[American Society of Civil Engineers World Environmental and Water Resources Congress 2007 - Tampa,...

Two-dimensional Simulation of Hydrodynamic and Sediment Transport in aGravel Bed Channel: The Salt River

Dong Chen1, Yaoxin Zhang2, Jennifer G Duan3, Mark Stone1, Kumud Acharya1

1 Division of Hydrologic Sciences, Desert Research Institute, 755 E. Flamingo Road,Las Vegas, NV 89119; PH (702) 862-5541; FAX (702) 862-5427; email:[email protected] Center for Computational Hydroscience and Engineering, The University ofMississippi, Carrier Hall, Room 102, University, MS 38677; PH (662) 915-5083;FAX (662) 915-7796; email: [email protected] of Civil Engineering and Engineering Mechanics, The University ofArizona, Tucson, AZ 85721-0072; PH: (520)-626-5946 Fax: (520)-621-2550email: [email protected]

Abstract

Flow hydraulics, sediment transport, and channel morphological changes ofthe Rio Salado, Salt River, Phoenix, Arizona were simulated using CCHE2D, adepth-averaged two-dimensional hydrodynamic and sediment transport modeldeveloped at the National Center for Computational Hydro-Science and Engineeringat the University of Mississippi. The bed material was treated as multiple grain-sizedsediment mixtures and simulated by a sediment transport module. Changes in bedelevation were calculated by solving the sediment mass conservation equation. Thecalculated results were compared with HEC-RAS results and field observation, whichshowed that the two-dimensional model was more applicable in simulating flood zonecoverage, non-uniform sediment sorting and channel geomorphology changes.

Study Area

Introduction of the Rio Salado (Salt) RiverThe Salt River (Rio Salado in Spanish) is a major tributary to the Gila River in

Arizona. The river originates in eastern Arizona and flows westward to its confluencewith the Gila River west of downtown Phoenix (Figure 1). The Phoenix basin consistsof Quaternary sediments that constitute the valley fill. Annual average rainfall in thelower Salt River Valley is approximately 8 inches and is less than theevapotranspiration rate in all months of the year. Summer thunderstorms in July andAugust are responsible for the majority of the total annual rainfall. Mean dailytemperatures in the summer and winter are 85°F and 53°F, respectively. The relativehumidity ranges from approximately 20 percent to 50 percent. Prior to agriculturaldevelopment and urbanization of the Phoenix metropolitan area, the Salt River was a

World Environmental and Water Resources Congress 2007: Restoring Our Natural Habitat © 2007 ASCE

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perennial stream fed by snowmelt from mountains in eastern Arizona. Cottonwoods,willows, and mesquites were the prevailing riparian habitat. In the early part of the20th century, major modifications to the river system occurred as part of the Salt RiverProject, which placed several dams along the Salt River to allow diversions of waterfor agricultural and urban uses. Sand and gravel mining operations and developmentalong the river induced additional changes to the river channel and hydrology. Sincethen, the Salt River has transformed to an ephemeral stream with severe degradationand loss of riparian habitat (USACE, 2005).

Fig. 1 Map of the Rio Salado, Salt River, Phoenix, Arizona

This study documents the hydraulic and sediment transport analyses conductedon a reach of approximately 9.5 miles of the Salt River extending from 19th Avenue onthe east to 91st Avenue on the west. The study area is within the boundaries of the Cityof Phoenix, Arizona. Three USGS gauges are marked at the attached map of study cite(Figure 1).

Bed Material Gradation along the River

Bed material gradations along the channel were obtained through acombination of Pebble Count Method and Sampling Materials at five accessiblelocations. First of all, pebbles bigger than 76 mm (3 inches) on the channel bed ateach data collection site were counted using the Pebble Count method followingguidelines from Bunte and Abt (2001). This method is widely used in geomorphic



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and hydraulic studies to determine the material gradation of the armored layer ingravel-bed rivers (Bunte and Abt, 2001). In this study, a sampling frame wasconstructed of four aluminum yard sticks bound together to make a 0.91 m x 0.91 msquare (Fig. 2). Strings that spanned horizontally and vertically were tied at 0.15 mincrements along the frame to create the sampling grid which aims to reduce operatorbiases. After counting and removing the big pebbles, samples of bed materials withthe sizes smaller than 76 mm (3 inches) were taken by digging approximately 5cmdown from bed surface within 1/16 frame area (Fig. 2). Each bed material sampleweighed about 0.5-1.0kg.

