Hydrological Study of the Colombo Harbour and its Watershed

Transcription

1 Hydrological Study of the Colombo Harbour and its Watershed Wijesekera, N. T. S. Wijesekera, N.T.S.,.(1998) Hydrological Study of the Colombo Harbour and its Watershed, Report submitted to the EIA of the Sri Lanka Ports Authority, Funded by World Bank, Colombo, Sri Lanka, November

2 HYDROLOGICAL STUDY OF THE COLOMBO HARBOUR AND ITS WATERSHED Dr. N.T.S Wijesekera Department of Civil Engineering University of Moratuwa Sri Lanka November 1998

3 HYDROLOGICAL STUDY OF THE COLOMBO HARBOUR AND ITS WATERSHED TABLE OF CONTENTS 1. INTRODUCTION 4 2. DATA AVAILABLE LITERATURE AGENCY INTERVIEWS 6 3. SITE INSPECTION GENERAL SITE INSPECTION OBSERVATIONS 7 4. IDENTIFICATION OF WATERSHEDS GENERAL MAJOR POLLUTION SOURCES LANDUSE PATTERN IN WATERSHEDS WATERSHED PARAMETERS RAINFALL IN THE STUDY AREA EVAPORATION IN THE STUDY AREA 9 5. ESTIMATION OF DISCHARGE THROUGH OUTLETS ESTIMATION OF PEAK STORM WATER FLOW METHODOLOGY DESIRED LEVEL OF SERVICE RAINFALL Rainfall Relationships Duration of Rainfall Time of Concentration PEAK FLOW ESTIMATION USING RATIONAL METHOD Rainfall Input Runoff Coefficient Peak Flow Estimates PEAKFLOW ESTIMATION USING HEC1 MODEL PEAKFLOW ESTIMATION USING SCS- HYDROGRAPH MODEL RUNOFF CURVE NUMBER COMPUTATION OF PEAK FLOW USING THE SNYDER'S SYNTHETIC UH METHOD COMPARISON OF PEAK FLOW COMPUTATIONS ADEQUACY OF EXISTING DRAINAGE STRUCTURES DRY WEATHER FLOW ESTIMATION HYDROLOGICAL ENVIRONMENTAL IMPACTS MITIGATORY MEASURES ACKNOWLEDGEMENT 38 2

4 12.-REFERENCES 38 ANNEX 1: DETAILS OF SITE INSPECTION 40 ANNEX 2: TIME OF CONCENTRATION USING COMMONLY APPLIED METHODS 40 ANNEX 3: WATERSHED PARAMETERS USED FOR TIME OF CONCENTRATION COMPUTATIONS 41 ANNEX 4-HEC-1 INPUT FILE (SAMPLE) 42 ANNEX 5-SCS METHOD SAMPLE OUTPUT 43 ANNEX 6-POPULATION PROJECTIONS IN THE CMC FOR

5 HYDROLOGICAL STUDY OF THE COLOMBO HARBOUR AND ITS WATERSHED 1. INTRODUCTION The Colombo Harbour is faced with an acute environmental problem with a threat of undesirable pollution of its waters. The watershed that drains naturally into the harbour is highly urbanized and consists of housing complexes, motels and retail & wholesales trade establishments. The watershed area has a separate storm water drainage system and a separate sewerage disposal system. The sewerage disposal system collects and transfers the pollution loads to the sea via an out-fall located to North of the harbour. The storm drainage system collects the water from respective watersheds and releases them to the sea under gravity. The storm drainage system regularly discharges a significant quantity of sediments from watersheds out side the port area and it is suspected that some illicit sewerage disposal outlets are also connected to the storm drains thereby leading to heavy pollution of the harbour water. Since the harbour and the surrounding area are paved thus making the drainage system buried and concealed leaving only a few manholes; it is not possible to clearly identify the outlet connections. In order to look at the environmental problems it is necessary to identify the quantities of flow in to the harbour through the drainage system both during the dry season and the wet season. The dry season flows would reflect the pollution due to seepage or any illegal wastewater connections draining into the harbour. The wet weather flow or the peak flows would provide information about anticipated sediment loads that would be carried to the harbour. The drainage network also require to be designed to pass through anticipated peak flows so that no inundation or blockage of outlets would cause harm to the surrounding environment by means of overflowing etc. In this background the hydrological study looks at the watersheds draining into the port area and the storm drainage network in the watersheds in conjunction with the climatic parameters. Data from the Survey Department, Colombo Municipality, available reports and site visits were used to establish the watershed boundary maps. Accepted design guidelines, data from Meteorology Department and available study reports were incorporated in the estimation of flows through the storm drainage system. It this study the anticipated peak flows had been computed and verified using four hydrological models. The study also discusses the concealed and the submerged nature of the outlets along with the possible upper bound of the quantities from wastewater, thereby indicating the degree of seepage or illegal wastewater discharges into the system. Under these circumstances Sri Lanka Ports Authority (SLPA) requested to conduct a hydrological study of the harbour and its watershed, to fulfil the following objectives 4

