# Presentation: 257904

## CBRFC/Western Region 

- CBRFC/Western Region

- Flash Flood Analysis Project

- Gypsum Wash – Near Las Vegas, NV

- Presented to: SVR WX/ FF WDM – COMET

- Greg Smith                                 Colorado Basin River Forecast Center

- Flash Flood Guidance Issues

- July 2002

## Limitation: FFG development process inconsistent from RFC to RFC 

- WHY ?

- Historically – Little or no coordination between RFC’s regarding FFG methods

-  Different perceptions of what constitutes a flash flood
-  Methods developed independently to meet local needs
-  No national program or methodology

- Methods:

-  Empirical – precipitation return frequency
-  Develop runoff curves (typically large basins > 100 mi2 )
-  Other
-  National program implemented in 2000 (limitations)

## 1-Hour CBRFC Flash Flood Guidance 

**1-Hour CBRFC Flash Flood Guidance**

- August 2001

## A Comparison of Flash Flood Guidance

## POINT B

**POINT B**

**POINT A**

- Parunuweap Canyon on the East Fork of the Virgin River – well known classic flash flood canyon about 10 miles northwest of point B.

- Sand dunes near Moquith Mountain.

- 1-Hour Flash Flood Guidance on this date = 1.10” for both point A and B.

**Notes:**
Point B are sand dunes near Moquith Mountain
Point A is Parunuweap Canyon – about 10 miles northwest of point B.
It is a well known and classic flash flood canyon on the east fork of the Virgin River.
Flash Flood guidance for these two points was essentially the same.

## 1 Hour Flash Flood

- Guidance = 1.10”

- FFG for 8/15/2001

## 1 Hour Flash Flood

- Guidance = 1.00”

- FFG for 8/15/2001

## 1 Hour Flash Flood

- Guidance = 1.00”

- FFG for 8/15/2001

## 1 Hour Flash Flood Guidance = 1.00” for both the barren clay hills in the foreground and alpine mountainous country in the background

- Photos courtesy Southern Utah Wilderness Alliance

- FFG for 8/15/2001

## Flash Flood near Hanksville, UT  July 1990

## Tools like this emphasize the need for greater spatial detail flash flood potential or guidance information  

- KICX Amber basins overlayed with current zone guidance
**Tools like this emphasize the need for greater spatial detail flash flood potential or guidance information**

## Modernized Guidance – ThreshR/FFG System

- Modernized program attempts to do this by providing guidance on 4km HRAP Grid

- Threshold Runoff:

- A fixed value of _runoff _required to initiate flooding. It is based on geographic and hydrologic features of the stream channel and basin.

- Flash Flood Guidance System:

- Derives an amount of rainfall that is controlled by soil moisture state from the SAC-SMA model at the RFC and the threshold runoff value.

**threshold**
**  ****runoff**

**rainfall-runoff curve generated by sac-sma model independent of threshr value.**

- (Input to FFG System)

## Modernized Guidance – ThreshR/FFG System

**Q10 = .071A ****0.815 ****E ****2.70**
**Q10 = 10 yr peak discharge   A = Area  E = Elevation**

- Primary Limitation
- Use of SAC-SMA model at a flash flood scale

## Amber (flash flood) basin size vs. NWSRFS calibrated basins

## SAC-SMA Issues

**Calibrations for this model are typically for large basins (frequently exceeding 100 sq. miles) vs. flash flood basins that occur on basins as small as 5 sq. miles.**

**Many calibrations are primarily developed for seasonal events such as snowmelt, volumetric water supply and synoptic scale events and do not produce realistic runoff values for short duration precipitation input.**

**Calibrations are based on historical 6 hour precipitation and temperature data (much of it derived from daily data) as well as mean daily streamflow.  The model executes on 6 hour time steps - unrepresentative of western flash flood events.**

**Precipitation catchment and intensity will be underrepresented due to the time scale and spatial scale of MAP areas that are much larger than individual convective cells.**

**Upper zone tension water tanks that are required to fill before generating runoff will not react properly to high intensity short duration rainfall. Deficits are frequently high in semi-arid areas and following extended periods of dry weather.**

**Parameters are not on a scale for flash flood application**

## SAC-SMA rainfall-runoff curve in the Gila River Basin

- Due to tension water deficits 4” of precipitation is required before runoff is generated

- Even with Threshold Runoff set to zero !

