Stream Restoration Load Reduction Estimates (Protocols 1 - 5)

A Unified Guide for Crediting Stream and Floodplain Restoration Projects in the Chesapeake Bay Watershed (September 17, 2021) provides the scientific basis for this model. The guidance includes five crediting protocols: prevented sediment (Protocol 1), hyporheic denitrification (Protocol 2), floodplain pollutant trapping (Protocol 3), dry channel RSC as a stormwater retrofit (Protocol 4), and outfall and gully stabilization (Protocol 5).

Land-river segment data and loading rates are sourced from the Chesapeake Assessment Scenario Tool (CAST) Phase 6 model, maintained by the Chesapeake Bay Program.

Protocol 1: Prevented Sediment

Awards credit for stream restoration projects that stabilize banks and prevent sediment erosion in degrading channels. Most projects will be located on first- to third-order streams, but fourth and fifth order streams may also qualify. Inputs include bank measurements (bulk density, erosion rate, length, height) and nutrient concentrations.

tss_lbs_reduced = bulk_density_of_soil * bank_erosion_rate *
                  eroding_bank_length * eroding_bank_height

tn_lbs_reduced = (tss_lbs_reduced / 2000.0) * nitrogen_concentration
tp_lbs_reduced = (tss_lbs_reduced / 2000.0) * phosphorus_concentration

# Apply 50% efficiency factor (expert panel baseline)
tss_lbs_reduced = tss_lbs_reduced * 0.50
tn_lbs_reduced = tn_lbs_reduced * 0.50
tp_lbs_reduced = tp_lbs_reduced * 0.50

All proposed implemention with greater than >60% design plan in place must fill out the higher resolution input table.

Protocol 2: Hyporheic Denitrification

Provides credit for stream restoration projects with design features that promote denitrification during base flow conditions. Inputs include restored floodplain and channel areas (sq ft), plus site-specific discount factors.

floodplain_tn = floodplain_sq_ft * 0.00269
channel_tn = channel_sq_ft * 0.00269

total_floodplain_tn = brf * fhf * acrf * floodplain_tn
total_channel_tn = brf * fhf * acrf * channel_tn

tn_lbs_reduced = total_floodplain_tn + total_channel_tn

Site-Specific Discount 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 factor (Hf)	Aquifer conductivity reduction factor (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

Protocol 3: Floodplain Pollutant Trapping

Awards credits for projects reconnecting streams with their floodplains, facilitating deposition, plant uptake, and denitrification processes. The expert panel methodology uses a six-step framework; FieldDoc implements the calculation as treatable flow credit, treatable loads, and wetland pollutant removal efficiency.

Treatable Flow Credit

treatable_flow_credit = post_restoration_treatable_flow
                       - pre_restoration_treatable_flow

# Treatable flow percentages are determined externally
# via H&H modeling (e.g. HEC-RAS) per expert panel guidance.

Treatable Loads

tn_load = (avg nitrogen_loads) * upstream_miles
tp_load = (avg phosphorus_loads) * upstream_miles
tss_load = (avg sediment_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

Wetland Pollutant Removal Efficiency

tn_lbs_reduced = sum(efficiency * tn_treatable_load for each category)
tp_lbs_reduced = sum(efficiency * tp_treatable_load for each category)
tss_lbs_reduced = sum(efficiency * tss_treatable_load for each category)

Pollutant Removal Efficiency Coefficients (Table 8):

• NTW Restoration (return natural functions to a former wetland where hydric soils are present): TN: 0.42, TP: 0.40, TSS: 0.31

• NTW Creation (develop a new wetland on a site where hydric soils are not present): TN: 0.30, TP: 0.33, TSS: 0.27

• NTW Rehabilitation (repair natural functions in a degraded existing wetland): TN: 0.16, TP: 0.22, TSS: 0.19

Protocol 4: Dry Channel RSC as a Stormwater Retrofit

Applies to qualifying dry channel regenerative stormwater conveyance projects in ephemeral channels within the headwater transition zone. The protocol uses retrofit rate adjustor curves from the Stormwater Retrofit Expert Panel to compute annual nutrient and sediment reduction rates for the contributing drainage area.

Calculation Modes:

• Runoff Reduction (RR): Practices that reduce runoff volume through infiltration, evapotranspiration, or harvest. Includes bioretention, permeable pavement, green infrastructure, and infiltration practices.

• Stormwater Treatment (ST): Practices that treat runoff without significant volume reduction. Includes constructed wetlands, filtering practices, and wet ponds.

The methodology uses adjustor curves to determine removal percentages based on the depth of rainfall captured over the contributing impervious area. Runoff storage volume is calculated by multiplying footprint area by ponding depth, then converting to inches for treatment depth analysis. Load reductions derive from segment-specific rates multiplied by removal percentages.

Protocol 5: Outfall and Gully Stabilization

Constructs a stable channel that dissipates energy from storm drain outfalls to the stream network. The channel design achieves equilibrium to minimize future sediment loss. Projects may only be applied within the headwater transition zone in ephemeral channels experiencing active headcutting or severe vertical erosion.

The expert panel methodology describes a multi-step process involving 3D surface modeling, equilibrium slope calculations, and bulk density conversion. These calculations are performed outside FieldDoc; enter the computed TSS, TN, and TP reduction totals directly.

Acronyms

BANCS	Bank Assessment for NPS Consequences of Sediment
BEHI	Bank Erosion Hazard Index
BMP	Best Management Practices
CAST	Chesapeake Assessment Scenario Tool
CBP	Chesapeake Bay Program
CBWM	Chesapeake Bay Watershed Model
EHZ	Effective Hyporheic Zone
FR-LSR	Floodplain Restoration: Legacy Sediment Removal
FR-RSB	Floodplain Restoration: Raising the Stream Bed
FRV	Floodplain Reconnection Volume
FTZ	Floodplain Trapping Zone
HA	Hyporheic Aquifer
HEZ	Hyporheic Exchange Zone
HTZ	Headwater Transition Zone
LEC	Limited Erosion Control
LSR	Legacy Sediment Removal
MS4	Municipal Separate Storm Sewer System
NBS	Near Bank Stress
NCD	Natural Channel Design
NTW	Non-Tidal Wetland
OGSP	Outfall and Gully Stabilization Projects
RR	Runoff Reduction
RSC	Regenerative Stormwater Conveyance
RTVM	Reporting, Tracking, Verification and Monitoring
ST	Stormwater Treatment
TMDL	Total Maximum Daily Load
TN	Total Nitrogen
TP	Total Phosphorus
TSS	Total Suspended Solids

Attribution

The Stream Restoration Load Reduction model estimates nutrient and sediment reductions using data and algorithms from the Chesapeake Bay Program's Unified Guide to Crediting Stream and Floodplain Restoration Practices. The Expert Panel was co-chaired by the Center for Watershed Protection and the Chesapeake Stormwater Network.