Coastal Hazards Study Guide
Infiltration Rates
Depends on land cover, nature of soils, and human factors such as pavement and ORV's. Controlled by rock properties and structure and by soil texture (Geological Factor).
Mitigation
the process of determining how to reduce or eliminate the loss of life and property damage resulting from natural and human-caused hazards.
Flood
A general and temporary condition of partial or complete inundation of normally dry land areas from overflow of inland or tidal waters or from the unusual and rapid accumulation of runoff or surface waters from any source."
Stream Gradient
A measure of the vertical distance that a stream channel falls over a given horizontal distance. Expressed in feet/mile or meters/kilometer.
Stream
Any permanent, channelized flow of water on the Earth's surface.
Landuse
Change in Landscape/Vegetative Conversion - Deforestation of hillsides and flood Plains (Human Landuse Factor).
What is meant by the terms, "bank full stage," and "flood stage?"
Discharge that fills a stream channel is called BANKFULL STAGE. When discharge exceeds the BANKFULL STAGE, the excess discharge flows out onto the floodplain of the stream and the stream is said to be in FLOOD STAGE.
What type of flooding occurred in Greenville along the Tar River, which crested about 6-7 days After Hurricane Floyd passed?
Downstream Flooding
FEMA
Federal Emergency Management Agency
What does FEMA stand for, and what is their primary role in responding to severe Flood Events?
FEMA = Federal Emergency Management Agency FEMA's primary role in responding to severe Flood Events is to coordinate relief efforts and to conduct buy outs of homeowners who have lost their home. Also to educate people in floodplain areas about flood mitigation approaches and general education about floods and their effects and hazards. FEMA encourages communities to establish sound floodplain management programs that go beyond the NFIP minimum requirements, the Community Rating System was created. The Mitigation Division at FEMA is the organization responsible for working with communities to encourage them to adopt and enforce ordinances that meet or exceed the minimum floodplain management requirements
Flood Insurance Rate Map (FIRM)
FEMA Flood Maps that show the 100-year and 500-year floodplain for Determining Flood Insurance Rates for homes and businesses.
What types of flooding occurred where Hardee Creek crosses NC 33 East/10th Street as a result of Hurricane Floyd?
Flash Flooding as Hurricane Floyd passed thru and then Backwater Flooding as the Tar River crested 5-7 days after the Flash Flooding.
What type of flooding occurred in Green Mill Run on ECU campus as Hurricane Floyd passed Greenville?
Flash Flooding or Upstream Flooding
What kind of information do Flood Hazard Maps portray? From what governmental agency Would you order Flood Hazard Maps for your community or town?
Flood Hazard Maps portray the location of the 100-year and 500-year floodplain surrounding various streams in each state in the USA. These can be ordered from FEMA although North Carolina has more up-to-date Flood Hazard maps for most of the state of NC than FEMA's 1972 maps.
List and describe the major landuse planning approaches for flood control and mitigation. What are the pros and cons of each landuse planning approach?
Flood Ways and Flood Plain Zoning - Areas of flood plain where no new structures are allowed And old ones may be removed in order to allow floodwaters to pass more freely and unobstructed. This reduces flood levels and submergence. Relocation of Homes and Businesses - Relocate out of the 100-year Flood Plain. If your home or business is damaged by a flood, you may be required to meet certain building requirements in your community to reduce future flood damage before you repair or rebuild. Flood insurance policyholders in high-risk areas, also known as special flood hazard areas, can get up to $15,000 to help pay the costs to bring their home or business into compliance with their community's flood plain ordinance. There are four options you can take to comply with your community's flood plain management ordinance and help you reduce future flood damage. You may decide which of these options is best for you: (1) Elevation - This raises your home or business to or above the flood elevation adopted by your community. (2) Relocation - This moves your home or business out of harm's way. (3) Demolition - This tears down and removes flood-damaged buildings. (4) Floodproofing - This option is available primarily for non-residential buildings. It involves making a building watertight through a combination of adjustments or additions of features to the building that reduces the potential for flood damage. Reforestation and Re-vegetation - Reduces runoff by increasing infiltration and interception Restoration of Wetlands - Restore Wetlands in the critical 50-year and 100-year Flood Plains. Wetlands are great water absorbers!
What condition must hold true for a dam to have any benefit for flood control and mitigation? What kinds of flood events (magnitude) can dams help control? Would a dam near the headwaters of the Tar River have substantially reduced the flood stage in Greenville, NC after Hurricane Floyd?
Flood impoundment dams are kept empty most of the time. These can mitigate the effects of flash/upstream flood events. Dams are not effective in preventing large, downstream flooding, such as the lower Tar River Basin in the Tarboro and Greenville NC areas.
