Isaac Odoi Danquah
ABSTRACT
Runoff generation within a catchment area is dependent on stormwater generated and overland flow after a precipitation for a given duration. Modelling using water resources engineering softwares becomes very import as one sorts for data, model and simulate into the future. This is able to give real time result after good work for future prediction and forecasting. This is the reason for this research work and the use of MIDUSS version 2.25 to model stormwater within three catchments. The 100min rainfall gave the maximum rainfall intensity to be 113.686mm/hr. This generated a runoff volume for pervious and impervious surface to be 268.92m3 and 239.17m3 respectively. Values of 9.606mm and 34.168mm are the runoff depths for pervious and impervious surfaces over areas of 2.8ha and 0.7ha. Stormwater volume of several depths will continue to be generated after a given duration of precipitation over impervious areas. This may lead to flooding hence the need for good hydraulic structures of adequate capacity for conveyance downstream for safety within a catchment.
Keywords: stormwater, catchment, modelling, Miduss, runoff, flooding, precipitation, impervious, pervious, abstraction.
1 INTRODUCTION
Every catchment area and the amount of runoff generated as stormwater after a given rainfall. It depends on two parameters which is the pervious and impervious nature of the catchment. In a given storm, initial abstraction or infiltration takes place for a given duration before runoff generation begins after saturation of the soil. This usually occurs on pervious areas where water can penetrate into the soil. On impervious surfaces such as parking lots, concrete areas, pavements etc, runoff generation begins instantly since there is no initial abstraction or infiltration. Runoff generation is dependent on time of concentration within the catchment area. The maximum time taken for a raindrop that falls farthest away in a catchment area to drain to the outlet point is called the time of concentration. The time of concentration depends on the distance and the water velocity. Runoff volume and depth is high for impervious areas than pervious areas within the same catchment area. Runoffs generated within a given catchment area ends up in drainage systems, conduits, networked pipe system and finally in streams and oceans downstream of the catchment. This is the case in undeveloped countries like Ghana. In the well developed countries like the USA, Scotland, UK, Germany, Dubai and others, they end up in sewerage systems in addition to streams, rivers and oceans for treatment before final disposal into other systems for recycling or into streams and oceans. Runoffs do not build up instantly upon rainfall but takes time within a catchment or forested area as abstraction is very high in such areas. High amount of runoff generation within an impervious area which is not collected and channeled into drains and conduits may result in flooding within the catchment. This usually happens in urbanized areas where most areas are impervious with areas such as parking lots, pavements, concrete areas etc. it therefore becomes important to model such stormwater within an urbanized catchment area for a given return period. There is the likelihood that within a given a return period, a rainfall of high intensity will happen. This will result in flooding because drainage systems of inadequate capacity will not be able to channel such stormwater downstream effectively. Hence for good drainage system design and construction, it’s important to model stormwater within catchments to avoid future contingencies. Stormwater tools such as StormTac, Infoworks Collection Systems (Infoworks CS), Sustainable drainage systems such as for example permeable pavings, ponds and green roofs (SuDS) Studio, Model for Urban Stormwater Improvement Conceptualizatio (MUSIC), SWMM, XPSWMM, StormCad, Civilstorm, Civil 3D – Storm and Sanitary analysis etc to model stormwater within a catchment area for prediction, forecasting and management of stormwater within the catchment for the benefit of mankind. Such tools are able to predict, forecast to a higher degree of which engineers, planners and urban developers are able to design and construct drainage systems and sewerage systems and hence flood controls within urbanized areas.