6 Collection of hydrological data of the port and its watershed Identification of major pollution sources, such as outfalls, canals, tunnels open channels etc., of harbour environs, and the impacts rendered by those sources to harbour waters. Collect information on water drainage system entering into the harbour, including runoff and seepage. Demarcate the overall watershed and sub basins with available maps. Estimate the quantities of discharge through different outlets. 2. DATA A significant amounts of primary data such as rainfall evaporation and secondary data such as maps of the area for the study were collected. A summary of different data sets collected is given in the Tables 1,2,and 3. Table 1 - Details of Rainfall and Evaporation Data TYPE OF DATA STATION PERIOD SOURCE Daily Rainfall Colombo Department of Meteorology Daily Evaporation Colombo Department of Meteorology Table 2- Details of Maps in the Study Area TYPE OF MAP SCALE SOURCE Topo sheets 1: Survey Department Contour and drainage maps 1: 1000 Colombo Municipal Council Contour 1: 2000 Author Harbour Map 1:2500 Sri Lanka Ports Authority 2.3 Available Literature Other than the literature cited in the text and listed as the references, reports indicated in the Table 3 were collected and information was extracted for the study. Table 3- Identified Literature Connected to the Study 5

7 Details of Identified Study Reports Study of the Canal and Drainage System in Colombo, Final Report, Sri Lanka Water Supply and Sanitation Rehabilitation Project, WS Atkins International, 1988 Beira Lake Restoration Study, Final Report, Urban Development Authority, ROCHE International, 1993 Greater Colombo Flood Control and Environmental Improvement Project, Final Report, Ministry of Housing and Construction and Public Utilities Sri Lanka Land Reclamation and Development Corporation, WS Atkins International, 1995 Greater Colombo Wastewater and Sanitation and Master Plan Draft Final Report, Engineering Science Inc., UNDP-SRL/90/006, 1992 Colombo Metropolitan Regional Structure Plan, Final Draft Report, Urban Development Authority, November 1996 Greater Colombo Water Supply System Master Plan Update Draft Final Report, Engineering Science Inc., 1991 City of Colombo Development Plan, Volume 1 and 2, Urban Development Authority, 1985 Beira Lake, Report of a Preliminary Study of Beira Lake, National Aquatic Resources Agency, Agency Interviews As a part of the data collection, representatives of different organizations were interviewed. Port Authority (SLPA), Sri Lanka Land Reclamation and Development Corporation (SLRDC), Colombo Municipal Council (CMC), NSWDB etc., were interviewed to collect information on drainage, water usage wastewater releases etc. Study area inspection was carried out after the literature survey. 3. SITE INSPECTION 3.1 General Several site inspections inside and outside the port, in the drainage watersheds of the port, adjacent watershed areas such as that of Beira Lake, were carried out in order to identify the existing drainage pattern and waste inflows. Site visit dates are presented in the Annex-1. 6

8 3.2 Site Inspection Following aspects were covered during site inspection. Confirmation of the main watershed boundaries and sub basin boundaries with on site terrain features and the corresponding features presented in maps ( The watersheds and their boundaries are indicated in Figure 1) Using actual features on ground (slopes etc.) demarcation of main and sub watershed boundaries in the cases where details were missing in topo maps. Identification of main drains, their nature and distribution pattern in sub watersheds leading to outlets through the port premises. Identification of flow directions in the drains within the system including the drains originating from watersheds other than already identified sub basins. Identification and confirmation of land use pattern of sub basins of the upper reaches. Observation of wastewater flow in the outlets and sediment quantities in drains. Identification of major pollution sources in the main watershed and outside. Collection of information about flooding, drainage and pollution aspects form the residents and other users of the watersheds. Identification of the functionality of the outlets and their key features. 3.3 Observations Given below are the details of observations made during site inspection It was observed that there is a significant amount of paved areas in the watersheds draining towards the port area. Pooling effect is prominent in some sub basins of the watersheds, thus increasing the time of concentration. Most of the storm water drains are old and damaged. Seepage could be expected through these. About 90% percent of the drains inside and outside the port are laid underground and these could not be fully inspected. The rest were inspected. The stench, debris, periodical submergence of outlets, significant sediment flows and inaccessibility were common features of the outlets and drains. The drain system that is meant for storm water appears to carry both wastewater and storm water. High quantities of waste plastics, polythene, paper products, domestic wastes etc., were visible near trash racks. Similar sediments could be observed from the manholes. Most trash racks prevented easy flow of storm water runoff due to blockage by sediments and silt. At some locations overflowing of storm water drains had taken place leading to a spreading of debris inside the port premises. A few open drains in the watershed area R (Figure 1) within the port premises contained high quantities of oily liquid. 7

9 4. IDENTIFICATION OF WATERSHEDS 4.1 General Nineteen watersheds that contribute to drain water into the port premises were identified using the drainage pattern from maps. The outer periphery and its watershed boundaries were first identified using storm drainage maps of scale 1: 1000 and 1: These areas were further confirmed and revised during site inspection. The drainage systems of each watershed were reconciled with the corresponding outlets belonging to each basin. (Figure 1) 4.2 Major Pollution Sources Details observed during site inspection, and information extracted form available maps and reports were compiled. Major pollution sources and their key information pertaining to each watershed based on the above are presented in the Table Landuse Pattern in Watersheds The general landuse pattern of watersheds is urban with business and contiguous residential premises. Table 5 shows the major land use pattern in Watersheds, identified from maps and site inspection. After confirmation of the main boundary, coupled watersheds, drainage network and outlets, the following parameters were calculated for the use of runoff calculations. Watershed area Length of the longest stream Slopes in of different segments of the longest stream Land Use Pattern 4.4 Watershed Parameters Surface area of each watershed, lengths of different stormwater stream segments, and elevations pertaining to each stream segment were extracted from the maps (Table 6). The canal segments were identified taking into account the representative slopes of the drain. The segment boundary was identified as a changing point of the slope. Using the identified information, the average slopes for the watersheds were calculated. Averaged slopes are based on the sloping segments of a particular drain. When the number of segments was observed to be less than three, such situations are indicated by **. In the watersheds Q1 and Q2, there are no defined drainage networks. The watersheds A-G which consisted of cemented drains had an average slope of one in 200 while the 8

