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that the site LID objectives have been achieved. If not, additional IMPs
are located on the site to achieve the optimal condition.
7. Evaluate supplemental needs. If supplemental control for either volume or
peak flow is still needed after the use of IMPs, selection and listing of
additional management techniques should be considered. For example,
where flood control or flooding problems are key design objectives, or
where site conditions, such as poor soils or a high water table, limit the
use of IMPs, additional conventional end-of-pipe methods, such as large
detention ponds or constructed wetlands, should be considered. In some
cases their capacity can be reduced significantly by the use of LID
upstream. It may be helpful to evaluate several combinations of LID
features and conventional stormwater facilities to determine which
combination best meets the stated objectives. Use of hydrologic
evaluations can assist in identifying the alternative solutions prior to
detailed design and construction costs.
For residential areas, Prince George's County, Maryland, has developed a
detailed illustration of an approach for conducting a hydrologic evaluation based on the
NRCS TR-55 method. Where NRCS methods (TR-20, TR-55) are accepted for
hydrologic evaluation, the effect of LID features should be reflected in the curve
numbers and times of concentration selected for the analysis. A full description of this
process is available from Prince George's County.37
Step 4: Design LID Site or Master Plan
1. Sketch a design concept that distributes the LID practices appropriately
around the project site. Try to use all surface types (built, hardscape, and
landscape). Keep in mind the multifunctional capability of LID
technologies (i.e., parking lot with detention facility underground).
2. Develop a master plan that identifies all key control issues (water quality,
water quantity, water conservation) and implementation areas. Specify
specific LID technologies and any connections they have to stormwater
overflow units and sub-surface detention facilities.
Step 5: Develop Operation and Maintenance Procedures
Develop operation and maintenance procedures for each of the LID practices
implemented in the site plan. Different types of IMPs will have different maintenance
requirements, but some general principles will apply:
Keep IMPs and flow paths clear of debris.
Water vegetation regularly during dry periods.
Grassed areas should be mowed regularly.
Plantings should be pruned as needed.