Stormwater generated within a catchment comes with a higher degree of contaminants and pollutants since runoffs generated collects a lot of debris and waste materials. Pollutants can be nutrients, heavy metals, bacteria, organic compounds and oils. These contaminants may originate from for example traffic pollutions, sewage systems, atmospheric deposition and oil leakage from vehicles. If such untreated stormwater do not end up in storm treatment facility and treated before discharge, they end up polluting water bodies and inhabitants. Storm treatment facilities therefore becomes a needed facicility in every country for stormwater treatment but that cannot be seen in Ghana. Such pollutants even end up in homes during storm because of bad housing systems, drainage designs and construction especially those living in flood prone areas within a catchment. Very high intensity rainfall for long duration generates runoff of high depth and volume which requires fast and correct runoff collection into drainage systems. If this is not rightly done within the catchment, flooding occurs posing threats to people living in flood prone areas. Wrong catchment delineation and time of concentration determination will result in wrong runoff determination hence inadequacy in drainage designs and resultant construction. This is why engineers and planners in developed countries are in constant stormwater modelling using different models and tools since there is constant increase and decrease in impervious and pervious areas respectively.
This research work therefore sorts to model stormwater generation within three catchment using MIDUSS version 2.25 to predict and forecast what is going on and can be done in a community within Ghana during a given rainfall duration. Ghana as a country has two rainfall seasons in the southern part. The June – July peak rainfall results in a given high rainfall duration over the region and resultant stormwater generation. This storm generates runoffs of given depths which ends up in drains and conduits of varied capacity. Some of these drains are able to channel the stormwater downstream safely into streams, rivers and ocean. Drians of inadequate capacity and full of debris and contaminants ends up channeling stormwater on roads and homes. Modelling such stormwater will help to forecast, predict and obtain real time knowledge on how to manage stormwater in Ghana and other developing countries where there are no proper sewerage systems and storm treatment facilities to treat such storm waters before channeling downstream and into streams, rivers and oceans.
2. RELATED WORKS ON STORMWATER MODELLING
2.1 Stormwater modelling and management within catchments
During precipitation within a catchments for a given duration, storms of varied depths and volumes are generated. These are channeled into drains of varied sizes to be moved downstream as proper management practices. Precipitated water that does not infiltrate into the ground or is intercepted by vegetation may become surface runoff (Ven Te Chow, 1988). Precipitation includes both liquid and solid water particles, such as rain, snow or hail. When stormwater is formed as a result of rainfall or snowmelt it runs off to a recipient. The land area whose water drains into a particular watercourse is called the catchment of the watercourse (Hendriks, 2010). Principally all rainwater that rains into a catchment, which does not evaporate or is absorbed by plants, will eventually drain into the same watercourse (SMHI, 2009).The size of the catchment affects how much water will be present in the watercourse. Emissions of pollutants in the catchment area will likely have an impact on the recipient. Three catchments are separated by a water divide and a catchment can be divided into sub-catchments (SMHI, 2013) as in this research work which considers three catchments or sub basins (Danquah, 2013). Stormwater modelling is very important as it results in proper management of stormwater to avoid contingencies and havocs within a catchment area. Modelling stormwater within catchments helps forecast and predict the nature and what will happen when runoffs are generated after a severe runoff within the catchment. Such modellings allows storms to be channeled into watercourses or to stormwater treatment facilities for safe storage and treatment before use again. Good stormwater treatment facilities from well designed and constructed hydraulic structures by engineers in developed countries have served the purpose of safe stormwater channeling into safe systems.
2.2 Stormwater management and importance
Stormwater management within catchments especially in well urbanized areas is very important in avoidance of future contingencies. Stormwater management and channeling by drains and conduits into watercourses and stormwater treatment facilities is very important to avoid flooding and making the environment dirty and full of debris after rainstorms (Danquah, 2013). Development of natural areas and densification of cities increases the percentage of impervious surfaces and at the same time vegetation and permeable surfaces decrease which affect the natural water balance (Svenskt Vatten AB, 2004). In forested areas, often only a few percent of the precipitation becomes surface runoff, while in an urban area with a high proportion of impervious surfaces the surface runoff can be up to 80-90% of the precipitation (Adielsson, 2012). When rain falls over an urban area large amounts of stormwater are formed and the runoff occurs faster than for example in a forested area (Svenskt vatten AB, 2011a) since the area is well urbanized. When developing or redeveloping an area it is therefore important for land use planners to investigate how the change might affect the stormwater flows and the stormwater quantity (Lind, 2015). An assessment can then be made regarding the capacity of the existing stormwater system and if the system is in need of an expansion or modification. The amount of pollutants on the surfaces that the stormwater possesses during the runoff will determine how much pollution the water will contain, and different surfaces can contain different amounts and types of pollutants (Lind, 2015). For example, a small country road with a small amount of traffic will contribute with less pollution than a highway with heavy traffic (Persson, et al., 2009). It is therefore also important to study how changes in land use in an area may affect the amount of pollutants in the stormwater (Lind, 2015).
