Methods to Simulate the Impact of BMPs

Transcription

1 LA221 Project Ortega Watershed Demonstration Area, Quito Ecuador Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley December 2011

2 Ortega Watershed Demonstration Area Quito Ecuador Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley Introduction This paper will present the method that will be used to simulate the implementation of Stormwater Best Management Practices (BMPs) in the Ortega watershed in the City of Quito, These methods are a section of a project on Integrated Stormwater Management (ISM) for the City of Quito, which will be my final professional project to fulfill the requirements for my Masters in Landscape Architecture & Environmental Planning. Due to the fact that the satellite imagery necessary to realize this work wasn t yet available, this paper will go through the idealized procedures to perform the simulation and it will use an example of satellite imagery from San Pablo, CA, to demonstrate the results of image analysis. Ecuador. The objective is to calculate the resulting stormwater discharges and to demonstrate how much runoff could be detained and infiltrated through the use of these BMPs. Eventually a successful implementation of these projects will reduce natural disasters, such as floodings and landslides, improve the water quality and the living conditions of local communities. Objectives Since the 1970s in developed countries, the concept of stormwater management has evolved through the application of new technologies, also called BMPs, such as detention and retention ponds, permeable surfaces, infiltration trenches and other source control measures. This approach has been Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 2/12

3 implemented through municipal regulations in developed countries and the land developer pays the cost of implementation and control (Tucci 2007). However, in developing countries, this type of control usually does not exist and the impacts are transferred downstream into the major drainage system. The cost of the control of this impact is transferred from the household to the public domain, since the municipality has to invest in hydraulic structures to reduce the downstream flood impacts (Tucci, Goldenfum, and Parkinson 2009). It is common sense that where urban runoff occurs, a disturbance has taken place; therefore mitigation and restoration are necessary. In the midst of a city, restoration of ecological processes depends on human protection and management. The characteristics of a place can make the processes through which hydrologic and ecological restoration takes place visible and comprehensible. Design is capable of revealing and integrating (Ferguson 1998). Urban infiltration constitutes the restoration of a site s hydrology. It restores groundwater to the earth and balanced flow regimes to streams. In addition to addressing flooding and erosion, which are targeted by conveyance and detention systems, infiltration supports groundwater recharge, stream base flows, water quality, aquatic life, and water supplies. Because it turns the hazard of storm flows into the resource of base flows, it is environmentally the most complete solution to the problem of urban stormwater (Ferguson 1998). Although it s known that these stormwater BMPs increase the storage and infiltration capabilities in the watershed, reducing flooding and erosion, and improving the general environment and thus the quality of urban life, the challenge presented is how to implement them in a growing developing country city, with few resources and where these technologies are unheard Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 3/12

4 of. To do so, a method of calculating and displaying these results has to be considered. The objective of this paper is to come up with a method to simulate the capacity of BMPs to A relatively low density and the existence of several open spaces, although not treated, adjacent to the streams characterize the 2400 ha of the watershed. decrease the total discharge volumes from large storm events and display these results effectively and comprehensively to engineers and general public. Study Site Located southwest of the city limits the Ortega watershed is mainly occupied by low-income communities occupying risk areas. The precarious conditions and geographic location, extending up the slopes of the Atacazo Mountain plus the fact that it is one of the few areas in the city that still has most of their streams daylighted, makes it a compelling study site to implement some BMP ideas. Photo of the southwest limits of the City of Quito. Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 4/12

5 Photo of the Ortega watershed with streams. Photo of the Ortega watershed with 10 m contours. Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 5/12

6 Method There is a series of data necessary to be able to design BMPs and also to properly run the specific hydrologic model. This includes delineating the watershed, sub-watersheds, and subbasins. Determining existing and future land uses. Determining the hydrologic-hydraulic parameters like, the dominant soil type (sand, silt, and clay), the soil permeability, the nominal slope, the percentage of imperviousness based on satellite imagery, the runoff coefficient, and the time of concentration and flow through time. It is necessary to assemble precipitation data. Two categories of precipitation data are typically needed. The first is intensity-duration-frequency curves or tables, which are often used for sizing the BMPs. The second are hyetographs or other historic data on storms. Data should be gathered on recent major storm events widely recognized by residents to have caused floodings. Searching for groundwater-level data, as this will affect the construction and operability of the BMPs, is also important. Land ownership should be determined. Determining, if existent, applicable surface water management regulations. Identifying related problems and opportunities, i.e. communities with flooding problems may have needs in one or more other areas of community services, such as providing open recreation areas, reducing water pollution, improving water supply, etc. These needs may be relevant to the surface water management and therefore should be noted and understood when planning and designing BMPs. Through remote sensing and GIS analysis it is possible to calculate the current and proposed discharges of urban runoff. This can be done using the US Army Corp of Engineers software developed by the Hydrologic Engineering Center, the Hydrologic Modeling System (HEC-HMS), that was conceived to simulate the precipitation and runoff processes of branched watershed systems (USACE-HEC). Engineers commonly use it to Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 6/12

