Essay: Reconstructing Past Flood Events Q & As

Question Sheet One (ex1_sheet1.doc) GG2024 Exercise 1. Reconstructing Past Flood Events

When citing sources:

Cite the exact source of the information (i.e. the author(s) and year) and then reference it.

Cite the author(s) in the text. And then list the reference in a bibliography using the Harvard System of referencing.

e.g. in a journal: author. year. title. journal. volume. pages

Put the rainfall rate of this flood and the flood magnitude into an Irish context (max 200 words). Cite texts you have used to make the comparison (at least 3 references)

In Irish context, the Yellow River flood event of 1986 was of a high magnitude. The Yellow River flood has an expected return period of “…hundreds if not thousands of years” (Coxon et al., 1989). This is unusual for Ireland as there have been few events with return periods of 100 years or more (Flood Policy Review Group, 2004). Events with 100 year return periods are so rare that they have a 1% chance of occurring each year (Walsh, 2010).

The rainfall rate of the Yellow River flood, which exceeded 100 mm possibly in less than four hours, is highly unusual for Ireland (Coxon et al., 1989). During June in Ireland, the monthly rainfall rate is approximately 80 mm (Walsh, 2012). This means that the rainfall rate of the Yellow River flood event exceeded the average rainfall amount predicted for the entire month. The average hourly rainfall rate in Ireland is between 1 and 2 mm, and rates of 15 to 20 mm may occur every 5 years. Rainfall rates of 25 mm or more per hour, like the rainfall rate of the Yellow River flood, are extremely rare (Walsh, 2010).  In Irish context, the Yellow River flood was very rare and of a high magnitude.

References:

• Coxon, P., Coxon, C. and Thorn, R.H. 1989. The Yellow River (County Leitrim, Ireland) flash flood of June 1986. in: Bevan,K and Carling,P. (eds.) Floods – Hydrological, Sedimentological and Geomorphological Implications of Floods. pp 199-217, Wiley.
• Flood Policy Review Group (2004). Report of the Flood Policy Review Group. [online] The Office of Public Works, pp.33-43. Available at: https://www.opw.ie/media/Report%20of%20the%20Flood%20Policy%20Review%20Group.pdf [Accessed 3 Dec. 2017].
• Walsh, S. (2010). Report on Rainfall of November 2009. Climatological Note 12. [online] Dublin: Met Éireann, p.2. Available at: https://www.met.ie/publications/Rainfall%20November%202009.pdf [Accessed 3 Dec. 2017].
• Walsh, S. (2012). A Summary of Climate Averages for Ireland, 1981-2010. Climatological Note 14. [online] Dublin: Met Éireann. Available at: http://edepositireland.ie/bitstream/handle/2262/70490/Climatological%20Note%20No.%2014.pdf?sequence=1&isAllowed=y [Accessed 3 Dec. 2017].

Who was Manning and what is Manning’s n equation? (cite your source to a hard copy reference):

Robert Manning was an Irish engineer who lived from 1816 to 1891. He is best known for his work in the development of Manning’s n equation. In 1846, Manning began to work for the Commissioners of Public Works in Ireland.  In 1846, Manning became a district engineer for the Board of Works, a position he held for several years before starting a private practice. He was later appointed to become second engineer in the Board of Works under William Forsyth, who he succeeded as chief engineer in 1874 (Tvedt and Oestigaard, 2006). Manning first presented his equation in 1889 during a meeting of the Institution of Civil Engineers of Ireland. His equation,

V=(R^0.67× S^0.5)/n

is used to estimate the velocity of liquid in an open channel flow. In this equation, V is the cross sectional average velocity, R is the hydraulic radius, S is the channel bed slope, and n is Manning’s empirical coefficient for bed roughness. The equation, derived from observation, was presented as a simpler alternative to the Chézy equation (Chadwick, Borthwick, and Morfett, 2004). Manning served as chief engineer of the Board of Works until 1891, when he retired (Tvedt and Oestigaard, 2006).