2.3 Storm Water Management Model (SWMM)
Increases in urbanization and conversion of land for urban use has resulted in increased areas of impervious surfaces and, as a consequence, increases in runoff volumes and peak flow rates (Chunlin et. al., 2014). Studies have shown that urban stormwater runoff contains a variety of pollutants such as sediment, organic material, microorganisms, nutrients, and heavy metals (Selvakumar et. al., 2006; Krometis et. al., 2009; Yang et. al., 2015) which can all seriously impact public health and threaten environmental quality (Rauch et. al., 2012). In urban areas, pollutants accumulate on urban surfaces and are then washed off by stormwater during wet weather (Zhang et. al., 2014). Urban runoff pollution problems are more difficult to control than steady-state point discharges because of intermittent and unpredictable rainfall and runoff, the large variety of pollutants involved, and the complex environmental setting (Characklis et. al., 1997; German et. al., 2002; Vaze et. al., 2004; Walsh et. al., 2012). Regular monitoring studies cannot provide sufficient data to support urban non-point pollution research. Because of the increased need to control non-point pollution and to reuse stormwater, hydrological models that can provide a thorough understanding of the basic hydrological and hydraulic processes are increasingly applied in urban settings (Peterson et. al., 2006). Field observation data have played an important role in developing urban hydrological models (Ouyang et. al., 2012). Since it was developed by the U.S. Environmental Protection Agency, the Storm Water Management Model (SWMM) has become one of the most widely used rainfall runoff models for simulating hydrological processes and water quality in urban areas (Barco et. al., 2008; Shorshani et. al., 2014; Walsh et. al., 2014). SWMM has been applied to all types of stormwater management – from urban drainage (Alias, 2014) to flood routing (Hsu, 2000).
2.3.1 Storm Water Management Model description
SWMM is a comprehensive hydrological and water quality simulation model used for single or continuous events of runoff in urban areas (Rossman, 2005). SWMM comprises four computational blocks, namely RUNOFF, STORAGE/ TREATMENT, TRANSPORT and EXTRAN. Hydrograph and pollutograph are generated by the RUNOFF block (Chow et. al, 2012). The basic input parameters required to simulate hydrograph are rainfall hyetograph and the sub catchments physical characteristics. In this analysis, the kinematic wave routing method with 5-min time steps was used for calculating runoff transport (Chow et. al., 2012). The infiltration loss on pervious area was estimated by Horton equation because of the availability of soil data. Pollutograph is generated by RUNOFF block based on the volume of storm runoff and catchment antecedent conditions (i.e. dry weather days, street sweeping data and land use). The stormwater pollutant loading is predicted based on the mechanism of build-up and wash-off processes (Chow et. al., 2012) For a given constituent, build-up can be computed either as a fraction of dust and dirt accumulation, or areal accumulation. The areal accumulation described by mass loading/curb length/dry day (kg/km/day) is used in most studies. The exponential build-up equation is used to simulate surface accumulation of constituent and the exponential wash-off equation for simulating the wash-off process (Chow et. al., 2012).