How is urbanization defined hydrologically? What are 3 impacts that increased urbanization have on flooding, especially small upstream flooding? (Sketch and label a hydrograph plot to show 2 of these 3 impacts)
Hydro-geologically, Urbanization is defined by: (1) the percentage of total land surface that is covered with impervious material. 60% is considered maximum urbanization. (2) the percentage of land area that is served by storm sewers. 100% is considered maximum urbanization. With increasing urbanization, 3 changes occur: (1) the Frequency of flooding increases. (2) the Lag Time between peak rainfall and peak discharge/flood stage decreases. (3) the Peak discharge is increases.
What is the Hydrologic Cycle, what is the energy source that drives it, and what are the major pathways/processes by which water is distributed/re-distributed within Earth systems?
Hydrologic Cycle = the circulation of water among the Earth's 4 major spheres (see below). Most of this circulation is driven by solar energy. However, the flow of water within a stream channel is driven by gravity. (See Notes Part I for a cartoon) Precipitation = Runoff→ + Infiltration↓ + Evapo-transpiration↑
What is meant by the term, "the 100-year flood?" <http://water.usgs.gov/pubs/FS/FS-036-98/text/what.html>
It is the flood elevation that has a 1 percent chance of being equaled or exceeded each year at a given location of a stream.
List and describe the major engineering approaches for flood control and mitigation. What are the pros and cons of each engineering approach?
Levées - Small ridges that are built naturally along banks of many flood-prone streams. These actually prevent flooding from small increases in stream flow. Levees are often built higher than natural levels by man to prevent all but the most severe floods from encroaching onto the flood plain. To Levée or not to Levée? (1) Reduces fertility of flood plain. (2) Prevents rapid return of flood waters to main channel. (3) Gives people along flood plain a false sense of security. (4) For large Magnitude Floods, Levees make them worse because Flood Peak or Flood Crest moves downstream like a large wave. Floodwalls - Elevated walls built to protect area behind them from flooding. These can prevent flood waters from returning immediately to main channel. Similar approach to levées. Channelization - Modification of channel by straightening, widening, clearing, deepening, and/or lining with concrete. Can cause a lot of ecological damage to a stream and its surrounding flood plain. Also impacts stream dynamics immediately upstream and downstream from the channelized portion of a stream. Also increases the frequency and magnitude of flooding downstream from the channelized portion of a stream. Storm Water Retention Ponds - Small ponds designed to temporarily retain excess surface runoff in urban areas. Dams - Flood impoundment dams are kept empty most of the time. These can mitigate the effects of flash floods. Dams have a negative impact on the river/fluvial geo and ecosystem both upstream and downstream from the dam site.
What are the factors that control the distribution of rainfall and runoff?
Rainfall - Both intensity and duration are important along with antecedent moisture conditions of the ground (Meteorological Factor).
Make sure you can interpret a Flood Frequency curve (Discharge vs R.I.) on log-linear graph paper and extrapolate the magnitude of a given flood event from your curve. Also, you should be able to calculate Flood Probability if given the Recurrence Interval of several flood events. Like the one you did in class Wednesday. (See 2 plots below).
Recurrence interval (R) for each flood is determined using the formula - RI = (N + 1)/M. Flood discharge data has been collected for N (in this case- 19) years of record for the drainage basin of interest. Magnitude of discharge is ranked from highest to lowest on an annual basis. Year with the highest magnitude is ranked as M = 1, while the year with the lowest magnitude is ranked as M = 10 in this case. Therefore, for the flood with the highest magnitude (M=1), RI = 20 years, and this recurrence interval would be referred to as a 20-year flood, which would have a 1 in 20 or 5% chance of occurring in any given year. For the flood with the rank of 10, RI = 2 years, and this recurrence interval would be referred to as a 2-year flood, which would have 1 in 2 or 50% chance of occurring in any given year. Flood recurrence values are just estimations, values averaged over a period of time. So under the right conditions, another 20-year flood could happen the very next year!!!
make sure you can calculate the yearly/annual probability in percent for the: 500-year Flood; 100-year Flood; 50-year Flood; 20-year Flood; 10-year Flood; 5-year Flood; 2-year Flood
Recurrence interval, in years Probability of occurrence in any given year Percent chance of occurrence in any given year 100 1 in 100 1 50 1 in 50 2 25 1 in 25 4 10 1 in 10 10 5 1 in 5 20 2 1 in 2 50
Upstream Floods or Flash Floods
Result from locally intense rainfall over small portion of drainage basin or dam failure. These rise and fall rapidly and sometimes are also called flash floods. Will have a steep rising and falling limb on a hydrograph plot.
Downstream Floods
Results from larger scale meteorological events that are longer lasting and produce persistent rainfall across a whole region or a large drainage basin (s). These develop more slowly but also wane more slowly. On a hydrograph, these will have relatively steep rising limb but a much less steep falling limb
Vegetation
Slows overland runoff and enhances infiltration and reduces flood risk (Botanical Factor).
Slope
Steep slopes produce greater runoff (Topographic Factor).
