Q4 interpretation of low frequency flood fraught with difficulty

Question 1

Explain why interpretation of floods of low flow frequency analysis may be fraught with difficulty

Answer 1

Intro/ Uses

• used to evaluate the ability of a stream to met a specified flow at locations - indicate adequacy of flow to meet a certain demand with probability of shortage
• dry spells increased water usage – reduced flow
• when deficiencies bulk users of water that suffer: agriculture, industry, urban population concentration - amount of storage required to keep up with demand
• hydroelectric dams – min flow for water quality, life

Data Length

• annual minimum series (independent of each other)
• At mid latitudes in the northern hemisphere a calendar year (low flow summer) - Elsewhere a hydrological year avoids splitting in the middle of a low flow period
• P(X) refers to the probability of an annual minimum greater than or equal to he value X. The formulae used are the same as for flood frequency analysis (Weibull e.t.c).
• Availability of data insufficient therefore statistical distributions are used to improve accuracy - Distribution must fit a finite limit of 0 (weibull distribution)

Statistical method

• finite limit on how low a flow can be (not on how high)
• most analytical methods cannot fit
• recommended graphical techniques be used with known geologic and hydrologic conditions - probabilities calculated through Weibull against minimum floods
• data fit a straight line - if extrapolated 0.95 – 5% of less than zero
• fit exponentially by eye, maths complicated

Limitations of frequency analysis

As with any estimation technique there are several limitations in the application of frequency analysis; three of these are major:

1) Estimation technique only as good as the records its derived from

Where the records are short or of dubious quality very little worth can be achieved through frequency analysis. As a general rule of thumb you should not extrapolate average recurrence intervals beyond twice the length of your data set.

2) Each storm or low flow event is independent of another

This relatively easy to guard against in annual maximum or minimum series but more difficult for a peak threshold series

3) Hydrological regime has remained static

There is the inherent assumption made that the hydrological regime has remained static during the complete period of record. This may not be true where land or climate change has occurred in the catchment

Application Problems

a) Basin Development

• basin developments significant for low flows
• moderate diversion can reduce or eliminate flow

b) Multi-Year Events

• in regions of water scarcity and high development
• limited number of flows - multiannual inconsistent
• synthetically generated streams to derive drought frequency

c) Regionalisation

• regional regression procedures are often employed at ungauaged sites by using basin characteristics - it regression poor then drainage area ratio or regional statistics
• regression models have large prediction errors - don’t capture important land-surface and subsurface geological
• it may be possible to estimate low-flow rates on a per unit area basis for a given exceedance if the study area is relatively homogeneous with respect to geology, topography and climate