FROM ROOFTOP TO DRAIN NON-TRADITIONAL APPLICATIONS OF HEC-RAS 2D FROM ROOFTOP. Calibration: A Hands-On Approach to Making a Model Reflect Reality

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1 FROM ROOFTOP TO DRAIN NON-TRADITIONAL APPLICATIONS OF HEC-RAS 2D Andrew C. Yung, PE, CFM Andres A. Salazar, PhD, PE April 26, FROM ROOFTOP 2 Amphitheater: 97,000 SF flexible engineered fabric roof Problem: During heavy rainfall events, water overflows the gutters collecting the roof runoff; additional structural support needed for roof and gutter system Tasks: Resize gutters Develop live loads for use in the structural engineering redesign of roof support system 3 SSPEED Short Course 1

2 Gutters 4 Drainage 5 Design Rainfall (Metal Buildings System Manual) 12 inches per hour (exceeds max. 100-year intensity) 6 SSPEED Short Course 2

3 PROPOSED APPROACH 2D Model = grid from Revit 3D Model of roof turned into a surface 7 PROPOSED APPROACH Rain on Grid 8 CHECKING THE DATA Comparison of HMS with HEC-RAS 2D Peak Flow = 8.3 cfs (HMS = 7.8 cfs) Total Volume = 0.87 ac-ft (HMS = 0.91 ac-ft) 9 SSPEED Short Course 3

4 RESULTS Gutters appear to have capacity 10 RESULTS New problem: downspouts 11 LIMITATIONS OF USING 2D FOR THIS ANALYSIS Surface had only the two central gutters Outside gutters had to be incorporated separately Geo-referencing these outside gutters relative to the surface was difficult (it could be done, but would require additional time and effort) Cannot account for downspouts 12 SSPEED Short Course 4

5 To DRain 13 Highly Urbanized Area Storm Sewers Undersized Built s Need Storm Water Detention for Improvements Lack of public ROW & Open Space COMMON SOLUTION UNDERGROUND DETENTION SSPEED Short Course 5

6 Flow (m3/s) COMMON SOLUTION UNDERGROUND STORAGE Saves space but expensive $8 / CF of storage $350,000 per acre-feet Inflow and Outflow Hydrographs Cost / acre-feet 70.0 Inflow Outflow 400, , , , , , , , :00 7:00 8:00 9:00 10:00 13:00 12:00 11:00 Time (hour) 14:00 15:00 16:00 17:00 Surface Underground 16 PROPOSED APPROACH MASS BALANCE 17 MASS BALANCE UNDERGROUND\ STORAGE Constants DOverland storage + Underground storage = 0 Overland storage + Underground storage = Constant 18 SSPEED Short Course 6

7 MASS BALANCE APPROACH Move storage from surface to underground OVERLAND STORAGE ASSESSMENT 20 OVERLAND STORAGE ASSESSMENT 21 SSPEED Short Course 7

8 Inches 2D MODELING APPROACH Use rainfall directly applied over the surface Rainfall Over Surface = Total Rainfall Losses Pipe Runoff Distribution of Rainfall Excess Runoff Pipe Runoff Infiltration Hours 22 USE HEC-RAS 2D TO ESTIMATE SURFACE STORAGE Surface storage = 68 acre-feet 23 CALCULATING STORAGE NEEDED TO MEET CRITERIA DOverland storage + Underground storage = 0 Overland storage + Underground storage = Constant Add = 40 acre-feet of underground storage Area = 524 acres Storage Rate = 0.07 acre-feet / acre Acceptable storage = 28 acre-feet 24 SSPEED Short Course 8

9 COMPARISON OF METHODS Comparison of volumetric hydrograph = 101 acre-feet Acre-Feet Required Hydrograph Methods HEC-RAS 2D 25 EXPLAINING THE DISCREPANCY Hydrographs assume pipe travel time, shorter time of concentration HEC-RAS 2D Modeling accounts for attenuation in overland flow (Ponding + Spread over surface) HEC-RAS 2D Modeling accounts for storage over the surface 26 LIMITATIONS Good for planning exercise Rainfall amount to move surface flow based on approximate pipe capacity, not modeling. Does not replace underground/surface interaction or full hydrodynamic model. 27 SSPEED Short Course 9