So now that we’re all well versed in hydrology (crawl my archives in the science or water category if you missed the previous posts), what exactly are the products of all that hydrology and what do we do with them?
Product number one is the peak flow. Peak flow is exactly what it sounds like, the maximum amount of water flow that will move through our drainage way for a given recurrence interval. We previously did all that work to get a flow frequency curve, so now we know what the peak flow is for a storm of any recurrence interval that we want to consider in our design.
We’re going to use the peak flow in the next part of our design process where we will take into account flow, channel size, channel vegetation, and various other factors and figure out how deep the water will be for that peak flow. If we’re designing a bridge or other type of drainage structure we’ll use the peak flow to figure out how big we want to make that structure.
A flow frequency curve can also be used in reverse. Say for instance, you developed a flow frequency curve for the Wolf River in Memphis. Then half of Shelby County floods as it did in spring of 2011. You go out to the US-51 bridge during the flood and measure the amount of water then take that back to your flow frequency curve to see how bad the flood is. That’s useful for telling people who live on the river how often they can expect the water to get that high again.
You can also use a flow frequency curve for big picture analysis like this color coded map from the USGS showing where flows are higher or lower than normal.
The other product of our hydrology study might be a hydrograph. A hydrograph is a visual representation of how the flow changes over time at a particular location. These are important for various reasons including charting how land use changes flood levels and determining how a tributary flood will affect the larger river it flows into (such as a flood on the Stones River affecting the Cumberland River at downtown Nashville). Hydrographs can also be used to give advance warning to people on properties that will flood soon.
This particular hydrograph shows a single storm, with rainfall amounts also plotted. The purple bars show the precipitation. You can see how the water starts to rise as rainfall increases. The flow on the hydrograph hits its highest point after the storm has ended because it takes some time for rainfall in the upper part of the watershed to runoff all the way down to the bottom of the watershed. Then flow slowly tapers off as everything begins to dry out again.