Transcript Surface Water

Surface Water
• Surface runoff - Precipitation or snowmelt which moves
across the land surface ultimately channelizing into
streams or rivers or discharging into lakes.
• Watershed - Land area which contributes surface runoff to
a specified point of interest  typically stream outlet Note:
entire area within watershed may not contribute runoff due
to depressions, detention ponds, etc.
• Watershed divide - Imaginary lines delineating adjacent
watersheds. Normally follow ridges and can be delineated
with topographical maps.
Surface Runoff
• Relationship between precipitation and runoff is influenced
by various storm and basin characteristics:
Storm
intensity
duration
areal extent
uniformity
Basin (or Watershed)
area, shape, slope
soil, vegetation, geology
stream patterns (length,branching)
antecedent moisture
land use
Surface Runoff
• Initially large portion of precipitation goes into surface
storage (initial abstraction), then soil moisture storage
(governed by infiltration equations).
• Both of these types of storage can be classified as either:
– i) retention storage - long term, depleted by evaporation
– ii) detention storage - short term, depleted by outflow
• After detention storage volume begins to fill, outflow
begins to occur. Can be
–
–
–
–
1)
2)
3)
4)
groundwater flow
unsaturated flow
overland flow
channel flow
Streamflow Hydrograph
• Plot of volumetric flow rate vs. time at a particular point in
stream or river. Gives spatially and temporally integrated
measure of runoff production at a point in stream.
• Annual streamflow hydrograph shows long term balance
of precipitation, evaporation and streamflow.
• Individual storm hydrograph - most widely used method of
evaluating surface runoff. Shows relationship between
peak streamflow and individual storms.
• Hydrographs are used to predict peak flow rates so that
hydraulic structures can be designed to accommodate flow
safely and to evaluate water quality effects associated with
surface runoff. Also integrating hydrograph over time
gives volumes needed to design reservoirs, detention ponds
etc.
Rainfall-Streamflow Relationships
• If we take a systems approach to rainfall-streamflow
relationship in a watershed:
interception
surface storage
rainfall
rate
observed
hyetograph
time
infiltration
subsurface storage
overland flow
channel flow
subsurface flow
stream
flow rate
direct
runoff
baseflow
time
• Excess rainfall - rainfall not retained as storage on land
surface or infiltrated into the soil, i.e., rainfall which
becomes direct runoff
• Graph of excess rainfall vs. time is called excess rainfall
hyetograph.
Rainfall-Streamflow Relationships
 Most excess rainfall-streamflow relationships use a systems approach
to develop a deterministic lumped unsteady model of the rainfalldischarge relationship.
 Watershed is treated as a “black-box” that produces output
hydrographs in response to input hydrographs, without detailed
consideration of the physical processes producing response.
 Systems approach is necessary because of difficulty in obtaining
spatial and temporal distribution of physical parameters and processes
which affect response (i.e., parameters needed for overland flow and
channel flow equations and to determine flow paths).
Best for predicting well-monitored watersheds within recorded bounds
of climatic record.
Rational Formula
• The simplest commonly used rainfall-discharge
relationship is the rational formula.
• Rational formula attempts to predict peak discharge for
extreme events in small urban areas. Usually used to
design ditches, canals and storm sewer networks.
• Assumes a storm of constant excess rainfall intensity
(precipitation-infiltration and initial storage losses) and
long duration completely covering watershed.
• Rational formula only gives peak discharge rate, no
information on time distribution of discharge.
Rational Formula
ie
Qp
lag time as
water
accumulates
on surface
input = output
storage filled
Qp = Aie
detention
volume
whole basin
responding
tc
drainage from
remote areas
reaches outlet
tr
response from nearby areas as
gravity force overcomes surface
resistance (overland flow)
• tc - time of concentration - The time at which entire
watershed begins to contribute runoff at outlet. Time of
flow from farthest point in watershed to outlet.
• tr - duration of storm
Time of Concentration
• Formula for the time of concentration
tc=0.00778L0.77S-0.385
where
L=hydraulic length (the distance from the most
remote point in the watershed to the outlet) in feet
S= average slope along the hydraulic length
expressed as a fraction
Rational Formula
•
Qp = CiA
• where Qp is the peak flow rate in cfs
C is an empirical runoff coefficient
A is watershed area in acres
• Note: 1cfs= 1.008 acre-in
• For the rational formula to be valid must have rainfall
duration > time of concentration for basin.
• Typically used to design structures to handle peak flows
from design storms with particular probability of
exceedence for time of duration  time of concentration
Example
• Determine the time of concentration and the peak
flow rate for a 1:25 year 30 minute design storm
with a rainfall intensity of 5 in/hr in a 100 acre
agricultural catchment having a 4000ft hydraulic
length with an average slope of 5%, hydrologic
soil group B.
tc=0.00778L0.77S-0.385=14.6 minutes
Qp = ciA = 0.21*5 in/hr*100 acre=105 cfs