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d Chapter 8Pollutant Loading CalculationsTis chapter describes the Simple Method or determining pre- and post-development pollutant loadings rom a project. It provides the event

mean concentrations (EMCs) o pollutants or each land use type, and thepollutant removal eciencies o various BMPs. Tis chapter also providesa link to the NHDES Simple Method Spreadsheet. Detailed calculationsand guidance on running the Simple Method are described in the NHDESInterim Guidance or Estimating Pre- and Post-Development StormwaterPollutant Loads, available rom NHDES.

At this time, NHDES requires pollutant loading analysis to include onlytotal suspended solids (SS), total phosphorus (P), and total nitrogen(N). Although projects may have other pollutants o concern, SS, P,and N will be used as surrogates or other parameters until such time that

additional data becomes available to suciently and condently model otherparameters.

8-1. Te Simple Method

Many models are available to estimate pre- and post-development pollutantloads, such as the SEPL, AVGWLF, WINNSLAMM, and the P8 UrbanCatchment Model. Each model varies in strengths and weaknesses. NHDESis reviewing various models and will consider the use o other models iproposed. At this time, however, NHDES recommends using the SimpleMethod, a spreadsheet based calculation, or comparing pre-development

to post-development pollutant loads. Tis section has been adapted rom theStormwater Centers Te Simple Method to Calculate Urban StormwaterLoads and describes the Simple Method loading analysis.

Simple Method Calculations

Te Simple Method estimates pollutant loading o stormwater runo orurban and developing areas. Te technique is recommended by NHDESbecause o the modest amount o inormation it requires, which includessubwatershed drainage area and impervious cover, annual precipitation, andstormwater runo pollutant concentrations.

Calculation or Chemical Constituents

Te Simple Method estimates pollutant loads or chemical constituents asa product o annual runo volume and pollutant concentration using theollowing equation:

L = 0.226 * R * C * A

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Where:L = Annual loads (lbs)R = Annual runo (inches)C = Pollutant concentration (mg/l)A = Area (acres)0.226 = Unit conversion actor

Calculation or Bacteria

For bacteria, the equation is dierent, to account or the dierence in units.Te equation or bacteria is:

L = 1.03 * 10-3 * R * C * A

Where:L = Annual loads (Billion Colonies)R = Annual runo (inches)C = Bacteria concentration (col/100ml)A = Area (acres)

1.03 * 10-3 = Unit conversion actor

Calculation or Annual Runo

Te Simple Method calculates annual runo as a product o annual runovolume, and a runo coecient (Rv). Runo volume is calculated as:

R = P * Pj

* Rv

Where:R = Annual runo (inches)P = Annual rainall (inches)P

j= Fraction o annual rainall events

that produce runo (usually 0.9)Rv = Runo coecient

Calculation or Runo Coefcient

In the Simple Method, the runo coecient is calculated based onimpervious cover in the subwatershed. Tis relationship is shown in Figure8-1. Although there is some scatter in the data, watershed imperviousnessdoes appear to be a reasonable predictor o Rv. Te ollowing equationrepresents the best t line in the dataset (N = 47, R2 = 0.71).

Rv = 0.05 + 0.9Ia

Where:Rv = Runo coecientI

a= Percent impervious area draining to the structure in decimal orm

Te Simple Method uses dierent impervious cover values or separateland uses within a subwatershed. Although representative impervious

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d cover data is available,DES recommends usingimpervious cover dataspecic to an individualproject site. owns mayhave detailed imperviouscover inormation i

they maintain a detailedland use/land cover GISdatabase, or imperviouscover can be measureddirectly rom the site orsite plans. Subdivisionplans typically do notinclude the exact size andlocation o the imperviouscover associated withrootops, driveways, andother impervious areas on

individual lots. Tese areas should be included in the Simple Method analysisby assigning an average house size and driveway length and width or eachlot. In addition, the sub-drainage areas to which each lot drains should beestimated.

Limitations to the Simple Method

Te Simple Method should provide reasonable estimates o changes inpollutant export resulting rom development activities. However, theollowing should be kept in mind when applying this method:

Te Simple Method is most appropriate or assessing and comparingthe relative stormfow pollutant load changes o dierent land useand stormwater management scenarios. Te Simple Method providesestimates o pollutant export that are probably close to the true butunknown values or a development site, watershed, or subwatershed.However, it is very important not to over emphasize the precisiono the results obtained. For example, it would be inappropriate touse the Simple Method to evaluate relatively similar developmentscenarios (e.g., 34.4% versus 36.9% impervious cover).

Te Simple Method provides a general planning estimate o likelypollutant export rom areas at the scale o a development site,watershed, or subwatershed. More sophisticated modeling may beneeded to analyze larger and more complex watersheds.

