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===Overland sheet flow===
<p>The following pretreatements are recommended where sheet flow is being directed to a BMP (typically a [[Bioretention cells| bioretention cell]]) from the edge of an impermeable surface such as an asphalt parking lot or a paved plaza:
<ul>
<li>[[Gravel diaphragms]]</li>
<li>[[Vegetated filter strips]]</li>
</ul>
A gravel diaphragm typically requires less setback between the catchment and the BMP. Where space permits, these two designs can be used in combination to even greater effect!</p>
===Concentrated overland flow===
Where a [[Curb cuts| curb cut]], [[Downspout disconnection| a disconnected downspout]] or a [[Swales| swale]] directs surface water to a point, increasing the flow rate, one or more of the following pretreatments should be considered:
<ul>
<li>[[Forebays]] (well suited for bioretention cells)</li>
<li>[[Sand filters]] (well suited for underground infiltration)</li>
<li>[[Screens and racks]]</li>
</ul>
===Concentrated underground flow===
<p>Oil and grit separators (OGS) are devices which use physical principles to improve water quality. Typically these are underground chambers installed in catchbasins or manholes. Many commercial and custom configurations exist for both new installations and retrofit scenarios.
<p>Oil and grit separators (OGS) are devices which use physical principles to improve water quality. Typically these are underground chambers installed in catchbasins or manholes. Many commercial and custom configurations exist for both new installations and retrofit scenarios.
*One example might employ baffles within a tank or chamber:
*One example might employ baffles within a tank or chamber:
**The first baffle slows the incoming water flow, encouraging any larger, heavier particles to drop out of the water,
**The first baffle slows the incoming water flow, encouraging any larger, heavier particles to drop out of the water,