Eric Suesz
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.
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 & flood protection: Without proper drainage, heavy rains can overwhelm systems, causing runoff, flooding, erosion, and property damage.
- Environmental stewardship: Good design helps reduce pollution (by filtering runoff), protect streams, and manage peak flows.
- Resilience & adaptation: With climate change increasing storm intensity and variability, drainage systems need to be robust and flexible.
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. 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.
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)
Each component must be sized and shaped to carry flows without overtopping or causing erosion.
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 — selecting pipe sizes, slopes, flow velocities, and ensuring connectivity. Hydraulic calculations ensure that flows are conveyed safely without excessive velocities or flooding.
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). 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.
8 practical steps in a drainage design workflow
In general, here is a list of steps that engineers typically follow when designing drainage for a specific site:
- Gather rainfall and climate data: Identify design storms (e.g. 10-year, 100-year) and rainfall distribution.
- Site and catchment study: Map slopes, land use, soils. Delineate drainage catchments.
- Estimate runoff: Use methods (eg, rational method, unit hydrographs) to compute peak flow rates per catchment.
- Layout drainage network: Draw preliminary pipes, channels, and inlets based on topography.
- 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).
- Check system performance: Run hydraulic models, check for backwater, flooding, velocities, energy dissipation, safety.
- Design for maintenance and access: Include manholes, cleanouts, inspection points, access ways.
- 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 follow along with this video.
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.