Source:

• Chadwick, A., Borthwick, M. and Morfett, J. (2004). Hydraulics in civil and environmental engineering. 4th ed. London [u.a.]: Spon Press, pp.129-138.
• Tvedt, T. and Oestigaard, T. (2006). A History of Water: The World of Water, Series 1, Volume 3. London: I.B. Tauris, p.221.

3. Give 4 completely separate examples from the literature of the use of ANY flood reconstruction techniques and cite the reference(s). Write a brief account of the flood and the methods used that would explain the techniques employed to someone who had not read the paper. As previously give the full reference to the article (not just a web address)

Example 1

On September 8 and 9, 2002, a catastrophic flash flood caused by a mesoscale convective system resulted in extensive damage over the Gard region of France (Bonnifait, et al., 2009).  There were twenty-four causalities as a result of the flash flood and an estimated 1.2 billion euros in economic damage. On September 8th, 2002, convection cells generated power over the Mediterranean Sea and proceeded to move northwards, until the quasi-stationary system was over the Gard region. The mesoscale convective system, which is a group of thunderstorms that interact and behave as one system, remained over the Gardon watershed until the morning of September 9, 2002 (Anquetin et al., 2005). While intense rainfall and flood events in the area are not unusual, the duration and extent of the rainfall was devastating.

This flood was reconstructed by Bonnifait et al., 2009 using observational data and n-TOPMODELSs that performed hydrologic modelling. Data and observations collected from an intense post-event field campaign (IPEC) were used to fully reconstruct the flood. The IPEC used eight stream gauges that were operational and undamaged to estimate water discharge rates at different points in the watershed. The IPEC also interviewed witnesses to help reconstruct the timeline of the flood. Bonnifait et al., 2009 combined the dataset collected by the IPEC with hydrological simulations produced by the n-TOPMODELSs. The n- TOPMODELSs are designed to simulate sub-surface and overland runoff flow. The parameters required by the model are surface hydraulic conductivity measurements, the rate of decline in the soil profile of hydraulic conductivity, and the initial deficit in root zone storage (Bonnifait et al., 2009). Although the n-TOPMODELS simulations and the IPEC estimates were at times inconsistent, the n-TOPMODELSs were able to reconstruct the dynamics of the flood with relative accuracy. The models also showed that the time and extent of stream flooding was determined by the trajectory of the mesoscale convective system. Observational data in combination with n-TOPMODELSs were used to reconstruct the catastrophic flash flooding that occurred in Gard, France on September 8 and 9, 2002.

Reference(s)

• Bonnifait, L., Delrieu, G., Lay, M., Boudevillain, B., Masson, A., Belleudy, P., Gaume, E. and Saulnier, G. (2009). Distributed hydrologic and hydraulic modelling with radar rainfall input: Reconstruction of the 8–9 September 2002 catastrophic flood event in the Gard region, France. Advances in Water Resources, [online] 32(7), pp.1077-1089. Available at: http://www.sciencedirect.com/science/article/pii/S0309170809000554 [Accessed 29 Nov. 2017].
• Anquetin, S., Yates, E., Ducrocq, V., Samouillan, S., Chancibault, K., Davolio, S., Accadia, C., Casaioli, M., Mariani, S., Ficca, G., Gozzini, B., Pasi, F., Pasqui, M., Garcia, A., Martorell, M., Romero, R. and Chessa, P. (2005). The 8 and 9 September 2002 flash flood event in France: a model intercomparison. Natural Hazards and Earth System Science, [online] 5(5), pp.741-754. Available at: https://www.nat-hazards-earth-syst-sci.net/5/741/2005/nhess-5-741-2005.pdf [Accessed 28 Nov. 2017].

Example 2

On June 18, 2006, a debris flood event occurred within the Negoiul basin of the Detunatelor Mountains in the Apuseni Mountains. While debris floods often affect the Apuseni Mountains, which are part of the Romanian Carpathians, the 2006 flood was especially severe (Văidean, et al., 2015). Socio-economic pressure in the area has increased the population’s economic vulnerability, and traffic infrastructure is often damaged by these events. (Arghiuş, et al., 2010). The debris flood begun when a thunderstorm occurred over an area of soil that had already been saturated by previous rainfall events. The thunderstorm resulted in 100-125 mm of rainfall within 2.5-3 hours over the small basin, flooding the Negoiu Stream. (Vădean, et al., 2015).

