Effects of a Low-Head Dam Removal on River Morphology and Riparian Vegetation: A Case Study of Gongreung River

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1 Journal of Water Resource and Protecton, 2014, 6, Publshed Onlne December 2014 n ScRes. Effects of a Low-Head Dam Removal on Rver Morphology and Rparan Vegetaton: A Case Study of Gongreung Rver S Nae Km *, Yuj Toda, Tetsuro Tsujmoto Department of Cvl Engneerng, Nagoya Unversty, Nagoya, Japan Emal: * flowerksn@hotmal.com Receved 19 September 2014; revsed 16 October 2014; accepted 11 November 2014 Copyrght 2014 by authors and Scentfc Research Publshng Inc. Ths work s lcensed under the Creatve Commons Attrbuton Internatonal Lcense (CC BY). Abstract The long term exstence of a low-head dam n the rver channel sgnfcantly affects rver geomorphology and rver ecosystem. Because more and more low-head dam structures have deterorated n recent years, the attenton for low-head dam removal s ncreasng as one of alternatves for rver restoraton. Thus, ths study ntends to nvestgate the mpacts of low-head dam removal on rver geomorphology and rparan vegetaton wth developng a quanttatve method to predct the changes of rver morphology as well as nvason, growth, expanson and destructon of rparan vegetaton after a low-head dam removal. To verfy the numercal smulaton model, the lowhead dam removal case n Gongreung Rver was employed wth nvestgaton of low-head dam removal responses on rver geomorphology and rparan vegetaton. Followng the low-head dam removal, the results of montorng and numercal smulaton ndcated that new sand bars has formed as well as ncreasng the extent of exstng sand bars n the upstream of the low-head dam. The sand bars have been colonzed n a year after the low-head dam removal by grass type plants. After a decade to several decades, the rparan vegetaton n sand bars often developed to tree type plants n several low-head dam removal cases. As other cases, Gongreung Rver also showed the growth of tree type plants n 5 years after the removal. Keywords Low-Head Dam Removal, Rver Morphology, Rparan Vegetaton, Numercal Smulaton 1. Introducton In recent years, the number of deterorated low-head dam structures s drastcally ncreasng due to ther lfe * Correspondng author. How to cte ths paper: Km, S.N., Toda, Y. and Tsujmoto, T. (2014) Effects of a Low-Head Dam Removal on Rver Morphology and Rparan Vegetaton: A Case Study of Gongreung Rver. Journal of Water Resource and Protecton, 6,

2 span rangng about 50 years. Partcularly, numerous exstng low-head dams whch were constructed between 1970s and 1980s are expected to be deterorated n the next decade. Many deterorated dams whch were abandoned n the rver channel cause serous problem for rver ecosystem and flood safety. To mprove rver ecosystem, low-head dam removal s emergng as an alternatve for rver restoraton. Prevous studes on a low-head dam removal documented that sgnfcant changes have been observed on rver morphology and rparan vegetaton followng a low-head dam removal. Especally, the channel evoluton by transportng stored sedment n upstream of a low-head dam greatly alters rver channel exposng large bare ground and often creatng floodplans. The stored sedment often forms a knck pont or head cut n the rver channel as soon as a low-head dam removed. Doyle et al., 2005 [1] found that the mgraton of head cut controls subsequent channel development n upstream of a low-head dam. The ntensty of a head cut mgraton to channel evoluton s mostly depends on the character of deposted sedment. Layered or cohesve deposts have been shown to produce stepped knck pont or head cut followng dam removal [2] resultng n channel ncson, wdenng by bank falure, and buldng floodplans [3] [4]. These morphologcal changes create sutable surface for rparan vegetaton [5]. Orr and Stanley, 2006 [6] mentoned that all dam removal stes had extensve vegetaton cover and stes retan hgh cover of vegetaton based on 30 dam removal stes n Wsconsn. Also, they found that the growth of tree plants s consderably related to tme snce dam removal. Whle a lot of low-head dam removals are expected n near future, only few studes take note of the responses on low-head dam removal. Wth nsuffcent montorng data on pre and post removal and researches, the mpacts on rver geomorphology and rparan vegetaton by the low-head dam removal have not been quantfed. Thus, t s necessary to develop a quanttatve method to predct the low-head dam removal responses on rver morphology wth rparan vegetaton development for rver management. To develop a quanttatve method, ths study ntends to 1) nvestgate a low-head dam removal mpacts wth the montorng results of Gongreung 2 dam removal; and 2) develop a numercal smulaton model for rver morphology and rparan vegetaton changes followng a low-head dam removal wth verfcaton by the case of Gongreung Rver. 2. Study Area Gongreung Rver s a trbutary of Han Rver located n Goyang-s, Gyengg provnce, Korea (Fgure 1). The rver s 45.7 km n length and km² n area of watershed wth 1/200 of average bed slope. The mean annual precptaton of the watershed s 1384 mm wth the maxmum and mnmum were mm n 1990 and mm n 1988 respectvely. Gongreung 2 dam s a low-head dam for the purpose of agrculture rrgaton constructed n 1970s. The lowhead dam was a 76 m wde and 1.5 m hgh concrete dam structure. The low-head dam was removed n Aprl, 2006 because t lost the functon for rrgaton by land use changes of surroundng agrcultural area and excessve sedment deposton n the reservor (Fgure 2). Another low-head dam n the 600 m downstream sde from the Gongreung 2 dam s stll n exstence. Fgure 1. Gongreung Rver n Republc of Korea. 1683

