ :10: Steady /05/ :50: Steady /06/ :03: Steady /06/ :40: Steady /07/ :00: Steady /07/ :40: Steady /08/ :31: Steady /08/ :05: Steady /09/ :05: Steady /09/ :40: Steady /10/ :30: Steady /11/ :31: Steady /12/ : Steady /01/ :25: Steady According to LFE, the lowest gauged flow of m³/s is lower than the 95th percentile flow and the highest gauged flow of m³/s is just higher than the Q6 flow, hence a wide range of flows has been covered. Moreover, there has been hardly ever managed to cover the whole range of LFE flows. The results from Table 2 are shown on the chart in Figure 2. P730-1 Garaidh Ghualaich Hydrology Report R0 Page 4

5 Figure 2 Stage Discharge Relationship 1.50 s)stage - Discharge1st approach Modelled Q Gauged Stage ( m) Q (m3 / Q The Single Rating Curve displayed above will be used for the further analysis. It is described bythe equation: Q = ( h ) Q. Discharge ( m3/ s) h... Logger stage ( m) When appliedto the water level record, this relationshipis usedto createa 15-minute flow recordat the gauging station. 6 Long Term Flow Measurement Station The gauging results provide useful information relative to the period over which data was collected. This however, isa relatively short period and to gaina more complete understanding of the flow regime in the Garaidh Ghualaich it is necessaryto establish a long- term flow record for the site by comparing it to a long- standing analogue flow measurement station. Daily Mean Flows from the close SEPA Analogue Stations Ardachy Bridge and Killilan were comparedto the Garaidh Ghualaich flowsin normalised hydrographs. These hydrographs are shown P730-1 Garaidh Ghualaich Hydrology Report R0 Page 5

6 Figure 3 Normalised hydrograph of Gairaidh Ghulalaich Gauging Station and Ardachy Bridge Analogue Station Figure 4 Normalised hydrograph of Gairaidh Ghulalaich Gauging Station and Killilan Analogue Station Both Analogue Stations show a good match with the pattern of Garaidh Ghualaich gauging station. As the proximity with the Garaidh Ghualaich catchment, Ardachy Bridge has a P730-1 Garaidh Ghualaich Hydrology Report R0 Page 6

7 relatively better fit due to possibly better the hydrological similarity. It has been also noticed that there is a limited calibration set of high flow gauge measurements and thus flows above 10m3/ s are of lower confidence than those flows at lower levels. However, both analogue stations will be used for the further analysis and FDC derivation. 7 Creating the FDC 7.1 FDC Creation Methods The SEPA template offers three methods to determine the long-term FDC at the target site: Matching Pairs Method A This method plots the log values of the target station against the log values of the analogue station and fits a trend line through the plot to determine a relationship between the flows at the two stations. Matching Pairs Method B Matching Pairs Method B first filters out the high flows to take only flows corresponding to an analogue flow of the 30th to 99th percentile. It then repeats method A, and is therefore only used if Method A produces unreliable results. Rescaling Method The Rescaling Method looks at the analogue station observed period ( short- term) FDC and compares it to the long- term FDC and then applies the appropriate scaling to the observed period FDC at the target site. With a long flow record and similar meteorological conditions at the target site and the analogue station, the Rescaling Method should produce very reliable results. However, in most cases the resulting FDC is wavy, therefore the Matching Pairs A Method is preferable Matching Pairs Method A Matching Pairs Method A plots the log values of the target station against the log values of the analogue station and fits a trend line through the plot to determine a relationship between the flows at the two stations. Figure 5 Matching Pairs Method A Garaidh Ghualaich log MDFs vs. Ardachy Bridge log MDFs P730-1 Garaidh Ghualaich Hydrology Report R0 Page 7

8 Figure 6 Matching Pairs Method A Garaidh Ghualaich log MDFs vs. Polloch log MDFs Rescaling Method The Rescaling Method looks at the analogue station observed period ( short- term) FDC and compares it to the long- term FDC and then applies the appropriate scaling to the observed period FDC at the target site. Figure 7 Rescaling Method Comparison of short- term ( observed flows) and long-term gauging station FDC ( Ardachy Bridge Station) P730-1 Garaidh Ghualaich Hydrology Report R0 Page 8

