The Islamic University of Gaza- Civil Engineering Department Sanitary Engineering- ECIV 4325 L5. Storm water Management

Transcription

1 The Islamic University of Gaza- Civil Engineering Department Sanitary Engineering- ECIV 4325 L5. Storm water Management Husam Al-Najar

2 Storm water management : Collection System Design principles The Objectives of storm water drainage To prevent erosion in hillside areas (paved roads and terracing are needed) To prevent land-slides To improve the hygienic conditions with regard to the conveyance of wastewater To limit inconvenience to people and traffic To limit damage to unpaved roads Prevent damage to housing, in case the elevation of ground floor is below street level. Collection for reuse purposes, Agriculture use, domestic use and recharge the aquifer

3 Basic Definitions Storm water: Precipitation or rainfall that does not infiltrate into the ground or evaporate into the air. Runoff: Storm water, and associated substances, discharged into streams, lakes, sewers or storm drains. Watershed: Land area from which water drains toward a common surface water body in a natural basin. Components of Storm water drainage system The main components of the storm water drainage system are: - Pipes -Channels -Culverts -Inlets -Pumping station -Manholes -Gutters

4 Methods of Storm Water collection 1. Road Drainage : a. Roof type roads b. Channel type roads 2. Open channel drainage 3. Sewer Drainage Circular sewers Elliptical sewers Box culverts 4. Individual property collection Roof collection: a. Roofs of the buildings b. Green house roofs (agriculture) Comparison criteria between the methods 1. Efficiency 2. Operation and maintenance 3. Public safety 4. Traffic requirements 5. Required space 6. Cost 7. Reliability

5 Box culvert Open channel Circular

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7 Example 1 Two types of concrete storm water drains are compared: Pipe, diameter 2.0m, running full Open channel, rectangular profile, bottom width 2.0m and water depth 1.0 m The drains are laid at gradient of 1.0%, manning coefficient = Determine the velocity of flow and discharge rate for the circular drain Determine the velocity of flow and discharge rate for the rectangular open culvert

8 Channel- type roads hydraulic calculation of road drainage W Road width= 6 m H A R V Q Width of street gutter= 0.6 m Super elevation= 0.08 m or 3% Kerb height= 0.30 m Road gradient 1% Friction factor= 50 (1/n Manning equation)

9 Roof- type roads hydraulic calculation of road drainage m W H A Section width m m 2 Road width= 6 m Width of street gutter= 0.6 m Super elevation= 0.07 m or 3% Kerb height= 0.30 m R m Road gradient 1% V m/s Friction factor= 50 (1/n Manning equation) Q m 3 /s

10 Channel type Roof type

11 Information needed for the design of storm water drainage system 1. Metrological and hydrological data Rainfall intensity Storm duration and occurrence 2. Topographical data Boundaries of the catchments areas Point of collection 3. Classification of catchments areas Industrial, domestic,.. Build up areas (run-off coefficient) 4. Soil investigations Permeability (run-off coefficient)

12 Methods of Run-off Computation Rational method Q = C i A Where; Q = is the run-off in m 3 /sec C = is the Run-off coefficient i = is the average rainfall intensity in mm/hr, A = is the drainage area in hectare (1 ha = 10,000 m 2 )

13 Runoff Coefficient (C) Only a part of the precipitation upon a catchments area will appear in the form of direct runoff. The runoff coefficient depends on: The slope of the area Type of roofs (flat or sloping roofs) Type of soil, absorption capacity of the soil Intensity of rain fall, duration of rain fall, previous rain fall. Composite runoff coefficient: When a drainage area consists of different surface types (or land use), a composite runoff coefficient is used by applying the weighted average method. Development, Pavement Parking/Road Public /Commercial lots Residential Communities Unimproved /Parks Areas Irrigation Areas Coefficient Natural Zones 0.05

14 Example 2: A catchments area has a total area of 0.2 Km 2. The land use of this area is distributed as follows: Area Code Area (m 2 ) Land Use Runoff- coefficient (C) A Buildings A Paved driveways and walks A Portland cement streets A4 190,000 Soil covered with grass Find the composite runoff coefficient for this catchment area. Solution A * C + A * C + A * C + A * C C com = A total Take the lower value for the range of the C: C com = 3000* * * *0.13 = Take the higher value for the range of the C: C com = 3000* * * *0.17 = (For conservative design use the higher value of C com.)

15 Drainage area The drainage area is determined according to the topography. The boundaries of each drainage area (catchment's area) are called watershed lines.

