Geomorphic Response of Rivers Below Dams by Sediment Replenishment Technique
Total Page:16
File Type:pdf, Size:1020Kb
River Flow 2010 - Dittrich, Koll, Aberle & Geisenhainer (eds) - © 2010 Bundesanstalt für Wasserbau ISBN 978-3-939230-00-7 Geomorphic response of rivers below dams by sediment replenishment technique S. A. Kantoush & T. Sumi Water Resources Research Center, Disaster Prevention Research Institute, Kyoto University, Japan A. Kubota Kizugawa Dams Integrated Operation & Management Office, Japan Water Agency, Japan ABSTRACT: Sedimentation problems on regulated rivers are considerably different from natural rivers. An effective means of reservoir sediment management is to excavate deposited sediments within reser- voir and transport to the river downstream of a dam. The flood is designed to erode the placed sediment at the banks of Nunome River, in an effort to restore the lower reach of the river to a more natural ecosys- tem. Since 2004, six attempts of sediment replenishment to rebuilding vital sediment sand bars below Nunome dam are in progressing. Monitoring measurements pre- and post- replenishment show the effect of improvements in riverbed formation, grain size distribution, and erosion processes. Actual field expe- riments are presented and compared with scaled laboratory experiments, with the objective of identifying parameters and process of sediment replenishment characteristics. The paper investigates the influence of sediment replenishment below Nunome Dam on changes of river bed. Appropriate sediment materials, which are excavated and treated from the upper part of the Nunome reservoir, and supplied to down- stream rivers contributes to rebuilt sand bars, mitigate the armouring of river bed, and management of re- servoir sedimentation. With the field experiments, the processes are directly visible and a wealth of valu- able data is obtained, and will be used for numerical modeling in future application. Keywords: Dam impacts, Reservoir sediment replenishment, Sediment management, River restoration, Revitalization of regulated rivers, Sediment feeding, Geomorphic response 1 INTRODUCTION Dam Impacts on: 1.1 Dam Impacts Morphological Hydrological Ecological Loss of aquatic and River bank Water table lowering Dams greatly influence flow and sediment dis- instability riparian habitats charge regimes and can have significant impacts on downstream reaches below dam. Bed load se- Bed armouring Reduction in overall flows oxygen stratification diment management is required to preserve the River incision and Changes in flood capabilities of water resources facilities and to bed degradation Nutrient exchange frequency and magnitude conserve the environment in rivers downstream of Impacts on the reservoir. The sediment deficit produces a infrastructures Changes in seasonal flow Gas-bubble disease large variety of physical, ecological and environ- Sediment deficit to Adverse effect on mental effects. To mitigate that coarse sediments, coastal zones Changes hydrodynamic within estuaries water quality by grain diameter, with specific grain size distri- butions artificially replenished to satisfy the river Figure 1. Possible impacts due to lack of sediment transport bed load capacity (sediment replenishment). Fig- in the downstream reach below dams ure 1 classifies the impacts of sediment deficit be- In addition, dams alter the downstream flow low dams to three groups; morphological, hydro- regime of rivers (Williams and Wolman, 1984), logical, and ecological effects. Sediment deficit is which controls many physical and ecological as- not only an environmental issue but also a socio- pects of river form and processes, including sedi- economic problem, for instance due to loss of re- ment transport and nutrient exchange (Poff et al., servoir capacity (Fan and Springer, 1993). 1997). Morphological effects on the river channel 1155 (e.g., Kondolf and Matthews, 1993; Shields et al., geometry are influenced by variability in water 2000) that includes riverbed incision, riverbank and sediment release. Sediment replenishment instability, upstream erosion in tributaries, dam- scenarios may, induce undesirable morphological age to bridges, embankments and levees (e.g., and ecological consequences as well as significant Kondolf, 1997), and changes in channel width channel adjustments that can result in failure of (e.g., Wilcock et al., 1996). Hydrological effects the restoration project itself. That is, it is neces- caused by dams include changes in flood frequen- sary to better understand reversibility, direction cy and magnitude, reduction in overall flows, and time scale of changes, and the sustainability changes in seasonal flows, and altered timing of of a replenishment intervention before it is im- releases (Ligon et al., 1995). plemented. 