1

Flash Flood Modeling Flash Flood Modeling Using the DHM-TF Using the DHM-TF

ApproachApproachCurrent Status and Current Status and

Future PlansFuture Plans Brian Cosgrove, Seann Reed, Ziya Brian Cosgrove, Seann Reed, Ziya Zhang, Victor Koren, and Mike Zhang, Victor Koren, and Mike

SmithSmithNOAA/NWS/OHDNOAA/NWS/OHD

2

Distributed Hydrologic Model-Distributed Hydrologic Model-Threshold Frequency (DHM-TF) Threshold Frequency (DHM-TF)

ApproachApproach DHM-TF system: Combine a distributed hydrologic model DHM-TF system: Combine a distributed hydrologic model

with a threshold frequency post-processor to improve flash with a threshold frequency post-processor to improve flash flood forecasts at ungauged locationsflood forecasts at ungauged locations

DHM produces gridded DHM produces gridded flow forecastsflow forecasts, from which gridded , from which gridded frequency forecastsfrequency forecasts are derived using historical are derived using historical simulationssimulations

Historical simulations are conducted using DHM and same Historical simulations are conducted using DHM and same type of forcing data used in forecast simulations to type of forcing data used in forecast simulations to maintain consistencymaintain consistency

Threshold frequency grids are derived from local Threshold frequency grids are derived from local information and compared with frequency forecasts for information and compared with frequency forecasts for flash flood determinationflash flood determination

Sacramento with kinematic routing used in OHD’s Sacramento with kinematic routing used in OHD’s implementation, but any hydrologic model can be usedimplementation, but any hydrologic model can be used

3

DHM-TF MotivationDHM-TF Motivation DHM-TF is able to predict flash flooding at ungauged DHM-TF is able to predict flash flooding at ungauged

locationslocations System provides inherent bias correction to model flow System provides inherent bias correction to model flow

predictions (concept that the relative rank of events—and predictions (concept that the relative rank of events—and therefore frequency—can be well-simulated even if the flow is therefore frequency—can be well-simulated even if the flow is biased) biased)

Superior to standard flash flood guidanceSuperior to standard flash flood guidance Computed at 2km to 4km resolution versus > 260kmComputed at 2km to 4km resolution versus > 260km22 basin scale basin scale Can be updated every 15 minutes versus 1 to 6 hoursCan be updated every 15 minutes versus 1 to 6 hours Produces verifiable small basin flow estimatesProduces verifiable small basin flow estimates

Flash Flood Warning Lead Time

0

10

20

30

40

50

60

70

1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Lead

Tim

e (M

inut

es)

DHM-TF has the capability DHM-TF has the capability of providing flash flood of providing flash flood forecasts two hours into forecasts two hours into the future, thus improving the future, thus improving upon current NWS flash upon current NWS flash flood warning lead timesflood warning lead times

4

DHM-TF HistoryDHM-TF History 2003: Threshold frequency concept advocated at DOH workshop2003: Threshold frequency concept advocated at DOH workshop 2003: Concept expanded and initial development of software2003: Concept expanded and initial development of software 2004: Statistical flash flood modeling paper presented at AMS2004: Statistical flash flood modeling paper presented at AMS 2006: DHM/QPF flash flood prediction paper presented at AMS2006: DHM/QPF flash flood prediction paper presented at AMS 2006-2007: DHM-TF simulations conducted over Oklahoma-2006-2007: DHM-TF simulations conducted over Oklahoma-

domain, work is published in Journal of Hydrology domain, work is published in Journal of Hydrology 2007: Discussions with Sterling WFO concerning use of DHM-TF 2007: Discussions with Sterling WFO concerning use of DHM-TF 2007: Maryland case developed to support Sterling WFO 2007: Maryland case developed to support Sterling WFO

transfertransfer Now: Brian Cosgrove takes over DHM-TF work, Sterling WFO Now: Brian Cosgrove takes over DHM-TF work, Sterling WFO

activities commencing soon, will work with MARFC and OHRFC activities commencing soon, will work with MARFC and OHRFC as well. Discussions with Pittsburgh WFO underway.as well. Discussions with Pittsburgh WFO underway.

