Appendix 4 ADB. IN THE 6 CI's RIVER BASIN TERRITORY - PACKAGE B. Final Report B.2 - Strategic Spatial Planning

Transcription

1 ADB TA 7189-INO: INSTITUTIONAL STRENGTHENING FOR INTEGRATED WATER RESOURCES MANAGEMENT (IWRM) IN THE 6 CI's RIVER BASIN TERRITORY - PACKAGE B Final Report B.2 - Strategic Spatial Planning Appendix 4 Modeling for Peak Runoff Zoning

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3 FINAL REPORT B.2 Strategic Spatial Planning Appendix 4 MODELING FOR PEAK RUNOFF ZONING TA7189-INO: Institutional Strengthening for Integrated Water Resources Management (IWRM) in the 6 Ci's River Basin Territory - Package B Ministry of Public Works, Directorate General of Water Resources Asian Development Bank

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5 FOREWORD This note concerns part of the extra work for B2 which was proposed in October The objectives of the proposed activities were the following: A. Establish the methodology to simulate the peak flow response (sensitivity) of catchments to changes in land-use using a GIS based approach, providing sufficient detail necessary for zoning; followed by application to the relevant catchments in the 6 Ci s river basin territory; B. Preparation of zoning synthesis for the upper Citarum basin (upstream of Saguling) The present note concerns the methodology in component A). page i

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7 TABLE OF CONTENTS Page 1. Need for information on runoff sensitivity 1 2. Modeling approach - Concept distributed hydrograph 2 3. Rainfall input excess rainfall in cells SCS method 4 4. Modeling of flow in the watershed Types of flow Distributed hydrograph approach Introduction overview Flow-, velocity- and flow time in the cell Cumulative flow time along the flow path Watershed outflow hydrograph static flow condition Mapping of the catchment contributions to the flood peak Dynamic flow condition during the flood wave storage on the catchment Introduction Step-wise build-up of the flow condition Calibration of the model Application of the model to Upper Ciliwung (Katulampa), Cisadane (Serpong), and Ciujung (Pamarayan) ArcGIS Model implementation Peak Flow folder structure ArcGIS Model Builder project Preparation module Equilibrium module Zoning module Hydrograph module 25 page ii

8 LIST OF FIGURES Page Figure 2.1. Example grid of flow directions and the equivalent drainage network 2 Figure 3.1. Annual rainfall isohyet map for Java 5 Figure 3.1. Illustration of channel flow cells defined with a minimum upstream cells, all other cells are overland flow cells 7 Figure 4.2. Distributed outflow hydrograph for the watershed 10 Figure 4.3. Mapping of the cells contributing to the peak flow in the hydrograph 10 Figure 4.4. Pattern of flow (storage) in a particular cell during a flood wave 11 Figure 4.5. Effect of introducing a dynamic flow situation compared to the static situation 11 Figure 4.6. Concept of the distributed hydrograph: zones in the catchment generating to different intervals in the outflow hydrograph 12 Figure 4.7. Adjustment of the initial zones associated with a dynamic flow condition 12 Figure 4.8. Comparison of the static and dynamic flood wave 13 Figure 6.1. Hydrograph for Ciliwung catchment (at Katulampa) with current land-use 17 Figure 6.2. Hydrograph for Cisadane catchment (at Serpong) with current land-use 17 Figure 6.1. Hydrograph for Ciliwung catchment (at Katulampa) with current land-use 18 Figure 6.4. Zone with the highest influence on the peak flow at Katulampa (further settlement should be stopped in this zone) 18 Figure 6.5. Zone with the highest influence on the peak flow at Serpong (Cisadane basin) 19 Figure 6.6. Zone with the highest influence on the peak flow at Pamarayan (Ciujung basin) 19 Figure 6.7. Comparison of hydrographs for Upper Ciliwung catchment (Katulampa) for different land-use 20 Figure 6.8. Current land-use (Bakosurtanal) Upper Ciliwung catchment 20 Figure 6.8. Current land-use (Bakosurtanal) Upper Ciliwung catchment 21 Figure 6.7. Hydrograph for Upper Ciliwung catchment (Katulampa) with a higher rainfall input 21 Figure 7.1. ArcToolbox 22 Figure 7.2. Preparation module 23 Figure 7.3. Equilibrium module 24 Figure 7.4. Zoning module 24 Figure 7.5. Hydrograph module 25 LIST OF TABLE Table 3.1. Curve Nr s and flow resistance (Manning s n) associated with different land-uses 4 Table 4.1. Flow resistance in channels in different parts of the catchment 9 page iii