3 Fgure 2. Gongreung 2 dam before (left) and after (rght) the dam removal. 3. Low-head Dam Removal Montorng 3.1. Methods To analyze the low-head dam removal effects, montorng has performed for rver morphology and rparan vegetaton on 1 km secton from 400 m far upstream to 600 m far downstream of the Gongreung 2 dam. The feld measurement for morphologcal changes had performed n before removal on Aprl 1, 2006, just after removal on May 30, after floods on August 4, 2006, and November 1, To analyze the morphologcal mpacts, cross secton of rver, rver bed elevaton and sedment characterstcs were observed. The montorng of rparan vegetaton performed n March 2006 before removal, November 2006, and May 2007 after removal Rver Morphology The reservor n upstream of the low-head dam was accumulated wth fne sedment as hgh as the heght of the dam structure. To prevent drastc downstream mpacts by excessve sedment transport, some of the accumulated reservor sedment was artfcally dredged durng the low-head dam removal constructon. On rver morphologcal changes followng a low-head dam removal, a headcut mgraton s a common and crtcal phenomenon of the upstream channel evoluton, and a headcut mgraton s governed by the characterstcs of accumulated sedment n the reservor [3] [4]. In Gongreung 2 dam removal, montorng results of rverbed cross secton showed that a headcut mgraton started after removal movng toward 25 m upstream n 2 months. However, the morphologcal mpacts by the headcut mgraton have not sgnfcantly affected the channel evoluton n case of Gongreung 2 dam removal, because the accumulated sedment was dredged before the removal constructon. Followng the low-head dam removal, the rver bed elevaton n upstream has degraded 0.67 m and 0.84 m at m upstream pont from the dam and the pont of prevous dam, respectvely. In downstream, the rver bed elevaton has aggraded 0.52 m and 0.27 m at the pont of m and m downstream sde, respectvely (Fgure 3). However, farther downstream has no sgnfcant change of rver bed elevaton due to the mpacts by another low-head dam exstence located n 600 m downstream sde from the removed low-head dam. Moreover, the number and sze of sand bars have subsequently ncreased n upstream of the low-head dam. The transported sedment from upstream reservor also ncreased the sze of sand bars n downstream of the lowhead dam. These changes contrbute to velocty dstrbuton to be varous mprovng dversty of flow structures such as pool and rffle. The sedment both upstream and downstream has coarsened followng the low-head dam removal. In upstream of the low-head dam, the medan gran sze has coarsened from 0.51 mm before removal to 1.96 mm after removal exposng underlyng coarse sand n the reservor. The medan gran sze n downstream of the lowhead dam also became coarser from 2.74 mm before removal to mm after removal exportng large amount of fne sedment by flood n July Rparan Vegetaton Followng the low-head dam removal, the number of rparan vegetaton speces n upstream of the low-head dam drastcally ncreased from 32 speces before removal (March 2006) to 54 speces n November 2006 and

