Core Concepts for the Civil PE Exam Water Resources and Environmental Depth
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Civil PE Exam Water Resources and Environmental Depth

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Additionally, another 40 Water Resources and Environmental Depth practice problems with detailed solutions

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Civil PE Exam Water Resources Online Study Guide
Click below to expand each topic. All of this is included in the paperback version of the Water Resources Depth
Analysis and Design
Mass Balance
Mass balance refers to the conservation of mass as it enters and exits a system. This concept is essential to the Water Resources and Environmental exam as it is a common method in solving for unknowns in many types of problems. This principle simply put is that mass is always conserved in fluid systems regardless of the properties of that system. Therefore what enters the system must also exit the system. The equation can be represented as mass:
M1 = M2
M1 = Mass entering the system
M2 = Mass exiting the system
The equation can also be represented by equating flow rates. In this case the equation is commonly known as the continuity equation:
Q1 = Q2
Q1 = Flow rate into system
Q2 = Flow rate out of a system
Given that the flow rate is equal to the area multiplied by the velocity, the equation can be used to equate these variables as well:
A1V1 = A2V2
Hydraulic Loading
Hydraulic loading refers to the flows in MGD (Million Gallons per Day) or cu. Ft./day to a treatment plant or treatment process. The equation is as follows:
HLR=QA
HLR = Hydraulic loading rate
Q = Flow rate
A = Surface area of the wet basin
Detention time is the amount of time it takes a given volume of wastewater to pass through the clarifier:
td=VQ
V = Volume of clarifier
Solids Loading
Solids loading similarly to hydraulic loading is the amount of suspended solids in a substance as it flows to the treatment facility. Solids loading is expressed as the following:
Hydraulic Flow Measurement
Many flow devices are available to measure either the flow rate or velocity of a given system. These methods are often used in conjunction with the laws of energy and mass conservation to be able to analyze a system. Here are a few of the more prevalent ones for the purposes of the PE exam:
 Pitot Static Tubes
g = Force effect due to gravity (32.2 ft/s2)
h = Difference in elevations of the fluid columns (ft)
ρ = Density of water (62.4 lb/ft3)
ρm = Density of manometer fluid (lb/ft3)
v = Velocity (ft/s)
Cf = Flow coefficient
Ao = Orifice area
A1 = Pipe area
p = Pressure
ρ = Density
g = Force due to gravity
Cd = Discharge coefficient
Cc = Coefficient of contraction
Fva =Velocity of approach factor
HydraulicsClosed Conduit
Storm Characteristics
A design storm must be specified when performing any calculations. The design storm is defined by its recurrence interval which is the given amount of time it is likely to see a storm of a certain intensity. Design storms are often 10, 20, 50, or 100year storms meaning a storm of a certain intensity would only occur once within the given duration.
Runoff Analysis
The rational method can be used to determine the flow rate from runoff of a drainage area. The equation is:
Q = ACi
Q = Flow Rate (cfs)
A = Drainage Area (Acres)
C = Runoff Coefficient
i = Rainfall Intensity (in/hr)
NRCS/SCS Runoff Method
This is an alternative method for determining runoff:
S = Storage Capacity of Soil (in.)
CN = NRCS Curve Number
Q = Runoff (in.)
Pg = Gross Rain Fall (in.)
Hydrographs
Hyetographs – Graphical representation of rainfall distribution over time
Hydrograph – Graphical representation of rate of flow vs time past a given point often in a river, channel, or conduit. The area under the hydrograph curve is the volume for a given time period
Parts of a Hydrograph are shown graphically:
Unit Hydrographs can be determined by dividing the points on the typical hydrograph by the average excess precipitation.
Synthetic Hydrographs are created if there is insufficient data for a watershed. This method uses the NRCS curve number and is a function of the storage capacity.
To develop the synthetic hydrograph, you must calculate the time to peak flow:
tR = Storm duration (time)
Lo = Length overland (ft)
SPercentage = Slope of land
The equation for peak discharge from a synthetic hydrograph then is:
Rainfall
Storm characteristics include duration, total volume, and intensity
Duration: The length of time of a storm. Often measured in days and hours.
