VIEWS: 12 PAGES: 3 POSTED ON: 3/25/2010
INTRODUCTION 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. Presents: 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 (cont.) 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 possibility. Larger than predicted tides occur Top of Dyke UPLAND RUNOFF UPLAND RUNOFF (con.t) 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 Occur 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. THANK YOU - THE END -
"Tide Cycles and Flooding"