The design project involved the construction of SMK Puncak Jalil at the site located near to Faculty of Law. This project site is adjacent to the existing road leading to Faculty of Law. In this project, we need to design sufficient drainage system to carter the quantity of runoff which will be increase after the development due to the increase in impervious surface. Furthermore, it is important to avoid erosion of soil, sedimentation, flash flood and other problem in future.
4.2 Scope of Works
The scopes of works involved in designing the drainage system are as follows:
i. The setting of network and direction of flow that take into consideration of certain factors such as contour, platform level, road level and slope that will affect the flow of the water direction.
ii. The estimation of the total flow rate for the whole project area. iii. Take consideration of quantity and quality of the water.
iv. Determination of types and sizes of drains, culvert and sumps that need to be used. v. The estimation of depth and area required for detention pond.
The main concept and design of the drainage system and on site detention for this project is based on the guidelines that had been provided by Department of Irrigation and Drainage. The guideline being used is “Manual Saliran Mesra Alam (MSMA) aligned with hydrological procedure and rules set by local authorities. The information’s from MSMA that had been taken into consideration are as follows:
Chapter 4 Acceptance Criteria Chapter 13 Design Rainfall
Chapter 14 Flow Estimation Routing Chapter 19 On-site Detention
Chapter 26 Open Drains Chapter 27 Culvert
Surface water from drains will be channeled into the detention pond before being discharged into the nearby river. The design of the drainage and detention pond follows the guidelines that had been provided.
4.4 Design Procedure
4.4.1 Estimating the Peak Runoff
Rational Method is the simplest method which can be used to calculate runoff. This is the most commonly used method of determining peak discharge from small drainage areas. Peak discharge is the greatest amount of runoff coming out of the watershed at any one time.
Step 1: Specify the catchment area (project boundary) Step 2: Draw the drainage layout
Step 3: Divide the catchment area into several sub-catchment areas according to the particular drainage area being considered
Step 4: Determine the particular drainage area Step 5: Estimate runoff coefficient
Estimate C values for each segment if there are different land covers according to the table below.
Table 4.1 Recommended Runoff Coefficients for Various Landuses (DID, 1980; Chow et al. 1988; QUDM, 2007 and Darwin Harbour, 2009)
(Table 2.5 in MSMA)
For this project, 10 years ARI is adopted for the planning and design of minor storm water system accordance with table below as this development considered commercial.
Table 4.2 Design Storm ARIs for URBAN Stormwater Systems (Table 4.1 in MSMA)
Step 7: Determine average rainfall intensity, yIt
Calculate 10I10 for design ARI 10 years and duration 10 minutes by using formula 13.2
and 13.3 in MSMA. Coefficients of the fitted IDF equation for Kuala Lumpur is selected because the catchment area is located near to Bangi compared to Kuala Kubu Bharu. The value of storm duration factor, FD is taken from table 13.3 in MSMA as 1.28.
Table 4.3 Coefficients of the Fitted IDF Equation for Kuala Lumpur (Table 13.2 in MSMA)
Table 4.4 Values of FD for equation 13.3 (Table 13.3 in MSMA)
Step 7: Input the drainage area, C value, and intensity into the rational method formula to determine the peak rate of runoff. (Equation 14.7 in MSMA)
4.4.2 Estimating the Drain Dimension
Open drain may be sized by Manning’s formula using the roughness values provided in MSMA. Step 1: Estimate Manning’s n of the lining material.
n = 0.015 and n = 0.035 is selected for lined drains and grassed swales respectively. Table 4.5 Suggested Values of Manning’s Roughness Coefficient, n
Step 2: Use design Chart 26.2 or 26.4 in MSMA to determine the flow depth, y, or use Design Chart 26.3 to determine the minimum base width for a trapezoidal shaped grassed swale. Estimate y from the charts or calculate manually. (In this project, we choose to calculate manually)
Step 3: Check if y is within required limits for the open drain type. If not, adjust the drain dimensions and return to step 2.
