Coastal Resource Guide Pleasant Bay and ... - Town to Chatham

Coastal Resource Guide for Pleasant Bay and Chatham Harborwere located on a UMass tower twenty meters off the ground,near Arey’s Pond in Orleans. The data have been filtered by RERL’squality control software, filtered to hourly average, and enteredinto Lakes Environmental Software WRPLOT View which isdesigned to created wind rose plots for meteorological data.The direction and intensity of estuarine wind energy has beendepicted by a wind rose (Figure 8). The wind rose summarizeswind speed, duration and direction. Additional detail is providedby providing three roses, one annual average, one just for Maythrough October, and one just for November through April.3.3.1 Findings - Inner Wave ClimateThe Orleans station has a broad, but fairly consistent, annualwind direction from the northwest to the southwest. Annually,approximately 70% of the time winds are between four andeleven knots. The summer is characterized by lower wind speedand a dominant wind direction from the southwest. The winter ischaracterized by a northwest dominant wind direction and higherwind speeds. Summer winds only exceed twelve knots 6.6% ofthe time, compared to 21.5% for winter winds. The southwestwinds do not happen as often in the winter as they do in thesummer, but when they occur in the winter they are frequentlystronger.Pleasant Bay is a complex estuarine system. In addition to anirregular shoreline, it contains numerous islands and mobileshoals, all of which may block or redirect surface winds and/or theresultant waves. The inner wind roses are not a perfect proxy forinner wave climate, however they illustrate the seasonality of thewind, which drives wave generation within Pleasant Bay. A latersection (4.3 - Longshore Sediment Transport) further discussesinner wave climate as it relates to sediment transport.3.4 Tidal Cycle in Pleasant Bay and Chatham HarborTides within the Pleasant Bay and Chatham Harbor estuary aresemi-diurnal (two tidal cycles per day) with an average range ofsomewhat less than six feet in Chatham Harbor and about fourand one-half feet at Meetinghouse Pond in Orleans at the veryhead of Little Pleasant Bay (Figure 9). These tides are producedby the open continental shelf (Gulf of Maine) tides which, as theyrise and fall, cause seawater to flow into and out of the estuarythrough the tidal inlets that connect the two systems.Passage of the tidal wave through the inlets and inner channelsdelays and distorts the estuary’s tides. While high tide at ChathamHarbor occurs less than an hour following Boston high tide (Bostonis the location of the region’s tidal reference station), Chatham’slow tide occurs more than an hour after Boston’s low. AtMeetinghouse Pond the delay and distortion are greater. Highs andlows occur about two and one-half and three and one-half hours,respectively, after those at Boston.reducing ranges and increasing phase lags (time delay betweenthe Boston and local tides) over most of the 21 st Century.This discussion has concerned “astronomical” or “predicted” tidesin the estuary, but the actual water level at any time and placemay differ markedly from the predicted tides due to atmosphericconditions. A common example would be the effect of stormsurges along the outer coast, but even greater anomalies canresult from direct wind stress acting along the length of thesystem, either “setting-up” or “setting-down” the water level,especially in the ponds and narrows.Chatham Harbor tides are recorded (at the time of this writing) bythe National Ocean Service (NOAA) at Chatham’s municipal fishpier (Aunt Lydia’s Cove). Both “real-time” and historic sea leveldata are available for this station on-line at http://tidesandcurrents.noaa.gov/. In addition, the Cape Cod National Seashore (CCNS)presently operates a tide recorder at Meetinghouse Pond inOrleans. Tidal data for a single day from both stations arepresented together in Figure 9.Elevation (feet - navd88)6543210-1-2-3-428Tide Records:Chatham Harbor (red) and Meetinghouse Pond (blue)28.5 29 29.5 30October 2011Figure 9 shows changes in tide elevation over October 28–29, 2011 forChatham Harbor (red) and Meetinghouse Pond (Orleans) at the headof Little Pleasant Bay (blue).13As discussed in Section 2, the inlets and channels within theestuary continually shift position and these changes producecorresponding changes in Pleasant Bay and Chatham Harbor tides.The present (2011) configuration is probably near-optimal withrespect to tide range within the estuary, so we anticipate slowlyPleasant Bay Alliance 2011