Limitations of pipe-based systems

Pipe-based systems have been a common way of dealing with stormwater in urban areas for many years. However, recently there has been more attention for the limitations that these systems have, and alternatives have become more popular. Before looking at the alternative solutions in the next step, this step will go over some of the limitations of pipe-based solutions.

Construction and maintenance costs

Burying stormwater pipes underground creates a significant amount of work. A trench needs to be dug to the right depth, then the pipe is placed and the trench is filled up again. Frequently, pipes are placed below streets. This is common because often no other space is available to the local authority of water utility company. This also means that replacing stormwater pipes requires the road to be closed and broken up before the work and restored afterwards. All this work costs a significant amount of money and resources and generates considerable greenhouse gas emissions.

Maintenance of stormwater pipes can also be challenging. Over time, pipes may deteriorate and lose some of their capacity, for example due to:

• Sedimentation
• Blockages, for example caused by tree roots growing into the pipe
• Subsidence of pipes if the surrounding soil is not supportive enough
• Leakage of groundwater into pipes
• Deterioration of the pipe material

This means pipes will need maintenance. In some cases, it can be sufficient to flush the pipes when needed, but this still requires specialized equipment and labour. More structural problems (e.g. blockages caused by tree roots or subsidence) may not be possible to fix without replacing the pipe by digging it up.

Limited resilience

As discussed last week, we can use rainfall data and information on the catchment (e.g. pervious/impervious areas) to make estimates of runoff from different catchments and flow rates in sewers. It is then relatively straightforward to determine what size of pipe is needed to handle a certain flow rate.

However, there will be cases where the flow rate is bigger than what the pipe is designed for. This can be because:

• There is a more extreme rainfall event than what the pipe was designed for. For example, a pipe designed for a 10-year rainfall event will likely be too small for a 100-year event.
• Future changes in climate conditions can change today’s extreme events into tomorrow’s more common events. For example, what is a 30-year event now may be a 10-year event in the future.
• Connecting new areas (e.g. a new urban area upstream) to an existing pipe network will cause higher flows and thus often under-dimensioned pipes.

When the flow in a pipe system starts exceeding the capacity, water will become “backed up” in the system, so even pipes with sufficient capacity before may be entirely full because of higher water levels downstream. The pipe-based system can store some additional water in the manhole, but this is only a relatively small volume. Once the water level reaches the surface, the drainage network will not carry away water from the surface anymore and can instead overflow and deliver even more water onto the surface.

In other words, pipe-based systems have very little spare capacity compared to what they are designed for. This means they are not resilient to larger rainfall events, which are expected to become more and more common in many places around the world due to climate change.

Flooding problems are moved downstream

Pipe-based systems are focused on removing water from an area quickly. This means that they are focused on moving water downstream. This can have the effect of causing more problems in the downstream area. For example:

• Other urban areas may need even larger pipes to handle the increased flows (often from several upstream areas).
• Small creeks may receive relatively high flows from urban areas, which can cause problems with high flow velocities which can damage the creek.
• Even for larger rivers the increased flows from urban areas along the length of the river can lead to noticeable higher flows, which can increase flooding risks along the river.

Negative effects on the water cycle

The urban hydrological cycle was already discussed last week, but it is worth recalling some of the effects that urbanization incorporating stormwater discharge with pipe-based systems can have. These are:

• Lower groundwater tables
• Less evapotranspiration
• Lower base flow in rivers and creeks
• Higher max flows in rivers and creeks

Stormwater pollutants are not removed

As we will discuss later in this course activity, stormwater contains many different pollutants from e.g. building materials, traffic, or road wear. Pipe based systems are not designed to remove these pollutants, and they instead simply carry them to the receiving water. This can have negative effects on the environment including aquatic animals.