Rainfall and Design Storms

Transcription

1 Methods in Stormwater Management Using HydroCAD Rainfall and Design Storms H03 Rainfall and Design Storms.pdf 1

2 Topics Covered 1. Rainfall characteristics 2. Rainfall sources 3. Design Storms 4. Example: Design Storm for Echo Valley WS 2

3 Outcomes 1. Be able to select a design rainfall based upon storm duration and frequency 2. Be able to select an appropriate rainfall data source 3. Be able to create a design storm for a specific location, return period and duration using HydroCAD. 3

4 1. Rainfall Characteristics A. Volume B. Intensity C. Duration D. Frequency E. Spatial/Geographic 4

5 A. Volume Total amount of rainfall falling during an event Expressed as inches Really a volume: rainfall depth x watershed area More correctly expressed as acre-inches B. Intensity Rate of rainfall falling during an event Expressed as inches/hour (depth/time) Really a flow: intensity (ft/s) x watershed area (ft 2 ) More correctly expressed as ft 3 /s 5

6 C. Duration Total time period of rainfall event Expressed as minutes or hours (typically) Defines the critical storm for watershed response 6

7 Volume, Intensity and Duration Largest intensity: 0.57 inches/hour Rainfall (in/hr) Total storm volume: Sum all intensities times the time step gives 5.41 inches Time (hours) Storm duration: 24 one-hour time steps equals 24 hrs 7

8 C. Frequency How often does this rainfall depth/intensity occur? Return period (years) Based upon probability of occurrence (statistics) Requires long-term rainfall records D. Spatial/Geographic Variation Rainfall intensity and volume varies with location Local variation: Watersheds > 1 mi 2 Regional/state: PA, MD, VA, etc. IDF curves National variation: NRCS Type I, IA, II, III National variation: NOAA Type A, B, C, D 8

9 Volume Duration Frequency Chart 9

10 Intensity Duration Frequency Chart 10

11 National variation: NRCS Type I, IA, II, III 11

12 National variation: NRCS Type I, IA, II and III 12

13 National variation: NOAA Type A, B, C and D 13

14 2. Rainfall Data Sources A. TP-40 (TR-55, 1986) B. NOAA Atlas 14 C. PennDOT Pub

15 A. TP-40 (TR-55, 1986, Appendix B) 15

16 B. NOAA ATLAS

17 ATLAS 14 Data 17

18 ATLAS 14 Data State College, PA

19 C. PennDOT Pub. 584 Pennsylvania Variation of Atlas 14 Step 1: Select appropriate Region Map from Table 7A.1 19

20 Step 2: From regions map, select appropriate region (1-5) 20

21 Step 3: From regions rainfall curves, read rainfall intensities using return period and storm duration OR.. 21

22 Step 3: From region rainfall tables, read rainfall depths using return period and storm duration 22

23 Example 1: Estimate 1-yr, 10-yr and 100-yr 24-hr rainfall depths for State College, PA Use Map F for all three return periods 23

24 Example 1: Estimate 1-yr, 10-yr and 100-yr rainfall depths for State College, PA Use Region 2 Rainfall Data 24

25 Region 2 VDF (volume) Table

26 Example 1: Estimate 1-yr, 10-yr and 100-yr rainfall depths for State College, PA Summary Results Storm duration: 24 hours Return Period... TP Pub Atlas 14 1-yr yr yr

27 3. Design Storms A. Definitions B. Design Storm Graphics C. Design Storm Characteristics D. Limitations of Design Storms E. Constructing a Design Storm 27

28 3. Design Storms A. Definitions Design Rainfall: total rainfall depth over some duration Design Storm: hyetograph, or time distribution, of the design rainfall over a specific duration 28

29 B. Design Storm Graphic Simple alternating-block design storm Rainfall Intensity (in/hr) Area = Design Rainfall Total Time (hours) 29

30 Duration (minutes) Duration (minutes) 30 Rainfall Intensity (in/hr) Runoff Rate (ft 3 /s)

31 C. Design Storm Characteristics Design storms are used in absence of actual rainfall records in the design area A design storm is synthesized from pieces of extreme rainfall taken from many actual storm events in region Sequencing and distribution is arranged somewhat arbitrarily to form a design storm, aka hyetograph 31

32 D. Limitations of Design Storms Runoff peaks are assumed to have same return period as the design storm. Doesn t happen that way! Runoff peaks from design storms may not agree with those from observed rainfalls having the same approximate return period The use of a design storm implies a single-event analysis; it does not account for changes in soil moisture during the inter-event times 32

33 E. Constructing a Design Storm Example: Develop an 80 minute, 10-year design storm for Region 5. Use the following PA Region 5 chart (1986). Steps: 1. From the depth vs. duration chart, obtain 10-year rainfall amounts for durations of 10, 20, 30,, 80 minutes (put in column 2) 2. Compute the incremental rainfall amounts between consecutive durations (put in column 3) 3. Rearrange the rainfall increments from column 3 in a quasisymmetrical pattern (put in column 4) 4. Compute the rain intensities during the time increments, dividing column 4 by the 10 minute time step and multiplying by 60 minutes (put in column 5) 33

34 Region 5 Depth-Duration Chart 34

35 - 10 x 60 From Depth- Duration Chart 35

36 Finished Design Storm Hyetograph 36

37 4. Example: Design Storm for Echo Valley WS 37

38 Atlas 14 Data for Echo Valley WS

39 Design Storms in HydroCAD for Echo Valley 1. Calculation Settings 2. Rainfall tab 3. Storm Type: Type II 24 hr 4. Storm Duration: 24 hr 5. Back-to-Back Storms: 1 6. Depth: AMC: 2 8. Rainfall Event Name: 25-yr 9. Click Apply and OK to save 39

40 Summary Topics 1. Rainfall characteristics 2. Rainfall sources 3. Design Storms 4. Example: Design Storm for Echo Valley WS Outcomes 1. Be able to select a design rainfall based upon storm duration and frequency 2. Be able to select an appropriate rainfall data source 3. Be able to create a design storm for a specific location, return period and duration using HydroCAD. 40