Protection of downstream channels and reduction in flooding can potentially be improved by evaluating alternative site stormwater management (SWM) strategies at a watershed scale and selecting the optimal strategy for a subject watershed. Tailoring a management strategy for a specific watershed may be worthwhile to minimize development costs and maximize downstream benefit. A hydrologic/hydraulic model for a watershed in Blacksburg, Virginia, is used to evaluate downstream results based on implementation of several alternative SWM strategies currently practiced within the United States. Results show none of the strategies meet the goal of maintaining the baseline goal at the watershed POI for the full range of design storms. Modification to the strategy that performs best at the watershed scale did meet the watershed goal for all design storms except the 1-year. For smaller storm events, it appears that increasing the volume of an initial capture and the drawdown time to release that volume does not increase performance downstream. This is potentially significant as extra dollars spent on site would not provide extra benefit downstream. When post-development peak runoff rates are detained to the predevelopment rate for larger storm events, whether based on a site or watershed focused strategy, the watershed goal can be met. A volume reduction strategy performs well, but implementation is hindered by soils with poor infiltration and the presence of karst. Other insight to watershed based management strategies, the role of regional facilities and predevelopment condition assumptions at the site scale to maintain a baseline condition downstream are discussed. / Master of Science

This dissertation develops a new distributed split-flow stormwater management strategy and compares its site design feasibility and construction cost to existing stormwater management methods. The purpose of the split-flow strategy is to manage stormwater by preserving predevelopment flows in terms of rate, quality, frequency, duration and volume. This strategy emulates the predevelopment hydrology: it retains and infiltrates additional runoff volume created by development by using bioretention and paired weirs as proportional flow splitters connected to small infiltration facilities distributed throughout a site. Results show that 1) the distributed split-flow stormwater management strategy can provide a higher level of environmental protection at comparable construction cost to existing detention-based methods, 2) split-flow systems are less expensive to construct than current truncated hydrograph-based bioretention and infiltration systems and 3) non-point source water pollution-reduction objectives, currently achieved with either detention with first flush or comparable bioretention and infiltration systems, could be achieved in a more cost-effective manner using distributed split-flow stormwater management strategy. / Ph. D.

Watershed-based stormwater management (WSM) has been proposed as more effective for stormwater management than traditional methods of controlling stormwater, which are carried out based on jurisdictional lines at the parcel-scale. Because WSM considers the watershed as a total unit, this method is considered to be more effective in reducing problems associated with stormwater management including environmental degradation and flooding. However, larger watersheds encompass smaller watersheds, and therefore WSM can be implemented at a wide range of scales. There has been little research on what scale is most appropriate, and more specifically, only a modest amount of work has taken stakeholder opinion into account. The specific objectives of this study are to determine: 1) if watershed scale is an important factor in WSM, 2) whether stakeholder opinion has an effect on the appropriate scale used in WSM, and 3) what scale is most appropriate for WSM, if scale is an important factor. To meet these objectives, we delineated sub-watersheds within a watershed in southwestern Virginia, surveyed stakeholders within the watershed on their opinions of stormwater management methods, and compared the results at both watershed scales using statistical tests and decisions support software. The results of this study have important implications for geographic scale in WSM as well as the use of qualitative data in determining appropriate geographic scale in matters of implementation in the field of planning. / Master of Science

The responsibility for mitigation of the ecological effects of urban stormwater runoff has been delegated to local government authorities through the Clean Water Act's National Pollutant Discharge Elimination Systems' Stormwater (NPDES SW), and Total Maximum Daily Load (TMDL) programs. These programs require that regulated entities reduce the discharge of pollutants from their storm drain systems to the "maximum extent practicable" (MEP), using a combination of structural and non-structural stormwater treatment — known as stormwater control measures (SCMs). The MEP regulatory paradigm acknowledges that there is empirical uncertainty regarding SCM pollutant reduction capacity, but that by monitoring, evaluation, and learning, this uncertainty can be reduced with time. The objective of this dissertation is to demonstrate the existing sources and magnitude of variability and uncertainty associated with the use of structural