Dewatering Pumps and Considerations

Construction dewatering is a common occurrence on construction sites. Rainwater and groundwater are often trapped within the excavation and this water must be removed before certain operations can be performed or to keep work conditions safe. Optimum construction project conditions can be maintained with the use of dewatering pumps.

Pumps and Construction Applications

Selecting a dewatering pump for an application can be a challenging endeavor. Along with the multitude of models of pumps available, there are many different types of applications where pumps are applied. Knowing your application and being able to report this information to pump experts when you request their assistance can help you be assured that you’re using the best pump for your application.

Portable trash pumps are commonly used in construction projects where there is an open body of water that can be accessed by the piping or hose. Trash pumps are centrifugal pumps ideal for handling water with solids and debris. Trash pumps can be used with wet priming systems or with dry priming systems. Wet priming systems do not have an external priming system and require the pump housing to be manually filled with water. Dry priming systems have a compressor or vacuum-assisted external priming system and do not require filling the housing.

Other types of pump technologies for construction dewatering include diaphragms, hydraulic submersibles and wellpoints.

Diaphragm pumps use rubber diaphragms inside the pump casing that open and close to create a vacuum and draw in water. They are low volume but are excellent for muddy materials.

Hydraulic submersible pumps use hydraulic power units to drive pump ends that are submersed in the water. These pumps are good for applications where the pump unit cannot be located close to the water.

Wellpoint systems are regularly used in trenchless applications where the ground must be dry to allow pipe bursting or other processes. Any project where the groundwater level is close to the surface is a good application for wellpoints. Wellpoint pumps are specially designed to handle a high percentage of air within the water that is drawn in from the ground material.

Suction Lift Considerations

When choosing a dewatering pump for an application it is important to consider your suction lift requirements. Suction lift is the vertical distance of suction the pump has to overcome to bring the fluid into the pump. Suction lift spans from the top of the pumping fluid, into the pump casing at the eye of the impeller. The laws of physics pertaining to atmospheric pressure only allow a pump to lift the water approximately 28 ft in ideal conditions, which means the pump cannot lift the water into the pump if it is more than 28 ft higher than the pumping fluid.

Depending on the type of pump you choose, you may have to take some precautions when dealing with high suction lifts. With self-priming centrifugal and end-suction centrifugal pumps, you may have to create a platform that your pump can safely rest on that is closer to the pumping fluid to reduce the suction lift. This is frequently referred to as “benching down.” With submersible pumps, where the pump end is submerged directly into the pumping fluid, the suction lift issue is completely eliminated, but enough power must exist to overcome the discharge head (the vertical distance that the fluid is discharged).

Altitude also affects suction lift. An application in Florida, where the elevation is close to sea level, may not work as well in Denver, where the elevation is considerably higher.

Dewatering Best Management Practices

Often, pumped water from a construction site finds its way to sensitive down gradient areas such lakes, wetlands, and streams that must be protected. Stormwater runoff from construction activities can have a significant impact on water quality by contributing sediment and other pollutants to waterways.

Discharge of the pumped site drainage requires particular attention. It is important to pay attention to the location of the outlet, where the pumped water goes, and if it requires filtering or other processing. It is common before releasing dewatering discharge to rivers, lakes or wetlands that some form of settling or filtering sediment occurs. One common filter process directs the discharge through bags made of geotextile fabric. Sediment and erosion control is impeded by the use of dewatering and sediment filter bags for pumping and filtering sediment laden water. These bags retain solid, sediment, and other particles allowing only clean water out of the bag.

When dewatering, the implementation of Best Management Practices (BMPs) is beneficial. BMPs are methods that have been determined to be the most effective, practical means of preventing or mitigating environmental affects. BMPs also help dewatering contractors stay in compliance with relevant laws and rules. Please refer to the U.S. Environmental Protection Agency at epa.gov for the most up-to-date regulations and practices.

Common Water-Related Construction BMPs:

• Protect Natural Features: Minimize clearing and the amount of exposed soil.
• Construction Phasing: Sequence construction activities so that the soil is not exposed for long periods of time.
• Vegetative Buffers: Protect and install vegetative buffers along water bodies to slow and filter stormwater runoff.
• Stabilization: Techniques such as sodding, seeding/ mulching and stone cover, which reduce the erosion of exposed soils and steep grades.
• Storm Drain Inlet Protection: Use and maintain inlet filters regularly.
• Slopes: Break up long slopes with sediment barriers, or under drain, or divert stormwater away from slopes.
• Silt Fencing: A temporary erosion and sediment control used to prevent dirt from entering waterways before bare soil is stabilized with vegetation.
• Berms: A temporary erosion and sediment control that physically prevents polluted runoff from entering nearby storm drain inlets and waters. During the active dewatering process, inspection of the dewatering facility should be reviewed frequently.
• Retention Ponds: Permanent structures designed to allow time for sediments to settle and water to infiltrate the ground.
• Temporary Sediment Basins: Structures designed to detain sediment-laden runoff from disturbed areas long enough for sediments to settle out and control the release of stormwater.
• Good Housekeeping: Techniques such as oil and fuel containment, spill prevention and clean-up, and street sweeping of “tracked-out” soils, which help prevent the contamination of stormwater runoff.
• The discharge areas should be chosen with careful consideration to the down gradient water resources and the landscape ability to treat water flows from the dewatering process. A wooded buffer is best.
• Special attention should be paid to the buffer area for any sign of erosion and concentration of flow that may compromise the buffer area.
• Observe where possible the visual quality of the effluent and determine if additional treatment can be provided.
• If the collected runoff is contaminated with oil, grease, or other petroleum products, oil/water separator or a filtration mechanism is likely necessary prior to the discharge.

In addition to BMPs, always remember that dewatering activities are regulated by various laws and regulations and all requirements of state law and permit requirements of local, state, and federal agencies must be met.

Kirsten Petersen Stroud is marketing manager for Thompson Pump, a full-service manufacturer and provider of heavy-duty portable pumps, pumping equipment and engineering expertise.