10 watersheds J1, Q1 and Q2 which did not have such specific drains had flat area where the slope was taken as one in Rainfall in the Study Area Rainfall measured at the Meteorological Department Premises Colombo, which is located approximately 2 km from the Colombo Port, was considered as representative for this study. Daily rainfall (Figure 2), Annual rainfall and Monthly rainfall (Figure 3) pertaining to a ten year period from 1987 to 1997 were studied. Rainfall in the study area is due to the Northeast and Southwest monsoons that are experienced from September to February and March to August respectively. Both monsoons provide similar quantities of rain into the study area. Average annual rainfall during this period had been between 2000 to 2500 mm. Daily rainfall in the area also indicates a consistent two seasonal rainfall pattern. 4.6 Evaporation in the Study Area Annual evaporation in the study area is also considered to be similar to the evaporation values measured at the Meteorological Department Colombo. Annual and Monthly variations are shown in Figure 4 and Figure 5 respectively. Mean annual evaporation in the region is approximately 1190mm. Monthly evaporation is high during less rain periods of December to April and July to September. 9

11 Table 4 - Details of Major Pollution Sources Watershed Pollution Sources Outlet Number Outlet Dimensions at Port- CMC Boundary Dimensions of Outlet from Port Premises There are no external pollution sources. However some form of pollution 1 Not Applicable 2/0.2mx0.7m Box drain A occurs due to oil spillage B There are no external pollution sources. However some form of pollution 2 Not Applicable 0.7mφ Pipe occurs due to oil spillage C Pollution due to waste from canteen. No pollution from external sources. 3 Not Applicable 0.75mφ Pipe D Pollution due to wastes from canteen No pollution from external sources. 4 Not Applicable 0.75mφ Pipe E Pollution from passenger terminal No pollution from external sources. 5 Not Applicable 0.75mφ Pipe F No apparent major pollution source. No pollution from external sources. 6 Not Applicable 0.3mφ Pipe G No apparent major pollution source. No pollution from external sources mφ Pipe 0.5mx0.8m Box Drain H No apparent major pollution source. No pollution from external sources mx0.8m Box Drain 0.6mx1.0m Box Drain I Pollution from upstream motels, sewers, residential areas etc mx1.5m Box Drain 1.7mx1.0m, 1.1mx1m Box Drain,0.7mφ Pipe J Watershed confined to SLPA premises. No major pollution source mφ Pipe 1.5mφ Pipe Through J1 Pollution from Beira Watershed through MacClum gate which is kept closed Lock Gate Beira Canal J1 No apparent major pollution source. No pollution from external sources Not Applicable No defined drains K Heavy pollution from outside watershed. Sources such as sewers, shops, motels and other residential sources. 11 2/1.0mφ Pipe 0.75mφ Pipe 1.2mx1.5m Box Drain L Largest watershed; Pollution is heavy. Fish Market is a prime source of mx2.4m Box Drain 1.2mx1.5m Box Drain pollution especially solid waste. Other sources such as sewers, motels, shops contribute to pollution. Overflowing of sanitary sewers is common. M Except for the Fish Market; Conditions prevailing in this sub basin are same as mx1.2m Box Drain 1.5mx1.5m Box Drain those of L N Same as M above x1.75m Box Drain N,O,P Combined to O Second largest sub watershed. Pollution is similar to M above mx1.75m Box Drain A Box Drain of 3.6mx1.6m P Same as O above. Pollution sources are same mx1.2m Box Drain Q1 Watershed lies in the port premises. No major pollution. No concentrated Not Applicable No defined drains drainage paths. Drains over the quay to harbour waters. Q2 A small part of the watershed lies outside the port. No major pollution sources. Not Applicable No concentrated Drains R Oil pollution from different sources inside the port premises mx1.0m Drain 0.75mφ Pipe 10

12 Table 5 Major Landuse Pattern in Watersheds Watershed A B C D E F G H I J J1 K L M N O P Q1 Q2 R Description of Landuse Pattern Main breakwater and coastal infrastructure. Main breakwater and coastal infrastructure Main breakwater and coastal infrastructure Harbour infrastructure such as jetties. Harbour infrastructure such as jetties Harbour infrastructure such as jetties Harbour infrastructure such as jetties Harbour infrastructure such as jetties Urban infrastructure in the upstream are such as shops, residential units, motels etc., downstream coastal and other harbour infrastructure. Coastal and harbour infrastructure. Urban infrastructure. Coastal and harbour infrastructure. Upstream area; 95% of total area consists of urban infrastructure. Rest is harbour infrastructure. Same as L Upstream area; 90% of total area consists of urban infrastructure. Rest harbour infrastructure. Upstream area; 95% of total area consists of urban infrastructure. Rest harbour infrastructure Same as O Harbour infrastructure 80% Harbour infrastructure and rest urban infrastructure. 60% Harbour infrastructure and rest urban infrastructures. 11