9 1. NEED FOR INFORMATION ON RUNOFF SENSITIVITY The strong influence of land-use on runoff is observed in many river basins. There is however very little quantitative and detailed information on the phenomenon. There is a lot of literature on rainfall-runoff modeling but only a few on the modeling of the influence of land-use changes on runoff. There is practically no literature on quantitative approaches to identify sensitive zones in the catchment which should be protected or which will be especially effective to provide retention. Such information is however essential to address zoning for runoff protection in the spatial plans. Runoff is a complex function of rainfall, slope, soil, and land-use; characteristics which are generally varying over the watershed. Further land-use has been changing over time. Measurements of runoff which occur at the outlet point of the watershed can only measure the aggregate effect of the above processes; those processes (land-use) have been varying over time. This situation makes it practically impossible to derive sufficiently detailed models using empirical data. A conceptual approach, representing as much as possible information on the functioning of the system and minimal use of measured data, is needed in such situation. Such approach, using a detailed representation of the catchment and its physical properties, is possible using a GIS approach (see Section 2). In view of the above the objective of the present study has been to set up an analysis of the flood wave in a watershed in function of the physical characteristics of the watershed, which will allow an assessment of the runoff sensitivity and a delineation of areas to be protected and/or especially suited to create retention. This note describes the runoff modeling as follows: Section 2, introduces the concept of distributed hydrograph. Section 3, describes the input of rainfall to the modeling using the SCS (curve Nr) approach. Section 4, describes the modeling of the flow over a watershed using a raster of cells and considering the different types of flow (overland, gully, channel, groundwater), and the determination of the hydrograph at the outlet point of the watershed. It further describes the mapping of cells contributing to the hydrograph onto the watershed, allowing to identify the set of cells contributing to the peak flow, and thus identify the sensitive part of the catchment. Section 5, discusses the calibration of the model followed in Section 6 with application to some example catchments. Section 7, describes the implementation of the model using ArcGIS 9.3 with Spatial Analyst.. page 1

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11 2. MODELING APPROACH - CONCEPT DISTRIBUTED HYDROGRAPH The basic technique of the spatially distributed unit hydrograph (SDUH) will be adapted and expanded to delineate areas in the catchment for which peak runoff is most sensitive. The basic modeling approach is to establish a spatially distributed unit hydrograph is to use the raster GIS functions of ArcGIS version 9.1 to establish the excess rainfall in each raster cell and calculate the travel time of surface water flow from each raster cell in the watershed to the outlet, by determining the flow path and the travel time through each cell along this path. The travel time through each individual cell along the flow path will be summed up to estimate the cumulative travel time from the start of each flow path to the outlet. Different types of land-use influence the excess rainfall in each raster cell as well as the flow properties of overland flow. Runoff is routed over the surface using DEM. The total travel time to the outlet from each grid cell is then estimated based on the runoff pathway and the travel time through each grid cell along the path using the path distance function in ArcGIS 9.3 with Spatial Analyst. Figure 2.1 illustrates the determination of the flow direction and the drainage network using the above mentioned GIS functions. Figure 2.1. Example grid of flow directions and the equivalent drainage network page 2

12 The excess rainfall and total travel time can then be used to compose the runoff hydrograph at the catchment outlet. The distributed parameter estimation capability of the SDUH modeling helps to understand how the development in the different parts is affecting the hydrology in the catchment: the contributions from raster cells to the hydrograph at various stages are mapped on the catchment. This provides information on the sensitivity of outflow for changes in the catchment. Ultimately this provides basic information for controlling land-use development in the catchment. The character of the SDUH is basically different from a classical rainfall-runoff model. A classical rainfall-runoff model is a lumped model and uses a segmentation of the catchment and observed data matching those segments, to estimate parameters; the number of segments is basically determined/limited to the number of rainfall stations. Such segmentation is relatively course, for example a percentage impermeable area can be specified for each segment, this is too rough to differentiate essential details of land-use and delineate zones for particular runoff control action. The SDUH approach strongly limits the need for monitored runoff data but this is replaced with a description of the hydrology/hydraulics of the generation of excess rainfall and runoff at the micro scale (raster cells); this provides at the same time the possibility to include land-use in a detailed way. The SDUH approach makes full use of GIS capabilities. The proposed approach makes use of available data in terms of areal characteristics (slope, landuse, ) available for the raster as well as hydraulic characteristics which can be observed in the catchment. page 3

13 3. RAINFALL INPUT EXCESS RAINFALL IN CELLS SCS METHOD Rainfall falling on each raster cell partly infiltrates and partly causes surface runoff; the proportion between the two is determined by a large number of factors (land-use, soil type, slope, antecedent moisture condition, ). For peak flow considerations we are particularly interested in this direct runoff; infiltration water contributes to slow runoff (base flow). To determine the proportion of direct runoff the SCS approach is considered. The US Soil Conservation Service (SCS) has determined the surface runoff for different land-uses, soils and antecedent moisture conditions. The curve Nr s used in the modeling corresponding to the land-uses in the available land-use mapping are presented in Table 3.1. Table 3.1. Curve Nr s and flow resistance (Manning s n) associated with different land-uses Land-user category Curve number Manning Settlements Other buildings Barren Technical Irrigated Paddy Semi-technical Irrigated Paddy Other agriculture Grass Shrub Forest Wetland Mangrove Water The aim with the modeling is to establish the unit hydrograph for the watershed, meaning the response of the catchment of a rainfall of one hour (more complex storms can be simulated by superposition). For the intended application of the unit hydrograph, that is to establish the sensitivity of catchments for runoff, but also enable comparison of different watersheds, a rainfall input should be chosen which is representative for different watersheds. The annual rainfall isohyet map, see Figure 3.1 with the map of Java, expresses well the different rainfall regimes. The rainfall value from this map, for each relevant cell of the catchment, will be used here as rainfall input. This annual rainfall input should be proportioned to a representative input for the (one hour) rainfall storm which will generate the response of the watershed. For the present application the storm input has been chosen as the annual rainfall value divided by 365. It is obvious that a different rainfall reference map could be used, for example a 1 in 10 year daily maximum rainfall map. It should be noted that the aim of the present analysis is to establish the relative sensitivity of runoff for different parts of the catchment using a (derived) unit hydrograph approach; the magnitude of the rainfall input is then less relevant page 4

