Jökull - 01.12.1999, Blaðsíða 50

Figure 3a. Pre-jökulhlaup panorama of the
Gígjukvísl river from the western bank
showing channel morphology and fine-
grained nature of outwash sediments. — Ut-
sýni afvesturbakka Gígjukvíslarfyrir hlaup-
ið sem sýnir fremur fínkorna farvegi.
Figure 3b. Post-jökulhlaup panorama of the
Gígjukvísl river from a similar position on
the western bank showing channel bank ero-
sion and the deposition of numerous ice
blocks. Flow direction is from left to right.
— Utsýni eftir jökulhlaupið af svipuðum
stað sýnir bakkarof og fjölda ísjaka. Streymi
varfrá vinstri til hægri.
The slope-area technique was used to reconstruct
peak discharge within the Gígjukvísl river chan-
nel, where mean flow velocities are calculated from
water surface slopes, channel cross-sectional areas
and roughness coefficients (Chow, 1959; Henderson,
1966; Maizels, 1983). Because the study channel was
noted for its high ice block density both during and
after the flood (Figures 2 and 3b), it was felt most ap-
propriate to take into account the resistance effects of
the ice blocks. We used the modified Keulegan equa-
tion proposed by Thompson and Campbell (1979) in
order to calculate the friction factor / to be used in the
Darcy-Weisbach equation. Church et al. (1990) sug-
gested this approach as being the least empirical and
least site-specific equation available for flow recon-
structions. The Darcy-Weisbach equation was used to
obtain mean velocity values (Table 4).
v=JW (l)
where 5 is the slope (mm^1), d is the flow depth (m),
g is the acceleration due to gravity (9.81 ms~2) and/
is the friction factor calculated using Thompson and
CampbelFs 1979 equation;
77= (1-0-1^)* 2 logio(^) (2)
where R is the hydraulic radius and Ks is the size of
the roughness elements equal to 4.5 times the boulder
size Di. In this case we calculated Ks as the propor-
tion of the flow depth, d, occupied by flow resistance
elements (AT) (equation 3) (Tables 1-3)
Ks = 4.5dAA (3)
48 JÖKULL, No. 47