5

DHM-TF OverviewDHM-TF Overview

Historical AnalysisForecast

HPN

6

Real-time DHM-TF PrototypeReal-time DHM-TF Prototype

MARFC MPE(4km, 1hr, high confidence)

HPE (1km, 15min, lower confidence)

HPN (4km, 15min)No Precipitation

T-16 hours T-4 hours Present Time T+2 hours

HL-RDHM Run 1(state update)

HL-RDHM Run 2(forecast run)

T+6 hours

Model States

Saved

*Cycle automatically repeated every hour in current setup*Can be set to update forecast run every 15 minutes, using more HPE and less HPN data

Switch Tim

e

Precipitation Observations Precipitation Forecasts

7

Current: 4 km model with 28 outlets defined Planned: 2 km model with 12 outlets defined

DHM-TF Maryland Pilot Project

Maryland area well-suited for implementation of prototype DHM-Maryland area well-suited for implementation of prototype DHM-TFTF HPE/HPN prototype currently running over this region Includes Baltimore, a well-studied, high profile, flash flood prone area Local basins, MARFC, and Sterling WFO are accessible for site visits Large number of stream gauges are available in Baltimore County for

validation Mix of urban and suburban areas

Baltimore

Washington DC

Baltimore

Washington DC

8

Case Study: Tropical Storm Case Study: Tropical Storm HannaHanna

Precipitation (inches) associated with Tropical Storm Hanna from 12Z September 6th, 2008 through 12Z September 7th, 2008

Tropical Storm Hanna brought heavy rain to the Maryland Tropical Storm Hanna brought heavy rain to the Maryland region September 6th-7th, 2008region September 6th-7th, 2008

Eight flash flood warnings were issued by the Sterling WFO Eight flash flood warnings were issued by the Sterling WFO for areas within Maryland and Virginiafor areas within Maryland and Virginia

Several local roads and large parkways were shut down due Several local roads and large parkways were shut down due to flash flooding and several rescues and evacuations were to flash flooding and several rescues and evacuations were necessarynecessary

9

Tropical Storm HannaTropical Storm Hanna

Depiction of the areas covered by the three Tropical Storm Hanna flash flood warnings (outlined in Red) issued by the Sterling WFO that lie within the Maryland DHM-TF case study domain. Figure also depicts maximum return period values from DHM-TF covering a similar time period.

Maximum return period values from DHM-TF simulated during the period of time from 16Z on September 6th through 02Z on September 7th 2008. Spotter-confirmed flash flood events are indicated by the Blue wave symbols

10

Sample DHM-TF Output (TS Tammy, Sample DHM-TF Output (TS Tammy, 10/2005)10/2005)

Prototype GoogleEarth Images

Grass GIS

Images

Return Period (Years)

Return Period (Years)

11

Additional Maryland DHM-TF Additional Maryland DHM-TF Case StudyCase Study

Multiple rain events, Multiple rain events, May 9th-12May 9th-12thth, 2008, 2008

Several flash flood Several flash flood warnings issued by warnings issued by Sterling WFO for Sterling WFO for Washington DC and Washington DC and surrounding surrounding counties on May 9counties on May 9thth

Tornado warnings Tornado warnings for Prince Georges for Prince Georges and Charles and Charles Counties in Counties in MarylandMaryland

12

Maximum Discharge (m3/s) May 12-13, 2008

Sacramento UZFWC (fraction) 12Z May 12, 2008

Sacramento UZFWC (fraction) 00Z May 12, 2008

Average Recurrence Interval (Years) May 12-13, 2008

Maryland Case Study: May 12-13 2008

Frequencies are derived from routed flows, demonstrating capability to forecast floods in ungauged locations downstream of rainfall.

= Forecast Gauge Locations

13

DUTCH

0

0.2

0.4

0.6

0.8

1

1 10 100 1000

Flow (cms)

Prob

. of O

ccur

renc

e

Simulated161 cms

Adjusted271 cms

SimulatedObserved

DUTCH

0

0.2

0.4

0.6

0.8

1

1 10 100 1000

Flow (cms)

Prob

. of O

ccur

renc

e

Simulated161 cms

Adjusted271 cms

SimulatedObservedSimulatedObserved

Assessing the Inherent DHM-TF Bias Correction(Are flow ranks and frequencies correct even if flow magnitudes are biased?)