The flood was reconstructed by Văidean, et al., 2015 using dendrogeomorphic methods and by estimating the peak discharge and average flow velocity. Văidean, et al., 2015 began by calculating the average velocity of the flow by using Manning’s n equation with the Manning’s roughness coefficient value of 0.035. The average velocity of the flow was then multiplied by the cross-sectional area to estimate the peak discharge of the flow, which was 96 m^3/s (Văidean, et al., 2015). Next, dendrogeomorphic methods were used to reconstruct the debris flood. Trees are able to record debris flood events by displaying growth disturbances in their rings. Cores were removed from 20 Picea abies trees with a Pressler increment borer, and were then analyzed for growth disturbances. The 2006 debris flood in the Negoiul basin of the Apuseni Mountains was reconstructed with dendrogeomorphic methods, and with estimations of peak discharge and average flow velocity.

Reference(s)

• Văidean, R., Arghiuş, V. and Pop, O. (2015). Dendrogeomorphic reconstruction of past debris-flood activity along a torrential channel: an example from Negoiul basin (Apuseni Mountains, Romanian Carpathians). Zeitschrift für Geomorphologie, [online] 59(3), pp.319-335. Available at: https://www.researchgate.net/publication/271346838_Dendrogeomorphic_reconstruction_of_past_debris-flood_activity_along_a_torrential_channel_an_example_from_Negoiul_basin_Apuseni_Mountains_Romanian_Carpathians [Accessed 30 Nov. 2017].
• Arghiuş, V., Ozunu, A., Ştefănescu, L., Costan, C. and Arghiuş, C. (2010). Damages Associated to 1995-2006 Floods and Flash-Floods in the East of the Apuseni Mountains. Quaestiones Geographicae, [online] 29(3). Available at: https://www.degruyter.com/downloadpdf/j/quageo.2010.29.issue-3/v10117-010-0017-2/v10117-010-0017-2.pdf [Accessed 1 Dec. 2017].

Example 3

Historic flood events may be reconstructed by using flood marks, photographs, and other documentary sources. On June 12 and 13, 1910, high levels of rainfall triggered a flood of the river Ahr, a left tributary of the Rhine in Germany. The towns of Altenahr, Dernau, Walporzheim, and Neuenahr received very heavy rainfall and high water levels (Roggenkamp and Herget, 2014). As the river flooded these well-populated towns, much economic damage was caused. The floodwaters also transported wooden building materials, increasing the danger to the local population, and several structures along the river were greatly damaged (Roggenkamp and Herget, 2014).

As this flood occurred before the time of reliable instrumental gauging, documentary sources are used to help reconstruct the flood event. Flood level markings were found in Altenahr, Dernau, and Walporzheim, while the flood levels in Neuenahr were well documented by photographs taken throughout the days of the flood (Roggenkamp and Herget, 2014). These flood marks were found on houses in Walporzheim and Dernau, and were found inside a road tunnel in Ahrweiler. Although there were no flood marks found in Neuenahr, several photographs were used to quantify the discharge of the flow and show the flood’s progression over the course of June 12 and 13. Since Neuenahr was an urban area, several of the photographs include clocks so the timeline of the flood can be accurately reconstructed. To calculate the peak discharge of the flood, the historic floodplain first had to be reconstructed using sources such as historic maps and photographs. Once the historic floodplain had been reconstructed, Mannings’s n equation was used to calculate the maximum, minimum, and average discharges throughout the floodplain. The flood’s hydrograph curve could also be estimated from the available photographs. The 1910 flood of the Ahr was reconstructed with the use of flood marks, photographs, and Manning’s n equation.

Reference(s)

• Roggenkamp, T. and Herget, J. (2014). Reconstructing peak discharges of historic floods of the river Ahr, Germany. Erdkunde, [online] 68(1), pp.49-59. Available at: http://www.sciencedirect.com/science/article/pii/S0921818109001969 [Accessed 1 Dec. 2017].

