Retention Ponds

A retention pond holds water permanently and loses it to infiltration and evaporation, so there is a standing water level between storms. A detention basin holds a storm temporarily and releases it slowly through a restricted outlet, so it is usually dry in between and the open space can be used for something else. Retention needs soil that will take water and is the better answer where groundwater recharge is a goal. Detention is about cutting peak flow to the storm drain. Which one a site needs comes out of the drainage report and the soils, not out of preference.

If the project is new development or significant redevelopment in Los Angeles County, some form of on-site stormwater management is very likely required under the Low Impact Development ordinance, and the civil engineer will have shown it on the drawings. That might be a basin, or it might be bioswales, permeable paving, dry wells or a combination. What we can tell you is what the shown solution costs to build and whether the site conditions actually support it, particularly the infiltration assumption, which is the item most often taken from a table rather than from a test pit.

Inspect after every significant storm and at least quarterly. Clean out the forebay when it is a third full, which on a site with disturbed ground upstream can be annually and on a settled site every few years. Keep the outlet structure and the trash rack clear, that being the most common cause of a basin backing up. Mow or cut back the banks once or twice a year, keeping vegetation out of the flow path but leaving the deep-rooted planting that holds the slopes. Once a decade or so, expect a full sediment removal to restore design volume.

Yes, and on visible sites it should. A basin that holds a permanent pool can be built with a proper edge, set stone, aeration and planted margins, and read as an amenity while still doing its hydraulic job. The constraints are real: the design volume, the freeboard, the outlet and the maintenance access all have to survive the makeover, and the planting has to tolerate inundation. But the alternative, a bare fenced hole at the front of a property, is a design failure rather than a requirement, and it is usually the version an HOA board is unhappy about.

Sediment. Every storm delivers material into the basin, and if there is no forebay to catch it where it can be dug out, it spreads across the floor and quietly eats the design volume. On infiltration basins the same fine sediment seals the base and the infiltration rate collapses, so the basin holds water for days when it used to drain in hours. Both are fixable: remove the accumulated material, restore the elevations, rebuild or add a forebay, and where infiltration has sealed, scarify and reinstate the permeable base.

Yes. Course ponds usually serve as hazards, irrigation storage and stormwater management at the same time, and the work ranges from building new water into a redesign to restoring ponds that have silted up, lost their banks or gone anaerobic and started smelling in late summer. The common package is dredging or sediment removal, edge and bank rebuilding, aeration to keep the water column turned over, liner repair where there is one, and planting that survives both carts and inundation. We work around play schedules, which on a live course is most of the planning.

A well-designed one does not, and a neglected one does. The problem is water that sits still for more than a few days without predators, which is exactly what a detention basin with a partially blocked outlet produces. The answers are a functioning outlet so the basin drains in the design time, aeration or circulation on permanently wet basins, planting and edge conditions that support dragonflies and other predators, and mosquitofish where there is a permanent pool. The Greater Los Angeles County Vector Control District takes complaints about standing water on private property seriously, which is another reason the outlet is worth keeping clear.

A small residential or HOA basin is usually one to two weeks on site. A commercial basin with concrete inlet and outlet structures, engineered slopes and full planting runs four to ten weeks depending on the volume and how much spoil has to leave. Golf course restoration depends far more on access and play schedule than on the excavation itself. The permitting and drainage review in front of the work typically runs longer than the construction, so the useful thing to know early is where the project sits in that queue.