2.4 MIDUSS version 2.25
MIDUSS is the software obtained for the modelling of storm water within three catchments in this research work. MIDUSS is windows-based software that helps one to engineer complex drainage networks to convey flow hydrographs from single event storms. MIDUSS allows one to;
· Generate hydrographs using a variety of built-in hydrological models
· Design a network of pipes, channels, culverts, ponds, infiltration trenches, cascading pipes, diversions – all with preliminary sizing done automatically and flood routing where appropriate
· Use interactive design and immediate feedback to quickly optimize each element as you progress downstream
· Use automatic mode to analyze and adjust your complete design under more severe storms
· Concentrate on engineering; robust error detection and step-by-step prompts are provided throughout
· Import/export hyetographs or hydrographs at virtually any stage
MIDUSS is a toolkit that brings together many accepted hydrologic models and hydraulic design methodologies into one software package. MIDUSS is a balance between hydrologic simulation and drainage design engineering. It includes a lot of hydrologic models (Storm, runoff and infiltration) and it excels at helping one to design the best drainage network to convey your hydrographs (www.miduss.com).
2.5 Stormwater development from Rainfall
When designing stormwater systems the return period, the duration and the rain intensity are of importance in order to design a better stormwater system. The return period is how often a given rainfall, with a certain intensity and duration, recur (Lind, 2015). The return period of a rainfall is usually referred to as 1, 10, 50 or 100-year rainfall. A 10-year rainfall returns on average 1 time in 10 years and a 100 - year rainfall returns 1 time/100 year (Lind, 2015). This rainfall happens to be the highest rainfall likely to be experienced once a life time within the stipulated period. To select a suitable return period when designing a conduit, consideration need to be taken regarding to if the area in question is enclosed or not and if it is located inside city settlements. An enclosed area is an area where the water cannot be diverted by gravity (Lind, 2015). The duration of the rain is how long the rain lasts, and is usually expressed in number of minutes. Rainfall intensity shows how much it rains per unit time and area, and is expressed in l / s ∙ ha (Svenskt Vatten AB, 2004). The following types of rainfall data can be used for design and modelling of stormwater systems as can be seen in this modelling research.
• Historical rainfalls and rainfall series from precipitation measurements. Rainfall measurements can be made with a weighty rain gauge or a tipping bucket rain gauge. Historical rainfalls can be data for a single-event rainfall or it can be long-term continuous rainfall series (Lind, 2015).
• Uniform time distribution rainfall. Such rainfalls are the maximum average intensity for a single rainfall during a given duration (Lind, 2015).
• Design Storms. Design storms are special rainfalls used in the analysis and design of sewer systems. The flow that are linked to a certain design storm is assumed to have the same return period as the storm (Lind, 2015).
3 STORMWATER MODELLING USING MIDUSS V2.25
3.1 Catchment Analysis
Catchment is the land area whose water drains into a particular watercourse. With three catchment consideration, it implies the draining of all runoffs after stormwater within the catchment into another and final discharge into watercourses. Runoffs are always generated after a given rainfall duration where there is initial abstraction or infiltration on pervious surfaces and final runoff build up after saturation.
Fig. 1: Rainfall intensity over the catchment
A 100min time duration rainfall intensity was considered over the three catchment areas to access runoff generation, runoff volume and depth. In analyzing catchment three as shown in Fig 2 below, the total catchment area for the impervious areas is 0.7ha and pervious surface area is 2.89ha. The time of concentration for impervious and pervious areas is 26.857min and 3.823min respectively. Time of concentration is the maximum time taken for a raindrop that falls farthest away in a catchment area to drain to the outlet point. The time of concentration depends on the distance and the water velocity. Time of concentration analysis would have given a nice results if consideration is for the same catchment area for both impervious and pervious surfaces. The real time catchment areas considered for this modelling is very large as compared to impervious areas in communities in Ghana. Most of the pervious areas are compacted surfaces with stones coverage where there will be initial abstraction after precipitation before runoff generation into drains, conduits and water courses. The other pervious areas within catchment 3 and the other two catchment areas are vegetation’s comprising of plants and different species of crops. In such areas, there is high rate of infiltration after precipitation and runoff generation is around 5% which even end up being infiltrated into the soil. Therefore as can be seen in Fig. 2, the catchment area for the pervious surfaces is 2.8ha. The impervious area is 0.7ha during this modelling and simulation which is a real time scenario for most areas and communities in Ghana. These areas are usually made of concrete pavement, compacted clay areas where infiltration is 0% and 3% respectively after stormwater over the catchment. The total surface area over catchment 3 is 3.5ha as seen in Fig. 2 for this modelling. Total runoff generation after stormwater is dependent on catchment area characteristics. The maximum intensity over a 100min rainfall duration is 113.686mm/hr as depicted in Fig 1. Runoff generation in catchment areas for pervious areas in most communities in Ghana is very slow or not at all as compared to impervious areas. The southern part of Ghana as a country is forested and full of vegetation as compared to countries like Dubai. Hence one do not see much runoff over catchment areas during precipitations as compared to the western world countries like Germany, UK, Canada and USA. There are no stormwater treatment facilities to treat storm waters. Even the drains to take such storm waters downstream are not in good shapes and of inadequate capacities. And if of adequate capacities, full of debris and sand hence resulting in settling of stormwater in drains.
