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Tide Cycles and Flooding


            Nova Scotia Department                                Every day there are two tidal cycles consisting
                of Agriculture                                    of a high and a low tide.

                                                                  To be more accurate, this cycle occurs every
       Resource Stewardship - Land Protection                     12.5 hours, or twice every 25 hours.
                                                                  The time and height of the tide changes
                                                                  continuously based on daily, monthly, 4 ½
          TIDE CYCLES and FLOODING                                and 18 year cycles.

               PREDICTION TOOLS                                       PREDICTION TOOLS
          For Nova Scotia the most common method
          of prediction of the tide times and heights is to       The source of such tide tables as for Saint
          base them on Saint John, New Brunswick                  John comes from a large information data
          tide tables. For every location in Nova Scotia          base and complicated computer models.
          corrections need to be made to the Saint
          John predictions. Since the time                        Some computer models are more elaborate
          adjustments change with the height of the               than other, with respect to the number of
          tide, it takes experience to reasonably                 locations and the detail given for each site.
          interpret the table, but general rules of               An example of a prediction program output is
          thumb can be made.                                      given on the following slide...
          e.g. the tidal bore hits Truro about 1 ½ hours later

                                                                    FACTORS AFFECTING
                                                                 ACCURACY OF PREDICTIONS
                                                                  The wind can play a significant role in tide
                                                                  heights and the amount of time it takes the
                                                                  tide to rise and/or lower.

                                                                  The barometric pressure will also affect the
                                                                  height of the tide. A low barometric pressure
                                                                  will result in a higher tide.

                                                                  In unique circumstances the effect of the wind
                                                                  and barometric pressure in combination with a
                                                                  storm event can have devastating
                                                                  consequences. Luckily, such events are rare.
Nova Scotia Department of Agriculture                                               2005 Presentation - revised 2009
Relationship to Dyke Height                             Influential Factors
 Dykes were built high enough to protect         Upland Runoff
 against the maximum predicted tides.             too much
 However, due to the factors associated with      too little (SILITATION)
 the accuracy of prediction and others related
 to natural phenomenon, flooding is a            Winter Ice Conditions
                                                 Larger than predicted tides occur

          Top of Dyke

      UPLAND RUNOFF                                     UPLAND RUNOFF
The role of the dykes and aboiteaux are to       On the other end of the spectrum, if there is
allow fresh water to escape and to prevent       too little runoff the downstream end of the
salt water from flowing upland. Un order to      aboiteaux structures tend to silt up. This is
accomplish this goal the aboiteaux must be       typical during summers in the Maritimes.
situated at an elevation that only permits the   Enough silt may be deposited to completely
gate to be open during low tide periods. This    block the gate and prevent it from opening.
means that the fresh water is left to            In cases like this they must be pried opened
accumulate during high                           or dug out. This
tides. During large runoff                       can cause short-
                                                 term flooding. It only
events this may cause
                                                 takes a small opening
flooding of adjacent
                                                 that allows the fresh
                                                 water out, because
                                                 once it gets started
                                                 it will clear itself out

     Winter Ice Conditions                       Larger Than Predicted Tides
Ice can come from, or be formed on either
the upstream or downstream ends. Both            As mentioned previously, factors such as
cases, either alone or in combination, can       wind and barometric pressure, can cause
restrict or block flow. If a large freshet       tides to be higher than the predicted. when
occurs, flooding can result.                     storm surges run in the same direction as the
                                                 incoming tide, a flooding situation may also
                                                 result and dykes can even give out in
                                                 extreme cases.
                                                 The frequency of such events is not high, but
                                                 since the severity can be great, it is still
                                                 considered a risk. Due to the risk involved,
                                                 buildings are no longer allowed to be built on
                                                 the marsh.

  - THE END -

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