A hydrograph is a graph of the flow in a stream over a period of time. Reading horizontally to the left, you can determine the discharge of the stream for that date. “Flashy” To say that a hydrograph is flashy means that the graph depicts sharp vertical jumps and equally steep vertical declines.
What is the difference between a flashy and gentle hydrograph?
A gentle hydrograph shows the river is at low risk of flooding. Flashy hydrographs have a steep rising limb and a small lag time. This indicates the river discharge increases rapidly over a short period of time, indicating rainwater reaches the river very quickly. This means the river is more likely to flood.
What are the two types of hydrograph?
Types of hydrographs include: Storm hydrographs. Flood hydrographs. Annual hydrographs a.k.a. regimes.
Why are urban hydrographs flashy?
The Urban Hydrograph A Hydrograph is a graph which shows the discharge of a river, related to rainfall, over a period of time. As a result of the changes to urban drainage basins, urban hydrographs tend to be flashy, where rivers are more likely to flood due to lower interception and infiltration rates.
What is a flashy flow?
A “flashy” stream is one that experiences a rapid increase in flow shortly after onset of a precipitation event, and an equally rapid return to base conditions shortly after the end of the precipitation event.
What are Hydrographs used for?
In its simplest form, a hydrograph is a graphical representation of runoff rate against time (Figure 1). It shows the time distribution of runoff at the point of measurement or computation, reflecting the complex characteristics of the watershed by a single curve.
Why is lag time so important?
Lag time is an essential input to the most common synthetic unit-hydrograph models. The lag time for an ungaged stream must be estimated from the physical characteristics of the stream and its watershed. We determined lag times for 200 significant events and estimated the average lag time for each watershed.
Why do Hydrographs differ?
The shape of a hydrograph is altered by a few different things. One factor is the shape of the drainage basin. Drainage basins come in a wide assortment of shapes. Basins with steep slopes will have a high peak discharge and a short lag time because the water can travel faster downhill.
What is the opposite of a flashy hydrograph?
These are often referred to as ‘flashy’ hydrographs. In contrast, there are some hydrographs with gently inclined limbs, a low peak discharge and a long lag time. These are called ‘delayed’, or ‘flat’ or ‘subdued’ hydrographs.
What causes short lag time?
Slope – steep slopes lead to rapid water transfer and shorter lag times. Gentle slopes slow down water transfer making the lag time longer. Vegetation – if there is no vegetation in an area, the water runs off into the river quicker, therefore it would have a short lag time.
How does vegetation affect Hydrographs?
If a drainage basin has a significant amount of vegetation this will have a significant affect on a storm hydrograph. Vegetation intercepts precipitation and slows the movement of water into river channels. This increases lag time. Water is also lost due to evaporation and transpiration from the vegetation.
What are flashy streams?
Stream that rapidly collects flows from the steep slopes of its catchment (watershed, basin) and produces flood peaks soon after the rain. Its flow quickly subsides after the cessation of rainfall.
What are the characteristics of a flashy hydrograph?
Flashy hydrographs have a steep rising limb and a small lag time. This indicates the river discharge increases rapidly over a short period of time, indicating rainwater reaches the river very quickly. This means the river is more likely to flood.
What is a flood hydrograph and how does it work?
A flood hydrograph shows the amount of rainfall in an area and the discharge of a river. The discharge of a river is the volume of water passing a point each second. It is expressed in cumecs (cubic metres per second). River discharge is displayed as a line graph. Precipitation is shown as a graph and is usually displayed in millimetres.
What is a delayed hydrograph?
These are called ‘delayed’, or ‘flat’ or ‘subdued’ hydrographs. When it comes to evaluating the factors affecting the character of storm hydrographs, particularly their ‘flashiness’, none is more important than urbanisation.
What is the shape of a hydrograph?
The shape of the hydrograph also varies from one river to another. This is a result of the particular physical characteristics of individual drainage basins. Some hydrographs have very steep limbs, especially rising limbs, a high peak discharge and a short lag time. These are often referred to as ‘flashy’ hydrographs.