Example 4

On September 26, 2009, Tropical Storm Ketsana triggered a major flood event in Metropolitan Manila, the capital region of the Philippines. The tropical storm brought 347.5 mm of rainfall in 6 hours, exceeding the country’s forty-year record. The extent of rainfall caused devastating economic effects and casualties. More than one million people in the region were left homeless, and damage to infrastructure and property totalled an estimated US $43.5 million (Abon, David and Pellejera, 2010). A highly urbanized area of Metropolitan Manila, Marikina City, was located within the Marikina River floodplain, increasing the area’s vulnerability to damage during high precipitation events. The damage was especially high due to a lack of warning systems, leading the population to be unprepared for the storm and flooding. The urbanization of the area also increased the flood’s destruction as the anthropogenic surfaces of the city prevented soil absorption of the floodwaters.

Due to a lack of stream gauges, Abon, David and Pellejera, 2010 used anecdotal data, flood marks, and digital elevation models to reconstruct the flood. Flood marks found on bridges, houses, and trees provided information about the water levels throughout the floodplain, and anecdotal accounts were collected from interviews with members of the local population. These interviews helped to reconstruct the timeline of the flood, particularly flood wave velocity and the lag time between higher levels of rainfall and higher flooding. Hydrologic modeling was then conducted using information about the soil and the shape of the floodplain to produce hydrographs. The maximum discharge that was calculated from these models was 5921〖 m〗^3/s, which greatly exceeds the estimated 100 year flood discharge of 3440〖 m〗^3/s  (Abon, David and Pellejera, 2010). The Tropical Storm Ketsana flood event was reconstructed by using anecdotal evidence and flood markers to calculate parameters for hydrologic modeling.

Reference(s)

• Abon, C., David, C. and Pellejera, N. (2010). Reconstructing the tropical storm Ketsana flood event in Marikina River, Philippines. Hydrology and Earth System Sciences Discussions, [online] 7(4), pp.6081-6097. Available at: https://www.researchgate.net/publication/46056361_Reconstructing_the_Tropical_Storm_Ketsana_flood_event_in_Marikina_River_Philippines [Accessed 1 Dec. 2017].

3. Briefly describe the way that stream velocity varies with increasing wetted perimeter and with increasing bed roughness

Stream velocity decreases as wetted perimeter increases. As wetted perimeter increases, the hydraulic radius decreases, lowering stream velocity. Stream velocity and wetted perimeter are negatively correlated because as wetted perimeter increases, stream velocity decreases.

Stream velocity decreases as bed roughness increases. Stream velocity and bed roughness are negatively correlated, so as bed roughness decreases, stream velocity increases. Increasing bed roughness increases friction, which slows the rate that fluid can travel over a bed. This trend is shown in the reconstruction of the Yellow River flood event. When the maximum value of Manning’s n was used, the calculated velocity was lower than when the minimum value of Manning’s n was used. As wetted perimeter and bed roughness increase, stream velocity decreases.

4. Where did you find your values for Manning’s n and did you find the exercise of applying it difficult?

I found my values for Manning’s n from Ven Te Chow’s book Open-Channel Hydraulics. The values I used were located on a table on page 113. These values are the same as those from the table provided as those values were from Chow’s textbook as well. I did not find the exercise of applying the values difficult, although it took much contemplation to determine which of Manning’s n values were the best matches for each of the channel reaches.

Reference:

• Chow, V. (1956). Open-Channel Hydraulics. New York: McGraw-Hill Book Company, Inc, pp.110-113.

