Posts Tagged ‘flooding’

Estimating with sparse data

Wednesday, May 9th, 2012

June 2012 article (Early View): “A Design Peak Flow Estimation Method for Medium-Large and Data-Scarce Watersheds With Frontal Rainfall,” by Enrique Muñoz, José Luis Arumí, and José Vargas

It’s always nice to have a lot of long-term data. But, what do you do when the data are sparse? Here’s a nice solution applied to Chile.

The authors developed a reliable peak flow estimation method for the design of hydraulic structures. The method is valid in medium-large watersheds (100-5,000 km2) located in Chile between 32°45? and 43°50?S, with scarcity of hydro-meteorological information, and where frontal rainfall prevails. The proposed method requires only rainfall data and geomorphologic descriptors as inputs, and relates the instant peak flow with the time of concentration rainfall flux (the contributing watershed area multiplied by the rainfall). The parameters of the model were defined with peak flows obtained from statistical analyses of historical fluviometric records from 25 watersheds. The quality of the proposed method is evaluated by applying it to three external watersheds different from those used to define model parameters, and comparing it with three other indirect methods and with peak flows obtained from statistical analyses, which were also used as the benchmark. The proposed method estimates peak flows with mean differences of less than 10%, which is two times less than other similar indirect methodologies, making it a recommendable option for estimating design peak flows.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own.]

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New England flood frequencies increasing

Monday, March 12th, 2012

April 2012 article (Early View): “Increased Frequency of Low-Magnitude Floods in New England,” by William H. Armstrong, Mathias J. Collins, and Noah P. Snyder.

Although catastrophic floods are usually responsible for channel avulsions, stream barrier breaches (e.g., dams), and transporting large bed-load particles, floods occurring every one or two years are more active agents in shaping the prevailing channel dimensions because of their frequency and ability to erode and transport bank and bed material. Frequent, low-magnitude floods are also important for riparian and aquatic habitat.

The authors selected gauges from the USGS Hydro-Climatic Data Network (HCDN), a compilation of river gauges that are minimally affected by human use and are the best available representation of natural streamflow throughout the U.S. They found widespread upward trends in peak over threshold per water year (POT/WY) – a direct measure of increasing flood frequency – on New England rivers. Twenty-two of 23 study gauges selected for climate sensitivity show increasing trends in POT/WY through the analysis of long-term records. They found evidence for a step increase in POT/WY around 1970 on more than half of the rivers, and seven rivers show evidence for a step increase in flood discharge, supporting recent studies noting change points in time series of various hydrologic variables around that time.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own.]

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75 years of the U.S. flood control program

Friday, March 2nd, 2012

April 2012 article (Early View): “The U.S. Flood Control Program at 75: Environmental Issues,” by Peter E. Black.

Peter Black, a Past President of AWRA, is one of our organization’s treasures. Peter doesn’t have to read about historic water events, he was there! In this article, he examines the U.S. riverine Flood Control Program, starting by pointing out a notable misquote of Gilbert White. He shows how misconceptions and lack of understanding concerning fundamental ecology and hydrology of floodplains and watersheds have led to unwise or unintended consequences.

Including topics such as “How Realistic is Flood Control,” this summary makes for very interesting, sometimes opinionated reading. Black is senior enough to not hold back: “The time has come to recognize that there is pressing need to reconsider the value of ‘flood control’ as a misnomer, oxymoron, or fantasy: actually, all three.”

Only in JAWRA. Recommended reading.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own.]

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Flood frequency changes

Tuesday, June 14th, 2011

June 2011 Article: “Examining Flood Frequency Distributions in the Midwest U.S.,” by Gabriele Villarini, James A. Smith, Mary Lynn Baeck, and Witold F. Krajewski. Part of Featured Collection on “Nonstationarity, Hydrologic Frequency Analysis, and Water Management.”