4 Fgure 3. Montorng results of rver bed elevaton changes. speces n May 2007 after removal. Because the low-head dam removal exposed the deposted sedment n the prevous mpoundment, new vegetaton could be settled n the bare ground. In downstream of the low-head dam, 41 speces before removal (March 2006), 61 speces n November 2006, and 52 speces n May 2007 were observed. The percentages of poneer speces were more than 50% of total vegetaton speces n both upstream and downstream of the low-head dam. The domnance of poneer speces ndcates that sgnfcant morphologcal changes by low-head dam removal dsturbed and changed the physcal habtat condtons for rparan vegetaton. The area of vegetaton has ncreased followng the low-head dam removal wth the formaton of large sand bars n upstream and ncreasng the sze of sand bars n downstream (Fgure 4). Because the left bank of upstream s a rock mountan wth steep slope, most sand bars and rparan vegetaton developed n the rght bank sde. 4. Numercal Smulaton 4.1. Numercal Smulaton Model To analyze the post low-head dam removal mpacts, the numercal smulaton model has developed wth the calculaton of flow, sedment transport and rparan vegetaton n flood and ordnary water stage. Input data are ntal morphology, sedment dameter and dscharge. In flood stage, the smulaton model calculates the flow and bed load transport as well as the destructon of rparan vegetaton by flood. The nvason, growth and expanson of rparan vegetaton are desgned to calculate n ordnary water stage (Fgure 5) Flow Calculaton In order to smulate flow n flood stage, shallow water equaton was employed to calculate the water depth and depth-averaged flow velocty. Usng Mannng s resstance law, the bottom frcton and drag force of rparan vegetaton are calculated. In terms of sedment transport n flood stage, the bed load transport n lateral and longtudnal drecton s calculated wth the MPM equaton [7] and Hasegawa equaton [8], respectvely Growth and Expanson of Rparan Vegetaton The growth and expanson of rparan vegetaton are calculated n ordnary water stage. To smulate the vegetaton dynamcs, ths numercal smulaton model sorts the rparan vegetaton nto grass type and tree type plants brefly. For the nterspecfc competton of rparan vegetaton, t s assumed that the hgher vegetaton can take more lght for photosynthess. The growth of both types of rparan vegetaton s calculated by the balance of prmary producton and respraton [9]. The horzontal expanson of rparan vegetaton s formulated by dffuson type formula n the growth equaton. The equaton for growth of the rparan vegetaton s gven by: M M M = P R + kx ky t x x + (1) y y n whch M : bomass per unt area, P : prmary producton, R : respraton, K x, K y : the dffuson coeffcents for horzontal expanson of vegetaton n x and y drectons, respectvely. The subscrpt denotes t types of vegetaton. the ndex dentfyng the grass ( g ) and tree ( ) 1685

5 Fgure 4. Montorng results of rparan vegetaton n Gongreung Rver. Fgure 5. Flowchart of the numercal smulaton model Invason of Rparan Vegetaton In order to smulate the nvason of rparan vegetaton at the nterface between water body and the surroundng upland, t s assumed that the vegetaton nvason for bare ground takes place n seed dspersal perods, and the seeds are dspersed by flowng water. The nvason possble area s estmated from the dfferences wth ordnary water level and averaged water level of seed dspersal season. The rparan vegetaton s settled n the bare ground f the bare ground has not experenced morphologcal dsturbance for T years. Based on the aeral photograph analyss on Gongreung Rver, the requred tme for settlement T s determned. T g = 1 year for grass type and T t = 5 years for tree type, respectvely. The ntal bomass on the vegetaton settlement area s gven by: z0 z M = M0 + M0 mn (2) z 0 n whch z : relatve heght from the ordnary water stage, z 0 : relatve heght of the water level of the seed dspersal season from the ordnary water stage, M 0 : ntal bomass at the water edge of the ordnary water stage and M 0mn : ntal bomass at z = z0, respectvely (Fgure 6) Vegetaton Destructon Regardng the destructon of the rparan vegetaton calculated n flood stage, there are the wash out type and the bured type by flood n the numercal model. The wash out type destructon s occurred by the bed scour around the vegetaton stand, and the bured type destructon s caused by the sedment deposton around the vegetaton stand. The wash out type destructon s occurred ether f the local scour depth durng flood becomes larger than 1686

