Last Updated on September 27, 2022 by admin
Stormwater engineers are responsible for designing, constructing, and maintaining systems that control rainfall runoff from land surfaces. These systems, known as stormwater management systems, are designed to control the quantity and improve the quality of runoff that is discharged from a site. To be a successful stormwater engineer, you must have a strong understanding of the basics of stormwater engineering.
This blog post will cover some of the fundamental knowledge that all stormwater engineers need. We will briefly discuss stormwater system components and basic stormwater engineering concepts. By reading this article, you will better understand the basics of stormwater engineering and be well on your way to success in your career.
Stormwater Engineering Basics
Table of Contents
Stormwater Management System
A stormwater management system consists of structures and appurtenances designed to control rainfall runoff. It includes detention basins, retention basins, storm sewers, and other related facilities. The goal of a stormwater management system is to detain or store water for a period of time so that it can be filtered and eventually discharged into a water body or into the ground. Stormwater management systems are essential to any urban area because they help protect our waterways from pollution and keep our streets dry.
When a drop of water falls on the ground, it “runsoff” the street (assuming sufficient rain to saturate the surface) – this is called runoff. This drop of water makes its way to the nearest low point and stays there. Stormwater management is the science, and sometimes art, of intercepting that drop of water with a catch basin or another collection structure and conveying it towards a disposal area. Oftentimes, before disposing of the stormwater, the drop of water will be treated to remove contaminants it picked up along the way.
What is a Catch Basin
A catch basin is a type of inlet that is designed to collect stormwater runoff. It is typically a concrete or plastic structure with a grate on top. Water enters the catch basin through the grate and is then piped away to a centralized storm sewer. Catch basins are one of the main components of any stormwater management system because they serve as one of the main points of water collection. From the catch basin, water flows into the storm sewer.
What is a Storm Sewer
A storm sewer is a pipe that is designed to collect runoff from rainfall and melting snow. Storm sewers are usually located underground, and they are connected to catch basins, or other collection structures, such as curb inlets. When it rains or snows, the water flows into the catch basin and then into the storm sewer. The water is then carried away from homes, businesses, and other structures to a nearby body of water, such as a river, lake, or stream.
As water flows through the storm sewer, it can be treated and allowed to infiltrate into the ground. One example of this is an exfiltration trench.
Exfiltration Trenches
Exfiltration trenches are used to collect stormwater runoff and allow it to seep into the ground, where it can recharge groundwater aquifers. They are typically placed along a storm sewer to attenuate runoff.
Exfiltration trenches are often used to manage stormwater runoff. They are designed to allow water to seep out of the soil and into a lined trench filled with gravel. The gravel helps to provide an extra level of filtration, and the slow release of water helps to prevent flooding and erosion. Exfiltration trenches can be used alone or in conjunction with other stormwater management strategies, such as detention basins and infiltration galleries. When properly designed and installed, they can be an effective way to reduce the impact of stormwater runoff on our environment.
Stormwater Treatment
Stormwater treatment is the process of capturing and cleaning rainwater before it enters the storm sewer system or natural waterways. Stormwater can pick up pollutants like oil, fertilizers, and heavy metals from cars, sidewalks, and rooftops and deposit them into local streams and lakes. This can lead to water pollution, which is harmful to people, wildlife, and the environment. Stormwater treatment helps to reduce water pollution by removing pollutants from rainwater before it is released into the environment.
There are several different methods of stormwater treatment, including vortex units, filters, and bioretention cells. Vortex units are designed to remove large particles from rainwater using a series of spinning discs. Filters are usually made of materials like gravel or sand, and they help to remove small particles from rainwater. Bioretention cells are landscaped areas that allow rainwater to seep into the ground, where bacteria and plants help to remove pollutants. Stormwater treatment is an important way to protect our waterways from pollution. Stormwater Treatment Units are typically located before an outfall in a gravity system and before a Stormwater Pump Station if the stormwater system has pressure pipe.
Stormwater Pump Station
A stormwater pump station is a facility used to collect and pump runoff conveyed to it from a given area. Typically, stormwater treatment units are located ahead of a stormwater pump station. Pump stations are typically used in areas where the natural topography cannot provide enough drainage for an area, such as in low-lying areas or near bodies of water. The pumps at a stormwater pump station help to move the water to its disposal point, typically a body of water. In some cases, the stormwater may also be reused for irrigation or other purposes. Stormwater Pump Stations play an important role in managing stormwater and preventing flooding, making them an essential part of many communities’ infrastructure.
Stormwater Pump Stations push the water into a pressure pipe, known as a stormwater forcemain. This allows a large volume of water to be moved quickly to avoid flooding.
