Major Hydrologic Processes Precipitation (measured at rain gage) Evaporation or ET (loss to atmosphere) Infiltration (loss to subsurface) Overland Flow Stream Flow (measured at stage gage) Ground Water Flow
The Watershed or Basin Area of land that drains to a single outlet and is separated from other watersheds by a drainage divide. Rainfall that falls in a watershed will create runoff to that watershed outlet. All other rainfall falling outside a basin will not affect the runoff response.
The Hydrograph Graph of Discharge vs Time at a Single Location Rising Limb, Crest Segment, Falling Limb,and Recession Base Flow is Usually Subtracted to yield DRO Peak Gives the Maximum Flow for the Event
The Hyetograph Graph of Rainfall Rate vs Time at a Single Gage Usually Plotted as a Bar Chart Net Rainfall is Found by Subtr. Infiltration Losses Integration of Net Rainfall in Time yields the Total Rainfall Vol (DRO) in inches over a Watershed
Runoff in an Urban Basin A portion becomes pipe flow (storm water). The remaining portion becomes overland flow in streets and yards. The total runoff is the sum of both components Hydrograph Pipe Flow Overland Flow Outflow Time Pipe Flow (SWWM)
Major Causes of Flooding (Excess Water that Inundates) Highly Developed (urbanized) Area Intensity and Duration of Rainfall Flat Topography with Little Storage Poor Building Practices - Floodplains
Tailwater Effects HGL: Gravity Flow HGL: Surcharged Flow 1 Ground surface Flow Incoming Culvert Bayou Water Level 1 Receiving Bayou HGL: Surcharged Flow 2 Top of Culvert Bottom of Culvert Bayou Water Level 2 Bayou Water Level 3
Manning’s Equation V = Velocity of Flow, ft/s n = Manning’s Roughness Coefficient S = Slope of Channel, ft/ft R = A/P, where A = Cross-sectional Area of Flow P = Wetted Perimeter of Channel
Backwater Calculation Hydraulic Grade Line Datum (MSL) Flow z2z2 h p2 z1z1 h p1 hLhL z 1 + h p1 + h L = z 2 + h p2
HEC-HMS Model Hydrologic Eng Center - Army Corps of Eng. Converts Input rainfall into runoff hydrographs Uses either historical data or design storms Predicts the total outflow hydrograph for a basin Routing Surface Runoff + Combination Q t Surface Runoff t Q Losses Removed 12 Q t Q t
HEC-HMS Theory Input Rainfall Loss Rate Function Unit Hydrograph Computes Runoff Flood Routing Combination Steps Routing Surface Runoff + Combination Q t Surface Runoff t Q Losses Removed 12 Q t Q t
Loss Rate Method: Initial and Uniform Loss Rate Method Initial Amount Lost to Infiltration (in) Soil is Saturated. Uniform Loss of a Constant Rate (in/hr) Example: Initial Loss = 0.5 in, Uniform Loss = 0.05 in/hr
Unit Hydrograph Theory The unit hydrograph represents the basin response to 1 inch of uniform net rainfall for a specified duration. Linear method originally devised in 1932. Works best for relatively small subareas. Several computational methods exist.
Unit Hydrograph Method Snyder’s Method (1938) Clark TC & R Method (1945) Nash (1958) SCS Method (1964) Espey-Winslow (1968)
Surface Runoff Theory: Clark TC & R Method L CA L1L1 L2L2 channel SoSo S S outlet L = channel length (mi) S = slope of channel (ft/mi) L CA = length to centroid (mi) S o = watershed slope (ft/mi)
Surface Runoff Theory: Clark TC & R Method TC = Travel time of overland runoff from most remote point to the outlet R = Routing Coefficient; Relates Storage and Outflow where:L = channel length (mi) S = slope of channel (ft/mi) L CA = length to centroid (mi) D = 0.94 for developed watersheds of S o <20 ft/mi S o = watershed slope (ft/mi)
Verification Results Volume Error Could Be Due to Unmeasured Quantities Calibration Successful
Urban Basin - Low Flow Bayou Level Inlets to Pipes Pipe Elevations and Sizes Junction Locations Rainfall Pattern
Urban Basin - Flood Backflow at Outlet High Bayou Level Flooding Areas Pipe Capacity
Harris Gully Models Determine Pipe Capacities at Six Different Tailwater Elevations DIVERT this Amount from Total Runoff Computed in HEC-HMS HEC-HMS Model: Computes Total Runoff, Subtracts Amount Diverted to Pipes, Remaining is Overland Flow
HEC-1 Diversion Operation Diversion to Pipes Remaining Overland Runoff Total Runoff capacity
Conclusions for Harris Gully 67 Acre-ft Pond in Hermann Park AND 50 Acre-ft Pond at Rice University would have prevented 48% of Overland Flow in March 1997 Storm Ponds are imperative for Flood Relief!