This document describes a HEC-RAS 2D flood modeling case study of the Little River in Newburyport and Newbury, Massachusetts. It summarizes the advantages of 2D modeling, details the HEC-RAS model setup including terrain and hydrologic inputs, and presents calibration results from a historic 2006 rainfall event. Next steps include running additional storm simulations and using the model to evaluate potential flood mitigation measures under future sea level and climate change scenarios.
1. HEC-RAS 2D FLOOD
MODELING
CASE STUDY: LITTLE RIVER, NEWBURYPORT AND NEWBURY, MA
Prepared by: Bill Mullen, P.E.
River Hydraulics
978-235-0875
billmullen@riverhydraulics.com
December 2017
2. Advantages of 2D Flow Modeling
Takes advantage of the availability of digital elevation models
Typically results in more accurate flood modeling
Models flows throughout a flood event (i.e. unsteady flow), not just at its peak
Can model complex runoff and tide/surge scenarios
3. HEC-RAS model features
2-dimensional flow modeling using available DEM
Seamlessly combine LiDAR terrain with any available surveyed channel bathymetry
Models unsteady flow events
Automated floodplain delineation on aerial or topo maps
Fast, accurate, well-tested, it is the “gold standard”
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5. Develop hydrology using HEC-HMS
Rainfall-runoff modeling program
Model inputs: drainage area (acres), runoff curve number, Tc, also precipitation and its
timing
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10. Rainfall events modeled in HEC-HMS
Historic rainfall event associated with Little River flooding
“100-year” rainfall presumed to be associated with the “100-year” flood
11. Parker Street in Newburyport, photo taken by Joe Teixeira, Newburyport Conservation Commission
Historic rainfall event:
Mother’s Day storm of
May 2006
16. Modeled as one single 2D flow area
DEM used has a vertical accuracy of 5.9” with 1 meter resolution
Cell size of 250’ x 250’; smaller cells along embankments; nearly 6000 cells total
Each cell and cell face has its own highly detailed hydraulic rating curve
Routing of flows is done from cell to cell
Little River HEC-RAS model parameters
21. Culverts & bridge openings were modeled within the 2D flow area
Surveyed channel bathymetry merged with overbank terrain
Ten stream input locations and one tidal input location
Manning’s n roughness coefficients for all cells based on land use
Little River HEC-RAS model parameters (continued)
25. HEC-RAS model calibration:
Calibration done using the Mother’s Day storm of 2006
The model produces reasonable results
Minor improvement in the model might be possible by
adding culverts underlying driveways, etc.
28. Next Steps:
Run HEC-RAS for the range of events of interest, e.g. 100-year storm
Evaluate potential flood-mitigating measures such as:
• Changes in culvert sizes & bridge openings
• Changes in embankment heights
• The addition of flap gates
29. 1. Sea level rise & storm surge
2. Climate change (rainfall amounts & temporal patterns)
The HEC-RAS model can be used to assess: