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.
Aggradation	Permanent cross-section Size gradation under 100 count. Bed material cross-section. Re-survey annually. Bar sample.	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) Re-survey annually. 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 Bar re-sample 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 Bedload kg/s. Output bankfull discharge.
Resistance Relations (Relative roughness, friction factors)	Measure at various stages - energy slope - depth of flow - bed material of active bed - velocity	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). Hydraulic geometry. 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

1. 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.
2. 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.
   
   

3. Scour chains. (Figure 129) Scour chains are small diameter chains attached to a small duck-bill anchor and driven vertically into the bed at a specific location under the permanent cross-section tape. The chain is left flush with the surface and re-surveyed annually or after a large flow event to determine scour depth and entrainment sizes of bed material. Energy slope and hydraulic radius is obtained at the highest flow associated with bed scour to calculate shear stress. Back-calculation of dimensionless shear stress is obtained in conjunction with the bed material and bar material sampling along with the slope and hydraulic mean depth values.
4. Bank pins. Bank pins are smooth steel rods 4 feet in length driven horizontally into streambanks at positions corresponding to bank erosion locations. The amount of exposed pin upon re-survey is measured as the amount of lateral erosion at that site. Sod mats cannot be "pinned" nor cobble/gravel matrix bank material.
5. Toe pins. (Figure 130) Toe pins are bank offset pins driven vertically into the bed surface in order to "profile" the streambank with elevation rods and vertical measurements from the rod to the bank. The toe pin offers a permanent location with which to determine lateral erosion/unit time between surveys.

Discussion

For every prediction methodology, there is a procedure to validate the model. Some methods are more difficult and time consuming to validate than others, while some results can be determined on a short-term rather than a long-term basis. Monitoring will improve our predictive capability in the future and potentially screen out mitigation measures that would not be effective for continued implementation. Conversely, if management practices indicate that sediment and/or stability conditions create obvious impairment, revised practices or specific process-based mitigation such as restoration may be recommended. The user will gain the most confidence in the procedure only by field measurements which not only validate a prediction, but determine if the initial assessment objectives have been met.

Figure 128. Collecting a bar sample.


Figure 129. Installing a scour chain


Figure 130. Measuring the bank profile at a toe pin.