Fig. 2 Sampling Frame for Pebble Count used in the present study

Samples were analyzed at the soil laboratory of Desert Research Institute toobtain the grain size distribution. The American Society for Testing and Materials(ASTM) procedures were followed for sieving analysis as described under StandardTest Method for Particle-Size Analysis of Soils. All samples were oven dried at105°C for 24 hours, then placed in a series of ASTM approved sieves and positionedon a mechanical shaker for a minimum of 10 minutes each. The employed sieve sizesare 31.5mm, 9.5mm, 5.66mm, 2.0mm, 500µm, 425µm, 355µm, 150µm, 75µm and45µm. The resultant size distributions of bed surface material are plotted in Fig. 3.



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Bed Surface Material Gradation Along the Channel

0.01

0.1

1

10

100

0.01 0.1 1 10 100 1000

Particle Size (mm)

Per

cent

finer

(%)

19th Ave.35th Ave.51st Ave.67th Ave.91st Ave.

Fig. 3. Bed Surface Material Gradation Along the Channel

The bed surface material size compositions are shown in Fig. 3. Less than0.6% of surface material consists of wash load (clay and silt) whose size is less than0.0625 mm. There is about 2% bed material belonging to fine sand at 19th Avenueand 91st Avenue. Bed surface material mean size D50 ranges from 20mm to 40mm.More than 90% of the bed surface material consists of very coarse sand and gravels.Bed material at the 51st Avenue is obviously coarser than the bed material at otherlocations.

Hydrodynamic and Sediment Transport Simulation

Introduction to CCHE2D

CCHE2D is an integrated package for two-dimensional simulation and analysisof river flows, non-uniform sediment transport, morphologic processes, coastalprocesses, pollutant transport and water quality. These processes are solved with thedepth integrated Reynolds equations, transport equations, sediment sorting equation,bed load and bed deformation equations. This model is based on Efficient ElementMethod, a collocation approach of the Weighted Residual Method. Internal hydraulicstructures, such as dams, gates and weirs, can be formulated and simulatedsynchronously with the flow. Dry and wet capability enables one to simulate flowswith complex topography with ease. Three turbulence closure schemes,depth-averaged parabolic, mixing length eddy viscosity models. and k- e model are



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available. The numerical scheme can handle subcritical, supercritical flows, and theirtransition (NCCHE, 2005).

Generation of 2-D Computational Mesh

A 2-D computational mesh was generated based on channel topography dataextracted from a digital contour map which was built up by using the cross section datafrom a HEC-RAS model. The 2D mesh covers the entire study reach. 77 cross sectionsextracted from the digital contour map were used to specify the bed elevations of thestudy reach. Additional cross sections were interpolated based on these cross sections.The computational mesh has 678 cross sections with 120 computational nodes at eachcross section.

Hydrodynamic Simulation

Boundary and Initial conditions. Flow discharges at 5, 10, 20, 50, 100, and500-year frequency floods and associated initial water surface elevations were obtainedfrom HEC-RAS data. Table 1 gives the discharges and associated inlet and outlet waterstages at each recurrence interval.

Table 1 Boundary and Initial Conditions

Water Surface Elevation, Velocity Field and Flood Zones at EachDischarge. The simulated flood inundate areas under each discharge were comparedwith the results from the HEC-RAS model. The results calculated by the two modelsare similar under 50-year, 100-year, and 500-year flood event. Almost no over bankflows occur in the HEC-RAS simulation under 5-year, 10-year, and 20-year floodevent, whereas water on the flood plains were predicted by CCHE2D under alldischarges, especially south of the river between 43rd Avenue and 51st Avenue wheregravel mining operates are found. Fig.4 presents a comparison of the calculated resultsby the two models under the 5-year flood event. Photos for the flood event, taken onJan. 03, 2005 (approximate 12,000 cfs, less than the 5-year flood), reveal over bankflow which suggests that the CCHE2D results are more realistic.