1.2 Sediment Replenishment Projects in Japan 1.3 Objectives Large Japanese rivers are often trained to a large The study focuses on the design criteria for sedi- extent, to maintain services such as navigation, ment replenishment project in Japan to improve hydropower generation and flood defense. Okano river bed conditions. The investigations are based et al., (2004) summarized sediment replenishment on field and laboratory experiments. The objec- projects in Japanese Rivers, such as Tenryu, Ota- tives of this paper are: (a) to make a general re- kine, Abukuma, Ara, Oi, Naka, Kuzuryu, Yodo, view of impacts of dam on downstream reach; (b) Kanna, and Tone, have been conducted by Minis- to discuss the role of different factors influencing try of Land, Infrastructure and Transport (MLIT). the sediment replenishment research; (c) to illu- Kantoush et al. (2010) investigated the morpho- strate and analyze the sediment replenishment logical evolution and corresponding flow field through a regulated fluvial system during actual during replenishment experiments in Uda River, field tests. Japan. Sediment treatment system is applied by Sumi et al. (2009), to produce appropriate grain sized material with less turbidity. Seto et al. 2 FIELD EXPERIMENTS FOR SEDIMENT (2009), analyzed sediment replenishment effects REPLENISHMENT IN NUNOME RIVER on the downstream river of Yahagi dam. 2.1 Test Procedure and Replenishing Concept Factors of Sediment Replenishment In Japan, it is common practice to remove accu- Sediment Flow Physical River Physical Replenishment Monitoring Economics mulated coarse sediment by excavation and dredg- Physical Characteristics Characteristics Characteristics Characteristics Characteristics Characteristics ing, and to make effective use of the removed se- Flood flow Initial bed Placed sediment Water quality Excavation and Porosity and discharges (Flood morphology and volume and (turbidity, Ph, sediment diment. Sediment replenishment method is one of density hydrographs) slopes geometry shape temperature, treatment costs new measures of sediment management. In this Bed material size, Submerged or Environmental (fish Sediment type and Flow water depths gradation and partial or un species, stream Transportation cost method, trapped sediment is periodically exca- cohesion permeability submerged state invertebrates, algae) vated and then transported to be placed temporari- Grain size Flow duration Sediment rating Locations and Flow field and distribution, and curve and curve (suspended placement Feeding cost ly downstream of the dam. In a manner decided percentage of fine frequency and bed loads) arrangement along vertical velocity sediment the river profiles according to the sediment transport capacity of the Treatment of Flow patterns and River bank Cross sectional Frequency of Monitoring cost excavated and velocity profiles vegetation feeding per year survey and precise channel and the environmental conditions. There- dredged sediment and season surveying of riffles and pools fore, the sediment is returned to the channel Indirect (side bank Erosion rate and Volume of Trapped feeding) or direct Visual inspection Sediment source type (side bank or sediment in (in river) feeding and grain size Project benefits downstream in the natural flooding processes. The vortex types) reservoirs method analysis procedure of the experiments consists of four Figure 2. Main groups of sediment replenishment characte- steps: (1) extracting mechanically the accumu- ristics and managing factors lated sediment at check dam; (2) transporting it by Sediment transport and associated channel bed truck to downstream river; (3) placing the sedi- mobility are recognized as key processes for ment with specific geometry (Figure 3), and (4) creating and maintaining physical habitats, aquatic monitoring flow, sediment, and environmental pa- and riparian ecosystems. In Japan, sediment rep- rameters. lenishment projects are undertaking with different Placed sediment configurations and characteristics of sediment and geometry discharges. Figure 2 summarizes factors that in- Flood stage fluence the sediment replenishment research. These factors are classified in six groups as shown Normal stage in Figure 2. Several significant gaps in the scien- tific understanding of these processes remain, par- Figure 3. Concept of sediment replenishment and placed se- ticularly concerning how riverbed deposition and diment geometry on the bank of Nunome River 1156 The replenished sediment is placed at such an 2.4 Case Study of Nunome Dam Actual Sediment elevation; in order to reduce the turbidity during Replenishment Tests normal flow period. The top of the sediment is ad- Since 2004, six experiments of sediment reple- justed so that the sediment is completely sub- nishment projects with different sediment volumes merged during flood at several