13 Table 6- Identified Watersheds and Key Parameters Watershed Area Lengths of Different Stream Levels of Different Stream Total Average (ha) Segments(m) Segments(m) Length(m) Slope L1 L2 L3 H1 H2 H3 H4 A 4 ** ** ** ** ** ** ** B 4.22 ** ** ** ** ** ** ** C 1.63 ** ** ** ** ** ** ** D 0.85 ** ** ** ** ** ** ** E 1.55 ** ** ** ** ** ** ** F 0.95 ** ** ** ** ** ** ** G 0.41 ** ** ** ** ** ** ** H 3.35 ** ** ** ** ** I J ** ** ** ** J1 5.6 ** ** ** ** ** ** ** ** K ** ** L M ** ** N ** ** O ** ** P R ** ** Q ** ** ** ** ** ** ** ** Q ** ** ** ** ** ** ** **

14 Figure 2-Daily Rainfall in the Study Area ( ) 13

15 Figure 3: Annual and Monthly Rainfall in the Study Area 14

16 Figure 4: Annual Evaporation in the Study Area Figure 5: Monthly Evaporation in the Study Area 15

17 5. ESTIMATION OF DISCHARGE THROUGH OUTLETS 5.1 General The estimation of discharge through outlets requires modeling of the watersheds concerned since storm water from each outlet during critical events is required. For the environmental improvement of the port area, it is important to reliably estimate the peak flows to check for adequacy of capacity to drain storm water. Also it is necessary to estimate the quantity of dry weather flows in the drain system to establish environmental guidelines with respect to probable discharge of pollutants into the system. Historical flow records of watersheds draining into the port were not available. Field inspection revealed heavy loads of sediments in the drains and the drain network did not provide reasonable access to carry out reliable flow measurement, to capture a few observations. However floats and other crude techniques were used to obtain approximate velocities and discharges of flow during site visits to obtain an idea about dry weather flows (Table 7). Table 7 Estimated Flow in Major Watersheds Observed During Site Visits Watershed Extent (ha) Dimensions of Outlet K mφ Pipe 1.2mx1.5m Box Drain L mx1.5m Box Drain M mx1.5m Box Drain N x1.75m Box Drain O mx1.75m Box Drain P mx1.2m Box Drain Observed Values At Initial Visits (Litre/Sec) Observed Values on 6 th Oct 98 (Litre/Sec) Remarks Discharge from Pipe Submerged; Emerging Stench Submerged: Observed From Manholes; Emerging stench Dimension shown is inlet to port from CMC drains combined as a Box Drain of 3.6mx1.6m: Observed From Manholes Not Identified 1-2 Dimension shown is inlet to port from CMC drains combined as a Box Drain of 3.6mx1.6m Not Identified Not Dimension shown is inlet Identified to port from CMC drains: Combined as a Box Drain of 3.6mx1.6m R /0.75mφ Pipe 1 2 About 1/10 submerged 16

18 Site visits were carried out on days where no significant rain was experienced. Even the subsequent verification visit on the 6 th October 1998 was carried out in a rainless period. Therefore these flows reflect the probable magnitudes of the Dry Weather Flows in the area Estimation of Peak Storm Water Flow Estimation of peak flow is vital for the design of the drainage systems. These estimates enable an adequacy check of the existing system and also to decide on the new system if the existing system is found inadequate. The peak flow estimates would enable to identify the environmental concerns such as overflows and inundation in quantity and duration. Peak flow estimates are made either by analyzing the historical flow records, extrapolating representative flow records or applying rainfall-runoff relationships. Due to the lack of historical flow records, the estimations for the watersheds draining into the port need to be based on rainfall-runoff relationships. Such relationships either incorporate empirical parameters or use parameters that are based on watershed characteristics. The accuracy of estimates depends on the accuracy of the parameters and hence a verification process is often carried out using measured data. In the absence of measured values, flow estimates can be made using reliable watershed models and these could be subsequently verified with observed data during detailed designs. This work carried out an application of commonly used Peak Flow Estimation Hydrologic Models on each watershed concerned. The values obtained with each model were compared and judgements were made giving due considerations to the model structure, model parameters and the reliability of these inputs. The estimation of expected peak storm water flows were done using the following four models. 1. Rational formula which is widely used for Engineering Designs ( Chow 1988 ) 2. HEC1 flood model (Off-the-Shelf Software Public Domain, US Corps 1990) 3. SCS hydrograph Method ( Chow 1988 ) 4. Snyder s Unit Hydrograph Method (Irrig. Dept ) 5.3 -Methodology In general Rational Model is stated a reliable model to estimate peak flow in small watersheds. However due to the importance of these watersheds, being located in the capital city and adjacent to the major port it was decided that only the very small watersheds would be modeled using the Rational Formula. The rest of the watersheds were modeled using the four models mentioned above. 17

