2019-10-28-minutes-public-works-study-session

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nhc APPROACH For any given river under typical conditions, channel migration rates are proportional to bed material transport rates (Wickert et al., 2013; Constantine et a], 2014; Nelson and BOO, 2016). A standard practice in fluvial geomorphology Includes the estimation of bediaad transport rates at the reach -scale from the volume of material exchanged due to channel migration (e.g. Neill, 2983; Ham and Church, 2000). Here we apply the inverse reasoning as an estimate of the potential Impact of a change in hydrology on channel migration rates. More specifically, we assume an estimate of a potential Impact on the lateral channel migration rate Is that it is proportional to the change in sediment transport capacity for the reach. This is a conservative assumption because some bed material is conveyed directly down the channel without exchange between the active river and floodplain that is expressed as bar growth and bank erosion. This means that sediment transport estimates from lateral channel migration will be minimum estimates. in the present Investigation, the Implication is that the channel migration rate Is proportional, to a power less than 1, to the bedload transport rate. Bedload transport rates can be estimated from channel geometry and discharge (hydraulics), as well as from bed material composition. Here we use these parameters to determine the relationship between the bedload transport rate and discharge so that changes in the transport capacity, due to Increased discharge In the reach between the current and proposed lick Creek confluence locations, can be estimated. Bedload transport rates that are estimated In this way are subject to substantial (order -of magnitude) uncertainty without collection of calibration data; but relative changes In transport capacity, due to changes In hydraulics, can be much better constrained. Due to the character of the reach (described in the following section), and the fact that the relationship between the bedload transport rate and the channel migration Is expected to be proportional to a power less than 1, this procedure inherently provides a conservative estimate of the maximum plausible Impact of the added flow. of the required input parameters, the hydrologic change Imposed by relocating lick Creek is the least well known. At the county's request, we have not conducted any model -based hydrologic or hydraulic Investigation to estimate this Impact. The hydrology was estimated by regression relations, which have very broad confidence bounds, while the hydraulics were estimated by a normal depth calculation, which does not account for backwater conditions. The channel character was evaluated by reviewing available L1DAR data, walking a portion of the subject reach, defining the bed material and bed material grainsixe distribution, and mapping: historical channel positions from available aerial photos. General understanding of the channel character gained from this analysis supports the applicability c fthe theoretical framework (outlined above) that served as the basis for a quantitative estimate of the potential Impact on lateral channel migration rates. In addition to evaluating the potential Impact on channel migration rates, we evaluated the expected Impact of the additional flow on the channel width by applying the change in flow to a rational regime model of the channel. This analysis is described in Section 4. Potential impact of Lick Creek relocation an the NF Tean away River Morphodynamics 4