SEGUIMIENTO, EVALUACIÓN Y CONTROL

Time series volume changes constrained to dredging footprints are discussed here. Both John’s Pass and Blind Pass channels and ebb shoals have been dredged in the past for channel maintenance purposes and to mine sediment for renourishment of nearby beaches (see Appendix A). To gain insights into dredge pit infilling rates, volume changes in dredge pits excavated into the John’s Pass ebb shoal in 1988 and 2010, and Blind Pass in 2010 were examined.

In 1988 channel maintenance dredging and mining of sediment from the northern flank of the John’s Pass ebb shoal was conducted (CTC, 1993; Walther and Douglas, 1993) (see Figures 49 and 50c). The sediment was used to renourish Redington Shores beach on Sand Key (CTC,

1993). While no verifiable dredging volumes are available, ca 407,000 m3 of sand from that

dredging was placed on the Redington Shores beaches (Dean and Lin, 1990). Additionally, Walther and Douglas (1993) reported that: (1) the pre-dredging elevation of the borrow area was -4 m (no datum reported), (2) the post dredging average elevation of the ebb shoal borrow site was

-6.5 m (no datum reported), and (3) during the 4 years following the dredging ca 96,000 m3 _infilled

the dredge pit equating to ca 24,000 m3 year-1. While the precise position and dimensions of the

1988 dredge pit are unknown, the general location is based on CTC (1993) and Walther and Douglas (1993), and the obvious bathymetric depression that currently remains (Figure 48). As of July 2014, the latest bathymetric survey collected during this study, the minimum elevation in the excavation is -5.4 m (NAVD). Between June 2010 and July 2014 which represents post

dredging years 22-26, the dredge pit received ca 1300 m3.Walther and Douglas (1993), using the

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26 year post dredging period ranging from ca 5000 m3/yearduring year 22 to ca 3000 m3 /year

during year 26 with an average rate over the 4 year period of ca 4000 m3 _{/year, ca 80% higher than}

those estimated by this study. Similarly, as described in Chapter 2, the approach used by Walther and Douglas (1993) at Boca Raton Inlet overestimated the infilling rates there by ca 47%.

John’s Pass and Blind Pass ebb shoals and channels were dredged in June 2010 (Figure 75). The excavated sediment was used to renourish Sunshine, Sunset, and Upham beaches on Treasure Island and Long Key. Bathymetric surveys of both inlets ebb shoals and channels were conducted by the USFCRL in June 2010 prior to the dredging and again in October 2010 immediately post dredging. Subsequent bathymetric surveys of the inlet shoals and channels were completed in January 2011, September 2011, July 2012, and July 2014. The 2010 dredging program at John’s Pass included channel maintenance dredging and ebb shoal mining. Two dredge pits were excavated, one along the seaward most portion of the ebb shoal (referred to as the terminal lobe) and a second within the main channel which including portions of the channel margin linear bar (Figure 75 left panel). Based on pre- and post-dredging bathymetric surveys of

the John’s Pass ebb shoal and channel, ca 126,000 m3 _{of material was dredged from the inlet}

channel and channel margin linear bar, and ca 158,000 m3 was removed from the terminal lobe

yielding a total of 284,000 m3_{. During the first year post dredging (10/2010 –9/2011) 39,000 m}3

of sediment was deposited into the main channel dredge pit. During the subsequent 10 months

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Figure 75. Outline of 2010 dredge pits at John’s Pass (left image), and Blind Pass (right image).

(24 months) 30,000 m3 or 15,000 m3 year-1 was deposited in the channel dredge pit. Overall,

during the 4 years post dredging, the channel dredge received ca 22,000 m3/yr of sand equating to

ca 65% of the material removed. At that annualized rate, it would take 5.3 years to recover to pre-

dredging conditions. However, as shown in Figure 76, while the first year infilling rate was ca

39,000 m3 _{it declined to ca 15,000 m}3 _{in subsequent years. Assuming those infilling rates, it would}

take ca 7 years to recover to pre-dredging conditions. Figures 79 and 80 illustrate infilling patterns within the channel pit during the first and second year, respectively.

The rate of infilling at the terminal lobe dredge pit was substantially less than the channel pit, likely influenced by its distal position relative to the ebb jet in conjunction with a limited supply of sediment. The floor of the dredge pit lies at -4.2 m NAVD88. During the first year, the

dredge pit received ca 5,100 m3 of sediment (Figure 77), which equates to 13% of the volume

deposited in the John’s Pass channel dredge pit during the same time period. During the second

year (10 months to be exact), the pit received ca 1000 m3 of sedimentation (annualized rate = 1200

m3) or ca 10% of the volume of sediment deposited in the channel dredge pit during the same time

period. Infilling rates increased to 2500 m3 _{during years 3 and 4. Based on the current rate of}

infilling, it would take ca 63 years to recover to pre-dredging conditions. Figures 79 and 80

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illustrate the patterns of infilling during the first and second year, respectively. In the two years post dredging, most of the sedimentation occurred at the northwest corner and along the eastern and western pit margins.

In 2010, ca 121,000 m3 of sediment was dredged from the Blind Pass ebb shoal and inner

shoal (Figure 75 right panel). At its deepest point, the dredge pit extended to a depth of -4.75 m

NAVD88. Sediment infilling during the first year post-dredging was ca 20,000 m3 (Figure 78).

This is lower than the 35,000 m3 _{(46,000 yd}3_{) infilling obtained by Wang et al., (2007) following}

dredging in 2000, and is likely due to the fact that in 2000 the dredge pit extended much further seaward and deeper than the 2010 dredge pit. During the second year (10 months to be exact),

sedimentation of 21,000 m3 (28,000 yd3) was measured. This equates to an annualized volume of

25,200 m3, ca 25% greater than the amount deposited during the first year (Figure 80). During the

3rd _{and 4}th _{years, the infilling rate declined to 14,000 m}3_{/yr for each year. At the current infilling}

rate, it would take ca 7.53 years to recover to pre-dredging conditions. While that infilling rate appears to be lower than the generally accepted net longshore sand transport rate, it does not account for the additional volume associated with the continued growth of the Blind Pass ebb shoal, which when included yields a volume consistent with reported gross transport rates (CP&E, 1992). Figures 82 and 83 illustrate infilling patterns in the dredge pit during the first and second year, respectively. In general, sedimentation in the dredge pit spreads both landward and seaward over time and illustrates the temporal variability of depositional patterns. Deposition in the north central portion of the pit during the first year post dredging experienced erosion during the second year. The eroded sediment along with additional sediment delivered to the inlet from southward longshore transport moved landward and seaward during year two. A similar pattern of landward- seaward spreading was observed following dredging in 2000 (Wang et al., 2007). The seaward

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spreading is important to the continued development of the Blind Pass ebb shoal and illustrates how sediment bypassing is initiated. It is important to note that the 2000 dredging of Blind Pass effectively removed most of the ebb shoal that existed at the time, in addition to much of the inner shoal. Therefore, the current Blind Pass ebb shoal effectively represents sedimentation that has

occurred subsequent to the 2000 dredging, equating to a growth rate of ca 37,000 m3/year during

the 14 years between 2000 and 2014.

Figure 76. Annualized infilling rates of the John’s Pass inlet channel dredge pit during the 4 years following the 2010 dredging.

y = 31874x-0.688 R² = 0.6775 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 0 1 2 3 4 5 m 3 years post-dredging