It is Highly Recommended that the designer provides non-erosive flow velocities at the outlet point to reduce downstream erosion. During the 10-year or 25-year storm (depending on local drainage criteria), discharge velocity should be kept below 4 feet per second for established grassed channels. Erosion control matting or rock should be specified if higher velocities are expected.
Common overflow systems within the structure consist of a yard drain inlet, where the top of the yard drain inlet is placed at the elevation of the shallow ponding area. A stone drop of about 12 inches or small stilling basin could be provided at the inlet of bioretention areas where flow enters the practice through curb cuts or other concentrated flow inlets. In cases with significant drop in grade this erosion protection should be extended to the bottom of the facility.for more information u can read here:
Design criteria for bioretention
https://stormwater.pca.state.mn.us/index.php/Design_criteria_for_bioretention
Suggestions for Designing and Constructing Bioretention cells for a nordic climate
https://www.researchgate.net/publication/268803718_Suggestions_for_Designing_and_Constructing_Bioretention_Cells_for_a_Nordic_Climate
Article Suggestions for Designing and Constructing Bioretention Cell...
You need to take into account many design considerations:
Basically, Three design flows are required for designing bioretention swales:
Minor flood rates (typically 5-year ARI).
Major flood rates (typically 100 year ARI).
Maximum infiltration rate through the filtration media.
The link below gives an insight into the procedure of designing bioretention swales:
https://www.derwentestuary.org.au/assets/Water_Sensitive_Urban_Design_Guidelines_04_Chapter_4_Bioretention_swales.pdf
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