14 Figure 3.1. Annual rainfall isohyet map for Java The rainfall input for a cell which has an annual rainfall of 3000mm/year will then be 3000/365=8.2mm. If the land-use of this cell has a curve Nr of 65 % then the excess rainfall in this cell which will contribute to the surface runoff and the formation of a flood wave in the watershed is 8.2*0.65=5.3 mm/hour. Considering the surface of the cell in this case 28.5 * 28.5 = 812 m 2 (0.0053*812)/3600= m 3 /s during one hour. this means a flow of page 5

15 4. MODELING OF FLOW IN THE WATERSHED 4.1. Types of flow General: Considering runoff in a watershed a first distinction can made between surface flow and ground water flow. For flood flow computations only surface water is important in the time scale of a flood wave (hours); the contribution of groundwater (base flow) will be relatively small and can be considered constant. The flow of surface water over a catchment changes in character as flow accumulates. Overland flow (also called sheet flow) occurs in the upstream parts. As more and more flow accumulates moving downwards, this flow starts to concentrate and gully flow is formed; finally as flow further accumulates the flow ends up in channel flow. Hydro-morphologically channel formation takes place if sufficient flow accumulates from upstream. Those types of flow can be characterized as normal flow (stationary flow), which can be described by the Manning equation. This flow condition is characterized by a resistance, slope and hydraulic radius (see further below). At the lowly sloped downstream end of the basin non-stationary flow may likely occur for example caused by storage on floodplains or backwater effect from the sea. This type of flow will not be considered here. The focus is on the analysis of the origin of flood waves generated in the more upstream catchment. The different types of flows which will be represented in the present modeling are further described below. Channel flow: Equilibrium flow in a channel can be described by the Manning equation which reads as follows with: Q = equilibrium flow rate in the channel (m 3 /s)) A = channel cross-sectional area (m2) P = wetted perimeter (m) n = Manning's roughness coefficient SL = slope (m/m) The shape factor of the flow (determined by A and P) can also be expressed in terms of a hydraulic radius r which is defined as follows: r = A/P For a relatively wide channel compared to the depth the hydraulic radius can be very well approximated by the depth of water. Adopting further a square channel of flow (with A=d * W, with d= depth of water and W width of the flow) Manning becomes then: In computation of the water balance for the cells the depth (d) and velocity (V) will be computed on the basis of the flow passing in the cell, the Manning formula is then used as follows With V = velocity in the cell page 6

16 The sequence in the computations will be the following (see further for details): - n and SL are respectively estimated based on the land-use for overland flow cells and roughness of the channel for channel flow cells, and DEM, - W : for overland flow will be estimated based on its position in the overland runoff process; for channel flow will be estimated based on observations in the catchment, - Q: is the flow in the channel for a major part determined by the flow entering from upstream cells (determined with a GIS function). In the modeling, channel cells are identified (using a GIS function) as those cells which have a minimum amount (for example >60) of upstream cells. Figure 3.1 illustrates a channel network which has been derived in this way. Overland flow and gully flow: Overland flow from excess rainfall occurs over relative short distances (in the order of 50 to 100 m, dependent on catchment characteristics; longer distances for higher slopes). Overland flow can be characterized as a thin film of water moving at a relatively slow pace (especially over vegetated surface) over the land surface. As this flow becomes larger due to accumulation of upstream flow and additional rainfall, this flow will tend to concentrate into gullies. Following this runoff from upstream to downstream the flow gradually changes because of additional rainfall and runoff joining in from upstream. This gradual process can be represented well with the grid cell (28.5m x* 28.5m) representation of the catchment. In traditional approaches overland flow has been modeled with a kinematic wave approach; however this does not allow to represent the gradually growing flow in downstream direction. A more accurate approach is possible with the grid representation. In the present approach the flow situation in each cell is calculated and in this calculation the changing flow situation is considered. Manning, the basic equation to compute flow, can be applied to compute the flow condition (depth and velocity) in each cell. Figure 4.1. Illustration of channel flow cells defined with a minimum upstream cells, all other cells are overland flow cells With the grid cell representation the gradual change from overland to gully flow can also be represented very well; this can be done by considering a gradually changing width of the flow in the cell; for the top cell(s) overland flow occurs over the full width of the cell (28.5m) this can be gradually reduced as the flow moves downward to other cells; the flow width is then minimal (a certain proportion of the full width of the cell) at the place where flow enters into a channel. Again for each of the cells the flow condition can be computed with Manning using the appropriate width of the flow. Base flow: As mention above, the very slowly varying groundwater outflow from the watershed (base flow), can be considered constant and relatively small compared to the surface water flood flows. In the modeling a constant base flow will be considered in the flow condition of the cells; this base flow can be considered proportional to the size of the catchment upstream of the particular cell. It can be remarked that the proportions of the different types of flow on the different parts of the watershed strongly help determine the shape of the runoff hydrograph at the outflow of the watershed. page 7

