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<br />fans. "Young" fans were formed by relatively recent <br />tectonic motions and are fed by steep. sparsely vegetated <br />watersheds with high sediment production and lntense <br />flooding events. Young fans exhibit steep slopes. uniform <br />contours, and no incised channels. As a fan ages the depth <br />of deposition at the apex increases and downcutting of <br />feeder channels in the watershed above the apex continues. <br />At some point in time. the upstream channels become lower in <br />elevation than the apex and incision of a channel or fanhead <br />trench occurs. Such a channel is likely to be permanent and <br />will be cut deeper with time as watershed channels continue <br />to erode. <br /> <br />A fanhead trench can also be created when sediment <br />production in the watershed is reduced due to increased <br />vegetation. reduction in rainfall intensity. or progressive <br />expo~ure of less erodible materials. Since the sediment <br />transport capacity. or comoetence of the flow exceeds its <br />sediment load at the apex. it will scour the fan surface and <br />create an incised channel. As long as stream competence <br />exceeds sediment supply, the channel deepening and widening <br />will continue. A return to higher sediment productivity in <br />the watershed, due to forest fires or increased rainfall. <br />will often cause the channel to be backfilled and erratic <br />flood channels and more uniform sediment deposition will <br />again dominate. <br /> <br />figure 3.2 provides a comparison between young, unentrenched <br />fans (a) and entrenched fans (bl. While the unentrenched <br />fan forms a single smooth cone, the entrenchment conveys <br />water and sediment down fan to a new apex area where <br />deposition onto the new fan surface occurs. Hence the area <br />of flooding and active sediment transport is shifted by the <br />entrenchment away from the original fan apex. Such shifts <br />in areas of active flooding have important impacts on <br />development plans and flood risk analysis on fans. <br /> <br />28 <br />