Jurisdictional Boundaries. And. Establishment of Mean Low Water

Transcription

1 Jurisdictional Boundaries And Establishment of

2 Rights of owners to extend to mean low-water mark. A. The limits or bounds of the tracts of land lying on the bays, rivers, creeks and shores within the jurisdiction of the Commonwealth, and the rights and privileges of the owners of such lands, shall extend to the mean low-water mark but no farther, except where a creek or river, or some part thereof, is comprised within the limits i of a lawful lsurvey.

3 Adoption of wetlands zoning ordinance; terms of ordinance. "Vegetated g wetlands" means lands lying between and contiguous to mean low water and an elevation above mean low water equal to the factor one and one-half times the mean tide range at the site of the proposed project Nonvegetated wetlands" means unvegetated lands lying contiguous to mean low water and between mean low water and mean high water, including those unvegetated areas of Back Bay and its tributaries and the North Landing River and its tributaries t i subject to flooding by normal and wind tides but not hurricane or tropical storm tides.

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5 Different degrees of accurate for different projects Real Time Tidal Stations

6 (MLW): The average of all the low water heights observed over a 19-year period. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent of a mean 19-year value. Mean High Water (MHW): The average of all the high water heights observed over a 19-year period. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent of a mean 19-year value. Mean Tide Range: The difference in height between mean high water and mean low water. Wetland Jurisdiction extends to mean high water where no vegetation exists, and to 1.5 times the mean tide range where vegetated wetlands is present Datum: A reference point for vertical (elevation) measurements. Tidal epoch: A cycle of approximately 18.6 years of the principle tide producing forces.

7 The most recent National Tidal Datum Epoch (NTDE) of was implemented in April 2003 to reflect the latest variations in Mean Sea Level (MSL) along the nation's coasts. Past NTDEs have included the years , , , and most recently,

8 40+ Tidal Benchmarks In Virginia Tidal Epoch

9 Semidiurnal Tides HHW LHW MHW TIDAL RANGE LLW MLW HLW

10 Mean low water is not as easy as one might think to locate. Many instances are know in which one or more persons have acted as experts in legal situations calling for the location of mean low water, only to cite from memory as to where low water usually falls. It is not surprising that these experts often fail to agree. There should be no problem in defining or locating mean low water in any tidal waterway so long as the proper p procedures are relied upon. A Method for determining Mean low water: In General, this method will give you results to within 0.1 of a foot when a full month of data is used. 1) A control tidal station serves as a reference and must have 19-year tidal values or the equivalent. 2) A second tidal station is installed near the desired location. 3) Simultaneous observations take place over one month. 4) At both stations the high and low waters are recorded for the month. 5) From this information you can then compute the mean high and low mean low water at the second tidal station. This method hdworks well provided iddh the two stations being bi compared are not too far apart or have completely dissimilar tides.

11 Tide Staff Tide Gauge Tidal Benchmark Stilling Well Intake

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15 11.07 feet above mean low w

16 Stake set at mean low water Based off Bench Mark Wachapreague, (E.S. Seaside)

17 Stake set at mean low water

18 Low Water Stake set at mean low water Low water that day

19 Stake set at mean low water Stake set at mean low water

20 Low Water that day Stake set at mean low water Low Water that day

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23 Falling 1-3mm Rising i 1-3mm Rising 1-3mm Rising 3-5mm Falling 5+mm Rising Rising 1-3mm 5-10mm

24 Red is from NOAA epoch difference pages Blue is computed from other NOAA resources 0.37

25 Additional i information i

26 Here are the basic definitions for tidal datums commonly used in the U.S. and its territories: Mean Sea Level (MSL) Arithmetic mean of hourly water levels observed during current NTDE. Mean Higher ih High ih Water (MHHW) Mean of higher h high h water heights during current NTDE. Mean High Water (MHW) Mean of all high water heights observed during current NTDE. (MLW) Mean of all low water heights observed during current NTDE. Mean Lower Low Water (MLLW) Mean of lower low water heights during current NTDE. Mean Tide Level (MTL) A datum located midway between MHW and MLW; i.e., MTL = ½ (MHW+MLW).

27 That last bit of algebra is intended to underscore the fact that all tidal datums have elevations above some arbitrary but well-protected reference usually the station datum or staff zero. The intersection i of any of these tidal datum elevations with the land produces a line or contour bearing the name of the datum; e.g., Mean High Water Line (MHWL), Line (MLWL), etc.

28 Tidal Datum Transfers How to get them from here to there is important for two reasons: 1) Tidal datum elevations vary from place to place transfer by leveling li is not an option, 2) establishing the datum by direct means (19-year series every 20 years) is an exacting and expensive operation conducted at a relatively small number of primary tide stations. Fortunately, there s an easier way called simultaneous comparisons. At a primary tide station, get NOS tidal datum elevations for the current NTDE. Obtain a month of tidal observations at station B matched by simultaneous observations at station A. Calculate monthly mean tide level at both stations (MMTL A, MMTL B ). Calculate l the monthly mean range (MMR A = MMHW A -MMLW A, MMR B = MMHW B - MMLW B ) and the range ratio between the stations (RR = MMR B /MMR A ).

29 Now, if stations A and B are connected by a tidal waterway and we assume that both experience similar il monthly deviations i from mean tide level, then Applying the range ratio, and Mean higher high water and mean lower low water transfer from A to B in the same way using a mean tide level elevation midway between them.

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