Hydrology and Watershed Analysis
Hydrology is the scientific study of water movement, distribution, and quality across the earth’s surface and within the subsurface. The hydrologic cycle describes the continuous movement of water through evaporation from oceans and land surfaces, condensation into clouds, precipitation as rain or snow, infiltration into the soil, surface runoff into streams and rivers, and eventual return to the oceans. Understanding the hydrologic cycle at the watershed scale is fundamental to water resource engineering, flood prediction, and environmental management. The water balance equation accounts for all inflows, outflows, and changes in storage within a watershed over a specified time period. The components of the water balance include precipitation, evapotranspiration, surface runoff, groundwater recharge, and changes in soil moisture storage. The water balance must close within acceptable accuracy for the analysis to be reliable.
Watershed characteristics that affect the hydrologic response include the drainage area size and shape, the slope and aspect, the soil type and permeability, the land use and vegetation cover, and the drainage network density. The time of concentration is the time required for runoff to travel from the hydraulically most distant point in the watershed to the outlet. The time of concentration depends on the flow path length, slope, and surface roughness. Watersheds with shorter times of concentration produce higher peak runoff rates because more of the watershed contributes to the peak flow simultaneously. Urban development increases peak runoff rates by replacing pervious surfaces with impervious surfaces and by reducing the time of concentration through drainage improvements such as storm sewers and channelized streams.
Rainfall-runoff models simulate the transformation of precipitation into runoff based on the watershed characteristics and the rainfall distribution. The unit hydrograph method represents the runoff response of a watershed to a unit of excess rainfall distributed over a unit duration. The unit hydrograph can be derived from observed rainfall and runoff data for gaged watersheds or estimated from watershed characteristics for ungaged watersheds using synthetic unit hydrograph methods. The Soil Conservation Service method is widely used for estimating runoff volumes from rainfall based on the curve number that characterizes the runoff potential of the soil-cover complex. The curve number ranges from 30 for fully pervious surfaces with high infiltration rates to 98 for impervious surfaces. The SCS method is the basis for many stormwater management regulations and design standards in the United States.
Open Channel Hydraulics
Open channel flow occurs in natural streams, constructed canals, and stormwater conduits where the flow has a free surface exposed to atmospheric pressure. The flow regime in open channels is classified as subcritical, critical, or supercritical based on the Froude number that relates inertial forces to gravitational forces. Subcritical flow with Froude numbers less than 1.0 has low velocity and deep flow with water surface disturbances that propagate upstream. Supercritical flow with Froude numbers greater than 1.0 has high velocity and shallow flow with disturbances that only propagate downstream. Critical flow at Froude number 1.0 represents the transition between regimes and occurs at the minimum specific energy for a given discharge. natural resources conservation service curve number method for runoff estimation. manning equation for open channel flow design. darcy law for groundwater flow in porous media. The Manning equation is the most widely used formula for calculating flow velocity and capacity in open channels based on the channel geometry, slope, and hydraulic roughness.
The design of open channels for flood control and drainage must accommodate the design discharge with adequate freeboard above the design water surface elevation. The channel cross-section may be trapezoidal, rectangular, triangular, or natural depending on the site conditions and the aesthetic requirements. Lined channels with concrete, riprap, or gabion protection resist erosion at higher velocities than unlined earth channels. The permissible velocity for erosion resistance depends on the lining material and the soil type of the channel bed. Grass-lined channels provide erosion protection at moderate velocities while maintaining a natural appearance and providing ecological habitat. The channel slope must be sufficient to maintain the design velocity without excessive sedimentation or erosion.
