CETN IV-15
Rev. September 1999
∑Q
∑Q
- ∑∆V +
∑P - ∑R =
-
Residual = 0
source
sink
[Qnet _ A1]- [Qsl _ A1 + Qsl _ A2 + Qnet _ A2] - [∆VA1 + ∆VA2 + ∆Vfl + ∆Vch + ∆Vebb] + [ PA1 + PA2 ] - [Rfl + Rch + Rebb] = 0
[230] - [7.3+ 7.3+ Qnet _ A2] - [47+ - 48+15+19+ 77] + [13+ 25] - [0 + 2.4 + 0] = 0
Qnet _ A2 = 141
Applying Equation 2 at the western boundary gives,
Qnet _ A2 = QR _ A2 - QL _ A2
141 = 1.8 QL _ A2 - QL _ A2
QL _ A2 = 176 and QR _ A2 = 317
Now the control volume A1 can be considered. There are two unknowns, the rate of sediment
transport around the east jetty, Qj_A1, and sediment transport from A1 to the ebb-tidal shoal,
Qebb_A1 . Inspection of bathymetric charts and aerial photography shows no evidence of
morphologic pathways (e.g., shoal features) from A1 to the ebb-tidal shoal. Thus, one can
assume that Qebb_A1 ~ 0 and solve for Qj_A1 in Equation 1,
∑Q
- ∑ Qsink - ∑ ∆V + ∑ P - ∑ R = Residual = 0
source
[Qnet _ A1] - [Qsl _ A1 + Qj _ A1 + Qebb _ A1] -
[∆VA1 ] + [ PA1] = 0
[230] - [7.3 + Qj _ A1 + 0] - [47] + [13] = 0
Qj _ A1 = 189
Next a control volume for A2 is formulated, excluding the ebb-tidal shoal. There are also two
unknowns for this control volume, the rate of sediment transport around the west jetty, Qj_A2 ,
and the rate of sediment transport bypassed from the ebb-tidal shoal to A2, Qebb_A2. A more
detailed analysis of the shoreline position and beach-fill placement records for A2 indicates that
∆V - P = -16 for the region east of the bulge in the 1979 shoreline position, and ∆V - P = -58 for
the region west of the bulge. As a first estimate, one can set Qj_A2 = 16, implying that all
sediment lost from the region east of the bulge moved around the west jetty. This assumption
also implies that this morphologic feature represents a long-term nodal zone for net longshore
sand transport. Using Equation 1 to solve for Qebb_A2 gives,
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