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Abstract: Session B  11:30 am (Back to Session B)
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Hydrologic Characteristics of Bankfull (Floodplain) & Daily Bankfull (Inner Sediment Berm) Discharges in Mid-Atlantic Headwater Streams: Parts of I-A, I-B, and I-C in a series of IV on Inner Berm Characteristics

 Thomas J. Garday
River Analyses Inc. 
Little Rock AR

Part I-A of IV, presents a hydrologic procedure to estimate the sediment regime ratio: capacity of geomorphic inner berm sediment bench to bankfull channel (QInner Berm/QBankfull). By applying Part I-A, a hydrologic regime ratio {QDaily/QPeak}CFR, developed in the most probable channel-forming discharge range (Q1.25 to QMean Annual Flood), is derived from USGS annual peak (QPeak) and mean daily (QDaily) discharge records A multiplier to QBankfull, it estimates most probable (mean/averaged) ‘quickest 1-day’ runoff volume (QDaily Bankfull) associated with a minimum peak bankfull wave event. Station-averaged {QDaily/QPeak} CFR ratios have units of time expressed in percent of day (% of 24-hours). This hydrologic regime ratio indicates; a) the flashiness of a channel-forming (bankfull) event; b) approximates the basin’s response of routing quickest 24-hours of storm runoff volume moving with the channel-forming peak wave or bore event; c) approximates the sediment regime ratio QPoint Bar/QFloodplain which is QInner Berm/QBankfull. Numerically {QDaily/QPeak} CFR is a surrogate to {QDaily Bankfull/QBankfull}. Applied to sediment regime, QDaily Bankfull correlates to a depositional surface and channel capacity at the tops of point bar/inner berm sediment bench threshold, mapped by flow duration, (% annum). QBankfull correlates to an upper geomorphic threshold; channel capacity prior to flood plain overflow, and mapped by annual flood frequency, in years (return interval).

Part I-A hydrologic procedure is utilized to compute hydrologic regime ratios {QDaily Bankfull/QBankfull} and geomorphic inner sediment berm discharges QInner Berm for ~67 USGS stream gauges, 2/3rd headwater (cobble-bed) streams (~1/3rd gravel bed) in the Mid-Atlantic region. QBankfull return intervals and QInner Berm flow durations were averaged within physiographic provinces: the Piedmont, comprised of 20 USGS stream gauges spanning Maryland (MD), North Carolina and Pennsylvania (PA), averaged 1.46 year (QBankfull) and 0.54% annum or 47.3 hours/year (QDaily Bankfull); the Valley and Ridge (V&R) province, comprised of 21 gauges spanning MD, PA, Virginia and West Virginia, averaged 1.55 year (QBankfull) and 0.54% annum (QInner Berm); the Appalachian Plateaus (AP) province, comprised of 26 gauges spanning MD, New York, PA, and Ohio, averaged 1.54 year (QBankfull) and 0.50% annum (QInner Berm). Preliminary Regional QBankfull and QDaily Bankfull Curves (vs. Drainage Area) were developed using linear least-squares regressions on logarithmic scales and are presented for respective provinces along with flood ratios (i.e. Q100/Q1.5) for AP and V&R provinces.