and non-structural SCMs towards the MEP goal, and to examine the extent to which the MEP paradigm of iterative implementation, monitoring, and learning is manifest in the current outcomes of the paradigm in Virginia. To do this, three research objectives were fulfilled. First, the non-structural SCMs employed in Virginia in response to the second phase of the NPDES SW program were catalogued, and the variability in what is considered a "compliant" stormwater program was evaluated. Next, the uncertainty of several commonly used stormwater flow measurement devices were quantified in the laboratory and field, and the importance of this uncertainty for regulatory compliance was discussed. Finally, the third research objective quantified the uncertainty associated with structural SCMs, as a result of measurement error and environmental stochasticity. The impacts of this uncertainty are discussed in the context of the large number of structural SCMs prescribed in TMDL Implementation Plans. The outcomes of this dissertation emphasize the challenge that empirical uncertainty creates for cost-effective spending of local resources on flood control and water quality improvements, while successfully complying with regulatory requirements. The MEP paradigm acknowledged this challenge, and while the findings of this dissertation confirm the flexibility of the MEP paradigm, they suggest that the resulting magnitude of SCM implementation has outpaced the ability to measure and functionally define SCM pollutant removal performance. This gap between implementation, monitoring, and improvement is discussed, and several potential paths forward are suggested. / Ph. D. / Responsibility for mitigation of the ecological effects of urban stormwater runoff has largely been delegated to local government authorities through several Clean Water Act programs, which require that regulated entities reduce the discharge of pollutants from their storm drain systems to the “maximum extent practicable” (MEP). The existing definition of MEP requires a combination of structural and non-structural stormwater treatment – known as stormwater control measures (SCMs). The regulations acknowledge that there is uncertainty regarding the ability of SCMs to reduce pollution, but suggest that this uncertainty can be reduced over time, by monitoring and evaluation of SCMs. The objective of this dissertation is to demonstrate the existing sources and magnitude of variability and uncertainty associated with the use of structural and non-structural SCMs towards the MEP goal, and to examine the extent to which the MEP paradigm of implementation, monitoring, and learning appears in the current outcomes of the paradigm in Virginia. To do this, three research objectives were fulfilled. First, the non-structural SCMs employed in Virginia were catalogued, and the variability in what is considered a “compliant” stormwater program was evaluated. Next, the uncertainty of several commonly used stormwater flow measurement devices were quantified in the laboratory and field, and the importance of this uncertainty for regulatory compliance was discussed. Finally, the third research objective quantified the uncertainty associated with structural SCMs, as a result of measurement error and environmental variability. The impacts of this uncertainty are discussed in the context of the large number of structural SCMs prescribed by Clean Water Act programs. The outcomes of this dissertation emphasize the challenge that uncertainty creates for cost-effective spending of local resources on flood control and water quality improvements, while successfully complying with regulatory requirements. The MEP paradigm acknowledged this challenge, and while the findings of this dissertation confirm the flexibility of the MEP paradigm, they suggest that the resulting magnitude of SCM implementation has outpaced the ability to measure and functionally define SCM pollutant removal performance. This gap between implementation, monitoring, and improvement is discussed, and several potential paths forward are suggested.

Purpose: Climate change and increased proportion of hard surfaces due to urbanization is causing problems with flooding. Although it has been known for a long time that traditional stormwater management needs to be complemented, progress towards Sustainable Drainage Systems, SuDS, is slow. Techniques to locally handle storm water are available for most situations, but there is a lack of knowledge and experience. The aim of this thesis is to present a proposal on how implementation of SuDS can be promoted in urban development projects in Jönköping municipality. Method: Using literature review, interviews, document analysis and observation a qualitative study was conducted in two urban development projects; Ekostaden Augustenborg in Malmö and Munksjöstaden in Jönköping. Findings: The majority of the stormwater management techniques available in Augustenborg have also been discussed in Munksjöstaden by Jönköping municipality. Only one third of the techniques which has been discussed will be realized. Jönköping municipality lacks clear goals and a vision that is integrated into the planning process, which can explain this. During the interviews the site conditions were presented as arguments to why SuDS is difficult to implement. However the analysis showed that it is the other conditions that are