Eli Gruber, PE Christian Warren, PE, CFM
Transcript of Eli Gruber, PE Christian Warren, PE, CFM
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2017 Montana Floodplain Resource Seminar: Lecture 8 HEC-RAS 2D Applications and FEMA Standards
Eli Gruber, PE
Christian Warren, PE, CFM
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Lecture 8 Overview
• Choosing the correct model
• Large Scale Base Level Engineering (LSBLE)
• Small scale, detailed applications
• Montana examples
• FEMA Guidelines and Standards
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Choosing the Correct Model
• Is a 1D model sufficient and more efficient?
–The end goal will determine this
• Do we have the necessary data for a 2D model?
–Terrain data may be limiting factor
• 2D or coupled 1D/2D model
–Application and efficiency
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FEMA Large Scale Base Level Engineering (LSBLE) • LSBLE process has recently been adopted by FEMA
• Produce Zone A regulatory ready products for large areas
• Rain-on-grid approach
• Model up to ~1,200 sq. mi. with one model
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Accuracy & Efficiency
Cost & Time
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2D LSBLE Conceptual Overview
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Rain-on- Grid Hydro
2D Grid Mesh
Model Execution
Floodplain Mapping
Terrain
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South Dakota 2D LSBLE Application
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Flood damage in Brown County, S.D. in Apr. 2011 Image: South Dakota Public Broadcasting
Flooding on the James River in S.D. in Mar. 2011 Image: Mitchell Republic
• Perform 2D LSBLE for 27 eastern South Dakota Counties
• Increase and enhance the flood risk products in South Dakota
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Project Scope
• Hydrologic and hydraulic modeling – 10% through 0.2% annual chance
exceedance (ACE) events
• Mapping – FIRM database ready S_FLD_HAZ_AR
with Zone A (1%) and Shaded X (0.2%)
• Scalable products – Can be leveraged throughout the Risk
MAP program, eventual production of regulatory and non-regulatory products
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Annual Chance Exceedance
H&H Modeling
Floodplain Mapping
10% R --
4% R --
2% R --
1%-minus R --
1%-plus R --
1% R R
0.20% R R
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Terrain Data Overview
• High resolution 1-meter LiDAR available for entire project area (except half of Clark County)
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Non-model backed Zone A’s on 10m DEM
2D HEC-RAS 5.0.1 on high-res LiDAR
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Model Area Delineation
• Main considerations for model areas: 1. SIZE: Max model area ~ 1,200 sq mi
2. DRAINAGE: Account for all contributing basin area (Rain-on-Grid + external inflows)
3. DATA: Leverage gage data for inflows and calibration
• 26 total model areas
• Basis for Hydrology
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Hydrologic Challenges
• Data availability – Variable period of record
record extension methods employed
• Two-station extension
• Snowmelt
– Rain-on-grid model not representative
• Flow regulation
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POR: 15 years
POR: 62 years
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Calibration Methods
• Peak flow targets at USGS gage locations for 1% ACE event
• Adjustments to Curve Numbers and/or hydrograph timing
• 38 total calibration points
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12
5
10
4
7
0 T O 5 % 5 % T O 1 0 % 1 0 % T O 2 0 % 2 0 % T O 3 0 % > 3 0 %
NU
MB
ER O
F G
AG
ES
PERCENT DIFFERENCE
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Calibration Challenges
• Single CN value
• Accounting for sub-basin variability
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Gage Name
Period of
Record (yr)
Gage Analysis
Flow (cfs) Modeled Flow (cfs) Difference
Percent Difference
Big Sioux River at North Cliff Ave at Sioux Falls, SD 45 31,930 30,195 -1,735 -5%
Big Sioux River near Dell Rapids, SD 67 30,730 35,133 4,403 14%
Flandreau Creek above Flandreau, SD 34 5,939 15,985 10,046 169%
Skunk Creek at Sioux Falls, SD 65 18,020 17,070 -950 -5%
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Detailed 2D Application
• Reservoir operations analysis to determine capacity of the downstream channel and identify potential flood impacts at different reservoir release rates
• Inline 1D/2D coupled model
• 1D areas used to model bridges where the deck would be impacted
• Twenty-foot nominal grid cell size
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Model Considerations
• Significant mesh detailed required to define floodplain features
• Significant leakage before breaklines
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Calibration
Verification
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Model Outcome
• Provided updated information for the reservoir operator to manage operations
• Areas where model results were different from effective FEMA floodplains (1D model)
• Ongoing effort to reconcile the two sets of results
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Questions
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Photo from FEMA
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