Urban Drainage and Sewerage I – EPA-SWMM design exercise

Case study: Porto Alegre Municipality, Brazil

Porto Alegre is the capital city of Brazil’s southernmost state, Rio Grande do Sul. Porto

Alegre itself has a population of 1.3 million inhabitants. It covers an area of 470 square

kilometres, 40 per cent of which is urban and 60 per cent rural. The Areia basin is located

in the north of the city. It covers an area of 20,85 km2, of which approximately half

corresponds to the basin of Arroio da Areia, i.e. about 11,7 km2 and the rest belongs to

the Airport polder. The Arroio da Areia has an extension of 5,4 km and it descends 121

meter to the pumping station Silvio Brum.

The basin is naturally drained through the main stream Arroio da Areia that receives

water from various smaller canals along its course, among others the Canal Assis Brasil,

Carneiro da Fontoura, Menna Barreto and Cerro Azul. The drainage originates from six

springs in the mounts Rio Branco and Petropolis.

The main drainage network runs in general below the street grid, except for a small part

that intersects a housing block and passes under the foundations. The drainage system in

the basin of Arroio da Areia can be divided into two distinct systems: one drained on the

basis ofgravity and the other drained by the pumping station Silvio Brum. The areas with

a level above 8,13 meter are drained by closed conduits, while the pumping station drains

an area of 139,2 ha that is below the level of 8,13 meter. In the end the drainage from the

upstream basin flows inside a pressure pipe (which is located below the airport lanes) up

to the Jacui Delta. A pumping station serves to drain the lowest basin (‘Pôlder

Aeroporto’). In the entire basin with the exception of Higienópolis the rainwater as well

as the waste water is removed through one system of conduits.

The actual capacity of the urban drainage in some parts of the Areia basin is not enough

to discharge the upstream increase in flood peak and volume as a result of the

urbanization process. The heaviest inundations happen on the intersection of the roads

‘Nilo Peçanha’ and ‘Texeira’. On this point that is the lowest of the region the drainage

system, which transports the water to the ‘Arroio the Areia’, overflows and inundation

levels can reach one meter. Previous floods had resulted in damage to property and even

in the loss of life.

BackgroundThe Municipality of Porto Alegre has an urban development plan, which has its drainage system as an important component. The objective of the exercise is to find the required expansion of the drainage system to accommodate increased storm water flow due to the future urban expansion.

A simple map of the part of the drainage network involved in this study, overlaid with the street map is provided as an image.

Exercise

Design Storm Objective: To compute a design storm for the area.

The recommended Intensity-Duration-Frequency (IDF) curve for the area is given by the following equation:

where i is intensity in mm/h, t duration in min, and T is the return period in years. Use the alternating block method to compute 10 year design rainstorm for the area. Plot this storm as a bar chart.

Catchment PropertiesObjective: To compute various catchment properties for each sub-catchment.

The following tables and figures give the information on various sub-catchments of the area. Tabulate the values of catchment properties needed to run SWMM model based on SCS curve number method.

Impervious Fraction

The sub-basins A, B, C and E fall within the area with 175 hab/ha region.

Maximum allowed population densities (hab/ha)

Compute the approximate value of impervious fraction.

Infiltration Properties

(At this point you will need to install EPA-SWMM in your computer. Download it from http://www.epa.gov/ednnrmrl/models/swmm/index.htm and install)

Municipality engineers use the SCS curve number method for the computation of infiltration loss and it is recommended we use the same method in this task. The soil type for the area falls between type C and type D of SCS tables. Using the data given in the SCS soil type table (it is provided in EPA-SWMM help) draw a graph of Curve Number against impervious fraction. (Note: Catchments under consideration are completely residential)

Use your graph to compute a curve number value suitable for the analysis.

Other Properties of Sub-Catchments

There are certain sub catchment parameters that are difficult to give a value based on the physical properties of the catchment by a straightforward means. Examples are the characteristic width of overland flow, the slope and the roughness values involved in the

Population Density (ha/ha)

Impervious Fraction

Empirical relationship between population density and impervious fraction for the city

non-linear reservoir routing model that computes the catchment outflow. While it is always possible to recommend a relative magnitude of these values (e.g. for a long strip catchment where slope is in lengthwise direction, the flow width should be smaller compared to a catchment of similar area of a square shape.)