Fig. 2: Catchment three analysis
3.2 Runoff generation on pervious and impervious surfaces
The three catchment considered for this modelling comprises of pervious and impervious areas. In considering catchment 3 alone, the pervious and impervious areas are 2.8ha and 0.7ha respectively as depicted in Fig 2. This is comparable to real life catchment areas situation in most communities in Ghana. The pervious areas are very large and impervious areas very small comprising of concretes, cemented areas, roads with drains and compacted clay areas. The pervious areas comprises of vegetated areas, bare soil grounds which experiences initial abstraction before runoff generation. Runoff generation occurs on both pervious and impervious areas within the catchment areas under consideration. Fig 3 gives a clear indication of the runoff generation over the impervious and pervious areas within the catchment areas during the modelling. In sampling a runoff of 0.1714 over catchment 3 as in Fig 3, 0.1496 and 0.02174 runoff was generated over the impervious and pervious areas respectively. Runoff generated during a storm over a catchment on impervious areas end up collected and channeled into drains and conduits. Most drains on roads in communities in Ghana have been there for ages and hence in bad shapes. Newly constructed roads have drains filled with sand, debris, standing water breeding mosquitoes after bad engineering works and construction. Runoffs ending in such drains end up on roads. On a daily stormwater over a catchment analysis, one can see waste materials, debris, sand on roads and in the environment after runoff generation hence making the environment and roads unattractive after precipitations. In communities in Ghana, almost all runoffs generated ends up in watercourse or in water bodies. Observation on a daily stormwater and runoff generation into a water body or river will give you an interesting results in most communities in Ghana. One can see, plastic waste, sand, debris, food waste all swimming through the water downstream. This makes the water unattractive and unhealthy for human consumption and usage. Runoff generated by pervious areas end up been infiltrated into the soil and little runoffs generated after saturation. Vegetated areas do not experience runoffs but high rate of infiltration and evapotranspiration after a given storm.
Fig 3: Runoff generation over the pervious and impervious areas
3.3 Runoff volume and depth
Runoff volume and depth depends on stormwater and runoff generation for a given duration over a catchment area. For a given runoff volume and depth, catchment area characteristics such as imperviousness or perviousness and hydraulic structures will contribute to runoff generation hence volume and depth. Fig 2 above gives a clear indication of the runoff volume and depth over the pervious and impervious areas after 100min rainfall duration of which the maximum rainfall intensity is 113.868mm/hr. After the modeling, the runoff volume for pervious and impervious surface is 268.92m3 and 239.17m3 respectively. Values of 9.606mm and 34.168mm are the runoff depths for pervious and impervious surfaces over areas of 2.8ha and 0.7ha.
