How does municipal water systems work

Time series of rainfalls are generally used to look at aspects such as overflow spill frequencies and volumes. On the other hand, synthetic design storms can be generated for a wide range of conditions including the same conditions as represented by real rainfall. This is generally considered appropriate for looking at pipe network performance.

Rainfall varies in space as well as in time, and the two effects are related. Short duration storms typically come from small rain cells that have a short life, or that move rapidly over the catchment. As these cells are small of the order of a kilometer in diameter there is significant spatial variation in rainfall intensity. Longer duration storms tend to come from large rainfall cells associated with large weather systems.

These have less spatial intensity variation. Rainfall is generally measured at specific sites using rain gauges. The recorded rainfall amount and intensity will not be the same at each site. Thus the use of recorded rainfall data requires some way to account for this spatial and temporal variations. The average rainfall over the catchment in any period of time can be more or less than the measured values at one or more gauge sites. The runoff from a portion of a catchment exposed to a high intensity rainfall will more than the runoff from the same amount of rainfall spread evenly over the entire catchment.

A convenient way of using rainfall data is to analyze long rainfall records to define the statistical characteristics of the rainfall, and then to use these statistics to produce synthetic rainstorms of various return periods and durations. The frequency with which it is likely to occur, or the probability of it occurring in any particular year. In most of the work on urban drainage and river modeling, the risks of occurrence are expressed not by probabilities but by the inverse of probability, the return period.

An event that has a probability of 0. An event having a probability of 0. Rainfall data show an intensity—duration—frequency relationship. The intensity and duration are inversely related. As the rainfall duration increases the intensity reduces. The frequency and intensity are inversely related so that as the event becomes less frequent the intensity increases. An important part of this duration—intensity relationship is the period of time over which the intensity is averaged.

It is not necessarily the length of time for which it rained from start to finish. In fact any period of rainfall can be analyzed for a large range of durations, and each duration could be assigned a different return period. The depth of rainfall is the intensity times its duration integrated over the total storm duration.

Design rainfall events hyetographs for use in simulation models are derived from intensity—duration—frequency data.

A design storm is a synthetic storm that has an appropriate peak intensity and storm profile. Most models assume that the first part of a rainfall event goes to initial wetting of surfaces and filling depression storage.

The depth assumed to be lost is usually related to the surface type and condition. Rain water can be intercepted by vegetation or can be trapped in depressions on the ground surface.

Depression storage can occur on any surface, paved, or otherwise. Estimation of depression storage based on data from catchments in the UK Price As rainfall increases so does depression storage. The values of a and b depend in part on the surface type. Evaporation, another source of initial loss, is generally considered to be relatively unimportant. Continuing losses are often separated into two parts: evapotranspiration and infiltration. These processes are usually assumed to continue throughout and beyond the storm event as long as water is available on the surface of the ground.

Losses due to vegetation transpiration and general evaporation are not particularly an issue for single events, but can be during the interevent periods where catchment drying takes place. This is applicable to models where time-series data are used and generated. Infiltration is usually assumed to account for the remaining rainfall that does not enter into the drainage system. Effect of catchment wetness on runoff Q over time t Price It is impractical to take full account of the variability in urban topography, and surface condition.

Impervious paved surfaces are often dominant in an urban catchment and the loss of rainfall prior to runoff is usually relatively small. Runoff routing is the process of passing rainfall across the surface to enter the drainage network. This process results in attenuation and delay. These are modeled using routing techniques that generally consider catchment area size, ground slope, and rainfall intensity in determining the flow rate into the network.

The topography and surface channels and even upstream parts of the sewer system are usually lumped together into this process and are not explicitly described in a model. The runoff routing process is often linked to catchment surface type and empirical calibration factors are used accordingly. Various models for rainfall—runoff and routing are available and are used in different parts of the world.

Overland runoff on catchment surfaces can be represented by the kinematic wave equation. However, direct solution of this equation in combination with the continuity equation has not been a practical approach when applied to basins with a large number of contributing subcatchments.

Simpler reservoir-based models, that are less computationally and data demanding, represent the physical processes almost as accurately as the more complex physically based approaches Price In practice, models applied to catchments typically assume an average or combined behavior of a number of overland flow planes, gutters, and feeder pipes. Therefore, the parameters of a physically based approach for example, the roughness value as applied would not relate directly to parameters representative of individual surfaces and structures.

Many overland flow routing models are based on a linear reservoir-routing concept. To take into account the effects of depression storage and other initial losses, the first millimeter s of rainfall may not contribute to the runoff. The simplest models rely on fixed runoff coefficients K. They best apply to impervious areas where antecedent soil moisture conditions are not a factor.

Typical values of the runoff fraction coefficient K. The Horton model describes the increasing runoff from permeable surfaces as a rainfall event occurs by keeping track of decreasing infiltration as the soil moisture content increases.

The runoff from paved surfaces is assumed to be constant while the runoff from permeable surfaces is a function of the conceptual wetting and infiltration processes. Flow chart of Horton model infiltration algorithm used in each time step of a simulation model. It has also been used for the permeable component in a semi-urban environment.