17 4.2. Distributed hydrograph approach Introduction overview The approach in the distributed hydrograph method is to derive for each cell the flow condition to route the excess rainfall in the cell on its way to the outlet of the watershed. This flow condition provides the depth and velocity and thus the time spend in each cell. Based on the flow path from a particular cell towards the outlet point of the watershed (using a GIS function) the total time to the outlet point of the watershed can be determined. In a final step the contribution of the cells of the watershed (excess rainfall in the cells) can be set out in a histogram with a horizontal time axis, and vertical flow axis. This represents the derived outflow hydrograph of the watershed. Various GIS functions are indispensable to identify cells in the catchment, determine flow directions, and flow paths. The method is elaborated in different sections below Flow-, velocity- and flow time in the cell Based on the DEM of the watershed the flow direction and the cells upstream of a particular cell can be identified. The flow condition in a particular cell can then be identified as follows: - if a cell has a particular minimum number of upstream cells (for example >80) then the cell is considered a channel cell, - in the (upstream) cells with less than (for example) 80 upstream cells the flow condition is expected to vary gradually from overland flow (top of the catchment) to gully flow near the channel. The cells can be further characterized as follows: All cells: Considering an inflow to cell i in time period n from the upstream cells (sum of excess rainfall on these cells) and a contribution from ground water (constant over time) then the total flow in the cell i is = + The (equilibrium) flow condition is then (Manning) With: = equilibrium flow rate in the channel (see above) (m3/s) = slope in cell i (m/m) = flow resistance in cell i W cell,i = width (m) of the flow in cell i = depth of the water (m) in cell i during period n The physical characteristics of the cell can be derived from maps and specified in tables related for example to the position in the watershed. The slope is derived from the DEM for the watershed. The flow resistance is determined by the land-use for the particular cell, see Table page 8

18 For a channel cell a distinction can be made for different parts of the watershed; resistance will be larger in upstream (steeper) sections (caused by armoring of the flow bed). The flow resistance in the channel can be linked to the size of the upstream catchment with resistance values specified in a table such as presented in Table 3.1. Table 4.1. Flow resistance in channels in different parts of the catchment Upstream area (number of cells) Manning s n for flow in the channel cells > Overland/gully flow cells: The flow width W cell,i of a particular cell i, corresponding to the gradually concentrating flow in downstream direction, can be determined proportionally based on its position between the top of the catchment and downstream channel cell based on the number of upstream cells for the particular cell i. The flow width can then be expressed as follows W cell,i = 28.5 (28.5 minimum width) * #upstream cells/60 A value of 5 m was adopted for the minimum width. The size of the upstream catchment for channel flow was set at 60 cells. Both parameters are expected to vary with slope and land-use ; the above parameters were selected based on some limited sensitivity analyses. Channel cells: The width of channels will gradually increase from a minimum upstream to a particular value downstream at the outlet point. These widths can be measured in the field for the particular watershed at hand. A proportional change between those two value, proportional to upstream catchment size has been adopted for channel width Cumulative flow time along the flow path The water velocity in a particular cell i based on Manning can be expressed as The time spend in cell i is then With: = flow time in cell i in period n D = flow distance in the cell; depending on the flow direction this distance is either the length of the cell (28.5m) or the diagonal length (28.5 * As mentioned above a GIS function can be used to find the flow path of a particular cell towards the watershed outlet. Once the path is determined, the GIS function can be used to sum the travel times through each cell in the flow path. The total or cumulative travel time for cell i is the sum of the time of travel through each cell along the route. with = cumulative flow time from cell i to the catchment outlet point (in hours) page 9

19 Watershed outflow hydrograph static flow condition The contributions to flow from the cells on the watershed can be characterized by their to reach the outlet of the watershed as indicated in previous section. This is based on a particular flow situation in the watershed. time A simplified way (a first approximation) to consider such flow situation is to consider the flow entering the cell as the sum of the excess rainfall/flow from the upstream cells. This is a reasonable assumption for the upstream cells but considerably overestimates the flow in the downstream cells, in particular during initial and final hours of the flood wave, when respectively either upstream flow has not yet reached downstream cells, or upstream flow is depleting. An adapted estimation of the flow situation is elaborated in Section 6. Figure 4.2. Distributed outflow hydrograph for the watershed Considering the excess rainfall/flow in each of the cells of the watershed and their time to reach the outlet point, the outflow hydrograph of the watershed is determined by plotting these contributions against time in a histogram. Figure 4.2 illustrates the concept. A hydrograph shape as shown in Figure 4.2 can be expected as result, reflecting a typical relatively wide range of flow times (as well as flow contributions) for various parts of the catchment to reach the outlet point, obviously the shape of the catchment will also have considerable influence Mapping of the catchment contributions to the flood peak For spatial planning the interest is in identification of those parts of the catchment which contribute most to the peak discharge, or i.o.w. the most sensitive parts of the catchment for contribution to floods. This is essential information to formulate controls, w.r.t. runoff, on the development of the catchment. Such controls comprise avoiding/prohibiting/compensating the land-uses with a high excess rainfall (curve Nr) and fast runoff (Manning n) such as urban settlement. Creation of (extra) infiltration/retention in this zone will have the highest efficiency (direct influence on the peak flow). Figure 4.3 illustrates the mapping of the cells contributing to the peak onto the watershed. The GIS system which has been previously used to characterize the cells and set up the flow system can subsequently be used to keep track of the position of the set of sensitive cells, and thus carry out such mapping. Figure 4.3. Mapping of the cells contributing to the peak flow in the hydrograph page 10

