Tidal Flooding

Pictured: NPS Headquarters in Potomac Park (2005)

Jason ElliottNWS Baltimore/Washington

Media WorkshopMarch 29, 2013

Easy to understand…

…if only it was really this easy!

Tidal Flooding

Winds can greatly affect tides…even without a “storm”. The shape of bays/inlets/etc. can also funnel or channel

water.

Worst of all…it’s a “game of inches”.◦On some days, tide levels only a few inches above the

astronomical prediction are enough to produce inundation.

Tides love to misbehave…

Tropical cyclones:Favorable if stormmoves west of DC

Most favorable ifstorm moves westof mouth of Ches.Bay and DC

Tidal Floods – Tropical Storms / Nor’easters

If the center of thestorm stays eastof DC, impacts willbe lessened

If the center of thestorm stays eastof the Bay and DC,impacts will bevery little◦ Still some minor

effects, and watchfor freshwater!!

Tidal Floods – Tropical Storms / Nor’Easters

Was it favorable orunfavorable?

Actually, both!

So what happened in Sandy?

Due to the movement of the storm, tides actually dropped significantly as the storm approached.

Phase 1: The Drop

Astronomical TideMinor

FloodLevel

But as the low center passed us to the north, a significant surge of water came in, causing flooding.

Phase 2: The Surge

MinorFloodLevel

Mod.Flood L

Freshwater inundation then followed from upstream rains above thefall line.

Phase 3: The Freshwater

pre-storm

blowout

TidalSurge

MinorFresh-water

influence

1942 Freshwater vs. 1933 HurricaneAll heights in common tidal datum (low water datum) except Little Falls (which is non-tidal)

Freshwater Inundation of Tidal Zones

Location Distance below Chain Bridge

1942 Freshwater Peak Elevation

Little Falls 1.5 miles above 26.9 feet

Key Bridge 3.3 miles 19.5 feet

Wisconsin Avenue 3.6 miles 17.8 feet

Memorial Bridge 4.7 miles 14.0 feet

17th Street 5.3 miles 13.6 feet

Hains Point/W.C. 7.4 miles 12.1 feet

Bolling AFB 9.3 miles 10.7 feet

Alexandria 11.6 miles 8.4 feet

New Alexandria 13.3 miles 7.9 feet

1933 Hurricane Peak Elevation

10.6 feet

11.1 feet

11.1 feet*

11.0 feet

10.9 feet

11.0 feet

10.7 feet

10.6 feet

10.5 feet

* For comparison, Isabel (2003) was 11.2 feet atWisconsin Avenue

You might be surprised!

This map shows the extentof where freshwater wouldcreate ½ foot rises or morein the Tidal Potomac, basedon NWS modeling.

How far can the freshwater go?

= 10 ft at Little Falls

= 12 ft at Little Falls

= 14 ft at Little Falls

= 28.1 ft at Little Falls

Minor tidal flooding happens frequently (usually at least once per month).◦ This minor tidal flooding is not a concern for most people.

On the Potomac River, river flood products stop at Little Falls. Below Chain Bridge, coastal flood products take over…even for freshwater inundation.

NWS Coastal/Tidal Flood Products

NWS BulletinsCoastal Flood Advisory – keep an eye on things but effects expected to be limited to nuisance flooding (low-lying Old Town Alexandria, East Potomac Park, etc.)Coastal Flood Watch or Warning – Need to take notice, could/will be significant

20-50 timesper year

1-5 timesper year

Forecast model was developed for the uppermost Tidal Potomac (Washington DC). ◦ Key limitation: This model cannot account for significant wind.◦ Second key limitation: The ocean model must pick up on a wind-

based tidal increase before the Potomac model will see it.

New Forecast Information Available

Potomac River/Washington Channel Forecasts Little Falls Wisconsin Avenue SW Waterfront

Chesapeake Bay Operational Forecast System

Blue = ObservedYellow = Astro.Red = CBOFS

http://tidesandcurrents.noaa.gov/ofs/cbofs/cbofs.html

Model Output Available

Key point:CBOFS or the Upper Potomac model showing conditions that reach minor/moderate flood and our issuance of coastal flood products are not always coincident.

Model output is still just guidance!

Model Output Available

It doesn’t take a tropical storm / Nor’easter to produce coastal flooding – even moderate-level flooding.

Although the Potomac is the most visible example, freshwater inundation affects other areas too.

Final Notes

Sediment spreads down the Susquehanna, Potomac, and Rappahannock tidal zones after Lee (2011).

Questions?Jason ElliottSenior Service HydrologistNWS Baltimore/Washingtonjason.elliott@noaa.gov

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