Total Volume: The entire amount of precipitation throughout the duration of the storm in a defined area.
Storm Intensity: Total volume of the storm divided by the duration of the storm event. Intensities can be averaged over the entire storm or at shorter intervals to provide instantaneous high intensity portions of the storm. Hyetographs are bar graphs used to measure instantaneous rainfall intensities over time and are covered below.
Stormwater Management
Detention and retention ponds are often used to collect water for flood control and stormwater runoff treatment.
Detention Ponds: Also known as dry ponds. These are ponds which are often kept dry except during flood events. The pond will fill up during increased precipitation to control the flow intensity. This is common in dry, arid, or urban areas to prevent excessive flooding. The ponds typically will be designed to hold water for about 24 hours. Detention ponds also control the amount of sediment since it is captured in the pond and then typically becomes accessible after the pond has dried.
Retention Ponds: Also called wet ponds since they contain a volume of water at all times. The elevation of the water will vary depending on precipitation but will always maintain a permanent amount of water based on low flow conditions. This allows sediment control since the deposits will settle to the bottom and allow for collection.
Infiltration is the rate of which water seeps into the ground. The Horton equation can be used to approximate this rate. This assumes that the water supply is infinite and the ground is saturated:
Time of Concentration
Depletions
The change in storage for a body of water can be approximated from the following equation:
ΔS=P+R+GIGOETO
S = Storage
P = Precipitation
R = Runoff
GI = Groundwater inflow
GO = Groundwater outflow
E = Evaporation
T = Transpiration
O = Surface water release
Stream Gauging
Stream gauging is the measurement of a stream channel to determine the discharge by obtaining the depth and velocity of the channel over time. The channel can be approximated by areas created by connected the dots of the measured depths. The discharge can be calculated by the following:
w = Width of cross section (ft)
y = Height of cross section (ft)
v = Velocity at indicated cross section (ft/s)
HydraulicsOpen Channel
Open Channel Flow
For open channel flow use the ChezyManning equation:
Q = (1.49/n)AR2/3S1/2
Q = Flow Rate (cfs)
n = Roughness Coefficient
A = Area of Water (ft2)
R = Hydraulic Radius (ft)
S = Slope (decimal form)
The hydraulic radius is the area of water divided by the wetted perimeter which is the perimeter of the sides of the channel which are in contact with water.
Hydraulic Energy Dissipation
A weir is a low dam used to control the flow of water. Weirs have shaped outlets notched into the top of the dam to allow water to flow out. The most common shapes are triangular and trapezoidal:
Triangular Weir
H = Height of water (ft)
θ = Weir angle
Trapezoidal Weir
b = Width of base (ft)
Broad Crested Weirs (Spillways)
Spillways are used to control the flow of excess water from a dam structure. Essentially they are large weirs and therefore can be called broad crested weirs. The calculation of discharge for spillways is taken as:
However, often in a dam situation the approach velocity can be taken as zero since it is so small and the equation becomes:
Cs = Spillway coefficient
Stormwater Collection and Drainage
There are many components used in the collection of stormwater. Some examples include:
Culverts: A pipe carrying water under or through a feature. Culverts often carry brooks or creeks under roadways. Culverts must be designed for large intensity storm events.
Stormwater Inlets: Roadside storm drains which collect water from gutter flow or roadside swales.
Gutter/Street flow: Flow which travels along the length of the street. Gutter flow can be approximated often by an adaptation of the Manning Equation:
K = Gutter flow constant = 0.56 ft3/(s*ft)
z = Inverse of the cross slope of the gutter (Decimal)
n = Roughness coefficient
s = Slope of the gutter (Decimal)
y = water depth at the curb (ft)
Storm Sewer Pipes: Pipes installed under the road which carry the water from inlets to a suitable outlet.
