DES 606 : Watershed Modeling with HEC-HMS Module 2: Hydrology Principles, HMS Overview Theodore G. Cleveland, Ph.D., P.E, M. ASCE, F. EWRI 26-28 August 2015 Module 2

Outline for Module 2 Hydrologic Principles Overview of HEC-HMS Watershed Hydrologic Cycle Overview of HEC-HMS Projects Components Module 2

Hydrologic Principles The Watershed The Hydrologic Cycle Precipitation Losses Storage and Routing Runoff Module 2

Watershed The fundamental unit in surface water hydrology is the watershed. A watershed is defined as the area on the surface of the earth that drains to a specific location. The watershed is defined both by the drainage location as well as topographic features that govern flow to that point. Module 2

Watershed Watershed physical properties are characteristics such as: Area Main channel length (if a main channel exists) Slope (requires the specification of path), Soil moisture/permeability, and similar measurable characteristics. Module 2

Watershed Watersheds also have descriptive properties such as: %-developed %- polluted, and so forth. These properties are certainly physical, but are called descriptive because there will be analyst interpretation in the specification of the values. Module 2

Watershed Physical properties are those things than can be measured from a topographic map. Area, slope, length Descriptive properties are everything else. Soil texture (and infiltration rate) Fraction developed Module 2

Watershed An analysis or even design will likely start with watershed delineation. Aerial imagery Topographic map Sewer drawings Module 2

Watershed What is the process of delineating watersheds? Manual delineation involves drawing lines on a topographic map, and connecting the slope or ridge tops. Assuming the water will drain away from those points, the watershed is delineated by enclosing a polygon. Module 2

Watershed What is the process of delineating watersheds? Automated delineation involves some technical skills with GIS-like databases. Digital Elevation Model (DEM) Stream network, and stream outlets. The DEM and stream outlets can either be downloaded or constructed. If a new DEM must be constructed, consider cost-sharing with the U.S. Geological Survey to do so. Module 2

Manual delineation example in course reference Topowatershed.pdf Module 2

Watershed Watershed physical and descriptive characteristics determined after delineation. Areas and lengths Slopes Change in elevation along a path Special concerns Sewers can cross topographic watershed boundaries Flat terrain – flow paths hard to define Module 2

Watershed Watershed physical and descriptive characteristics determined after delineation. Descriptive characteristics Google Earth/aerial imagery can be used to define cover types and fractions. Soil maps for selected properties. Module 2

Watershed Minimal Description Watershed boundary on a map Outlet Subsurface storm sewer network Area that drains to the outlet Main channel length Outlet to highest point in watershed Slope(s) Descriptive (any or all) Soil type Fraction developed/impermeable etc. Module 2

Example/Exercise 2 Harden Branch Creek, Concho County Texas Module 1

Hydrologic Cycle Precipitation (Input) Loss Runoff (Output) Module 2 Subsurface storm sewer network if appropriate and available. Runoff (Output) Module 2

Precipitation Precipitation Meteorology Data Rainfall (by far most important in Texas) Snow, Sleet, Hail Meteorology Synoptic storms Cyclonic storms Data NWS, local networks, SAO, NCDC (historical) Module 2

Losses Losses Infiltration Evapotranspiration Hortonian Loss Model Green-Ampt Loss Model NRCS Runoff Generation Model Initial Abstraction, Constant Rate Model Evapotranspiration Thornwaithe Energy Balance Models Module 2

Transformation Transform the spatially distributed precipitation input to the outlet Unit Hydrograph Module 2

Storage and Routing Storage Routing Reservoirs, ponds, depressions store water and release later in time (as compared to the input) Routing Moving water from one location to another on the watershed occurs over a path (route). Routing develops the temporal relationship of input to the outlet from this process Module 2

Rainfall-Runoff Process Precipitation Meterology, Climate Watershed Losses Transformation Storage Routing Runoff Fraction of precipitation signal remaining after losses Module 2

HEC-HMS Overview History HMS is a complex and sophisticated tool Evolved from HEC-1 as part of “new-generation” software circa 1990 Integrated user interface to speed up data input and enhance output interpretation HMS is a complex and sophisticated tool Intended to be used by a knowledgeable and skilled operator Knowledge and skill increase with use Module 2

HEC-HMS Conceptualizes precipitation, watershed interaction, and runoff into major elements Basin and sub-basin description Supply how the system components are interconnected Loss model Supply how rainfall is converted into excess rainfall Transformation model Supply how the excess rainfall is redistributed in time and moved to the outlet Module 2

HEC-HMS Conceptualizes precipitation, watershed interaction, and runoff into major elements Meterological model Raingage specifications and assignment to different sub-basins Time-series models Supply input hyetographs Supply observed hydrographs Simulation control Supply instructions of what, when, how to simulate Module 2

Summary Watershed is fundamental unit Area, Length, Slope, etc. Exercise 2: Delineate and Measure Area Hydrologic Cycle and Processes Rainfall is the input function Losses and Storage are watershed functions Runoff is the excess rain redistributed in time HEC-HMS is a computer program for rainfall-runoff modeling Module 2