Sediment and Erosion Control Techniques on Stream Restoration Projects

Size: px

Start display at page:

Download "Sediment and Erosion Control Techniques on Stream Restoration Projects"

Susanna Davidson
6 years ago
Views:

1 Sediment and Erosion Control Techniques on Stream Restoration Projects D. R. Clinton, G. D. Jennings, R. A. McLaughlin, D. A. Bidelspach 1 ABSTRACT Erosion control, sediment loss and turbidity control are important considerations during construction of stream restoration projects. Standard practices such as basins, traps, check dams, and silt fences may reduce sediment losses but may not be effective at reducing turbidity impacts to downstream water. Innovative techniques for improved sediment control and turbidity reduction include flocculants, sediment basin enhancements, baffles, outlet modifications, micrograding, and construction phasing. The purpose of this paper is to describe the application of innovative erosion and sediment control techniques for stream restoration construction projects. KEYWORDS Fluvial geomorphology, turbidity, sedimentation, stability, watershed INTRODUCTION Stream restoration projects typically involve earth moving activities such as channel relocation, shaping of streambanks and floodplains, and installation of in-stream structures. These projects often result in large disturbed land areas with the potential to cause downstream water quality impacts due to erosion and sedimentation. Traditional engineering approaches to controlling sediment impacts may not meet water quality goals, especially with regard to turbidity impacts. Innovative techniques for reducing sediment loss and turbidity control during construction activities may be necessary to effectively control sediment and turbidity. This paper describes the application of innovative erosion and sediment control techniques for stream restoration in three categories: (1) site preparation; (2) construction; and (3) post-construction stabilization. Site Preparation Typical site preparation practices include stone construction entrance, demarcated staging/stockpiling areas, and haul road establishment (Smolen et al, 1988). Maintenance is critical with all erosion control but the following can aid in the effectiveness of each. A wash station added to construction entrance can add to its effectiveness and can aid in a positive public perception particularly in urban settings. Wash stations typically include hoses, a pump, a splash station, a collection facility, a settling area and a return channel. In addition to public perception, maintaining a wash station reduces soil from leaving the construction site therefore reduces potential turbidity and sediment damage off site. Sediment on the surrounding roads can also be a safety (and liability) hazard if it accumulates enough to become slick. Connecting the wash station return channel to a sediment basin can increase its effectiveness by allowing the sediment 1 Biological & Agricultural Engineering Department, North Carolina State University, Raleigh, NC dan_clinton@ncsu.edu 0

2 laden water time to settle out larger particles prior to it returning to the stream. Polyacrylamide (PAM) can be added to the pump water to further reduce turbidity by flocculating out finer particles. Demarcating staging and stockpile areas can benefit a construction site by minimizing disturbed areas and focusing stockpile areas to controllable limits. Water flow coming from stockpile areas should be directed into a sediment basin to reduce the risk of sediment reaching the stream. Construction haul roads are areas of high sediment movement because they tend to intercept runoff and bring it downslope at high velocities. Intermediate road berms which direct sediment laden water off the road prior to it accumulating into significant volumes can reduce potential stream sedimentation. Extra care should be taken in areas where haul roads cross stream channels. In these areas, diversion berms should be established to direct sediment laden water toward sediment basins where it can be clarified prior to entering the stream channel. A stable, clean pad of stone should be maintained at crossings to reduce sedimentation into stream channels. Project clearing can enhance or counter sediment and erosion control efforts on a restoration project. Recommendations for typical clearing techniques include phased clearing in conjunction with construction activities, salvaging of transplant materials and spot clearing to maintain a viable seed bank post construction. Phasing of clearing operations can reduce open area thus reducing sedimentation risk. Phased clearing can also improve the viability of transplant materials by keeping the transplants in the ground for a longer period of time. Prior to construction on most stream restoration projects, a diversion is typically required if inchannel work is necessary. Although no national standards have been established, channels with drainage areas less than 4 square miles are typically required to be diverted in North Carolina. The diversion can be accomplished through an open channel, pipe, or by pumping. Within the diversion area, water that has infiltrated into the construction area from ground water or seepage through diversion dikes needs to be pumped out in order to work. The pumped water often has high sediment concentrations which should be removed prior to release downstream. Sediment filter bags are often used for this purpose because they are simple and do not require a lot of space. However, they are only partially effective and the water can still be quite turbid after passing through the bag. A stilling basin may be more effective, particularly if it is designed with porous baffles (Thaxton et al, 2004). The most successful material for these has been a combination of jute matting and coir erosion control blanket, strung across the basin on hog wire or similar. The effect is to force the water to spread across the entire cross-section of the basin and to reduce turbulence that interferes with settling. Providing a floating outlet, or skimmer, attached to a solid riser or similar, can also greatly improve sediment capture (McLaughlin, 2003; Millen et al., 1997; Figure 1) The addition of polyacrylamide (PAM) has substantial potential for reducing turbidity (McLaughlin, 2003). We have had some success in metering in PAM solutions at the pump as well as running the water across PAM logs or PAM powder sprinkled in a pipe or on jute matting. However, each sediment material is different and the correct PAM needs to be matched 1

