IDEAL URBAN HYDROLOGY, SEDIMENTOLOGY AND WATER QUALITY SPREADSHEET MODEL

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IDEAL URBAN HYDROLOGY, SEDIMENTOLOGY AND WATER
QUALITY SPREADSHEET MODEL

• Greenville County Greenville, SC
• August 22, 2002
• Billy J. Barfield
• John C. Hayes

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Modeling Framework For Watershed
Pervious and Unconnected Impervious
Directly Connected Impervious
Impervious
Veg Buffer/ Filter
Veg Buffer/ Filter
Imp
Dry/Wet Detention Basin
Outflow From Watershed
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GENERAL APPROACH

• Each element is modeled with approximations to
  state-of-the-art procedures
• In many cases, used generated data to develop
  explicit prediction equations that match more
  complex trial and error procedures

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RAINFALL INPUTS

• Rainfall is driving force
• Amount of runoff for a given rainfall depends on
• Soil
• Cover
• Antecedent moisture which depends on season and
  recent rainfall
• Model develops predictions for an average storm,
  using statistical averages

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Modeling Rainfalland Antecedent Moisture
12 Storms 0.25 to 10.5
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Rainfall Interactions With Runoff, etc
Runoff
Sediment
Nutrients
Pathogens
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Modeling Rainfall
12 Storms 0.25 to 10.5
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Looking At Rainfallon theSpreadsheet

• Located in the Storm Data Worksheet
• Not Viewed by User

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User Inputs
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Modeling Rainfalland Antecedent Moisture
Condition (AMC)

• Runoff for a given rainfall depends on antecedent
  moisture
• Rainfall required for a given antecedent moisture
  depends on season
• Developed probabilities for rainfall occurring in
  a given season and for a given AMC

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Modeling Rainfalland Antecedent Moisture
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Modeling Runoff
Runoff
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Runoff Inputs

• Areas and Land Use
• Hydrologic Parameters
• Curve Number for each land use
• Time of concentration for each area

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Modeling Framework For Watershed
Pervious and Unconnected Impervious
Directly Connected Impervious
Impervious
Imp
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MODELING RUNOFFVolume

• NRCS Curve Number Method

Curve Number Depends on Land Use, Hydrologic Soil
Group, and Antecedent Moisture
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Looking At Runoffon theSpreadsheet

• Located in the Qqp Worksheet
• Not Viewed by User

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Information Only
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MODELINGPEAK DISCHARGE

• NRCS TR55 Method

Q Runoff Volume (in) Aw Area (mi2)
Qu given in graphical form, but was parameterized
for this model
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Looking At Peak Dischargeon theSpreadsheet

• Located in the Qqp Worksheet

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MODELING SEDIMENT YIELDPervious Areas

• MUSLE

The MUSLE is only used for pervious areas.
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MODELING SEDIMENT YIELDImpervious Areas
EMC Approach

• EMC varies with type of impervious area
• Modeling dependability improves as local data is
  collected

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Looking At Sediment Yieldon theSpreadsheet

• Located in the Sediment Concentration, Sediment
  Yield, And Trapping Worksheet

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MODELING SEDIMENT SIZE DISTRIBUTION
Why is it important?

• Size determines sediment trapping in VFS and in
  pond
• Nutrients and bacteria are absorbed on the
  exchange phase of the clay particles, hence need
  to know concentration of clay size particles

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MODELING SEDIMENT SIZE DISTRIBUTION
Types of Particles in Eroded Sediment

• Primary - basic clay, silt and sand broken down
  to the single particles
• Aggregates - particles made of multiple primary
  particles bonded together by clay or organic
  matter
• Both are present in eroded material from pervious
  areas
• Only primary particles are assumed to be washed
  from impervious areas

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MODELING ERODED SEDIMENT SIZE DISTRIBUTION
Pervious Areas

• Model Uses the CREAMS Equations to predict
  fraction of particles in the following size
  classes

• Predictions are based on fractions of original
  silt, sand and clay in the parent soil

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MODELING PERCENT CLAY IN ERODED PARTICLES
Pervious Areas

• Aggregates contain clay particles used to cement
  the silt and sand particles together
• Model Uses the CREAMS Equations to predict
  fraction of clay in the aggregates

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MODELING SEDIMENT SIZE DISTRIBUTIONImpervious
Areas

• Based on data from NURP study, have the following
  input values

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Looking At Eroded Size Distributionon
theSpreadsheet

