# Near-Nature Ecological Technique for Pier Scour Countermeasure in a Submerged Overfall
## 1 Introduction  
Overfall often takes place at the downstream of a grade-control structure, dam, or reservoir, and scours the riverbed, thus posing a risk to the stability of local structures (i.e., bridge piers) [1, 2, 3, 4]. The investigation of local scour has been researched for many years. According to [PERSON] (1932) [5], the maximum scour depth induced by drop structures and bridge piers is related to water table differences between its downstream and upstream, discharge per unit width, tailwater depth, and median diameter of riverbed sediment. The above pioneering work generated much scientific and engineering interest in the following years. Varied empirical formulas have been proposed for the estimation of local scour due to drop structures and bridge piers under different conditions of structure types, approach flow, and bed material [6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Such contributions are important to effectively control overall-induced scour and diminish the scour around piers, an area that is influenced by scour hole. To prevent local scour below drop structures and at bridge piers, construction work for protection and/or stilling basin is generally conducted in engineering practice to resist scour and/or diminish overall energy [16]. The frequently applied protective construction involves the use of rectangular concrete block, irregular block, and ding-shaped block to resist scour and energy dissipation [17]. A flow-altering device can be another option to minimize scouring through flow adjustment [16], such as by mitigating the down-flow motion by using a collar plate near the base of piers, decreasing the flow velocity by using a series of surface guide panels around the pier, and decreasing the eddy near thepier by using counter-eddy bottom panels [18; 19; 20; 21; 22]. Among the engineering methods for natural river management, near-nature ecological engineering has been attracting attention, especially in developed and developing countries [23; 24; 25]. This study aims to develop a near-nature ecological structure to control the local scour induced by drop structures and bridge piers. The use of geosynthetic components to control erosion and sediment materials has been considered a more economic and effective solution than costly concrete structures in some circumstances since the late 1950s [26; 27]. Generally, geosynthetic components range from temporary work such as hydraulic mulch geofibers, plastic erosion control meshes and netting, erosion control blankets and reinforcement mats, geotextiles, and fabric-formed revetments [25; 28]. Their applications were introduced and discussed in past studies [29; 30; 31; 32]. With the awareness of the need for environment protection nowadays, geosynthetic materials need to be less disruptive to the environment. Many naturally occurring textile products can be used to solve geo-environmental engineering problems. For example, southeast and southwest Nigeria have an abundance of vegetation from which soft fibers such as flax, hemp, jute, and coir are obtained [33]. Many of these natural fibers, such as malvacae and penduculata, are used by farmers in these regions to stabilize soil and control erosion. To solve river erosion, the authors have conducted many field investigations and found that the sediment protected by hard man-made concrete structure sometimes performs worse than sediment protected by water weeds growing in the river. The effect of vegetation on the change of flow features, i.e., flow velocity, flow turbulence, wave dissipation, and drag force, has been studied widely [34; 35; 36; 37; 38; 39]. Due to the effect of vegetation, the flow-induced bed shear stress can be reduced so that the capacity of sediment motion can be weakened [35; 36; 37; 38; 39; 40; 41]. Therefore, the authors introduced a new way to control sediment and erosion in a river. This paper proposes a new technique composed of easily obtained materials and installation method aimed at controlling erosion and sediment movement in the river.