Engineering Hydrology Class 14: Basic Hydrograph Analysis Class 14: Hydrograph Analysis Learning Topics and Goals: Objectives 1. Explain how hydrographs relate to hyetographs Hydrograph 2. Create DRO (direct runoff) hydrographs by separating baseflow Description 3. Relate runoff volume to watershed area and create UH (next time) Unit Hydrographs Separating Baseflow DRO Hydrographs Ocean Class 14: Hydrograph Analysis Learning Gross rainfall = depression storage + Objectives evaporation + infiltration Hydrograph + surface runoff Description Unit Hydrographs Separating Baseflow Rainfall excess = (gross rainfall – abstractions) DRO = Direct Runoff = DRO Hydrographs = net rainfall with the primary abstraction being infiltration (i.e., assuming depression storage is small and evaporation can be neglected) Class 14: Hydrograph Hydrograph Defined Analysis Learning • a hydrograph is a plot of the Objectives variation of discharge with Hydrograph Description respect to time (it can also be Unit the variation of stage or other Hydrographs water property with respect to Separating time) Baseflow DRO • determining the amount of Hydrographs infiltration versus the amount of runoff is critical for hydrograph interpretation Class 14: Hydrograph Meteorological Factors Analysis Learning • Rainfall intensity and pattern Objectives • Areal distribution of rainfall Hydrograph • Size and duration of the storm event Description Unit Physiographic Factors Hydrographs Separating • Size and shape of the drainage area Baseflow • Slope of the land surface and channel DRO • Channel morphology and drainage type Hydrographs • Soil type and distribution • Storage detention Human Factors • Land use and land cover Class 14: Hydrograph Hydrograph with Hyetograph Analysis Learning Objectives Hydrograph Description Unit Hydrographs Separating Baseflow DRO Hydrographs Class 14: Hydrograph Analysis Learning Objectives Hydrograph Description Unit Hydrographs Separating Baseflow DRO Hydrographs Class 14: Timing parameters Hydrograph Analysis - Duration of rainfall excess (D): time Learning from start to finish of rainfall excess Objectives Hydrograph Description - Lag time (L or tp) : time from the center of mass of rainfall excess to Unit the peak of the hydrograph Hydrographs Separating Baseflow - Time of rise (TR) : time from the start of rainfall excess to the peak of the DRO hydrograph Hydrographs - Time of concentration (tc) : time for a wave (of water) propagate from the most distant pt in the watershed to the outlet; one estimate is time from the end of net rainfall to the inflection point - Time base (Tb) : total duration of the DRO hydrograph Class 14: Hydrograph Unit Hydrograph Analysis Learning Objectives Hydrograph Description Unit Hydrographs Separating Baseflow DRO Hydrographs Class 14: Hydrograph Unit Hydrograph Analysis Learning Why do we want to derive the “Unit Hydrograph”? Objectives Hydrograph Description Unit Hydrographs Separating Baseflow DRO Hydrographs Class 14: Hydrograph Unit Hydrograph Analysis Learning Why do we want to derive the “Unit Hydrograph”? Objectives Hydrograph The Unit Hydrograph may Description be used as a “transfer Unit Convolve function” to convert Hydrographs the input observed hyetographs hyetograph with the unit (rain intensity vs. time) Separating hydrograph into real-time predictions Baseflow (transfer of ultimate discharge. function) DRO Hydrographs to predict a runoff hydrograph Such predictions are (output function). essential for effective engineering design for detention basins, drainage systems, etc. Class 14: Hydrograph Constructing the Unit Hydrograph Analysis Learning Objectives From BHV, p. 117 (chapter 2): Hydrograph Description Unit Hydrographs Separating Baseflow DRO Hydrographs Class 14: Hydrograph Constant Discharge Analysis Method Learning Objectives Hydrograph Description Unit Hydrographs Separating Baseflow DRO DRO Hydrographs baseflow Point of lowest discharge at start of runoff The constant discharge method assumes that baseflow is constant during the storm hydrograph. The minimum streamflow immediately prior to the rising limb is used as the constant value. Class 14: Hydrograph Constant Slope Analysis Method Learning Objectives Hydrograph Description Unit Hydrographs Separating Baseflow Inflection point = end of DRO DRO Hydrographs The constant slope method connects the start of the rising limb with the inflection point on the receding limb. This method assumes an instant response in baseflow to a rainfall event. Class 14: Hydrograph Concave Method Analysis Learning Objectives Hydrograph Description Unit Hydrographs Separating Baseflow DRO Hydrographs The concave method attempts to represent an assumed initial decrease in baseflow during the beginning of a storm (climbing limb) by projecting the declining hydrographic trend prior to the rainfall event (to) to directly under peak runoff. This minima is then connected to the inflection point on the receding limb of storm hydrograph to mimic what’s thought to be a delayed increase in baseflow..