‘Fundamentals of Drainage Design’: a framework with 8 steps and a video

If you are a new civil engineer, urban planner, or simply interested in understanding how drainage systems work, we have a new course that’s designed to provide you with the knowledge and skills necessary to tackle drainage design challenges. By understanding key concepts and considerations in this video, you will be equipped to create sustainable and efficient drainage solutions for various land uses and scenarios. We also created this blog post that summarizes some of the key points from the video so you can follow along if you wish.

The importance of stormwater in design

Drainage design, while simple in theory, covers a wide range of topics like hydrology and hydraulics, site analysis and planning, drainage system components, stormwater management, regulatory considerations, and maintenance practices. Hydrologic factors, drainage patterns, and environmental conditions can significantly affect engineering design decisions and water flow behavior, influencing the stability of infrastructure and the health of surrounding ecosystems. Managing fluids effectively is essential in drainage design, as proper understanding of hydrology and hydraulics ensures that water flow is controlled to prevent erosion, sediment buildup, and road failure.

Before you begin, it’s important to know about the high-level concepts of drainage design. There can be many goals, but these are the 3 biggest goals drainage designers typically seek to achieve:

• Stormwater control and flood protection: Without proper drainage, heavy rains can overwhelm systems, causing runoff, flooding, erosion, and property damage. Inadequate drainage can also lead to road surface erosion and weakened subgrades, resulting in mass failures.
• Environmental stewardship: Good design helps reduce pollution (by filtering runoff), protect streams, and manage peak flows.
• Resilience and adaptation: With climate change increasing storm intensity and variability, drainage systems need to be robust and flexible.

Important to note: Excess water or moisture will adversely affect the engineering properties of the materials with which it was constructed.

As you survey a site, you will want to pay attention to these key details…

Hydrology & rainfall input

Understanding how much water you expect (rainfall depth, intensity, duration) is the starting point. The inflow, or the water entering the drainage system as a result of rainfall and runoff processes, is a key factor in hydrologic and hydraulic analysis. Your design must consider how precipitation turns into surface runoff given soil, land cover, and terrain.

Site analysis and catchment characterization

You must assess:

• Topography (drainage paths, slopes)
• Land use and impervious surfaces (roads, roofs, pavements)
• Soil permeability
• Existing drainage features (ditches, culverts)

This lets you break the area into catchments and decide how water moves across and off your site. What is a catchment? The catchment area is the geographical region that collects and channels surface water flow toward a common outlet, such as a river or lake. Identifying and mapping your drainage areas is crucial for accurate hydrologic analysis and effective stormwater management system design. Beyond catchments, the watershed defines the drainage boundaries and helps in understanding runoff characteristics and managing water resources. The perimeter of each catchment or watershed is the boundary – often composed of ridges or hills – that separates one drainage basin from another and determines the extent of water flow convergence.

Soil type and drainage

Soil type is a critical factor in drainage design, as it directly affects how water moves through and across the land. Sandy soils, for instance, are highly permeable, allowing water to infiltrate quickly and reducing the amount of surface runoff and the risk of flooding. This means that in areas with sandy soil, drainage systems can often be designed with a focus on infiltration, using structures like infiltration trenches or permeable storm drains to efficiently manage excess water. On the other hand, clay soils have much lower permeability, causing water to run off the surface rather than soak in. This increases the potential for flooding, erosion, and the need for larger capacity drains, culverts, and other structures to safely convey stormwater away from the site.

Understanding the soil type and its properties is essential for selecting the right drainage system and ensuring it operates efficiently. The topography of the site, combined with the soil’s infiltration capacity, will determine the layout and sizing of storm drains, culverts, and other drainage structures. Properly matching the drainage design to the soil type helps prevent erosion, maintain water quality, and ensure the system can handle excess water during heavy storms. Ultimately, considering soil type in drainage planning leads to more effective, sustainable, and resilient drainage solutions that protect both property and the environment.

