University Park ONLY: Formative Assessment 2

Shoreline change due to the presence of a groin

Assignment:

The goal of this activity is to help you understand the impact of hard structures on the evolution of a coastline. Groins are primarily used to stabilize a natural or nourished beach that is eroding due to a net loss of sediment from the coastal cell due to longshore transport. Groins are built perpendicular to the shoreline, extending from the beach berm out to the surf zone, and block longshore sediment transport to varying degrees depending on their length. Shoreline change occurs when there is a spatial gradient in longshore transport. If more sediment is entering the surfzone fronting a small portion of the beach than is leaving, then the shoreline will accrete. If more sediment is leaving than entering, the shoreline will erode.

For a groin that completely blocks longshore transport, updrift areas of the beach will have a net gain of sediment and accrete. The shoreline progresses seaward so that the orientation more closely matches the dominant incident wave angle and continues to accrete until it approaches the seaward end of the groin. At this point, sediment can bypass the groin and reach the downdrift coast.

On the downdrift side of the groin, sediment input from the updrift direction is completely blocked. Oblique waves are still reaching this section at a certain distance from the groin, driving longshore transport in the downdrift direction. The net flux of sediment in this region is negative, so erosion of the downdrift shoreline occurs. In principle, the erosion on the downdrift side should mirror the accretion on the updrift side; however, in reality the groin creates a small shadow zone on the downdrift side. The incident waves diffract around the groin tip into the shadow zone with altered incident angles, and the shoreline adjusts to more closely match the diffracted wave angle. Figure 7.10, below, gives results from a coastline model for a long and short groin, where the updrift accretion and downdrift erosion are clear.

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Figure 7.10: Shoreline development exposed to oblique incident waves with and without groins of varying lengths.
Credit: Caitlin Pilkington http://www.coastalwiki.org/wiki/File:Shoreline_development_oblique.jpg the Flanders Marine Institute (VLIZ)

Figure 7.11, below, shows updrift accretion and downdrift erosion at each groin within a groin field on Long Island, NY.

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Figure 7.11: Updrift accretion and downdrift erosion at a groin field on Long Island, NY.
Credit: USGS: EarthExplorer

Analytical solutions to predict the shoreline evolution based on longshore transport gradients have been developed for simple cases. These so-called "one-line" models describe the long term variation in shoreline position, assuming that the cross-shore beach profile maintains a constant shape and that longshore transport occurs uniformly across the whole beach profile down to the depth of closure (depth where seaward limit of longshore transport occurs). The model treats each small section of beach as a control volume, where the sediment fluxes into and out of the volume are calculated and the net flux determines the shoreline change.

Instructions

Using the provided excel spreadsheet (Download groin_updrift_model.xlsx (Excel 2007 (.xlsx) 26kB Sep7 16)), compare the performance of a 50 m groin and a 15 m groin.

1. Approximately how long does it take from the shoreline directly updrift of the groin to accrete at least 10 m seaward? Evaluate two groin lengths (15 m, and 50 m). Using the corresponding time from the previous question, report the shoreline accretion at a location that is 500 m updift of the groin for each groin length. (Select your time from the available solutions in the spreadsheet, corresponding to 5,10,25,100 and 365 days.)
2. What has a greater effect on the shoreline and produces more accretion – a doubling of the incident wave angle or a doubling of the wave height? Evaluate this question using a groin that is 50 m long.
3. With H_b of 1 m, α_b of 10 °, and Groin Length of 15 m, how does reducing the beach grain size by 0.05 mm affect the shoreline evolution? Does it erode or accrete? (Note: it may help if you focus at one location updrift of the groin to evaluate this)

Files

Download this worksheet (Microsoft Word 2007 (.docx) 70kB Sep7 16)

Download the Excel file (Excel 2007 (.xlsx) 26kB Sep7 16)

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