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1、精选优质文档-倾情为你奉上专心-专注-专业Relevance between Stream Barb Length and Bank ErosionAnalysis Using Delft-3D Flow Model at Sawmill CreekAbstractThe Sawmill Creek is a small creak which is located in Ottawa, Canada. It generatesfrom the south of Lester Road, flowing through several primary residential area andf
2、inally drains into Rideau River. There is a wide range of discharge and water levelfluctuation taking place in sawmill creek. And the watershed of Sawmill Creekresponds quickly to big rainfall events across the year. Several bends in Sawmill havewitnessed notable band erosion and aquatic habitats re
3、duction in the recent years dueto high flow rate and distinct secondary flow which is towards the outer bank near thewater surface and towards the inner bank near the bed at the bends. Several barbshave been constructed in two primary bends in October, 2009 and the result andfollowing measurement sh
4、ow improvements in the band erosion and aquatic habitatsproblems. The objective of this paper is trying to study the relevance between thelength of barbs and band erosion reduction effect to find the proper design plan forstream barbs and obtain the best effect of band erosion reduction and aquatic
5、habitatsprotection. The three dimensional hydraulic software, Delft-3D, is applied to study thebarbs effect in this paper.1. INTRODUCTION1.1. Introduction to Sawmill CreekAs It is shown in Figure1. Sawmill Creek begins in a wetland south of LesterRoad, Ottawa, flows towards west and then towards nor
6、th through South Keys andFigure 1. The Location and Flow Path of Sawmill Creek(Sawmill Creek 2014 Summary Report, 2014)Heron Park and finally flows into the Rideau River near the intersection of BankStreet and Riverside Drive. The total watershed of Sawmill Creek is 27.73km2and the精选优质文档-倾情为你奉上专心-专注
7、-专业total length is approximately 11km. The surficial geology condition has a high degreeof diversification, consisting of 40% sand, 29% clay 12% gravel, 9% diamicton, 6%organic deposits and 4% Paleozoic bedrock. The surficial geology in the watershedarea is mainly marine clay plains with sand and ro
8、ck ridges (RVCA,2008).The naturalwatercourse accounts for 41% of the total water course and the rest section ischannelized.According to Sawmill Creek 2014 Summary Report, there were 26 aquaticspecies , including 4 game fish species, observed in 2014 and It is shown in Table 1.Table 1. Fish Species O
9、bserved in Sawmill Creek in 2004(Sawmill Creek 2014 Summary Report, 2014)Figure 2. The Fluctuation of Discharge and Water Surface Elevation(WSE) inOne Year for 2009 (E. C. Jamieson et al.,2013)Although Sawmill Creek is one of the last free-flowing cool water streams left in theurban core of the City
10、 of Ottawa, the lower and middles reaches of the creek are精选优质文档-倾情为你奉上专心-专注-专业highly urbanized and the creek corridor is degraded and confined by development andtransportation infrastructure (RVCA 2012).The land use in Sawmill Creek watershedincludes 48% urban/rural, 16%wooded area, 12% transportat
11、ion area, 11% wetland,1% agriculture, 1% water body and 11% unclassified area. 54% of the land use wasmade up of residential, industrial or commercial, infrastructure and recreation. Thehigh degree of the urbanization and quick respond to rainfall in heavy rainfall eventsdirectly causes wide range o
12、f water level fluctuation in Sawmill Creek watershed.Figure 2 shows the discharge and water surface elevation fluctuation condition acrossone year in Sawmill Creek.1.2 Bank Erosion in Sawmill CreekBank Erosion is a kind of flow-bank interaction change process due to stronglysecondary flow when water
13、 flows in the bend.As flow approaches the bend, the flowrate near the outer band is faster than that near the inner bank, which causes the lowcohesive material at the surface of the outer bank eroded and gradual sedimentation atthe inner bank.Figure 3. Erosion along Sawmill Creek(Sawmill Creek 2014
14、Summary Report, 2014)Excessive erosion and deposition of sediment within a stream have a detrimentaleffect on the bank stability, channel change and aquatic habitat. And poor bankstability caused by bank erosion can make the bank become easier to be eroded. Withincrease in runoff during rain season
15、in Ottawa will adjust to accommodate theadditional flow, increasing stream bank erosion. Accelerated stream bank erosion ispart of the process as the stream seeks to reestablish a stable size and pattern.精选优质文档-倾情为你奉上专心-专注-专业Damaging or removing streamside vegetation to the point where it no longer
16、providesfor bank stability can cause a dramatic increase in bank erosion(Steam Notes, Volume1 Number 2). The loss of bank vegetation is also a significant problem due to theinteraction between bank failure and bank erosion, resulting in trees falling into thestream and the potential to impact in aqu
17、atic habitat and migration path. Figure 3shows high levels of bank erosion were observed along many sections of SawmillCreek downstream of Walkley Road. Figure 4 shows the image of bank erosionhappening in one bend of Sawmill Creek.Figure 4. Erosion in One Bend of Sawmill Creek(Sawmill Creek 2014 Su
18、mmary Report, 2014)1.3. Stream BarbsStream barb a type of groyne which has been used for centuries under a varietyof purposes ranging from river training to stream bank protection. It is a kind of linearrock structure connected with the bank and stretching into the channel. It is typicallyanchored.