This runoff model allows for variation in runoff depending on catchment wetness. The model relies on what are called curve numbers, CN. However, other studies suggest that k values between 0.

The storage variable, S , itself is related to an index known as the runoff curve number, CN , representing the combined influence of soil type, land management practices, vegetation cover, urban development, and antecedent moisture conditions on hydrological response. SCS hydrologic soil groups used in Tables The Green—Ampt model is similar to the Horton model, in that it has a conceptual infiltration rate that varies with time.

It is therefore applicable to pervious or semipervious catchments Huber and Dickinson ; Roesner et al. The modeling of surface pollutant loading and washoff into sewer systems is very imprecise. Pollutants that build up on the surface of an urban area originate from wind blown dust, debris that is both natural and human-made, including vehicular transport emissions.

When rainfall takes place some of this material, as dissolved pollutants and fine solids, is washed into the stormwater sewers or gullies. During buildup time many of the pollutants degrade. Deposition of this material is not homogeneous but rather is a function of climate, geography, land use, and human activity. The mechanism of washoff is obviously a function of location, land use, rainfall intensity, slope, flow rate, vehicle disturbance, etc. None of these factors are explicitly modeled in most washoff and sewer flow models.

Measurements made of pollutant accumulation and washoff have been the basis of empirical equations representing both loading and washoff processes. In practice the level of information available and the complexity of the processes being represented make the models of pollutant loading and washoff a tool whose outputs must be viewed for what they are, merely guesses. Modeling does not change this, it only has the potential of making those guesses better.

Pollutant loadings and accumulation on the surface of an urban catchment occur during dry periods between rainstorms. This assumed constant loading rate on the surface of the ground can vary over space and is related to the land use of that catchment. In reality these loadings on the land surface will not be the same, neither over space nor over time. Hence to be more statistically precise, a time series of loadings may be created from one or more probability distributions of observed loadings.

Just how this may be done is discussed in Chaps. Different probability distributions may apply when, for example, weekend loadings differ from workday loadings. However, given all the other uncertain assumptions in any urban loading and washoff model, the effort may not be justified. Sediments that become suspended solids in the runoff are among the pollutants accumulating on the surface of urban catchments. They are important by themselves, but also because some of the other pollutants that accumulate become attached to these sediments.

Sediments are typically defined by their medium diameter size value d Normally a minimum of two sediment fractions are modeled, one coarse high-density material grit and one fine organics. The sediments of each diameter size class are commonly assumed to have a fixed amount of pollutants attached to them. The fraction of each attached pollutant, sometimes referred to as the potency factor of the pollutant, is expressed as kg of pollutant per kg of sediment.

Potency factors are one method for defining pollutant inputs into the system. Pollutants in the washoff can be dissolved in water, or they can be attached to the sediments.

Many models of the transport of dissolved and particulate pollutants through a sewerage system assume each pollutant is conservative—it does not degrade with time.

For practical purposes this is a reasonable assumption when the time of flow in the sewers is relative short. Otherwise it may not be a good assumption, but at least it is a conservative one. As the storm event proceeds and pollutants are removed from the catchment, the quantities of available pollutants decrease, hence the rate of pollutant washoff decreases even with the same runoff.

When runoff occurs, a fraction of the accumulated load may be contained in that runoff. This fraction will depend on the extent of runoff. If a part of the surface loading of a pollutant is attached to sediments, its runoff will depend on the amount of sediment runoff, which in turn is dependent on the amount of surface water runoff. As the sediments are routed through the system those from different sources are mixed together.

The concentrations of associated pollutants therefore change during the simulation as different proportions of sediment from different sources are mixed together.

The results are given as concentrations of sediment, concentrations of dissolved pollutants, and concentrations of pollutants associated with each sediment fraction. Flows in pipes and sewers have been analyzed extensively and their representation in models is generally accurately defined. The hydraulic characteristics of sewage are essentially the same as clean water. Time-dependent effects are, in part, a function of the change in storage in manholes.

Difficulties in obtaining convergence occurs at pipes with steep to flat transitions, dry pipes, etc. Pollutant transport modeling of both sediment and dissolved fractions involves defining the processes of erosion and deposition and advection and possibly dispersion. One-dimensional models by their very nature cannot predict the sediment gradient in the water column.

In addition the concept of the sewer being a bioreactor is not included in most simulation models. Most models assume pollutants are conservative during the time in residence in the drainage system before being discharged into a water body.

All these processes that take place in transient are generally either ignored or approximated using a range of assumptions. Manholes, valves, pipes, pumping stations, overflow weirs, etc. The impact of some of these structures can only be predicted using 2- or 3-dimensional models. However, the ever-increasing power of computers is making higher dimensional fluid dynamic analyses increasingly available to practicing engineers. The biggest limitation may be more related to data and calibration than to computer models and costs.

Slime can build up on the perimeter of sewers that contain domestic sewage. The buildup of slime may have a significant effect on roughness. In a combined system the effect will be less as the maximum daily flow of domestic sewage will not usually be a significant part of pipe capacity. The extent to which the roughness is increased by sliming depends on the relation between the sewage discharge and the pipe-full capacity.

