9/5/2011. The flow regime of a stream is characterized by the magnitude, duration, timing, and variability of flows throughout the year.

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1 Lecture 3: Floods (readings Leopold 30-55: ) Topics to Cover: Techniques for describing the flow regime Gaged systems Mean Annual Discharge vs. Mean Annual Flood Flood Frequency Analysis Flow Duration Curves Strategies for Ungaged systems Multiple regression (type of regional curve) National Streamflow Statistics Program StreamStats Concept of Bankfull Discharge Field indicators of BF Example of BF cross sections The flow regime of a stream is characterized by the magnitude, duration, timing, and variability of flows throughout the year. Two types of flow: Base flow and flood flow Site hydrology: each part of the flow regime plays a different ecological role: magnitude, duration, frequency, duration, timing and predictability Little variability because large groundwater inputs; precipitation occurs all year Augusta Crk, MI Depicting all of the data Satilla R, GA High variability; no snow, small groundwater input, precipitation all year Some flows have critical roles in habitat formation and maintenance Overbank flows maintain floodplain features Bankfull flows maintain bars, pool/riffle sequences Moderate and high flows transport sediment delivered from upstream Flow variability matters because many species evolved to exploit a mosaic of habitats Seasonal variation similar year to year due to annual snowmelt Colorado R., CO S. Fk McKenzie R., OR A welldefined low flow season; rain at other times Poff et al.,

2 Envelope curves: Evaluation of the largest potential flood in a watershed Mean Annual Flow vs. Mean Annual Flood Mean annual flow is the discharge that if continued constantly during a year would equal the total volume. For the Logan River the mean annual discharge for water years is cfs Mean Annual Flood is the mean of the peak flow series. For the Logan River the mean annual flood is 977 cfs. Flood frequency analysis is completed on two types of data, annual flood series or a partial duration series. Recurrence interval (T r ):is the average interval, in years, between annual maximum events equaling or exceeding a given magnitude. Exceedenceprobability (p):is the chance that the annual maximum event of any year will equal or exceed some given value. Annual flood series for T r >2. Also for T r >10 years both data sources should return the same values. Steps in flood frequency analysis Download data from USGS web site, or otherwise obtain data Make sure you select the tab-delimited file format Calculate the Weibulldistribution, based on plotting position Order the data from largest to smallest annual value Assign rank (m) to each value, with the largest discharge assigned the rank of 1 The recurrence interval plotting position (T r ) is calculated where n= the total number of years for which there are data Once the data is plotted, read the discharge for a given T r off the plot 2

3 Ranked Flood Q Plotting (cfs) Rank position (Tr) Flood frequency estimation based on plotting position (Weibull distribution) What distribution fits the data? Common Distributions include: 1. Lognormal 2. Gumbel Type I extremevalue distribution 3. GumbelType III extreme-value distribution 4. Log Pearson Type III FFA using Log-Pearson Type III distribution (follow steps in handout) Log transform the ranked discharges Calculate the mean of the log transformed data Calculate the standard deviation Calculate the skew coefficient Determine K Finally calculate the log(q) for given return intervals Transform the calculated Q back to normal discharge units. 3

4 Example of FFA spreadsheet (Blacksmith Fork) Determining K from calculated skew values Calculated based on table in the handout. You must interpolate between points. Kennedy: Regional Skew Coefficients Calculated skew values can be highly influenced by extreme events. Predicted peak flows for Big Creek 4

5 Flow duration curve: a concise picture of the temporal variability of stream flow [but does not reflect seasonality or autocorrelation]. A cumulative frequency curve showing the fraction of time flows exceed a specific value average flow = 13 ft 3 /s; occurs % of the time median flow = 9 ft 3 /s median flow is typically less than average flow because distributionof stream flow is rightskewed Steps for completing a FDC (explained in handout) Download daily average flow data from USGS site for the gage of interest and over the time periods of interest(this technique can be used to look at trends in any type of continuous data). Analysis can be done for the period of record or chunks of time (example decade) The equation you use will depend on the time period of interest. Rank discharges from smallest to largest Calculate non-exceedance probability Plot Exceedance Probability (x) vs. Discharge (y) Median flow (138 ft 3 /s) Average flow (225 ft 3 /s) Ways to plot flow duration curves Howto compare flow regimeof two streams of very different size? Blacksmith Fork A D = 263 mi 2 Qavg= 127 ft 3 /s Big Creek A D = 52 mi 2 Qavg= 14 ft 3 /s 5

