ECM of Urban Land - Impoundments and Wetlands

Impoundments are man-made ponds or lakes constructed to control storm run-off and/or trap sediments. Before soil or sediment reach the drainage system, detention ponds can be placed to trap and settle sediments. This in effect is NOT an erosion preventive measure but a measure to minimize the already detached or eroded soil from entering waterways. It is a sediment control measure. There are two kinds of impoundments: permanent ones called retention ponds and temporary ones called detention ponds.

A good example of a retention pond is Holmes Lake, which receives storm sewer water and runoff from the City of Lincoln. Holmes Lake serves to retain sediments before the water drains into Antelope Creek, Salt Creek, and eventually into the Platte River. Click Here for a Geographical Map of Holmes Lake

Retention ponds collect run-off from surrounding areas. They control flooding and collect sediment. Image by M. Mamo

Detention ponds collect runoff and sediments from small areas during land disturbance such as construction (84th and Pioneer St, Lincoln, NE). Image by M. Mamo

Detention ponds are small in area because they drain a relatively smaller area. Because construction changes the topography, or lay of the land, drainage characteristics of the land change. Detention ponds are often constructed during road or building construction because the removal of topsoil and compaction reduces water infiltration into the soil and increases the risk of runoff. In addition, paved roads and streets are impervious to water and often become a conduit for runoff and sediment to move into drainage systems.



What are the soil or sediment sizes that will likely dominate the sediment in a detention pond? Justify your answer.

Looks Good! Think about the erosion process. As water carries soil from the land, particles are deposited along the way. As the runoff velocity decreases, the largest sand sized particles will get deposited first along the runoff path. The smallest particles will travel longer in the runoff because they are smaller and will likely get end up in detention ponds. Thus, clay and silt size particles will likely dominate detention ponds.


Wetlands are areas that are frequently saturated by surface water or groundwater, staying wet at least part of the year (i.e., swamps, bogs, fens, marshes, and estuaries). Wetlands are often natural but can also be constructed. Wetlands are effective at controlling sediment and filtering nutrients and other chemical pollutants because they are a biologically active system.