One of the most important design aspects of any land development project is managing the flow of water across, and sometimes under, the site. To do this, engineers use the sciences of hydrology and hydraulics. But what’s the difference? If you’ve ever wondered if you’re using the right term, read on!

Hydrology Hydrology is the study of rainfall and water, especially its movement, in relation to land. It represents the quantity of water (runoff) generated from a specific area or watershed. It deals with the circulation of water through the hydrologic cycle and the quantification of flows that are produced by rainfall. In land development, hydrology typically refers to the rate of precipitation, quantity of water, rate of surface runoff, and timing of its arrival at a point of interest (the project site).

How do civil engineers use hydrology in land development? Any development activity will change the flow of water on a site. If you clear cut a parcel of land, rainfall is more likely to run off the site rather than be absorbed by grass and trees. If left unmanaged, this newly created runoff can negatively affect neighboring parcels. To combat this domino effect, local development authorities (city, county, state, etc.) require site designs to meet certain water-related criteria – namely that the site design will not negatively affect nearby sites through increased runoff.

Engineers use hydrology to provide reports detailing the surface water flow changes that are anticipated with the proposed site design. The goal is typically to determine if the post-development stormwater flow is equal to or greater than the pre-development stormwater flow. Typical hydrologic design requirements include providing water quality treatment and runoff reduction through detention of a series of 24-hour storm events, such as the 1, 2, 5, 10, 25, 50, and 100-year return intervals. Engineers use data related to precipitation, drainage basin sizes and land covers, soil type(s), appropriate runoff coefficient(s), conveyance elevation and geometry, etc. to determine this information. A drainage report is typically required in support of the stormwater management plan for a proposed project.

Once this data is compiled, the engineer moves on to the hydraulic design of any stormwater systems needed to manage the difference in water flow that the proposed development would cause.

Hydraulics Hydraulics is concerned with the conveyance of water through pipes and channels (stream, river, lake, ocean). In land development, hydraulic analysis is used after the hydrology reporting process in order to design stormwater conveyance networks, calculate the depth of flow in pipes, and determine open channel flow in ditches and swales. This is often referred to as “stormwater management design.”

How do civil engineers use hydraulics in land development? Once the difference between pre-development stormwater flow and post-development stormwater flow is determined, civil engineers use hydraulics to design stormwater management systems to account for the difference to minimize a development’s effect on neighboring sites. These systems, both pipes and open channels, are designed to accommodate all intercepted water flow from rainfall events. A typical project includes the design of many hydraulic components, such as pipe sizes, pipe alignments, flow line elevations, end treatments, inlet and outlet protection, channel geometry, and channel slope, to name a few.

No one wants to interact with a site suffering from drainage and flood issues, and civil engineers play a vital role in the design of surface water management systems. Hydrology helps assess all possible conditions of a proposed development during multiple environmental scenarios. Hydraulics is then used to design appropriate systems to mitigate any changes from the site’s pre-development state. While the names are easy to confuse, I hope this helps delineate the two practices and show how, while different, they are used together to create a well-designed site that plays nicely with its neighbors.

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