16 Precipitation and evapotranspiration Rainfall can occur in several ways from very short rains with high intensity (tropical storms) to rains even during several days with low intensity (drizzle) In hydrologic studies the following aspects are important: Annual rainfall and distribution over the year Short term intensity Arial rainfall Quality of rainfall Measurement of rainfall: Rain gauges: The ordinary rain gauge for manual observation is normally standardized within a country.

17 Analysis of rainfall data Estimating areal rainfall from point rainfall: Arithmetic mean Thiessen method: depends on the area Isoyetal method: depends on the area Effective Rainfall Assessments of effective rainfall provide an indication of how much of the rainfall over an aquifer outcrop actually contributes to the recharge of groundwater The effective rainfall from year 1982 till year 2004 is calculated based on the FAO general formula for effective rainfall (Pe.) : 17.5 Pe. = 0.8 * P - 25 for average rainfall (P) > 75 mm/month Pe. = 0.6 * P - 10 for average rainfall (P) < 75 mm/month

18 Intensity return period I=aT b Where; I is the rainfall intensity (mm/min), T is the duration time (min), and a, b are constants and related to the number of return years. This equation is fit for Gaza Strip rainfall condition Design frequency of rainfalls sewers in residential areas: T= 1 to 2 years sewers in business areas: flooding caused by rivers: T= 2 to 5 years T= 10, 25, 50, 100, 500 years

19 Return Period: 2 years a: 4.06 b: Duration 5 min 15 min 30 min 1 h 2 h 3 h 6 h 12 h 18 h 24 h P j = p24h X Rainfall (mm) Return Period: 5 years a: 6.18 b: Duration 5 min 15 min 30 min 1 h 2 h 3 h 6 h 12 h 18 h 24 h P j = p24h X Rainfall (mm) Return Period: 10 years a: 7.95 b: Duration 5 min 15 min 30 min 1 h 2 h 3 h 6 h 12 h 18 h 24 h P j = p24h X Rainfall (mm)

20 Design Periods of storm water facilities Drains: years Sanitary sewers: concrete, asbestos cement pipes: years glazed stone ware pipes: years Plastic (PVC, PE): years Pumping Stations: buildings, concrete works: years equipment (pumps, drives, etc.,) years

21 Time of Concentration (T c ) The time of concentration is the time associated with the travel of run-off from an outer point, which best represents, the shape of the contributing areas. The Kirpich formula will be suitable to be used in determining the concentration time for over land run-off flows: T c = (L) 1.15 / ( 52 (H) 0.38 ) Where; T c is the Concentration time in minutes, L is the Longest path of the drainage area in meter, H is the Difference in elevation between the most remote point and the outlet in meters.

22 If the duration of the rainfall (tr) is equal to the time of concentration (tc), then the total run-off gradually increase to the peak discharge. Q Q tc=tr tc tr

23 Example 3 Triangular basin of 20 km2 surface area. A1= 2 km2 Run-off coefficient= 0.8 A2= 4 km2 constant rainfall intensity= 0.1m/hr A3= 6 km2 Time of concentration= 2 hours A4= 8 km2 A4 A3 A2 0.5 hr 0.5 hr 0.5 hr Time in hr. A1 A2 A3 A4 Total A1 0.5 hr

24 Example 4 Use the rational method to find the 10 years design runoff for the are showing in the figure. Time of concentration: Tc = t1 + t2 = 15+5 = 20 min Runoff coefficient: C = {(3x0.3)+ (4x0.7)}/7 = 0.53 Rainfall intensity: I = 65.1 mm/hr. Design peak runoff: CIA= x0.53 x 65.1 x 7= 0.67 m 3 /s. A1= 30 du C1= 0.3 T1= 15 min A2= 40 du C2=0.7 T2= 5 min Duration Rainfall (mm) h 2 h 3 h 6 h 12 h 18 h 24 h min min min From the table: intensity at 20 minute = 21.7 mm/20 min = 65.1mm /hr

25 Example 5 A storm water line is used to collect storm water from three catchment areas (A1, A2, and A3) as shown on the figure. Find the storm water quantities at the three inlets (I 1, I 2, I 3). Assume the velocity in the pipes as 1 m/s. L=50 m S=0.9% C=0.5 L=90 m L=70 m, S=0.17%, I1 I2 C=0.7 I3 S=1%, C= m 25 m 50 m M1 M2 M3 25 m M4 M5 A1= 2 ha A2= 3 ha A3= 4 ha

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28 Time of Concentration I Q Inlet Area C (minutes) mm/h m 3 /s /h Code Code T inlet T travel T C I 1 A I 2 A1& A2: C com = 0.44 *A1+pipe *A2 For: A1,A /60= I 3 A1+ A2+ A3: C com = 0.56 *A1+pipe *A2+pipe *A3 For: A1,A2 A /60=2 50/60= Q = C i A