20 4.4. Dynamic flow condition during the flood wave storage on the catchment Introduction In the distributed runoff representation in Section 4, a static flow condition was used to represent the flow which was used to estimate the time to reach the watershed outlet. In reality this flow varies considerably during the time that the flood wave builds up and recedes. The flow in a particular cell will have a pattern as sketched in Figure 4.4, the flow (and associated storage in the cell) build up as more flow from upstream cumulates. This flow (or associated storage) in a certain cell at a particular time will not only accumulate but also naturally dissipate downstream and this will occur at a stronger rate if storage is larger. The actual storage in a cell will be a balance between building up due to inflow and dissipation from storage. The effect of a dynamic flow situation on the outflow hydrograph, compared to the static flow situation is presented in Figure 4.5, increasing storage during the build-up of the wave will decrease the initial outflow and increase the outflow when storage is depleted during recession of the wave. The build-up and recession of the flow condition will create a delayed peak in the flood wave. Figure 4.4. Pattern of flow (storage) in a particular cell during a flood wave Flow/ storage Time Figure 4.5. Effect of introducing a dynamic flow situation compared to the static situation Below a method is proposed to approximate the changing flow condition. This consists of a combination of stepwise build-up of the flow and a linear reservoir representation of the storage in each cell Step-wise build-up of the flow condition The method, to establish a dynamic flow situation to estimate more accurately travel times, starts with determination of the runoff and mapping of the contributing cells using the static flow condition as presented in previous sub-sections. The resulting zones in the catchment which contribute flow to the outflow hydrograph in the different time periods is presented in Figure 4.5. The approximation in this setup is that the flow/velocity in the downstream part of the catchment, during the initial and final stages of the flood wave are overestimated. Further this flow velocities are kept constant during the runoff process. This distorts the zones of cells contributing to a particular outflow period. The deviation will be strongest for the downstream zones and gradually decrease for the more upstream zones. In reality, as indicated above, the velocity at any point (cell) in the catchment (and more pronounced in the downstream parts) will have a wave type shape during the runoff process, which is also different for each cell. For example zone Z1 (cells which contribute to the outflow in the first hour) will be much smaller in reality because the velocity is much lower (most flow from upstream has not yet arrived in this first hour), and consequently many cells of zone 1 will not reach the outlet point during the first hour; zone Z1 will therefore be considerably smaller. page 11

21 The actual distribution of the flow over the catchment varies with time and place in the catchment. The zones as identified in Figure 4.6 are used to build up the flow in subsequent time steps (hours) and to approximate the space-time varying flow distribution. This can be seen as a numerical scheme to approximate this flow distribution. The scheme is elaborated below for subsequent time steps. Figure 4.6. Concept of the distributed hydrograph: zones in the catchment generating to different intervals in the outflow hydrograph Time step 1: Adjustments in the first time step comprise the following (Figure 9 illustrates the adjustments): 1) determination (for each cell i, using GIS function) of the flow (Q upstr,i ) from upstream for zone Z1 ; but the cells which provide input to this flow consist only of the cells zone Z1, those are the cells active in zone Z1 in time period 1; in the first hour the flow from further upstream has not reached this zone yet and can thus be ignored in this first hour 2) computation of the depth/velocity in the cells is carried out as presented in previous sections depth - velocity - cell time 3) computation of the total time along the flow path for each cell i to reach the outlet point (carried out as before) 4) selection of the cells contributing to the outflow in the first hour based on the new timing ( values). This results in an updated zone Z1, named UZ1 5) summing up the contributions from UZ1 results in a new (reduced) flow in the outflow hydrograph for the first hour (as indicated in Figure 9 in time interval 0-1 (by testing T for all the cells; again this can be limited to all cells in zone 1). After selection of the cells corresponding to T<1 (UZ1) there will be a set of remaining cells from zone Z1 as illustrated in Figure 4.7, those will be considered as part of the next adjustment step Figure 4.7. Adjustment of the initial zones associated with a dynamic flow condition page 12