Sub and Supercritical Flow
Sub and super critical flows are relative to the critical flow depth. This depth is defined as that which minimizes the energy of the flow of water for a given channel cross section and slope. The critical flow depth is important because since the energy is minimized the flow rate is maximized.
So when the flow depth is greater than the critical depth, the flow is subcritical and the velocity is less than the critical velocity. When the depth is less than the critical depth, the flow is supercritical and the velocity is faster than the critical.
For a rectangular channel, the following equations can be used to determine the critical depth and critical velocity:
dc = Critical depth of flow (ft)
Q = Flow rate (ft3/s)
w = Width (ft)
The Froude number is used to qualify a flow channel and can be used to determine if it is sub or supercritical. The number is dimensionless:
v = Velocity (ft/s)
L = Characteristic length and is determined based on the channel geometry.
For a rectangular section L = d
For a circular section flowing half full L = πD/8
For trapezoidal and semicircle sections L = the area of flow/top width of channel
If the Froude number is less than 1, the flow is subcritical
If the Froude number is greater than 1, the flow is supercritical
A hydraulic jump is a rise in water elevation due to a supercritical flow abruptly meeting a subcritical flow. The following equations can determine the heights and velocities in a hydraulic jump for rectangular sections:
d1=depth of supercritical flow (ft)
d2=depth of subcritical flow (ft)
Fr = Froude Number for associated depth
v = Velocity at associated depth (ft/s)
g = Force effect due to gravity (32.2 ft/s2)
Hydrology
Storm Characteristics
A design storm must be specified when performing any calculations. The design storm is defined by its recurrence interval which is the given amount of time it is likely to see a storm of a certain intensity. Design storms are often 10, 20, 50, or 100year storms meaning a storm of a certain intensity would only occur once within the given duration.
Runoff Analysis
The rational method can be used to determine the flow rate from runoff of a drainage area. The equation is:
Q = ACi
Q = Flow Rate (cfs)
A = Drainage Area (Acres)
C = Runoff Coefficient
i = Rainfall Intensity (in/hr)
NRCS/SCS Runoff Method
This is an alternative method for determining runoff:
S = Storage Capacity of Soil (in.)
CN = NRCS Curve Number
Q = Runoff (in.)
Pg = Gross Rain Fall (in.)
Hydrographs
Hyetographs – Graphical representation of rainfall distribution over time
Hydrograph – Graphical representation of rate of flow vs time past a given point often in a river, channel, or conduit. The area under the hydrograph curve is the volume for a given time period
Parts of a Hydrograph are shown graphically:
Unit Hydrographs can be determined by dividing the points on the typical hydrograph by the average excess precipitation.
Synthetic Hydrographs are created if there is insufficient data for a watershed. This method uses the NRCS curve number and is a function of the storage capacity.
To develop the synthetic hydrograph, you must calculate the time to peak flow:
tR = Storm duration (time)
Lo = Length overland (ft)
SPercentage = Slope of land
The equation for peak discharge from a synthetic hydrograph then is:
Rainfall
Storm characteristics include duration, total volume, and intensity
Duration: The length of time of a storm. Often measured in days and hours.
Total Volume: The entire amount of precipitation throughout the duration of the storm in a defined area.
Storm Intensity: Total volume of the storm divided by the duration of the storm event. Intensities can be averaged over the entire storm or at shorter intervals to provide instantaneous high intensity portions of the storm. Hyetographs are bar graphs used to measure instantaneous rainfall intensities over time and are covered below.
Stormwater Management
Detention and retention ponds are often used to collect water for flood control and stormwater runoff treatment.
Detention Ponds: Also known as dry ponds. These are ponds which are often kept dry except during flood events. The pond will fill up during increased precipitation to control the flow intensity. This is common in dry, arid, or urban areas to prevent excessive flooding. The ponds typically will be designed to hold water for about 24 hours. Detention ponds also control the amount of sediment since it is captured in the pond and then typically becomes accessible after the pond has dried.
