13 
Methods : 
At many locations where stream and tide are both semi-diurnal, there is a definite relation between the 
times and directions of the tidal stream and the times of high and low water. It is difficult to visualize the 
tidal stream conditions using the harmonic tidal constituents calculated for the stream components (east- 
west, north-south). The following procedure is therefore used; the time series of stream components are 
smoothed by means of spline functions to eliminate current oscillations with periods shorter than 1.5 hours. 
From 6 hours before until 6 hours after each high water (HW) at Aberdeen, the stream components are de 
termined by means of approximating splines at hourly intervals. This yields 13 pairs of components for each 
HW. Currents with periods longer than a semi-diurnal cycle are eliminated by subtracting the mean over the 
13 values of each current component from the individual values. 
The NORA data cover about 60 HWs, i.e. there are 60 pairs of components for each of the 13 hours 
from 6 hours before until 6 hours after HW Aberdeen. The relationship between each stream component at 
the measuring station and the coincident tidal range at Aberdeen is estimated by a least squares linear re 
gression. The regression equation is used to calculate mean spring and neap rates of the components from 
the mean spring and neap tidal ranges at Aberdeen. Taken together, they give the speed and direction of the 
tidal stream at hourly intervals from 6 hours before until 6 hours after HW Aberdeen for spring and neap 
conditions. Results of the measurements between 20 and 25 m depth at positions N1 to N4 are given as ex 
amples in Table 4. 
Table 4 
Direction and speed of tidal streams from 6 hours before to 6 hours after high water at Aberdeen 
betweeen 20 and 25 m depth at positions N1 to N2 during spring and neap time 
time 
dir. 
SPRING 
speed 
NEAP 
dir. speed 
dir. 
SPRING 
speed 
NEAP 
dir. speed 
h 
0 
cm/s 
O 
cm/s 
O 
cm/s 
O 
cm/s 
N1 water depth 24 m 
N3 water 
depth 25 m 
-6 
69 
27.2 
64 
16.6 
23 
22.0 
29 
13.8 
-5 
131 
30.3 
102 
16.1 
81 
15.9 
86 
11.4 
-4 
158 
49.2 
138 
17.9 
130 
19.9 
124 
15.5 
-3 
178 
63.2 
167 
22.8 
151 
25.3 
150 
18.7 
-2 
194 
62.6 
179 
26.8 
164 
29.2 
166 
18.2 
-1 
208 
50.7 
197 
24.4 
176 
28.2 
172 
16.9 
0 
229 
32.6 
218 
16.1 
201 
24.1 
198 
15.3 
1 
294 
26.9 
269 
12.2 
260 
20.0 
249 
11.6 
2 
336 
41.7 
308 
17.0 
306 
22.1 
298 
14.4 
3 
352 
57.8 
335 
23.2 
329 
28.2 
327 
16,8 
4 
3 
65.0 
348 
25.5 
330 
20.7 
334 
18.7 
5 
17 
55.5 
6 
23.1 
5 
20.6 
352 
15.0 
6 
47 
35,2 
34 
17.8 
13 
23.9 
21 
15.6 
N2 water depth 22 
m 
N4 water 
' depth 22 m 
-6 
29 
16.9 
45 
8.0 
17 
15.6 
33 
7.1 
-5 
101 
14.6 
118 
9.7 
69 
9.7 
97 
5.0 
-4 
136 
24.5 
148 
12.6 
143 
13.8 
146 
6.3 
-3 
147 
33.0 
148 
15.8 
157 
18.3 
179 
10.5 
-2 
161 
36.1 
157 
15.8 
159 
20.8 
182 
13.1 
-1 
175 
31.5 
165 
14.0 
171 
21.8 
185 
13.3 
0 
200 
23.1 
188 
9.6 
188 
19.0 
191 
10.0 
1 
263 
17.5 
222 
5.2 
217 
16.5 
214 
6.4 
2 
309 
22.0 
311 
10.3 
288 
11.7 
335 
4.9 
3 
330 
29.9 
321 
17.2 
332 
17.1 
354 
11.7 
4 
337 
35.2 
333 
18.0 
348 
22.0 
358 
14.2 
5 
347 
31.1 
343 
15.2 
354 
22.4 
8 
13.3 
6 
12 
20.1 
10 
10.8 
7 
17.7 
30 
9.5