Jökull - 01.12.1999, Blaðsíða 55

Figure 5. Photograph showing erosional terraces on the westem Gígjukvísl channel margin. Surveying staff is 1
m high. Palaeoflow direction is towards the viewer. — Ljósmynd afþrepum í vestanverðum Gígjukvíslatfarvegi.
Horft er á móti straumstefiiu á myndunartíma laganna. Mœlistikan er 1 m að lengd.
Table 1. Input data for Gígjukvísl cross profile 1.
— Gildi sem notuð eru íþversniði 1.
West East
Sector A Sector B Sector C Total or Mean
Width (m) 130 300 65 495
Mean depth d (m) 6.5 10.5 9 9.25
Hydraulic radius 5.91 9.81 7.05 8.92
Roughness Ks (m) 1.4625 2.3625 2.025
Proportion of flow depth occupied by resistance elements AA 0.05 0.05 0.05
Slope (mm_1) minimum 0.0023 0.0023 0.0023
Slope (mm-i) maximum 0.0049 0.0049 0.0049
Cross-sectional area (m2) 845 3150 585 4579
Wetted-peri- meter (m) 143 321 83 513.5
the cross-sectional area of each channel sector occu-
pied by roughness elements, cross-sectional areas and
water surface slopes. Table 4 contains friction fac-
tors, mean flow velocities, associated Froude num-
bers and discharges calculated for the three cross-
sections. Flow velocities and associated Froude num-
bers ranged from 3.6 to 6.7 ms-1 and 0.43-0.66 re-
spectively for cross-section 1 (Table 4). Calculations
for cross-sections 2 and 3 give velocities of 0.7 to
5.0 ms"1 with associated Froude numbers of 0.13 to
0.65. Discharge figures ranged from 10,513 m3s-1 to
28,791 m3s_1, with an average of 19,500 m3s_1.
Boundary shear stresses ranged from 200 Nm"2
at the moraine constriction to average values of 166
Nm"2 and 223 Nm"2 for cross-sections 2 and 3 re-
spectively. Comparing these values with published
competence relationships indicates that the jökulhlaup
was capable of transporting sediment of up to 2 cm
diameter in suspension (Komar, 1988; O'Connor,
1993). Stream powers at the Gígjukvísl moraine con-
striction ranged from 8,000 to 16,000 Wm"2 (Figures
JÖKULL, No. 47 53