CETN IV-20
September 1999
Step 2: Pre-Inlet. Within SBAS, the conceptual sediment budget was copied to another
alternative and titled "Pre-Inlet: Apply macrobudget." Values within the budget were estimated
from the given values as in the following.
At the north boundary, Equation 2 of CETN-IV-15 (Revised) gives Q L-N = Q R-N Q net-N . Thus,
Q L-N = 50,000 m3/year. Applying Equation 5b of CETN-IV-16 gives
δQR- N +δQnet - N = 80, 0002 + 60, 0002 = 100, 000 m3/year
δQ L-N best =
2
2
For all transport rate uncertainties, we have assumed that they were approximately 40 percent of
the given value (implying δQ L-N = 0.4(50,000) = 20,000 m3/year), which differs from the
uncertainty calculated above. For consistency within this example problem, we will adopt the
40-percent value; thus Q L-N = 50,000 20,000 m3/year. However, for a site-specific project,
other information might indicate the higher value was more appropriate, or perhaps the engineer
would create another alternative with the higher value to determine its impact on the uncertainty
of the macrobudget.
For the entire pre-inlet barrier, Equation 5 of CETN-IV-15 (Revised) gives ∆V =
(∆y/∆t)(DA)(∆x) = (-0.3 m/year/m)(10 m)(10,000 m) = -30,000 m3/year. According to
Equation 9b of CETN-IV-16, the associated uncertainty is
FG δ∆y IJ + FG δD IJ + FG δL IJ
FG IJ FG IJ FG
IJ
2
2
2
2
2
2
0.1
1
0
A
= 10,400 m3/year
H ∆y K H D K H L K
H K H K H
δ∆Vbest = ∆V
= - 30,000
+
+
K
0.3
10
10,000
A
This volume change and associated uncertainty were portioned for each segment of the barrier
according to its length, ∆Vupdrift = ∆Vdowndrift = -13,500 4,700 m3/year and ∆Vinlet = -3,000
1,000 m3/year. The pre-inlet ebb and flood shoals do not exist; thus ∆Vebb = ∆Vflood = 0.
The beach fill was converted to a placement rate, P = F/∆t = 200,000 m3/10 years =
20,000 m3/year. Applying Equation 6b of CETN-IV-16,
FG 10,000 IJ + FG 0 IJ
FG δF IJ + FG δ∆t IJ
2
2
2
2
= 1,000 m3/year
H F K H ∆t K
H 200,000K H 10K
δPbest = P
= 20,000
This placement rate was portioned to the pre-inlet barrier according to each cell length, Pupdrift
= Pdowndrift = 9,000 450 m3/year and Pinlet = 2,000 100 m3/year. No beach fill was placed
within the ebb- and flood-shoal regions.
All known data were entered into SBAS in units of 1,000's of m3/year, done for convenience but
not being necessary. Within SBAS, the macrobudget Option combines all cells. The residual for
the macrobudget indicates that the only unknown for the macrobudget QL-S = 100,000 m3/year
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