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Module 4: Loss Models Theodore G. Cleveland, Ph.D., P.E, M. ASCE, F. EWRI 26-28 AUG 2015 Module 4 1
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Rainfall-Runoff Process Excess precipitation, it is what’s left after abstractions (losses) are satisfied HEC-HMS Runoff Generation Infiltration loss models Runoff generation (NRCS) model Module 4 2
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Watershed –Losses –Transformation –Storage –Routing Precipitation –Meterology, Climate Runoff Fraction of precipitation signal remaining after losses Module 4 3
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Hydrologic Cycle Components in HEC- HMS (circa 2008) Land Surface and Vegetation ChannelsReservoirs Infiltration Loss Snowpack Rainfall, P(t) Snowfall Snowmelt Runoff Percolation Loss Evapo- transpiration Discharge, Q(t) Module 4 4
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Precipitation (rainfall) is the raw input Distributed in space and time Commonly assumed uniform in space for hydrologic computations (refine later) A component of this signal never appears as runoff, it is “lost” Excess precipitation (rainfall) is the component of the signal remaining after losses. Module 4 5
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As a process diagram: Loss Model Precipitation Losses Excess Precipitation Module 4 6
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Vital to achieve volume balances Cannot easily measure Biggest source of uncertainty in hydrologic modeling Used in calibration where data are available Loss models get used to “tune” a model Module 4 7
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Infiltration is water that soaks into the ground. This water is considered removed from the runoff process. Largest contribution to losses during a storm event, hence most loss models are some form of an infiltration accounting Module 4 8
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HEC-HMS Losses are infiltration losses. Evaporation is modeled as a component of meterology. Infiltration accounting defined by soil properties and ground cover. Soil type (sand, clay, silt, etc.) Land use (percent impervious, etc.) Module 4 9
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Pedagogical Rate has an initial and asymptotic value. Integral of rate is total depth (volume) lost Module 4 10
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Detailed Discussion Initial Abstraction, Constant Loss NRCS Curve Number Green-Ampt Other Methods Exponential Model Phi-Index (and proportional rainfall) Soil Moisture Accounting Deficit/Constant Module 4 11
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Assumes soil has an initial capacity to absorb a prescribed depth. Once the initial depth is satisfied, then a constant loss rate thereafter. No recovery of initial capacity during periods of no precipitation. Module 4 12
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Typical values, Ia: Sandy soils: 0.80 to 1.50 inches Clay soils : 0.40 to 1.00 inches Typical values, Cl Sandy soils: 0.10 to 0.30 inches/hour Clay soils : 0.05 to 0.15 inches/hour Module 4 13
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Two parameters, the initial abstraction and the constant loss rate. Parameter estimation: Calibration TxDOT 0-4193-7 (HEC-HMS Example 2) Local guidance (i.e. Harris County, circa 2003) Module 4 14
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Advantages Simple to set up and use Complexity appropriate for many studies Disadvantages Parameter estimation (outside of 0-4193-7) May be too simplified for some studies HEC-HMS User Manual 3.5, pg 136 “Initial and Constant Loss” Module 4 15
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NRCS Runoff Curve Number Is really a runoff generation model, but same result as a loss model. Uses tables for soil properties and land use properties. Each type (A,B,C, or D) and land use is assigned a CN between 10 and 100 Module 4 16
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The CN approaches 100 for impervious The CN approaches zero for no runoff generation. Reminder: The CN is NOT a percent impervious. The CN is NOT a percent of precipitation. Module 4 17
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NRCS CN method Separate computation of impervious cover then applied to pre-development land use or Use a composite CN that already accounts for impervious cover. Composite CN described in TxDOT Hydraulic Design Manual (circa 2009) Composite common in TxDOT applications Module 4 18
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Rural: Table from NEH-630-Chapter 9 (included on reference flash-drive) Module 4 19
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Urban: Table from NEH-630-Chapter 9 (included on reference flash drive) Composite CN equation Module 4 20
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Runoff generated by where, q = depth of direct runoff (inches) P = precipitation depth (inches) Module 4 21
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Graphical runoff generation model From NEH-630- Chapter 10 Depth Module 4 22
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Parameter Estimation NEH 630 Chapters 9 and 10 ▪Detailed development of the model, Chapter 10 ▪Estimation of CN, Chapter 9 FHWA-NHI-02-001 (Highway hydrology) Most hydrology textbooks TxDOT Hydraulics Design Manual (circa 2009) Module 4 23
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Advantages Simple, documented approach Widely used and established across the USA Disadvantages Losses approach zero for moderate duration storms Same loss for given rainfall regardless of duration. HEC-HMS User Manual 3.5 pg 137 Module 4 24
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Infiltration model based on constant head or constant vertical flux into a porous medium. Assumes soil behaves like a permeameter. Uses Darcy’s law (adjusted for soil suction). Four parameters: Initial and saturated water content Soil suction and saturated hydraulic conductivity Module 4 25
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Volume infiltrated over time; Governed by flux, change in water content. Flux (infiltration rate); Governed by saturated hydraulic conductivity, soil suction, and accumulated infiltration. Module 4 26
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Parameter estimation Initial water content ▪wilting point is a good lower bound for modeling Saturated water content ▪porosity is a good approximation Saturated hydraulic conductivity ▪Infiltrometer measurements Soil suction ▪Textural description ▪Hanging column measurements Local guidance (e.g. Harris County has suggested GA parameter values) Module 4 27
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Advantages Documented soil saturation theory Parameters can be estimated either by measurement or textural soils description Disadvantages Parameter estimates NON-TRIVIAL. More complex than rest of hydrologic model. HEC-HMS User Manual 3.5, pg 133 Module 4 28
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Deficit and Constant Exponential Model Smith Parlange Soil Moisture Accounting Phi-Index (and proportional rainfall) Not in HEC-HMS, analyst prepares excess precipitation time series externally. Documented in most hydrology textbooks. Module 4 29
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Deficit and Constant Similar to IaCl. Ia “rebounds” after period of zero precipitation. HEC-HMS User Manual 3.5 pg 130 Exponential Model Exponential decay of infiltration rate Needs local calibration, popular in coastal communities (long history of calibration) HEC-HMS User Manual 3.5 pg 130 Module 4 30
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Smith Parlange A soil science approach more complex than Green- Ampt, similar concepts. Nine parameters HEC-HMS User Manual 3.5, pg 138 Soil Moisture Accounting Three-layer soil storage model. Evapotranspiration used to dry upper layer. 14 parameters HEC-HMS User Manual 3.5, pg 139 Module 4 31
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Example 4 – Extending the minimal HEC- HMS model Ash Creek Watershed Learn how to incorporate real runoff and rainfall. Estimate CN for the watershed Estimate GA parameters for the watershed Compare 3 Loss Models (without calibration) Module 4 32
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Rainfall-runoff process determines excess precipitation. Excess precipitation is the portion of the input that is available for runoff. The conversion is via a “loss” model There are many different loss models Module 4 33
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Many different loss models are available, ranging from the simple to the complex. Example 4 illustrates three different loss models and compares their performance for a real storm. Module 4 34
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