19、It is one of the most reliable and economically attractive approaches forstabilizing eroding banks in incised channels. The general configuration of barbs isshown in Figure 5.As for the bend area of the small stream, 3 or 4 stream barbs are generallyinstalled at the outer bank and parallel with each
20、 other with a spacing ranging from1m to 3m, determined by specific condition. Barbs are generally constructed out oflarge rock riprap, between 500 and 600mm in diameter, with additional smaller riprap(d:50 230mm) along the bank side slope (50% above/below bank full) upstream ofeach barb to provide a
21、dditional protection in the area.( E.C. Jamieson et al.,2009)Spurs were spaced at roughly twice the average baseflow channel width ( 7 m) withlengths roughly 40 percent of the average width. Crests were level, 2 m wide, and 1 mabove the bed, or about 60 cm above baseflow water surface elevation. Sto
22、ne sizeranged from 0.2 to 450 kg, with 50 to 85 percent of the stones weighing less than 36kg (F D. Shields et al.,1998).精选优质文档-倾情为你奉上专心-专注-专业Figure 5. Conceptual Drawing of Spurs (darker rock)(F D. Shields et al.,1998)This configuration redirects the attacking current and the primary secondary flow
23、cell away from the outer bank towards the centre of the channel (Minor et al., 2007,Jamieson et al. accepted). Moreover, unlike other bank protection measures (e.g.riprap, concrete paving and gabion walls), stream barbs require less material and canpromote vegetated stream banks, maintain deep pool
24、habitat through the developmentof scour holes at barb tips and increase aquatic species diversity (Shields et al., 1998;Engelhard et al., 2004).However, Stream barbs do not address bank failure due to soil instability andDrawdown (Technical Note 23: Design of Stream Barbs, U.S, 2005).Therefore,addit
25、ional work should be done before the barbs installation such as stream clean up,vegetative planting and site monitoring. The plan is to incorporate vegetative plantingand other bioengineering practices during the construction phase to address theseadditional mechanisms of bank failure( E.C. Jamieson
26、 et al.,2009). Moreover,vegetation provides additional roughness to dissipate energy along the stream bankand enhances wildlife habitat and water quality( E.C. Jamieson et al.,2009).1.4 Target Reach InformationThe target reach of this study is a 50-m reach in Sawmill Creek, which generallyconsists o
27、f two small bends and receives approximately 90% of the contributingwatershed area. The reach is experiencing bank erosion and mass wasting at twoconsecutive bends. The linear distance and area of eroding bank in the first (second)bend were 13.4m and 41.7m2(28.0 m and 46.0 m2)(E. C. Jamieson et al.,
28、 2013).Anumber of large boulders dominated the morphology of the second (downstream)bend, and a riffle had formed close to the apex of the bend. (E. C. Jamieson etal.,2013) target reach was selected not only because the eroding banks indicated thepresence of erosion processes but also because of rel
29、ative ease of site access and lackof proximity to critical infrastructure. (E. C. Jamieson et al.,2013)The site image ofthe target reach and two bends are presented in figure 2.精选优质文档-倾情为你奉上专心-专注-专业Figure 6. (A) Aerial photo of the Sawmill Creek study area; red dashed lineindicates right bank and fl
30、ow is from south to north. (B) Bend 1 lookingupstream, and (C) outside bank of bend 2 looking downstream.( E.C.Jamieson et al., 2009)In September 2009, a series of seven stream barbs were installed to protect thetwo eroding outer channel banks and to serve as a demonstration project for the use ofth
31、ese structures in a semi-alluvial channel( E.C. Jamieson et al., 2009). Three barbswere placed in the first upstream) bend, and four barbs were placed in the proceeding(downstream) bend (labeled B4B7) (Figures 1 and 2) ( E.C. Jamieson et al., 2009).Figure 7. Stream barbs at Sawmill Creek: (a) bend 1
32、 and (b) bend 2.Barbs arenumbered in the downstream direction. Photos taken on 9 November2009,精选优质文档-倾情为你奉上专心-专注-专业during low flow conditions (discharge, Q 0.3m3/s). (E. C. Jamieson et al.,2013)2. OBJECTIVETable 2.Barb Installation Plan In the Two Bends of Sawmill Creek(E. C. Jamieson et al.,2009)Ac
33、cording to the barb design plan in previous study (E. C. Jamieson et al.,2009),the barb installation angle was determined due to laboratory experiments (Matsuura, Tet al., 2004 and numerical modeling (Minor, B. et al,. pp1087-1095). In theseexperiments, which considered 90 and 135 channel bends, opt
34、imum bank protectionwas achieved for a series of three barbs, each with an alignment angle of 30 (E. C.Jamieson et al.,2009).However, the length of stream barbs are determined due to theUSDAguidelines recommendation value which says barb lengths should not exceedone-third of the cross section top wi
