Weber River Flood Damage Assessment - Riverdale City

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Riverdale City 2011 Flood Assessment October 13, 2011 Figure 1-Example of large alluvial deposits upstream of Riverdale City. Figure 2-Active channel upstream of Riverdale City As the valley was urbanized over the last 100 years the river’s ability to meander through Riverdale City has been significantly reduced by urbanization. Aerial photographs from 1953, 1954, 1971, 1973, 1979, 1980, 1997, 2003, 2006, and 2009 were reviewed to understand the geomorphic evolution of the reach through Riverdale City. In the photos from the 1953 and 1954 it is apparent that Riverdale City 3
Riverdale City 2011 Flood Assessment October 13, 2011 was largely an agricultural community with farmlands dominating the landscape (Figure 1). While the river was somewhat constrained by the railroad and Highway it was free to migrate across much of the valley. Historic flow paths are evident throughout the 1953 photo. Historic channel alignments appear as dark oxbow shaped areas, which are older alignments that have become vegetated, or light oxbow shaped areas, which are more recent (see Figure 3). These historic channels indicate that the Weber River may have had a meander belt width in excess of 1500ft. Analysis of Figure 3 indicates that the Weber River had a meander belt width over 1000 ft wide in 1953. In this active system as the main channel aggrades and a new main channel is formed the old channel is still available to convey water at high flows. These overflow channels relieve pressure within the main channel. Shear stress in the main channel is thus reduced resulting in less bank erosion than if all flood flow remained in the main channel. Loss of these overflow side channels results in increased pressure in the main channel causing excessive bank erosion and down cutting (degradation) of the channel bed. Although riparian areas represent less than one percent of the land within Utah, they are among the most diverse and densely used wildlife habitat in this region. Animals including migratory songbirds, deer, elk and amphibians rely on these riparian corridors for food, cover, breeding habitat and movement corridors. Riparian areas also provide many important functions to humans, from recreation to providing clean and abundant water, to reducing flooding. Disturbance of a riparian area also has a cause and effect that changes the characteristic of these environments. The aggradation of historic flow paths and the resultant migration of the river to new flow paths create a mosaic of diverse habitat. Some remnant channels remain open, filling with water and becoming oxbow lakes or wetlands. Newly exposed point bars provide new habitat for riparian plants. Diversity is one of the most important measures of habitat quality. Riparian forest species such as Fremont Cottonwoods (Populus fremontii) have evolved with channel migration as a critical component of their life cycle. High flows during spring runoff mobilize sediments at the edge of active channels and create point bars. Additionally overbank flows may scour areas of vegetative cover and mulch. These exposed areas are essential for cottonwood establishment. The cottonwood seeds are deposited in these areas which remain wet as the river recedes. The seeds germinate at this time and grow a tap root that follows the water table as the hydrograph declines, creating new riparian forests. These forests in turn capture sediment and retain floods. Many other plant and animal species are similarly adapted to this dynamic environment. 4
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Weber River Flood Damage Assessment - Riverdale City

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