Arkansas Heavy Rainfall and Flooding

Heavy Rainfall and Flooding

Data Limitations

There is a relatively long historical record of precipitation data. However, a lack of spatial density of stations combined with highly variable precipitation across the state means that some rainfall events, including high rainfall amounts, may not be adequately represented in the data. Also, flood risk depends on a precipitation event, preceding events, the built environment and flood mitigation techniques. Flooding can and does occur outside of the Federal Emergency Management Agency (FEMA) Special Flood Hazard Areas. Flood impacts are extremely localized, so the data listed below may not adequately represent a single community or neighborhood flood risk or history.

Definition and Description

Heavy Rainfall and Flooding
Definition

Heavy rainfall is rain with a rate of accumulation exceeding a specific value that is geographically dependent (AMS 2012). Flooding is any high flow, overflow, or inundation by water which causes or threatens damage (NWS 2009).

Description

Heavy rainfall is a subjective term, but is rain falling at a rate more than the underlying surface can handle, causing runoff, inundation of low-lying areas, and flooding. This may include short-duration thunderstorms lasting a few hours or rainfall accumulating over several days. Flooding is the result of heavy rainfall but also the underlying surface. The rate of infiltration (how quickly it is absorbed by the soil), how quickly runoff reaches the creeks and rivers, if there had been prior rainfall, if the ground is frozen, and other local factors affect runoff and flooding. Consequently, a rainfall of a given rate and amount may cause flooding in one circumstance but not in another. Flooding is most likely in low-lying areas, along the edges of water bodies (ponds, lakes, rivers), and over impermeable surfaces (such as streets and parking lots). Primary causes include slow-moving thunderstorms and storms that track over a location in rapid succession, or tropical systems. Flash flooding may occur with intense thunderstorms while river flooding usually requires rainfall accumulated over a longer duration.

Rainfall accumulations may be compared against previous occurrences through the concept of “return-period values”. This is a statistical assessment of the frequency with which similar amounts have been recorded in the past at a specific location. These return periods, such as 1 in 25 years (a 4% chance of occurring in any given year), are not predictive tools – a large event occurring in the recent record does not prevent another similar or larger event occurring shortly after. Also, if heavy rainfall has already occurred, stormwater retention systems may be filled to capacity allowing a smaller event to cause flooding impacts similar to a much larger event. Rainfall rates and accumulations are usually greatest during the spring, summer, and fall, when warm air can hold more water vapor to produce greater rain rates. River flooding is most likely in spring or fall, when fronts may stall giving a focus for thunderstorm development. Tropical systems, either from the Gulf of Mexico or from the Pacific Ocean, can produce among the highest rainfall rates in the state.

Historical Data

Historical Climate Trends Tool

(1895-present)Southern Climate Impacts Planning Program

Shows precipitation trends and its average over the period of record by state and climate division, annually, seasonally, and by month. Years, seasons, or months with high precipitation totals may be indicative as years with flood events during one or more parts of the year, however, must be correlated with other data.

1. On the left side of screen select Arkansas. 2. Select climate division of interest or entire state. 3. Select annual, season, or month of interest. 4. Select Precipitation. 5. The horizontal line indicates the average annual amount of precipitation. The brown and green lines represent 5-year averages and indicate wetter and drier periods within the record.

Climate Extremes Tool - Precipitation

(Period of record varies by station; up to ~140 years)Southern Regional Climate Center

This interactive map shows precipitation extremes at airport weather stations, which can be used to show some previous heavy rainfall occurrences (i.e., the highest rainfall totals within a single storm do not necessarily occur at airport weather stations).

1. Pan and zoom to location of interest. 2. To obtain High precipitation records by month: On left side of screen select Records In A MonthHigh PrecipitationMonth of interest → Go. 3. Measurement unit is inches. Mouse over icon for record details (date of occurrence and station record). 4. To obtain All-time records: Select All-Time RecordsHigh PrecipitationGo.