19 All the watersheds were classified to two groups based on their extent of surface coverage. Relatively smaller watersheds were classified as one group and the larger ones were classified as the other. The first group comprises of watersheds A, B, C, D, E, F, G, H, J (Area less than or very close to 4 Ha) and the second group comprises watersheds I,K,L,M,N,O,P,R. The watersheds J1, Q1 and Q2 were watersheds within the port premises, and did not have a clearly defined drain system. Since the McCallum Lock gates are not kept open for the flows to pass into the port area, such estimates through the gates were not carried out. For the first group only the results of the rational formula is used, as the discharge generated is small. The watersheds J 1, Q 1 and Q 2 were not considered relevant for modeling because of the absence of a clear drain network. In the second category consisting of larger watershed areas, all four methods mentioned earlier were applied to estimate the design discharge through the storm drains. 5.4 Desired Level of Service The peak flow of a particular watershed varies from the Peak Flow corresponding to a specific return period to Estimated Limiting Value (ELV) which is the largest magnitude of flow possible at a given location. As such it is necessary to identify the intended or desired level of service from the drainage system for the assessment of impacts to the environment. Therefore it is a prerequisite to establish the acceptable risk levels or the desired return periods prior to estimating the peak flows. Peak flow into the harbour is fed from urbanized watersheds and flow passes through the port where the drainage lines are crossed by significant traffic. In urban storm drainage design, typical design levels for minor structures vary from 2-25 year return period (Chow 1988). If the drainage structure does not cause a backwater situation to effectively halt traffic, then the structure is treated as a minor structure and the recommended design period varies between 2-10year (Viessmann et al 1989). The soil conservation service recommends the use of a 25-year frequency for minor urban drainage design if there is no potential loss of life or risk of extensive damage. Due to the varying nature of the design levels depending on the importance of the locality and the involved resources it is necessary to study peak flow corresponding to a few design return periods providing the flexibility for requisite decision making. As such this study looks at peak flows pertaining to 2year, 5 year, 10year, and 25year return periods. At each watershed of the Colombo Port, peak flows for all aforementioned return periods were estimated. 18

20 5.5 Rainfall Rainfall Relationships Application of Hydrologic mathematical models for the computation of peak flow requires the estimation of rainfall to be fed into the models. Intensity-Duration-Frequency (IDF) relationships developed for Colombo (Dharmasena & Premasiri 1990, Obeysekera ) were used for the computations Duration of Rainfall Peak flow from a watershed occurs only when the furthest point of the watershed also has contributed to he runoff. To ensure this situation rainfall event needs to operate on the watershed for a period at least equal to the required time for the furthest point to contribute. As such the critical rainfall duration for a watershed should be more than the Time of Concentration for that particular watershed Time of Concentration Initially time of concentration for significantly large watersheds were computed using three empirical methods commonly used in practice. The values thus obtained along with the method used are shown in the Annex 2. Due to the significant variation of values from each method, computations were carried out for each watershed considering the actual drainage paths and sizes. The parameters used for individual computations in case of major watersheds are given in the Annex 3. In this analysis it was assumed that pipes or drains flow half full and that the cemented surface roughness has a Manning Coefficient of For larger watersheds inlet time was taken as 15 minutes while for smaller watersheds it was taken as five minutes considering travel time to defined stream channels in a given land use. Computed Time of Concentrations for all watersheds are also shown in the Annex Design Rainfall Event Determination of a particular runoff event through a model requires computation of the rainfall event and this rainfall event needs to satisfy the typical temporal distribution and should last beyond the critical time of concentration. A comparison of time of concentration values from Federal Aviation Administration Method (FAA 1970), UK Flood Studies Method (Watkins and Fidds 1984), Dept of Irrigation (ID 1984) and the individual computations (Annex 3), indicates that a one hour rainfall event would be best suited to compute the peak flow from the watersheds. Values for a common rainfall event with 10-minute time intervals was derived using the IDF curves for Colombo. The rainfall hyetograph was developed using the Alternating Block Method and it is shown in the Table 8 and Figure 6. 19

21 Table 8-Design Rainfall Event Time Design Rainfall (mm) (min) 2Year 5Year 10Year 25Year Design Rainfall Events Rainfall (mm) Time (Minutes) 2Year 5Year 10Year 25Year Figure 6: Design Rainfall Events for 2,5,10,25 year Return Periods 5.6 Peak Flow Estimation Using Rational Method Rainfall Input Time of Concentrations for all watersheds were used as the critical duration of rainfall for the Rational Method. Rainfall intensities corresponding to each Time of Concentration in case of each return period was obtained from the IDF curves for Colombo. These values are shown in Table 9. 20

22 5.6.2-Runoff Coefficient Runoff coefficient that represents a multiplier in the rational formula depends on the characteristics of the drainage area or in other words the land use characteristics of the watershed draining through a particular point. The choice of a value for runoff coefficient is the most intangible aspect in the use of the rational method for peak flow estimate (Chow 1964). In this study runoff coefficient was estimated after a comparison of results from three different methodologies. The methodologies adopted were, 1 Tabulated Values from Handbooks 2 Based on Sample Areas 3 Based on Remote Sensing Data (a) Tabulated Values from Handbooks Typical values given in the Handbook of Hydrology (Maidment 1993), runoff coefficients recommended by the American Society of Civil Engineers and Water Pollution Control Federation, for business down town areas range between 0.7 and Since land area draining into the port premises can be classified as business down town area and as such the average value in the above range which is 0.8 could be used as the average runoff coefficient for the study area. (b) Based on Sample Areas A sample area that is typical to the drainage watersheds was selected and pervious and impervious areas were manually computed. The selected sample area from the watershed P drawn to a scale of 1:2000 is shown in the Figure 7. Computed land use in the sample is shown in the Table 10. Values from Sample Areas were averaged using a runoff coefficient of 1 for the Type 1, and a coefficient of 0.5 for the land use Type 2. The weighted average runoff coefficient for the sample area amounted to 0.8. Table 10-Typical Land Use in the Sample Area Type of Land Use Area (sq m) Type 1 Impervious: Roads, Concrete, Buildings Type 2 Semi Pervious - Gardens, Bare Land, Intermittently Paved Total Sample Area Based on Remote Sensing Data Land Classification done using remote sensing data indicate that impervious fraction of highly dense regions in the Colombo region amounts to 0.65 (Silva 21

