Effects of Influential Parameters on Long-Term Channel Evolution Following Low-Head Dam Construction and Removal

The long-term existence of dam structures significantly modified the river channel. In accordance with a drastic increase of low-head dams under consideration for removal in recent years, it is important to predict the effects of low-head dam removal from the modified river channel by the low-head dam construction. This study intends to investigate the long-term channel evolution process following low-head construction and removal and to find out the influential parameters (sediment diameter, river bed slope, dam height) for those channel evolution by two-dimensional numerical simulation model. Following the low-head dam construction, sediment deposition rates in upstream of the low-head dam are varied with the influential parameters. The sediment deposition rates and sandbar formation with riparian vegetation settlement on sandbars have significantly affected for channel evolution following low-head dam removal. Especially the knickpoint formation and the types of vegetation (grass type and tree type) on the sandbars are critical factors for channel evolution following low-head dam removal. Through the numerical simulation results of low-head dam construction (50 years) and low-head dam removal (50 years), it is identified that the modified river channel by low-head dam may not be easily restored to pre-dam conditions following its removal especially in river geomorphology and riparian vegetation. Consequently, this study found that the reversibility following low-head dam construction and removal depends on the sediment deposition rates in upstream of the low-head dam.


Introduction
Dam structures obstruct a connectivity of river corridor affecting river ecosystem. However, these dam structures are indispensable elements in river for water resources and flood control. The long-term existence of dam structures affects structure and function of river ecosystem [1] with modification of flow and sediment flux. Especially, abandoned dam structures can cause serious problems for flood safety and river ecosystem. Thus, it is necessary to remove the dam structure as soon as the end of function or life span of dam structures.
Since there was a huge number of dam construction works until 1980s, the number of dam structures completed their life span or function has been drastically increasing in recent years. However, studies for low-head dam removal are insufficient to predict the effects of low-head dam removal with lack of quantitative methods. A few studies have investigated and documented short-term geomorphic changes following low-head dam removal. Reference [2] established channel evolution model to explain channel development in a reservoir following dam removal adapted from Reference [3] channel evolution model for incising channel.
Once riparian vegetation begins to establish on the river channel, the riparian vegetation also can affect the fluvial processes as well [4]. Therefore, it is important to investigate fluvial processes along with the riparian vegetation establishment. Reference [5] suggested a numerical simulation model to predict both geomorphic and riparian vegetation changes following the low-head dam removal.
Using the two-dimensional numerical simulation model [5], this study intends to investigate the long-term channel evolution process following low-head construction and removal and to find out influential parameters on channel evolution following low-head dam removal. The critical parameters considering for this study are dam height, sediment diameter, and river bed slope.

Numerical Simulation Model
The numerical simulation model calculates the depth-averaged flow, bed load transport and bed elevation change in flood stage with the destruction of riparian vegetation. In ordinary stage, riparian vegetation invasion, growth and expansion are calculated in this numerical model ( Figure 1). Initial river bed morphology, sediment diameter and discharge are necessary for calculation as input data.

Flow and Sediment Transport Model
In flood stage, the water depth and depth-averaged flow velocity are calculated S. Kim, Y. Toda Journal of Water Resource and Protection where V i : u when i = x, V i : v when i = y, n: Manning roughness coefficient, D C : drag coefficient (=1.0), χ : vegetation density parameter (=0.02) and l: vegetation height in flow, respectively.
To calculate bed load transport rates in longitudinal and lateral direction, the MPM equation [6] and Hasegawa equation [7] are applied respectively.
where bx q , by q : bed load transport rate in x and y direction, λ : bed porosity, bs q : longitudinal bed load transport rate, bn q : lateral bed load transport rate, * τ : critical tractive force, *c τ : dimensionless critical tractive force, s R : specific gravity of bed load, d: diameter of sediment, r: streamline curvature of radius,

Treatment of Low-Head Dam
If there is a low-head dam, the bed load will be transported to downstream when the sediment deposition height in upstream of dam reaches to the same height of the low-head dam (Figure 2(b)). Unless upstream of the low-head dam is fully deposited as high as the low-head dam, there is no bed load transport across the low-head dam (Figure 2(a)).

Vegetation Model
The numerical simulation model calculates the growth, expansion and invasion of riparian vegetation (grass type, tree type) in ordinary stage. It is assumed that the higher vegetation can take more light for photosynthesis for the interspecific competition [8]. The growth of riparian vegetation is calculated by the balance of primary production and respiration [9]. The horizontal expansion of riparian vegetation is formulated by diffusion type formula in the growth equation. The equation for growth of the riparian vegetation is given by: where i M : biomass per unit area, i P : primary production, i R : respiration, xi k , Based on the analysis of reference case study (Gongreung River), the required time for settlement i T is determined as 1 g T = year for grass type and 5 t T = years for tree type, respectively. The initial biomass on the vegetation settlement area is given by: in which z: relative height from the ordinary water stage, 0 z : relative height of the water level of the seed dispersal season from the ordinary water stage,   if the bottom friction on the vegetation stand exceeds critical wash out shear stress for vegetation [10]. The buried type destruction takes place when the sediment deposition depth is higher than vegetation height. Also the buried type destruction calculated the vegetation biomass depend on the vegetation height from ground due to the bed elevation changes during flood.

