CONCENTRATED FLOW BREAKTHROUGHS MOVING THROUGH SILVICULTURAL STREAMSIDE MANAGEMENT ZONES: SOUTHEASTERN PIEDMONT, USA¹

Abstract

Geomorphic characteristics and spatial frequency of ephemeral concentrated flow paths entering streamside management zones (SMZs) were evaluated to determine the efficiency of best management practices (BMPs) in preventing concentrated overland flow and associated sediment from reaching stream channels. Specifically, SMZs of 30 recently clearcut and site prepared commercial forestry units in the Georgia Piedmont were surveyed to find two types of locations: those where flow and/or sediment from the adjacent silvicultural site entered and moved through SMZs into stream channels (breakthroughs), and those where either flow and/or sediment entered SMZs without reaching stream channels or where no overland flow entered SMZs (successes). A total of 187 breakthroughs were identified on 3,773 total acres. On average, sites featured one breakthrough for every 20 acres of clearcut or site prepared area. The average hydrologic contributing area to a breakthrough was 1 acre. The percentage of the total clearcut or site prepared area contributing to breakthroughs was 5 percent. Approximately 50 percent of all breakthroughs occurred in areas of convergence (swales) and gullies, while 25 percent of all breakthroughs occurred where runoff from roads or skid trails was concentrated. Breakthroughs tended to occur in areas with large contributing area, low litter cover, and steep slopes. However, individually these variables did not differentiate well between breakthroughs and successes. The variables that discriminated best between successes and failures were the product of contributing area and percent bare ground, and the same variable multiplied by average slope. Fourteen percent of the breakthroughs traveled more than 100 feet through SMZs before reaching streams. Results imply that reduction of bare ground, better dispersal of road runoff, introduction of hydraulic resistance to likely flow paths, and targeted extensions of SMZ width may be warranted in improving BMPs on Piedmont forests.

Estimating Costs and Effectiveness of Upgrades in Forestry Best Management Practices for Stream Crossings

“…In a separate study, Lang et al [74] compared USLE erosion estimates from forest road stream crossing approaches in the Piedmont of Virginia and found that sediment delivery rates from forest approaches to streams were nearly 100%. Rivenbark and Jackson [44] examined causes of SMZ failures in the Piedmont and concluded that roads and stream crossings were directly associated with 25% of all SMZ failures. Thus, our USLE-Forest estimates potentially provide the approximate quantity of sediment delivery that could be expected at these crossings.…”

“…In addition, several researchers [42][43][44] concluded that road stream crossings can compromise the sediment trapping ability of streamside management zones.…”

The effects of riparian forest disturbance on stream temperature, sedimentation, and morphology

“…Riparian disturbance has been linked to increased sedimentation and turbidity (Jones et al, 1999;Swank et al, 2001), mainly due to the exposure of mineral soil when building of roads and log landings during forestry operations (Kochenderfer et al, 1997;Rivenbark & Jackson, 2004). Increased sediment loads can cause shifts in AI communities (Wood & Armitage, 1997), resulting in fewer Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa (Davies & Nelson, 1994;Kaller & Hartman, 2004).…”

“…The riparian zone entraps and retains small particles, reducing sediment loads in streams (Rivenbark & Jackson, 2004, but see Keim & Schoenholtz, 1999). Riparian disturbance has been linked to increased sedimentation and turbidity (Jones et al, 1999;Swank et al, 2001), mainly due to the exposure of mineral soil when building of roads and log landings during forestry operations (Kochenderfer et al, 1997;Rivenbark & Jackson, 2004).…”

Prescribed burning effects on the hydrologic behavior of gullies in the South Carolina Piedmont

“…Many remnant agricultural gullies become more active after timber harvest (Rivenbark and Jackson, 2004) and such gullies have been identified as significant contributors to nonpoint source pollution after timber harvest (Hewlett and Doss, 1984). The contribution of these gullies, through continued erosion, to the overall sediment production of a watershed is typically unaccounted for in erosion models (e.g.…”