58
ERDC/CRREL TR-02-14
The velocity is practically constant, compared to the variation of the velocity
of propagation in the mixed soil medium surrounding the buried portion of the
probe. The TDR data were analyzed in the manner described below.
Depth of scour, ds, in meters is calculated from the TDR measurement as
ds = R ∆pt vf
(6)
where
R = spatial distance (in meters) represented by each discrete point on
the TDR display. It is calculated by dividing the full-screen spatial
distance by 251, the number of discrete points used to digitize the
full-screen image.
∆pt = number of points between the reflection from the top of the probe
and the reflection from the watersoil interface at the bed.
vf = velocity of propagation or ratio of the electrical length of the probe
in air (le(a)) to the electrical length of the probe in water (le(w)), or
vf = le(a) / le(w).
Images from TDR 1, on the north bank of the north channel, using the 1503
for the period 11 January and 7 April are shown in Figure 39. The top of the
probe is indicated by the signal fluctuation at the top. The watersediment inter-
face appears as a change in slope, with the end of the probe appearing where the
signal returns to its nominal value. The upper shaded area on Figure 39 repre-
sents the distance between the top of the probe and the watersediment interface,
or the riverbed. The depths of erosion and redeposition were computed using the
propagation velocity in water, which is slower compared to that of the sediment.
Consequently, as more of the probe becomes exposed to the water, the round-trip
travel time of the signal also increases. This increase makes the probe "electri-
cally longer," causing the inflection representing the bottom of the probe to shift
down. Because of the differences of the velocity of propagation, the implied scale
in the exposed region of the probe cannot be used for determining the depth of
sediment around the probe. The exposed/buried length of the probe has to be
calculated using eq 6. The accuracy of the TDR in various media is described by
Yankielun and Zabilansky (1999).
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