M E MO RA ND U M
TO:
SLRT Technical Advisory Committee (TAC)
FROM:
SLRT Development Team
SUBJECT:
SLRT Approach
DATE:
3/29/2012
STREAM LOAD REDUCTION TOOL (SLRT) OVERVIEW SLRT Objective: Develop a reliable, repeatable and cost-effective method to estimate water quality benefit of
SEZ restoration actions that is:
• Applicable to range of SEZ scales
• Inclusive of best available data, models and hypotheses of system function
• Consistent with accepted stormwater tools and programs within Lake Tahoe Basin
• Improvable and adaptable over time
Intended users: Stream restoration practitioners and design engineers; secondary users: restoration planners
Potential uses:
Lake Clarity Crediting Program; grant applications that require estimation of water quality
benefit of SEZ restoration; Tahoe Basin pollutant reduction opportunity analyses
Platform:
Excel spreadsheet [with potential to develop a custom macro-enabled spreadsheet]
Output:
Average annual load at downstream boundary of a stream or SEZ reach
Pollutants:
Current version: Fine sediment particles (FSP < 16μm) only
Rationale: FSP is the TMDL’s primary pollutant of concern. TSS will be included, but the quality
and breadth of data to calculate floodplain retention is less than FSP. Funding constraints
dictate development for a single pollutant; however the method can easily expand additional
pollutants.
Time of interest: All scenarios use hydrology from WY1999-WY2006.
Rationale: This timeframe (1) is consistent with PLRM meteorology inputs, (2) reduces
variability across users, (3) makes use of available basin-wide datasets (USGS), (4) differences in
average annual load at downstream boundary of SEZ across scenarios will dependent upon
changes in morphology and floodplain characteristics as a result of restoration and differences
of inflowing hydrology or FSP loads.
Time steps:
Frequency of occurrence rather than continuous simulation.
Rationale: Frequent low flow conditions provide insignificant signal on SEZ pollutant load. This
approach provides a significant reduction in data and processing needs.
2NDNATURE draft product CALCULATIONS The SLRT method uses the following series of equations to calculate average annual load reductions. Definitions
of each variable are provided in the table below. For simplicity only the FSP pollutant is denoted below, but
other pollutants of concern can replace FSP in any variable.
EQ1
FSP Load Reduction (MT/yr) = AA OUTfsp Pre Restoration (MT/yr) – AA OUTfsp Post Restoration (MT/yr)
EQ2
AA OUTfsp (MT/yr) = (INfsp– SFPfsp + SCEfsp)/18 yr
EQ3
INfsp =
EQ4
Qfsp (MT/day) = f(Q)
EQ5
SFPfsp =
EQ6
EQ7
DFPfsp = (Qfsp – Qfsp-cc) * ti
Rfsp = f (Q:Qcc)
EQ8
SCEfsp =
Q
t
DFP
E
t
SLRT variables and associated units.
Definition
Variable
Average annual pollutant (FSP) load estimated at the downstream extent of the
AA OUTfsp
restored SEZ reach
Total pollutant (FSP) load estimated at downstream extent of restored SEZ reach
OUTfsp
over time interval
INfsp
Qmd
FSP(Q)
Qfsp
ti
SFPfsp
Qcc
Q:Qcc
Rfsp
DFPfsp
Qfsp-cc
Q
Qmax
FPz
Dv
SCEfsp
Efsp
Units
MT/yr
MT
Total pollutant (FSP) load delivered to subject reach over time interval of interest
Mean daily discharge
FSP to discharge rating curve
Pollutant (FSP) loading rate per day delivered to SEZ as a function of discharge
Time of Qmd occurrence
(SLRT scenarios are evaluated using the hydrologic record between 10/1/88 and
9/31/06).
MT
cfs
equation
MT/day
Pollutant (FSP) load stored on the floodplain over time interval
Channel capacity discharge
Ratio of discharge to the channel capacity discharge of SEZ of interest
Pollutant (FSP) retention coefficient expressing the fraction of the pollutant that
is delivered to the floodplain that is retained. Ri is expressed as a fraction (0‐1) of
the total DFPfsp.
Pollutant (FSP) load delivered to the floodplain as a function of discharge
Pollutant (FSP) daily loading rate for the channel capacity discharge
Instantaneous discharge
Maximum mean daily discharge over the time interval
Average water depth on floodplain for a given Q
% of floodplain cross sectional flow path occupied by vegetation for a given
floodplain water depth (FPz)
MT
cfs
unitless
Total pollutant (FSP) load generated from stream channel erosion over time
interval
Total pollutant (FSP) load generated from stream channel erosion as a function
of discharge
2NDNATURE draft product days
%
MT
MT/day
cfs
cfs
ft
%
MT
MT