Field Methods and Procedures
The prediction methods used in the PLA methodology include many models, empirical relationships, and "reference" stable channel analog descriptions. Even a basic prediction of mean bankfull velocity can be verified by direct measurements with a flow meter by measuring a series of velocity readings across the stream channel at specific depths at the bankfull stage to calculate "mean" velocity. Procedures for velocity measurements and determination of streamflow are also necessary for sediment monitoring. The appropriate reference for these measurements is the U.S.Geological Survey manual Surface Water Investigations Measurement of Stream Discharge, (1980) Chapter A8, Book 3.
The following table shows typical monitoring procedures for various processes and sediment sources including hillslope, hydrologic and channel process relations.
Table 22. Effectiveness Monitoring
|Process/Source||Monitoring Methods||Monitoring Outputs|
|Surface Erosion||Frame & pin (1) and precipitation gage (can)||Surface erosion rate|
|Sediment delivery from surface erosion||Discontinuous contour trench||Sediment yield|
|Mass wasting||Total station survey of site||Erosion in cubic yards, tons|
|Sediment delivery||Replicate re-survey to map "removed" or delivered sediment||cubic yards/tons of sediment|
|Road - surfaces, ditch line erosion, fill failures||Measurement (physical survey) of rill depth, ditch line depth||cubic yards/tons of erosion|
|Sediment delivery - roads||Installation of sediment basin above vs below road or before vs after. Install crest gage.||cubic yards/tons of sediment delivered to channel.|
Size gradation under 100 count.
Bed material cross-section.
|Rate of change of channel.
Dimension, pattern, profile, and materials.
|Degradation||Install permanent cross-section, bed material particle size gradation (100 count under tape at cross-section)
Bar sample (2)
Scour chains (3)
|Rate of change in channel dimension, pattern, profile & materials. Bank height ratio (degree of incision). Rate of incision.|
|Lateral accretion/stream bank erosion||Install permanent cross-section with annual re-survey bank pins (4), toe pin/bank profile (5)||Lateral, annual erosion rates and tons/yr from stream bank erosion.|
|Enlargement.||Permanent cross-section with annual re-survey bed material gradation under tape at cross-section. (100 count)||Rate of change in channel dimension/pattern/profile and materials.|
|Riparian vegetation||Install permanent plant stratified by transects. Stream types/stream conditions.
Install permanent photo points with each transect.
|Changes in plant composition, vigar and density. Overhead and ground cover percentages|
|Sediment competence||Install scour chains under permanent cross-section.
Survey longitudal profile, install crest gage for depth of flow
Measure at high flow. Bed load sample with Helly Smith or comparable sampler. Measure discharge.
|Size of particle moved a given shear stress to entramnent verify calculations.
Largest size (Di) on bar.
Largest size caught bank full
Output bankfull discharge.
|Resistance Relations (Relative roughness, friction factors)||Measure at various stages
- energy slope
- depth of flow
- bed material of active bed
|Plot data and compare predicted versus observed values.|
|Sediment Transport||Measure suspended and bedload sediment for wide range of flows using appropriate samples.
Measure stream flow, velocity, slope, hydraulic geometry of bedload.
Measure grain size distribution of separate wash load from suspended sediment. Install staff plate, crest gage (Highest stage)
|Sediment rating curve (Both suspended and bedload).
Largest size bedload transported from a given stream power/discharge shear stress.
Total annual tons suspended and bedload
|Stream flow||This represents a longer period of record to determine time trends in stream flow change.
-Install stream gage
-Establish rating table. Measure discharge for a wide range of stages. It is often advisable to install a precipitation gage network above stream gage to relate changes in runoff to changes in precipitation.
|Measure changes in hydro graph peaks, duration timing and base flow. See USGS A6, Book 3: Gaging Streams|
- Frame and pin. A rectangular rigid frame is set on four bench marked corners. An interior wire mesh with fixed conduit pipe allows a calibrated pin to be lowered through the conduit to the ground surface. The pin depths within the fixed frame determine soil loss between runoff periods following storms or annual re-survey.
- Bar sample. (Figure 128) A core sample is taken on the downstream 1/3 of a bend on a point bar at an elevation halfway between the thalweg and the bankfull stage. The core sample generally represents the size gradation of bedload at the bankfull stage.