CARSON CREEK 2D Model Approximation within a 1D Model

1. CARSON CREEK2D Model Approximation within a 1D Model TFMA Fall Technical Seminars

2. Here today: • Katina Bohrer PE, CFM • City of Austin • Scot Laun PE, CFM • Atkins

3. Carson Creek Watershed Montopolis Trib 4 Carson Creek Main Stem Trib 3 Trib 2 Trib 1

4. Potential Regulatory Issues - Quarries • The Quarries to the northeast are large enough to affect the 500-year floodplain. They are, essentially, detention ponds. • Quarries are regulated by the Railroad Commission and there are no requirements of what happens to them when mining is complete. • As the City regulates to Fully Developed conditions, it is important for us to have a pretty good idea as to what happens to the quarries after their ‘useful life.’ • The quarries are currently in City of Austin 2-mile ETJ. • Based on the size of the quarries, the City has determined that chances are likely that they will not be backfilled.

5. Carson Creek Quarry Areas

6. Potential Regulatory Issues – Airport Buyout • Jet/Patton Area was bought out by the Airport for noise abatement. • A noise abatement buyout is not the same as a floodplain buyout – when a grant is given for noise abatement, part of the agreement is that the recipient of the grant at least attempts to sell any properties that were bought with the grant money.

7. Airport Buyout Areas

8. Previous Flood Risk Management Areas • 13 homes in the floodplain were bought out by the City on Harrison and Hyman (mid 2000’s) • Hoeke and Posten were upgraded to allow safe passage during storm events (2012) • Thornberry Tributary channelization project (mid 2000’s)

9. Previous Flood Risk Management Areas

10. Known Hydraulic Issues • Montopolis Tributary centerline disappears prior to the confluence with Carson Creek Main Stem. • The watershed, in general, is very flat – especially in the area where Montopolis and the Overflow Channel confluence with the Main Stem. Naturally, the water spreads out in all directions. • This would be an area that would benefit from 2D modeling. However, due to funding and a low cost/benefit ratio, 2D modeling is not a viable solution at this time.

11. Carson Creek – Project Description Montopolis Trib 4 Carson Creek Main Stem Trib 3 Trib 2 Trib 1

12. Major Project Components

13. Cross section Layout

14. Stream Diversions and Intermingling Montopolis to Carson – XS 3580 Carson and Montopolis combined – XS 8855

15. Project Modeling Approach • Iterative approach to solving Peak Flow rates using HEC RAS and HMS • HMS used to generate flows using initial rating curves from RAS • RAS components • 2 overflows into Quarry • 1 diversion from Montopolis to Main Stem • 1 diversion from Main Stem to Overflow Channel • Split Flow for Main Stem and Trib 4 • RAS used to generate updated rating curves and split flow elevations • RAS and HMS rating curves compared. Rating curves adjusted if necessary. HMS rerun with new rating curves and process repeats

16. Project Challenges • Split flow for Main Stem and Trib 4 were to match elevations upstream • Overflows into quarries drives the downstream split flow • With all the flow diversions, there resulted in many knobs to turn\ • What was good enough?

17. Project Results

18. Lessons Learned • Limit number of diversions • Be aware for placement of diversions and model junctions • Optimization