Protocol 3: Floodplain reconnection - ChesapeakeCommons/stream-restoration-model GitHub Wiki

Protocol 3 provides credit for projects that reconnect the stream channel with its natural floodplain, encouraging floodplain deposition, plant uptake and denitrification.

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

View source code here, here, and here.

Protocol 3 requires multiple input steps to derive values for downstream calculations. The sequence is organized as follows.

Calculate treatable flow credit.

Inputs

Outputs

Formulas

See design example.

existing_percent_flow_treated = (
    float(existing_treated_discharge) /
    float(existing_total_discharge)
)

proposed_percent_flow_treated = (
    float(proposed_treated_discharge) /
    float(proposed_total_discharge)
)

treatable_flow_credit = (
    proposed_percent_flow_treated -
    existing_percent_flow_treated
)

Calculate treatable loads for TSS, TN, and TP.

Calculations depend on treatable_flow_credit and load rates for each land river segment from the 2019 CAST Scenario.

Inputs

Outputs

Formulas

See design example.

tn_load = (sum(nitrogen_loads) / float(len(n_loads))) * upstream_miles

tp_load = (sum(phosphorus_loads) / float(len(p_loads))) * upstream_miles

tss_load = (sum(sediment_loads) / float(len(s_loads))) * upstream_miles

tn_treatable_load = tn_load * treatable_flow_credit

tp_treatable_load = tp_load * treatable_flow_credit

tss_treatable_load = tss_load * treatable_flow_credit

Calculate load reduction credits for TSS, TN, and TP.

Calculations depend on tn_treatable_load, tp_treatable_load, and tss_treatable_load values from step 2.

Inputs

¹ A percent of area implemented must be entered for at least one of the wetland practice categories. Credit is based on the flow volume in the total Floodplain Trapping Zone (FTZ). Allowed values are 0 to 100 and the sum of all values cannot exceed 100. Enter 0 for those wetland practices that do not apply.

Outputs

Formulas

See design example.

# Wetland pollutant removal efficiencies.

COEFF = {
    "restoration": {
        "tn": 0.42,
        "tp": 0.4,
        "tss": 0.31
    },
    "creation": {
        "tn": 0.3,
        "tp": 0.33,
        "tss": 0.27
    },
    "rehab": {
        "tn": 0.16,
        "tp": 0.22,
        "tss": 0.19
    }
}

tn = []
tp = []
tss = []

# Apply the appropriate wetland pollutant removal efficiencies.

for key, value in COEFF.items():

    tn.append(
        value['tn'] * tn_treatable_load
    )

    tp.append(
        value['tp'] * tp_treatable_load
    )

    tss.append(
        value['tss'] * tss_treatable_load
    )

tn_lbs_reduced = sum(tn)

tp_lbs_reduced = sum(tp)

tss_lbs_reduced = sum(tss)

• Project extended hyporheic zone and floodplain treatment zone boundaries must be assessed with hydrologic and hydraulic models to demonstrate whether it increases water surface elevations or has adverse downstream flooding impacts.

• Project must avoid extended ponding / inundation of the floodplain to assess 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, such as aquatic passage and potential water quality loss. 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.

• Any wetlands that fall within the boundaries of the floodplain treatment zone and are reported for credit under Protocol 3 should not also be reported using the Non-Tidal Wetlands Expert Panel.

• Project meets all of the qualifying conditions listed in the original Stream Restoration Protocol Expert Panel report outlined in Appendix B.

• 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 stream bed projects, the EHZ 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. For example, change from perennial to seasonal intermittent flow.

• Confirm the presence of legacy sediment deposits.

• Demonstrate that the design approach restores channel and floodplain connection with the hyporheic aquifer and restores processes within a hyporheic exchange zone. The EHZ is a maximum of 18 inches deep in the floodplain soil profile. When modern site constraints prevent directly connecting the restored channel and floodplain to the hyporheic aquifer, the design should include measures to interrupt flow within the hyporheic aquifer and elevate the hyporheic exchange zone into the restored floodplain.

• Project has defined EHZ boundaries across channels/floodplain.

• Legacy sediment removal is the primary floodplain restoration technique.

Restoration

Manipulate physical, and biologic characteristics of a site with the goal of returning natural/historic functions to a former wetland:

• No wetland currently exists or has been extensively degraded
• Hydric soils are present
• “prior converted”

Creation

Manipulate site characteristics to develop a new wetland that did not previously exist at the site:

• No wetland currently exists
• Hydric soils are not present
• Functional gain due to new wetland features

Rehabilitation

Manipulate site characteristics with the goal of repairing natural/historic functions to a degraded wetland:

• Wetland present
• Wetland condition or function is degraded