decisive, something that also the scientific studies indicated. In order to promote the implementation of SuDS general and project-specific actions were elaborated. Implications: The local plan is the municipality’s sharpest tool for control of the physical development and demands must be submitted in the local plan in order to promote SuDS. The municipality needs to reprocess a practice level and equate SuDS with traditional stormwater management in order to make relevant demands on developers. To promote the implementation of SuDS, the authors suggests that Jönköping municipality carry out a pilot project focusing on sustainable stormwater management in an upcoming urban development project. Limitations: Since the thesis only includes two projects, there is a limitation of the conditions treated. Since the other conditions and the recommended actions are based on the factors identified in a national survey, the applicability is considered to be good. Although the project-specific actions are based on Jönköping municipality, it is possible for other municipalities to apply the PDSA-wheel to the extent that is suitable for the municipality. / Syfte: Klimatförändring och urbanisering med ökad andel hårdgjorda ytor orsakar problem med översvämningar. Trots att det sedan länge är känt att den traditionella dagvattenhanteringen behöver kompletteras går utvecklingen mot Sustainable Drainage Systems, SuDS, långsamt. Tekniker för att ta hand om dagvattnet lokalt finns för de flesta situationer men det saknas kunskap och erfarenhet. Målet med arbetet är därför att presentera ett förslag på hur implementering av SuDS kan främjas i stadsbyggnads-projekt i Jönköpings kommun. Metod: Med hjälp av litteraturstudie, intervjuer, dokumentanalys och observation har en kvalitativ studie genomförts på två stadsbyggnadsprojekt; Ekostaden Augustenborg i Malmö och Munksjöstaden i Jönköping. Resultat: Majoriteten av de tekniker för dagvattenhantering som finns i Augustenborg har diskuterats även i Munksjöstaden från Jönköpings kommuns sida. Det visade sig dock att endast en tredjedel av det som diskuterats kommer att förverkligas. Detta kan bland annat förklaras av att Jönköpings kommun saknar tydliga mål och en vision som är integrerad i planeringsprocessen. Vid intervjuerna lyftes platsen förutsättningar fram som argument till varför SuDS är svårt att tillämpa. Analysen visade dock att det är de övriga förutsättningarna som är avgörande, något som även de vetenskapliga studierna pekat på. För att främja implementeringen av SuDS har därför generella och projektspecifika åtgärder riktade mot de övriga förutsättningarna utarbetats. Konsekvenser: Detaljplanen är kommunens skarpaste verktyg för att styra den fysiska bebyggelsen och för att främja SuDS är det därför viktigt att det finns krav i detaljplanen. Kommunen behöver upparbeta en praxisnivå och likställa SuDS med traditionell dagvattenhantering för att kunna ställa relevanta krav på exploatörer. För att främja implementeringen är författarnas förslag att Jönköpings kommun genomför ett pilotprojekt med fokus på hållbara dagvattenlösningar i ett kommande stadsbyggnadsprojekt. Begränsningar: Eftersom arbetet endast innefattar två projekt finns det en begränsning i vilka förutsättningar som behandlats. Eftersom de övriga förutsättningarna och de rekommenderade åtgärderna utgår från faktorer som identifierats i en nationell enkät-undersökning bedöms ändå tillämpligheten vara god. Även om de projektspecifika åtgärderna utgår från Jönköpings kommun, är det möjligt för andra kommuner att tillämpa PGSA-hjulet i den omfattning som passar den aktuella kommunen.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: In the context of rapidly expanding cities, it is imperative that urban planning in South Africa has sufficient guidance regarding stormwater and river corridor management, in order to provide solutions that address issues of flood risk and the environmental health of river systems. Attenuation of stormwater runoff, the focus of this study, is one of the most important structural mechanisms used for the mitigation of many of the negative impacts caused by uncontrolled urban runoff. Typically, it involves the use of attenuation ponds or wetlands, which temporarily store runoff during a storm and release flow downstream at a reduced rate so as to mimic natural flow patterns. The focus of urban stormwater management and flood control has historically been on the protection of human life and property. However, in recent decades, through growing environmental awareness and the advancement of the concept of sustainable development, urban stormwater management has become a growing field of research worldwide, with a broader focus which considers not only flood control, but also water quality, aquatic biodiversity and the amenity value of urban drainage systems. Flood attenuation controls are becoming more widely used within South African urban areas, primarily due to policies or legislation brought into effect by local authorities. However, there is often little understanding regarding the positive and perhaps negative effects that these attenuation controls are having on receiving watercourses downstream. Three case studies were assessed by means of stormwater modelling simulations to evaluate various flood attenuation practices which are currently in use in South Africa. Two of the study areas, the Mosselbank