Drainage system components

Typical elements include:

• Pipes, culverts, and channels
• Manholes, drop inlets, junctions
• Detention or retention basins
• Swales, infiltration trenches, and sustainable drainage features (SuDS)

Selecting appropriate materials, such as rocks for riprap, is essential for structural stability and effective erosion control around inlets and outlets. Each component must be sized and shaped to carry flows without overtopping or causing erosion, ensuring smooth passage of water and debris through culverts and storm drains. Proper culvert design, based on sound hydraulic design principles and established criteria for sizing and placement, is critical to maintain system capacity and prevent blockages or failures.

Debris control and management

Debris control and management are critical aspects of effective drainage design, especially in areas where stream crossings, culverts, and storm drains are exposed to high debris loads. When debris such as leaves, branches, and sediment accumulates at inlets or within drainage structures, it can cause blockages that lead to flooding, damage, and reduced system performance. To address this, engineers incorporate inlet and outlet protection measures – such as grates, screens, and debris barriers – at key points in the drainage system. These features help prevent debris from entering and clogging the system, ensuring that water can flow freely even during intense storms.

Regular maintenance and inspection of drainage structures, including storm drains and culverts, are also essential for effective debris control. By identifying and removing blockages early, engineers can prevent excess water from backing up and causing flooding or erosion. Integrating robust debris control strategies into drainage design not only protects the system’s capacity and efficiency but also safeguards people, property, and the environment from the impacts of uncontrolled runoff and stormwater. Inlet and outlet protection, combined with ongoing debris management, are critical for maintaining the long-term reliability and functionality of any drainage system.

Sustainable drainage / Low-Impact Design (LIDs, SuDS, BMPs)

Our approach is focused on using sustainable, nature-based drainage features, which are sometimes called LIDs, BMPs, or WSUDs in different geographies. These features slow water, encourage infiltration, and provide treatment (filtering pollutants) before discharge.

Hydraulics and network design

Once catchment flows are estimated, you design the network by determining appropriate pipe sizes, slopes, and where drainage structures are located based on hydraulic calculations. Gravity-driven drainage systems are preferred for their efficiency and reliability, reducing the need for pump stations. It is crucial to direct water efficiently through the network to prevent flooding and erosion. Managing water flowing through storm drains and culverts is essential to ensure hydraulic efficiency, control sediment and debris, and prevent erosion or structural failure. Hydraulic analysis determines the water levels and how fast the water moves based on the calculated Annual Exceedance Probability (AEP). Hydraulic design may result in the need for drainage easements in areas not already owned by or classified as waters of the State. Hydraulic design also includes determining culvert sizes and grades to handle design stormwater flows. Selecting the right location for drainage components ensures effective water conveyance and minimizes risks. The slope of pipes and channels plays a key role in controlling water velocity and preventing erosion, contributing to the overall stability of the drainage system.

Regulatory and maintenance considerations

Drainage is not just about making it work today — systems must comply with local regulations (stormwater management, runoff limits, water quality) and be maintainable over their life span (access, sediment removal, inspection). Adhering to established criteria and following official guidance is essential for regulatory compliance and effective hydraulic design. It is also important to develop a comprehensive drainage plan as part of the design and regulatory process to ensure proper water management and structural integrity. While regulations vary by locale, the core design philosophy is broadly applicable. Note that if you are using software like InfoDrainage, it allows you to input regional standards or customize the details to fit your specific regulatory needs.

Drainage design principles

Effective drainage design is built on a set of core principles that ensure a drainage system can reliably manage excess water and protect property and infrastructure from flooding. At the heart of any drainage design is the careful consideration of the catchment area—the region where water collects and flows into the system. Understanding the size and characteristics of this area is essential for determining how much water the system must handle during a storm.

Equally important is the drainage field, which is the area where water is directed away from critical assets, helping to prevent water accumulation and potential damage. The outlet, typically a storm drain or underground pipe, is the final discharge point for water leaving the system. Ensuring proper outlet protection is critical to prevent erosion and maintain the stability of the surrounding land.

Inlet and outlet protection measures, such as riprap, grates, or energy dissipators, are essential for safeguarding the entry and exit points of the drainage system. These features help to control the flow of water, reduce the risk of erosion, and prevent debris from entering or clogging the system. Debris control is another critical aspect—by incorporating screens, barriers, and regular maintenance, designers can maintain system flow and prevent blockages that could lead to flooding.