FEMA Flood Map Service Center

Federal Emergency Management Agency

Website can be used to locate and identify flood hazard zones in a jurisdiction and produce maps for inclusion in a hazard mitigation plan. When combined with other map layers it can provide a spatial relationship between flood hazard zones and critical facilities and infrastructure. Note that the 100- yr floodplain is an estimate used for insurance and regulatory purposes. Floods can and do occur outside of the areas depicted. Note: This tool is a little more involved than some of the others and it is helpful to use a larger computer screen because of the amount of data shown.

1. Enter an address, place, or coordinates in the search bar near the top of the page. 2. Click Search. 3. Click Streets view in upper right corner of the map. 4. The panel of land outlined in light blue is the one that will be mapped. If you need a different panel, click on the one of interest (Zoom out if needed. It may take a few seconds for it to be selected.). 5. Zoom in to view details in map such as those shown at right. Note legend below map and effective date noted on the map. 6. To download a black & white static image of full original FIRM panel, click on Map Image icon. 7. To access a colored map, click on Dynamic Map icon. You may need to disable your browser’s pop-up blocker.

Climate Explorer – Extreme Events Tool

(Period of record varies by station; up to ~140 years) NOAA Climate Program Office and National Environmental Modeling and Analysis Center

The Climate Explorer is an interactive tool that allows you to view the number of extreme rainfall events per year for a station’s period of record.

1. Type in the city or county you are interested in. 2. Click Extreme Events. 3. From the top menu, select a Threshold in inches (e.g., 1 in). 4. You can choose whether or not to select an option from the Percentiles drop-down menu, from the 10th to 90th percentile. 5. Choose at least (>=) from the next drop-down menu. 6. Select a Duration in days (e.g., 1 day). 7. Choose a station (red dot) on the map. 8. The resulting plot displays a bar chart with days per year (or multiday duration you chose) with precipitation reaching your selected threshold. Hover your mouse over the bars to view yearly information. Under the chart on the right side is a Plain-language description that summarizes the number of times the threshold was reached over the station’s period of record and the yearly average occurrence.

Flood Impacts by River Crest Height

(Period of record varies by gauge; up to ~100+ years)National Weather Service Arkansas-Red Basin River Forecast Center

This interactive tool shows a summary of flood impacts for location of interest. It can be used to show the extent of flood events.

1. On map, click forecast center of interest, Arkansas Red Basin or Lower Mississippi. A new page will load. 2. On map, pan and zoom to area of interest. 3. Double-click on stream gauge of interest (small circle) on the map. 4. Click River at a Glance tab near top of page. 5. Left column: Select gauge of interest. Right column: At a minimum, select Flood Impacts, Location Map, Record Crest History. 6. Click Make my River Page! 7. Information you selected will be displayed on a new page.

Historical Flood Risk and Costs

(1996-present) Federal Emergency Management Agency

Map and graphs show state and county flood events that are documented in NOAA’s Storm Events Database. It shows the number of flood events by county and costs of flooding based on average National Flood Insurance Program and FEMA’s Individual and Household Program payments.

1. Under Choose a State, select Arkansas. 2. Arkansas statistics will be displayed on the page. 3. If you wish to view statistics by county, click on a county on the map displayed on the page.

NOAA Atlas 14 Precipitation Frequency Data Server

(Last updated in 2013) NOAA Hydrometeorological Design Studies Center

Interactive tool shows rainfall frequency estimates for select durations (e.g., 3-, 12-, and 24 hours) and recurrence intervals (e.g., 100-, 500-, and 1000-years) with 90% confidence intervals. Probable maximum precipitation (PMP) values are not represented in this tool. Such values will be available through an additional tool in the near future.

1. Click on Arkansas from the map. A new screen will open. 2. To select a location, either enter the desired location, station, or address manually OR select a station from the interactive map. 3. Precipitation frequency estimates will be displayed in both table and graph forms below. 4. For additional help, select FAQ from the left-hand menu, then refer to the Section 5 link under section 1.1.

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