## current vs. modernized method

- Modernized guidance using sac-sma soil moisture tank deficits

- 100 year 1-hour precip return frequency for NW Wyoming is ~1.5 inches

## Modernized vs. Current Flash Food Guidance Output

**FOUS65 KSR 220825**
**FFGAZ**
**ZONE FLASH FLOOD GUIDANCE**
**COLORADO BASIN RIVER FORECAST CENTER...SALT LAKE CITY UT**
**ISSUED 0800 AM MDT TUE MAY 22 2001**
**Flash Flood Guidance is primarily dependent upon terrain and rainfall intensity.**
**Flash Flood Guidance for urban areas and steep mountainous terrain may be less than**
**indicated.**
**.B SLR 20010522 Z DH12/DC200105220825 /DUE/PFH/PFT/PFQ**
**:IDENT     1HR     3HR   6HR**
**:======  ====    ====  ====**
**AZZ001****	**** 3.4/       3.6/      3.7**
**AZZ002     4.3/       4.5/      4.5**
**AZZ003     4.3/       4.5/      4.5**
**AZZ004     3.4/       3.6/      3.7**

**Modernized vs. Current Flash Food Guidance Output**

- Threshold Runoff is set to zero

**:IDENT     1HR     3HR   6HR**
**:======  ====    ====  ====**
**AZZ001****	**** 1.4/       1.5/      2.0**
**AZZ002     1.4/       1.5/      2.0**
**AZZ003     1.4/       1.5/      2.0**
**AZZ004     1.6/       2.1/      2.3**

## FFG Quotes

- It is better that FFG is absent than inaccurate.

- 			-Brian McInerney, SH SLC

- A constant frame of reference (of 1 inch per hour) allows the forecaster using Amber to self-calibrate.

- With the advent of FFMP, (i.e. the widespread use of amber), FFG will become much more important and will be reviewed much more critically.  (We need to be careful about what we give them).

## “For some of the Narrower canyons, as little as a 30 cfs flow can cause significant difficulties.  In 1993, two people drowned in Kolob Creek when the stream was flowing at less than 40 cfs.  And, many of the narrowest canyons are located in areas where their entire drainage is made up of slickrock.

- We have a lot of flash floods that we consider significant because they cause flows through tributaries of the North Fork yet do not show up as a large rise on the North Fork river gauge.”
- Ray O’Neil, Backcountry permit office supervisor, Zion Nat’l Park

**FFG Quotes, cont.**

## Where does this leave us ?

- _Current FFG Method_

- Empirical in nature – based on precipitation frequency studies

- Not very robust - lacks spatial variation

- More emphasis on rainfall intensity as the driving force behind flash flooding

- _Modernized FFG Method_

- Severe scale limitations due to its dependence on SAC-SMA

- More emphasis that soil moisture is the driving force behind flash flooding

- Lacks verification / reality checks along the way

- Dependent on unrealistic long term drought index for temporal variation

- Assumes a single uniform method is applicable across the nation

- Application and scale/dataset concerns associated with ThreshR

**Notes:**
FFG Modernized – These are the primary limitations we have encountered.
There are others with the ThreshR calculation itself, however removing TR
completely from the process left sac-sma as the big player for us. Central
and Eastern region offices have concerns with other parts of ThreshR.