River Catchment and the Bayside Canal Catchment, were selected in areas of Cape Town where future development has been proposed by spatial planners. The third study area, the Upper Kuils River Catchment, was evaluated in terms of the performance of existing attenuation facilities in an area which is already almost completely developed. The study found that attenuation facilities constructed with a single culvert-type outlet structure, designed to reduce flows during large storm events, do not mitigate the impact of post-development runoff occurring during lower recurrence interval storm events. Attenuation facilities with multi-stage outlet structures were found to be much more effective at mimicking pre-development flow during a range of storm events. It was also found that because attenuation does not reduce post-development runoff volumes to pre-development levels, but merely reduces peak flow rates, the cumulative runoff from multiple attenuation controls across a large (>30 km2) urban catchment resulted in higher runoff peaks in downstream watercourses. The study concluded that more widespread use of stormwater Best Management Practices (BMPs) and Sustainable Drainage System (SuDS) controls allows a greater portion of runoff to infiltrate, resulting in less runoff volume and therefore reduced peak flows downstream, especially during low recurrence interval storm events. In addition, the study recommended the use of detailed catchment-wide stormwater modelling to understand specific catchment dynamics holistically, thus increasing the potential for designing effective attenuation controls in urban stormwater systems. / AFRIKAANSE OPSOMMING: In die konteks van die vinnige tempo van stedelike uitbreiding, is dit noodsaaklik dat stedelike beplanning in Suid-Afrika plaasvind met in aggenome van voldoende riglyne vir die bestuur van stormwater en rivierkorridors, ten einde oplossings te vind vir die kwessies van vloedrisiko en die omgewingsgesondheid van rivierstelsels. Vloedvertraging, wat die fokus van hierdie studie is, is een van die belangrikste strukturele meganismes wat gebruik word vir die verligting van talle negatiewe impakte wat veroorsaak word deur onbeheerde stormwaterafloop in stedelike gebiede. Tipies behels dit die gebruik van vloedvertragingsdamme of vleilande, wat afloop vertraag tydens 'n storm en dus vloei stroom-af teen 'n verlaagde tempo uitlaat met die doel om natuurlike vloeipatrone na te boots. Die fokus van stedelike stormwaterbestuur en vloedbeheer was in die verlede hoofsaaklik op die beskerming van lewe en eiendom gefokus, maar het egter die afgelope dekades verskuif na water gehalte, die biodiversiteit van waterekosisteme en die geriefswaarde van stedelike dreineringstelsels. Hierdie verskuiwing van fokus is weens die groeiende omgewingsbewustheid en die bevordering van die konsep van volhoubare ontwikkeling wat wêreldwyd 'n groter navorsingsgebied geraak het. Vloedvertraging beheermeganismes word al hoe meer algemeen gebruik in Suid-Afrikaanse stedelike gebiede, hoofsaaklik as gevolg van die beleide of wetgewing wat deur plaaslike owerhede in werking gestel is. Daar is egter dikwels min begrip vir die positiewe en moontlike negatiewe gevolge wat hierdie vertragingsmeganismes op stroom-af sisteme het. Drie gevallestudies is geëvalueer deur middel van numeriese modelstudies wat verskeie benaderings van vloed beheer, wat tans in Suid-Afrika gebruik is, in ag neem. Twee van die studie areas, naamlik die Mosselbank en die Bayside-kanaal opvanggebiede in die Kaapse metropool, is gekies in areas waar toekomstige ontwikkeling in die vooruitsig gestel is deur stadsbeplanners. Die derde studie area, die opvangsgebied van die bolope van die Kuilsrivier, is in terme van die prestasie van bestaande stormwater infrastruktuur in 'n gebied wat reeds byna heeltemal ontwikkel is, geëvalueer. Die studie het bevind dat vloedvertragingsfasiliteite met 'n enkele duiker uitlaatstruktuur, wat ontwerp is met die doel om die vloeispitse tydens groot storms te demp, nie die impak van die na-ontwikkeling afloop, wat gedurende storms met laer herhalingsinterval voorkom, verminder nie. In terme van vloedvertragingsfasiliteite met 'n veelvuldige uitlaatstruktuur, is dit bevind dat voorontwikkelingsafloop tydens 'n reeks van groot en kleiner storms veel meer effektief nageboots word. Daar is egter ook bevind dat die demping van die vloedspitse nie die naontwikkeling afloopvolumes verminder tot voorontwikkelingsvlakke nie, maar slegs tot die vermindering van maksimum snelhede lei. Die gevolg is dat die totale afloop van ‘n kombinasie van ‘n aantal vertragingsdamme oor 'n groot (> 30 km2) stedelike opvanggebied ‘n hoër spitsvloei in die stroom-af riviere tot gevolg het. Die studie het bevind dat die wydverspreide gebruik van bestebestuurspraktyke (BMPs) en volhoubare stedelike dreineringstelsels (SuDS) tot die infiltrasie van ‘n groter gedeelte van die afloop lei, wat laer afloopvolume en dus verminderde spitsvloei stroomaf tot gevolg het, veral gedurende storms met ‘n lae herhalingsinterval. Daarbenewens word die aanwending van gedetailleerde modellering van stormwatersisteme binne die groter opvangsgebied aanbeveel ten einde ‘n meer holistiese begrip van spesifieke aspekte van die opvangegebied dinamika, om sodoende die potensiaal vir die ontwerp van effektiewe vloedvertragingskontroles in stedelike stormwaterstelsels te verbeter.

