eh l4 runoff and hydrograph analysis
DESCRIPTION
EH L4 Runoff and Hydrograph AnalysisTRANSCRIPT
LECTURE 6
RUNOFF AND HYDROGRAPH
ANALYSIS
At the end of the week, students should be able to:
draw the resultant hydrograph based on catchment characteristics and rainfall –runoff conditions;state and apply base flow separation techniques and estimate direct runoff values;derive Unit Hydrograph from historical data;change unit hydrograph time lag;derive Unit Hydrograph using synthetic methods.
Runoff and Surface RunoffRunoff (Discharge/Streamflow) includes all the water flowing in the stream channel at any given section.Consist of 3 constituents :
1. direct precipitation over surface of the stream – small portion of total flow where precipitation balances by evaporation and can be ignored.
2. surface runoff – true surface runoff and subsurface storm flow
3. groundwater inflow or base flow
RUNOFF = SURFACE RUNOFF + GROUNDWATER INFLOW
Hydrograph of Stream FlowGraphical representation of discharge flowing in a river at a given direction with passage of time.A plot between time (X-axis) and discharge (Y-axis)Represent discharge fluctuations in the river at a given site; indicates peak flow that governs the design of given hydraulic structure.Peak flow – maximum flow in the river due to any given storm.
4 types of hydrograph : a) annual hydrograph ; b) monthly hydrograph ; c) seasonal hydrograph d) flood hydrograph.
Runoff Characteristics of Streams
A. Perennial StreamsB. Intermittent StreamsC. Ephemeral Streams
Flood HydrographsRepresent the short-term runoff phenomenon where it is the response of a given catchment to a rainfall input.A typical single-peaked skew distribution of discharge.Consist 3 characteristic regions:
1. rising limb2. crest segment3. falling limb/recession limb (depletion
curve)
A. Shape of basinB. Size of basinC. Land use D. SlopeE. Drainage DensityF. Climatic Factors
Factors Affecting FloodHydrograph
Baseflow Separation
Establishing a relationship between surface-flow and effective rainfall.
Separation of quick-response flow (surface flow and subsurface flow) from slow response flow (base flow).
A. Method I-Straight-Line Method
Joining beginning of surface runoff, Point Ato a point on recession limb (end of direct runoff), Point B with a straight line. (Fig. 6.5, pg202)
Point A can be identified when sharp change of runoff rate at beginning of hydrograph while Point B can be determined from
daywhere A=drainage area in km2
2.083.0 AN
B. Method II
Extend base flow curve at beginning Point A till it intersects with ordinate drawn at the peak discharge at Point C. (Fig. 6.5, pg202)Connect Point AC and CB with a straight line that demarcate base flow and surface runoff.
C. Method III
Extend backward of base flow recession curve at Point F till it intersects the ordinate at point of inflection.
F with a straight line and Point F and A with an arbitrary smooth curve.
COMPUTING RUNOFF USING UNIT HYDRODGRAPH THEORY
A T-hour unit hydrograph is defined as the hydrograph of runoff produced by an intense excess rainfall of 1cm occurring uniformly over the entire drainage basin and at a uniform rate for the short specified duration of T-hour (unit duration).
There are 4 aspects of this definition that should be given special notice :
i. 1 cm depth of rainfall excess over basin area.
ii. Uniform spatial distribution of rainfall over the watershed.
iii. A rainfall excess rate that is constant with time
iv. Specific duration of rainfall excess.
Assumption in unit hydrograph :i. time invariance – runoff produced from a given
drainage basin due to a given effective rainfall shall always be the same irrespective of the time of its occurrence.
ii. Linear response-the runoff response of a drainage basin to the excess rainfall is assumed to be linear in which if an input x1(t) causes an output y1(t), and an input x2(t) causes an output y2(t), then an input x1(t) + x2(t) will cause an output y1(t) + y2(t).
Limitation of U.H. :i. excess rain only occurs uniformly over the
entire basinii. intensity should be constant during the
entire duration.iii. Unreliable for basins exceeding about 5000
km2 or less than 2km2.iv. Precipitation only considered from rainfallv. Catchment should not have unusual large
storages which will affect the linear relationship between storage and discharge.
vi. If precipitation is decidedly non-uniform, UH will not give good result.
S-Curve Hydrograph(Summation Curve Hydrograph)
Used for deriving unknown U.H. of desired unit duration. The duration of unknown hydrograph is either shorter or not an integral multiple of duration of known hydrograph.
S-curve is produced by continuous effective rainfall representing by a continuous rising curve, which ultimately attains a constant value when equilibrium discharge reached (entire catchment starts contributing to runoff).
When using S-curve for determining U.H. of unknown duration (t1), S-curve lagged by t1 hr when subtracted from origin S-curve is the unit hydrograph of t1 hr.
Synthetic Unit Hydrograph-Snyder’s Method
Apply for basins which are not gauged. U.H. are synthesized from known U.H. of a meteorologically homogeneous basin.
Most appropriate for large watersheds, but calibration of coefficients is recommended.
Formation of U.H. includes time to peak, time base, duration of rainfall excess, peak discharge, width of unit hydrograph at both 50% and 75% of peak discharge.
Time of peak depends on 2 elements: duration of rainfall excess (td) and time lag (tp).
Time lag,tp is the time interval from midpoint of unit rainfall excess to the peak of unit graph, tp = Ct (L Lca)0.3.
Duration of rainfall excess of t hours is given by
pp t
tt
112
5.5
Peak discharge Qps for unit hydrograph of standard unit duration of tr hour is given by
– For non-standard unit duration,tR, time to peak
and
- Peak discharge,
ppps t
ACQ 78.2
ppp t
ACQ'
78.2
42221
4'
Rp
rRpp
tt
tttt
Time base for this U.H, tb is given by
for large catchment while
for smaller catchment
The shape of Synder’s U.H. is largely controlled by 2 times parameters, W50 and W75 which represent the time widths of U.H. at discharges of 50% and 75% of peak discharge.
where and
hrstt pb '372
08.15087.5
qW
75.150
75WW
AQ
q pp
hrsttt Rpb
2'5
Caution should be used in applying Snyder’s method to a new area without first deriving coefficients for gauged streams in the general vicinity of the problem basin. The coefficients Ct and Cp have been found vary considerably form one region to another.