Flash Flood Guidance & Threshold Runoff Concepts

1. Flash Flood Guidance &Threshold Runoff Concepts Dr. Dennis L. Johnson Asst. Professor Juniata College

2. Overview • Floods  Flash Floods • Flash Flood Guidance • Threshold Runoff • Future Issues/Developments

3. What’s a Flood? • What is a flood????? • A rather elusive definition • Generally contains terms like: • High water • High flows • Normal water course • Human impact(s) • Etc…

4. Recipe(s) for a Flood • What causes a flood? • What are the conditions? • What are the types of flooding situations? • Your area or other areas…..

5. My Recipes • “BIG” heavy soaking rains… • Low infiltration rates • Snow melt • Rain on snow • Very intense precipitation • Dam failure • Others….??

6. Does a Flood Have to Happen in a Defined Water Course or Waterway? ….and If a Flood Does Occur in an Overland Situation – Does the Nearest Stream Even Feel It?

7. What’s a Flash Flood? • This is your part…….

8. What Are the Defining Characteristics of a Flood? • Timing – rise time, recession, duration. • Flows – peak flows, magnitude (statistical). • Precipitation – intensity, duration, frequency….

9. What Controls the Timing, Flow, and Precipitation? • The hydrology – short term and long term. • The meteorology – short term (weather/storm type) and long term (climate). • ………………………. • So from a prediction standpoint – we must be aware of the defining mechanisms in the short term worlds of BOTH hydrology and meteorology!

10. Enter FFG Flash flood guidance is the amount of rainfall needed in a specified period of time to initiate flooding on small streams.

11. “A” or “The” Basic Definition FFG is defined as the average basin rainfall (ABR) required over a specified time interval to begin flooding. In the northeast United States, FFG is produced for each county for a 1, 3, 6, 12 and 24 hour time duration. The FFG is updated once per day, based on the average rainfall in a Mean Areal Precipitation (MAP) area.

12. FFG is traditionally a “region” or “regional” term. Flash flood guidance is computed for small, ungaged streams in areas (grids, zones, and counties) and for gaged streams (primarily headwaters) for 1-, 3-, and 6-hour durations, (12- and 24-hour are optional).

13. Apparent Lack of Hydrologic Input/Relevance • On the surface it may appear that the “county” or regional FFG had or has little hydrologic relevance. • This could easily be disputed as these #’s most likely came from a vast personal knowledge pool of observations. • The basic problem is not necessarily the lack of hydrologic relevance – but rather the lack of sub-basin definition/variance.

14. Basic Computation of FFG • In most hydrologic modeling situations: precipitation is input and runoff or excess precipitation is the (or an) output. • FFG is done in reverse: current states and runoff are input and precipitation is the output.

15. General Goal of Most Models Infiltration Excess Precip. Interception Storage Basin Process Representation Time Series Time Series We must begin to think of the basin as a “whole”

16. The Basic Process Excess Precip. Model Excess Precip. Basin “Routing” Unit Hydrograph Runoff Hydrograph Excess Precip. Downstream Hydrograph Stream “Routing” Runoff Hydrograph

17. The Unit Hydrograph • The way the basin reacts (streamflow) to 1 inch of excess precipitation. • So if the basin reaches 1000 cfs for 1 inch of excess – it will most surely reach 2000 cfs for 2 inches! 2 inches 1 inch

18. From A Basin View Excess Precip. Excess Precip. Model Basin “Routing” Unit Hydrograph Runoff Hydrograph Stream “Routing”

19. Mathematically • Multiply the excess precipitation by the UHG ordinates to get the predicted streamflow In this case we would predict that the 1 hour burst that caused 1.5” of excess would cause the stream to reach 150 cfs at hour 3 1.5” X =

20. Revisit the Basic Process Excess Precip. Model Excess Precip. = Basin “Routing” Unit Hydrograph Runoff Hydrograph X Excess Precip. Downstream Hydrograph Stream “Routing” Runoff Hydrograph

21. FFG Works this in Reverse • Knowing, assuming, or estimating the flow needed to initiate flooding: • We “back out” the excess rainfall necessary to cause this flooding and: • We “back out” the actual precipitation necessary to cause the excess computed in Step #2.

22. The Basic Process – in Reverse Excess Precip. Model Excess Precip. / Peak of Unit Hydrograph PEAK FLOW = Excess Precip.

23. Change Terminology Iterative Threshold Runoff SAC-SMA/API / Peak of Unit Hydrograph Flood Flow = Threshold Runoff

24. Iterative Process • Enter rainfall into RFC Model (SAC-SMA)….. • Get out Runoff or Excess • Do it again…. • And again…. • And again… • Build a graph (table)

25. FFG Threshold Runoff FFG!

26. Elements of Success • Predict the rainfall – spatially & temporally • Predict the initiation • Know what flooding means • Know characteristics that make a basin entirely or temporarily susceptible to a flash flooding situation – model states….

27. A Basic Requirement The correct application of FFG requires that rainfall be uniformly distributed across the watershed. The forecaster can help satisfy this FFG factor by examining ABR in progressively smaller watersheds.

28. New Techniques/Acronyms • AMBER • FLARE • ThreshR • SCAN • Gridded FFG – GFFG? • FFMP • Others?

29. Gridded FFG Rainfall estimates from the WSR-88D radars are derived on the Hydrologic Research Analysis Project (HRAP) grid, nominally 4 km on a side. To more effectively use the radar rainfall estimates in the flash flood program, flash flood guidance is needed on the same spatial grid scale.

30. Gridding is Simple • Simply “pass” grid over basin or county • All grid values are assigned according to the value of the basin, county,region in which the center of the grid falls

31. AMBER AMBER uses the Digital Hybrid Scan Reflectivity (DHR) product from the (WSR-88D) The DHR product uses a polar grid of 1 o x 1 km for all radar azimuths from 0 o to 359 o and for all radar ranges from 1 to 230 km. A single rainfall estimate is computed for each 1 km range bin. A rainfall amount is calculated every 5-6 minutes for each range bin. All range bins, whose center point falls in a stream watershed are averaged to compute the ABR for that watershed. The small DHR rainfall grid allows AMBER to compute ABR in watersheds as small as 1 mi 2 in area.

32. Enter……. SCAN/FFMP!

33. Regardless of ABR Method: • Must still estimate the Threshold Runoff • This is a basin property • Requires estimation of flooding flow • Requires UHG (peak) • Back out Threshold Runoff

34. A bit of History In 1993 OH developed an objective and hydrologically-based procedure to derive threshold runoff values on the HRAP grid for the entire country. Input parameters included 3 arc-second digital elevation data (approximately 90-m spacing), land use land cover (LULC) data, and EPA river reach files (RF3). The GRASS Geographic Information System (GIS) software was used to delineate small watersheds and to display input, intermediate, and output data. This GRASS-based procedure is known as threshR and will be referred to as threshR93

35. Current ThreshR • Arcview (ESRI) environment • Some “C” coding • Uses USGS Regression Equations for Bankfull flooding estimate • Q2 = Bankfull

36. Works on All or Part of an RFC

37. Sub-Basins for Small area in Oklahoma

38. 2-Year Flows are Estimated Using USGS Equations

39. Threshold Runoffs are then Computed(a UHG is determined internally) Gridding is final step

40. Current Work • Current Dem is 400m • NSSL working on 30m (NED) • This will combine with SCAN and provide “good” basin definition down to relatively small watershed sizes.

41. Thanks…. Questions?