Modeling a Complex Hydraulic Environment Using a 1-D Approach Supplemented with Simple 2-D Principles Manas Borah Ed Dickson June 5, 2014

Transcription

1 ASFPM 2014 Annual Conference Modeling a Complex Hydraulic Environment Using a 1-D Approach Supplemented with Simple 2-D Principles Manas Borah Ed Dickson June 5, 2014

2 Agenda Overview and Background Hydrology Hydraulics: Unsteady HEC-RAS and FLO-2D Input Data Results Conclusions 2

3 The Congaree River Watershed

4 Project Background Project area consists of several Levee Systems just DS of the fall line of the Piedmont and Sandhill geographic region in Columbia, SC Drains over 7,000 square miles Majority of agricultural levee system was built from the 60 s to the 70 s National Levee Standards were formalized in the 1980 s Main Levee has never overtopped but did breach in

5 Ring Levee Main Levee Farm Levee Congaree River Levee System 5

6 Areas of Interest Main Levee Wateree Watershe d Heathwood Hall School Riverland Park City of Columbia WWTP Congaree River Middle Savannah Watershe d 6

7 Project Background City of Columbia built sewer plant in the 1960s Heathwood Hall School built in the 1960 s Both rely on levee for protection Accurate modeling is essential 7

8 Congaree River Levee Typical River Side of Levee 8

9 Congaree River Levee Typical River Side of Levee 9

10 Project Details River transitions from Piedmont to Sandhills Geographic Region through downtown Columbia Modeling Impacts Typical Cross sections 10

11 Piedmont Region Shallow, rocky channel Hilly with thin clay soils Can have Cliff-like channels Typically narrow floodplains contained in or near channel Elevations from 375 1,000 feet 11

12 Piedmont Region 12

13 Sandhills Region Smooth, sandy channel Sandy with silt and clay soils Wide flat floodplains Elevations from feet 13

14 Sandhills Region 14

15 Hydrology Flood Frequency Analysis (Bulletin 17B) USGS gage station Congaree River at Columbia ( ) Entire available period of record ( ) unregulated and regulated flow conditions Adjustments made to account for the effects of regulation on unregulated flows 100-year discharge: 286,000 cfs 500-year discharge: 414,500 cfs Ignored discharges contributed by the drainage area along the modeled reach of the River 15

16 Hydraulics Data Unsteady HEC-RAS FLO-2D Topographic Data (LiDAR) N-value (aerial imagery) Bridges across the Congaree River (survey) Levees (LiDAR) Channel Geometry (survey + LiDAR) Major Road Embankments (LiDAR) 16

17 Hydraulics Additional Data needed for more accurate 2-D modeling Smaller Road Embankments Bridges and Culverts Tributaries Area and width reduction factors Streets Cell Size: 250 ft 17

18 100-year Floodplain Unsteady HEC-RAS Levees 18

19 100-year Floodplain Unsteady HEC-RAS 19

20 100-year Floodplain Unsteady HEC-RAS 20

21 100-year Floodplain Velocity Grid FLO-2D 21

22 100-year Floodplain FLO-2D & HEC-RAS 22

23 Final 100-year Floodplain FLO-2D & HEC-RAS 23

24 BFEs HEC-RAS Zone AE (Elev 129.6) Zone AE (Elev 129.6) Zone AE (Elev 129.6) Zone AE (Elev 126.8) 24

25 Variability in WSEL Zone AE (Elev 129.6) Zone AE (Elev 129.6) Zone AE (Elev 126.8)

26 Flow Pattern near I year vs 500 year 286,00 cfs 414,500 cfs 26

27 Without Levee Simulation FLO-2D mapping and HEC-RAS Storage Area Designations 27

28 Conclusions 2-D simulation assisted in o differentiating areas of ineffective flow and storage in HEC-RAS model in the vicinity of Interstate 77 o Identifying ineffective flow areas behind the levee for HEC-RAS model o Identifying ineffective flow areas on the left overbank further downstream from Interstate 77 Overall, 2-D simulation provided a sound basis for making engineering judgments associated with 1-D simulation Showed general flow pattern and variation in water surface elevations across HEC-RAS cross sections that may assist with mitigation planning 28

29 Questions? 29