ERDC/CHL CHETN-VII-5

December 2003

the application of the ISSDOT method. To illustrate this, consider the three data collection trips to

Pool 8 in which there was no drawdown. These are trips 1, 3, and 4. The data obtained during these

trips were taken with a downstream pool elevation at L&D 8 of about el 630.1 to 630.5. However,

the flows through L&D 8 were significantly different for the three trips. Trip 1 had a flow of

about 3,555.6 cu m/sec (96,000 cfs). Trip 3 was 1,074 cu m/sec (29,000 cfs), and trip 4 was

2,185.2 cu m/sec (59,000 cfs). If the data collection and ISSDOT method are consistent as

previously proposed, then this fact should be reflected in increasing transport gradients for

increasing flow rates; all other factors being equal.

The data in Figure 5 seem to justify this consideration. ∆*q *for trip 4 falls between that of trip 3 and

trip 1. Clearly for the three trips, ∆*q *increases with increasing flow. Since curves were fitted through

each data set, it is easy to see the differences. The method appears to be consistent in that, as flow

rate increases, ∆*q *increases. It also seems reasonable to allow that for similar flow rates the sand

transport should be the same at a given site if all other factors are held constant. In the case of trip 2

and trip 4, the flow rates were indeed the same. One factor was not held constant, that was the

drawdown. As stated earlier, it was a reduction of the pool water level at L&D 8 of about 0.5 m

(1.5 ft). In the vicinity of the study area near Brownsville, this caused a local drawdown of about

0.3 m (0.9 ft). The data lines for trip 2 and trip 4 seem to indicate clearly that the drawdown did in

fact have a net effect of increasing the ∆*q *in the vicinity of the study area. This could be true not

only because the cross-sectional area at the study site was reduced, but also because the percentage

600

550

trip4: 59,000 cfs

500

2

R = 0.993

Trip2: 59,000 cfs

450

trip3: 29,000 cfs

400

trip1: 96,000 cfs

350

Power (trip4: 59,000 cfs)

300

Power (trip1: 96,000 cfs)

Power (trip3: 29,000 cfs)

250

Power (Trip2: 59,000 cfs)

200

150

100

50

R2 = 0.9988

0

0

2

4

6

8

10

12

∆**t in hours**

Figure 5. ∆*q *versus ∆*t *computed using ISSDOT for stated conditions (Tons/day vs. time span between

measurement of two sections. Average of 30 rows at each ∆*t*. Data from trip 1, 2, 3 and 4)

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