BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

unoff over a period of time to compute the total weight, or load, of theentrained pollutant. In general terms, this relationship can be written:Pollutant Load = Runoff Volume x Pollutant ConcentrationThis is an important relationship in estimating the water quality effects ofrunoff. In the Lake Tahoe Region, a major effort is in progress to define andadopt a Total Maximum Daily Load (TMDL) to improve and protect theclarity of Lake Tahoe 5 . Loads in the equation above may be computed atdifferent time intervals (e.g. hourly, daily, or annual loads). In planning effortsfor the Lake Tahoe TMDL program, loads of pollutants carried by stormwaterand other mechanisms have been computed as annual or average annual loads.While conceptually simple, calculations on long time scales for annual oraverage annual loads are computationally difficult due to concentrations andflow rates that are highly variable with time. The accuracy of load estimatesdepends on long-term data collection and careful analysis to define the effectsof meteorological and land cover conditions. Site specific data may be helpfulin developing or validating pollutant load estimates, but short term data maybe misleading if it does not reflect long term averages or characteristics.1.2.4 HYDROLOGIC DATA AND MODELINGStatistical metrics are used inhydrologic design as measuresof performance. For example,a BMP may be designed tostore or convey a storm eventwith the probability ofoccurring, on average, onceevery twenty years. This event(20-year storm) has a 5 percentannual exceedence probabilityor a recurrence interval of 20years.The process of designing stormwater facilities and water quality BMPs involvesestimation or forecast of runoff, as well as pollutant concentrations or loads,based on known characteristics of the tributary watershed. The flow of waterthrough the storage, infiltration, and runoff pathways shown in Figure 1-avaries significantly in response to changes in key process variables such asprecipitation rates and saturation of soils. Thus, measurements at a particularpoint in time may not represent long term average characteristics. For example,a particular measurement of runoff rate or quality cannot directly be used toestimate average annual runoff volume or annual pollutant load because bothrate and quality have naturally high variability through seasons, between stormor runoff events, and even within runoff events. Similarly, a particularmeasurement cannot be directly used to estimate appropriate design flows orperformance standards for BMPs.A systematic series of measurements or observations can be used to estimateaverage or other statistical metrics for use in hydrologic design. Whereavailable, runoff or pollutant load data may be analyzed statistically toextrapolate to design events of interest (e.g., the 1 percent annual chance or100-year peak flow may be estimated from a gaging station record that does notinclude an event of such magnitude). If sufficient data are available for a specificwatershed (or can be collected in anticipation of project needs), this type ofanalysis is powerful, accurate, and the logical first choice when selectinghydrologic methods. However, sufficient data are seldom available for urban5LRWQCB and NDEP, 2009, Lake Tahoe Total Maximum Daily LoadCHAPTER 1: Urban HydrologyTRPA BMP HandbookPage 1-6 2012