Compute peak flow adjustments at gauged validation points using

observed and modeled flow frequency distributions

DUTCH

0

200

400

600

0 200 400 600

Obs Qpeak (cms)

Sim

Qpe

ak (c

ms)

• Supports concept that frequency is well-simulated even if flows are biased • Potential exists for implementing DHM-TF approach even if model is not well calibrated Raw simulationAdjusted

1

2

3

Dutch Mills Basin Flow Frequency Distributions

Dutch Mills Basin

14

•Analysis of Monocacy River flow shows reduction in cumulative bias and improved consistency when bias corrected precipitation is used• Consistent bias can be removed through calibration or through DHM-TF approach

Monocacy at Jug Bridge (2116 km2)

• Bias detected in MARFC MPE archives prior to 2004• Bias corrected precipitation needed to support unbiased simulation statistics for a reasonable historical period (~10 years)

Bias Bias CorrectionCorrection of of PrecipitationPrecipitation

Cumulative Bias, Monocacy River at Jug Bridge (2100 km2)

15

Monthly RFC MPE Precipitation 03/97 (mm)

Monthly PRISM Precipitation 3/97 (mm)

Monthly Bias (ratio)RFC Hourly MPE

Precipitation 03/01/97 12z (mm)

Adjusted RFC Hourly MPE Precipitation 03/01/97 12z (mm)

Yu Zhang

Bias Bias CorrectionCorrection of Precipitation of Precipitation

16

Impact of Precipitation Bias Impact of Precipitation Bias CorrectionCorrection

Monocacy River Flow: April 12 through June 11, 2003

Re-analysis

Original

Reanalysis

Original

Observations

Improved precipitation input leads to improved

simulation of stream flow

17

Prototype Tools Available Prototype Tools Available from the DHM-TF Projectfrom the DHM-TF Project

Several new routines were needed for DHM-TF Several new routines were needed for DHM-TF approachapproach monthlySum: generates gridded monthly total precipitationmonthlySum: generates gridded monthly total precipitation xmrgAdjust: bias corrects hourly precipitation data with xmrgAdjust: bias corrects hourly precipitation data with

PRISMPRISM AnnPeaks: produces annual grids of maximum peak AnnPeaks: produces annual grids of maximum peak

dischargesdischarges FreqParams: reads annual peak files, computes frequency FreqParams: reads annual peak files, computes frequency

distribution at each grid cell and outputs distribution distribution at each grid cell and outputs distribution parametersparameters

getMaxRet: computes maximum discharge and return period getMaxRet: computes maximum discharge and return period during forecast periodduring forecast period

GRASS and GrADS scripts for visualizing input and GRASS and GrADS scripts for visualizing input and output. Google Earth and AWIPS being investigated.output. Google Earth and AWIPS being investigated.

Cron scripts for real-time prototypeCron scripts for real-time prototype Full set of DHM-TF components available through LADFull set of DHM-TF components available through LAD

18

Future WorkFuture Work Continue pilot real-time DHM-TF runs at OHDContinue pilot real-time DHM-TF runs at OHD Increase run resolution to 2km (1/2 HRAP) Increase run resolution to 2km (1/2 HRAP)

over Maryland domainover Maryland domain Add Snow-17 into prototype DHM-TF systemAdd Snow-17 into prototype DHM-TF system Refine graphical tools and scripts needed for Refine graphical tools and scripts needed for

output visualization at RFCs and WFOsoutput visualization at RFCs and WFOs Coordinate with Sterling and Pittsburgh Coordinate with Sterling and Pittsburgh

WFOs, and MARFC and OHRFC on use of WFOs, and MARFC and OHRFC on use of experimental DHM-TF systemexperimental DHM-TF system

Work to increase portability of system Work to increase portability of system (parameter and forcing derivation, expansion (parameter and forcing derivation, expansion to additional RFCs)to additional RFCs)