Te Simple Method only estimates pollutant loads generated duringstorm events and does not take into consideration backgroundpollutants associated with basefow volumes. Basefow is typically

Figure 8-1. Relationship between watershed imperviousness (I) and thestorm runo coecient (Rv) (Source: Adapted rom Schueler, 1987).

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negligible at the scale o a single development site and can usuallybe saely neglected. However, watersheds and subwatersheds dogenerate basefow pollutant loads. Basefow pollutant loads usuallymake up only a small raction o the total pollutant load deliveredrom a developed area. Still, it is important to remember that the loadestimates reer only to storm event derived loads and should not beconused with the total pollutant load rom an area.

8-2. Simple Method Input Data

Tis section discusses and provides or reerences the input data requiredto run the Simple Method analysis including a link to the Simple MethodSpreadsheet, event mean concentrations or stormwater pollutants,stormwater BMP pollutant removal eciencies, and precipitation data.Additional guidance on conducting pollutant loading analysis using theSimple Method is provided by NHDES.

Simple Method Spreadsheet

Te Simple Method spreadsheet was developed by NHDES to calculatepre- and post-development pollutant loads using the Simple Method.Te spreadsheet is in Microsot Excel ormat and includes an instructionworksheet (Instructions), two data input worksheets (Input_LU_A_Ia_Cand Input BMPs), a summary worksheet (SUMMARY), and worksheetsor each pollutant showing pre- and post-pollutant loading (ie, Pre_SS,Post_SS, Pre_BOD, etc.).

Te Input_LU_A_Ia_C worksheet requires the input o general projectinormation (e.g., date, project name, town, etc.) as well as the pre- andpost-development conditions or each subwatershed, including the nameo the subwatershed, the area (in acres), and the impervious raction oreach applicable land use in the subwatershed. Te worksheet is set up orup to ten dierent subwatersheds; however, more can be added i necessary.Ater the pre- and post- construction inormation is entered, the pollutantconcentrations or each applicable pollutant and land use should be enteredor each subwatershed. Tis is the C value in the Simple Method equations,but it also known as the event mean concentration (EMC). A table o EMCvalues is included in the worksheet.

Te Input BMPs worksheet is where descriptions and removal eciencieso existing and proposed BMPs is entered. Tis inormation should beentered or the pre- and post-development condition or each subwatershed.

Once this inormation has been entered, the SUMMARY worksheet showsthe total pre- and post-development loading results. Te individual pre- andpost- pollutant worksheets show the loads and removals or the individualsubwatersheds. Additional inormation, instructions, and the Simple Methodspreadsheet are available rom NHDES.

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Event Mean Concentrations o Stormwater Constituents

Appendix D shows the list o pollutants with their event mean concentration(EMC) values that should be included in pre- and post-developmentpollutant loading estimates. Te project area can either be grouped intogeneral land use types, such as residential, commercial, industrial, androadway, or broken down into specic components such as residential

roos, commercial streets, or lawns, to calculate annual pollutant loads. It isimportant that the approach used to model the pre-condition is also used tomodel the post-condition. NHDES will consider other EMCs i requested bythe applicant and i sucient documentation is provided.

Best Management Practices (BMP) Pollutant Removal Efciencies

I you are using the Simple Method, the pollutant removal eciencies inAppendix E should be used. NHDES will consider other BMP removaleciencies i requested and i sucient documentation is provided.

Precipitation Data

In addition to the Simple Method, many other models or calculatingpollutant loads require input o the average annual precipitation. Averageannual precipitation or various towns and cities in New Hampshire or theperiod o 1971 2000 may be obtained rom http://www.erh.noaa.gov/gyx/climo/NH_SAS_NEW.htm. Te average annual precipitation value ininches or the community closest to your project should be used.

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Chapter 8 Reerences

Camp, Dresser, and McKee. Merrimack River Watershed Assessment Study,Drat Screening Level Model. Jan. 2004.

Center or Watershed Protection. A Stormwater Design Manual oolbox.2006. 24 Feb 2006. http://www.stormwatercenter.net/intro_manual.htm#about.

Horsely Witten Group. Model Stormwater Regulations; Duxbury,Marsheld, and Plymouth, MA. December 31, 2004.

NH Department o Environmental Services, Interim Guidance orEstimating Pre- and Post-Development Pollutant Loads. 17 Oct. 2005.

Stocker, Joel. Nonpoint Education or Municipal Ocials. echnical PaperNumber 3.Methods for Measuring and Estimating Impervious Surface Coverage.1998.

United States Environmental Protection Agency. National ManagementMeasures to Control Nonpoint Source Pollution rom Urban Areas. 2006.