Experimental Investigation of Flood Energy Dissipation by Single and Hybrid Defense System

Abstract

In this study, a series of laboratory experiments were conducted to investigate the energy loss through the hybrid defense system (HDS) in the order of dike, moat, and emergent vegetation in steady subcritical flow conditions. The results of HDS were compared with a single defense system (SDS) comprising only vegetation (OV). The dimensions of dike were kept constant while two different shapes (trapezoidal and rectangular) of moat were considered. The impacts of vegetation of variable thickness and density were investigated. Two combinations of HDS were investigated including the combination of dike and vegetation (DV) and the combination of dike, moat, and vegetation (DMV). The effect of backwater rise due to the vegetation, hydraulic jump formation and the impact of the arrival time of floodwater on energy dissipation were investigated. It was observed that on the upstream side of obstructions, the backwater depth increased by increasing the Froude number in both the SDS and HDS. The hydraulic jump observed in HDS was classified and the energy dissipation due to it was calculated. Under various conditions investigated in this paper, the maximum average energy dissipation was 32% in SDS and 46% in HDS. The trapezoidal moat performed better than rectangular moat as energy dissipater. The delay time was also greater with trapezoidal moat as compared to that in rectangular one. The maximum delay time was 140 s in the case of HDS. Hence, the hybrid defense system offered maximum resistance to the flow of water, thus causing a significant energy loss. For each case of SDS and HDS, empirical equations were developed by regression analysis to estimate the energy dissipation amounts.

Experimental investigation of flood energy reduction through vegetation at various angles

“…However, the variation of relative backwater rise is very small against the selected range of Froude numbers as compared to backwater rise as shown in Figure 2b. It is resulted that the initial depth (ho) has a very small effect on backwater rise as similar results are reported previously (Ahmed & Ghumman, 2019;Anjum & Tanaka, 2019;Pasha & Tanaka, 2017).…”

“…The density of vegetation can be defined by the ratio of spacing between two cylinders in the cross stream direction (B) to the diameter of cylinder (d) (Takemura & Tanaka, 2007) number of trees in a rectangle with a frontage of unit length along the bank of the river and depth equal to the width of vegetation (Wv) (Pasha & Tanaka, 2019). The selected spacing between the vegetation cylinders (D) are 1.88 cm and 1.25 cm for sparse and intermediate vegetation cases, respectively, same as used previously (Ahmed & Ghumman, 2019). The density (B/d) and thickness (d n ) of vegetation are kept constant for all sparse vegetation cases with angles 90 (90VS), 45 (45VS), and 30 (30VS) and also for all intermediate cases with angles 90 (90VI), 45 (45VI) and 30 (30VI), respectively.…”

“…When water spreads toward the floodplain, the angle of flow toward the floodplain is usually oblique for straight (Keramaris, Pechlivanidis, Kasiteropoulou, Michalolias, & Liakopoulos, 2016;Mahtabi & Arvanaghi, 2018) and meandering channels (River, 1969;Van Dijk et al, 2013). Previously, some authors studied perpendicular weir (Ahmed & Ghumman, 2019;Mahtabi & Arvanaghi, 2018) and some others used oblique weirs in their studies of open channel hydraulics (Aichel, 1953;Tuyen, 2007). The combine effect of vegetation and weir (placed in perpendicular direction) on energy head loss is also studied previously (Ali & Uijttewaal, 2013;Ali & Uijttewaal, 2014 ).…”

“…When water flows through emergent vegetation, the water surface elevation increases on the upstream side of the vegetation forming a backwater rise. Water flows with decreasing slope inside the vegetation cover and on the downstream side a hydraulic jump may form, if flow regime transitions from supercritical to subcritical, otherwise, oscillating flow may occur which dissipates after some distance from the last row of vegetation (Ahmed & Ghumman, 2019;Pasha & Tanaka, 2017). In the case of catastrophic floods in straight channels (Tuyen, 2006) and especially in meandering channels (River, 1969; Van Dijk, Teske, Van de Lageweg, & Kleinhans, 2013), the flow may propagate downstream through the floodplain at a perpendicular or an oblique angle (Tuyen, 2006).…”

Experimental Investigation of Flood Energy Dissipation through Embankment Followed by Emergent Vegetation

“…For example, such experimental studies have been conducted to quantify the reduction of velocity and fluid force of a tsunami wave behind a double layered vegetation [3,4], and to analyze in detail the energy dissipation through composite defense systems comprising of vegetation and backward facing steps [5,6]. Moreover, the combined effects of sea embankments and coastal forests has also been tested [7] against moats with and without vegetation [8] and a double embankment system has been studied experimentally by varying the spacing between the embankments to understand the energy dissipation through it [9]. Many other studies of composite defense structures [10,11] including the analysis of embankment and vegetation with moat [12], embankment upstream barriers [13] have been conducted, in the recent past.…”