7 estimate discharges based on watershed conditions and therefore study different drainage scenarios (Jencks). To calculate the impacts of BMPs, HEC-HMS conceptually subdivides the watershed into separate basins by grouping together areas based on the runoff that they are likely to generate (Jencks). First it is necessary to calculate the percentage of imperviousness in the watershed. This can be done with the use of 4 bands satellite imagery, i.e. satellite imagery with at least the NRI spectrum. Well, the water company in Quito does not have this type of imagery, so it will have to be bought from a private satellite imagery company. These companies own, for instance, orthorectified imagery from GeoEye-1 or Ikonos satellites with 1m resolution. With this information it is possible to reclassify the image and quantify the impervious surfaces. Satellite image of the City of San Pablo (green) and the Wildcat Creek Watershed (blue). The following example shows the capabilities of this type of image manipulation for the City of San Pablo, CA. Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 7/12

8 Infra-red imagery of the same site. Already noticeable the difference between hard and soft surfaces. Zoom into the site. After reclassifying the image it is possible to clearly distinguish pervious (green) from impervious (orange). Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 8/12

9 As it can be seen in the last image there are some black areas caused by shade that GIS cannot distinguish and therefore classifies it has no data. For this problem there are a few tricks that can be used to reduce this lack of data. One is adding the road layer. One knows that all the black areas that fall onto the road layer will be roads too and therefore can be added to the impervious areas. Another trick is to measure the average building, for instance, in the last image it is easy to distinguish, being a residential neighborhood, how all the houses have approximately the same size. It is possible to measure it and assign an average radius for them. Then edit points on top of each house and buffer those points by the previous calculated average radius. Again the shadows that fall onto those buffered points can be assumed as falling onto a house and therefore be part of the impervious areas. The remaining data needed, such as precipitation is already Closer image of the site. available and can just be used in HEC-HMS. Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 9/12

10 Another information needed is soil permeability. This will be calculated on site by using a buried bottomless cylinder in the ground and then dump one liter of water into it. Obviously the idea is to calculate how long it takes to the water to infiltrate. This procedure will be repeated in several points within the watershed so it can give a rough estimate of the general soil permeability. Before running HEC-HMS it is necessary to run HEC-GeoHMS. HEC-GeoHMS builds a spatial model of the watershed that is later exported to HEC-HMS to run the hydrologic calculations. HEC-GeoHMS builds a spatial model of the watershed based solely on a DEM of the area. To do this I had to the DEM. First I created a TIN from 10 m contours. TIN of the Ortega watershed. After creating the TIN and clipping the edges, I convert it into a Raster. Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 10/12

11 The current combined sewer system in Quito is frequently overloaded and overflows. Therefore, the HEC-HMS analysis will be used to examine the 1, 2, 5,10, 25, 50 and 100-year storm events to estimate the relative effectiveness of the system. A successful implementation is judged to be any storageoriented system strategy that reduces runoff discharges below the 5-year storm discharge under larger storm events, which is the standard for developed cities. Conclusions The results of this study will hopefully serve as an DEM of the Ortega watershed. encouragement for the application of this method not only in Quito, but also in other cities in developing countries that struggle with floodings and related natural disasters. The main idea is to develop a method that can resort to satellite imagery, available for the entire world, and from there quickly Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 11/12

12 simulate stormwater volumes. Present the results in a clear way is also essential for these issues to be understood and solutions to be implemented. One of the key problems in cities in developing countries is the lack of knowledge about the hydrologic processes and the sustainable solutions that exist for them. References Ferguson, Bruce K Introduction to Stormwater: Concept, Purpose, Design. New York: Wiley. Jencks, Rosey A Finding Room for Stormwater: A Review of Site and Design Opportunities in San Francisco. Tucci, Carlos Urban Flood Management. GWP-SAMTAC. Tucci, Carlos, Joel Avruch Goldenfum, and Jonathan N. Parkinson Integrated Urban Water Management: Humid Tropics: UNESCO-IHP. 1st ed. CRC Press, July 3. USACE-HEC. Hydrologic Engineering Center - Hydrologic Modeling System (HEC-HMS) Website. Ricardo da Cruz e Sousa Master of Landscape Architecture & Environmental Planning University of California, Berkeley 12/12