Retention Ponds: Also called wet ponds since they contain a volume of water at all times. The elevation of the water will vary depending on precipitation but will always maintain a permanent amount of water based on low flow conditions. This allows sediment control since the deposits will settle to the bottom and allow for collection.
Infiltration is the rate of which water seeps into the ground. The Horton equation can be used to approximate this rate. This assumes that the water supply is infinite and the ground is saturated:
Time of Concentration
Depletions
The change in storage for a body of water can be approximated from the following equation:
ΔS=P+R+GIGOETO
S = Storage
P = Precipitation
R = Runoff
GI = Groundwater inflow
GO = Groundwater outflow
E = Evaporation
T = Transpiration
O = Surface water release
Stream Gauging
Stream gauging is the measurement of a stream channel to determine the discharge by obtaining the depth and velocity of the channel over time. The channel can be approximated by areas created by connected the dots of the measured depths. The discharge can be calculated by the following:
w = Width of cross section (ft)
y = Height of cross section (ft)
v = Velocity at indicated cross section (ft/s)
Groundwater and Wells
Mass Balance
Mass balance refers to the conservation of mass as it enters and exits a system. This concept is essential to the Water Resources and Environmental exam as it is a common method in solving for unknowns in many types of problems. This principle simply put is that mass is always conserved in fluid systems regardless of the properties of that system. Therefore what enters the system must also exit the system. The equation can be represented as mass:
M1 = M2
M1 = Mass entering the system
M2 = Mass exiting the system
The equation can also be represented by equating flow rates. In this case the equation is commonly known as the continuity equation:
Q1 = Q2
Q1 = Flow rate into system
Q2 = Flow rate out of a system
Given that the flow rate is equal to the area multiplied by the velocity, the equation can be used to equate these variables as well:
A1V1 = A2V2
Hydraulic Loading
Hydraulic loading refers to the flows in MGD (Million Gallons per Day) or cu. Ft./day to a treatment plant or treatment process. The equation is as follows:
HLR=QA
HLR = Hydraulic loading rate
Q = Flow rate
A = Surface area of the wet basin
Detention time is the amount of time it takes a given volume of wastewater to pass through the clarifier:
td=VQ
V = Volume of clarifier
Solids Loading
Solids loading similarly to hydraulic loading is the amount of suspended solids in a substance as it flows to the treatment facility. Solids loading is expressed as the following:
Hydraulic Flow Measurement
Many flow devices are available to measure either the flow rate or velocity of a given system. These methods are often used in conjunction with the laws of energy and mass conservation to be able to analyze a system. Here are a few of the more prevalent ones for the purposes of the PE exam:
 Pitot Static Tubes
g = Force effect due to gravity (32.2 ft/s2)
h = Difference in elevations of the fluid columns (ft)
ρ = Density of water (62.4 lb/ft3)
ρm = Density of manometer fluid (lb/ft3)
v = Velocity (ft/s)
Cf = Flow coefficient
Ao = Orifice area
A1 = Pipe area
p = Pressure
ρ = Density
g = Force due to gravity
Cd = Discharge coefficient
Cc = Coefficient of contraction
Fva =Velocity of approach factor
Wastewater Collection and Treatment
Storm Characteristics
A design storm must be specified when performing any calculations. The design storm is defined by its recurrence interval which is the given amount of time it is likely to see a storm of a certain intensity. Design storms are often 10, 20, 50, or 100year storms meaning a storm of a certain intensity would only occur once within the given duration.
Runoff Analysis
The rational method can be used to determine the flow rate from runoff of a drainage area. The equation is:
Q = ACi
Q = Flow Rate (cfs)
A = Drainage Area (Acres)
C = Runoff Coefficient
i = Rainfall Intensity (in/hr)
NRCS/SCS Runoff Method
This is an alternative method for determining runoff:
S = Storage Capacity of Soil (in.)
CN = NRCS Curve Number
Q = Runoff (in.)
Pg = Gross Rain Fall (in.)
