Appendix 2. Hillslope and stream channel coupling

Church (1983) developed a classification of landscape units (decoupled, partially coupled, and coupled hillslopes and channels, described below) that considers the sediment transfer routes from hillslope to stream channel. Generally, the degree of coupling determines the short-term response of the stream channel to events that occur on the hillslope, and the importance of the valley flat as a buffer to sediment transfer.

The relations among the hillslope, valley flat, and stream channel associated with each reach identified in the CAP should be determined. Refer to Figure A2-1 and assess the potential for sediment and debris mobilized on the hillslope to enter the stream channel. Note that the presence of one or more landslides along or in a reach is not required. The objective is to assess only the connections between the channel and the hillslope and between the channel and the upstream reaches.

Figure A2-1. Hillslope and stream channel coupling (after Church 1983). Decoupled hillslopes are identified when sediment or debris, mobilized by a landslide, is interrupted by a valley flat. Partially coupled hillslopes are identified when only a portion of a landslide deposit can be stored on a valley flat. Coupled hillslopes are identified when sediment or debris directly enters the stream channel.

Decoupled hillslopes

Generally, a channel is considered decoupled from a hillslope when sediment mobilized on the hillslope by a landslide normally would not enter the stream channel. Sediment movement through a decoupled channel is dominated by normal fluvial processes. The dominant ways sediment is delivered to the channel are bank erosion and delivery from upstream. Channel gradient is typically <2 per cent. Some indicators of decoupled reaches are as follows.

• Sediment or debris mobilized by a landslide is intercepted by the valley flat, preventing material from directly entering the stream channel.
• Surrounding slopes are steep (>65 per cent gradient) but of sufficiently limited extent to prevent the delivery of significant volumes of sediment to the channel (e.g., the channel is incised into a relatively wide valley flat).
• The channel is large relative to the volume of sediment and debris that may be transferred from the surrounding hillslopes.
• The surrounding slopes are gentle (<35 per cent gradient) and unlikely to initiate landslides.
• There are no surrounding hillslopes (a valley flat is not necessarily present).

Partially coupled hillslopes

Generally, a channel is considered partially coupled to a hillslope when a portion of the sediment mobilized on the hillslope by a landslide directly enters the stream channel. Sediment delivery to a partially coupled channel is dominated by debris flows or persistent erosion from fluvially undercut till or outwash bluffs. The transfer of sediment out of the reach is by normal fluvial processes. Channel gradient ranges from 2 to 10 per cent. Partially coupled channels are identified by the following indicators.

• Landslides either directly enter the channel or are buffered by the valley flat (only some of a landslide deposit is stored on the valley flat).
• The surrounding slopes are steep (>65 per cent) and likely to initiate landslides.
• A reach can be impacted from upstream by a debris flow or torrent.
• A reach with a discontinuous valley flat impinges on some parts of the hillside, so landslides mobilized from some hillside locations are apt to enter the channel.

Coupled hillslopes

Generally, a channel is considered coupled to a hillslope when sediment mobilized on the hillslope by landslide activity directly enters the stream channel. Sediment delivery to coupled reaches is dominated by landslides, while sediment movement through the reach is by debris flow and fluvial processes. Channel gradient is typically >5 per cent. Coupled reaches are identified by the following indicators.

• There is no valley flat; sediment or debris mobilized by landslides directly enters the stream channel.
• The surrounding slopes are steep (>65 per cent) and likely to initiate landslides that can transfer sediment directly to the stream channel.
• The channel is small relative to the volume of sediment and debris that may be transferred from the surrounding hillslopes.
• Debris flows may be initiated from within the reach.