RESTORATION PLAN Glade Creek Stream Restoration Alleghany ...

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3.0 Project Site Streams (existing conditions)3.1 Channel ClassificationAs part of the field reconnaissance, the Rosgen classification system (Rosgen, 1994) was used todetermine channel type at each field cross section on the basis of existing morphological features ofthe stream channel. Two stable, representative riffle cross sections were surveyed on Glade Creek.The measured channel dimensions for the representative cross sections generate a C4 classification;except for width/depth ratio at the cross sections were 9.1 and 11.1, just under the classificationminimum for a C channel, which is 12. A width/depth ration of less than 12 is characteristic of an Echannel. For this reason, the existing channel is classified as a C E 4 channel, in deference to the lowwidth/depth ratios at these representative riffles. However, in other sections of the Glade Creekchannel, where downcutting and widening have occurred, decreasing the entrenchment ratio andincreasing the width/depth ratio, the channel classification is F4 or G4. The F4 and G4 classificationsare indicative of channel instability. Only one stable, representative cross section could be located onthe unnamed tributary. Based on the measurements taken in the field, the unnamed tributary also wasclassified as a C4.3.2 DischargeIn adjustable, alluvial, transport-limited rivers in temperate climates, flows of moderate frequency(e.g., the 1.5- to 2-year storm event) and magnitude perform most of the geomorphic work (Wolmanand Miller, 1960). This concept of the “dominant discharge” provides a statistical index for the flowthat corresponds with the peak volume of sediment transported. Dominant discharge is the maximumpossible product of the frequency of a flow occurrence and the amount of sediment transported bythat flow event. Channel morphology is ultimately a result of all flows above a sediment transportthreshold that do some geomorphic work. However, the dominant discharge is commonly used as asingle-value estimate for a flow that may be largely responsible for resulting geomorphic form.It is thought that, in many cases, the morphological feature of a bankfull elevation corresponds fairlywell to the flow stage of the dominant discharge. This has led to the concept of bankfull elevation asa tool in stream restoration design. However, the concept should be applied cautiously in streamrestoration design. It should be noted that as channel boundaries are more resistant or less adjustable(i.e., bedrock, hillslope constraints, or large bed material) or in more arid environments, the majorityof geomorphic work is more likely to be performed by larger and rarer flood events. For the purposesof this restoration plan, here the bankfull discharge is considered to be essentially equivalent to thedominant discharge, and serves a guiding value in many aspects of the restoration design.As mentioned in Section 3.1, the bankfull elevation was identified in the field by Biohabitatspersonnel at surveyed cross sections. The bankfull elevation at each cross section was derived fromall available indications including depositional features, changes in bank angle, vegetation, scourlines, and storm debris lines. Bankfull discharge was estimated by solving the Manning equation fordischarge given the bankfull elevation, local channel geometry, slope, and roughness. Channelroughness, represented by Manning's "n", was approximated using the standard references Chow(1959) and Barnes (1967) based on field observations of bed material, channel geometry, andadjacent riparian vegetation.For the purpose of comparison, a predicted bankfull discharge was also calculated for Glade Creekand the unnamed tributary using available North Carolina regression relationships for rural streams in
the Mountain physiographic province (Harman et al., 1999). The rural regression relationships areexpressed by the following equations:A bkf = 22.1 A w0.67Q bkf = 115.7 A w0.73W bkf = 19.9A w0.36D bkf = 1.1A w0.31where A w is watershed area in square miles (mi 2 ), A bkf is the bankfull cross-sectional area in squarefeet (ft 2 ), Q bkf is the bankfull discharge in cubic feet per second (cfs), W bkf is the bankfull width in feet(ft), and D bkf is the bankfull mean depth in feet (ft).For the purpose of comparison, a predicted bankfull discharge was also calculated for Glade Creekand the unnamed tributary using available North Carolina regression relationships for rural streams inthe Piedmont physiographic province (Harman et al., 1999). The rural regression relationships areexpressed by the following equations:A bkf = 21.43 A w0.68Q bkf = 89.04 A w0.72W bkf = 11.89A w0.42D bkf = 1.50A w0.32The discharge was also estimated using the hydrologic model TR-20. The results of deriving Q bkffrom the different methods are listed below, along with the chosen design discharges.StreamManning’sEquationBankfull DischargeMountain RegionalCurvePiedmontRegional CurvePeak Flow,TR-20DesignDischargeGlade CreekDischarge (cfs) X/S 1: 228 352 267 1-yr: 204 200X/S 2: 153 2-yr: 335Unnamed TributaryDischarge (cfs) 27 98 76 1-yr: 16 202-yr: 23The existing channel cross section geometry data was used in the Manning’s Equation and TR-20estimates, making those estimates site specific. Bankfull indicators were more reliable at crosssection 1 on Glade Creek. The Manning’s Equation estimate at cross section 1 also falls within thebounds of the 1-year and 2-year discharge predictions from the TR-20 model output. The RegionalCurve predictions are above the Manning’s and TR-20 values, but the Manning’s and TR-20 valuesfall reasonably within confidence intervals on the Piedmont curve, and within a reasonable range onthe Mountain Curve. Therefore, the design discharges were determined using a combination of theManning’s Equation and TR-20 estimates.3.3 Channel Morphology (pattern, dimension, profile)The existing channel morphology exhibits several Rosgen Classifications over the project reaches ofGlade Creek and the unnamed tributary (channel geometry is summarized in Table 4). The diagnosticclassification, measured at stable riffle cross sections for both Glade Creek and the unnamed tributaryis C4. C4 is what the entire channel would have been classified as before it experienced degradation.17
Page 1 and 2: RESTORATION PLANGlade Creek Stream
Page 3 and 4: Table of ContentsExecutive Summary.
Page 5 and 6: Executive SummaryThe project site a
Page 7 and 8: 1.0 Project Site Identification and
Page 9 and 10: By far the most prevalent soil seri
Page 11 and 12: N.C. Natural Heritage Program List
Page 13 and 14: Bog turtles inhabit slow, shallow,
Page 15: control measure. There is no guaran
Page 19 and 20: 3.6 VegetationThe riparian area alo
Page 21 and 22: 4.5 VegetationThe forest canopy is
Page 23 and 24: 6.0 Project Site Restoration Plan6.
Page 25 and 26: o n' nDwhere all terms are as de
Page 27 and 28: 6.5 Natural Plant Community Restora
Page 29 and 30: 8.0 ReferencesBarnes, H.H. Jr., 196
Page 31 and 32: Wolman, M.G., and Miller, J.P., 196
Page 33 and 34: Table 4. Morphological TableTable 4
Page 35 and 36: Table 6. Designed Vegetative Commun
Page 37 and 38: TEMPORARY HERBACEOUS SEEDDateSpecie
Page 39 and 40: P a r k w a y¬« 18 £¤ 21SpartaD
Page 41 and 42: CeCCeECeCCdGWaFCaFWaEGlade CreekTlC
Page 43 and 44: WhiteheadWhitehead 7.5 Min Topo Qua
Page 45 and 46: ChDCeFCeFGrDChDCrFCoveChEDoughtonRe
Page 47: 11.0 Designed SheetsSheet 1. CoverS

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