Using Streambank Stabilization to Generate TP and TSS Credits in Water Quality Trades Introduction The Wisconsin Department of Natural Resources (WDNR) offers watershed based alternatives for meeting requirements associated with strict effluent limits in Wisconsin Pollution Discharge Elimination System (WPDES) permits. These programs operate on the premise that pollution reductions from nonpoint sources can be achieved at lower cost than further pollutant reductions from existing point sources. This document details the necessary steps for quantifying pollutant reductions achieved from streambank stabilization. While the steps and examples are specific to water quality trading, similar methods of documentation are required for adaptive management and multi-discharger variance projects. 283.84(1m)(a), Wis. Stats. requires trades to result in water quality improvement. For WDNR to assert that this requirement has been met, pollutant reductions must be properly quantified, assigned a representative trade ratio, and accompanied by supporting documentation. For a broader overview of water quality trading, please consult the WDNR Guidance Documents: Guidance for Implementing Water Quality Trading in WPDES Permits or Water Quality Trading How-To Manual. Specific questions can be directed to your regional or statewide Water Quality Trading and Adaptive Management Coordinators .
Transcript
Using Streambank Stabilizationto Generate TP and TSS Credits in Water Quality Trades
IntroductionThe Wisconsin Department of Natural Resources (WDNR) offers watershed based alternatives for meeting requirements associated with strict effluent limits in Wisconsin Pollution Discharge Elimination System (WPDES) permits. These programs operate on the premise that pollution reductions from nonpoint sources can be achieved at lower cost than further pollutant reductions from existing point sources. This document details the necessary steps for quantifying pollutant reductions achieved from streambank stabilization. While the steps and examples are specific to water quality trading, similar methods of documentation are required for adaptive management and multi-discharger variance projects.
283.84(1m)(a), Wis. Stats. requires trades to result in water quality improvement. For WDNR to assert that this requirement has been met, pollutant reductions must be properly quantified, assigned a representative trade ratio, and accompanied by supporting documentation. For a broader overview of water quality trading, please consult the WDNR Guidance Documents: Guidance for Implementing Water Quality Trading in WPDES Permits or Water Quality Trading How-To Manual. Specific questions can be directed to your regional or statewide Water Quality Trading and Adaptive Management Coordinators.
Streambank erosion has long been identified as having negative impacts on water quality. Unnatural quantities of sediment entering streams impacts aquatic habitat and other physical characteristics. Nutrients associated with soil particles enter the stream and become available to aquatic plants and algae, ultimately contributing to eutrophication of local and downstream waters.
Erosion of streambanks is a naturally occurring process for most waterways, but human impacts can exacerbate erosion. Removal of vegetation, watershed hydrologic alteration, foot or vehicle traffic, and channel modifications can contribute to erosion.
Causes and Impacts of Streambank Erosion
Streambank Stabilization OverviewConsiderations for Project Selection
When planning streambank stabilization projects, keep the following points in mind:• Is the project upstream from the credit user? Selecting a project upstream from
the credit user location will avoid adding a downstream factor to the trade ratio.• Same HUC 12? Delivery Factor?• Other?
By implementing a project that stops streambank erosion, soil particles that would have entered the waterway now remain on land. A series of measurements and calculations quantify the mass of phosphorus or TSS kept out of the waterway. Values are reported in pounds per year, and generate credits annually for the life of the practice.
Several key steps are needed to quantify pollutant reductions:
• Identification of water body and erosion areas
• Measurement of erosion areas
• Soil P sampling and lab analysis
• Calculating Reductions
Identifying water body and erosion areas
Quantifying Pollutant Reductions
Identifying water body and erosion areas
Quantifying Pollutant Reductions
Erosion sites are most commonly identified by a lack of vegetation. This condition is merely a symptom of erosion as streambanks lacking vegetation may not be eroding, or may be eroding very slowly. This will be further addressed through quantification of recession rate. Other symptoms of erosion include:
The ideal location for streambank stabilization depends on several factors. The waterbody should be upstream from the WPDES outfall, have landowner approval, and be located in the same HUC 12 watershed. Sites with greater geographic separation may be subject to a higher trade ratio. Consultation with the local County Land and Water Conservation Department, or other environmental groups, may turn up priority areas for erosion control, with landowner contact already established.
A comprehensive erosion survey will provide the baseline data needed to plan a project. Walking a section of stream and completing a standardized streambank erosion survey form will provide the most structured approach. Beyond detailed measurements (discussed next), data fields should include: a unique site ID, centerpoint GPS coordinates, right or left bank (facing downstream), apparent cause of erosion, and vegetative condition on and above the bank.
Site Delineation
On certain streams, erosion may be concentrated in localized “pockets”, with stable bank conditions found between sites. Other streams may have a continual stretch of eroding bank, making delineation of individual sites more difficult. Each site (and associated measurements) should accurately represent the conditions at hand. A degree of variability along an eroding bank is expected, which should be addressed by averaging the height, recession rate, and other measurements. However, large scale averaging across a highly variable bank does not properly quantify pollutant reductions. When deciding where to “split or lump”, consider bank height, recession rate, soil texture, and cause of erosion. If any of these observations substantially change, it is best to split the bank into a new site. Photo documentation may be required to support large sites with averaged measurements.
