Sep. 28, 2021
Performance of notched groins within a groin field and their impact on the morphological evolution of a beach nourishment. By Taylor Zimmerman

Taylor Zimmerman
Marine Engineer

Expert Insight
Notched Groins
Impact on Beach Nourishment

UAS-SfM approach to evaluate the performance of notched groins within a groin field and their impact on the morphological evolution of a beach nourishment.

Figure: Identification of Traditional and Notched Groins at Deal beach
Identification of Traditional and Notched Groins at Deal beach

A research paper by Taylor Zimmerman, Marine Engineer at McLaren has been published by Elsevier’s Coastal Engineering Journal! Taylor is a civil and coastal engineer with a Federal Aviation Administration (FAA) UAS Remote Pilot certification, specializing in marine and coastal projects.

Her co-authored paper highlights:

  • Groin notches function optimally when the notch is located in the swash zone.
  • Improperly constructed notches can fill-in with sediment and behave like traditional groins.
  • Notch specifications can vary to account for position and uncertainty in the equilibrated beach.
  • Notch designs should reflect the direction and magnitude of net longshore sediment transport.

Abstract

Beaches are dynamic systems characterized by frequent morphological changes due to storm events, gradients in longshore sediment transport, seasonal variations in wave climate and more. The complexity of variations in beach morphology can increase significantly after a beach nourishment, particularly when coastal structures are involved. This study investigates the impacts of a groin field, consisting of both notched and traditional groins, on the morphological evolution of a beach in Deal, New Jersey, USA. More specifically, an evaluation of the performance and specifications of the notched groins is undertaken. In 2016, the beach in Deal was nourished and three of the eight groins comprising the groin field were concurrently notched in order to promote increased movement of longshore sediment transport while maintaining a stable shoreline. The study site was monitored nine times over a period of two years (2018–2019) using Unmanned Aerial Systems Structure from Motion (UAS-SfM) photogrammetry. The near-term morphological response of the study site was evaluated by analyzing high-resolution digital elevation models (DEMs) and longshore sediment transport estimates based on measured wave data.

Figure: Notched Groin and Traditional Groin at Deal beach
Notched Groin and Traditional Groin at Deal beach

The elevation change maps established that in general, the groin notching was partially successful in reducing shoreline asymmetry in consequence of sediment transport bypassing through the notches. Differences in behavior and patterns of elevation change adjacent to each individual notched groin was observed even though they were all notched according to the same specifications. While all three notched groins were effective in stabilizing the adjacent shoreline, they varied in their effectiveness at promoting longshore sediment transport through the notch. This is likely attributable to the relative location of the notched groins within the groin field and the characteristics of the post-nourishment morphological evolution; the direction of net annual longshore sediment transport at Deal beach is northward, which caused the southern areas of the study site to narrow and the northern areas of the study site to widen. Consequently, the notched groins in the wider sections of beach did not function as intended, as the notches filled in with sediment causing them to mimic the behavior of a traditional groin, including restricted longshore sediment transport and significant erosion/accretion offsets. In the narrower section of beach, the notched groin functioned as designed, such that the notch was located in the swash zone and therefore allowed bi-directional sediment transport and maintained a stable and relatively symmetrical shoreline. Thus, the notched groins performed most effectively when the notch was consistently located toward the seaward end of the swash zone.

Figure: Sections of Elevation Change Map
Sections of Elevation Change Map 5: Duration 5
a) abrupt erosion/accretion pattern shown by TG5.
b) bi-directional sediment transport shown by NG2
(not to scale)

Additional Information

Publication: Coastal Engineering, Volume 170, December 2021, 103997

Authors: Taylor Zimmerman, Jon K. Miller

Outline:

  • 1. Introduction
  • 2. Site Description
  • 3. Methodology
  • 4. Results
    • 4.1. Beach response during non-storm conditions4.2. Beach response during hurricane conditions4.3. Beach response during Nor’easter conditions4.4. Summary of overall beach response to various climate conditions
  • 5. Notched Groin Discussion
    • 5.1. Wave characteristics of analyzed durations
    • 5.2. Notched groin behavior
      • 5.2.1. Notched Groin 2
      • 5.2.2. Notched Groin 3
      • 5.2.3. Notched Groin 6
    • 5.3. Assessment of Notched Groins Summarized
  • 6. Conclusion
  • Funding
  • CRediT authorship contribution statement
  • Declaration of competing interest
  • References