6 Fgure 6. Modelng of vegetaton nvason to bare ground. the root depth or f the bottom frcton on the vegetaton stand exceeds crtcal wash out shear stress for vegetaton [10] [11]. The bured type destructon takes place when the sedment deposton depth s hgher than vegetaton heght. Also the bured type destructon calculated the vegetaton bomass depend on the vegetaton heght from ground due to the bed elevaton changes durng flood Computatonal Condtons A numercal smulaton has performed under the condtons of Gongreung Rver n Korea, n whch the average bed slope s and mean dameter of bed materal s 0.5 mm. Measured flood dscharge data from 2006 to 2010 n Sngok observatory have been appled for the calculaton of flood stage. Except flood stage, t s assumed that dscharge s unform n ordnary stage. Also, measured data n 2001 for rver master plan were appled to create ntal morphologcal data of numercal smulaton. Montorng data of rver morphology n May 30, 2006 (just after removal) for adjacent area of the low-head dam were reflected as ntal condtons. Table 1 shows the value of vegetaton parameters appled for the numercal smulaton Results As a result of numercal smulaton model, the bed elevaton degraded 0.84 m n the pont of prevous low-head dam whle the rver bed elevaton aggraded 0.3 m n 40 m downstream sde from the low-head dam after removal (Fgure 7). Montorng results showed 0.84 m degradaton n the pont of prevous low-head dam and 0.27 m aggradaton n m downstream sde of the dam. Fgure 8 shows the results of numercal smulaton model for rver morphology and rparan vegetaton changes. In upstream of the low-head dam, the numercal smulaton results shows that large scale of sand bars have been created wth exposng deposted sedment after the low-head dam removal. The newly formed sand bars n upstream sde (reservor) of the low head dam were colonzed by grass type plants. Compare to montorng results, the tree plants were slghtly overestmated n numercal smulaton model due to the assumpton of bare ground. In present numercal smulaton, f the bare ground has not experenced morphologcal experence for 1 year, the grass type plants are settled n the bare ground. The sand bars n the downstream of the low-head dam developed n both sdes of the rver banks wth new grass type vegetaton. In contrast to the upstream results, the results of sand bars and rparan vegetaton n numercal smulaton were slghtly under estmated than montorng results. For the reason, t s assumed that the lmted cross secton data caused lmtaton for smulatng detal sand bar formaton. Nevertheless, the numercal smulaton model could smulate the general geomorphologcal changes and rparan vegetaton changes followng low-head dam removal. Accordng to the results of numercal smulaton and montorng, t was found that the low-head dam removal contrbute to form new sand bars and expand the extent of exstng sand bars wth rparan vegetaton colonzaton. However, the geomorphologcal changes by the lowhead dam removal was restrcted only n vcnty of the low-head dam as the montorng results denoted that the mpacts by a head-cut mgraton was not sgnfcant because of dredgng the deposted sedment n the reservor. In addton, t was smulated that the area of tree type plants s ncreasng after 5 years later of low-head dam removal. Aeral photo n 2012 (Fgure 9) also shows that tree type plants are newly establshng and expandng 1687

7 Fgure 7. Numercal smulaton results on rver bed elevaton changes. Fgure 8. Numercal smulaton results on Gongreung Rver for rver morphology and rparan vegetaton. Fgure 9. Aeral photo of Gongreung Rver n