35、dth at bank full flow but does not specify theoptimum value of barb length. The barb length design in the two bends of SawmillCreek was not taken into account in detail. It can be concluded that there may be abetter barb design plan with the optimum length value.The objective of this paper is trying
36、 to study the relevance between the length ofbarbs and band erosion reduction effect to find the proper design plan for stream barbsand obtain the best effect of band erosion reduction and aquatic habitats protectionusing Delft-3D hydraulic flow model.3. METHODDelft-3D hydraulic flow model (Delft-3D
37、-FLOW) is a unique, fully integratedcomputer software suite for a multi-disciplinary approach and 3D computations forcoastal, river and estuarine areas. It can carry out simulations of flows, sedimenttransports, waves, water quality, morphological developments and ecology. It hasbeen designed for ex
38、perts and non-experts alike(Delft3D-FLOW_User_Manual,Deltares System. The Delft-3D suite is composed of several modules, grouped arounda mutual interface, while being capable to interact with one another(Delft3D-FLOW_User_Manual, Deltares System. It is a multi-dimensional (2D or 3D)精选优质文档-倾情为你奉上专心-专
39、注-专业hydrodynamic (and transport) simulation program which calculates non-steady flowand transport phenomena that result from tidal and meteorological forcing on arectilinear or a curvilinear, boundary fitted grid(Delft3D-FLOW_User_Manual,Deltares System).The general process is to simulate several fl
40、ow conditions with different value ofbarb length and analyze the velocity and shear stress profile to see the differences.3.1 Grid GenerationThe Delft-3D-RGFGRID interface is used to generate splines and grids based onthe channel, which is the first step of the flow simulation.Figure 8. Grid Generat
41、ionABCD精选优质文档-倾情为你奉上专心-专注-专业(A: Spline Generation; B: Change Spline into Grid; C: Orthogonalisation;D: Refinement .)The channel ichnography is obtained from the Geographic information System (GIS)which can be used to determine the land boundary in The Delft-3D-RGFGRIDinterface.After determining the
42、land boundary, several splines can be drawn along thechannel and perpendicular to the channel, roughly. The splines should be as smooth aspossible and overlap with the channel boundary as much as possible when drawingthe boundary splines.After splines are determined, grid can be generated from the s
43、plines. In order tomake the grid cell uniform and smooth, orthogonalisation should be done severaltimes until getting the optimum grid. The grid was refined two times to improve theresolution in these regions for better flow and sediment transport calculations. Thefirst time of refinement sets the M
44、 refinement factor and N refinement factor equal to3 and 3,and the second time equal to 2 and 2, respectively. Finally, a optimum gridwith 74 points in M-direction and 14 points in N-direction is generated successfully.This process is shown in Figure 8.3.2 Bathymetry GenerationThe Delft-3D-QUICKIN i
45、nterface is used to generate continuous bathymetryimage based on the depth data and the gird generated previously, which is the secondstep of the flow simulation.Figure 9. Bathymetry ImageAB精选优质文档-倾情为你奉上专心-专注-专业(A: Colored view; B: 3D view)The depth data are obtained from field measurement in 36 cro
46、ss sections in thetwo bends area and several thalweg lines on the river bed. After the gird and depthdata input is done, the size of gird cell can be averaged using Grid Cell Averagingfunction. Then, the continuous bathymetry image can be displayed through triangularinterpolation and internal diffus
47、ion. The internal diffusion is used to fill the blankdepth data in the channel.This process is shown in Figure 93.3 Virtual Barbs InstallationThe Delft-3D-FLOW_INPUT interface is used to simulate of flow and record thedata in observation points and cross sections based on the bathymetry image and th
48、egird generated previously, which is the third step of the flow simulation.ParameterValue/SettingParameterValue/SettingSimulation Start Time13/04/2016 00:00Water Density1000 kg/m3Simulation Stop Time14/04/2016 01:00Temperature15 Duration15 minRoughness (Manning)0.05Time Step0.01 minHorizontal Eddy V
49、iscosity2 m2/sTime Interval for Map Store120 minHorizontal Eddy Diffusivity10 m2/sHistory Interval for MapStore10 minVertical Eddy Viscosity2 m2/sWater Level1.0 mVertical Eddy Diffusivity10 m2/sSalinity30 pptOzmidov Length Scale11 m2/sUpstream Discharge2.5 m/s3Model for 3D turbulancek-EpsilonGravity
50、9.81 m/s2Threshold Depth0.05 mTable 3. Channel and Flow Parameters Setting in Flow SimulationAfter the grid and bathymetry image input is done, several channel and flowparameters can be determined, which are shown in table 3. The initial water level anddischarge were determined using the average val