Hydrographs
Hyetographs – Graphical representation of rainfall distribution over time
Hydrograph – Graphical representation of rate of flow vs time past a given point often in a river, channel, or conduit. The area under the hydrograph curve is the volume for a given time period
Parts of a Hydrograph are shown graphically:
Unit Hydrographs can be determined by dividing the points on the typical hydrograph by the average excess precipitation.
Synthetic Hydrographs are created if there is insufficient data for a watershed. This method uses the NRCS curve number and is a function of the storage capacity.
To develop the synthetic hydrograph, you must calculate the time to peak flow:
tR = Storm duration (time)
Lo = Length overland (ft)
SPercentage = Slope of land
The equation for peak discharge from a synthetic hydrograph then is:
Rainfall
Storm characteristics include duration, total volume, and intensity
Duration: The length of time of a storm. Often measured in days and hours.
Total Volume: The entire amount of precipitation throughout the duration of the storm in a defined area.
Storm Intensity: Total volume of the storm divided by the duration of the storm event. Intensities can be averaged over the entire storm or at shorter intervals to provide instantaneous high intensity portions of the storm. Hyetographs are bar graphs used to measure instantaneous rainfall intensities over time and are covered below.
Stormwater Management
Detention and retention ponds are often used to collect water for flood control and stormwater runoff treatment.
Detention Ponds: Also known as dry ponds. These are ponds which are often kept dry except during flood events. The pond will fill up during increased precipitation to control the flow intensity. This is common in dry, arid, or urban areas to prevent excessive flooding. The ponds typically will be designed to hold water for about 24 hours. Detention ponds also control the amount of sediment since it is captured in the pond and then typically becomes accessible after the pond has dried.
Retention Ponds: Also called wet ponds since they contain a volume of water at all times. The elevation of the water will vary depending on precipitation but will always maintain a permanent amount of water based on low flow conditions. This allows sediment control since the deposits will settle to the bottom and allow for collection.
Infiltration is the rate of which water seeps into the ground. The Horton equation can be used to approximate this rate. This assumes that the water supply is infinite and the ground is saturated:
Time of Concentration
Depletions
The change in storage for a body of water can be approximated from the following equation:
ΔS=P+R+GIGOETO
S = Storage
P = Precipitation
R = Runoff
GI = Groundwater inflow
GO = Groundwater outflow
E = Evaporation
T = Transpiration
O = Surface water release
Stream Gauging
Stream gauging is the measurement of a stream channel to determine the discharge by obtaining the depth and velocity of the channel over time. The channel can be approximated by areas created by connected the dots of the measured depths. The discharge can be calculated by the following:
w = Width of cross section (ft)
y = Height of cross section (ft)
v = Velocity at indicated cross section (ft/s)
Water Quality
Storm Characteristics
A design storm must be specified when performing any calculations. The design storm is defined by its recurrence interval which is the given amount of time it is likely to see a storm of a certain intensity. Design storms are often 10, 20, 50, or 100year storms meaning a storm of a certain intensity would only occur once within the given duration.
Runoff Analysis
The rational method can be used to determine the flow rate from runoff of a drainage area. The equation is:
Q = ACi
Q = Flow Rate (cfs)
A = Drainage Area (Acres)
C = Runoff Coefficient
i = Rainfall Intensity (in/hr)
NRCS/SCS Runoff Method
This is an alternative method for determining runoff:
S = Storage Capacity of Soil (in.)
CN = NRCS Curve Number
Q = Runoff (in.)
Pg = Gross Rain Fall (in.)
Hydrographs
Hyetographs – Graphical representation of rainfall distribution over time
Hydrograph – Graphical representation of rate of flow vs time past a given point often in a river, channel, or conduit. The area under the hydrograph curve is the volume for a given time period
Parts of a Hydrograph are shown graphically:
Unit Hydrographs can be determined by dividing the points on the typical hydrograph by the average excess precipitation.
Synthetic Hydrographs are created if there is insufficient data for a watershed. This method uses the NRCS curve number and is a function of the storage capacity.
