Donegal County Council Hydrodynamic Assessment
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DONEGAL COUNTY COUNCIL HYDRODYNAMIC ASSESSMENT PROPOSED PROMENADE DOWNINGS CO DONEGAL JANUARY 2017 DONEGAL COUNTY COUNCIL HYDRODYNAMIC ASSESSMENT PROPOSED PROMENADE DOWNINGS CO DONEGAL JANUARY 2017 151052 DA GGN CD Job No Prepared Checked Approved Status Issued to No of Date by by by copies HYDRODYNAMIC ASSESSMENT PROPOSED PROMENADE, DOWNINGS, CO DONEGAL CONTENTS PAGE NO. 1.0 INTRODUCTION 1 2.0 ANTICIPATED WAVE CONDITIONS 2 3.0 OFFSHORE SWELL CONDITIONS 5 4.0 LOCALLY GENERATED WAVES 8 5.0 INFLUENCE OF WAVE CONDITIONS ON PROPOSED PROMENADE 9 6.0 REVETMENT WALKWAY 10 7.0 AREAS OF FURTHER STUDY 12 Appendix A - General Arrangement of Proposed Promenade Hydrodynamic Assessment Proposed Promenade, Downings, Co Donegal 1.0 INTRODUCTION 1.1 Donegal County Council propose to create a promenade in Downings Co. Donegal to connect the pier area to the caravan park. It is proposed that the promenade will be located around the top edge of the beach at the foot of the rock slopes. 1.2 Prior to the detailed design of the proposed promenade it is important to assess the wave climate in the area where the promenade will be located. A preliminary evaluation of the wave climate in this area is presented within this report based on a desk study of the site and a site visit during heavy swell conditions on the north coast. 1.3 This Hydrodynamic Assessment aims to: Review storm wave and surge conditions. Establish the extreme wave and water levels required for the design of the marine works. Inform the design and specification of the proposed marine works. Hydrodynamic Assessment Proposed Promenade, Downings, Co Donegal 1 2.0 ANTICIPATED WAVE CONDITIONS 2.1 Downings is located on the eastern shores of Sheephaven Bay on the Northern coast of Co Donegal. See Figure 2.1 below. Figure 2.1 Admiralty Chart of Donegal Coast Hydrodynamic Assessment Proposed Promenade, Downings, Co Donegal 2 Figure 2.2 below presents the admiralty chart for Sheephaven bay. Figure 2.2 Admiralty Chart of Sheephaven Bay 2.3 The proposed location for the promenade may be affected by two types of wave conditions; offshore swell and locally generated waves. 2.4 The location for the proposed promenade is sheltered firstly by the existing pier at Downings and secondly by the headland on which the monument and picnic area are located (see Photograph 2.1). Hydrodynamic Assessment Proposed Promenade, Downings, Co Donegal 3 Photograph 2.1 View of Downings Beach and Pier Hydrodynamic Assessment Proposed Promenade, Downings, Co Donegal 4 3.0 OFFSHORE SWELL CONDITIONS 3.1 Ocean waves are generated by the action of wind on water. The waves are formed initially by a complex process of resonance and shearing action, in which waves of differing wave height, length and period are produced. For a given wind speed, the waves produced will depend on the duration and fetch (the distance the wind blows over the sea in generating the waves). The longer the fetch or duration, the bigger the waves produced. 3.2 Due to the orientation of Rosguill peninsula, the only swell waves that would reach Downings are propagated from the North West. With a relatively large fetch length available between Iceland and Ireland, swell waves of several metres height could enter Sheep Haven Bay and propagate towards Downings Pier. 3.3 Due to the location of the proposed promenade being sheltered from north western swell waves by first the pier and secondly the monument and picnic area, swell waves cannot enter directly into the promenade area. However, swell waves will change direction through both the processes of diffraction and refraction to enter the promenade area during heavy swell conditions. 3.4 In order for swell waves to reach the section of water at Downing’s Pier, the depth of water in front of the structure must be suitably deep to ensure the waves do not break before reaching the structure. Therefore, only waves smaller than a certain wave height will reach the structure, with the larger waves breaking and expending most of their energy before they reach the pier. This limits the wave height of swell waves that may eventually reach the proposed promenade area. 3.5 Whenever the restricted wave height swell waves reach Downings’ Pier, the wave energy will mostly be reflected from the smooth near-vertical structure. However, the wave segment that passes the tip of the structure will have a portion of its energy transfer along the wave crest into the lee of the structure. As a consequence, this wave segment will have its height reduced. The diffracted wave crest pattern in the lee of the structure will form approximately concentric circular arcs with the wave height decreasing