Clogging due to construction phase sedimentation is one of the greatest risks of bioretention failure.  In the majority of cases, this is the result of improper construction phasing and insufficient knowledge of construction personnel.

There are several methods to minimize sedimentation failure of bioretention facilities.  In cases where Low Impact Development is used, refer to the Prince George’s County Low Impact Development Manual for additional information.

Regardless of the approach used to minimize sedimentation damage, it is the designer's responsibility to clearly show the intention on the construction drawings.  Inadequate sequencing, notation, and specifications on construction and in project manuals is a common mistake that often leads to unnecessary construction-phase problems and/or failed facilities. 

It is likewise important that the critical nature of these issues - and the ramifications - be clearly conveyed to the responsible construction personnel prior to and throughout the construction process.

CONSTRUCTION SEQUENCING

To ensure the greatest potential of success of a bioretention facility, it should not be constructed until construction activity is completed within the entire contributing drainage area and all disturbed areas have been stabilized.  In this case, stabilization means having all pervious areas either fully landscaped (with mulch installed) or having well established grass or other ground cover. 

If this method is employed, the construction drawings should clearly state the designer's intentions and an appropriate sequence of construction should be shown on the plans.  This sequence should then be the topic of a detailed discussion at the pre-construction meeting (that must include the on-site responsible construction personnel) and then enforced by an appropriate inspection program throughout the construction period.

Although this makes sense for the success of the bioretention, the reality is that some owners or construction contractors may be resistant to this type a sequencing or may ignore it altogether if an enforceable inspection program does not exist.  This can be for several reasons, including:

• Inconsistency with typical "get in and get out" construction methods

• Delays in the completion of construction

• The need to remobilize construction equipment and personnel

• Re-installation of sediment controls for the bioretention construction

• Overall increase in costs

Each site is unique, and this strict construction sequencing approach may be achievable, such as in the case of bioretention in parking lot islands with little contributing pervious areas.   However, It is critical that the designer understand these realities and plans accordingly.  In cases where the above construction sequencing approach is not practical, several other methods may be considered.

DRAINAGE DIVERSION

The bioretention facility is constructed in conjunction with the overall site construction, but drainage from up-gradient disturbed areas is diverted around the bioretention location.  The diversion must remain in place until all disturbed areas contributing to the facility are stabilized.  The diversion  can employ common sediment control devices such as earth dikes, diversion swales, etc.  Even in this case, sequencing should be established to delay the bioretention construction as long as possible and timed to minimize the potential of sedimentation. 

An enforceable inspection program is also a critical component of this approach.  In the event that a breach of the diversion occurs that results in sedimentation of the facility, mandatory reconstruction of the facility should be enforceable, if needed.

MULTI-STAGE SEDIMENT CONTROLS

In this approach, sediment control devices are employed upstream of the bioretention location (to protect it from potential sedimentation) and downstream of it (to protect downstream undisturbed areas).  This effectively treats the bioretention area as an undisturbed area or as a protected natural resource.

Care should be exercised when designing these sediment controls to reduce the potential of a breach in the sediment controls in the event of an unusually extreme or long duration storm event.

As in the case of the above, an enforceable inspection program with mandatory reconstruction authority is important to the success this approach.

CONTINUE TO SOIL MEDIA

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The information on this website was compiled by T.E. Scott & Associates, Inc. from various sources.

We make no guarantees or warranties to the applicability of this material to specific applications. 
Last modified: 10/23/09