3 to the sediment on the project. There is no one way to introduce the PAM to the water, but the system will require high turbulence to form the flocs followed by a settling area with minimum turbulence. One method we ve used is to add PAM solution at the pump intake and release the pumped water into a corrugated pipe followed by a stilling basin (Figure 2). PAM injected into the pumped water can improve the performance of sediment bags, but there may be problems with the flocs clogging the bag long before it is filled. This may be acceptable, however, since the bags are relatively inexpensive. The outfall area of the diversion should be adequately protected to avoid unnecessary scour from the diversion water. Typical outfalls consist of rip-rap pads or stilling basins. Construction Activities Erosion control during stream restoration construction involves stabilizing stream banks immediately after grading and prior planning of construction sequence. Stabilizing stream banks involves installing erosion control blankets (ECBs) and developing a temporary herbaceous cover as soon as possible while the woody materials are becoming established. ECBs should cover all open stream banks to provide adequate protection until vegetation becomes established. Various grades of ECBs can be utilized in higher and lower stress areas of the stream banks. For example, a tougher grade of ECB is needed on the outside meander bend compared to the lower stress point bar areas. The point bar area ECB should decay at a much higher rate since that area of the stream scours and redeposits during large storm events. ECBs should overlap the top of the stream bank by a minimum of one foot to reduce the potential of rills and gullies forming under the matting. Upstream and top of bank edges of the ECB should be well keyed-in to reduce lifting of the matting during storm events. ECBs should be trimmed such that they are tightly secured around rootwads. This minimizes potential scour. In our experience, straw mulch placed under an ECB with large openings will enhance the protection of the bank and increase the herbaceous cover success rate. Herbaceous plugs inserted through the ECB can also be used to provide faster ground cover on stream banks. ECBs, particularly on riffle and outside meander banks, should be rated to withstand average predicted velocities. Securing the ECB with either metal sod staples or biodegradable wood stakes should follow manufacturer s guidelines to ensure stability. The sequence of events is critical to provide effective sediment and erosion control on stream restoration projects. Stream restoration projects often involve moving significant volumes of soil. Soil should be placed in its final location or secured in stockpile areas behind silt fence at the completion of each day s construction or before the threat of rain. The clearing sequence is also a critical part of the erosion control plan. Minimizing the duration of cleared land reduces the risk of sedimentation downstream. Typically 500 feet of cleared stream length is adequate to maintain construction timeline. Reduced clearing also increases the viability of potential transplants because the reduced amount of time they are removed from the ground. 2

