ANDRES BONIFACIO COLLEGE
College Park, Miputak, Dipolog City
SCHOOL OF ENGINEERING
CE 328 – HYDROLOGY
9:00-11:00 A.M. FRIDAY
RUNOFF
GROUP 7
ABAYA, MARY GRACE S.
ABELLANOSA, ARTFEL
APILAN, ROMEO
ARWAJIB, ASIMUDDIN
BURGUITE, CLENTON S.
BUTALID, MARKY VINCE
ELOPRE, KEYSHA
INSTRUCTOR:
GERALD E. SALAVERIA, RCE, RMP
A. Runoff
- Runoff occurs when there is more water present than the land can absorb. The
excess liquid flows across the surface of the land and into nearby creeks,
streams, or ponds.
B. Overland Flow
- It is the water that run across the land after the rainfall, either before it enters a
watercourse, after it leaves a watercourse as floodwater, or after it rises to the
surface naturally from underground. It does not include: water that has naturally
infiltrated the soil in normal farming operations.
C. Surface Runoff
- A flow travelling all the time over the surface as overland flow and through the
channel as open channel flow and reaching the catchment area.
D. Interflow
- It is a part of precipitation that infiltrates and moves laterally through upper
crusts of the soil and returns to the surface at some location away from the
point of entry into the soil. This sometimes called as through flow, storm
seepage, subsurface flow, or quiz return flow.
E. Groundwater Flow or Groundwater Runoff
- It is the slowly movement of groundwater through the spaces and cracks
between the soil particles on its journey to lower elevations.
F. Classification of Runoff
1. Direct Runoff
- The part of runoff that enters the stream immediately after the precipitation. It
includes surface runoff, prompt interflow, and precipitation on the channel
surface. This is sometimes called as direct storm runoff or storm runoff .
2. Baseflow
- It is the delayed flow, which after falling on the ground surface, infiltrates into
the soil and meets the water-table, and flow to the streams, oceans, etc.
G. Runoff Hydrograph
- It is an expression for surface water discharge over time. It is an expression of
the watershed characteristics that invariably govern the relationship between
rainfall and the resulting runoff. It represents the integrated effects of rainfall
and watershed characteristics, such as area, shape, drainage patterns, land
use, and channel properties.
H. Watershed Characteristics Affecting Runoff Hydrograph
a. Area (A) – the total volume of runoff and peak discharge are proportional to the
area of the watershed.
b. Overland Slope (So) – this is the average vertical elevation change per
horizontal distance. In the general case, the time it takes water to get to a point
of discharge is the sum of overland flow and channel or pipe flow. As watershed
size increase, overland slope decreases in significance. The greater the
overland flow time, (less slope), the less is the peak discharge.
c. Channel Slope (Sc) – the average vertical elevation change per horizontal
distance along a watercourse bed. The steeper the channel slope, the greater
the velocity and the peak discharge.
d. Channel Area (Ac) – the cross-sectional has an effect on storage and thus
attenuation of the hydrograph can be expected as storage increases.
Attenuation is the term used to express a hydrograph shape with no longer
base time and less peak discharge.
e. Soil Type and Vegetation Cover – this affects the amount of rainfall excess and
thus the peak of a hydrograph. The greater the initial abstraction and infiltration,
the less the rainfall excess.
f. Basin Length (L) – the travel length associated with the longest time it takes for
a particle of water to flow overland. The basin length and slope determines the
watershed time of concentration.
g. Stream Pattern and Watershed Shapes – a fan-shaped area with streams
radiating from the same points suggests contributing incremental areas
increasing with time such that a late but high peak in the hydrograph is
suggested, whereas an elongated area traversed by one major stream with
some relatively uniformly spaced tributaries suggest a less pronounced rise
and fall of the hydrograph.
h. Channel Roughness – the frictional resistance, and its moderating effect on
flow velocities, provided by: the sizes of instream material on stream beds and
banks. The lower the roughness the higher the velocity and possibly the peak
discharge.
I. Time of Concentration
- It is defined as the longest travel time it takes of a particle of water to reach a
discharge point in a watershed.