23 1997). Weighted average runoff coefficient computed using a runoff coefficient of 1 for the impervious fraction and a runoff coefficient of 0.5 for the rest was 0.8. The results of the three methods show a reasonably consistent run off coefficient of 0.8 that can be used as the average value for flow estimations. Hence the average runoff coefficient value of 0.8 was taken to estimate the peak flows using the Rational Formula Peak Flow Estimates Using the rainfall intensities pertaining to each return period, and a runoff coefficient of 0.8 common for all watersheds (based on comparative analysis), peak flow estimates were computed. The numerical values of peak flows for each return period is shown in Table 9. 22

24 Figure 7-Selected Sample Area for Runoff Coefficient Computations 23

25 Table 9-Rainfall Intensities and Corresponding Peak Flow Estimates Using Rational Method Watershed Area (ha) Tc (min) Rainfall Intensity (mm/hr) Corresponding to Time of Concentration (Tc) Peak Flow from Rational Method (cu.m/sec) 2Yr 5Yr 10Yr 25Yr 2Yr 5Yr 10Yr 25Yr A B C D E F G H I J K L M N O P R

26 5.7-Peakflow Estimation Using HEC1 Model HEC1 model developed by the engineers of the US Army Corps is designed to simulate surface runoff of a watershed by schematizing the watershed as an interconnected system of hydrologic and hydraulic components. Model parameters representing the watershed can incorporate temporal as well as spatial averages. Simulations are limited to a single storm since there is no provision for soil moisture recovery during rainfall (US Army 1990) Data extracted from the topographic and detail maps for the use in the HEC1 model are shown in the Table 12. In this application the pervious and impervious ratio in all watersheds were taken as 80:20, and the roughness coefficients in the impervious area corresponds to that of Asphalt/Concrete. Values in the Table 12 are in metric except for the extent of the collector canal contributory area that is expressed in square miles. A sample-input file used in the HEC 1 Model is shown in the Annex 4, Estimated peak flow values and time to peak values in each watershed for different return periods are shown in the Table 11. Watershed Table 11-Peak Flow Estimates Using the HEC1 Model 2 Year Return Period Peak Flow (cumec) Time to Peak (min) 5 Year Return Period Peak Flow (cumec) Time to Peak (min) 10 Year Return Period Peak Flow (cumec) Time to Peak (min) 25 Year Return Period Peak Flow (cumec) Time to Peak (min) I K L M N O P R

27 Table 11-Watershed Parameters Used forhec1 Computations Watershed I Flow Length (m) Slope Roughness Extent Bed Width (m) Paved Area % Bare Surface % Collector Canal Main Canal Watershed K Flow Length (m) Slope Roughness Extent Bed Width (m) Paved Area % Bare Surface % Collector Canal Main Canal Watershed L Flow Length (m) Slope Roughness Extent Bed Width (m) Paved Area % Bare Surface % Collector Canal Main Canal Watershed M Flow Length (m) Slope Roughness Extent Bed Width (m) Paved Area % Bare Surface % Collector Canal Main Canal Watershed N Flow Length (m) Slope Roughness Extent Bed Width (m) Paved Area % Bare Surface % Collector Canal Main Canal Watershed O Flow Length (m) Slope Roughness Extent Bed Width (m) Paved Area % Bare Surface % Collector Canal Main Canal Watershed P Flow Length (m) Slope Roughness Extent Bed Width (m) Paved Area % Bare Surface % Collector Canal Main Canal Watershed R Flow Length (m) Slope Roughness Extent Bed Width (m) Paved Area % Bare Surface % Collector Canal Main Canal