Sliming will occur over the whole of the perimeter below the water level that corresponds to the maximum daily flow. The slime growth will be heaviest in the region of the maximum water level. Over the lower part of the perimeter, the surface will still be slimed, but to a lesser extent than at the waterline.

Above the maximum waterline the sewer surface will tend to be fairly free of slime. When sediment is present in the sewer the roughness increases quite significantly. It is difficult to relate the roughness to the nature and time history of the sediment deposits. Most stormwater sewers contain some sediment deposits, even if only temporarily.

The only data available suggest that the increase in head loss can range from 30 to mm, depending on the configuration of the deposit and on the flow conditions. The higher roughness value is more appropriate when the sewer is flowing part full and when considerable energy is lost as a result of the generation of surface disturbances.

In practice the lower roughness values are used as flow states of interest are usually extreme events and therefore sewers are operating in surcharge. The effects of combined sewer outflows CSOs or discharges are particularly difficult to quantify and regulate because of their intermittent and varied nature.

Their immediate impact can only be measured during a spill event, and their chronic effects are often difficult to isolate from other pollution inputs. Yet CSOs are one of the major causes of poor river water quality.

Standards and performance criteria specifically for intermittent discharges are therefore needed to reduce the pollutants in CSOs. These can be either organic, such as fecal matter, or inorganic. Heated discharges, such as cooling waters, reduce the saturated concentration of dissolved oxygen ,.

Problems arise when pollutant loads exceed the self-purification capacity of the receiving water, harming aquatic life, and restricting the use of the water for consumption and many industrial and recreational purposes. The assimilative capacity for many toxic substances is very low.

Water polluted by drainage discharges can create nuisances such as unpleasant odors. It can also be a direct hazard to health, particularly in tropical regions where waterborne diseases such as cholera and typhoid prevail.

The aim of good drainage design, with respect to pollution, is to balance the effects of continuous and intermittent discharges against the assimilation capacity of the water, so as to achieve in a cost and socially effective way the desired quality of the receiving water. In Fig. This is because there is a time lag of up to several days while the bacteria, which digest the pollutant, multiply. The suspended solids SS settle relatively quickly and they can then be a source of pollutants if the bed is disturbed by high flows.

This can create a subsequent pollution incident especially if the suspended solids contain quantities of toxic heavy metals. The rise in the ammonium concentration downstream of the discharge is relevant due to the very low tolerance many aquatic organisms, particularly fish, have to the chemical. The ammonium concentration rises if the conditions are anaerobic and will then decline once aerobic conditions return and the ammonium ions are oxidized to nitrates.

The increased quantities of phosphate and nitrate nutrients that they consume can lead to eutrophication. The fauna show perhaps the clearest pattern of response.

The predictability of this response has lead to the development of the many biological indices of pollution. The rapid succession of organisms illustrates the pattern of dominance of only a few species in polluted conditions. For example, tubificid worms can exist in near anaerobic conditions, and having few competitors, they can multiply prolifically. As the oxygen levels increase these organisms are succeeded by Chironomids midge larvae and so on until in clean well-oxygenated water, there is a wide diversity of species all competing with each other.

This maximum saturation concentration is temperature dependent. The hotter the water is the lower the DO saturation concentration. All higher forms of life in a river require oxygen. In the absence of toxic impurities there is a close correlation between DO and biodiversity. Perhaps of more pragmatic significance is the fact that oxygen is needed in the many natural treatment processes performed by microorganisms that live in natural water bodies. The quantity of oxygen required by these organisms to breakdown a given quantity of organic waste is, as previously discussed, the biochemical oxygen demand BOD.

It is expressed as mg of dissolved oxygen required by organisms to digest and stabilize the waste in a liter of water. These organisms take time to fully digest and stabilize the waste. There are 6 key stages in our municipal water systems: Source water — the lakes, rivers and underground aquifers that are the source of our water supply, fed by rain and melting snow. Water treatment — the processes to filter and purify water so that it is safe for human use.

Water distribution systems — the pipes and pumps that deliver clean water to our taps. Wastewater collection systems — the pipes and pumps that take away used water from our toilets, drains, bathtubs, and laundry. These are also called sewers. In BC, there are over 26, km of municipal sewer pipes underground — that is enough pipe to circle two-thirds of the way around the earth! Wastewater treatment — the processes to remove contaminants from our used water so that it can be safely returned to the environment.

This is also called sewage treatment. Stormwater systems — the pipes, ditches and natural systems that channel our rain water and snow melt away from our homes and businesses and back to the natural environment. This is also called sewage treatment. Stormwater systems — the pipes, ditches and natural systems that channel our rain water and snow melt away from our homes and businesses and back to the natural environment.

In Madera there are over 45 miles of stormwater pipes underground. How does water get from the source to our taps? How do we know our water is safe to drink? Click to view our most commonly asked water rate questions. Please click here to see any active alerts. Providing safe drinking water is a partnership that involves EPA, the states, tribes, water systems, and water system operators.

The public drinking water systems regulated by EPA and delegated states and tribes provide drinking water to 90 percent of Americans.