Stormwater Forcemain
A stormwater forcemain is a pipe that is used to convey stormwater from one location to another. It is pressurized and it is the discharge pipe of a stormwater pump station. The pipe is typically made of ductile iron, PVC, or HDPE. Stormwater forcemains may discharge stormwater into natural waterways or into wells. Regardless of their specific application, all stormwater forcemains share one common goal: to efficiently transport large volumes of stormwater from one place to another.
Stormwater forcemains are sometimes manifolded to drainage wells to dispose of the stormwater. The end of a stormwater forcemain, where the water is discharged, is known as a stormwater outfall.
Stormwater Outfalls
A stormwater outfall is a pipe or other drainage structure that is used to convey stormwater runoff from an urban area to a receiving water body. In many cases, stormwater outfalls are equipped with some type of control device, such as a baffle or grate, to help protect the receiving water body from pollutants that may be carried in the runoff.
Outfalls are also often used to discharge stormwater from detention basins and other types of stormwater management facilities. While stormwater outfalls can be found in both rural and urban areas, they are typically more common in urban areas due to the high density of impervious surfaces, such as pavement and rooftops, which can cause large volumes of runoff during rain events.
Stormwater outfalls can discharge water into a receiving water body; they can also discharge into infiltration basins.
Drainage Wells
Drainage wells are often used to improve the drainage of an area of land. They are generally artificial, although they may be created by natural processes. Drainage wells typically collect water from a particular catchment area and then release it into the ground. This can help to improve the overall drainage of an area and reduce the risk of flooding. In some cases, drainage wells may also be used to recharge groundwater aquifers. This can help to ensure a consistent supply of water in dry periods and reduce the need for irrigation. Drainage wells are an important tool in managing the water resources of an area.
Drainage wells are often used ahead of stormwater outfalls to provide water quality treatment by collecting the “first flush” of stormwater runoff. The first flush carries the largest concentration of contaminants.
Infiltration Basin
An infiltration basin is a type of detention basin that is designed to infiltrate water into the ground. It is typically located downstream of a storm sewer. Water enters the basin through the pipes and seeps into the ground below. This process helps recharge groundwater and improve soil health. Infiltration basins are an important part of any stormwater management system because they help reduce flooding and improve water quality.
Drainage Design
Drainage design is the process of designing a drainage system that will allow water to be safely and effectively removed from an area. A properly designed drainage system will minimize the risk of flooding, erosion, and other problems that can occur when water is allowed to accumulate in an area. Drainage design involves many different factors, including the size and shape of the area to be drained, the type of terrain, the amount of rainfall that typically occurs in the area, and more. Thus, it is essential to perform the proper calculations before constructing a drainage system to ensure it is effective and that the system can handle the amount of water that it will need to drain.
Tributary Area
A tributary area is the area of land that drains to a particular point on a water body. It is important to know the size of the tributary area because this will determine the amount of runoff that needs to be managed. The tributary area can be calculated using topographic maps or aerial photography. When working on an urban drainage system, the tributary area can be the entire area that drains to a particular pump station or an outfall. On the other hand, it can also be the area that drains to a specific catch basin or inlet. For example, a tributary area can be specific to a particular catch basin if you are working to ascertain a time of concentration for the catch basin.
The runoff coefficient determines the amount of water that flows out of a tributary area.
Runoff Coefficient
The runoff coefficient is a number that represents the portion of rainfall that will become runoff from a given land use. Runoff coefficients can range from 0 to 1. The lower the runoff coefficient, the less water will become runoff. Runoff coefficients are important because they help engineers estimate the amount of runoff that will need to be managed. They are also used in conjunction with rainfall data to design stormwater management systems.
Time of Concentration
Time of concentration, the time it takes for water to travel from the most remote point on a site to the drainage outlet, is a key parameter in stormwater management. This parameter is important because it influences the size of the stormwater management system that needs to be designed. Time of concentration can be calculated using a variety of methods, including TR-55, and SCS-CN. It is important to use the correct method for your site so that the results are accurate. It will also influence your hydrograph.
What is a Hydrograph
A hydrograph is a graph that shows how the discharge of a watershed changes over time in response to a precipitation event. Hydrographs are important because they help engineers understand how watersheds respond to rainfall. They are also be used to design stormwater management systems.
Peak Flow
Peak flow is the maximum discharge that occurs during a precipitation event. It is important to know the peak flow because this will determine the size of the stormwater management system. Peak flows can be estimated using hydrographs or by measuring the rainfall during a precipitation event and estimating a runoff coefficient.
The Rational Method
The rational method is used to estimate the amount of runoff from a given drainage area. This formula takes into account the rainfall intensity and the runoff coefficient. The formula shown below is one of the cornerstones of stormwater engineering.
