Blind Pass Stability Analysis

Date of Award

Fall 2011

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Civil & Environmental Engineering

Program/Concentration

Civil Engineering

Committee Director

David R. Basco

Committee Member

Jaewan Yoon

Committee Member

Tal Ezer

Call Number for Print

Special Collections LD4331.E542 N43 2011

Abstract

The stability of Blind Pass has been subjective since the opening of Redfish Pass in the 1920's. The inlet's stability, defined as its ability to maintain a consistent or equilibrium cross-section, is reviewed in terms of its historic performance and that provided through a 2009 maintenance project. Pre-project expectations are compared with physical results to illustrate the initial success of construction. The Escoffier relationship of tidal prism and cross-section area is implemented to gauge the 2009 projects long term stability. Additional monitoring within one year of construction is analyzed to further realize anticipated stability trends. The hydraulic benefit of a dual jetty system is also predicted through use of Coastal Modeling Software (CMS).

With no previous management, Blind Pass has alternated between a stable and unstable condition since the opening of Redfish Pass in the 1920' s. The pass has closed on multiple occasions as a result of a loss in tidal prism to Redfish Pass. Local interest supports an active initiative to maintain the pass and have pursued authorizations to dredge the channel in 2001 and 2009. Results of the 2009 event show the tidal prism is sufficient to maintain the pass above the critical cross-section area. However, shoaling occurring within 12 months of construction creates concern regarding sediment loading that may be overburdening the inlet.

Analysis of dual jetty alternatives indicate minor benefits are achievable. These include forcing the ebb jet seaward and increasing scouring potential. The structural alignment necessary to achieve these minor benefits may be unrealistic and the benefit level may not justify the risk.

Constricting the channel to the required cross-section to recognize the benefit of a dual jetty system may limit the channels ability to adjust to sediment overburden and also limit the adjacent shorelines natural function. Numerical modeling results show the channel width must be constricted to approximately 160 ft, assuming a 10 ft depth, to maintain scouring velocities. Forcing this channel width would encumber the inlet's innate capacity to shift along the downdrift shoreline. It could not compensate with lateral shoreline movement during periods of increased sediment loading. Historically, the lateral migration is believed to have been a very effective means of maintaining an open Blind Pass.

Structure placement may also have severe impacts on sediment bypassing rates. Additional analysis should verify structural implementation will not further impede southerly transport and create additional erosion stresses on Sanibel Island. Decreased transport to Sanibel would reduce the shorelines natural means of balancing the sediment supply. The shorelines recreational, storm protection, and habitat value would be impacted with the loss of material.

Rights

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DOI

10.25777/nk86-vn82

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