Annual maximum peak discharge time series from 196 stream gage stations with a record of at least 75 years from the Midwest United States is examined to study flood peak distributions from a regional point of view. Change-points rather than monotonic trends are responsible for most violations of the stationarity assumption. The abrupt changes in flood peaks can be associated with anthropogenic changes, such as changes in land use?land cover, agricultural practice, and construction of dams. The trend analyses do not suggest an increase in the flood peak distribution due to anthropogenic climate change.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]

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Getting from here to there

Friday, June 10th, 2011

June 2011 Article: “Getting From Here to Where? Flood Frequency Analysis and Climate,” by Jery R. Stedinger and Veronica W. Griffis. Part of Featured Collection on“Nonstationarity, Hydrologic Frequency Analysis, and Water Management.”

The authors note, “Flood-risk computations by United States (U.S.) federal agencies follow guidelines in Bulletin 17 for which the latest update 17B was published in 1982. Efforts are underway to update that remarkable document. Additional guidance in the Bulletin as to how to address variation in flood risk over time would be welcome.”

If stationarity is dead, what should we do? The authors address this question by showing some possible ways Bulletin 17 could be modified to account for some concepts of climate change. They don’t reject the value of historical data, but realize we can’t take it at face value: “Does climate change mean we are not interested in historical records? Of course not: historical data tell us what flood series look like and help us to estimate where we start. We then need to estimate what would change, which describes where we go from there. The critical concern is whether the contribution of nonstationarity to our uncertainty is large enough, and the signal sufficiently clear, that we would do better by including it in our analyses.”

They conclude, “In water-supply management, we often use period-of- record planning, which hides uncertainty. Now with climate change, we need to project from the uncertainty of our current knowledge based upon the past record to estimate the risk in the future. … Change in flood risk is not new.”

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]

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Flood magnification

Wednesday, June 8th, 2011

June 2011 Article: “Nonstationarity: Flood Magnification and Recurrence Reduction Factors in the United States,” by Richard M. Vogel, Chad Yaindl, and Meghan Walter. Part of Featured Collection on“Nonstationarity, Hydrologic Frequency Analysis, and Water Management.”

This study takes a different approach by exploring trends in floods in watersheds which are subject to a very broad range of anthropogenic influences, not limited to climate change. A simple statistical model is developed which can both mimic observed flood trends as well as the frequency of floods in a nonstationary world. Nonstationarity in floods can result from a variety of anthropogenic processes including changes in land use, climate, and water use, with likely interactions among those processes making it very difficult to attribute trends to a particular cause.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]

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Urban Drainage Networks Matter

Tuesday, August 24th, 2010

October 2010 Article: Analyses of Urban Drainage Network Structure and its Impact on Hydrologic Response, by Katherine L. Meierdiercks, James A. Smith, Mary Lynn Baeck, Andrew J. Miller

Urban flood studies have linked the severity of flooding to the percent imperviousness or land use classifications of a watershed, but relatively little attention has been given to the impact of urban drainage networks — including storm pipes, surface channels, street gutters, and stormwater management ponds — on hydrologic response. The authors demonstrate that drainage networks, like natural river networks, exhibit characteristic structures and that these features play critical roles in determining urban hydrologic response.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]

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Modeling an Extreme Flood in Texas

Monday, August 23rd, 2010

October 2010 Article: Hydrologic Modeling of an Extreme Flood in the Guadalupe River in Texas, by Hatim O. Sharif, Almoutaz A. Hassan, Sazzad Bin-Shafique, Hongjie Xie2, Jon Zeitler.

"Flash Flood Alley"

One of the most flash flood prone areas in the U.S. is a region of South-Central Texas, called Flash Flood Alley (see figure) by local residents and weather experts. This dubious honor is the result of both climatic and geomorphic factors. In November 2004, a moist air mass from the Gulf of Mexico combined with moist air from the Pacific Ocean resulted in the wettest November in Texas since 1895. This paper examines the meteorological conditions that led to this event and applies a two-dimensional, physically based, distributed-parameter hydrologic model to simulate the response of a portion of the basin.

The study results clearly demonstrate the ability of physically based, distributed-parameter simulations, driven by operational radar rainfall products, to adequately model the cumulative effect of two rainfall events and route inflows from three upstream watersheds without the need for significant calibration. Results of simulation by the distributed model demonstrate the advantage of using quality-controlled radar products, which typically better characterize the spatial and temporal distribution of rainfall than rain gauges.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]

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