# Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds
## 1 Introduction  
State and municipal governments are facing depleting natural resources, adverse climate change impacts, and many socio-economic challenges. Continuation of industrial, transportation, and utility services is critically dependent on continued construction and maintenance of roads, drainage infrastructure, culverts, etc., commonly known as \"grey infrastructure\". Despite being essential for economic growth, these infrastructure investments are significant and continue throughout the life of any such project, through annual maintenance costs [1]. This point is of particular importance in infrastructure designed to provide protection from natural hazards, and specifically flood hazards. Large drainage systems and structural flood protection are being exposed to natural decay and require constant investment. They are characterized by a limited life span and declining benefits and are under additional stress by the changing climate [2; 3]. Moreover, the human and the material assets protected by grey infrastructure are becoming more valuable over time, so the exposure to flood hazard grows even faster. Grey infrastructure also has a very limited role in ecosystem functions and can lead to degradation of local and regional systems [3]. In the process of looking for an alternative solution, attention is turning towards nature-based solutions (\"green infrastructure\") that could be used for mitigation of flood risks [4]. Green infrastructure (GI) is defined as \"strategically planned network of natural and semi-natural areas that... deliver a wide range of ecosystem services\" [5]. Examples of applicable GI for flood risk mitigation are reclaimed wetlands, riverine floodplains, natural detentions, etc. Because of the growing challenges related to urban areas, such as population growth, rapid urbanization, and climate change effects, there has been abundant emergence of projects and research on coping with flood risk in urban areas using GI [6; 7; 8; 9; 10; 11]. It has been shown in many cases that nature-based GI solutions are less expensive than grey infrastructure (e.g., [12]) and provide a wide array of co-benefits for local economies, the social fabric, and the broader environment [13]. This should be of particular interest to decision-makers, as GI can provide comparable benefits to grey infrastructure at reduced costs in the long term. On the other hand, there has been very little research on the quantitative and cost effectiveness of GI for flood mitigation in settings other than urban, such as flat valleys of large rivers or small hilly watersheds prone to flash floods. One of the reasons for this is that improved understanding of trade-offs between flood risk reduction and economic consequences in rural settings is needed [2] because the assets in rural settings considerably differ from the ones in urban areas. Recently, the term \"large-scale nature-based solutions\" is commonly used for the flood risk mitigation measures in rural settings on either watershed or regional scale [4]. In small to medium size watersheds, flood risk reduction measures can vary from conventional grey infrastructure to various scales and levels of GI. In some cases, depending on the scale of the watershed, solutions gravitate towards larger scale nature-based solutions, or some combination of the green and grey infrastructure approach. Both green and grey infrastructure as flood mitigation measures have their specific advantages and drawbacks and either may not be a universal solution to be applied indiscriminately to all types of watersheds and local conditions. Therefore, the two types of flood mitigation measures should not be a replacement for each other but should rather be considered as complementary measures [12; 14; 15]. For rural watersheds with some flood mitigation measures already present, the combination of green and grey infrastructure may be the most suitable solution.