In serious design computations such values should always be examined during model calibration, provided that runoff data for known rainfall events are available. However, for the present problem, as in the case of many small-scale engineering computations, no such data is available.

Population Densities and areasSub basin Density Area

[inhab/ha] [km2] A1 46.00 0.338A2 72.47 0.969A3 51.62 0.507A4 39.19 0.300A5 21.57 0.255B1A 41.30 0.280B1B 41.30 0.653B2 66.63 0.273B3 93.40 0.154E1 0.00 0.349E2 0.00 0.121

Computing characteristic width of overland flow: Characteristic width of the overland flow path for sheet flow runoff (feet or meters). An initial estimate of the characteristic width is given by the sub-catchment area divided by the average maximum overland flow length. The maximum overland flow length is the length of the flow path from the inlet to the furthest drainage point of the sub-catchment. Maximum lengths from several different possible flow paths should be averaged. These paths should reflect slow flow, such as over pervious surfaces, more than rapid flow over pavement, for example. Adjustments should be made to the width parameter to produce good fits to measured runoff hydrographs.

Sub-Catchment

Approx. Geometry Flow Length* (m) Slope (%)

A1 Channel 1: 900Channel 2: 700

4.13

A2 Channel 1: 800 4.01

A3 Channel 1: 720Channel 2: 615Channel 3: 834

4.28

A4 Channel 1: 548 4.09

A5 Channel 1: 500Channel 2:600

B1A Channel 1: 1912 3.01

B1B Channel 1: 3.18

B2 Channel 1: 836 3.30

B3 Channel 1: 802 2.99

E1 Channel 1: 278 1.75

E2 Channel 1:507 1.95

Total length from upstream most point to the pour point.

Use the manning’s roughness values for watersheds as: Impervious 0.01, Pervious 0.1, Depression storages as 0.5mm and 1.5mm respectively for impervious and pervious fractions.

The Drainage network Objective: To construct the drainage network in SWMM

The backdrop image shows how the drains are connected. Except T5 T9 and T8 all other drains are open. Following table summarize the conduit properties and where the each sub basin contributes to.

Section Length Slope

PipesIn Parellel Unit Dimensions Roughness

Basins pouring on upstream

Conduit shape

∆L [m] [m/m]W [m] h or D [m]

Basin A T1-1 242 0.038 1 0.80 0.020 A1 Circ.T1-2 242 0.038 1 0.80 0.020 Circ.T2-1 288 0.019 1 0.80 0.020 A2 Circ.T2-2 288 0.019 1 0.80 0.020 Circ.T2-3 288 0.019 1 0.80 0.020 Circ.T3-1 428 0.016 3 0.80 0.020 A3 Circ.T3-2 342 0.015 2 1.00 0.020 Circ.T4-1 117 0.045 1 1.00 0.020 A4, A5 Circ.T4-2 117 0.045 1 1.00 0.020 Circ.T4-3 60 0.045 1 1.00 0.020 Circ.Basin B T6 870 0.018 1 1.00 0.020 B2, B1B Circ.T7 584 0.014 1 1.50 0.020 B3, B1A Circ.Basin E

T5 710 0.011 1 3.00 3.00 0.050 E1Rect. Open

T8 210 0.027 1 2.50 1.50 0.050Rect. Open

T9-1 187 0.005 1 3.00 3.50 0.035 E2Rect. Open

Specify Model ParametersObjective: Provide required model data like rainfall.

Steps: 1. Create a rain gauge object and assign your design rainfall2. Associate the rain gauge object to all the sub catchments. 3. Switch off the groundwater flow and water quality computations. 4.

Running your modelObjective: Decide on simulation parameters like routing method, runoff time step, etc. and run the model; to obtain and analyze the results.

Steps: 1. First investigate all the model parameters and make sure that they represent the

values you intend them to. Since SWMM produce default values for each parameter, it is quite easy to make a mistake.

2. Select full dynamic equation as routing method and set the computation time step to 5 minutes.

3. Run the simulation and examine the simulation report. Discuss your observations. 4. Examine the simulation results. 5. Adjust the time-step to a suitable value. Justify your selection. 6. Run the simulation and find the flooding locations.

Revise the design

Objective: Re-size the conduits so that there will be no flooding for the 10year storm.

1. By trial and error find the sizes of pipes that will completely eliminate flooding in the system for the 10 year storm.Note: Do not select larger-than-necessary pipes in consideration for cost.

Appendix: Alternating block method for computing design storms