Hydraulic structures such as weirs, flumes, and culverts control and measure flow in open channel systems. Weirs are overflow structures that create a hydraulic control section where the flow depth is directly related to the discharge rate. Sharp-crested weirs with a thin crest are used for flow measurement in laboratories and small channels. Broad-crested weirs with a wide crest are used for flow control in larger channels and at dam spillways. Culverts convey flow under roadways and embankments, operating under inlet control or outlet control conditions depending on the culvert geometry and the headwater and tailwater elevations. The culvert design must accommodate the design discharge while maintaining acceptable headwater elevations and preventing roadway overtopping for the design storm event.
Groundwater Hydrology
Groundwater is water stored in the pore spaces and fractures of soil and rock beneath the earth’s surface. The study of groundwater hydrology includes the occurrence, movement, and quality of groundwater in aquifers. Aquifers are geologic formations that contain sufficient saturated permeable material to yield usable quantities of water to wells. Unconfined aquifers have a water table that is free to rise and fall in response to recharge and pumping. Confined aquifers are bounded above and below by low-permeability aquitards that restrict vertical water movement and create artesian pressure conditions. The storage properties of aquifers are characterized by the specific yield for unconfined aquifers and the storativity for confined aquifers. The sustainable yield of an aquifer is the rate at which groundwater can be withdrawn without causing unacceptable depletion, water quality degradation, or environmental impacts.
Darcy’s Law describes the flow of groundwater through porous media as proportional to the hydraulic gradient and the hydraulic conductivity of the medium. The hydraulic conductivity depends on the pore size distribution and connectivity, with values ranging from less than 10 to the minus 7 centimeters per second for clays to greater than 1 centimeter per second for clean gravels. The transmissivity of an aquifer is the product of the hydraulic conductivity and the saturated thickness and represents the capacity of the aquifer to transmit water. Groundwater flow patterns are controlled by the distribution of hydraulic head, which represents the total energy per unit weight of water at each point in the aquifer. Flow nets combining equipotential lines and flow lines provide a graphical representation of groundwater flow patterns that is used for analyzing seepage through earth dams, beneath cutoff walls, and into excavations.
Well hydraulics analyzes the drawdown of the water table or piezometric surface around pumping wells. The Theis equation provides the transient response of a confined aquifer to pumping based on the aquifer transmissivity, storativity, the pumping rate, and the distance from the well. The Cooper-Jacob approximation simplifies the Theis equation for late-time conditions where the drawdown varies linearly with the logarithm of time. The results of pumping tests analyzed using these equations provide the aquifer parameters needed for well field design and groundwater modeling. The radius of influence of a pumping well is the distance from the well at which the drawdown becomes negligible. The interference between multiple pumping wells in a well field reduces the specific capacity of each well compared to isolated wells.
Water Quality and Treatment
Water quality is determined by the physical, chemical, and biological characteristics of water that affect its suitability for specific uses. Surface water quality is influenced by natural factors including soil and rock types, climate, and vegetation, as well as human activities such as agriculture, urban development, and industrial discharges. The Clean Water Act establishes the regulatory framework for water quality protection in the United States, requiring states to establish water quality standards for all water bodies and to develop total maximum daily loads for waters that do not meet the standards. Common water quality parameters include dissolved oxygen, pH, temperature, turbidity, nutrients such as nitrogen and phosphorus, pathogens such as bacteria and viruses, and toxic substances such as heavy metals and organic compounds.
Water treatment processes are selected based on the source water quality and the intended use of the treated water. Conventional drinking water treatment includes coagulation, flocculation, sedimentation, filtration, and disinfection. Advanced treatment processes such as activated carbon adsorption, membrane filtration, and advanced oxidation are used to remove contaminants that are not effectively removed by conventional treatment. The Safe Drinking Water Act establishes maximum contaminant levels for over 90 contaminants in public drinking water supplies and requires monitoring and reporting to verify compliance. Wastewater treatment removes pollutants from sewage to produce effluent that can be safely discharged to receiving waters or reused. Secondary treatment with biological processes removes biodegradable organic matter, and tertiary treatment provides additional removal of nutrients and pathogens for discharge to sensitive waters or for water reuse applications.