Revision as of 14:41, 22 January 2016

Reduce undesired sediment input

Category 02. Sediment flow quantity improvement

General description

Erosion and deposition of bed and bank material are natural processes in rivers. However, excess sediment input, especially of fine sediments, has severe detrimental effects like clogging of interstitial spaces, limiting spawning success of fish and habitats for macroinvertebrates, and filling of pools.

The main sources of such an undesired sediment input are soil erosion from agricultural areas and bank erosion (see fact-sheet on the pressure of (Sedimentation and sediment input for more details) but also agricultural drains, which are often neglected and poorly studied but may contribute high fine sediment loads (Chapman et al., 2005). Knowledge on the importance and contribution of these three main pathways is essential for an effective reduction of undesired (fine) sediment input.

Soil erosion: Measures to reduce the input of sediment from agricultural areas due to water erosion can either tackle the source or the sink. There are several well-studied practices and techniques to reduce water erosion at the source like no-tillage or counter farming, cover crops, and vegetation buffers. There are several websites and reports which give an overview on these measures, practices, and techniques, e.g. FAO, the EU-RECARE project and the respective RECARE-Report. Riparian buffers are widely promoted as an end-of pipe technique to reduce the input of fine sediments. Buffer strips are most effective when the flow is shallow (non-submerged grassy vegetation), slow, and enters the buffer strip uniformly along its length as sheetflow. Moreover, effectiveness decreases with sediment particle size since the main process is the reduction of flow velocity and associated settling of sediment particles. As a consequence, the recommended width of riparian buffers to filter fine sediments increases with slope, flow, decreases with particle size. As a consequence, the recommended buffer with is highly variable (3-200 m) but a minimum width of 15-30 m seems to be sufficient to provide the multiple functions of riparian buffers (including nutrient retention and habitat provision) under most conditions. For more information see the reviews in Barling and Moore (1994), Castelle et al. (1994), Dosskey (2001), Broadmeadow and Nisbet (2004), Hickey and Doran (2004), Feld et al. (2011).

Bank erosion: Erosion and deposition of bed and bank material are natural processes in rivers. However, excess bank erosion leads to lateral instability and excess input of (fine) sediment. Excess bank erosion increases the input of fine sediment, clogging interstitial spaces and limiting spawning success of fish and habitat for macroinvertebrates, and filling pools. The restoration measures should first try to address the causes for excess bank erosion like reducing non-natural peak flows (Modify hydropeaking, (Reduce anthropogenic flow peaks) or livestock fencing to reduce stream bank trampling damage. If habitat conditions are severely affected by excess bank erosion and the above mentioned measures cannot be applied, river banks may be stabilized using bio-engineering techniques.

Important note: Bed- and bank erosion and deposition of sediment are natural processes needed to develop and sustain a natural planform and channel features. Therefore, many restoration projects aim at initiating natural morphodynamics rather than limiting it (Initiate natural channel dynamics). The paradigm of the “stable channel design”, assuming that river channels should be stable and can be “restored” by creating a desired “stable” channel, ultimately resulted in questionable projects like building meandering channels with fixed banks. This approach has been heavily criticized since it ignores that channels are naturally dynamic, such projects are prone to failure (Kondolf 2006, Simon et al. 2007) and should not be applied.

Applicability

Restoration measures to limit excess bank erosion are especially important in gravel-bed rivers where fine sediment is clogging interstitial spaces and altering habitat for fish and macroinvertebrates. Especially livestock fencing is very easy to implement.

Pressures that can be addressed by this measure

• Alteration of instream habitat
• Reservoir flushing
• Sediment discharge from dredging
• Sand and gravel extraction
• Impoundment
• Sedimentation and sediment input

Case studies where this measure has been applied

• Doñana. Restauración del arroyo del Partido
• River Wensum Rehabilitation Project - Bintree
• Sweden- Restoration of the Freshwater Pearl Mussel and its habitats (LIFE04/NAT/SE/000231)

Useful references

Barling, R. D., Moore, I.D. (1994). Role of buffer strips in management of waterway pollution: a review. Environmental Management, 18, 543-558. Broadmeadow, S., Nisbet, T.R. (2004). The effects of riparian forest management on the freshwater environment: a literature review of best management practice. Hydrology and Earth System Science, 8, 286-305. Castelle, A.J., Johnson, A.W., and Conolly, C. (1994). Wetland and stream buffer size requirements—A review. Journal of Environmental Quality, 23, 878–882. Chapman, A.S., Foster, I.D.L., Lees, J.A., Hodgkinson, R.A. (2005). Sediment delivery from agricultural land to rivers via subsurface drainage, Hydrological Processes, 19, 2875–2897. Dosskey, M. G. (2001). Toward quantifying water pollution abatement in response to installing buffers on crop land. Environmental Management, 28, 557-598. Feld, C., Birk, S., Bradley, D., Hering, D., Kail, J., Marzin, A., Melcher, A., Nemitz, D., Pedersen, M., Pletterbauer, F., Pont, D., Verdonschot, P., Friberg, N. (2011). From natural to degraded rivers and back again. A test of restoration ecology theory and practice. Advances in Ecological Research, 44, 119-202. Hickey, M.B.C., Doran, B. (2004). A review of the efficiency of buffer strips for the maintenance and enhancement of riparian ecosystems. Water Quality Research Journal of Canada, 39, 311-317.