Components of Storm-induced Water Level along the Coastal Margin and Related Effects on the Nearshore Wave Environment Hans R. Moritz and Heidi P. Moritz U.S. Army Corps of Engineers, Portland District Portland, Oregon USA OUTLINE Review Water Level Components – as Affected by Storm Wave Action Examine Observed Infragravity Transients – Produced by Shoaling Bound Waves Examine Consequences of Infragravity Transients – Effects on Coastal Margin Use Consider Hypothesis: Storm Surge Enhanced by Infragravity Transients Conclusions The storm water level that acts upon the coastal margin is a product of many components (processes). A basic understanding of these components should be attained before initiating significant costal zone planning or implementing the design and construction of coastal infrastructure, at a given location. Infragravity Transients (Δη) of 1-2 meters and associated rip currents elevate the RISKS to life and property within the active coastal margin. More work is needed to fully parameterize the estimation of Δη and use this information to reduce risk along the coastal zone. Hypothesis: Δη may be responsible for a considerable fraction of the storm surge which affects coastal margins. Based on an initial assessment, this “Δη-Surge” warrants further evaluation. BC WA Pacific OR Ocean CA 3 MAR 1999 - Extr Trop Low 29 AUG 2005 - Hurricane image courtesy of NOAA image courtesy of NOAA Water Level and Waves Offshore SW Pass, LA: 26-31 Aug 2005 Waves, 60 mi. offshore 18 Wave Height, m, Hsig 16 14 12 10 8 Wave Height, m 6 Surge (above Predicted Tide), ft 4 2 0 2 2.5 3 3.5 4 4.5 5 Days after 26 AUG 2005 Water Level and Waves Offshore SW Pass, LA: 26-31 Aug 2005 18 Storm Surge, max=5.8 ft 16 Continental shelf break is 30 km offshore Wave Height, m, Hsig 14 12 10 8 6 Wave Height, m 4 Surge (above Predicted Tide), ft 2 0 -2 22.533.544.55 Days after 26 AUG 2005 Water Level and Waves Offshore SW Pass, LA: 26-31 Aug 2005 18 Storm Surge, max=5.8 ft 16 Wave Height, m, Hsig Predicted Tide El, ft, MLLW 14 12 10 8 6 Wave Height, m 4 Surge (above Predicted Tide), ft 2 0 -2 22.533.544.55 Days after 26 AUG 2005 Water Level and Waves Offshore Mouth of Columbia River, OR / WA: 3 Mar 1999 Waves, 18 mi. offshore 18 16 Wave Height, m, Hsig 14 12 10 8 Wave Height, m 6 4 2 0 1 1.5 2 2.5 3 3.5 4 Days after 1 MAR 1999 Water Level and Waves Offshore Mouth of Columbia River, OR / WA: 3 Mar 1999 18 Storm Surge, max=5.2 ft 16 Continental shelf break is 30 km offshore Wave Height, m, Hsig 14 12 10 8 6 Wave Height, m 4 Surge (above Predicted Tide), ft 2 0 -2 1 1.5 2 2.5 3 3.5 4 Days after 1 MAR 1999 Water Level and Waves Offshore Mouth of Columbia River, OR / WA: 3 Mar 1999 18 Storm Surge, max=5.2 ft 16 Wave Height, m, Hsig Predicted Tide El, ft, MLLW 14 12 10 8 6 Wave Height, m Tide Elevation, ft 4 Surge (above Predicted Tide), ft Predicted Surge (above 2 0 -2 1 1.5 2 2.5 3 3.5 4 Days after 1 MAR 1999 "Open" Coast Storm Surge Comparison Hurricane (GOM) vs. Extr. Low (PacNW) 13 12 Toke Pt, WA 11 SW Pass, LA 10 Potential Surge Limit ? 9 8 FOR STEEP “Open” COASTAL MARGINS 7 6 5 4 Total Surge ft Total Level, 3 2 1 0 -1 11.522.533.54 Days during Storm Willapa Bay =NOAA Tide Gauge =NDBC Wave Buoy N Pacific =USACE Instruments 15 km Washington Ocean Mouth of Columbia River Columbia Oregon River Cross-Shore Profile: 5 km South of Columbia River Mouth 50 0 13 m Sep-Dec03 -50 -100 Nov98 – Mar99 35 m R C Elevation (ft, MLLW) f M o -150 S km 5 -200 -250 10 9 8 7 6 5 4 3 2 1 0 Distance Offshore, west, miles 10 8 Hmo = 11.5 m, Tp = 17 sec 6 4 2 0 WSE, m -2 -4 -6 -8 -10 1 101 201 301 401 501 601 701 801 9011001 1.5 1 WSE - Infragravity Time, sec 0.5 Filtered for LOWPASS at 120 sec. 0 IG --0.5 WSE , m -1 35 m water depth -1.5 30 150 20 IG-Current: U: Onshore - Offshore 100 10 Onshore 50 0 0 IG- U, cms -10 Offshore -5 0 -20 Current: U, cms U, Current: -1 0 0 -1-30 5 0 30 1 101 201 301 401 501 601 