4 Sediment control basins may be necessary depending on the size of the disturbed area. The sediment capture rate can be greatly increased through the enhancement mentioned previously (floating outlet, porous baffles). A standing pool has also been demonstrated to increase sediment capture (Fennessey and Jarrett, 1997). To further reduce turbidity, PAM can be used to flocculate the suspended fines. It is important to allow a zone of mixing and contact time followed by a settling zone prior to entering the stream channel. Alternative bank stabilization methods include root wrap and brush mattresses used for higher stress areas along outside meander bends. Root wrap, where root wads and tightly packed along an outside meander, can provide excellent bank protection and enhance the habitat values of the pool area. Brush mattresses, although labor intensive to install, provide an almost instantaneous living stream bank capable of withstanding high stream velocities, if constructed properly. Post-Construction Stabilization Beyond establishing a stand of herbaceous cover upon completion of construction activities, ensuring that the floodplain is graded smoothly and free of high pockets can strongly aid in the bank stability. High areas within the floodplain can cause water to concentrate and enter the stream in large volumes in local areas. These overland flow interception points often form gullies where vegetation cannot become established. Often ECBs are not sufficient to protect the bank in these areas. Attention should be directed to areas where water is likely to intercept the stream channel. Hardened inlet features such as rock-lined channels can be used to ensure erosion will not occur. CONCLUSION Effective control of soil erosion, sediment loss, and downstream turbidity impacts is critical for the success of stream restoration construction. Designers must look beyond standard practices and consider innovative techniques such as flocculants, sediment basin enhancements, baffles, outlet modifications, micro-grading, and construction phasing. These approaches are being evaluated in several projects in North Carolina and should be considered for further research and development as components of stream restoration projects. REFERENCES 1. Smolen, M.D., R.G. Jessup, L.C. Wyatt, D.W. Miller, J. Lichthardt, A.L. Lanier, W.W. Woodhouse, and S.W. Broome North Carolina Erosion and Sediment Control Planning and Design Manual. NC Department of Environment, Health, and Natural Resources, Division of Land Resources, Land Quality Section, Raleigh, North Carolina. 572 pp. 2. Thaxton, C. S., J. Calantoni, and R. A. McLaughlin Hydrodynamic assessment of various types of baffles in a sediment retention pond. Transactions of the ASAE Vol. 47(3):

5 3. McLaughlin, R. A The potential for substantial improvements in sediment and turbidity control. Pp in Total Maximum Daily Load (TMDL) Environmental Regulations II, Conference Proceedings, 8-12 November 2003 (Albuquerque, New Mexico, USA), ed. Ali Saleh.,8 November ASAE Pub #701P Millen, J. A., A. R. Jarrett, and J. W. Faircloth Experimental evaluation of sedimentation basin performance for alternative dewatering systems. Trans. Am. Soc. Agr. Eng. Vol. 40: Fennessey, L. A. J., and A. R. Jarrett Influence of principal spillway geometry and permanent pool depth on sediment retention of sedimentation basins. Trans. ASAE 40(1): Mostaghimi, S., T.M. Gidley, T.A. Dillaha, and R.A. Cooke Effectiveness of different approaches for controlling sediment and nutrient losses from eroded land. J. Soil and Water Cons. 49 (6) NCDENR (North Carolina Department of Environment and Natural Resources), NC Sediment Control Commission, and NC Agricultural Cooperative Extension Practice Standards and Specifications Surface Stabilization. In North Carolina Sediment and Erosion Control Planning and Design Manual. 8. NCDENR (North Carolina Department of Environment and Natural Resources), Division of Water Quality Surface Water and Wetland Standards (NC Administrative Code 15A NCAC 02B.0100 &.0200). p.22. Accessed at 4

Corrugated pipe for PAM mixing and contact with pumped water.

6 Porous Baffle Faircloth Skimmer Emergency Spillway Figure 1. Example of a floating outlet design for a sediment basin. Corrugated pipe for PAM mixing and contact with pumped water. Porous baffles across entire width of basin. Figure 2. Example of a combination of PAM treatment and a large stilling basin at a borrow pit pumping operation. This can be scaled down considerably for lower pumping volumes. 5

Materials. Use materials meeting the following.

Materials. Use materials meeting the following. 208.01 Section 208. SOIL EROSION AND SEDIMENTATION CONTROL 208.01 Description. Install and maintain erosion and sedimentation controls to minimize soil erosion and to control sedimentation from affecting