Figure 2. Aerial view to the southwest of the tidal-flat study area at 



Anchorage, Alaska. Arrow identifies channel shown in Figure 3. 



area at Anchorage; the arrow identifies the tidal-flat drainage region shown 

 in Figure 3. Figure 3 also shows cross sections at 2-foot contour intervals 

 extending from one side to the other of the drainage region. Contours in Figure 

 3 are given in feet rather than meters because the original topographic computa- 

 tions from aerial photos and associated ground surveys were in feet. 



For computational purposes, channel areas were calculated to be the area en- 

 closed by the channel walls up to the obvious break in slope and extending up- 

 ward to the contour elevation at the width of the drainage basin. On the cross 

 section of the 22-foot contour (Fig. 3), the break occurs at 20.5-foot eleva- 

 tion, and the width is 35 feet. The cross-sectional area of the channel, defined 

 by the diagonal line pattern, is 156 square feet (14.6 square meters). This is 

 the area through which the discharge is assumed to pass. 



Water discharge through the channel becomes important when the water surface 

 elevation declines to the elevation of the cross section (Fig. 3). The ebbtide 

 discharge through the channel is then calculated to be that volume of water 

 which is upslope from the cross section. It includes the volume in the channel 

 and above the channel break, and the volume on the tidal-flat surface that may 

 be expected to move to the channel at or landward of the channel section. The 

 volume is computed by drawing orthogonals to the tidal-flat contours at the 

 cross section which intercepts the channel at its break. The easiest way to 

 derive the volume is to sum the volumes of thin horizontal slices in the upslope 

 volume. This volume for the channel at the 22-foot cross section is 8,800 

 cubic feet (250 cubic meters). The area-ebbtide discharge relationship is 

 shown in Figure 4. The contour at 4 feet is omitted because of a bichannel 

 condition. 