To develop the synthetic hydrograph, you must calculate the time to peak flow:
tR = Storm duration (time)
Lo = Length overland (ft)
SPercentage = Slope of land
The equation for peak discharge from a synthetic hydrograph then is:
Rainfall
Storm characteristics include duration, total volume, and intensity
Duration: The length of time of a storm. Often measured in days and hours.
Total Volume: The entire amount of precipitation throughout the duration of the storm in a defined area.
Storm Intensity: Total volume of the storm divided by the duration of the storm event. Intensities can be averaged over the entire storm or at shorter intervals to provide instantaneous high intensity portions of the storm. Hyetographs are bar graphs used to measure instantaneous rainfall intensities over time and are covered below.
Stormwater Management
Detention and retention ponds are often used to collect water for flood control and stormwater runoff treatment.
Detention Ponds: Also known as dry ponds. These are ponds which are often kept dry except during flood events. The pond will fill up during increased precipitation to control the flow intensity. This is common in dry, arid, or urban areas to prevent excessive flooding. The ponds typically will be designed to hold water for about 24 hours. Detention ponds also control the amount of sediment since it is captured in the pond and then typically becomes accessible after the pond has dried.
Retention Ponds: Also called wet ponds since they contain a volume of water at all times. The elevation of the water will vary depending on precipitation but will always maintain a permanent amount of water based on low flow conditions. This allows sediment control since the deposits will settle to the bottom and allow for collection.
Infiltration is the rate of which water seeps into the ground. The Horton equation can be used to approximate this rate. This assumes that the water supply is infinite and the ground is saturated:
Time of Concentration
Depletions
The change in storage for a body of water can be approximated from the following equation:
ΔS=P+R+GIGOETO
S = Storage
P = Precipitation
R = Runoff
GI = Groundwater inflow
GO = Groundwater outflow
E = Evaporation
T = Transpiration
O = Surface water release
Stream Gauging
Stream gauging is the measurement of a stream channel to determine the discharge by obtaining the depth and velocity of the channel over time. The channel can be approximated by areas created by connected the dots of the measured depths. The discharge can be calculated by the following:
w = Width of cross section (ft)
y = Height of cross section (ft)
v = Velocity at indicated cross section (ft/s)
Drinking Water Distribution and Treatment
Mass Balance
Mass balance refers to the conservation of mass as it enters and exits a system. This concept is essential to the Water Resources and Environmental exam as it is a common method in solving for unknowns in many types of problems. This principle simply put is that mass is always conserved in fluid systems regardless of the properties of that system. Therefore what enters the system must also exit the system. The equation can be represented as mass:
M1 = M2
M1 = Mass entering the system
M2 = Mass exiting the system
The equation can also be represented by equating flow rates. In this case the equation is commonly known as the continuity equation:
Q1 = Q2
Q1 = Flow rate into system
Q2 = Flow rate out of a system
Given that the flow rate is equal to the area multiplied by the velocity, the equation can be used to equate these variables as well:
A1V1 = A2V2
Hydraulic Loading
Hydraulic loading refers to the flows in MGD (Million Gallons per Day) or cu. Ft./day to a treatment plant or treatment process. The equation is as follows:
HLR=QA
HLR = Hydraulic loading rate
Q = Flow rate
A = Surface area of the wet basin
Detention time is the amount of time it takes a given volume of wastewater to pass through the clarifier:
td=VQ
V = Volume of clarifier
Solids Loading
Solids loading similarly to hydraulic loading is the amount of suspended solids in a substance as it flows to the treatment facility. Solids loading is expressed as the following:
Hydraulic Flow Measurement
Many flow devices are available to measure either the flow rate or velocity of a given system. These methods are often used in conjunction with the laws of energy and mass conservation to be able to analyze a system. Here are a few of the more prevalent ones for the purposes of the PE exam:
 Pitot Static Tubes
g = Force effect due to gravity (32.2 ft/s2)
h = Difference in elevations of the fluid columns (ft)
ρ = Density of water (62.4 lb/ft3)
ρm = Density of manometer fluid (lb/ft3)
v = Velocity (ft/s)
Cf = Flow coefficient
Ao = Orifice area
A1 = Pipe area
p = Pressure
ρ = Density
g = Force due to gravity
Cd = Discharge coefficient
Cc = Coefficient of contraction
Fva =Velocity of approach factor
Engineering Economics
Storm Characteristics
A design storm must be specified when performing any calculations. The design storm is defined by its recurrence interval which is the given amount of time it is likely to see a storm of a certain intensity. Design storms are often 10, 20, 50, or 100year storms meaning a storm of a certain intensity would only occur once within the given duration.