Fig 4: Rainfall nature on pervious and impervious surfaces
Runoff generation in most communities and cities in Ghana are comparable to what is happening in other developed countries but channeling downstream through drains and conduits is what is different. Drain construction in Ghana by Ghanaian engineers isn’t perfectly done as most drains end up with inadequate capacity and full of debris, sand and waste materials. So such drains are unable to carry runoffs from drains to downstream and into watercourses. Most areas in communities in Ghana are vegetated and pervious with little impervious areas. This results in more infiltration and evapotranspiration as compared to runoff generation within a catchment as considered in the three catchments in this modelling. Runoff generation on impervious areas after stormwater needs to be carried downstream by drains of adequate capacity if not flooding can occur. Rainfall intensity and duration over most communities are small hence drains and hydraulic structures are able to contain and carry stormwater downstream and into watercourses. Return period is very important during design and construction of hydraulic structures. This is the why flooding and bad rainfall situations are rare in life as it occurs once in a life time. Accra does experiences heavy rainfall during the June July rainfall in Ghana where flooding mostly occurs. Accra for instance have bad housing siting system, bad drainage system and drains full of waste materials, sand, plastic materials etc. All these materials reduces the drain capacity resulting in high runoff volume resulting in overland flows. Decrease in depth in drains which is a rare scenario in developed countries. This is why Accra in Ghana for instance experiences yearly flooding resulting in the loss of lives and properties worth thousands of cedis, dollars and pound sterling’s.
3.4 impacts of stormwater on impervious and pervious surfaces
Poor environment, impervious areas and hydraulic structures results in unhealthy generated runoffs into drains and watercourses. Most communities in Ghana have unhealthy and untidy environment which results in runoffs full of waste materials and plastic waste ending in rivers and streams after downpours. This reduces the beauty and aesthetic nature of the river making it unhealthy for drinking and meeting domestic water demands. Stormwater resulting in runoffs on impervious areas makes the environment untidy and unhealthy each time there is precipitation over a catchment area in most communities in Ghana. Stormwater on pervious areas is not much seen in most communities in Ghana as most areas are vegetated and small in imperviousness. Catchment areas for modelling are good vegetated land areas with high rate of abstraction and infiltration. Evapotranspiration also reduces the amount of stormwater reaching land to result in runoff generation. Since most communities in Ghana are full of vegetation and forest zones, interception of rainwater by plant leaves reduces the amount of water reaching land for infiltration or abstraction and further runoff generation after saturation. With this in mind, one expect runoff generation from a given stormwater over the same catchment area for both pervious and impervious areas not to be the same. Evapotranspiration will reduce the amount of generated runoff over the pervious area from stormwater as compared to impervious areas. Since impervious areas are free areas comprising of parking lots, concrete areas, compacted sands etc hence no resultant evapotranspiration from plants and vegetated areas. There is no or little runoff generation over pervious areas due to high rate of abstraction and infiltration over the catchment area.
4 Conclusion
Modelling has become an important tool to simulate into what can become reality and predict for the future. MIDUSS V2.25 hence becomes an important tool to model and simulate stormwater and runoff generation characteristics within a catchment. Modelling gave a beautiful results using MIDUSS after analyzing 100min rainfall within three catchment areas. The 100min rainfall gave the maximum rainfall intensity to be 113.686mm/hr. This generated a runoff volume for pervious and impervious surface to be 268.92m3 and 239.17m3 respectively. Values of 9.606mm and 34.168mm are the runoff depths for pervious and impervious surfaces over areas of 2.8ha and 0.7ha. Modelling and sampling communities in Ghana to access and analyze stormwater over catchment areas indicated that generated runoffs end up in drains of inadequate capacity. Some filled with sands, debris, bushes, plastic hence reducing drain capacity. This results in runoff ending on roads, car parks, pavements and important areas full of debris, plastic waste, food waste, human waste etc. It also leads to flooding in cities especially during rainy season and rainfall of high intensity over the catchment areas such as in areas like Accra in Ghana. Runoffs of high velocities are able to move such waste materials into streams and rivers distorting the beauty and aesthetics of the river and high rate of contamination hence unable to meet water demand. It therefore deems important to construct drains and conduits of adequate capacity to carry all stormwater downstream and into water courses safely. It is again of importance to design and construct stormwater treatment facilities to treat and recycle waste water and channeled back into the system for good usage such as in watering lawns and gardens.
Acknowledgment
I thank the Almighty God for the completion of this modelled research work successfully. God bless all those who contributed to the success of this work. God bless you all.
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