9 Figure 8 Rescaling Method Comparison of short- term ( observed flows) and long-term gauging station FDC ( Killilan Station) 7.2 Flow Duration Curves for the Garaidh Ghualaich Gauging Station using different Analogue Stations The choice of one or the other Analogue Station can have a significant impact on the resulting FDC for the site of interest. In order to assess this possible variation, the FDC analysis was conducted for all the three Analogue Stations described earlier. For each the comparison chart, showing the different FDC methods is shown. P730-1 Garaidh Ghualaich Hydrology Report R0 Page 9

10 Figure 9 Modelled Flow Duration Curve for Garaidh Ghualaich Gauging Station using Ardachy Bridge Analogue Station Figure 10 Modelled Flow Duration Curve for Garaidh Ghualaich Gauging Station using Killalan Analogue Station P730-1 Garaidh Ghualaich Hydrology Report R0 Page 10

11 The three methods have produced similar flows for the low flow end of the charts, and these low flows also agree fairly closely with LFE. In the high flow range the curves are not in as close agreement. However, Matching Pairs Method A agrees with LFE throughout the range of flows. For this reason it is recommended that the Matching Pairs Method A is used throughout the rest of the assessment. Both Analogue Stations show fairly good fits in the Matching Pairs analysis, between 0.76 and 0.69, so both of them will be used for the FDC Averaging. 7.3 FDC Averaging using different Analogue Stations The analysis shown in section 7.1 and 7.2 was done for the Analogue Stations Ardachy Bridge and Killilan. For all the Matching Pairs A Method seems to yield the most reliable FDC. In order to produce an even more robust FDC, the resulting FDCs from the respective Analogue Stations will be averaged into a single synthesised FDC. This is shown in Table 3. Table 5 Summary flow statistics for the Garaidh Ghualaich Gauging Station Q % ilesmatching PairsA FDC Ardachy Bridge Q ( m3/ s) Matching Pairs A FDC Killilan Q (m3/ s) Averaged FDC Q (m3/ s) LFE FDC Q ( m3/ s) Mean P730-1 Garaidh Ghualaich Hydrology Report R0 Page 11

12 Figure 11 Modelled Flow Duration Curve for Garaidh Ghualaich Gauging Station Figure 12 Flow Duration curves for Garaidh Ghualaich Gauging Station ( with logarithmic scale) P730-1 Garaidh Ghualaich Hydrology Report R0 Page 12

13 Both FDCs show a good similarity in shape, while the Synthesised FDC predicts higher flow values, except higher than 15-percentiles. This is supported by the good range of flows covered by the gaugings and the logger records of almost a full year. 8 Flow Assessment at the Points of Abstraction The synthesised FDC created in Section 7.3 represents the best estimate of the flow regime at the gauging station. The site of the proposed both intakes have a smaller catchment area and therefore lower flows. To create the FDC for the Intake, the Gauging Station FDC must be scaled down by the ratio of the mean flows given from LFE. Table 6 Catchment Scaling from the Garaidh Ghualaich Gauging Station to the Intake location LFE Q mean m³/s) Gauging Station Q mean ratio Intake GG Intake GG This mean flow ratio is used to scale down the FDC at the Gauging Station to give the FDC at the proposed Intake site ( see Table 5-6 and Figure 13 and 14). This synthesised FDC represents the best estimate of the flow regime at the Intakes. Table 7 Q %iles Summary Flow Statistics at the Intakes Synthesised m³/s) Intake GG1 LFE (m³/s) Synthesised m³/s) Intake GG2 LFE (m³/s) Mean P730-1 Garaidh Ghualaich Hydrology Report R0 Page 13

14 Figure 13 Comparison of the Modelled Intake GG1 FDC and the LFE FDC Figure 14 Comparison of the Modelled Intake GG2 FDC and the LFE FDC P730-1 Garaidh Ghualaich Hydrology Report R0 Page 14