22 Time step 2: Similar adjustments are made as in step 1, for a particular set of cells which are relevant for the downstream flow in the second hour. For computation of this flow the following cells are relevant: - rainfall which has fallen in the first hour on zone Z2 and the remaining part of zone Z1 which is now reaching zone UZ1 in the second hour, - the computation of the flow in zone Z2 plus Z1-UZ1 is computed for the first hour with no previous flow and for the second hour with remaining flow in UZ1 from the first hour - part of the remaining flow (storage) in zone UZ1 from rainfall in the first hour will however have dissipated from the cells; this process can be represented by a linear reservoir, which is the typical method to represent discharge from storage in hydrological models. The flow condition in the watershed in the different cells during the second time step (hour) is then * (1 R) With : R = outflow coefficient from storage in the cell (outflow = storage * R) will have a value in the zone UZ1 (from the computations in time step 1) and will be zero for the cells in Z2 and Z1-UZ1. Time steps 3,4,5.: Similar computations and re-arranging of zones as in Time step 1 and 2 are carried out for the subsequent time steps. Those result in updated values in the outflow hydrograph. For Time step 3 the flow in a cell i in UZ1 will consist of the following or * (1 R) * (1 R)) * (1 R) It can be observed that with this formulation the storage in the cell i over the different time steps will increase as long as the upstream flow is larger than the dissipation of storage, and decrease when this upstream flow becomes smaller. The storage in the cell will then behave as considered in Figure 4.4. Figure 4.8 illustrates the computed static versus dynamic flood wave for the Upper Ciliwung (Katulampa). Figure 4.8. Comparison of the static and dynamic flood wave page 13

23 5. CALIBRATION OF THE MODEL The validity of the model should be checked based on a comparison of the simulated hydrograph(s) and measured hydrograph(s). In particular the runoff at Katulampa in the upper Ciliwung catchment will be considered for such validation; this will be further complemented with a check on the hydrologic/hydraulic consistency of the runoff pattern for 2 other catchments with strongly varying characteristics (Cisadane and Ciujung). Two steps can be considered in the validation: 1) Checking if the representation of excess rainfall using the above approach is valid. This means basically comparing the excess rainfall in a measured situation, represented by the ratio between the rainfall input (measured at rainfall stations) and the volume of runoff in the measured hydrograph, is equal to the excess rainfall (basically based on curve Nr) applied to the input of rainfall and the excess portion of this rainfall should be compared to the volume of runoff in the runoff hydrograph, which is caused by the excess rainfall, at the outflow point. 2) A second step concerns the representation of the hydraulics of runoff which will determine (together with other factors) the shape of the hydrograph. Several factors influence this shape such as the proportion of overland flow and channel flow in the watershed, the shape of the catchment (wide or elongated), the flow resistance associated with different land-uses and the roughness of channels in different sections of the watershed. Quite some information can be found in the literature, the final calibration should be based on measured runoff (waves) for different catchments. Checking the consistency of the hydrologic/hydraulic response to different catchments characteristics forms an important part in validation of the model. Ad 1) The ratio of excess rainfall to rainfall was checked for the Upper Ciliwung based on a 24 hour period of measures rainfall and runoff in feb 2002; this period included a small flood wave. Hourly rainfall was measured at only 2 points in the catchment, viz Citeko in the middle of the catchment and Tugu in the top of the catchment. The ratio excess rainfall/total rainfall was found to be 83%. Simulation of the existing situation based on the (Bakosurtanal) land-use map and application of the curve numbers indicated a ratio of 67%. Several uncertainties should be mentioned in relation to those estimates, such as: - measurement of rainfall: only 2 rainfall stations (hourly data) are available. The ability of those stations to accurately capture the rainfall input on the catchment is quite limited. This is demonstrated by the observation in the total available series that a medium flood wave happened at Katulampa while no rainfall was indicated at the two stations; apparently rainfall storms occur (on part of the catchment) which are not captured by the rainfall stations. This points to the strong need in Indonesia for a fully instrumented experimental catchment (or several of them) for testing of models and deriving characteristic hydrologic/hydraulic parameters (see also further) - use of curve Nr: the curve Nr values have been empirically derived for US conditions. How far are these data applicable to Indonesian conditions? - antecedent moisture condition: in the original SCS set up of curve numbers a differentiation is made according to antecedent moisture condition; in the present runoff model an average antecedent moisture condition (and associated curve Nr ) has been assumed; the actual antecedent condition for the observed period is unknown; maybe for Indonesian conditions (climate, soils) a wet condition should be used resulting in higher excess runoff, and thus a higher ratio; this would bring the simulation result closer to the observed ratio. - land-use mapping: the Bakosurtanal land-use mapping has been used in the model; it is unknown how compatible those land-use categories are compared to the US categories Ad 2) The model has been applied to 3 catchments with strongly different characteristics (steepness, shape, rainfall, land-use), see next Section 6. Based on hydraulic/hydrologic principles different run off behavior can be deduced associated with those characteristics. The relationship between catchment characteristics and runoff pattern is discussed in Section 6. It can be inferred that the different model applications produce results which are in line with expectations. page 14

