An 18-section interactive reference guide covering the fundamentals of environmental and water resources engineering. Includes hydrology and the hydrologic cycle, rainfall IDF curves and return period, the rational method and detention basin design, weir and orifice flow, open-channel flow with Manning's equation, pipe flow and Hazen-Williams distribution networks, groundwater hydrogeology and Darcy's law, drinking water treatment and disinfection, wastewater treatment through tertiary and biosolids, hydraulic retention time, and air quality/NEPA permitting.
All the core areas of environmental and water resources practice are covered: Hydrology & Hydraulics (the hydrologic cycle, IDF curves and return period, the rational method Q=CiA, detention and retention basin design, weir and orifice flow, open-channel flow and Manning's equation, and pipe flow with Hazen-Williams and distribution network design); Water & Wastewater Treatment (the conventional drinking-water treatment train from coagulation through filtration, disinfection and CT values, drinking water standards under the SDWA, wastewater treatment from preliminary through tertiary, nutrient removal and biosolids handling, and hydraulic retention time / clarifier sizing); and Groundwater & Environmental Permitting (aquifer types and Darcy's law, wells and drawdown, groundwater remediation, NAAQS air quality permitting, and the NEPA environmental review process).
Use the Prev / Next buttons at the bottom, or press the arrow keys on your keyboard. Click the ☰ menu button in the top-right corner to open the table of contents and jump directly to any of the 18 sections. The teal progress bar at the top tracks your position through all sections. Scroll the mouse wheel to advance or go back.
This handbook is designed to pair with the Environmental & Water Resources Studio's calculators. Once you understand the rational method in Section 4, try the studio's Rational Method Runoff Calculator; once you understand Manning's equation in Section 7, try the Manning's Open-Channel Flow Calculator; the same pairing holds for Hazen-Williams, detention basin storage, weir flow, BOD removal, chlorine dosage, hydraulic retention time, and Darcy's law groundwater calculators. Reading the concept here first makes the calculator's inputs and outputs much easier to interpret correctly.
The rational method estimates peak stormwater runoff with Q = C × i × A, where Q is peak flow, C is a runoff coefficient reflecting how much rainfall becomes runoff, i is rainfall intensity for a storm duration equal to the time of concentration, and A is the drainage area. It is the standard tool for small drainage areas, generally under a few hundred acres, and drives storm sewer sizing, detention pond design, and culvert selection across most site-scale stormwater projects.
Manning's equation, V = (1.49/n) × R^(2/3) × S^(1/2), estimates the average velocity of water flowing in an open channel — a ditch, culvert running partly full, or natural stream — under uniform flow. R is the hydraulic radius (area divided by wetted perimeter), S is the channel slope, and n is a roughness coefficient that depends on the channel lining. Multiplying velocity by cross-sectional area gives discharge, letting engineers size channels and check whether a design achieves adequate velocity without eroding the lining.
CT is the product of the disinfectant residual concentration (C, in mg/L) and the contact time (T, in minutes) that water spends in contact with that disinfectant. Regulators publish required CT values for inactivating a target percentage of specific pathogens at a given temperature and pH, and a treatment plant demonstrates adequate disinfection by proving it achieves the required CT in its contact basins and pipelines before water reaches the first customer.
Darcy's law, Q = −K × A × (dh/dl), describes how groundwater moves through a porous medium such as sand, gravel, or fractured rock. Q is the flow rate, K is the hydraulic conductivity of the material, A is the cross-sectional area, and dh/dl is the hydraulic gradient — the change in head over distance. It is the foundational equation of hydrogeology, governing everything from well yield calculations to contaminant plume migration.
Hydraulic retention time (HRT = V/Q, tank volume divided by flow rate) is the average time a parcel of water spends inside a basin, clarifier, or reactor. It is a core sizing check across both drinking-water and wastewater treatment: sedimentation basins need enough HRT for particles to settle, biological reactors need enough HRT for microorganisms to consume organic matter, and disinfection contact basins need enough HRT (combined with residual concentration, as the CT value) to inactivate pathogens.
Disclaimer: This reference guide summarizes publicly available engineering standards and principles for educational purposes only. Always consult the official adopted edition of the applicable code, regulation, or standard for design, engineering, and compliance decisions. Standards referenced herein are copyright their respective organizations (EPA, AWWA, WEF, ASCE, NCEES, etc.).