SME
Hydrology · Problem type
A storm hydrograph
Use this for “Rain is heaviest at hour 2 and the stream peaks at hour 5. What is the lag time, and why is there one?”
Record and look
Discharge (m³/s)
3
3
5
12
25
34
30
22
15
10
7
5
4
Rain (mm)
2
10
14
6
1
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
7
8
9
10
11
12
0
1
2
3
4
5
6
7
8
9
10
11
12
Rain (mm)
2
10
14
6
1
0
0
0
0
0
0
0
0
Discharge (m³/s)
3
3
5
12
25
34
30
22
15
10
7
5
4
Rain (mm)
Discharge (m³/s)
The flow peaks 3 hours after the heaviest rain (hour 2 to hour 5), then falls more slowly than it rose.
Tap a bar at the height you want.
Assumptions
•
Columns are hours from the start of the storm: rain is the depth that fell in that hour, discharge the flow at the outlet.
•
The lag time runs from the heaviest rain to the highest flow: water takes time to cross the basin.
•
Before and after the storm the stream carries its base flow, fed by groundwater.
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Related lessons
Surface runoff
Main lesson
Storm runoff by the curve number
Use this for “A basin with curve number 80 gets 75 mm of rain in a storm. How much runs off?”, or the curve number from a storm’s rain and runoff.
Channel flow by Manning’s equation
Use this for “A channel 10 m wide flows 2 m deep on a slope of 0.001 with n = 0.035. What are its speed and discharge?”, or the depth that carries 50 m³/s.