Basics of Hydraulic Design Process

Porous Concrete Pavers

Two goals for alternative pavements are to allow water to pass through top layers easily and be able to store water in the gravel layer temporarily. An integral part of the design involves determining not only how much water can be stored in the system, but also how fast it will drain (exfiltrate) from the system. The following is an approach to achieving these goals.

Select the design storm and associated likely rainfall usually a one or two-year return period.

Determine the storage capacity of the permeable paving system using the depth of gravel and void space between the stones. Your gravel supplier can usually provide this information.

Compare rainfall and storage capacity. If storage capacity is greater than expected rainfall from design storm, design is adequate to this point.

Determine desired percolation capability of underlying in-situ soil. If it would take longer than 48 hours for the stored water to drain into the soils, this may not be a suitable project for stormwater control. 20 hours is suggested for design exfiltration purposes.

Calculate actual amount of time required for water to drain from gravel.

Compare design exfiltration time to actual drawdown time.

Note: Further information on hydraulic design for permeable pavements can be found from manufacturers and at the North Carolina State University site: http://www2.ncsu.edu/eos/info/bae/cont_ed/links/hydrdes.htm

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