• Located in the Ero Prtcle Size Worksheet

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MODELING NUTRIENTS

• Yield based on event mean concentrations (EMCs)
  for each chemical

• EMCs vary slightly based on land use
• Total Phosphorus 0.1 - 0.4 mg/l
• Total Nitrogen 1.6 - 2.0 mg/l

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MODELING INDICATOR BACTERIA

• Yield based on event mean concentrations (EMCs)
  for bacteria

• EMCs highly variable
• National average 15000 number/100ml
• Depends a great deal on presence of wildlife,
  leaky sewers, etc

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Looking At Nutrient And Bacteria Generation
Calculations on Spreadsheet

• Located in the Polnt Ldng Trpng Worksheet

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Modeling the Impact of Buffer Strips/VFS
Pervious and Unconnected Impervious
Directly Connected Impervious
Impervious
Veg Buffer/ Filter
Veg Buffer/ Filter
Imp
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FLOW HYDRAULICS IN VFS
Infiltration Rate
Flow Velocity
Flow Depth
The important hydraulic parameters
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Hydraulic Inputs for VFS

• Type vegetation
• Roughness (Mannings n)
• Density Ss (spacing of vegetated media)
• Slope S (ft/ft)
• Infiltration rate (iph)

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Hydraulic Calculations for VFS

• Velocity - Mannings Equation

• Equations are solved to determine velocity V and
  depth of flow df

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Hydraulic Calculations for VFS

• Infiltration and Outflow Volume

• Peak inflow rate modified for reduction in flow
  volume to get peak outflow rate, using triangular
  hydrograph approximation

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Looking At Flow Through Filter Strip on
Spreadsheet

• Located in the VFS Hyd Sed Worksheet

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Sediment Trapping in VFS
Tei Trap eff for particle i ReReynolds
Number NfFall Number RsSpacing Hyd
Radius VsPartcle Settling Velocity Fctn
(particle dia) LfFlow Length dfFlow
Depth VFlow Through Velocity

• Trapping by settling

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Sediment Trapping in VFS

• Trapping by infiltration
• Infiltrating water carries sediment into soil

Msi Mass of sediment infiltrated QinfVolume of
water infiltrated Cs,avgAvg sed conc on filter
strip
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Looking At Sed Trapping in Filter Strip on
Spreadsheet

• Located in the VFS Hyd Sed Worksheet

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Nutrient Trapping in VFS

• Trapping by settling of particulate nutrients
• Trapping by settling of nutrients sorbed to
  active clay
• Trapping by infiltration into VFS

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Nutrient Trapping in VFSTrapping by Settling of
Particulates

• Particulates particles of nitrogen or
  phosphorus that are blown in or fall from
  atmospheric dryfall
• Probably 33 percent of EMC for nitrogen and
  phosphorus

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Nutrient Trapping in VFSTrapping by Settling of
Particulates
FN/PS Fraction EMC that is
particulates YN/PYield of N/P to VFS

• MSN/P assumed to be clay sized particles
• Must be distributed among clay fraction in all
  particle classes

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Nutrient Trapping in VFSTrapping by Settling of
Particulates
Mass of Particulate Nutrient Trapped Equals the
Sum of

• Fraction in size class Fi
• Times fraction of size class that is clay sized
  particles, CFi
• Times fraction of CFi that is particulate
  nutrient, FCNSI
• Times the sediment yield, Y

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Nutrient Trapping in VFSNutrients Sorbed to Clay

• Use an isotherm approach

Actual isotherm
Linear isotherm CsKCl ltCs,max
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Nutrient Trapping in VFSNutrients Sorbed to Clay

• Trapping by settling calculated from mass of clay
  particles trapped in VFS

Mnut,s Mass of nutrient trapped by settling
(lbs) Mclay,s Mass of clay trapped by settling
(lbs) Cs Concentration of nutrient on
clay (mg/g) Cnst Constant to convert units
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Nutrient Trapping in VFSby Infiltration

• Trapping by infiltration
• Infiltrating water carries nutrients into soil

Msi Mass of nutrient infiltrated QinfVolume of
water infiltrated Cs,avgAvg nutr conc on filter
strip
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Bacteria Trapping in VFS

• Trapping by settling calculated from mass of clay
  particles trapped in VFS

Nnut,s Number of bacteria trapped by settling
Mclay,s Mass of clay trapped by settling
(lbs) Cbact Concentration of bacteria on
clay (no./g) Cnst Constant to convert units
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Bacteria Trapping in VFS

