A peak rainfall and peak discharge are two different graphs. Lag time is the term for the difference between the peak rainfall from the peak discharge. There is a little chance of flooding if the value of the lag time is great. A lag time that is short will show that water had already reached the river channel at a fast rate. The rise in the discharge shown in a graph is called the rising limb, whereas the decrease in the discharge is called the falling limb.

Areas of large basins normally receive more precipitation than the small ones and the they have a larger runoff. A bigger size of runoff would mean that there is longer lag time as water has a longer distance to reach the river trunk. The shape of the basin that is elongated and larger produces a lower peak flow and longer lag time than a circular basin with the same size (Gillesania, 2006). The effects of the slopes are very important. The channel flow would fall faster down a steep slope therefore it produces a steeper rising limb and a shorter lag time. Considering the soil as an important factor, infiltration is greater on thick soil even if clay would act as impermeable layer. A longer lag time happens when there are more infiltration and more shallow rising limb

A fast overland flow is observed with a higher drainage density. …

The volume of water that reach the river from the surface run off is the overland flow, and the through flow is the volume of water reaching the river through the soil and its underlying rock la

Cynon River Hydrograph

Time over Discharge

Cynon River hydrograph

Rainfall over time

The data gathered from the Cynon river study the results above were plotted. The study was performed within a period of 4 days (96hours) continuously, taking the height reading every hour. We can notice from the readings that there is a steady flow from the start of the reading up to its reading in the 44th hour, then water began to rise after the reading of the 42th hour. The rising limb takes place at this time. The time between the rise of water and the time the water reaches the peak flow is known as basin lag time. As it reached the peak on the 55th hour, water hasalready reached the peak discharge and it is now beginning to fall down. The time water height starts to fall down is called recession limb. After the4 recession limb, discharge of the water will go back to its original flow.. The storm flow is called the total of the overland flow and the through flow. The overland flow is the flow when water rises above the through flow, and the through flow is the water that rises above the base flow.

Channel Design

The discharge in the is Q m3. Apply the Manning’s formula to design a suitable breadth b of a channel with depth d

v = R2/3S1/ 2 where: v = velocity, m/s

n R = hydraulic radius, m

S = channel bed slope, m/m

n = Manning’s coefficient of roughness

A = db where: a = area, m2

b