28 5.8-Peakflow Estimation Using SCS- Hydrograph Model Soil Conservation Service (SCS) Method (Maidment 1993) is widely used for the estimation of peak flows in small to medium sized ungauged drainage basins. In the United States, it has replaced the rational method to a significant degree as a result of its apparent database and the manner in which physical characteristics are considered in its application. Runoff (Q) computations were carried out by using the SCS formulae Q=(P-0.2S) 2 /(P+0.8S) and CN=1000/(S+10), where P is the Depth of Rainfall (inch), Q is the Depth of Surface Runoff(inch), S is the Potential Maximum Retention(inch), and CN is the Run Off Curve Number. Curve Number depends on the antecedent wetness of the watershed, soil, land cover and the hydrologic conditions. The antecedent moisture condition refers to three classes of antecedent moisture conditions (AMC), namely dry, average and wet. Hydrologic condition is related to whether vegetation is dense and in good condition, and also whether the soil is rich in organic matter and has a well aggregated structure. The hydrologic condition identifies the watershed capacity to result in high infiltration and low runoff. 5.8-Runoff Curve Number Soils in the watersheds pertaining to the concerned area is treated as in the category C (Maidment 1993) which consists of slow infiltration, fine textured soils such as clay loam, shallow sandy loam, and soils low in organic content. SCS curve numbers for typical land use types having soil category C are shown in Table 13. Table 13 Typical Land Use, Corresponding Curve Numbers and Impervious Percentages in the Study Area Land Use Type Paved Roads and Parking lots, Curbs and Storm Sewers, Roofs, Driveways Residential with approximately 38% impervious SCS Curve Number Impervious % from Remote Sensing Data 98 65% 83 35% A composite curve number for the area was calculated using the impervious percentage as the weights. The composite curve number thus calculated was 93 and this was used for computations carried out for each watershed. SCS computations for each return period events were carried out using the curvilinear hydrograph conversion approach on each pulse in the rainfall hyetograph. Conversion of the trapezoidal runoff hydrograph to the curvilinear one was based on the dimensionless Unit Hydrograph conversion commonly adopted in the SCS method. Peak flow estimates using the SCS method and the other inputs used for the computations are shown in the Table 14. A sample computation out put from this method is shown in Annex 5. 27

29

30 Table 14-Peak Flow Estimates Using the SCS method Watershed Time of Basin Concentration Lag(tp) (Tc) Time to Peak (Tp) Surface Runoff for Each Return Period (Peak Flow in Cumec, Time to Peak in Hours) Identity Area 2-Year 5-Year 10-Year 25-Year Sq Km hrs hrs hrs Peak Time to Peak Time to Peak Time to Peak Flow Peak Flow Peak Flow Peak Flow Time to Peak I K L M N O P R

31 5.9-Computation of Peak Flow using the Snyder's Synthetic UH Method Snyder's standard unit hydrograph method (Chow et al 1988, ID 1988) which synthesizes a triangular unit hydrograph for a watershed was used to compute the peak flows in the larger watersheds. Computations for this study converted the triangular hydrographs to curvilinear hydrographs using the ratios given in the irrigation department guidelines as better suited for Sri Lankan watersheds (ID1988). C t and C p coefficients for Snyder s synthetic hydrograph computations were extracted from the values given in the Irrigation Department guidelines. The C t and C p values of adjacent Kelani, Kalu, Maha watersheds(table 15) were compared for their recessions and response patterns and values pertaining to Maha watershed was selected for the incorporation in the computations since those values were most representative of the study area. Table 15-Watershed Coefficients Compared for Snyder s UH Computations STATION RIVER BASIN DISTANCE FROM COLOMBO(Km) Cp Ct Glencourse Kelani Ganga Putupaula Kalu Ganga Badalgama Maha Oya The synthetic unit hydrographs developed for each watershed was used with the design rainfall events to compute the peak flows and the values are shown in the Table 16. Table 16-Peak Flow Estimates Using Synthetic Hydrograph Method Peak Flow Estimates Using Snyder's Method Watershed 2 Year 5 Year 10 Year 25 Year Peak Flow Peak Flow Peak Flow Peak Flow (cumec) (cumec) (cumec) (cumec) I K L M N O P R COMPARISON OF PEAK FLOW COMPUTATIONS Estimated peak flow values from each method for each return period is shown in Table 17. The results from all four models show consistency in the case of small watersheds but the estimates made using the Irrigation Department method appear to deviate in case of larger watersheds. The watershed coefficients in the Snyder-ID estimates were of low reliability 30

32 and therefore in evaluating larger watersheds these values were not taken into consideration. The graphical presentation of peak flow estimate fluctuations for the 2-year return period situation is shown in Figure 8. Figure 8-2-Year Peak Flow Comparison from Four Methods Peak Flow Estimates from Four Methods Peak Flow-2 Yr (cu.m/sec) I (14.5) K (9.4) L (34.9) M (11.7) N (11.3) O (33.8) P (32.0) R (12.9) Max Min Mean-4 31

33

34 Table 17-Comparison of Peak Flow Estimates from Four Methods Watershed 2 Year Peak Flow (cumec) 5 Year Peak Flow (cumec) 10 Year Peak Flow (cumec) 25 Year Peak Flow (cumec) Rational Hec SCS Snyder-ID Rational Hec SCS Snyder-ID Rational Hec SCS Snyder- ID Rational Hec SCS Snyder- ID I (14.5) K (9.4) L (34.9) M (11.7) N (11.3) O (33.8) P (32.0) R (12.9)

35 7.-ADEQUACY OF EXISTING DRAINAGE STRUCTURES The dimensions of the culverts discharging storm water runoff through the port premises were checked for the adequacy in cases of each return period floods. Size of culvert opening, effective length, outlet hydraulic conditions, slope of culvert barrels, head water & tail water elevations were extracted from the available maps and site visit information to estimate the maximum quantity which could be discharged during a flood situation. Parameters used for each culvert are shown in the Table 18. A software package called Flow Master was used for the hydraulic computations. Computed discharges for each culvert and the adequacy of same in case of 2,5,10 and 25 year return period floods are shown in the Table 19. The computations assumed an average tide level of 0.8m above mean sea level as the tail water elevation to consider the critical hydraulic condition. Manning roughness of the culvert flowing under full flow was taken as Out of the larger watersheds drainage outlets L and R appear to be inadequate even to cater to two year return period floods. 34