exponentially along the crest. Hydrodynamic Assessment Proposed Promenade, Downings, Co Donegal 5 3.6 This diffraction process will allow the concentric circular waves of decreased wave height to approach the section of water immediately adjacent to the monument and picnic area. As these waves can still not propagate directly into the majority of the proposed promenade area due to the location of the monument and picnic area, only waves affected by the process of refraction will enter this area. 3.7 Refraction allows alteration of the direction of waves due to the fact that the part of the wave crest in shallower water will travel more slowly than those parts in deeper water. This process means that as waves travel into shallow water there will be a gradual change in their direction, with the wave crests tending to align themselves to be more nearly parallel to the seabed contours. 3.8 Therefore as waves approach the water adjacent to the monument and picnic area (see Figure 1.2), they will be refracted around this area of land and follow the seabed contours to enter the proposed promenade area. Refraction causes the energy density of the refracted wave (and hence the wave height) to be reduced as the crest length of the refracted section of the wave is stretched. 3.9 Due to the above processes of wave breaking, wave diffraction around the pier and wave refraction into the proposed promenade area, it is anticipated that most of the swell wave’s energy will be lost. Photographs 2.1 and 2.2 present the areas of water between the pier and beach and proposed promenade area and beach respectively. These photographs were recorded on 24 February 2007 at high tide whenever a swell of approximate height 1.5m was present at the mouth of Sheephaven Bay. Photographs 2.1 and 2.2 Hydrodynamic Assessment Proposed Promenade, Downings, Co Donegal 6 3.10 The above photographs show that even during relatively high seas along the north coast, the sheltered position of the proposed promenade area means only small waves will enter this area. 3.11 For the purposes of presenting the preliminary design for the new promenade, it is assumed that this wave height will not exceed 0.6m within the area of the proposed promenade, regardless of wave conditions along the north coast. Hydrodynamic Assessment Proposed Promenade, Downings, Co Donegal 7 4.0 LOCALLY GENERATED WAVES 4.1 Due to the sheltered location of the proposed promenade area, the only waves that can approach the area directly without loosing energy through diffraction and refraction are those between SSW and S (see Figure 1.2). 4.2 Due to the limited fetch length over which the waves can be developed, it is anticipated that the waves that enter the proposed promenade area will have short wavelengths and small wave heights. These type of waves would be much more ‘choppy’ and erratic than swell waves from the Atlantic. 4.3 As with the swell waves, it is difficult to accurately estimate wave heights from desk study information without collecting all the appropriate information and undertaking either a mathematical or physical model tests. Therefore our assessment is based on experience of similar sheltered areas and by recording the views of local fishermen and residents. 4.4 Based on an assessment of available fetch lengths and discussions with both harbour users and local residents, it is assumed for the purposes of presenting design options for the new promenade, that the locally generated waves will not exceed 0.8m in wave height. These waves will produce a much more choppy sea state at the proposed promenade than would be produced by swell waves from the Atlantic. Hydrodynamic Assessment Proposed Promenade, Downings, Co Donegal 8 5.0 INFLUENCE OF WAVE CONDITIONS ON PROPOSED PROMENADE 5.1 The expectation of relatively small wave heights in the area of the proposed promenade will dictate both the height and construction of the walkway. 5.2 The promenade will be designed for pedestrian use in most sea states, but will not cater for extreme sea conditions. It is anticipated that in these extreme conditions, the extent of sea spray and wave overtopping would discourage people from using the promenade. 5.3 In order to determine the height of the proposed walkway, several factors need to be considered. With MHWS at +1.33m, if we assume a 300mm sea level rise over the next 40 years, a 400mm storm surge during poor weather conditions, and a wave height of 0.8m (only half this wave height will be above the tide level), the top water level at the peak of the waves will be +2.43m OD Malin. 5.4 The height of the walkway should be above this level but not to an excessive level, as it is anticipated that in the worst sea conditions it is unlikely that people will use the facility.