Atlas 14: The New Rainfall Atlas

Transcription

1 Atlas 14: The New Rainfall Atlas Olmsted County Environmental Commission Trends Subcommittee September 17, 2014 Presenter: Art Kalmes, PE, CFM Vice President Barr Engineering Co

2 agenda Overview of TP-40 and new Atlas 14 New Atlas 14 Rainfall Depths Midwest and Statewide Olmsted County Is this a Game Changer? Regional Scale Individual Projects Policies What are others doing? Conclusion and discussion

3 Framing this presentation.... this is an introduction of Atlas 14 our climate did not change the day Atlas 14 was released our understanding of the frequency of rainfall was update. this isn t a presentation on climate change. hydrologic modeling procedure, design practices, and protection standards have changed in the past. it is up to the County stakeholders to determine how they will use this information and their decisions there are few mandates please ask questions during the presentation if you like.

4 Technical Paper 40 (TP-40) created by NOAA in 1961 key document for hydrologists and water planners 63 storms that included dust bowl during 1930s gives rainfall data for every county in every state

5 Technical Paper 40 (TP-40) rainfall frequency or recurrence intervals: 1-year, 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year events rainfall durations: 30-minute, 1-hour, 2-hour, 3-hour, 6-hour, 12-hour, 24-hour, 2-day, and 4-day events

6 Atlas 14 (the new TP-40) 11 states (dark blue) pooled funds to update Source: NOAA, peer technical review document

7 TP-40 Minnesota daily stations Atlas 14 Minnesota daily stations

8 TP-40 Minnesota sub-daily stations Atlas 14 Minnesota sub-daily stations

9 Atlas 14 - improved science average record length now over 50 years more than double the record used in original studies oldest Minnesota data set from 1836 (Ft. Snelling / Minneapolis St. Paul Airport) detailed data provides a clearer picture of the rainfall distribution new detail shows higher intensity during short storms NRCS is developing regional relationship to replace previous methodology

10 Atlas 14 - improved usability interactive web interface click to a specific point: no more estimating s.noaa.gov/hdsc /pfds/

11 Atlas 14 - improved usability station data provided in downloadable tabular form over 190 storms data on extreme events (up to 1000-yr)

12 improved usability or as depthdurationfrequency graphs

13 improved usability Confidence interval information

14 Atlas 14 rainfall depths - Midwest some significant increases in each state vs. TP-40 most of Minnesota and Wisconsin, eastern North Dakota, and the Black Hills of South Dakota percent change in 24 hour/ 100 year depths: Minneapolis, MN to 7.5 inches (+25%) Dakota County, MN to 7.4 inches (+23%) Ashland, WI to 7.3 inches (+38%) Fargo, ND to 6.5 inches (+23%) Rapid City, SD to 6.2 inches (+35%)

15 Atlas 14 rainfall depths - Midwest some decreases for certain storms central Minnesota, western North Dakota, most of South Dakota, and east/central Wisconsin degree of change tends to increase as storm frequency decreases example: MSP International AP Frequency % Change

16 Atlas 14 rainfall depths - Midwest some surprises (large changes over short distances) difference in 24 hour/ 100 year depths (inches): Minneapolis, MN to St. Cloud, MN: 7.5 to 6.4 (1.1 dif) Worthington, MN to Sioux Falls, SD: 7.4 to 5.9 (1.5 dif)

17 St. Cloud (Frequency) (%Change) Minneapolis/St. Paul (Frequency) (%Change)

18 Moorhead (Frequency) (%Change) Alexandria (Frequency) (%Change)

19 Rochester Airport (Frequency) (%Change)

20 Atlas 14 rainfall depths Olmsted County

21 Atlas 14 rainfall depths Olmsted County

22 Minneapolis/St. Paul (Frequency) (%Change)

23 Albert Lea (Frequency) (%Change)

24 Duluth Airport (Frequency) (%Change)

25 any questions?... That concludes the intro to Atlas next we will talk about its implications.

