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Tidal Currents River Currents Gravity Pulls Water from Higher Elevation to Lower Elevations

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Tidal Currents River Currents Gravity Pulls Water from Higher Elevation to Lower Elevations Ocean Skills Workshops Welcome! We’ll get started at 6:30pm I. Navigation - April 14-15 II. Tides, Currents & Waves - April 21-22 III. Trip-Planning - April 28-29 Tides Why do tides matter to kayakers? Bay of Fundy, Nova Scotia Neap Tide vs. Spring Tide Neap = Smallest tide cycles (lowest highs and highest lows) Spring = Biggest tide cycles (highest highs and lowest lows) Tides are highest on the new and the full moon phases when the gravitational forces of the sun and the moon work together. Tides are lowest on the half moon phases when the gravitational pull of the moon and the sun work against each other. Gravitational Force Inertia Centrifical Force Proximity Tidal Cycles Diurnal Semi-Diurnal (Maine) Mixed Semi-Diurnal: Two highs and two lows A “tidal day” is 24.8 hours. There are 12.4 hours between one high tide and the next. Tidal Prediction - Resources and how to Use them Tide Stations in Southern Maine Tide charts and other data from each of these stations can be found on NOAA’s website. The link is on our course webpage. Maine Harbors Tide Chart The Rule of Twelfths A rule of thumb for predicting the height of the tide. The Rule of Twelfths Graphic: eOceanic Tidal Range Casco Bay’s mean tidal range = 9 feet Source: Researchgate.com Intertidal Zone Four Zones: ● Splash Zone. ● Upper Littoral Zone ● Mid-Littoral Zone ● Lower Littoral Zone Exercise #1: Assess Tides for a trip around Mackworth Island You are launching from the East End Beach at 9AM on May 31, 2021. ● Can you paddle around Mackworth Island? ● At noon how much can you expect the water level to have risen since the last low tide? Use the rule of twelfths. May Tide Chart, Portland Harbor Currents Offshore banks affect the flow of currents entering the Gulf of Maine Types of Currents Ocean Currents River Currents Rip Currents Tidal Currents River Currents Gravity pulls water from higher elevation to lower elevations. Near river mouths these currents interact with various ocean currents Rip Currents Strong, localized current of water moving directly away from the shore, cutting through breaking waves like a river running out to sea Source: Live Science Rip Currents Source: Live Science Tidal Currents Blue Hill Falls Photo:Andy Gagne When are tidal currents Fastest/slowest? Between high and low tide the water accelerates to reach maximum speed (max flood or max ebb) then slows until it changes direction (slack water). Geographic Features Casco Bay has that Affect Currents many features that ● Shallows constrict tidal flow, ● Surf causing ● Headlands faster ● Cliffs currents ● River mouths ● Ledges ● Bathymetry Lower Hell Gate Large volumes of water flowing through a narrow channel creates tidal rapid. SouthWest of Fort Gorges in Portland Harbor Currents from Fore River and eastern harbor combine to create turbulent waters Currents: Friend or Foe? Old Sow Whirlpool in Eastport Current Symbols on Charts These symbols are rarely found on charts that sea kayakers use in Maine, but are more common and useful in other regions. Flood tide stream with mean spring rate Ebb tide stream with mean spring rate Circumnavigating Manhattan Island, NYC Via the East River, Harlem River and Hudson River Exercise #2: Assessing Tides & Currents on a plotted course You are paddling the course you plotted from Mere Point to Whaleboat Island on May 26, launching at 10:30AM. 1. How will the water level change throughout your trip due to tides? 2. How will the currents be flowing throughout your trip? (Estimate direction & speed) 3. Where do you expect the strongest currents? Answers 1. The water will be rising for the first hour, then falling until 5:32pm. 2. The large tidal range, caused by a full moon, will create especially strong currents. You’ll be paddling against a mild flood current during the first hour, then very little current during slack tide, then an ebb current will peak at around 2:30pm. 3. The current is likely to be strong in the narrow channel between Whaleboat I. and Harpswell. Waves What are waves? Energy moving through water Words to Describe Waves lumpy, steep, choppy, frothy, standing, haystack, rooster tail, breakers, rollers, rebounding, refracting, glassy, towering, messy, spilling, collapsing.... Waves: Friend or Foe? Types of Waves: Ocean Swell Caused by an often distant gale or seismic disturbance. Only encountered in areas exposed to vast open waters. Types of Waves: WInd Waves (Chop) Caused by wind moving over water. Can be encountered on any body of water. You’re unlikely to encounter swell when paddling with land between you and the open ocean Today’s Marine Forecast for Casco Bay E WINDS 5 TO 10 KT, BECOMING SE EARLY THIS AFTERNOON, THEN BECOMING S 10 TO 20 KT WITH GUSTS UP TO 30 KT LATE. SEAS 1 TO 2 FT. Would a kayaker on Casco Bay encounter significant waves today? Types of Waves: Breaking Waves When the wave height becomes 1.3 times the water depth, the crest falls, forming a breaker Three main types of Breakers Spilling: Gently sloped beach. Breaks far from shore Plunging: Moderate sloped beach. Surging: Steeply sloped beach. Breaks close to shore. Where are you likely to find Breaking waves? ● Exposed rocks ● Shallow areas ● Beaches ● Sandbars Types of Waves: Refracting Waves Clapotis When waves moving in different directions encounter each other. Wave length – distance between crests or troughs. Wave height – distance from base to the crest. Wave period – Time for peaks to pass a point. Fetch The distance the wind has traveled over open water More Fetch = Bigger Waves In a westerly wind there is fetch of about .5 ½ Mile miles where the star is positioned In a northeasterly wind there is essentially no fetch where the star is positioned In southeasterly winds the fetch could be up to 4 miles where the star is positioned! Expect big waves if winds are strong. .
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