Protocol 2: Hyporheic exchange - ChesapeakeCommons/stream-restoration-model GitHub Wiki

Protocol 2 provides credit for projects that include design features to promote denitrification during base flow.

See complete 2020 Consensus Recommendations to Improve Protocols 2 and 3 for Defining Stream Restoration Pollutant Removal Credits.

View source code.

Inputs

Name Type Description
floodplain_sq_ft float/integer Area of the restored floodplain measured in square feet (ft2).
channel_sq_ft float/integer Area of the restored channel measured in square feet (ft2).
brf float/integer Baseflow reduction factor. (See Appendix A: Site-specific reduction factors.)
fhf float/integer Floodplain height factor. (See Appendix A: Site-specific reduction factors.)
acrf float/integer Aquifer conductivity reduction factor. (See Appendix A: Site-specific reduction factors.)

Outputs

Name Type Description
tn_lbs_reduced float Annual reduction in pounds of total nitrogen.

Formulas

See design example.

# Apply denitrification rate.

floodplain_tn = floodplain_sq_ft * 0.00269

channel_tn = channel_sq_ft * 0.00269

# Apply site-specific discount factors.

total_floodplain_tn = brf * fhf * acrf * floodplain_tn

total_channel_tn = brf * fhf * acrf * channel_tn

# Calculate total nitrate removed.

tn_lbs_reduced = total_floodplain_tn + total_channel_tn

Appendix A: Site-specific reduction factors

Site specific discount factors for adjusting the denitrification rate (Parola et al, 2019)

Effective hyporheic zone N credit = (Base rate) (EHZ) (Bf) (Hf) (Af)

Baseflow reduction factor Bf Floodplain height factor1 Hf Aquifer conductivity reduction factor2 Af
Perennial baseflow 1.0 0 - 0.75 ft 1.0 cobbly gravel, gravel, gravelly sand, sand and peat 1.0
Baseflow in all but late summer/fall 0.75 0.76 ft - 1.00 ft 0.75 gravelly silt, silty sand or loamy sand, sandy loam, and organic silt with no coarse material layer connected to the streambed 0.60
Baseflow in winter/spring 0.50 1.01 ft - 1.25 ft 0.50 clayey gravel, sandy silt, sandy clay loam, loam, silt loam, and silt with no coarse material layer connected to the streambed 0.40
Baseflow only during wet seasons 0.25 1.26 ft - 1.50 ft 0.10 sandy clay, clay loam, silty clay loam, organic clay with no coarse material layer connected to the streambed 0.10
Flow only during runoff events 0.10 > 1.50 ft 0.00 silty clay and clay with no coarse material layer connected to the streambed 0.01

1 The floodplain height factor is determined by the restored floodplain height (Hf) above the streambed riffle elevations or low flow water surface elevations. Additional streambed feature elevations, like those at a run in sand bed channels or streambeds comprised of silty clay, also may be used to determine the restored floodplain height. Low base-flow (lowest 10% of flows) could also be used as a suitable alternative. ↵

2 This refers to an aquifer capacity factor based on the dominant materials within the streambed and below the floodplain soil of the EHZ. Where coarse grain aquifer layers are not directly connected to the channel, the factor should be determined based on the soil texture at the elevation of the streambed using NRCS standard texture classifications (Schoeneberger, et al, 2012). ↵

"Base rate" is the mean areal floodplain denitrification rate (lbs/sq foot/yr), as recommended by Group 4.


Appendix B: Qualifying conditions

  • Project must meet applicable floodplain management requirements in the stream corridor. Any individual stream restoration project should be assessed with hydrologic and hydraulic models to demonstrate whether it increases water surface elevations or has adverse downstream flooding impacts. In general, these analyses are based on design storm events and flood risk conditions established by the appropriate local or state floodplain management agency (e. g., the 100-year storm event).

  • Project must evaluate the duration of floodplain ponding in the context of the restoration goals. Micro pools and long-duration ponding of water on the floodplain is essential for amphibian habitat, but large open water features may adversely impact the desired riparian vegetative community. In evaluating a potential restoration site and design, consider the potential adverse effects of extended open water ponding based on the soil characteristics, plant community, amphibian and other aquatic habitat goals.

  • Project must demonstrate consideration of potential unintended consequences of the restoration. The project should document that a site impairment exists and that the interventions or restoration work proposed are appropriate to address the impairment. The proposed design should demonstrate that a positive ecological functional uplift (or change) for the stream and associated riparian system will result.

  • Project must demonstrate that it either provides, or is tied into existing upstream and downstream grade controls to ensure the project reach can maintain the intended stream access to the floodplain.

  • Project must clearly define the boundary of the effective hyporheic zone. For Floodplain Restoration and Raising the Streambed projects the effective hyporheic zone is a maximum of 18 inches deep in the floodplain soil profile, and extends only to those areas that are regularly inundated after the streambed is raised. The actual dimensions must be confirmed by site investigations that define stream flow conditions, root zones, aquifer conditions and the pre-project water table conditions.

  • Project must demonstrate that baseflow conditions are not reduced as a result of the restoration (ex. change from perennial to seasonal intermittent flow).

  • Presence of legacy sediment deposits or other floodplain impairment. Legacy sediments must be present in the project reach to a depth that has impaired aquatic ecosystem function. Legacy sediment includes any deposits that have occurred since European settlement, including very recent sediment deposits, often created by features such as mill dams, road embankments, floodplain fill and other kinds of stream corridor impairment.

  • Floodplain connection to valley bottom aquifer. The design objective is to restore a plant/groundwater connection within the floodplain, so that most of the root mass of the floodplain vegetation is in direct contact with the underlying hyporheic aquifer. In cases where the historic hyporheic aquifer cannot be accessed due to modern controls (i.e., culverts or utility crossings), the objective is to plug the flow of the underlying aquifer so as to create a new hyporheic zone using cobbles, gravel and/or sandy materials.

  • Defined boundaries for the channel(s), floodplain and valley bottom. The restored channel and floodplain dimensions are based on field testing that define the key vertical and lateral sediment boundaries of the existing floodplain and the hyporheic aquifer beneath it.

  • Removal of legacy sediments is the primary means to restore floodplain reconnection at most sites.

  • Meet applicable floodplain management requirements in the stream corridor. Any individual stream restoration project should be assessed with hydrologic and hydraulic models to demonstrate whether it increases water surface elevations or adverse downstream flooding impacts. In general, these analyses are based on design storm events and flood risk conditions established by the appropriate local or state floodplain management agency (e.g., the 100-year storm event).

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