RecurrenceInterval (years)

Discharge(m3/s)

Inlet WaterStages (m)

Outlet WaterStages (m)

5 572.0 322.34 289.7210 1500.8 324.08 290.5920 2463.6 325.41 291.2650 3822.8 326.93 291.84100 4700.6 327.75 292.17500 6796.0 329.47 292.73



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Fig. 4. Comparison between the 5-Year Flood Zone Simulated by CCHE2D andby HEC-RAS (Water Surface Elevation and Velocity are also shown in the 2Dresult)



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Sediment Transport Simulation

Size Classification and Bed Material Samples. The sediment transport modeltreats suspended, bed load and bed material as mixed, grain-sized sediments anddivides bed load into ten groups based on quantity and fluvial characteristics. The Wu,Wang and Jia’s (2000) Formula and size distribution of substrate material are adoptedin the present study.

There is no field measurements of suspended load and bed load in the studyreach. In the present study, we simulate sediment transport by assuming anequilibrium sediment supply at the inlet boundary which is calculated by usingHEC-6T which uses Yang’s (1984) Equation.

Bed Load Transport Rate at Each Discharge. Bed load transport waspredicted in the main channel at each flood discharge. As an example, Fig. 5 showsthe bed load transport rate under 20-year and 500-year floods. The highest bed loadtransport rate occurs directly upstream of the 19th avenue bridge. This is likely due tothe gravel mining operations downstream of 19th Avenue and the effect of themeandering channel pattern. No significant bed load transport is calculated at otherlocations due to bed armoring. The heavy load at the first several cross sections islikely caused by inaccuracies in the boundary conditions, i.e., the equilibriumsediment transport rate was overestimated by Yang’s (1984) Equation.

Suspended load transport rates at each discharge were also calculated in thispresent study. However, the results were not shown in this report since suspendedload is minor (less than 10% in quantity) compared to bed load due to insufficientsupply from channel bed.

Bed Elevation Changes at Each Discharge. Bed degradation was predicted inthe main channel at each flood discharge. As examples, Fig. 6 shows the loadelevation change under the 20-year and 500-year floods. The most serious beddegradation occurs upstream of the 19th Avenue Bridge which is 1.2 m and 3.3 m atthe 20-year and 500-year floods, respectively. Reaches upstream of the 35th AvenueBridge and 75th Avenue are also under considerable degradation.

Deposition at the inlet reach in the present study is probably due toassumptions made in the setting of the boundary condition for sediment transport, i.e.,the equilibrium sediment transport rate was overestimated by Yang’s (1984) Equation.Slight deposition was also predicted downstream the serious bed degradation duringbig flood events (50-year, 100-year, and 500-year) at 19th avenue.



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(a)

(b)

Fig. 5 Bed Load Transport Rate (a) 20-year; (b) 500-year



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(a)

(b)

Fig. 6 Bed Elevation Change (a) 20-year; (b) 500-year



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Conclusions

Hydrodynamic and sediment transport in a reach of Salt (Rio Salado) Riverunder 5, 10, 20, 50, 100, and 500-year frequency floods were simulated by usingCCHE2D in the present study.

a). The flow depths range from 0 to 9.8 meters. The deepest flow depth islocated between 19th Avenue and 35th Avenue, in which the channel bed is nearlycompletely modified with sand and gravel mining operations.

b) The simulated flood inundate areas under each discharge were compared withthe results from a HEC-RAS model. Field observations suggest that the 2D resultswere more realistic.

c) The most serious bed degradation occurs upstream of the 19th Avenue Bridge,which is 1.2 m and 3.3 m at 20-year and 500-year floods, respectively. Reachesupstream of the 35th Avenue Bridge and 75th Avenue are also under noticeabledegradation.

Acknowledgement

The study is part of the Urban Flooding and Channel Restoration DemonstrationProgram for Arid and Semi-Arid Regions (UFDP).

References

Bunte, K. and Abt, S. R. (2001), “Sampling Frame for Improving Pebble CountAccuracy in Course Gravel-Bed Streams”, Journal of the American WaterResources Association, 37(4):1001-1014.

National Center for Computational Hydroscience and Engineering (NCCHE), TheUniversity of Mississippi,http://www.ncche.olemiss.edu/index.php?page=freesoftware#cche2d (Jan15,2007)

US Army Corps of Engineers (2005). Rio Salado Oeste Interim Feasibility Report.

Wu, W., Wang, S. S. Y. and Jia, Y. (2000) “Nonuniform sediment transport inalluvial rivers”, J. Hydraul. Res., Vol. 38, 2000, No. 6, pp. 427-434.

Yang, C. T. (1984) “Unit Stream Power and Sediment Transport”, J. of HydraulicEngineering, ASCE, Vol.110, No.12, pp. 1783-1797.



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