What Is A Dam Spillway?

What Is A Dam Spillway?

Dams are important structures that are constructed to store water. This water, generally from a channel, can later be used for work involving electricity and irrigation. Dams are also critical in flood control. What is a dam spillway and why are they important? 

Although the majority of dams are built using high tech designs, some experience an overflow of water. If this occurs, the whole structure and design of the dam can become damaged. Therefore, excess water needs to be discharged successfully to avoid any potential disasters. 

what are dam spillways

What Is A Dam Spillway?

What is a dam spillway? A spillway offers a way for the stored excess water to discharge. This is why most dams should be designed with at least one spillway. Whether water is required downstream or the stored water behind the dam has become overwhelming, just about every dam requires a spillway to safely discharge certain amounts of water. 

Here at Gracon Industrial Construction, we specialize in providing hydroelectric constructions, dam constructions, piping services, food manufacturing and process, and electrical services for various government agencies and privately held companies. 

Gracon has industry-leading capabilities which allow us to perform dam maintenance with our expert dam construction contractors. Our experts specialize in replacement and rehabilitation services for all sizes of dam mechanical equipment. Dam repair is what Gracon specializes in. Find out more about our services here

A Dam Spillway Explained and the Different Types of Spillways

Designing dams would be more straightforward if environmental factors such as rainfall and snowmelt were consistent throughout the year. As a matter of fact, since most dams exist because of hydrologic variability, they wouldn’t be necessary if rainfall was consistent. 

Dams exist to provide storage for water and to smooth out the varying nature of inflows. These structures protect us from flooding so the stored water can be used for different purposes during the year.

Those varying inflows are not always modest, however. Some can be gigantic with various watersheds generating the majority of their annual volume of water in just one storm event. In such a scenario, these inflows can race to a reservoir with little to no warning. Therefore, dams need to be ready to withstand and handle any major storms.  

Because of each season’s variability in terms of weather, engineers can not predict the exact volume of floodwater due to melting snow and rainfall. Therefore, designing a dam for a specific discharge of water through its channel is nigh on impossible. 

To combat this, spillways are included in most dams. If there is a sudden rise of water upstream of a dam, a spillway will allow the water to discharge through it and stop the dam from overflowing. 

Spillways are made with the aid of a penstock and a large cross-sectional tube. A connection is made between the inlet of the spillway upstream and the outlet of the spillway downstream from the dam. 

Spillways are classed into four distinct categories:

  • Overflow spillway
  • Chute spillway
  • Side-channel spillway
  • Limited service spillway

Each of these serves specific purposes for specific conditions. But, they all share the role of discharging water to inhibit any flooding from rising water levels. What is a dam spillway and different types? We’re explaining more here:

what is a dam spillway

Overflow Spillway

An overflow spillway is typically used with concrete gravity dams. These are usually gated or ungated and allow water to overflow if it builds up behind the dam.

Chute Spillway 

A chute spillway is usually constructed for earth or rock-filled dams. Nevertheless, some concrete dams also use chute spillways. If this is the case, the dam would typically be located in a small canyon where there wouldn’t be enough space for an overflow spillway.

You will generally find a chute spillway through the abutment. This is adjacent to the dam itself. It can also be located in a saddle, found a little away from the dam.

Side Channel Spillway 

Side-channel spillways are used in similar circumstances to chute spillways. Compared to other spillways, a side-channel has a more unique shape. This allows it to be built on a narrow dam abutment. 

In general, these spillways are ungated but they can be designed with gates if required.

Limited Service Spillways 

A limited service spillway is designed if the spillway will only be operated infrequently and, when this occurs, damage is possible. However, this damage can not be so bad that it causes a catastrophic release of water from the reservoir. 

Other types of spillways include:

  • Siphon spillway
  • Shaft spillway
  • Auxiliary spillway
  • Ogee spillway

Siphon Spillway 

A siphon spillway works as the name suggests, like a siphon. These are built with a hood over a standard spillway which forms a conduit. When the reservoir’s water level rises, it will start to flow over the crest. This is regarded as an ogee spillway.

This flowing water creates tiny air bubbles. Once the air in the crest is removed, the siphon action begins. This type of spillway has a larger discharging capacity meaning more water can flow.

Shaft Spillway 

This spillway uses a circular crest that is built into the surface of the reservoir. Here, the water flows over having been carried by a sloping or vertical tunnel that travels to a horizontal tunnel around the stream bed level. Eventually, this travels to the downstream side. Diversion tunnels constructed during the construction of the dam can later be used as the horizontal conduit in some cases. 

Auxiliary Spillway

Most large dams are constructed with two spillways with one called the service spillway and the other the emergency spillway. The emergency spillway, also known as an auxiliary spillway, is operated when there is an excess flow of water located upstream of the channel. 

Ogee Spillway 

Ogee spillways are usually constructed in rigid dams. These form part of the main dam if there is enough length to do so. 

This is a modified version of a drop spillway. In this case, the spillway’s downstream profile coincides with the lower nappe’s shape of the flowing, falling water jet from the crested weir.

defining a dam spillway

Dam Spillway & Gracon Services

What is a dam spillway? As explained, the main function of a spillway is to control the overflow of water that is stored upstream of a dam in a reservoir. It is simply constructed to maintain the level of water in the reservoir.

Spillways control this level of water without the need for moving parts or mechanical devices. If the level of water reaches above the spillway, then it will automatically fall through the pool of the spillway and escape through the outlet.  

However, on occasions, spillways will require maintenance in order to withstand and allow the safe flow of water. 

Here at Gracon, we provide dam construction and maintenance services. In the last ten years alone, Gracon has replaced or repaired over 100 gates in dams. Our team is one of the most knowledgeable and experienced throughout the entire United States. Our expert crews draw on years of project experiences to solve some of the most complicated and technical problems that arise with the complex nature of dams.

Gracon, dam construction company, provides solutions.