Assessing Stream Channel Stability At Bridges in Physiographic Regions

5. MODIFICATIONS OF THORNE'S RECONNAISSANCE SHEETS

The stream stability assessment method developed in chapter 4 is self-contained and does not require any other data or formal method of data collection other than the descriptors given in table 8. However, it is prudent to develop field forms that help observers focus attention on specific aspects of a stream, be consistent in those observations, and systematically record their observations. For this purpose, the Thorne reconnaissance field sheets are the best method available for systematically collecting stream channel data. There are several problems with using the reconnaissance field sheets developed by Thorne for highway-related purposes. (2) First, it is not clear that this highly detailed and time-consuming level of data collection is necessary to assess stream stability or stream response at a bridge. In addition, bridge inspectors generally cannot and will not take the time to collect this level of data, as it is out of line with the overall inspection process. Second, even when all of the detailed data are collected, there is no guidance within the method for interpreting the data. Third, few inspectors and hydraulic engineers have appropriate backgrounds to identify geological parameters, such as rock type. To develop a rapid stability assessment method, a simplified version of the Thorne reconnaissance sheets should be created specifically for use in training and data collection associated with assessing stream stability at bridges.

The stream stability method developed for this report is based on data collected through the reconnaissance. However, given that bridge inspection requires only assessment of stream stability in the short term and since each bridge is inspected at least every 2 years, data collection requirements can be simplified to reflect this less detailed and rapid assessment. In addition, several aspects of the sheets required minimal revision. Interpretive observations, while critical to communicating between observers, are neglected in the revised sheets because, in a rapid assessment, most of the qualitative data are collected by an inspector who is interpreting what he or she observes. All inspections require written reports in which the inspector provides his or her judgment on the status of the overall bridge condition and maintenance needs. In addition, items on the sheet that cannot be assessed in a very brief site visit are excluded from the revised sheets. The simplified and revised data collection sheets, based strongly on Thorne's reconnaissance sheets, are given in figures 18-20.

In addition to the stability assessment, keeping a record of channel dimensions upstream and downstream of the bridge will provide a history of changes in width and depth. Although detailed surveys are not part of a bridge inspection, a simple measurement of station and elevation upstream of the bridge taken annually will provide adequate cross-sectional information to assess longer term changes. Without this information, gradual but continual changes in the channel may be overlooked.

Figure 18. Simplified and revised reconnaissance sheets based on Thorne, sheet 1. (2)

STREAM RECONNAISSANCE Revised for Bridge Inspection Based on Thorne (1998)
SECTION 1-SITE DESCRIPTION
ROAD NAME/NUMBER		DATE
BRIDGE NUMBER
STREAM NAME
GPS COORDINATES
SECTION 2-REGION AND VALLEY DESCRIPTION
PART 1: WATERSHED		PART 2: RIVER VALLEY CONDITION
Land Use	Vegetation	Valley Side Failures	Failure Locations
Natural	None	None	None
Agricultural	Grass	Occasional	Away from river
Urban	Pasture	Frequent	Along river
Suburban	Crops
Rural	Shrubs
Industrial	Deciduous Forest/trees
Cattle grazing	Coniferous Forest/trees
PART 3: FLOOD PLAIN
Flood Plain Width	Land Use	Vegetation	Riparian Buffer Strip
None	Natural	None	None
< 1 river width	Agricultural	Grass	< 1 river width
1-5 river widths	Urban	Pasture	1-5 river widths
5-10 river widths	Suburban	Orchards	> 5 river widths
> 10 river widths	Rural	Crops
	Industrial	Shrubs
	Mining	Deciduous Forest/trees
	Cattle grazing	Coniferous Forest/trees

Figure 19. Simplified and revised reconnaissance sheets based on Thorne, sheet 2. (2)

PART 4: VERTICAL CONFINEMENT
Terraces	Levees	Levee Location
None	None	Along channel bank
Left bank	Natural	Setback < 1 river width
Right bank	Constructed	Setback > 1 river width
PART 5: LATERAL RELATION OF CHANNEL TO VALLEY
Planform	Meander Characteristics
Straight	Mild bends
Meandering	Moderate bends
Braided	Tight bends
Anastomosed	Engineered
SECTION 3-CHANNEL DESCRIPTION
PART 6: CHANNEL DESCRIPTION (select all that apply)
Bed Controls	Control Types	Width Controls	Control Types	Other
None	None	None	None	Debris
Occasional	Bedrock	Occasional	Bedrock	Mining
Frequent	Boulders	Frequent	Boulders	Reservoir
Confined	Gravel armor	Confined	Gravel armor	Knickpoint
	Bridge protection		Bridge protection
	Grade control		Bridge abutments
	Debris		Bank stabilization
	Dams (beaver, engineered)		Debris
Flow Habit	Channel Width = ___________
Perennial
Flashy perennial	M-B Classification	Corps Classification (Other)
Intermittent	Cascade or step-pool	Modified (engineered)
Ephemeral	Plane, pool-riffle, dune-ripple	Regulated
	Braided	Arroyo
PART 7: BED SEDIMENT DESCRIPTION (select all that apply)
Bed Material	Bar Types	Bar Material	Bar Vegetation	Bar Width
Clay	None	Silt	None	None
Silt	Alternate bars	Sand	Grasses	Narrow
Sand	Point bars	Gravel	Reeds/shrubs	Moderate
Gravel	Midchannel bars	Cobbles	Trees	Wide
Cobbles	Diagonal bars
Boulders	Irregular/combination
Bedrock	Braided
		Percent Sand in Bed = ____________%

Figure 20. Simplified and revised reconnaissance sheets based on Thorne, sheet 3. (2)

SECTION 4-BANK SURVEY (select all that apply)
Bank Characteristic	Left Bank	Right Bank
Bank material	Clay	Clay
	Silt	Silt
	Sand	Sand
	Gravel	Gravel
	Cobbles	Cobbles
	Boulders	Boulders
	Bedrock	Bedrock
Layer material	No layers	No layers
	Cohesive	Cohesive
	Sand	Sand
	Gravel	Gravel
	Cobbles	Cobbles
	Boulders	Boulders
Bank height
Bank slope	Steep	Steep
	Moderate	Moderate
	Shallow	Shallow
Bank vegetation	None	None
	Grasses/annuals	Grasses/annuals
	Reeds/shrubs	Reeds/shrubs
	Trees	Trees
	Falling trees? Yes No	Falling trees? Yes No
	Tree density: Sparse Dense	Tree density: Sparse Dense
	Tree health: Good Poor	Tree health: Good Poor
	Tree ages: Young Mature Old	Tree ages: Young Mature Old
	Tree diversity? Yes No	Tree diversity? Yes No
Bank erosion and failure location	Location of erosion:	Location of erosion:
	Outside meander bend	Outside meander bend
	Inside meander bend	Inside meander bend
	Opposite bar or obstruction	Opposite bar or obstruction
	General	General
	Type of erosion:	Type of erosion:
	Fluvial	Fluvial
	Geotechnical	Geotechnical

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