26 Is this a game changer? how does Atlas 14 data affect calculated flood levels? who (city, watershed org, state?) will decide which rainfall amounts will be used? permitting, ordinances, policies, and standards legal implications of using or not using Atlas 14 results when to start using Atlas 14 numbers? Barr Engineering Utica Ravine stabilization project in Savage, MN

27 Is this a game changer? How does it change flows and flood levels? Let s look at runoff. NRCS Rainfall Runoff Equation

28 Is this a game changer? How does it change flows and flood levels in residential watershed? NRCS Rainfall Runoff Equation

29 Is this a game changer? Residential Watershed Comparison, TP-40 vs. Atlas 14: 1.7 more precipitation produces 1.6 more runoff 42% increase in 100-yr runoff volume for residential watershed NRCS Rainfall Runoff Equation

30 Is this a game changer? How does it change flows and flood levels in rural (grassland) watershed? NRCS Rainfall Runoff Equation

31 Is this a game changer? rural (grassland) Watershed Comparison, TP-40 vs. Atlas 14: 1.7 more precipitation produces 1.2 more runoff 60% increase in 100-yr runoff volume for rural (grassland) watershed NRCS Rainfall Runoff Equation

32 Is this a game changer? how does it affect things on a regional scale? examined by the City of Rochester for South Fork Zumbro River hydrology based on stream gage data not rainfall Re-examined flood frequency using latest stream gage data

33 Is this a game changer? South Fork Zumbro River in Rochester FIS flows based on stream gage data

34 Is this a game changer? Conclusion: at flooding sources where hydrology was based on stream gage data, the previous evaluations (e.g. Flood Insurance Study or FIS) should be similar to updated evaluation. What Olmsted County stream/rivers are based on stream gage data South Fork Zumbro River Middle Fork Zumbro River (incl. S. Branch) Willow Creek (incl. E. Fork, S. Fork, SE Branch) South Fork of Bear Creek

35 Is this a game changer? How about if hydrology based on rainfall-runoff model (precipitation losses = runoff)? City of Rochester evaluated along Cascade Creek calculated peak flood discharges and flood levels using Atlas 14 rainfall depths compared these to previous values.

36 Is this a game changer? Cascade Creek in Rochester FIS flows based on rainfall-runoff model

37 Is this a game changer? Cascade Creek in Rochester FIS flows based on rainfall-runoff model

38 Is this a game changer? Atlas 14 data will predict more runoff and a larger floodplain than previous studies (e.g. FEMA floodplain maps) TP yr precipitation is similar to Atlas year Rochester Evaluation concluded: Future FEMA 100-year floodplain will be similar to current 500- year floodplain

39 Is this a game changer? the City of Rochester evaluation concluded that it is difficult to predict the amount the flood level will increase when Atlas 14 data is used. therefore, the establishment of future zoning restriction should be based on information specific to the flooding source

40 Is this a game changer? What Olmsted County stream/rivers are based on rainfallrunoff modeling and may be subject to future floodplain map changes? Cascade Creek (Incl. N Fork) Badger Run Bear Creek Hadley Valley Creek Silver Creek Carey Creek Mill Creek N. Branch of Root River S. Fork of Whitewater River

41 Is this a game changer? How about individual projects? newly placed systems (especially detention ponds) may now appear to be undersized for storms larger than 10-year modeled rainfall distribution can change since Atlas 14 provides actual data rather than assumed SCS distribution emergency overflows and freeboard protection will help needs to be assessed on an individual basis

42 Is this a game changer? Design and policy future storm sewer infrastructure sizing detention ponds future development possibly held to higher standards integrating current and future infrastructure capacities water quality features waste containment ponds animal waste storage systems roadway crossings

43 any questions?... that concludes the discussion about implications... next we will talk about what others are doing and conclude

44 possible approaches for water planners, designers, and regulators adopt Atlas 14 for new systems base level of protection on rainfall depth rather than event probability freeboard requirements take advantage of opportunities to mitigate impacts: safe overflow routes increase storage and infiltration larger easements increase conveyance

45 What others are doing? federal agencies have switched to Atlas 14 for design MnDOT has switched to Atlas 14 for design FEMA will incorporate when new maps are published Rochester is requiring on new stormwater infrastructure for design most LGUs and watershed districts are using or will be using for development reviews LGUs and watershed districts are transitioning for regional systems and floodplains most LGUs and watershed districts are assessing and planning

46 What others are doing City or Rochester Flood Forecasting display tool

47 Conclusions Atlas 14 increases rainfall depth for storms greater than 10-year; Rainfall pattern also is more intense the Olmsted County 100-year rainfall increases by approx. 30% runoff volume has larger increase up to 60% for rural grassland areas previous evaluations based on stream gage data will likely have little or no change previous evaluations based on a rainfall-runoff model will likely have measurable changes that could be significant there is no general rule for determining implications regional flood protection systems and infrastructure should focus on assessing risk

48 Conclusions implications will vary on an individual project or property basis local government policies will need to determine how to adopt Atlas 14 rainfall depths for new developments and projects Atlas 14 data, along with proper planning, will help us better manage our streams and rivers. South Fork Zumbro River, Rochester

49 so stay tuned very complex and broad-reaching issues to be addressed in the future Thank You!