Protocol 4: Stormwater performance standard - ChesapeakeCommons/stream-restoration-model GitHub Wiki

This protocol is also referred to as "Dry channel regenerative stormwater conveyance".

Protocol 4 is for zero-order channels with intermittent flow and is credited as a stormwater retrofit practice. These practices occur in the urban drainage zone to directly capture upland runoff or are used at the stormwater outfall to capture and treat stormwater in the headwater transition zone.

View source code.

Protocol 4 requires multiple input steps to derive values for downstream calculations. The sequence is organized as follows.

Step 1

Retrieve the land river segment(s) that intersect the practice footprint.

Step 2

Retrieve list of eligible load source(s) for the selected land river segment(s). This module draws from an abbreviated set of developed load sources. Note that the set does not include "CSS Construction" or "Regulated Construction".

Step 3

Pass values to model.

Input group

¹ Ponding depth (ft) = surface volume storage (ft) + (filter media layer depth (ft) * porosity). ↵

See design example.

# Function mapping

FUNCS = {
    'rr': {
        'tn': lambda x: (0.0308 * x ** 5) - (0.2562 * x ** 4) + (0.8634 * x ** 3) - (1.5285 * x ** 2) + (
                1.501 * x) - 0.013,
        'tp': lambda x: (0.0304 * x ** 5) - (0.2619 * x ** 4) + (0.9161 * x ** 3) - (1.6837 * x ** 2) + (
                1.7072 * x) - 0.0091,
        'tss': lambda x: (0.0326 * x ** 5) - (0.2806 * x ** 4) + (0.9816 * x ** 3) - (1.8039 * x ** 2) + (
            1.8292 * x) - 0.0091
    },
    'st': {
        'tn': lambda x: (0.0152 * x ** 5) - (0.1310 * x ** 4) + (0.4581 * x ** 3) - (0.8418 * x ** 2) + (
                0.8536 * x) - 0.0046,
        'tp': lambda x: (0.0239 * x ** 5) - (0.2058 * x ** 4) + (0.7198 * x ** 3) - (1.3229 * x ** 2) + (
                1.3414 * x) - 0.0072,
        'tss': lambda x: (0.0304 * x ** 5) - (0.2619 * x ** 4) + (0.9161 * x ** 3) - (1.6837 * x ** 2) + (
            1.7072 * x) - 0.0091
    }
}

# Calculate runoff storage volume (acre feet)

runoff_storage_volume = footprint_area * ponding_depth

treatment_depth = runoff_storage_volume * 12

inches_treated = adjust_inches_treated(
    treatment_depth / impervious_acres
)

tn_pct_reduced = FUNCS[mode]['tn'](inches_treated)

tp_pct_reduced = FUNCS[mode]['tp'](inches_treated)

tss_pct_reduced = FUNCS[mode]['tss'](inches_treated)

# Calculate load reductions

sediment_loads = []
nitrogen_loads = []
phosphorus_loads = []

for segment in segments:

    rate_query_result = object # Load rates for land river segment from database.

    load_rate = rate_query_result.load_rate

    nitrogen_loads.append(rate_query_result.n / load_rate)

    phosphorus_loads.append(rate_query_result.p / load_rate)

    sediment_loads.append(rate_query_result.tss / load_rate)

# Perform the following for each load reduction.

# Load rate (pounds/acre)

segment_avg_load = sum(loads) / Decimal(len(loads))

# Pre-restoration load (pounds/acre)

pre_load = segment_avg_load * source_acres

lbs_reduced = pre_load * Decimal(load_pct_reduced)

Runoff reduction

Total post-development runoff volume that is reduced through canopy interception, soil amendments, evaporation, rainfall harvesting, engineered infiltration, extended filtration or evapo-transpiration. Stormwater practices that achieve at least a 25% reduction of the annual runoff volume are classified as providing runoff reduction, and therefore earn a higher net removal rate.

BMPs to use with this module include landscape restoration/reforestation, riparian buffer restoration, rooftop disconnection (to amended soils, conservation area, or pervious area), sheetflow to filter/openspace, non-structural BMPs (chapter 5 of 2006 Pennsylvania Stormwater BMP manual), all ESD practices in MD 2007, bioretention/rain garden, dry channel regenerative stormwater conveyance, dry swale, expanded tree pits, grass channels (with soil amendments, aka bioswales), green roofs, green streets, infiltrations practices (infiltration basin, infiltration bed, infiltration trench, dry well, landscape infiltration), permeable pavement, and rainwater harvesting.

Estimated reductions calculated using acres treated by the practice.

Stormwater treatment

Total post-development runoff volume that is reduced through a permanent pool, constructed wetlands or sand filters have less runoff reduction capability, and their removal rate is lower than runoff reduction.

BMPs to use with this module include contructed wetlands, filtering practices (e.g. constructed filters, sand filters, stormwater filtering systems), proprietary practices (manufactured BMPs), wet ponds, and wet swales.

Estimated reductions calculated using acres treated by the practice.

• Eligible runoff reduction practices are: landscape restoration/reforestation; rooftop disconnection; sheetflow to filter/open space; all ESD practices in MD 2007; dry swale; expanded tree pits; grass channels; green roof; green streets; infiltration; rainwater harvesting; landscape restoration/reforestation.

• Eligible stormwater treatment practices are: constructed wetlands; filtering practices; proprietary practices; wet ponds and wet swales.