estimating extreme coastal flooding conditions for the ... · nathan dill, pe . what is the hazard?...

Estimating Extreme Coastal Flooding Conditions for the Maine Coast

2016 Maine Sustainability & Water Conference

March 29, 2016 Augusta Civic Center, Augusta, Maine

Nathan Dill, PE

What is the Hazard?

A increase sea level, above the normal tide, caused by wind stress and changes in barometric pressure during a storm event. AKA “Wind Setup”.

Highly energetic fluctuations in the water surface caused by local winds (seas) and/or generated from distant winds (swell).

Rising water at a range of time scales

Gradual increase in the mean water level due to increased volume of ocean water, subsidence, changing large scale ocean currents.

Hours

Seconds

Decades

Storm Surge:

Relative Sea Level Rise:

Storm Waves:

Storm Surge

“The flood in Bangor was due to a combination of strong, prolonged,

south-southeasterly winds and high astronomical tides. Storm

rainfall, ice jams, and streamflow were not major factors causing the

flood.”

“ Water surface elevation in downtown Bangor reached 17.46 feet (5.32 m) (NGVD),

approximately 10.5 feet (3.2 m) above predicted astronomical

tide.” Morrill et al. 1979. Maine Coastal Storm and Flood on

February 2, 1976, Geological Survey Professional Paper 1087, Joint report by the U.S. Geological Survey and the National Oceanic and Atmospheric Administration.

Storm Waves

“Waves caused by high winds can be more of a factor in flooding and damage than the

combination of surge and tide. Flooding from wave action can take many forms. The storm surge may not reach the height of a seawall, but waves may

overtop it. Water passing over (overwash) a barrier can damage structures behind it.”

- Gadoury, R.A. 1979. Coastal Flood of February 7, 1978 in Maine Massachusetts, and New Hampsihre. Water-Resources Investigations Report 79-61, U.S. Geological Survey.

Sea Level Rise

A gradual increase in the mean water level due to increased volume of ocean water, subsidence, changing large scale ocean currents, etc.

The magnitude of sea level change over the past century has been very small compared to sea level change experienced on a twice daily basis in Maine.

Sea Level Rise

Sea Level rise impacts will almost certainly manifest through discrete extreme events, rather than a gradual “flat” increase in the mean high water. Although interesting, tools such as this have limited utility since they cannot take into account the highly dynamic nature of actual coastal flooding. The flooding they portrait is an unlikely future reality.

Storm Surge – Gauge Data Analysis For FEMA FIS purposes Gives the Still Water Elevation (SWEL) 5 stations in Maine

Strategic Alliance for Risk Reduction. (2012). Updated

Tidal Profiles for the New England Coastline

Storm Surge – Gauge Data Analysis



L-Moment Analysis Annual Maximum Series Wakeby Distribution

Storm Surge – Gauge Data Analysis



Gives the Still Water ELevation (SWEL) for FIS purposes

Storm Surge – Single Event Numerical Modeling

FEMA used the RMA2 model to simulate a single “100-year” storm, recognizing that the gage analysis

Does not necessarily give accurate results complex Maine coastline

Strategic Alliance for Risk Reduction. (undated). Coastal Hydraulics and Hydrology.

Storm Surge – Single Event Numerical Modeling

Results give a spatially variable estimate for the still water level associated with a 100-year storm

-No waves -No direct wind forcing -Only a single high tide adjusted peak at the 100-year SWEL at the model boundary

Strategic Alliance for Risk Reduction. (undated). Coastal Hydraulics and Hydrology.

Storm Surge – Probabilistic Modeling SLOSH

• Dynamic modeling of Storm Surge including important meteorological forcing (Wind and Atmospheric Pressure)

• Hurricane Inundation maps by Saffir-Simpson Category • Used for Evacuation Planning • MEOWs and MOMS • National Hurricane Center Forecasts • Maine Geological Survey Provides updated maps at

http://www.maine.gov/dacf/mgs/hazards/slosh/index.shtml

• Saffir-Simpson Category does not easily translate to any risk level

• Does not include Extratropical storms • Does not include Wave processes



Storm Surge – ADvanced CIRCulation Modeling ADCIRC + STWAVE/SWAN

• State of the practice for storm surge modeling • Includes all pertinent physical processes (including wave setup) • High spatial resolution • Requires high performance computing

• Applied by FEMA for Flood insurance studies in all East Coast and

Gulf Coast Regions except Region I (New England)

• Determine the 100-year storm surge and and associated wave conditions rather than the surge and waves from a single example of a 100-year storm.

Storm Surge – Probabilistic Modeling Joint Probability Methods (JPM)

• Simulate hundreds to thousands of storms that could occur as well as many that have.

• Based on Joint probability of storm size, intensity, speed, approach angle, landfall location. e.g. North Atlantic Coast

Comprehensive Study (NACCS). Simulate Sea Level Rise cases to assess one aspect of climate change

Nadal-Caraballo et al. 2015. Coastal Storm Hazards from Virginia to Maine, ERDC/CHL TR-15-5

Storm Surge – Probabilistic Modeling Statistical-Deterministic approach

• Simulate a large set of hypothetical tropical storms that could occur and simulate hundreds to thousands of them.

• Storm set based on statistical-deterministic model of storm generation and evolution.

Bosma et al. 2015. MassDOT-FHWA Pilot Project Report Climate Change and Extreme Weather Vulnerability Assessments and Adaptation Options for the Central Artery. Massachusetts Department of Transportation

• WindRiskTech model allows for generation of storm sets based on future climate scenarios by downscaling from global climate models

• Simulate Sea Level Rise cases. • For example, applied in the recent Boston

Harbor Flood Risk Model (BH-FRM) effort

Storm Surge – Storm Set Optimal Sampling Methods

• Reduce computational requirements by selecting an “optimal” sample of storms for high fidelity modeling.

Response Surface Method Bayesian Quadrature Method

Risk Engineering, Inc. (2007). Joint Probability Analysis Of Hurricane Flood Hazards For Mississippi. Report Prepared for URS Group in support of the FEMA-HMTAP flood study of the State of Mississippi, October.

Resio, D. T., S. J. Boc, L. Borgman, V. J. Cardone, A. Cox, W. R. Dally, R. G. Dean, D. Divoky,

E. Hirsh, J. L. Irish, D. Levinson, A. Niederoda, M. D. Powell, J. J. Ratcliff, V. Stutts, J.

Suhayda, G. R. Toro, and P. J. Vickery,. (2007). White Paper on Estimating Hurricane

Inundation Probabilities. Vicksburg, MS: US Army Corps of Engineers.

If uncertainty can be quantified can be quantified in terms of its probability, (i.e. you need to estimate a probability distribution for the uncertainty), It can be accounted for mathematically when determining recurrence intervals associated with the coastal flood hazard. For example:

Storm Surge – Incorporating Uncertainty The Epsilon Term

FEMA, 2012. Joint Probability-Optimal Sampling Method for Tropical Storm Surge Frequency Analysis, Operating Guidance 8-12

Storm Surge – Advanced Analysis for Maine Based on NACCS study

NACCS grid In New York City


NACCS grid In Portland


• Best Use of NACCS Data for Maine will use nested modeling for storm surge and waves to downscale NACCS results to the local scale

• E.g. a model for Scarborough Marsh

estimating extreme coastal flooding conditions for the ... · nathan dill, pe . what is the hazard?...

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