Visual effects of waterfalls affected by water diversion

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1 Visual effects of waterfalls affected by water diversion R.M. Boes 1, P.H. Hiller 2, Å. Killingtveit 3 1 Laboratory of Hydraulics, Hydrology and Glaciology (VAW) ETH Zurich 8092 Zurich SWITZERLAND 2 Sweco Norge AS (formerly 3 ) 7030 Trondheim NORWAY 3 Department of Hydraulic and Environmental Engineering Norwegian University of Science and Technology 7491 Trondheim NORWAY boes@vaw.baug.ethz.ch Abstract: When considering the renewal of a hydropower or other water withdrawal concession, the general benefits of the scheme have to be weighed against its ecological disadvantages. Loss of water from a scenic waterfall will often be considered as a considerable drawback. Also, if granting concession, the licensing authority will limit the minimum release discharge, which may have a considerable impact on project economy. The visual impact of reducing the discharge in a waterfall is significant for the feasibility of hydropower or irrigation projects, but no generally accepted guidelines for quantifying the relationship between discharge and visual appearance currently exist. There is thus a need for a methodology to determine the minimum residual flow of waterfalls. Based on results from a case study covering seven waterfalls in Norway and Austria, such a methodology is presented accounting for the visual effects of flow changes at waterfalls. A guideline for stepwise residual flow determination at waterfalls is given. Keywords: extent, residual flow, visual flow appearance, waterfall, water diversion, waterfall magnitude 1. Introduction If water is extracted from a watercourse, the appearance of a downstream waterfall will change. The conflict of individual conception of pristine nature and technical use of water is currently a matter of intense discussion. There is thus a need for a generally accepted methodology to determine the minimum residual flow of waterfalls. This methodology should find a compromise between the diverging interests involved, e.g. allowing economical operation of hydropower plants or irrigation schemes, while avoiding a devaluation of waterfalls affected by water diversion. A case study covering six waterfalls in Norway and one in Austria was conducted to develop a methodology for determining the visual effects of discharge reduction at waterfalls (Hiller 2009). Although these changes are in relation to hydropower development in this study, the proposed residual discharge determination may as well be applied to other water extraction purposes such as irrigation and drinking water, navigation, recreation, or flood protection. The residual flow of interest here is merely related to the visual flow appearance of waterfalls, as these are special elements of a landscape. Reduced discharge in a waterfall will result in a change in the scenic component and thus leads to a modification of the landscape s character, typically of relevance in environmental impact assessments. The minimum residual flow required for the sake of flora and fauna along a watercourse is not further treated below.

2 2. Classification of waterfalls There are many descriptions of waterfalls, but there is still no generally used classification. Since waterfalls are changing their appearance continuously, it is hard to find relevant criteria, and literature is spare. According to Plumb (1993), the visual magnitude of a waterfall m s,f,g can be expressed by m s,f,g = m s a f a g, (1) where m s is a log-scaled factor quantifying the waterfall height h and width w, a f represents the effect of discharge, and a g the gradient of the waterfall. The larger m s,f,g, the higher is its visual impact. In the international waterfall classification system (Beisel, 2006), waterfalls are rated in 10 classes. The rating value is the natural logarithm of the water volume of the waterfall, corresponding to the product of the mean discharge Q m and the time the water needs to fall down, depending on its geometry. Waterfalls with Q m < 1 m 3 /s are excluded from this classification and are rated in class 0. Niagara Falls on the US/Canadian border get a Beisel rating of 10.0, thus becoming the world s most important falls according to this classification, whereas their magnitude according to Plumb (1993) amounts to m s,f,g = 130. The waterfalls of the present case study are evaluated in Table 1 after Plumb (1993) and Beisel (2006). The two classifications do not closely correspond. Plumb s method prioritizes high vertical waterfalls. After Beisel all present waterfalls with Q m > 1m 3 /s are of class 3. Stuibenfall in the Austrian Alps scores highest with m s,f,g = 50 after Eq. (1) and is second only to Prestfossen according to the international waterfall classification system (Beisel, 2006). With a height of 150 m, it is one of the largest waterfalls in Austria. Table 1: Waterfalls classification of the present case study after Plumb (1993) and Beisel (2006) height h width w average discharge Q m Plumb (1993) Beisel (2006) Waterfall [m] [m] [m 3 /s] magnitude m f,s,g rating class Hokfossen (N) * Prestfossen (N) Dølanfossen (N) to Tverrgjuvlo (N) * Ingdalen (N) Råna (N) Stuibenfall (A) to * excluded due to Q m < 1 m 3 /s 3. Methodology of case study The relationship between discharge and appearance of waterfalls was observed in seven cases, six located in Norway and one Alpine case in Austria: Hokfossen on Sagelva River, Trondheim community, Sør-Trøndelag: small waterfall close to a steep river. No water diversion planned. Prestfossen on Garbergelva River, Selbu community, Sør-Trøndelag: wide waterfall. Application to build a 6.82 MW hydropower plant processed to the authorities. Dølanfossen on Homla River, Malvik community, Sør-Trøndelag: wide waterfall with a rock nose dividing the water and featuring a partly destroyed timberbox dam at the top. Application to develop hydropower refused in The river has been protected by law since Waterfall on Tverrgjuvlo River, Voss community, Hordaland: Narrow waterfall with free-falling water. Application to build a 7 MW hydropower plant processed to the authorities. Waterfall on Ingdalelva River, Agdenes community, Sør-Trøndelag: Flat waterfall with a natural rock channel. Ideas to develop hydropower. Ongoing acoustic sampling of waterfall.

3 Waterfall on Råna River, Selbu community, Sør-Trøndelag: Narrow waterfall with free-falling water in several steps. Hydropower development under design. Stuibenfall on Horlachbach Creek, Tyrol, Austria: Highest waterfall in the Tyrol with free-falling water in two steps, being a regional attraction. A portion of the Horlachbach flow has been diverted to a reservoir for hydropower use since 1980 some 5 km upstream. The maximum discharge withdrawn amounts to 4900 l/s, representing 258% of the mean discharge at the falls. Discharge-related pictures from 1971/1972 (Figure 1) were part of the residual flow determination in the frame of the concession application (Bundesministerium für Land- und Forstwirtschaft, 1973). a) b) Figure 1. Discharge-related image series of Stuibenfall at a) Q = 650 l/s, b) Q = 2250 l/s. For all Norwegian cases, pictures of the waterfalls were regularly taken by automatic cameras, and the river discharge was measured simultaneously, resulting in a discharge-related picture series. Each series shows the appearance of the waterfalls at different discharges (Figure 1). For the Austrian case, a series of photos of the waterfall for a number of known discharges has been evaluated. To evaluate the area of the waterfall wetted by water, the images at selected discharges were printed on checked paper (Figure 2). The water-covered area was thereafter measured in squares. The applied method is influenced by the perspective and zoom of the camera. Discharges Q i and water-covered areas A i were normalized with the respective values at median discharge Q 50% and the corresponding area A 50%, respectively. Figure 4 illustrates that waterfalls change their appearances most visibly from low relative discharges up to about Q i /Q 50% = 1.0, whereas the curves level off for higher relative discharge. Narrow and steep waterfalls like Tverrgjuvlo (Figure 1) and Råna show a more distinct bend than wider and flatter falls like Prestfossen (Figure 2). Here, Q clf denotes the so-called common low flow required by Norwegian legislation as residual flow, representing approximately a low flow Q 95% that is not undercut during 95% of the year. From Figure 4, the median discharge Q 50% is taken as a typical reference in the further evaluation process of residual flows below.

4 a) b) c) Figure 2. Discharge-related image series of Tverrgjuvlo Waterfall at a) Q clf = 91 l/s, b) Q = 293 l/s, and c) Q 50% = 519 l/s. a) b) Figure 3. Water-covered area in Prestfossen for Q = a) 726 l/s, and b) 7926 l/s. Water coverage in squares is indicated in colours: red = 100%, orange = 75%, yellow = 50%, green < 25% and no colour for no coverage.

5 Figure 4. Relative discharge versus relative water-covered area for all cases of the study. 4. Results The procedure developed from the case study to determine a residual flow value for a given waterfall contains nine steps described hereafter. All steps should be considered consecutively Step 1: Provision of basic data The basic data contain: (i) an image series of the waterfall considered, particularly in the low flow season; (ii) data of simultaneous discharge measurements, and (iii) specifications on the expected design discharge Q d of the hydropower development and residual flow Q r Step 2: Selection of discharge-related images Images are selected showing the waterfall with the probable residual flow demanded in the regulatory approval procedure, mean discharge Q m and median discharge Q 50%. Additional images are selected at 40% and 60% of Q 50% as well as the image showing the waterfall at the lowest observed discharge Step 3: Determination of wetted area covered by water, A i A i is determined on each selected image and related to the area at Q 50%, as shown in Figure 4. The checked paper method described above can be used (see Figure 3) Step 4: Duration of quasi unchanged appearance, t na As outlined in Figure 5 for the Hokfossen case, the time tna (subscript na = not affected) a waterfall is not significantly affected by water diversion is determined from t na [-] = 1 [t(q r ) t(q d )] (2)

6 Here, Q d denotes the design discharge of a potential diversion scheme, e.g. of the turbines in case of hydropower use, and Q r stands for a possible residual flow. The assumptions underlying Eq. (2) are (i) there is sufficient waterfall discharge from a visual appearance perspective once the design discharge is surpassed, i.e. for Q > Q d, and (ii) there is no water withdrawal for Q < Q r. The time t na from the duration curve is a dimensionless number between 0 and 1. Figure 5: Duration of unchanged appearance t na for Hokfossen with Q r = Q clf and Q d = 2Q m Step 5: Extent of appearance impact, E With the increase of environmental relevance, impacts on the environment have to be dealt with. Nonmonetary impacts are judged on the basis of extent, value and consequences. Extent E characterizes the changes due to the impact. The extent E in the appearance of the waterfall depends both on the duration (1 t na ) and the amount of Q r. It may be expressed by the product of the relative area covered by water A r /A 50% and t na from Eq. (2), where A 50% = area covered by water at Q 50%, as E = (1 A r /A 50% ) t na (3) Here, E 0 represents a regime close to natural conditions without diversion. For A r A 50%, i.e. typically for Q r Q 50% = median discharge, the extent E of the impact on waterfall appearance varies between 0 and 1. According to Eq. (3), E = 1 for either A r = 0, i.e. for Q r = 0, or for t na = 0; i.e. there is either no residual flow, or water is constantly diverted from the watercourse considered. In these cases the waterfall would be highly affected in its visual flow appearance. For the waterfalls of the case study the E-values of each image are plotted against the ratio Q r /Q 50%, as shown in Figure 5. The trendline shown in Figure 5 can be approximated by E = Q r /Q 50%. (4) 4.6. Step 6: Determination of waterfall value Value and extent are evaluated for each protected commodity of a project, e.g. natural scenery. Value has a scale from small over medium to large. The consequences of an impact are divided into five categories from "strongly negative" to "strongly positive". If value and extent of one topic are fixed, the consequence is evaluated with a so-called consequence matrix (Figure 7).

7 The consequence of the extent E on the appearance of a waterfall depends on its value. The classification systems of Beisel (2006) and Plumb (1993) can be taken into account to evaluate the waterfall value. Further evaluation criteria are location, accessibility, contribution to the surrounding landscape, seasonal discharge variability and cultural or touristic value of the waterfall. It is recommended to set up a list of criteria to determine the waterfall value and to apply it for each evaluation. Figure 6: Relative Q-E diagram for waterfalls of case study assuming Q d = 2Q m, except Stuibenfall with Q d = 2.58Q m Step 7: Residual flow to keep a sufficient appearance, Q r Residual flow Q r is recommended to be selected such that for a given waterfall value the water diversion has little or medium negative consequences on its visual appearance according to the consequence matrix in Figure 6. For instance the extent E may be chosen up to about 0.6 in case the waterfall has a large value. Depending on the waterfall value and the admitted extent E, one subsequently obtains the ratio Q r /Q 50% from the relative Q-E diagram (cf. Figure 6) and can determine Q r Step 8: Evaluation of consequences for selected Q r For the selected Q r, the consequence is evaluated from Figure 6, combining the extent E from Eq. (3) and the waterfall value. The lower the value, the higher the extent E may be selected in step 7 to limit the negative consequences of water extraction from a waterfall Step 9: Review of selected residual flow, Q r Reviewing the discharge-related images and comparing those with the selected Q r helps to check if the selection is also adequate from a limnology point of view. Possibly, there is need to analyze more images to increase the number of data. Plotting A i /A 50% against Q i /Q 50% may help to recognize critical discharges where the waterfall suddenly changes its appearance significantly, as shown in Figure 4 for the case study. The characteristic hydrological regime of the relevant watercourse may be accounted for here. For this step, people who have not been involved previously in the evaluation process should join to eliminate possible subjective influences.

8 Figure 7: Consequence matrix, adapted to E-values, after Statens vegvesen (2006) 5. CONCLUSIONS The nine step procedure presented above describes how to observe, measure, analyze and evaluate the appearance of a waterfall related to discharge and water utilization schemes. The visual effects of flow changes are quantified, based on the median discharge as the reference. The main interest lies in highlighting the differences in the appearance of a waterfall at different discharges, rather than comparing various waterfalls with each other, using the methods of Beisel (2006) and Plumb (1993). The extent of the reduced discharge is combined with the waterfall value to evaluate the consequences of water extraction. This allows for incorporating the developed methodology in ongoing proceedings dealing with the environmental effects related to water utilization developments. 6. ACKNOWLEDGMENTS The data on the Stuibenfall case were provided by the Utility TIWAG-Tiroler Wasserkraftwerke AG, Innsbruck, Austria. The support of Dr. Robert Reindl of TIWAG is kindly acknowledged. The results of the present study were elaborated in the framework of a joint Master Thesis (Hiller, 2010) between the Norwegian University of Science and Technology (NTNU) in Trondheim and the Swiss Federal Institute of Technology, ETH Zurich. The support of Dr. Lars Jenssen, formerly at NTNU, is kindly acknowledged. 7. REFERENCES Beisel, R. H. (2006), International waterfall classification system, Outskirts Press Inc., Denver CO, USA. Hiller, P.H. (2010), Flow and appearance of waterfalls, Master Thesis, Norwegian University of Science and Technology (NTNU) & Swiss Federal Institute of Technology, ETH Zurich. Plumb, G. A. (1993), A scale for comparing the visual magnitude of waterfalls, Earth-Science Reviews, 34, Statens vegvesen (2006), Konsekvensanalyser, veiledning. ( Consequence assessment guideline ), Vegvesenets håndbokserie nr 140 [in Norwegian]. Bundesministerium für Land- und Forstwirtschaft (1973), Kraftwerksgruppe Sellrain-Silz der Tiwag, wasserrechtliche Bewilligung, Wasserrechtsbescheid, [in German].