701 801 901 1001 20 150 North 10010 500 -10 IG-Current: V - Alongshore IG - cms V, 0 South -20-5 0 Current: V, cms V, Current: -1-30 0 01 101 201 301 401 501 601 701 801 901 1001 -1 5 0 Time, sec 1 101 201 301 401 501 601 701 801 901 1001 6 Hmo = 5 m, Tp = 17 sec 4 2 0 WSE, m -2 -4 -6 0 50 100 150 200 250 300 350 400 450 500 1.5 Time, sec 1.5 1 WSE - Infragravity 1 0.5 0.5 0 0 -0.5 13 m water depth -0.5 -1 -1 WSE - m Infragravity, -1.5 -1.5 0 50 100 150 200 250 300 350 400 450 500 30 20 onshore IG-Current: U: Onshore - Offshore 10 0 -10 Infra-Current: U, cms U, Infra-Current: -20 offshore -30 30 North 20 IG-Current: V - Alongshore 10 0 -10 South -20 -30 cms V, Infra-Current: Location =M Deployment =3 12 10 8 6 4 2 18 Tp, sec Hmo, m 0 12 6 0 20 40 60 80 100 120 2 0 2 -2 1 WSE-Tide, m IG-Waves, m 0 0 20 40 60 80 100 120 40 40 20 20 0 0 -20 -20 IG - V, cm/s IG - U, cm/s -40 -40 0 20 40 60 80 100 120 Days After :30-Oct-1998 Longwave (IG, η) Propagation in Nearshore and Shoreface 8 km = still water level (non-storm perturbed) = short waves (sea/swell) = long (bound) waves - water level transients, η Longwave Propagation, Nearshore, based on Solitary Wave behavior Depth-limited Translation speed = √ g × (depth + wave height) Excursion = 10s - 100 meters Persists for 1-2 minutes lerating pe - Dece Departure ing upslo Bore mov point Δη =1 m horz. vert : 70 lope = 1 Beach S Still water level Wave (Bore) Speed at “0” Still Water level – Departure point Translation speed = √ g × (longwave height, Δ η) = 3 m/sec…….. for Δη = 1 m SNEAKER WAVES An “Unpredictable” Occurrence along the Coastal Margin Along the Pacific NW coast of the US, several people each year succumb to “sneaker waves”……. Appear to be associated with transient water levels (Δ η ) produced by groups of large waves. Landward Speed = 2-4 m/sec Excursion Distance = 10-100 meters Bore height = 0.3 – 1.5 m Return Flow more dangerous than run-up Duration = 1-2 minutes Components of Coastal Margin Water Surface Elevation at MCR 8 7 Storm water level = WSE due to tides + storm surge + infragravity transients (waves) 6 5 4 3 2 1 0 -1 Tidal WSE results are presented in terms of an annualized percent excedance for Astoria 1987-2007, applied to MCR using a 0.87 modulation factor. The Tidal WSE , ft NGVD -2 hourly high tide level exceded 10% of the time during a given year = 3.4 ft NGVD (6.9 ft MLLW). The 1% annual high tide is 4.6 ft NGVD (8.1 ft -3 -4 -5 WSE due to Tides at MCR, ft NGVD -6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Annual % Excedance for TIDE Components of Coastal Margin Water Surface Elevation at MCR 8 8 Storm water level = WSE due to tides + storm surge + infragravity transients (waves) 7 7 6 6 5 5 4 4 3 3 Storm Surge, ft 2 2 1 1 0 0 -1 Tidal WSE results are presented in terms of an annualized percent excedance -1 for Astoria 1987-2007, applied to MCR using a 0.87 modulation factor. The Tidal WSE , ft NGVD -2 hourly high tide level exceded 10% of the time during a given year = 3.4 ft -2 NGVD (6.9 ft MLLW). The 1% annual high tide is 4.6 ft NGVD (8.1 ft -3 Storm surge results are based on a partial-duration frequency -3 analysis for a 20-yr period of record using data that was recorded at -4 Toke Pt, WA (1-hr interval). Results were extrapolated to a 100-yr -4 WSE due to Tides at MCR, ft NGVD frequency of occurrence and are presented here in terms of a -5 -5 Storm Surge, ft above observed tide cummulative distribution. The 10-year (0.9) storm surge = 4.7 ft -6 -6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Annual % Excedance for TIDE & Cummulative Distribution for SURGE and IG Transient Components of Coastal Margin Water Surface Elevation at MCR 8 8 Storm water level = WSE due to tides + storm surge + infragravity transients (waves) 7 7 6 6 5 5 4 4 3 3 Storm Surge, ft 2 2 Infragavity transient Infragravity Transient, ft 1 results are based on an estimated 1 cumulative distrubution.