Step 4: Calculate the average flow velocity from V = Q/A and check that it is within the maximum and minimum velocity criteria for the open drain type. If not, adjust the drain dimensions and return to step 2.
Step 5: Add required freeboard to y and calculate top width of drain for drains with sloping sides. 4.4.3 Estimating the On-site Detention (OSD) Pond Size
On-site Detention (OSD) is a way of collecting the rain that falls on a site (known as storm water), storing it temporarily and then releasing it slowly so that it doesn’t worsen downstream flooding
Step 1: Determine storage volume required a) Select type of storage
b) Determine the area of the site that will be directed to the OSD storage system
c) Determine the amount of impervious and pervious area draining to the OSD storage system.
d) Determine the time of concentration, tc and tcs
e) Calculate the pre-development and post development flows for the area of the site draining to the OSD storage.
f) Determine the required Permissible Site Discharge (PSD). g) Determine the required Site Storage Requirement (SSR). Step 2: Determine size of primary outlet
Step 4: Sizing of secondary outlet
4.5 Design Criteria
4.5.1 Lined Drain
The dimensions of lined open drains have been limited in the interest of public safety and to facilitate ease of maintenance. The minimum and maximum permissible cross sectional dimension are illustrate in figure below and described as follows.
The maximum depth for lined open drains shall be in accordance with Table 26.1 in MSMA
Table 4.6 Recommended Maximum Depths (Table 26.1 in MSMA)
The width of lined open drains may vary between a minimum width of 0.5 m and a maximum width of 1.0 m.
c. Side slope
The recommended maximum side slopes for lined open drain are indicated in Table 26.2 in MSMA
Table 4.7 Recommended Maximum Side Slopes (Table 26.2 in MSMA)
Drain Lining Maximum Side Slope Concrete, brickwork, and block work Vertical
Stone pitching 1.5(H):1(V)
Figure 4.1 Dimension Limits for Open Lined Drains Cover Condition Maximum Depth (m) Without protective covering 0.5
(Figure 26.3 in MSMA)
ii. Velocities and Grades
To prevent sedimentation and vegetative growth, the minimum average velocity shall not be less than 0.6 m/s. The maximum average flow velocity shall not exceed 4 m/s.
The depth of an open lined drain shall include a minimum freeboard 0f 50 mm above the design storm water level in the drain.
4.5.2 Grass Swale i. Location
A grassed swale is generally located within parkland, open space areas, along pedestrian ways, and along roadways with limited access to adjacent properties. Grassed swales should not be provided in urban street verges with adjacent standard density residential and commercial properties where on-street parking is permitted.
Standardized alignment for grassed swales are provided to limit the negotiations needed when other services are involved. In new development areas, the edge of a grass swale should generally be located 0.5 m from the road reserve or property boundary.
The preferred shapes for grassed swales are shown in Figure 26.2. The flow depth shall not exceed 0.9 m. A ‘vee’ shaped section will generally be sufficient for most
applications; however, a trapezoidal section may be used for additional capacity or to limit the depth of the swale.
iv. Velocities and Grades
The average flow velocity in a grassed swale shall not exceed 2 m/s. Figure 4.2 Recommended Grass Swale Cross Section
(Figure 26.2 in MSMA)
The depth of a grassed swale shall include a minimum freeboard of 50 mm above the design storm water level in the swale.
4.6 Analysis and Calculation
All the analysis and calculations are shown in the drawing, tables and documents attached.
After making calculations and analysis, as well as taking into account the design criteria that have been set by authority, the proposed drainage system for this project are as follows:
i. Drainage networks
a. 600 U 300 U-Shape RC Drain b. 600 U 450 U-Shape RC Drain c. 300 Diameter Concrete Pipe Culvert d. 450 Diameter Concrete Pipe Culvert e. Cascading Drain at hillside
f. Grassed Swale ii. Detention Facilities
The proposed OSD have optimum size to cater in 50 year peak flow rate of the development area.