Runoff Analysis
The rational method can be used to determine the flow rate from runoff of a drainage area. The equation is:
Q = ACi
Q = Flow Rate (cfs)
A = Drainage Area (Acres)
C = Runoff Coefficient
i = Rainfall Intensity (in/hr)
NRCS/SCS Runoff Method
This is an alternative method for determining runoff:
S = Storage Capacity of Soil (in.)
CN = NRCS Curve Number
Q = Runoff (in.)
Pg = Gross Rain Fall (in.)
Hydrographs
Hyetographs – Graphical representation of rainfall distribution over time
Hydrograph – Graphical representation of rate of flow vs time past a given point often in a river, channel, or conduit. The area under the hydrograph curve is the volume for a given time period
Parts of a Hydrograph are shown graphically:
Unit Hydrographs can be determined by dividing the points on the typical hydrograph by the average excess precipitation.
Synthetic Hydrographs are created if there is insufficient data for a watershed. This method uses the NRCS curve number and is a function of the storage capacity.
To develop the synthetic hydrograph, you must calculate the time to peak flow:
tR = Storm duration (time)
Lo = Length overland (ft)
SPercentage = Slope of land
The equation for peak discharge from a synthetic hydrograph then is:
Rainfall
Storm characteristics include duration, total volume, and intensity
Duration: The length of time of a storm. Often measured in days and hours.
Total Volume: The entire amount of precipitation throughout the duration of the storm in a defined area.
Storm Intensity: Total volume of the storm divided by the duration of the storm event. Intensities can be averaged over the entire storm or at shorter intervals to provide instantaneous high intensity portions of the storm. Hyetographs are bar graphs used to measure instantaneous rainfall intensities over time and are covered below.
Stormwater Management
Detention and retention ponds are often used to collect water for flood control and stormwater runoff treatment.
Detention Ponds: Also known as dry ponds. These are ponds which are often kept dry except during flood events. The pond will fill up during increased precipitation to control the flow intensity. This is common in dry, arid, or urban areas to prevent excessive flooding. The ponds typically will be designed to hold water for about 24 hours. Detention ponds also control the amount of sediment since it is captured in the pond and then typically becomes accessible after the pond has dried.
Retention Ponds: Also called wet ponds since they contain a volume of water at all times. The elevation of the water will vary depending on precipitation but will always maintain a permanent amount of water based on low flow conditions. This allows sediment control since the deposits will settle to the bottom and allow for collection.
Infiltration is the rate of which water seeps into the ground. The Horton equation can be used to approximate this rate. This assumes that the water supply is infinite and the ground is saturated:
Time of Concentration
Depletions
The change in storage for a body of water can be approximated from the following equation:
ΔS=P+R+GIGOETO
S = Storage
P = Precipitation
R = Runoff
GI = Groundwater inflow
GO = Groundwater outflow
E = Evaporation
T = Transpiration
O = Surface water release
Stream Gauging
Stream gauging is the measurement of a stream channel to determine the discharge by obtaining the depth and velocity of the channel over time. The channel can be approximated by areas created by connected the dots of the measured depths. The discharge can be calculated by the following:
w = Width of cross section (ft)
y = Height of cross section (ft)
v = Velocity at indicated cross section (ft/s)