Mary Ann Madej

Variability of bed mobility in natural, gravel-bed channels and adjustments to sediment load at local and reach scales

Local variations in boundary shear stress acting on bed-surface particles control patterns of bed load transport and channel evolution during varying stream discharges. At the reach scale a channel adjusts to imposed water and sediment supply through mutual interactions among channel form, local grain size, and local flow dynamics that govern bed mobility. In order to explore these adjustments, we used a numerical flow model to examine relations between model-predicted local boundary shear stress (тj( and measured surface particle size (D50) at bank-full discharge in six gravel-bed, alternate-bar channels with widely differing annual sediment yields. Values of тj and D50 were poorly correlated such that small areas conveyed large proportions of the total bed load, especially in sediment-poor channels with low mobility. Sediment-rich channels had greater areas of full mobility; sediment-poor channels had greater areas of partial mobility; and both types had significant areas that were essentially immobile. Two reach-mean mobility parameters (Shields stress and Q*) correlated reasonably well with sediment supply. Values which can be practicably obtained from carefully measured mean hydraulic variables and particle size would provide first-order assessments of bed mobility that would broadly distinguish the channels in this study according to their sediment yield and bed mobility.

Erosion and sediment delivery following removal of forest roads

Erosion control treatments were applied to abandoned logging roads in California, with the goal of reducing road-related sediment input to streams and restoring natural hydrologic patterns on the landscape. Treatment of stream crossings involved excavating culverts and associated road fill and reshaping streambanks. A variety of techniques were applied to road benches, which included decompacting the road surface, placing unstable road fill in more stable locations, and reestablishing natural surface drainage patterns. Following treatment and a 12-year recurrence-interval storm, some road reaches and excavated stream crossings showed evidence of mass movement failures, gullying, bank erosion and channel incision. Post-treatment erosion from excavated stream crossings was related to two variables: a surrogate for stream power (drainage area channel gradient) and the volume of fill excavated from the channel. Post-treatment erosion on road reaches was related to four explanatory variables: method of treatment, hillslope position (upper, mid-slope or lower), date of treatment, and an interaction term (hillslope position method of treatment). Sediment delivery from treated roads in upper, middle and lower hillslope positions was 10, 135 and 550 m 3 of sediment per kilometre of treated roads, respectively. In contrast, inventories of almost 500 km of forest roads in adjacent catchments indicate that untreated roads produced 1500 to 4700 m 3 of sediment per kilometre of road length. Erosion from 300 km of treated roads contributed less than 2 per cent of the total sediment load of Redwood Creek during the period 1978 to 1998. Although road removal treatments do not completely eliminate erosion associated with forest roads, they do substantially reduce sediment yields from abandoned logging roads. Published in 2001 by John Wiley & Sons, Ltd.

Benefits and impacts of road removal

Road removal is being used to mitigate the physical and ecological impacts of roads and to restore both public and private lands. Although many federal and state agencies and private landowners have created protocols for road removal and priorities for restoration, research has not kept pace with the rate of removal. Some research has been conducted on hydrologic and geomorphic restoration following road removal, but no studies have directly addressed restoring wildlife habitat. Road removal creates a short-term disturbance which may temporarily increase sediment loss. However, long-term monitoring and initial research have shown that road removal reduces chronic erosion and the risk of landslides. We review the hydrologic, geomorphic, and ecological benefits and impacts of three methods of road removal, identify knowledge gaps, and propose questions for future research, which is urgently needed to quantify how effectively road removal restores terrestrial, riparian, and aquatic habitat and other ecosyste...

Development of channel organization and roughness following sediment pulses in single‐thread, gravel bed rivers

Large, episodic inputs of coarse sediment (sediment pulses) in forested, mountain streams may result in changes in the size and arrangement of bed forms and in channel roughness. A conceptual model of channel organization delineates trajectories of response to sediment pulses for many types of gravel bed channels. Channels exhibited self-organizing behavior to various degrees based on channel gradient, presence of large in-channel wood or other forcing elements, the size of the sediment pulse, and the number of bed-mobilizing flows since disturbance. Typical channel changes following a sediment pulse were initial decreases in water depth, in variability of bed elevations, and in the regularity of bed form spacing. Trajectories of change subsequently showed increased average water depth, more variable and complex bed topography, and increased uniformity of bed form spacing. Bed form spacing in streams with abundant forcing elements developed at a shorter spatial scale (two to five channel widths) than in streams without such forcing mechanisms (five to 10 channel widths). Channel roughness increased as bed forms developed.

TEMPORAL AND SPATIAL VARIABILITY IN THALWEG PROFILES OF A GRAVEL-BED RIVER

This study used successive longitudinal thalweg profiles in gravel-bed rivers to monitor changes in bed topography followingfloodsandassociatedlargesedimentinputs.Variationsinchannelbedelevations,distributionsofresidualwater depths,percentage of channellengthoccupied byriffles,andaspatial autocorrelationcoefficient (Moran’sI) wereused to quantify changes in morphological diversity and spatial structure in Redwood Creek basin, northwestern California. Bed topographyinRedwoodCreekanditsmajortributariesconsistsprimarilyofaseriesofpoolsandriffles.Thesize,frequency and spatial distribution of the pools and riffles have changed significantly during the past 20 years. Following large floods and highsediment input inRedwood Creek and its tributaries in 1975, variation in channelbed elevations was low and the percentageofthechannellengthoccupiedbyriffleswashigh.Overthenext20years,variationinbedelevationsincreased while the length of channel occupied by riffles decreased. An index [(standard deviation of residual water depth/bankfull depth) 100]wasdevelopedtocomparevariationsinbedelevationoverarangeofstreamsizes,withahigherindexbeing indicativeofgreatermorphologicaldiversity.Spatialautocorrelationinthebedelevationdatawasapparentatbothfineand coarsescalesinmanyofthethalwegprofilesandtheobservedspatialpatternofbedelevationswasfoundtoberelatedtothe dominant channel material and the time since disturbance. River reaches in which forced pools dominated, and in which large woody debris and bed particles could not be easily mobilized, exhibited a random distribution of bed elevations. In contrast, in reaches where alternate bars dominated, and both wood and gravel were readily transported, regularly spaced bed topography developed at a spacing that increased with time since disturbance. This pattern of regularly spaced bed featureswasreversedfollowinga12-yearfloodwhenbedelevationsbecamemorerandomlyarranged.Copyright#1999 John Wiley & Sons, Ltd.