## CBRFC/Western Region

** ****Flash Flood Analysis Project**

** ****What physiographic properties make an area susceptible to flash  **
**   ****flooding – can we identify these ?**
** ****What changes in these features or properties increase/decrease an   **
**   ****area’s susceptibility to flash flooding.**
**  ****Identify areas susceptible to flash flooding, relative to one another, **
**   ****based solely on these properties.**

**Take a big step back**** – View from a flash flood potential perspective**

**Is it even possible to create accurate guidance values ?**

**Notes:**
With the need to improve upon existing procedures and
uncertainty with the national program a Grass roots project focusing on
western flash floods.

## CBRFC/Western Region

- - **Basin geography (slope and shape information)**

**Soil information & derived hydrologic properties**
** ****Pedotransfer functions required for certain soil datasets**

- - **Vegetation coverage information**

- - **Forest coverage/canopy information**

- - **Land use information, etc.**

** ****Perform analysis on raster datasets using GIS map algebra**

- - **Assign FF potential indicators based on combined properties**

**CBRFC/Western Region**
** ****Flash Flood Analysis Project**

** ****Utilize GIS tools/methodology to carry out such an analysis**

## CBRFC/Western Region

**CBRFC/Western Region**
** ****Flash Flood Analysis Project**

## CBRFC/Western Region

** ****Add basin geometry component to FFG output weighting**

**Verify results: **

** ****Based on local knowledge of flash flood prone areas**

** ****Based on documented flash flood events**

**CBRFC/Western Region**
** ****Flash Flood Analysis Project**

**Output – Thematic layer of relative flash flood potential**

** ****A data layer for spatial variation of FFG**

** ****Gridded output if desired**

** ****Interpolate to Amber basin layer if desired**

## Amber Basin Flash Flood Potential

- hypothetical example

## CBRFC/Western Region

** ****Incorporate observed flash flood event information**

**CBRFC/Western Region**
** ****Flash Flood Analysis Project**

** ****Incorporate precipitation return frequency information**

- - **May vary regionally by climate, etc.**

- - **May vary by physiographic characteristics**

- - **Important to ground in observational truth**

** ****Incorporate distributed model component**

** ****Assign a FFG value to each of the categories**

**Simple assignment**
**Regression approach using layer info and observed info**
**Other?**

**Notes:**
How de we do this.. we’re not entirely sure.. several ideas are being
postulated. Regression of data layers, climate zones, against observed events
to come up with a relationship. Using a distributed model component perhaps.
Despite the method – it should be grounded in observational truth.

## How does this differ from the ThreshR component?

**Flash Flood Project: **

**ThreshR: **

** ****Uses Unit Hydrograph Theory and USGS statistical procedures**

** ****Focus is constant runoff value to achieve bankfull flow**

** ****Assumes uniform application across all areas**

** ****Limited use of physiographic datasets  **

** ****Complex? – Addresses features affecting western flash floods?  **

## CBRFC/Western Region Flash Flood Analysis Project

**Numerous GIS considerations to keep in mind**

** ****Error Propagation**

- - **Quantitative attributes, positional, categorical**

** ****DEM uncertainties and derived attributes**

**Notes:**
You can also call these limitations

## CBRFC/Western Region Flash Flood Analysis Project

** ****Scale Limitations**

** ****Computational concerns**

**1 arc-second (~30m) delineate to:****	****     5 km****2****  (min < 1 km****2****)**
**3 arc-second (~100m) delineate to:****	****   40 km****2****  (min 5 km****2****)**
**15 arc-second (~400m) delineate to: ****	****1000 km****2**** (min 60 km****2****)**
**30 arc-second (~1 km) delineate to:****	****4000 km****2**