6 Compare watershed runoff regime by eliminating the effect of drainage basin size by dividing discharge by drainage basin area (A D ) Since the upper end of the curve is primarily determined by regional climate. The two curves are essentially the same. Blacksmith Fork Big Creek The lower ends of the curves are primarily determined by geology and topography. The flat slope of BSF indicates a significant ground-water input. Blacksmith Fork Big Creek Here, we can see that BSF watershed yields more water overall, and especially yields higher base flows. Compare flow regime by eliminating the effects of watershed size and unit area precipitation and evapotranspiration by dividing discharge by average annual discharge Multiple regression techniques are available for predicting peak flows in ungaged systems. Peak flows equations have been developed for every state and some urban areas as part of the National Streamflow Statistics Program (NSS) Equations are updated regularly by USGS Equations use watershed and climate characteristics to predict flows Estimates of accuracy are provided for each predicted statistic Important to realize these predictions can have a very inaccurate Other flow characteristics are also predicted Low flow frequency and durations Mean annual flow Monthly mean flows Equation are developed based on multiple gaging station records in a region In some states the NSS equations have been programmed within a map-based user interface: StreamStats A map-based Web application that provides information that can be used by engineers, managers, planners, and others to make informed decisions on water-related activities Primary product is streamflow statistics Runs better in internet explorer Make sure to add Stream gauge layer Within the map content tabs on the right To delineate watershed must zoom to 1:24,

7 StreamStats results for Big Creek Delineation of flow path and calculation of longitudinal profile Before you get too excited Strategies for ungaged watersheds Drainage area vs. flow duration curve Regionalized duration curves 7

8 Alluvial valley is the part of the valley filled by alluvium (river transported sediment) There is a landformcalled the active floodplainthat is being constructed in the present hydrologic regime. This landform is inundated by frequent floods. The elevation of flow that just begins to inundate this feature is the bankfull stage. The size of the channel, defined by the point at which flow just begins to inundate the flat surfaces of the point bars, contains a moderate flood, such as ones whose recurrence is less than 5 yrs. Active floodplain T r of 1.5 years is considered a close approximation of Q BF for many river systems but some studies have shown it can vary between 1.5 and 25 years. Studies with higher T r often misidentified an abandoned terrace as the active floodplain. 8

9 The bankfullstage corresponds to the Q at which channel maintenance is the most effective, that is, the Q at which moving sediment, forming or removing bars, forming or changing bends and meanders, and generally doing work that results in the average morphologic characteristics of channels (Dunne and Leopold, 1978). Stroubles Creek, VA 2008, Ridge and Valley of VA Local land use = pasture, residential, urban Watts Branch, Maryland mid-1950s, Maryland piedmont local land use = grazed pasture Field indicators of bankfull stage include: Presence of a floodplain at the elevation of incipient flooding (easy one not always present) Elevation associated with the top of the highest depositional features within the active channel A break in the slope of the banks and/or a change in the particle size distribution (coarser material deposited in active channel, finer material above active channel) Evidence of an inundation feature such as small benches Staining of rocks Exposed root hairs below an intact soil layer indicating exposure to erosive flow Lichens and certain riparian vegetation species (use care in identifying bankfullstage only by plant species, it will vary by location). Determination of bankfull discharge at a site near a gaging station The Natural Flow Regime -- Cub River nr Preston, ID Hydrology of the site: Understand the flood regime Understand the rating relation Identify the location of bankfull at many places Plot on a longitudinal profile, average those values and extend the relation through the gage rating relation The stage, at the gage, of this relation is the bankfull stage, and its associated Q is the bankfull discharge 9

10 Determination of bankfull discharge at a site near a gaging station Calibrating your eye: what elevation does the water get to during floods? Hydrology of the site: Understand the flood regime Understand the rating relation Identify the location of bankfull at many places Plot on a longitudinal profile, average those values and extend the relation through the gage rating relation The stage, at the gage, of this relation is the bankfull stage, and its associated Q is the bankfull discharge 10

11 2003 December 2008 October 2009 March 2010 July 2010 floodplain terrace riffles Identify the location of bankfullat many places, plot on a longitudinal profile, average those values and extend the relation through the gage rating relation. The stage, at the gage, of this relation is the bankfull stage, and its associated discharge is the bankfull discharge. 11