and Flooded Structures Mapping John S. Grounds, III, PhD, PE, CFM, - - PowerPoint PPT Presentation

CR 288 B Bridges at Bastrop Bayou Hydraulic Analysis, Diversion Channel to Big Slough, and Flooded Structures Mapping

John S. Grounds, III, PhD, PE, CFM, D.WRE Vice President Director, Water Resources

March 28, 2018

Project Scope

• Site Reconnaissance
• Data Collection and Review
• Survey
• Hydraulic Modeling of CR 288 B Bridges at Bastrop Bayou
• FEMA/TxDOT/Updated Survey
• Pump Capacity Determination
• Diversion Channel Conceptual Design
• Hydrology and Hydraulics
• Channel Cross Section for 25 and 100-Year Events
• Cost Estimates
• Harvey Flooded Structures Map

Site Reconnaissance

Bastrop Ragsdale Rosen

Survey

• Surveyed approximately 2,500 LF along the

upstream and downstream faces of all three bridge openings

• 2 Cross Sections across Big Slough

TxDOT

Angleton Area Office

FEMA Model

• Confirm Flow through all three (3) bridge openings. FEMA only had the Bastrop

Bayou opening modeled.

• Updated HEC-RAS 5 2 D Hydraulic Model

Cross Section Comparison: Bastrop Bayou

• 10
• 5

5 10 15 20 100 200 300 400 500 600 700 800 900

Elevation (ft) Relative Station (ft)

Southern Bridge (Bastrop Bayou)

TX DoT Plans FEMA Model Survey

Scour/Cut Scour/Cut Siltation

Cross Section Comparison: Ragsdale Creek

• 2

2 4 6 8 10 12 14 16 50 100 150 200 250 300 350 400 450

Elevaton (ft) Relative Station (ft)

TX DoT Plans FEMA Model Survey

Cross Section Comparison: Rosen Creek

• 2

2 4 6 8 10 12 50 100 150 200 250 300 350 400 450 500

Elevation (ft) Relative Station (ft)

TX DoT Plans FEMA Model Survey

HEC-RAS 2D Updated Model Results

Ragsdale Creek Max Flow (cfs): 2522 Rosen Creek Max Flow (cfs): 1191 Bastrop Bayou Max Flow (cfs): 2797 Combined Peak Flow (cfs): 6484

• 500

500 1000 1500 2000 2500 3000 200 400 600 800 1000 1200 1400

Flow (cfs) Time (t)

Individual Bridge Flows

Bastrop Bayou Ragsdale Creek Rosen Creek

• 1000

1000 2000 3000 4000 5000 6000 7000 200 400 600 800 1000 1200 1400

Flow (cfs) Time (t)

Combined Peak Flows

HEC-RAS 1D Updated Model Results

• For the 10, 50, 100, and 500-Year Events, there is a 0.01 feet

decrease in the water surface elevation when the TxDOT bridges are updated with the surveyed topography.

• Updating the FEMA model with the updated survey, TxDOT

bridges, and opening all 3 bridges results in a deceases as follows:

• 0.07 feet for 500-Year (~12.7 Feet)
• 0.03 feet for the 100-Year (~12.0 Feet)
• 0.03 feet for the 50-Year(~10.9 Feet)
• 0.05 for the 10-Year (~8.9 Feet)
• These are all dominated by the Combined Probability (Riverine +

Coastal) (100-Year / ~12 Feet)

• The Flood Insurance Rate Maps would be unchanged so a Letter of

Map Revision is NOT needed.

HEC-RAS 1D Updated FEMA Model Conclusions

• Siltation in channel is offset by wider cross

section in overbanks.

• FEMA vs. TxDOT vs. Survey results in no

significant difference in Water Surface Elevations for 10, 50, 100, and 500-Year Events

• Pumping is not needed to offset the loss in

conveyance from siltation. It would increase flooding potential downstream.

Diversion Channel to Big Slough: 25-Year Capacity Layout

• 360 CFS Capacity
• 4 Feet Deep
• 4:1 SS (H:V)
• 65 Foot Bottom
• 30’ Maintenance Berms
• 158 Feet ROW
• 3 – 4’ x 10’ RCB Culverts
• 3 – 4’ x 10’ RCB Siphon
• Channel Slope = 0.018%
• Channel Velocity = 1.5 FPS (SLOW)

Diversion Channel to Big Slough: 100-Year Capacity Layout

• 455 CFS Capacity
• 4 Feet Deep
• 4:1 SS (H:V)
• 85 Foot Bottom
• 30’ Maintenance Berms
• 178 Feet ROW
• 4 – 4’ x 10’ RCB Culverts
• 4 – 4’ x 10’ RCB Siphon
• Channel Slope = 0.018%
• Channel Velocity = 1.5 FPS (SLOW)

Diversion Channel to Big Slough: Cost Estimates

Flooded Structures Mapping

• Report from the City of Richwood
• Highlighted Brazoria County

Appraisal District parcels

Questions, Comments, and Discussion