Quantifying Pollutant Reductions
[2 pics demonstrating pockets and continual]
Measuring Height and Length
Quantifying Pollutant Reductions
Basic measurements required for quantifying pollutant reductions include bank height and length. As discussed previously, reasonable averaging across a site should occur, but large-scale averaging does not properly quantify pollutant reductions.
[pic for bank height w/measure
bar]
[pic for bank length w/measure
bar]
Measuring Lateral Recession Rate
Quantifying Pollutant Reductions
Lateral recession rate, or depth of bank loss, has a large impact on the amount of material entering the stream from a given site. Lateral Recession Rate is most commonly measured in feet per year, and ranges from 0 (no erosion occurring) to greater than 1 ft/year (very severe erosion). Various methods exist for measuring recession rate. These involve taking a baseline reading with survey equipment or LiDar at two points in time, and calculating bank surface differences before and after surveys. Bank pins or stakes may also be used as a benchmark for comparison of current and future conditions.
• [pic of measuring ]
Estimating Lateral Recession Rate
Quantifying Pollutant Reductions
If it is not feasible to measure lateral recession rate, this value may be estimated through various means. Vegetative indicators are moderately useful, as the most commonly recognized symptom of erosion is a lack of vegetation. A lack of vegetation does not necessarily guarantee that erosion is occurring, so additional methods should be used to quantify erosion rate. The following points should be observed and compared to the NRCS erosion severity chart below.
• Exposed Roots
• Physical deformation of the bank surface including gullies, rills, or slumping
• Bank grade steeper than what is sustainable
• Channel shape indicates active cutting
Soil Sampling for Phosphorus Concentration
Quantifying Pollutant Reductions
By measuring length, width, and lateral recession rate of an erosion site, the amount of soil entering a stream can be determined. Once the phosphorus concentration of the soil is known, the amount of phosphorus entering a stream can be calculated.
Composite soil samples must be taken from each site to represent an average phosphorus concentration. Samples should be taken from each soil horizon, ensuring that variability in soil texture is captured. Samples must be analyzed for total phosphorus (%). This is also known as total leachable P. The Bray-1 soil test is inappropriate for this purpose and will underestimate total phosphorus contributions resulting from erosion. Samples should be analyzed at a certified laboratory. More information on soil sampling is available at:
• [pic of erosion w/ layered textures and points]
A simple, mass-based equation is used to calculate phosphorus reductions from bank stabilization:
The NRCS has created a spreadsheet that estimates soil weight at an erosion site using all above variables, with the exception of soil total phosphorus (%). Standardized soil weights are pre-programmed into the spreadsheet, and are applied once a site soil texture is selected. The NRCS spreadsheet can be found here: ______INSERT LINK ON WQT TOOLS PAGE_________.
Once soil weight is calculated in the spreadsheet, it can be multiplied by soil total phosphorus (%) results from the lab. This final result represents the pounds per year of phosphorus reduction, but does not represent phosphorus credits generated. Reductions are subject to trade ratios for credit generation.
Trade Ratio Overview
Translating Pollutant Reductions to Credits
Trade ratios are used to ensure that the load reduction provided by the credit generator results in a net water quality improvement at the point of standards application. Factors affecting the trade ratio include pollutant delivery, downstream distance from credit user, pollutant equivalence, and practice uncertainty. This document will address erosion specific components of the trade ratio:
• Pollutant equivalence
• Uncertainty as it relates to the habitat adjustment
Consult the water quality trading guidance documents referenced on page 1 for an overview of all components of the trade ratio.
Pollutant equivalence for streambank stabilization
Translating Pollutant Reductions to Credits
Stabilizing streambanks results in less sediment entering the water body. The properties of the sediment may be different than the properties of the pollutant this reduction is intended to offset, requiring an equivalency rate.
Total Phosphorus: No equivalency rate required. Lab analysis of total phosphorus in sediment is comparable to analysis of total phosphorus in wastewater.
Total Suspended Solids: Sediment produced from eroding streambanks varies in particle size. Larger particles such as coarse sand and gravel will not be suspended in the water column (except at very high velocities), and therefore will not offset discharges of TSS from wastewater facilities.
Equivalence rates for TSS are based upon soil texture. For each site entered into the NRCS spreadsheet, note the soil texture, and use the adjacent table to apply the appropriate equivalence factor for TSS.
Soil TextureEquivalence Factor (TSS)
Gravel 3.00
Sand 0.90
Loamy Sand 0.67
Sandy Loam 0.43
Fine Sandy Loam 0.00
Sandy Clay Loam 0.43
Silt Loam 0.11
Silt Clay Loam 0.05
Silt Clay 0.03
Clay Loam 0.18
Organic 0.00
Habitat Adjustment
Equation for Phosphorus Reduction
Translating Pollutant Reductions to Credits
Habitat Adjustment continued
Equation for Phosphorus Reduction
Translating Pollutant Reductions to Credits
Required Standards:
All other laws/regs apply
Avoid T/E impacts, and cultural/heritage site impacts
Identify the cause of erosion
Structural Stability / Channel Compatibility
Area should be re-vegetated after construction
Streambank Projects Must not upset geomorphic processes
In-stream habitat should be retained unless specific obstructions threaten bank stability
Bank treatments must be able to withstand high flows