What is the numero uno clue you would look for in the field in order to determine whether an area surrounding a particular stream had experienced flooding in the recent past?
Stranded debris and/or mud lines on buildings or trees or stranded sand deposits
What is the relationship between stream discharge and stream stage or gage height?
Stream Discharge is proportional to stream stage, or as Stream Stage increases so does Stream Discharge.
. How is peak flow/discharge for a flood event determined? <http://water.usgs.gov/pubs/FS/FS-036-98/text/determination.html
Stream stage (or water level) and streamflow (or discharge) are measured at locations called streamflow gaging stations. Stage is measured and recorded continuously by electronic instruments to an accuracy of 0.01 foot. Stage information is transmitted daily by telephone line to USGS computers. Flow is more difficult to measure accurately and continuously than is stage. Discharge for a gaging station is typically determined from an established stage-discharge relation, or Rating Curve. Individual discharge measurements are made by USGS personnel at a gaging station by using standard procedures (Rantz and others, 1982); ideally, these measurements are made when the stage is not changing. A series of these measurements made over a range of flow conditions defines the rating curve, which is used to convert continuous measurements of stage to a continuous record of discharge. Channel changes, resulting from scour, deposition, vegetation, or other processes, alter the stage-discharge relation, so that discharge measurements must be made routinely and continuously to ensure that the rating curve remains accurate.
Base Flow
Streamflow coming from ground-water seepage into a stream.
What magnitude of flooding did rainfall associated with Hurricane Matthew (Oct 2016) cause in the lower parts of the Tar and Neuse River Basins?
Tar River at Greenville - RI = 100 years (100-Year Flood)
What magnitude of flooding did rainfall associated with Hurricane Floyd (Sept 1999) cause in most of the Tar and Neuse River Basins?
Tar River at Greenville - RI >100 years but <500 years (100- to 500-year Flood) Tar River at Tarboro - RI > 500 years Neuse River at Kinston - RI >100 years but <500 years (100- to 500-Year Flood)
Stream Stage
The height of the water surface above the gage datum (zero point).
What are the pitfalls of over-reliance on flood frequency curves to determine the magnitude and frequency of future flood events?
The major pitfalls of over-reliance on flood frequency curves to determine the magnitude and frequency of future flood events is that over a period of several decades (1) their accuracy can be negated by the effects of climate change on the rainfall characteristics for a given region; (2) their accuracy can be negated by the effects of human landuse change and alteration of the floodplain and drainage basin environment.
What is Recurrence Interval, and how is it calculated for a given flood event? <http://water.usgs.gov/pubs/FS/FS-036-98/text/what.html>
The recurrence interval (sometimes called the return period) is based on the probability that the given event will be equalled or exceeded in any given year. Streamflow recurrence intervals are based solely on the magnitude of the annual peak flow. Ten or more years of data are required to perform a frequency analysis for the determination of recurrence intervals. More confidence can be placed in the results of a frequency analysis based on, for example, 30 years of record than on an analysis based on 10 years of record. Make sure you can use the formula, R.I. = (n + 1)/m to calculate R.I. when given the value of "n" and "m"
Drainage Basin/Watershed
The total area drained by a stream and its system of tributaries. (See Notes Part I for a cartoon)
How is Stream Discharge or Flow measured?
The width of the stream is measured; the average depth is determined by measuring the depth of the flow at several intervals across its width; the velocity is measured with a flow meter at multiple depths. Then the Discharge is determined by multiplying the Width x Average Depth x Average Velocity. The equation defining discharge is: Q = V x W x D where Q = discharge (ft3/sec), V = velocity (ft/sec), W = width (ft), D = avg depth (ft)
Stream Discharge
Volume of water passing a fixed reference point per unit time. This is a volumetric variable and is expressed cubic meters/sec or cubic feet/sec.
Backwater Flooding
Water that is retarded, backed up, or turned back in its course by an obstruction, such as rising water levels in the trunk stream channel of a drainage basin.
What are the major causes of stream flooding?
What are the major causes of stream flooding? Heavy rains - High Intensity and/or Long Duration. Often related to tropical depressions but can occur in winter when there is less vegetation to slow excess rainfall. Rapid Snowmelt - Has both local and regional effects Dam failure - Produces flash floods including Landslide Dams.
What combination of engineering and landuse planning approaches would you recommend for Greenville, such that it will survive the next BIG FLOOD EVENT better than this last one? (This one was not in original Study Guide but is useful fodder)
Zoning ordinances for 100-year floodplain areas to prevent more development on them and to create Floodway Districts More retention water ponds to collect excess runoff Make ECU obey Greenville City Flooding Ordinances Encourage people who already live there to move off the floodplain or elevate their homes/businesses Buy Flood Insurance if your home lies in the 100-year floodplain
Hydrograph
a Plot of Lapsed Time versus Stream Stage or Stream Discharge. Thus, the hydrograph can be used to show changes in stream stage or discharge over lapsed time.