Time of Concentration Formula for Overland Flow:
1. Izzard’s Formula
𝟒𝟏 𝑲𝑳𝟏/𝟑
tc =
(for i x L < 500)
𝒊𝟐/𝟑
where:
tc = time of concentration (min)
L= overland flow (ft)
i = rainfall intensity (in/hr)
K = (0.0007i + Cr) / s1/3
and
s = slope (ft/ft)
Cr = retardance coefficient, given as:
Very smooth asphalt
0.007
Tar and sand pavement
0.0075
Crushed-slate roof
0.0082
Concrete
0.012
Tar and gravel pavement
0.017
Closely slipped sod
0.046
Dense grass
0.060
Sample Problem:
Estimate the time of concentration using Izzard’s formula. There is 350 ft overland flow,
10% flow slope and the rainfall intensity, 1.5 in/hr is for short duration storm. The area is
closely-slipped sodded.
Given:
L = 350 ft
i = 1.5 in/hr
S = 0.10
Cr = 0.046
k=
k=
(𝟎.𝟎𝟎𝟎𝟕 𝒊+𝑪𝒓)
𝟏
(𝒔)𝟑
(𝟎.𝟎𝟎𝟎𝟕 (𝟏.𝟓)+𝟎.𝟎𝟒𝟔)
(𝟎.𝟏𝟎)𝟏/𝟑
k = 0.101 in/hr
tc =
tc =
𝟒𝟏 𝑲𝑳𝟏/𝟑
𝒊𝟐/𝟑
𝟒𝟏 (𝟎.𝟏𝟎𝟏) (𝟑𝟓𝟎)𝟏/𝟑
(𝟏.𝟓)𝟐/𝟑
tc = 22.35 min
2.Kerby’s Equation
𝑳𝒏 𝟎.𝟒𝟔𝟕
tc =𝒄 ( 𝒔)
√
for L < 365 m (1000ft)
where:
tc = time of concentration (min)
L = length of flow (ft) (generally less than 1000 ft)
s = slope (ft/ft)
c = 0.83 (when using in feet) or 1.44 (when using meters)
n = retardance roughness coefficient, given as:
smooth pavement
0.02
poor grass, bare sod
0.30
average grass
0.40
dense grass
0.80
Calculation of Runoff by Rational Method
Q=ciA
where:
c = runoff coefficient, variable with land use
i = intensity of rainfall of chosen frequency for a duration equal to the time of concentration
tc(in/hr)
tc = time for rainfall at the most remote portion of the basin to enter or travel to the outlet
(min,hr)
A = area of the watershed (acres, sq.m)
If c is not given:
c = t / (8 + t)
for impervious surface
c = 0.3 t / (20 + t)
for pervious surface
Intensity Formula (Talbot’s Formula)
𝟏𝟎𝟓
𝒊 = 𝐭+𝟏𝟓 inches/hr
𝒊=
𝟑𝟑𝟎𝟎
𝐭+𝟏𝟗
mm/hr
Basic assumptions for using the rational formula
1. The rainfall intensity must be constant for a time interval at least equal to the time
of concentration.
2. The runoff is a maximum when the rainfall intensity lasts as long as the time of
concentration.
3. The runoff coefficient is constant during the storm volume.
4. The watershed area does not change during the storm.
Sample Problem:
A 310,000 sq.ft. pervious dense grass surface has an overland length of 890 ft with slope
2.5 x 10-4. Estimate the time and discharge using Kerby’s equation.
Given:
𝑳𝒏 𝟎.𝟒𝟔𝟕
L = 890 ft
tc =𝒄 ( 𝒔)
√
s = 2.5 x 10-4
tc =0.83 (
c = 0.83 ft
890 𝑥 0.80 0.467
√2.5 x 10−4
)
tc = 123.67 min
n = 0.80
Rational Method
𝟎.𝟑 𝒕
c = 𝟐𝟎+𝐭 =
𝟏𝟎𝟓
𝟎.𝟑 𝒙 𝟏𝟐𝟑.𝟔𝟕 𝒎𝒊𝒏
𝟐𝟎+𝟏𝟐𝟑.𝟔𝟕 𝐦𝐢𝐧
𝟏𝟎𝟓
= 𝟎. 𝟐𝟓𝟖
𝒊𝒏
𝒊 = 𝐭+𝟏𝟓 = 𝟏𝟐𝟑.𝟔𝟕+𝟏𝟓 = 𝟎. 𝟕𝟓𝟕 𝒉𝒓
Q = 𝒄𝒊𝑨
𝑖𝑛
1𝑓𝑡
1ℎ𝑟
1𝑚𝑖𝑛
=(0.258) (0.757 ℎ𝑟 ) (310,000 𝑠𝑞. 𝑓𝑡) (12in) (60𝑚𝑖𝑛) (60𝑠𝑒𝑐)
Q = 1.40 ft3/s