36 Table 18-Parameters Used for Culvert Adequacy Checking WATERSHED AND OUTLET NUMBER EXISTING OUTLET SIZE CULVERT LENGTH (m) HYDRAULIC STATUS H.W.E (m) T.W.E (m) SILL ELEVATIONS(m) INLET OUTLET SLOPE MANNING ROUGHNESS COEFT A-1 2/0.2mx0.7m ** Free Surface Flow ** ** ** ** drain B-2 0.7mφ Pipe ** Free Surface Flow ** ** ** ** C mφ Pipe ** Free Surface Flow ** ** ** ** D mφ Pipe ** Free Surface Flow ** ** ** ** E mφ Pipe ** Free Surface Flow ** ** ** ** F-6 0.6mφ Pipe ** Free Surface Flow ** ** ** ** G-7 0.6mφ Pipe ** Free Surface Flow ** ** ** ** H-8 0.5mx0.8m Drain ** Free Surface Flow ** ** ** ** I-9 1.7mx1.5m Drain 100 Submerged Flow ** J mφ Pipe 140 Submerged Flow ** J1 Lock Gate N.O Submerged Flow N.O N.O N.O N.O N.O N.O K mφ Pipe 1.2mx1.5m Drain 140 Submerged Flow ** ** L-12a&b 1.2mx1.5m Drain 125 Submerged Flow ** M mx1.5m 275 Submerged Flow ** N x1.75m 325 Submerged Flow ** O x1.75m 135 Submerged Flow ** P mx1.2m 120 Submerged Flow ** Q1 N.O N.O Submerged Flow N.O N.O N.O N.O N.O N.O Q2 N.O N.O Submerged Flow N.O N.O N.O N.O N.O N.O R mφ Pipe 40 Submerged Flow ** **- Parameters not required for calculations. N.O No well-defined outlets HWE Headwater elevation, TWE Tail water elevation 35

37 Table 19-Comparison of Present Status of Culverts for Storm Water Drainage Watershed and Outlet Present Capacity (Cumecs) Adequacy to Drain Flood Runoff 2Year 5Year 10Year 25Year A X X X X B X X X X C X X X D E X X F X X X G H X X X X I J-10 N.O N.O N.O N.O N.O K ( ) L X X X X M N X X X O X X X P X Q1 N.O N.O N.O N.O N.O Q2 N.O N.O N.O N.O N.O R X X X X N.O-No Well Defined Outlet -Adequate -Marginal X-Inadequate 8.-DRY WEATHER FLOW ESTIMATION Peak flow estimation computations show that for design return period floods considered in the study, the direct runoff hydrographs decay within several hours. Since the watersheds in size vary from 0.4ha to 35ha having approximately 65% paved, the seepage of runoff which is quite small would become zero during dry weather spells. However if the groundwater levels are high in the area there rises a possibility to receive seepage flows at the out lets during dry weather. Site visits indicated that such a possibility has a very low probability in the watersheds under this study. Therefore any dry weather flow into the drains would lead from water used in the municipal wards out side the port premises and drain to the sea through the port. The watersheds under this study occupy land space identified as an area covered by sewers. Therefore the sewage and wastewater from the watersheds into the storm water drainage network would need to be either zero or negligible. Since no data pertaining to dry weather flow from the watersheds were available, and also since the manholes and high sediment loads did not provide easy access to carryout flow measurements, the only information available are those presented in the Table 7. 36

38 Since the flow observed during the dry weather was of considerable magnitude, an attempt was made to identify the wastewater flow into the watersheds if the sewer system was not in existence. This quantity would provide an upper bound value to compare the magnitudes of flow observed during the site visits. This estimation was based on the consumption levels of occupants of watersheds and the average consumption per capita was assumed as 120 Litre/day. Total population in the port watershed was estimated from the demographic values indicated in the Greater Colombo Wastewater and Sanitation and Master Plan (ECI 1992) and shown in the Annex 6. Due to lack of information regarding the drainage dry weather flow contribution from the total population was apportioned to each watershed assuming a homogenous distribution of water users. A factor of 0.75 was applied on the water supply to compute the wastewater quantity. Computed upper bound values of wastewater at each watershed were further increased by another 50% to cater to the hotels and shops located in the watershed (Table 7). The maximum wastewater flow prediction values (upper bound) for the year 2000 (assuming gravity flow to port area) when compared with the point observations during site visits reflect that prevailing dry weather flows are a significant fraction of the upper bound values. Table 20-Wastewater through each Watersheds in the Absence of a Sewer System Watershed Area (ha) Wastewater (l/s) from Settlements (120x0.75) Wastewater (l/s) including other users (120x0.75x1.5) I K L M N O P R HYDROLOGICAL ENVIRONMENTAL IMPACTS The drainage paths in port watersheds show the existence of well-defined drainage net work to drain storm water to the sea. Flow computations through the culvert systems showed that in some cases the culverts were of insufficient capacity even to cater to the 2 year return period storms. This is quite unsatisfactory since it is usual to design important urban infrastructure to cater to larger return period floods. This inadequacy leads to upstream inundation either within or outside the port premises, for prolonged periods thereby causing damage to property and disruption of traffic. Site visits revealed a significant quantity of sediments such as waste polythene, paper wrappings, silt from domestic waste, get washed downstream with the showers and tend to 37