Pre-Restoration Geomorphic Characteristics of Minebank Run, Baltimore County, Maryland, 2002-04

Abstract

Data collected from 2002 through 2004 were used to assess geomorphic characteristics and geomorphic changes over time in a selected reach of Minebank Run, a small urban watershed near Towson, Maryland, prior to its physical restoration in 2004 and 2005. Longitudinal profiles of the channel bed, water surface, and bank features were developed from field surveys. Changes in cross-section geometry between field surveys were documented. Grain-size distributions for the channel bed and banks were developed from pebble counts and laboratory analyses. Net changes in the elevation of the channel bed over time were documented at selected locations. Rosgen Stream Classification was used to classify the stream channel according to morphological measurements of slope, entrenchment ratio, width-to-depth ratio, sinuosity, and median-particle diameter of the channel materials. An analysis of boundary shear stress in the vicinity of the streamflow-gaging station was conducted by use of hydraulic variables computed from cross-section surveys and slope measurements derived from crest-stage gages in the study reach. Analysis of the longitudinal profiles indicated noticeable changes in the percentage and distribution of riffles, pools, and runs through the study reach between 2002 and 2004. Despite major changes to the channel profile as a result of storm runoff events, the overall slope of the channel bed, water surface, and bank features remained constant at about 1 percent. The cross-sectional surveys showed net increases in cross-sectional area, mean depth, and channel width at several locations between 2002 and 2004, which indicate channel degradation and widening. Two locations were identified where significant amounts of sediment were being stored in the study reach. Data from scour chains identified several locations where maximum scour ranged from 1.0-1.4 feet during storm events. Bank retreat varied widely throughout the study reach and ranged from 0.2 feet to as much as 7.9 feet. Sequential measurements of bed elevation in selected locations indicated as much as 2 feet of channel degradation in one location during a storm event in May 2004 and identified pulses of sediment that were gradually transported through the study reach during the monitoring period. Particle-size analyses of channel bed materials indicated a median particle diameter of 20.5 millimeters (coarse gravel) for the study reach, with more than 24 percent being sand particles (greater than 0.062 millimeters). Analyses of bank samples showed finer-grained material composing the channel banks, predominantly silt/clay or a mixture of silt/clay (less than 0.062 millimeters) and very fine to coarse sand. The Minebank Run stream channel was classified as a B4c channel, based on morphological descriptions from the Rosgen Stream Classification System. The B4c classification describes a single-thread stream channel with a moderate entrenchment ratio of 1.4 to 2.2; a width-to-depth ratio greater than 12; moderate sinuosity of 1.2 or greater; a water-surface slope of less than 2 percent; and a median-particle diameter in the gravel range of 2 to 64 millimeters. Analysis of boundary shear stress indicated larger mean velocities and boundary shear stress values for Minebank Run when compared to relations for non-urban B channel types developed by Rosgen. The slope of the regression line for mean velocity versus boundary shear stress at Minebank Run was considerably less than slopes developed by Rosgen for non-urban channel types. This indicates that relatively small increases in mean velocity can result in large increases in boundary shear stress in stream channels with highly developed watersheds, such as Minebank Run.