Computational Conditions
Simplified channel designed for the simulation is 50 m in width 1000 m in length ( Figure 4). On the point of 700 m from upstream end, the low-head dam is located. The unit grid size is 5 m in length and width. The boundary conditions for the numerical calculation are decided by referring the case of Gongreung River (averaged river width 70 m [5]). Averaged annual maximum discharge is designed as 300 cms and occurred once in a year. The ordinary discharge is designed as 2 cms. To find the influential parameters and channel evolution processes, the numerical simulations have been performed through the 2 stages of low-head dam construction (stage I) and low-head dam removal (stage II) ( Table 1). In stage I, the numerical model has simulated with a low-head dam. The simulations have been conducted for 50 years considering the life span of a low-head dam structure. Stage II is performed to identify the low-head dam removal effects. The initial conditions is the final results of stage I (50 years) extracting the Journal of Water Resource and Protection  low-head dam. Three parameters (dam height, river bed slope and sediment diameter) which can be influential for channel evolution following low-head dam construction and removal are chosen for this study based on previous studies.

Long-Term Effects of Low-Head Dam Construction
The construction of a low-head dam and its long-term existence in the river channel substantially alter river hydraulic features, sediment transport rates, and river geomorphology. The numerical model has been simulated with the low-head dam for 50 years to clarify the long-term effects of low-head dam construction with a low-head dam installation in the river channel. By the low-head

Influential Parameters
The effects on upstream of the low-head dam are evident following a low-head dam construction. The sediment deposition in upstream begins from the upstream end of the backwater pool and gradually expands toward the dam with time sequence. As a result of numerical simulations, the sediment deposition characteristics on upstream reservoir have some differences depends on the parameters (sediment diameter, river bed slope and dam height).

Low-Head Dam Removal Effects
River channel development and evolution following dam removal are strongly  [19]. The stored sediment in the upstream of the low-head dam frequently forms the knickpoint as soon as the low-head dam is removed. Stepped knickpoints (Head cut migration) take place during channel evolution following low-head dam removal in terms of cohesive, consolidated, or layered deposits [18].
From the results of numerical calculation for low-head dam removal, the knickpoints have been formed (Figure 7). In upstream of the removed dam, the primary knickpoint has been created by deposited sediment just after the low-head dam removal and gradually higher until 12 months after removal. Af-

Influential Parameters
In order to identify the influential parameters for channel evolution after the low-head dam removal, the final (50 years) results of dam construction simulation for each case have been applied for low-head dam removal calculation as initial conditions except the low-head dam structure.

Discussion; Reversibility Following Low-Head Dam Construction and Removal
Low-head dam removal has been regarded as an effective alternative to restore  river ecosystem. Unlike large dam (more than 15 meters in height), the effects following low-head dam removal have been overlooked with expectation that the river channel will be restored as pre-dam conditions. To clarify the reversibility of river channel following low-head construction and removal, this study performed the comparison analysis for the final results of low-head dam construction (Stage I) and low-head dam removal (Stage II). As a result, the overall bed elevation has been aggrades as 20 -30 cm except the lower gradient case (D-3, R-3) following low-head dam construction and removal. Cases R-1 and R-2 have shown the sandbar formation in downstream of the removed dam with riparian vegetation colonization. The water channels of these 2 cases (R-1, R-2) have had sinuosity after low-head dam removal with sandbar formation changing the straight channel to meandering channel. The case R-1 which had most sediment deposition rates with dam construction has shown the biggest distinction between before low-head dam construction and after low-head dam removal.
Meanwhile, the case R-3 with the least sediment deposition rates in the simulation of dam construction has restored near pre-dam conditions. Consequently, the reversibility following low-head dam construction and removal depends on particular parameters which decide the sediment deposition rates in upstream of the low-head dam. In this research, the sediment diameter and river bed slope significantly attribute to increase the sediment deposition rates in upstream of the low-head dam. Moreover, the riparian vegetation settlement and development of tree type plants are crucial for durability of downstream sandbars.

Conclusions
This study intends to investigate the long-term channel evolution process following low-head dam construction (50 years) and removal (50 years) and to find out the influential parameters (sediment diameter, river bed slope, dam height) for those channel evolution by numerical simulation model.