Bibliography:

• Abon, C., David, C. and Pellejera, N. (2010). Reconstructing the tropical storm Ketsana flood event in Marikina River, Philippines. Hydrology and Earth System Sciences Discussions, [online] 7(4), pp.6081-6097. Available at: https://www.researchgate.net/publication/46056361_Reconstructing_the_Tropical_Storm_Ketsana_flood_event_in_Marikina_River_Philippines [Accessed 1 Dec. 2017].
• Anquetin, S., Yates, E., Ducrocq, V., Samouillan, S., Chancibault, K., Davolio, S., Accadia, C., Casaioli, M., Mariani, S., Ficca, G., Gozzini, B., Pasi, F., Pasqui, M., Garcia, A., Martorell, M., Romero, R. and Chessa, P. (2005). The 8 and 9 September 2002 flash flood event in France: a model intercomparison. Natural Hazards and Earth System Science, [online] 5(5), pp.741-754. Available at: https://www.nat-hazards-earth-syst-sci.net/5/741/2005/nhess-5-741-2005.pdf [Accessed 28 Nov. 2017].
• Arghiuş, V., Ozunu, A., Ştefănescu, L., Costan, C. and Arghiuş, C. (2010). Damages Associated to 1995-2006 Floods and Flash-Floods in the East of the Apuseni Mountains. Quaestiones Geographicae, [online] 29(3). Available at: https://www.degruyter.com/downloadpdf/j/quageo.2010.29.issue-3/v10117-010-0017-2/v10117-010-0017-2.pdf [Accessed 1 Dec. 2017].
• Bonnifait, L., Delrieu, G., Lay, M., Boudevillain, B., Masson, A., Belleudy, P., Gaume, E. and Saulnier, G. (2009). Distributed hydrologic and hydraulic modelling with radar rainfall input: Reconstruction of the 8–9 September 2002 catastrophic flood event in the Gard region, France. Advances in Water Resources, [online] 32(7), pp.1077-1089. Available at: http://www.sciencedirect.com/science/article/pii/S0309170809000554 [Accessed 29 Nov. 2017].
• Chadwick, A., Borthwick, M. and Morfett, J. (2004). Hydraulics in civil and environmental engineering. 4th ed. London [u.a.]: Spon Press, pp.129-138.
• Chow, V. (1956). Open-Channel Hydraulics. New York: McGraw-Hill Book Company, Inc, pp.110-113.
• Coxon, P., Coxon, C. and Thorn, R.H. 1989. The Yellow River (County Leitrim, Ireland) flash flood of June 1986. in: Bevan,K and Carling,P. (eds.) Floods – Hydrological, Sedimentological and Geomorphological Implications of Floods. pp 199-217, Wiley.
• Flood Policy Review Group (2004). Report of the Flood Policy Review Group. [online] The Office of Public Works, pp.33-43. Available at: https://www.opw.ie/media/Report%20of%20the%20Flood%20Policy%20Review%20Group.pdf [Accessed 3 Dec. 2017].
• Roggenkamp, T. and Herget, J. (2014). Reconstructing peak discharges of historic floods of the river Ahr, Germany. Erdkunde, [online] 68(1), pp.49-59. Available at: http://www.sciencedirect.com/science/article/pii/S0921818109001969 [Accessed 1 Dec. 2017].
• Tvedt, T. and Oestigaard, T. (2006). A History of Water: The World of Water, Series 1, Volume 3. London: I.B. Tauris, p.221.
• Văidean, R., Arghiuş, V. and Pop, O. (2015). Dendrogeomorphic reconstruction of past debris-flood activity along a torrential channel: an example from Negoiul basin (Apuseni Mountains, Romanian Carpathians). Zeitschrift für Geomorphologie, [online] 59(3), pp.319-335. Available at: https://www.researchgate.net/publication/271346838_Dendrogeomorphic_reconstruction_of_past_debris-flood_activity_along_a_torrential_channel_an_example_from_Negoiul_basin_Apuseni_Mountains_Romanian_Carpathians [Accessed 30 Nov. 2017].
• Walsh, S. (2010). Report on Rainfall of November 2009. Climatological Note 12. [online] Dublin: Met Éireann, p.2. Available at: https://www.met.ie/publications/Rainfall%20November%202009.pdf [Accessed 3 Dec. 2017].
• Walsh, S. (2012). A Summary of Climate Averages for Ireland, 1981-2010. Climatological Note 14. [online] Dublin: Met Éireann. Available at: http://edepositireland.ie/bitstream/handle/2262/70490/Climatological%20Note%20No.%2014.pdf?sequence=1&isAllowed=y [Accessed 3 Dec. 2017].

Originally published 15.10.2019