Enhancing flood hazard estimation methods on alluvial fans using an integrated hydraulic, geological and geomorphological approach

Zeinab Mollaei,Alireza Faridhosseini,Yavar Pourmohamad,Kamran Davary,Seyed Majid Hasheminia

doi:10.5194/nhess-18-1159-2018

Zeinab Mollaei, Alireza Faridhosseini + Show 3 more

Open Access

https://doi.org/10.5194/nhess-18-1159-2018

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Abstract

Abstract. Due to the uncertainty concerning the location of flow paths on active alluvial fans, alluvial fan floods could be more dangerous than riverine floods. The United States Federal Emergency Management Agency (FEMA) used a simple stochastic model named FAN for this purpose, which has been practiced for many years. In the last decade, this model has been criticized as a consequence of development of more complex computer models. This study was conducted on three alluvial fans located in northeast and southeast Iran using a combination of the FAN model, the hydraulic portion of the FLO-2D model, and geomorphological information. Initial stages included three steps: (a) identifying the alluvial fans' landforms, (b) determining the active and inactive areas of alluvial fans, and (c) delineating 100-year flood within these selected areas. This information was used as an input in the mentioned three approaches of the (i) FLO-2D model, (ii) geomorphological method, and (iii) FAN model. Thereafter, the results of each model were obtained and geographical information system (GIS) layers were created and overlaid. Afterwards, using a scoring system, the results were evaluated and compared. The goal of this research was to introduce a simple but effective solution to estimate the flood hazards. It was concluded that the integrated method proposed in this study is superior at projecting alluvial fan flood hazards with minimum required input data, simplicity, and affordability, which are considered the primary goals of such comprehensive studies. These advantages are more highlighted in underdeveloped and developing countries, which may well lack detailed data and financially cannot support such costly projects. Furthermore, such a highly cost-effective method could be greatly advantageous and pragmatic for developed countries.

Highlights

Alluvial fan floods are considered to be serious hazards, since the flooding that emerges from the apex of an alluvial fan, moves fiercely downstream while carrying a large amount of substrate load and debris (King and Mifflin, 1991; Coe et al, 2003; Garfi et al, 2007; Merheb et al, 2016)
Five steps have been followed to achieve this goal: 1. the FAN model was established using the 100-year return period hydrograph and the average slope which was extracted from topographical maps; 2. the FLO-2D model was executed by using the 100-year return period hydrograph and topographical maps; 3. active and inactive areas were distinguished by considering topographical maps, normalized difference vegetation index (NDVI), and field investigations; 4. thereafter, all the models, results, and field investigations were georeferenced and overlapped in geographical information system (GIS) as sep
FAN model results are illustrated in Fig. 7a, b, and c for the Ferizy, Ardak, and Sarbaz fans, respectively

Summary

Introduction

Alluvial fan floods are considered to be serious hazards, since the flooding that emerges from the apex of an alluvial fan, moves fiercely downstream while carrying a large amount of substrate load and debris (King and Mifflin, 1991; Coe et al, 2003; Garfi et al, 2007; Merheb et al, 2016). Alluvial fans are made of larger size sediments which may have some degree of cohesive material that lies on a steep slope. These two characteristics could lead to “avulsion”, formation of new channels during flooding events, which causes major flow path displacement (Blair and McPherson, 2009). Due to “avulsion” the characteristics of alluvial fan flooding are more important than other flooding features, which could increase the risk of damage (Calcaterra et al, 2000, 2003; Lancaster et al, 2012; Santangelo et al, 2012). As stated by National Research Council: “the area of deposition on an alluvial fan shifts with time, but the episode of flooding is more likely to occur where the most recent deposits have been laid down, than where deposits of greatest antiquity occur” (National Research Council, 1996)

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