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”

5. Develop hydrology using HEC-HMS
 Rainfall-runoff modeling program
 Model inputs: drainage area (acres), runoff curve number, Tc, also precipitation and its
timing

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

15. HEC-RAS model

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

20. Cross-section
location

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)

23. 2400 1200 2400 1200 2400 1200 2400 1200 2400
-6
-4
-2
0
2
4
6
SA: shrunk2Dflowarea BCLine: tidal input
Stage(ft)
Legend
Stage
W

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.

27. Other HEC-RAS model output:
 Flood Depths
 Flood Velocities

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: