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Water: WARSSS

Variables Influenced

Tables 7 through 9 in the RLA identify various land use activities and their influence on the variables related to potential adverse change. The list of processes/land use interrelations (Table 9) is important to revisit during this step, as this list is augmented with several RRISSC prediction methods below. The user should also review the relevant RLA inventory information for links among Table 9 variables and specific land uses or changes. For example, a reservoir located on an unstable G4 stream type helps explain the channel degradation and enlargement below the dam.

Some of the variables influenced are magnitude, duration and timing of stream flow, clear water discharge, increased shear stress, increased stream power, change in local base level, energy slope, and other interrelated process variables. It is obvious in the example that this combination of reservoir use and G4 stream type represents a high risk of both excess sediment supply as well as reduced channel stability. Specific mitigation effects on the operational hydrology of the dam would require a much more detailed assessment at the PLA. If the reservoir mentioned above was on a B2 stream type, however, the response to the change in flow and sediment would be entirely different than on a G4 stream type, requiring very little mitigation for channel stability. A list of land use activities and their influence on process variables and potential consequences, along with the corresponding RRISSC prediction methods, appears in Table 11 below.

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TABLE 11. Relationship Among Land Uses/Activities, Process Influences, Consequences and Assessment Methods
Land Use/Activity Variables and Factors Influenced Risk Prediction Method
Potential Change
From Land
Use/Activities
Processes Influenced Potential Consequence
Stream flow decrease in magnitude, duration and timing, due to reservoirs, diversions Shear stress
Stream power
Competency
Sediment transport capacity
Excess sediment deposition
Accelerated bank erosion
Widening channel
Successional state
Aggradation
Figure 71
Urban areas - stream flow discharge increase due to high % impervious cover & storm water drains; clear water discharge Shear stress
Stream power
Sediment transport capacity
Degradation
Channel enlargement
Bank erosion
Channel successional state
Increased sediment load
Figure 72
Stream flow increase from vegetative alteration, clear cutting, land clearing, roads Shear stress
Stream power
Magnitude of flow
Duration of flows
Channel enlargement
Bank erosion
Degradation
Channel successional state
Increased sediment load
Figure 73
Worksheet 2 (PDF) (1 pg, 36K)
Riparian vegetation alteration (% of channel length by stream type) Bank erodibility
Sediment transport capacity
Stream power
Shear stress
Bank erosion
Aggradation
Enlargement
Channel successional state
Figure 74
Worksheet 3 (PDF) (1 pg, 119K)
Surface disturbances (% of ground cover) and roads Surface runoff
Sub-surface flow intercept (roads)
Deposition
Sediment transport capacity (aggradation)
Excess scour(degradation)
Surface erosion delivered to stream
Road source sediment
Gully erosion
Aggradation
Degradation
Streambank erosion
Figure 75
Figure 76
Figure 77
Figure 78
Table 12 (PDF) (1 pg, 17K)
Figure 79
Water yield - harvest and roads - add to soil water influencing slope stability Surface/sub-surface hydrology
Soil saturation
Internal strength by roots
Slope equilibrium
Mass wasting
  slump earthflow
  debris torrent
  sediment supply delivered to channel
  aggradation
  confinement channel evolution shifts
  enlargement (debris torrents)
Figure 77
Figure 80
Figure 81
Figure 82
Direct channel impacts
   channelization
   levees
   straightening
   dredging
Shear stressUp and down arrows
Stream powerUp and down arrows
Width
Confinement
Incision
Gully erosion
Bank erosion
Channel enlargement
Degradation
Aggradation
Channel successional state
Worksheet 4 (PDF) (1 pg, 42K)
Figure 74
Figure 77
Figure 78
Figure 79
Figure 83
Figure 84
Figure 85
Table 13
Channel clearing, cleaning, grubbing, large woody debris removal Stream power
Shear stress
Sediment transport capacity
Competence
Degradation
Energy dissipation
Sediment deposition
Degradation
Bank erosion
Channel enlargement
Increased sediment supply
Aggradation
Figure 77
Figure 78
Figure 79
Figure 85
Figure 86

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