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Last modified
1/26/2010 10:10:41 AM
Creation date
10/5/2006 4:40:29 AM
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Floodplain Documents
County
Statewide
Basin
Statewide
Title
Forest- Fire Devegetation and Drainage Basin Adjustments in Mountenous Terrain
Date
1/1/1977
Prepared By
Department of Geology University of New Mexico
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />w. D. White and S. G. Wells <br /> <br />population in the light-burn watershed compared to the other watersheds. <br />Gopher rehabitation and subsequent burrowing in this area after the fire <br />produced unexpectedly high volumes of easily-transported sediment. Thus <br />erosion may not be directly proportional to the extent of de vegetation because <br />other activities, such as animal burrowing. may produce significant amounts of <br />sediment. <br /> <br />Sediment Delivery Rates <br /> <br />The rate of movement of individual grains oft hillslopes can be studied by <br />tracer (colored) sand. This technique was used to gather data on the rate <br />of sediment transport across the interfluves. Tracer sand (0 to I~ grain size) <br />was placed along transect A on a hillcrest in the intense-burn watershed of <br />Burnt Mesa on May 11. 1978. One month later (June 14. 1978) distribution <br />of the tracer sand showed the effects of raindrop impact. Some particles <br />were transported at least 0.11 m downslope from the drainage divide. Two <br />months later (July 14, 1978) the sand was transpor~ed by sheetwash during <br />a summer thunderstorm. Some particles were moved 0.77 m downslope from <br />the hillslope divide. Whether this transportation occurred during a single <br />runoff event or several is not known. By August 1. 1978. the tracer sand <br />was difficult to locate because of dispersion dn the hillslope i however. grains <br />were found at 0.82 m downslope of the hillslope divide. One grain was found <br />only 0.08 m from the initial starting point on the divide. indicating selective <br />or random transport of similar sized grains. An approximation of the rate <br />of sediment transport by overland flow can be derived from this test. The <br />mean rate of transport is approximately 3.0 m/yr. Obviously, sediment move- <br />ment on hillslopes is not constant. For example. faster movement should " <br />occur 011_ steep reaches and slower movement on gentle slopes. However. 3.0 <br />mlyr may be representative of the mean transport rate for the intense-burn <br />watershed. <br /> <br />FLUVIAL ADJUSTMENTS TO FOREST-FIRE DE VEGETATION <br /> <br />Types of Fluvial Adjustments <br /> <br />Adjustments of fluvial systems to changeS- in geomorphic variables have <br />been described in terms of complex responses (Schumm, 1973, 1977; Womack <br />and Schumm, 1977). In the_ complex-response model, variations fn sediment <br />supply from tributary streams to trunk streams cause modifications of the <br />larger fluvial systems which are. in turn.. transmitted back "through the tri- <br />butaries. Adjustment of Rita de los Frijoles to increased sediment load has <br />resulted in frequent overbank events as well as channel modifications. These <br />adjustments are not transmitted back to the tributary' watersheds on the mesas <br />because 20 m-high bedrock nickpoints separate the two systems. Thus.. the <br />mesa top watersheds can be..considered semi-closed systems which progressi- <br />vely adjust to a single chaIi.ge....~.Z~rainage-baSin variable (devegetation). <br /> <br />Post-fire adjustments of fluvial systems on the mesa tops involve altera- <br />tions from stable channel forms (shallow, parabolic cross-section) to unstable. <br />degrading or aggrading channels. In general. "low-order channels of Burnt <br />Mesa are scouring as the high-order channels are cutting and filling (fig. 9); <br />however, net filling is typical in the higher-order streams (fig. 9). Most <br /> <br />214 <br />
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