8 Table 1. The value of vegetaton parameters. Parameter Value (grass/tree) Explanaton Reference χ C d 1.0 Drag coeffcent a. 1 ( m ) 0.02 Vegetaton densty parameter b. k, k 4.0/2.0 Dffuson coeffcent for horzontal expanson b. x 0 mn y 3 ( g cm ) M 6.0 Mnmum ntal bomass b. 3 ( ) M g cm Intal bomass at the water edge b. T 1/5 Requred tme for settlement Measured a. Toda et al [12]; b. Toda et al [11]. n the rver channel. For the effectve rver management followng low-head dam removal, t s necessary to predct long term changes of rver morphology and rparan vegetaton. 5. Conclusons Ths study ntends to develop the numercal smulaton model for analyss of low-head dam removal mpacts on rver morphology and rparan vegetaton. To verfy the numercal smulaton model, the low-head dam removal case n Gongreung Rver was appled for the numercal smulaton wth nvestgaton of low-head dam removal responses on rver morphology and rparan vegetaton. The results of montorng and numercal smulaton ndcated that there are sgnfcant changes on rver geomorphology and rparan vegetaton followng the low-head dam removal. Above all, upstream of the low-head dam has sgnfcant morphologcal changes wth exposng deposted sedment. New sand bars have formed by ths morphologcal changes as well as ncreasng the extent of exstng sand bars n upstream of the low-head dam. These sand bars have been mostly colonzed n a year after the low-head dam removal by grass type plants. The area of sand bars and rparan vegetaton have ncreased n upstream and downstream of the low-head dam followng low-head dam removal. After a decade to several decades, the rparan vegetaton n sand bars often develops to tree type plants n several low-head dam removal cases. As other cases, Gongreung Rver also showed the growth of tree type plants n 5 years after the removal. Therefore, t s mportant to study the geomorphologcal changes wth long term rparan vegetaton successon followng a low-head dam removal for rver management as a future study. References [1] Doyle, M.W., Stanley, E.H., Orr, C.H., Selle, A.R., Seth, S.A. and Harbor, J.M. (2005) Stream Ecosystem Response to Small Dam Removal: Lessons from the Heartland. Geomorphology, 71, [2] Sawaske, S.R. and Freyberg, D.L. (2012) A Comparson of Past Small Dam Removals n Hghly Sedment-Impacted Systems n the US. Geomorphology, 151, [3] Pzzuto, J. (2002) Effects of Dam Removal on Rver form and Process. BoScence, 52, [4] Doyle, M.W., Stanley, E.H. and Harbor, J.M. (2003) Channel Adjustments Followng Two Dam Removals n Wsconsn. Water Resources Research, 39, [5] Shafroth, P.B., Fredman, J.M., Auble, G.T., Scott, M.L. and Braatne, J.H. (2002) Potental Responses of Rparan Vegetaton to Dam Removal. BoScence, 52, [6] Orr, C.H. and Stanley, E.H. (2006) Vegetaton Development and Restoraton Potental of Draned Reservors Followng Dam Removal n Wsconsn. Rver Research and Applcatons, 22, [7] Meyer-Peter, E. and Muller, R. (1948) Formulas for Bed Load Transport. Proceedngs of 2nd meetng of the Internatonal Assocaton for Hydraulc Structures Research, Delft, 7 June 1948, [8] Hasegawa, K. (1981) Bank-Eroson Dscharge Based on a Non-Equlbrum Theory. Proceedngs of JSCE, 316,

9 (In Japanese) [9] Jensen, P.B. (1932) De Stoffprodukton der Pflanzen. Fscher. [10] Yagsawa, J., Tanaka, N. (2009) Dynamc Growth Model of Rver Vegetaton Consderng the Destructon by Floods and Regeneraton Process of Trees. Annual Journal of Hydraulc Engneerng, 53, (In Japanese) [11] Toda, T., Km, S.N., Tsujmoto, T. (2014) Relaton between Sandbar Mode and Vegetaton Expanson n Sand-Bed Rver. Proceedngs of Rver Flow 2014, Lausanne, 3-5 September 2014, 429. [12] Toda, Y., Ikeda, S., Kumaga, K., Asano, T. (2005) Effects of Flood Flow on Flood Plan Sol and Rparan Vegetaton n a Gravel Rver. Journal of Hydraulc engneerng, 131,

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