24 Based on the above observations it can be concluded that the modeling provides an acceptable representation of the (storm) runoff process. Obviously further calibration of the modeling is warranted, which was not possible in the short time frame of the present study. Note on the application of the modeling to large catchments: In the runoff modeling a one-hour rainfall input is considered for the total catchment; this corresponds to the concept of unit hydrograph; with reference to the methodology in the runoff modeling the present modeling could be called a distributed unit hydrograph. For larger catchments it is increasingly unlikely that an actual storm will cover the total catchment. The model can be used to simulate the catchment response to any rainfall input. An appropriate design storm including the areal extend and depth of the rainfall may be formulated as an input to the model. In the present analysis no attention has been paid to the derivation of an appropriate rainfall storm input for larger catchments. Requirements on the DEM Determination of the flow pattern in the watershed is based on the relative height differences in neighboring cells of the DEM. The DEM should be sufficiently consistent/smooth to establish a logical upstream-downstream flow pattern. This may be a problem in relatively (very) flat areas where height differences become (very) small. A combination of limited accuracy of the DEM and small changes in the terrain may then cause an erratic pattern of heights in the cells and prevent establishment of consistent flow pattern page 15

25 6. APPLICATION OF THE MODEL TO UPPER CILIWUNG (KATULAMPA), CISADANE (SERPONG), AND CIUJUNG (PAMARAYAN) Introduction The storm runoff model has been applied to the Upper Ciliwung (Katulampa), Cisadane (Serpong), and Ciujung (Pamarayan) basins. For the Upper Ciliwung the model was applied to different (future) land-uses; those were the following: present land-use, future land-use according to the current spatial plan (with larger settlement), future potential land-use according to the proposed zoning for re-forestation and upgraded management of dry land agriculture (see B2 main report). Those will have impact on erosion as well as runoff. For runoff this results in improved infiltration (less excess rainfall) and surface runoff properties. Also a test was made with a higher rainfall input to test the effect on runoff. Runoff pattern for different catchments: The catchments have strongly different characteristics. The Upper Ciliwung (151 km 2 at Katulampa) is small and steep, the Cisadane (1094 km 2 at Serpong) is much larger and has some steep sections. The Ciujung basin (1494 km 2 at Pamarayan) is the largest and has the lowest slope. Rainfall input to the 3 catchments is also quite different. The Cisadane catchment has the highest range of rainfall on the catchment, from very high on the slopes of mount Salak in the top of the basin, to moderate in the low section. The Ciujung has on average the lowest rainfall and the lowest range. As can be observed in Figure 6.1, 6.2 and 6.3 the runoff pattern in the three basins is strongly different. The difference in the patterns can be explained on the basis of the above mentioned characteristics. The following observations can be made: Katulampa has a single wave with the shortest time to peak, corresponding to the small and steep catchment. The Katumlampa response is very fast with a relatively very high peak flow. Its peak flow is only about half of the one at Pamarayan while the catchment is 10 times larger. The peak runoff at Serpong is 50 % larger that at Pamarayan while the catchment at Serpong is only about 2/3 of the catchment upstream of Pamarayan. This reflects the steep slopes and higher rainfall in part of the Cisadane basin. The double peak in the Cisadane can also be explained by the fast reaction of the steep and high rainfall zone and the later response of the other part of the basin (considerable fast runoff takes place which further delays the response of the slow part of the basin). The response of the Ciujung is the slowest with a very long, relatively flat wave, corresponding to the wide shape of the basin. Figures 6.4, 6.5 and 6.6 illustrate the zones contributing most to the peak flow in each basin. The zones are located in the mid sections of the basins. The zones demonstrate upwards excursions following different side rivers; those channels are speeding up the response of the surrounding areas (channel flow is relatively fast compared to overland flow). Effect of different land-use: Figure 6.7 compares the runoff hydrographs for the three land-use situations for the Upper Ciliwung as indicated above. The present land-use (Bakosurtanal) is presented in Figure 6.8 and 6.9 presents the (potential) land-use according to the current spatial plan. As can be expected the larger settlement in the spatial plan causes a substantial increase in the peak flow while the conservation reduces the peak flow. A difference of 30% can be observed between the two extremes: the zoning for peak runoff should prevent an increase in the peak while the conservation zoning will reduce the peak; an optimal zoning for the Upper Ciliwung should include both types of zoning. Effect of higher rainfall: As can be expected a higher rainfall and associated flow cause a faster response of the catchment and consequently the peak increases more than proportional with rainfall. Figure illustrates the result for a doubling of the rainfall for the Ciliwung basin: the peak flow has increased by 2.75; the difference in peak flow of the current land-use and (future) land-use according to the spatial plan remains 20%. It can be concluded that there are substantial scale effects in runoff w.r.t. rainfall. This also means that it is important to select a proper design rainfall to determine/design the protection zone. page 16

26 Figure 6.1. Hydrograph for Ciliwung catchment (at Katulampa) with current land-use Figure 6.2. Hydrograph for Cisadane catchment (at Serpong) with current land-use page 17

27 Figure 6.3. Hydrograph for Ciliwung catchment (at Katulampa) with current land-use Figure 6.4. Zone with the highest influence on the peak flow at Katulampa (further settlement should be stopped in this zone) page 18

28 Figure 6.5. Zone with the highest influence on the peak flow at Serpong (Cisadane basin) Figure 6.6. Zone with the highest influence on the peak flow at Pamarayan (Ciujung basin) page 19