By applying these drainage design principles—focusing on the catchment area, ensuring robust inlet and outlet protection, and prioritizing debris control—engineers can create drainage systems that are resilient, efficient, and capable of handling the challenges posed by storms and excess water. These principles are essential for maintaining the long-term performance and reliability of any drainage system.

8 practical steps in a drainage design workflow

IIn general, here is a list of steps that engineers typically follow when designing drainage for a specific site:

1. Gather rainfall and climate data: Identify design storms (e.g. 10-year, 100-year) and rainfall distribution.
2. Site and catchment study: Map slopes, land use, soils, and delineate drainage catchments. Consider the impact of flat terrain, as flat areas tend to have slower runoff and different drainage characteristics compared to steeper slopes. For roadway projects, map the roadway alignment and assess how traffic volumes may influence drainage needs.
3. Estimate runoff: Use methods (eg, rational method, unit hydrographs) to compute peak flow rates per catchment. When designing for roads, factor in the effects of surface water on the roadway and ensure the drainage system can handle runoff generated by both the road surface and adjacent land. Traffic loads and roadway design should be considered to ensure culverts and channels are appropriately sized for safety and durability.
4. Layout drainage network: Draw preliminary pipes, channels, and inlets based on topography. Pay special attention to managing surface water, intercepting and channeling it efficiently to prevent erosion, structural damage, and maintain road stability.
5. Size components: Calculate diameters, slopes, capacities, ensuring that each link in the network can carry design flow with safety margin. Integrate SuDS where possible (eg, infiltration swales, retention ponds). Consider ground infiltration, especially in endorheic basins, where water may evaporate or infiltrate into the ground, affecting features like playas or salt flats.
6. Check system performance: Run hydraulic models, check for backwater, flooding, velocities, energy dissipation, safety.
7. Design for maintenance and access: Include manholes, cleanouts, inspection points, access ways.
8. Ensure compliance and adjust: Compare against local stormwater and environmental regulations. Adjust detention, restrict discharge, or add treatment features as needed.

All of these steps become easier to tackle when you have a good computer-aided design tool. Of course, we use InfoDrainage when designing, but you do not need the software to accomplish all of these steps.

Following a structured drainage design workflow in this way offers many benefits, including improved regulatory compliance, long-term value, and reliable system performance.

Conclusion and final considerations

In summary, successful drainage design is essential for protecting properties and infrastructure from the risks associated with excess water, flooding, and erosion. By carefully analyzing the catchment area, drainage field, and outlet, and by applying sound drainage design principles, engineers can develop systems that efficiently manage surface water and withstand the challenges of varying climate and precipitation.

Key considerations such as inlet and outlet protection, debris control, and understanding soil type are critical for ensuring the drainage system remains functional and effective over time. The use of storm drains, culverts, and bridges, combined with a thorough understanding of hydrology, watershed, and basin characteristics, allows for the creation of systems that can handle high flows and prevent flooding.

Hydraulic design plays a vital role in determining the capacity and efficiency of the system, ensuring that water flows safely and reliably through all components. Regular maintenance and attention to debris control are essential for preserving system capacity and preventing blockages.

Ultimately, the many benefits of proper drainage design include improved safety, reduced property damage, and enhanced resilience to storms and changing climate conditions. By following the guidance and principles outlined in this framework, and by tailoring each design to the unique characteristics of the site, engineers can deliver drainage solutions that are efficient, effective, and essential for long-term protection against flooding and water damage.

Go deeper into sustainable drainage design

• We have in-depth articles on the stormwater features that drainage designers use when designing project sites. Learn about bioretention systems, swales, infiltration trenches, cellular storage, porous pavement, soakaways, rain gardens, and wet ponds and infiltration basins.
• Download a 30-day free trial of InfoDrainage – no credit card required. (Are you a student or educator? You may be eligible to use this software for free.)
• Download the comprehensive Guide to Representing SuDS in InfoDrainage in accordance with the SuDS Manual Ciria 753.
• Dig into the InfoDrainage Technical Information Hub and examine how the software conforms to industry standards.