Methods Of Estimation Of Flood Discharg
Methods of Estimation of flood discharge
4.1 Introduction
After estimating the change in rainfall, as described in chapter 3, the next step is to convert that rainfall change into a flood flow (an amount of water flowing in a river). This chapter looks at both screening and advanced tools that can be used to help river managers estimate changes in flood flows.
Historical data and ongoing data campaigns are vital components of any forecasts of flood flows. Although climate change means that future flow statistics will be different from those in the past, they are necessary to calibrate any model of river flow. Past extreme events can be used as indicators of future trends and are invaluable for assessing how climate change has affected river flows.
4.2 Screening methods
There are many different screening methods available to assess changes in flows in a changing climate. Common to each is relative ease of use and the ability to tune the method to replicate historical events, because they are all simple empirical methods. This ease, however, comes at a cost. By restricting themselves to historical data and by only vaguely representing real-world processes, screening methods generally offer less confidence in making forecasts for events that fall outside the range of historical observations.
Empirical screening methods generally draw on a few basic approaches: the Rational Method, the US Soil Conservation Service (SCS) method, and the unit hydrograph. Here we discuss the Rational Method and one approach presently used in New Zealand that employs both the SCS and unit hydrograph methods.
4.2.1 Unit hydrograph and SCS methods
The unit hydrograph method reflects how a catchment converts a hyetograph (a graph of the distribution of rainfall over time) into a hydrograph (a graph showing changes in river flow over time), while the SCS method empirically relates peak flood flow to rainfall using land-cover-related parameters. To illustrate how the unit hydrograph and SCS
4.1 Introduction
After estimating the change in rainfall, as described in chapter 3, the next step is to convert that rainfall change into a flood flow (an amount of water flowing in a river). This chapter looks at both screening and advanced tools that can be used to help river managers estimate changes in flood flows.
Historical data and ongoing data campaigns are vital components of any forecasts of flood flows. Although climate change means that future flow statistics will be different from those in the past, they are necessary to calibrate any model of river flow. Past extreme events can be used as indicators of future trends and are invaluable for assessing how climate change has affected river flows.
4.2 Screening methods
There are many different screening methods available to assess changes in flows in a changing climate. Common to each is relative ease of use and the ability to tune the method to replicate historical events, because they are all simple empirical methods. This ease, however, comes at a cost. By restricting themselves to historical data and by only vaguely representing real-world processes, screening methods generally offer less confidence in making forecasts for events that fall outside the range of historical observations.
Empirical screening methods generally draw on a few basic approaches: the Rational Method, the US Soil Conservation Service (SCS) method, and the unit hydrograph. Here we discuss the Rational Method and one approach presently used in New Zealand that employs both the SCS and unit hydrograph methods.
4.2.1 Unit hydrograph and SCS methods
The unit hydrograph method reflects how a catchment converts a hyetograph (a graph of the distribution of rainfall over time) into a hydrograph (a graph showing changes in river flow over time), while the SCS method empirically relates peak flood flow to rainfall using land-cover-related parameters. To illustrate how the unit hydrograph and SCS