29 Figure 6.7. Comparison of hydrographs for Upper Ciliwung catchment (Katulampa) for different land-use Figure 6.8. Current land-use (Bakosurtanal) Upper Ciliwung catchment page 20

30 m3/s Appendix 4 - Modeling for Peak Runoff Zoning Figure 6.9. Current land-use (Bakosurtanal) Upper Ciliwung catchment Figure Hydrograph for Upper Ciliwung catchment (Katulampa) with a higher rainfall input current land-use urbanized Tim e (hours) page 21

31 7. ARCGIS MODEL IMPLEMENTATION 7.1. Peak Flow folder structure The Peak Flow model is setup in a folder structure to manage both the required data sets and the model project. The folder structure is as follows: Model This folder contains the project workspace, model builder project and some other files used by the Model. The way you use the model is that for each catchment you wish to model you make a copy of this folder an rename it to the name of the catchment (i.e. Katulampa) DEM This folder contains the 30 m SRTM elevation mode, projected to UTM (zone 48 South) and derived hydrological datasets typically used for hydrological analysis; the filled DEM, flowdirection and FlowAccumulation rasters, ready to be used for all of Java. Land use This folder contains a 30 meter raster version of the 1:25,000 BAKOSURTNAL RBI land use map. The attribute file for this layer has already been provided with the SC Curve and overland Manning coefficient required by the model. Katulampa, Serpong, Pamarayan These are project folders containing existing Peak Flow model output files. These folders were originally a copy of the Model folder that was renamed and then used to run the model ArcGIS Model Builder project The current Peak Flow runoff model has been implemented using ArcGIS 9.3 using Spatial Analyst extension for the Raster processing and using Model Builder to organize the workflow and model iteration. The Model builder project is stored in the file PeakFlow.tbx file in the Model folder and will be opened automatically if you open the PeakFlow.mxd workspace included with the model. Alternatively you can add it to the ArcGIS toolbox by Figure 7.1. ArcToolbox opening the ArcToolbox window in ArcMap, then right clicking on the topmost ArcToolbox: icon and selecting Add Toolbox to open the PeakFlow.tbx file. The Peak flow model consists of several modules, which need to be executed in this order: 1) Data preparation 2) Equilibrium 3) Zoning 4) Hydrograph Each of these modules will be explained in a separate section below. The Toolset folder contains two utility models used in the main modules, one of them is used to cut out a specific section of the DEM and the other calculates the flow velocity for both the Equilibrium and Zoning modules. You can open each of these modules by right clicking it and selecting Edit and you will see the module schematic appear in a separate window. page 22

32 7.3. Preparation module Figure 7.2. Preparation module After creating a copy of the Model folder and renaming this for the catchment to be analyzed, you open the Peak Flow workspace in this folder to get started in a new catchment. The preparation module helps you to properly prepare the datasets for a catchment. After making sure the Working folder is set properly (under model Properties > Environments > General settings) you follow the workflow from left to right to delineate the catchment to be analyzed and cut out subsets of the Java datasets included with the model, making sure all the rasters are properly aligned. The main input for the Preparation is a point on the main river for which the catchment is to be delineated, you create this by editing the included (empty) Pourpoint layer and digitizing a single point on the main river. Having created the Pourpoint, it is recommended to manually run the Snap Pour Point and Watershed steps by right clicking these yellow boxes, selecting Run from the menu and taking a checking if the process of delineating the catchment is successful. You can speed up these first two steps considerably by manually setting the extent under Properties > Environments > General. When the catchment has been created properly, you can run the rest of the model from the model main menu, watch the model complete each step until done. Once the Preparation module has been completed, you will need to adjust the channel width at the outlet of the basin, to do this zoom in on the upper right part of the model and double click on the yellow square which is labeled Channel Width. By comparing the value found in the WChannel layer at the outlet of the basin and comparing this with the observed with of the river in Google Earth, you should be able to adjust the Channel Width formula and recalculate WChannel to make it match the observed river width more closely. page 23

33 7.4. Equilibrium module The equilibrium module is used to make a first estimate of the flow velocities in the catchment assuming a storm event lasting one hour. Opening this model for the first time, make sure to set the working folder and analysis extent properly, from the Model menu under Properties > Environments > General. After running the Equilibrium model, you should see a layer named Iso_0 that contains a first (over) estimate of hourly zones, a so-called Isochrone map. Figure 7.3. Equilibrium module 7.5. Zoning module After having created the first Isochrones map using the Equilibrium model, the Zoning module will run a loop iterating over each hour to adjust each of the hourly zones. Again, before using this model you need to open the model Properties > Environments dialog to set the proper working folder and analysis extent. Additionally, based on the number of hourly zones in the initial Iso_0 isochrones layer, you may want to change the number of iterations to a higher value then the default value of 12. Figure 7.4. Zoning module page 24

34 7.6. Hydrograph module The Hydrograph module is the simplest one, it creates a DBase file with hourly runoff values. The Zonal Statistics as Table command is used to summarize the excess rainfall per hourly zone of the final Isochrones output layer. Figure 7.5. Hydrograph module page 25