Where:
Q= Peak Flow in cubic feet per second (cfs)
C= Runoff Coefficient
i = Average Rainfall Intensity
A = Tributary Area in acres
Open Channel Flow
Open channel flow occurs when water flows through an open channel, such as a river or stream. In stormwater management systems, open channels are sometimes the method used to convey water. For example, a ditch can be constructed to convey stormwater runoff. It’s important to note that flow through a gravity pipe is also open channel flow, as long as the pipe is not surcharged.
The preferred method for calculating open channel flow is Manning’s equation.
Manning’s Equation
Where:
Q = flow rate in cubic feet per second (cfs)
n = Manning’s Roughness Coefficient
R = Hydraulic Radius in feet (ft)
S = Channel Slope (ft/ft)
Pipe Slope
Pipe slope is the slope of a conduit. It is important to know the pipe slope because it determines the velocity of flow in a conduit. A steeper slope will cause water to flow faster through the pipe. Pipe slope is typically expressed as a percentage but for many engineering calculations, it is expressed as a unitless ratio in ft/ft. For example, a pipe with a slope of 2% means that for every 100 feet of pipe length, the pipe will drop 2 feet in elevation.
Pipe Velocity
Pipe velocity is the speed at which water flows through a conduit. There are many reasons to know the velocity of the water, for example, it is important to know the velocity because it determines the amount of sediment that will be transported by the water. This is called scour velocity. The slower water flows the more sediment will deposit in the pipe. Pipe velocity can be calculated using the following equation:
Where:
Q = flow rate in cubic feet per second (cfs)
A = cross-sectional area of the pipe in square feet (sqft)
V = velocity in feet per second (fps)
Design Velocity for Water Pipes
Although the design velocity for water pipes depends on a large number of factors, some parameters can help guide a design. Firstly, whether designing a gravity or pressure pipe you want to ensure you achieve scour velocity. Typically this is done by ensuring your pipe velocity is above 2 feet per second. Second, if designing a pressure pipe, you should design for a maximum velocity of 8 feet per second as stipulated in the 10 State Standards. This will minimize headloss and protect the system’s appurtenances.
Pipe Capacity
Pipe capacity is the maximum amount of water that can flow through a conduit. It is important to know the pipe capacity because it determines the size of the stormwater management system. Pipe capacity can be estimated using the Manning equation or the Hazen-Williams equation.
Hazen Williams Equation
The Hazen Williams equation calculates the flow of water in a pressure pipe, such as a stormwater forcemain. When water flows through a pipe, there is friction between the water and the pipe walls. The equation considers pipe diameter, pipe length, and friction loss due to the pipe’s walls. This friction slows down the flow of water and causes pressure losses. These pressure losses need to be accounted for when designing your stormwater system.
The equation, shown below, is named after engineers Robert Hazen and John Williams, who developed the equation in 1918. The Hazen Williams equation is used in a variety of applications, including engineering and environmental science.
Where:
V = velocity in feet per second (fps)
C = Hazen Williams Friction Factor
R = Hydraulic Radius in feet
S = Slope of hydraulic grade line
The Hazen Williams Equation can also be rearranged as follows to calculate head loss.
Where:
hf = headloss due to friction in feet
Q = flow rate in cubic feet per second (cfs)
C = Hazen Williams friction factor
d = inside diameter of the pipe in inches
L = length of pipe in feet
Hydraulic Grade Line
The hydraulic grade line is the line that represents the water level in a conduit. Knowing the hydraulic grade line is important because it determines the direction of flow in a conduit. The hydraulic grade is also used in many engineering calculations, such as the Hazen Williams Equation. Furthermore, it is often visually depicted to demonstrate if the gravity pipes are surcharged.
Water quality
Water quality is an essential part of proper stormwater management. Since stormwater runs off from impervious surfaces like roofs, pavements, and roads during rainfall or snowmelt. The runoff can pick up pollutants like oil, grease, litter, and chemicals as it flows. If this contaminated stormwater is not properly managed, it can pollute our waterways and adversely impact public health and the environment.
One way to improve water quality is by using blue-green infrastructure (BGI). BGI is a holistic approach that uses both natural and built systems to manage stormwater. For example, BGI solutions include rain gardens, bioswales, green roofs, and permeable pavement. These solutions help to reduce runoff and filter pollutants out of stormwater before it enters our lakes, rivers, and streams. Furthermore, BGI can also provide other benefits such as reducing flooding, improving air quality, and creating habitats for local wildlife.
Implementing BGI solutions is one way to improve water quality in stormwater management.
While stormwater engineering may seem complex, its not very difficult to get a handle on the basics – rain falls, you catch it and deviate it to a disposal point; along the way you may perform some treatment or introduce green stormwater infrastructure to improve water quality. That’s it. You, of course, have to do some math, and we’ll get deeper into actual designs in future posts.
For now, by understanding how stormwater moves through our environment and what factors affect its flow, we can begin to design systems that help keep our communities healthy and our waterways clean.
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