• Trapping by infiltration
• Infiltrating water carries bacteria into soil

Nsi Number of bacteria infiltrated QinfVolume
of water infiltrated Cbac,avgAvg number conc
filter strip
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Looking At Nutrient and Bacteria Trapping in
Filter Strip on Spreadsheet

• Located in the Polnt Ldng Trpng Worksheet

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Modeling Trapping in Ponds
Pervious and Unconnected Impervious
Directly Connected Impervious
Impervious
Veg Buffer/ Filter
Veg Buffer/ Filter
Imp
Dry/Wet Detention Basin
Outflow From Watershed
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Inputs for Pond

• Ponds (dry and wet detention)
• Outlet types, sizes, crest elevations,
  hydraulic constants
• Drop inlet
• Orifice
• Weir
• Emergency spillway
• Stage and area information
• Average interval (hrs) between storms for wet
  detention

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Pond Hydraulics

• Solve continuity equation

• Storage and outflow depend on stage
• Non-linear relationships
• Solved by iteration for peak outflow

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Looking At Pond Hydraulics on Spreadsheet

• Located in the Pond Hydr worksheet

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Sediment Trapping in PondsDry Detention

• Modified overflow rate equations using peak
  discharge as the flow rate

A Surface area of pond

• or

A Pond Inefficiency Parameter
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Sediment Trapping in PondsDry Detention -
Continued

• Calculated for each class of particles and for
  clay fraction within aggregates
• Overall trapping efficiency calculated by

Fi Fraction particles of size i
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Looking At Pond Sediment Trapping on
SpreadsheetDry Detention

• Located in the Pond Sed worksheet

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Nutrient Trapping in PondsDry Detention

• Based on settling of particulate nutrients,
  trapping of nutrients sorbed on clay as defined
  by isotherms

Cs Concentration of nutrient on clay
fraction in mg/g
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Looking At Pond Nutrient Trapping on
SpreadsheetDry Detention

• Located in the Polnt Ldng Trpng worksheet

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Bacteria Trapping in PondsDry Detention

• Trapping by settling
• Based on trapping of clay and isotherms

Cbac,s Concentration of bacteria on clay
fraction in number/g
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Bacteria Trapping in PondsDry Detention

• Death by natural mortality
• Based on temperature and residence time in pond
• Input parameter is temperature

• r is death rate (number/day)
• T is temp in deg C

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Mortality Due to Light Penetration

• Based on light intensity, penetration into pond,
  and duration of light penetration

Rbac,light mortality due to light penetration
(No./day) Io Light intensity
(ly/day) ke Light penetration
constant 0.55 CTSS H Depth of water
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Looking At Pond Bacteria Trapping on
SpreadsheetDry Detention

• Located in the Polnt Ldng Trpng worksheet

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Sediment Trapping in PondsWet Detention

• Uses the same calculations as dry detention for
  stormflow
• Between stormflow, calculates removal rates due
  to settling for each particle class, using
  settling velocity and surface area
• Corrects for variations in inter-arrival times
  between storms by using a probabilistic approach
• Modification of procedure proposed by EPA to
  evaluate wet detention

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Nutrient and Bacteria Trapping in PondsWet
Detention

• Uses same calculations as dry detention for
  stormflow
• For periods between stormflow, calculates
  trapping based on settling of particulates, the
  mass of nutrients on clay trapped during the same
  period

Cnut/bac,s Concentration of nutrient or
bacteria on clay fraction
calculated using an isotherm
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Looking At Pond Sediment, Nutrient and Bacteria
Trapping on SpreadsheetWet Detention

• Located in the Sed Conc, Yield and Trap worksheet

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What is Missing?

• Does not account for re-suspension,
  remobilization, of sediment or nutrients
• Does not account for growth of bacteria in the
  deposited sediment or in moist areas in VFS
• Does not account for the deposition of bacteria
  by feces of animals attracted to the VFS or pond
• Does not account for denitrification that may
  occur in deposited sediments
• Does not consider bioswales

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Summary

• Presented a process based model for runoff,
  sediment, nutrients, and bacteria
• Predicts sediment by EMC for impervious areas and
  MUSLE for pervious areas
• Predicts nutrient and bacteria yield by
  particulate matter, sorbed on clay by isotherms,
  using clay content of the sediment
• Routes through VFS using the KY VFS model
• Routes through ponds using the modified overflow
  rate, combined with the SC design aid approach
• Predicts nutrient and bacteria removal using
  fraction of particulate matter and isotherms
  (values are needed)