** ****Storage-Space concerns**

**CBRFC/Western Region Flash Flood Analysis Project**

**Numerous GIS considerations to keep in mind**

## CBRFC/Western Region Flash Flood Analysis Project

_**Example**_

## CBRFC/Western Region Flash Flood Analysis Project

**CBRFC/Western Region Flash Flood Analysis Project**

_**Example**_

## Percent Slope

**Percent Slope**
**Grid**

- Re-sampled
- 400 meter
- DEM

## Reclassified Percent Slope Grid

**Reclassified Percent Slope Grid**

- Relative Flash Flood Potential 1-10

- Low

- High

## CBRFC/Western Region Flash Flood Analysis Project

**CBRFC/Western Region Flash Flood Analysis Project**

_**Example**_

## Rock Volume Grid

**Rock Volume Grid**

**Rock fragments in the soil > 2mm**

**source: STATSGO**

## Reclassified Rock Volume Grid

**Reclassified Rock Volume Grid**

- Relative Flash Flood Potential 1-10

## CBRFC/Western Region Flash Flood Analysis Project

** ****Fractional soil grid (STATSGO)**

**- Percent of sand, silt, and clay in the soil layer**

**- Top STATSGO layer(s) used**

**- Pedotransfer functions exist for this data type**

**- Bilinear method used for re-sampling**

**- Data 1 km resolution re-sampled to 400 meter**

**CBRFC/Western Region Flash Flood Analysis Project**

_**Example**_

## CBRFC/Western Region Flash Flood Analysis Project

** ****Percent forest cover**

**- Remote sensed data – NOAA AVHRR**

**- Bilinear method used for re-sampling**

**- Data ~ 1 km resolution re-sampled to 400 meter**

**CBRFC/Western Region Flash Flood Analysis Project**

_**Example**_

## Percent Forest Cover

**Percent Forest Cover**

## Reclassified Percent Forest Cover

**Reclassified Percent Forest Cover**

- Low

- High

## Analysis based on four themes:

- Volume of rock
- Fractional Soil
- Slope
- Forest Density

- Flash Flood Indicators

- static relative flash flood potential

## Flash Flood Indicators

- static relative flash flood potential

- North and East Fork
- Virgin River

## Flash Flood Indicators

**Sheep Creek **
**Canyon**

- static relative flash flood potential

## Slide 46

## CBRFC/Western Region Flash Flood Analysis Project

**CBRFC/Western Region Flash Flood Analysis Project**

_**Example**_

** ****Only visual analysis at this point in time**

**- Comparison with known/expected flash flood areas**

** ****Need for data layers of observed/documented events**

**- Some positives but inconclusive **

**- Starting point for guidance values **

** ****Determine valid datasets for use**

** ****Determine weighting schemes for data layers**

## CBRFC/Western Region Flash Flood Analysis Project

**How best to document these events ?**

** ****Can we get the WFO SH or Hydro Focal Point involved ?**

**- Assist in documenting event parameters**

**- Parameters that could be derived would be determined by the RFC **

**- A simple interface to document these events – databased at RFC **

**- Future and at least some historical information is desired **

_**It is imperative observed information be collected if this program is to improve**_

## CBRFC/Western Region Flash Flood Analysis Project

**To document or not to document – what do we call a flash flood ?**

- It's probably best just to focus on the initial concepts we are working with when deciding whether to document an event.
- Primarily trying to relate surface physiographic characteristics conducive to a hydrologic response of exceptional high and/or sudden discharge that is on a similar scale as the short duration high intensity rainfall. If an event falls into this type of hydrologic response category.. document it.
- If it is questionable.. document it.

## CBRFC/Western Region Flash Flood Analysis Project

**Where Next? **

** ****Continue with analysis – More rigorous**

**- Utilize observed event information**

**- Seek out and test additional datasets **

**- Re-visit assumptions regarding hydrologic relationships of datasets  **

**- Create a layer of flash flood potential for interested WFO **

** ****Finer resolution DEM**

**- Identify a sub area for more in depth analysis **

**- Utilize finer resolution DEM and other data if available  **

## Peter Fickenscher (CNRFC)

- Greg Smith (CBRFC)

- James Fahey (CNRFC)

- Steve King (NWRFC)

- Melissa Goering (WFO Tucson)

**Team Members**