This paper serves as a “kicking-the-tires” analysis of low impact developments as a method of stormwater management. Specifically, this paper examines the feasibility, benefit, and current practice of low impact developments in Austin, Texas. Merits, strengths, and weakness are comparatively determined primarily on the basis of the impact and efficiency of design, particularly relating to ability to handle water volume and potential to improve water quality. By examining case studies and “applied” examples the potential of low impact development application is considered for the expected, potential, and/or alleged benefits of low impact implementation. / text

Master of Landscape Architecture / Department of Landscape Architecture/Regional and Community Planning / Jason Brody / The way America thinks about and develops with water is not sustainable (Mouritz et. al. 2003). These thoughts and actions embody a paradigm that does not value ecological functions necessary to maintain water quality and quantity for future generations (Ahern et. al. 2010). Linear water infrastructure systems of collect, treat and convey lead to issues of flooding and contamination. These systems are reaching the end of their life span in American cities. Instead of replacing them using the current development approach, which treats water as a nuisance, this study argues for a new approach, developing with water as a resource; water-centric development. People have different perceptions regarding water resources and sustainability (Pahl-Wostl et. al. 2007). These perceptions affect the acceptance and support of public projects. Commonly, these perceptions are based upon people’s personal values and the immediate benefits they reap from the project. In order for communities to shift towards a water-centric development approach, demonstration projects must work to communicate the social value in the development’s hydrological functions (EPRI 2009). This project investigates emerging urban water management paradigms and synthesizes relevant knowledge to create a comprehensive new paradigm—New Urban Water Management (NUWM). This project focuses specifically on landscape architecture’s role in catalyzing the adoption of NUWM in Kansas City by applying the paradigm as a design approach to water-centric urban development. This approach employs environmental psychology strategies to append “Motivational Aspects” to the traditional social, ecological and economical aspects of sustainable development. The methodology provides the steps and tools for designers to apply the design approach. A three part design model of 1. Hydrologic Function 2. Social Amenity, and 3. Personal Relevance guide designers in developing water infrastructure systems as social amenities that objectively connect ecological functions with personal relevance. Washington Square Park in Kansas City, Missouri functions as a case study in the application of the design approach.

Increased urbanization leads to an increasing amount of impervious surfaces and a decrease ofthe natural hydrological function. Urban stormwater does thus risk to create high surface flows which could damage the receiving water bodies (e.g. erosion) or the urban area itself (flooding). Integrating more nature-based systems into the urban area increases the natural hydrological function and the risks for high surface flows are lowered. One way of implementing nature-based systems in the urban environment is to install green roofs. Most of the research and development done on green roofs have been focusing on the conditions of central Europe. Installing green roofs with the same vegetation in the subarctic climate of northern Sweden would expose it to a climate it might not be suited for, and growth would be limited. The vegetation helps increase the retaining and detaining capabilities of the green roofs and therefore the purpose of this thesis was to examine if planting native vegetation would help increase thegreen roofs performance in a subarctic climate. Conventional green roofs vegetated with sedum was hypothesized to have lessened retaining and detaining capabilities when placed in cold climates since the vegetation was exposed to a climate it probably was not suited for. It was examined whether planting more native vegetation could help increase green roofs performance. The vegetation was chosen based on Grime´s “universal adaptive strategy theory”, which describes competitors, stress tolerators and ruderals as three different vegetation groups with different survival strategies. Different species from each strategy were selected and planted on the roofs. There were five roofs per survival strategy and five roofs where all strategies were mixed. Conventional sedum vegetation was planted on five roofs to be able to compare green roofs performance. Five control roofs with substrate only and one reference roof made of steel were installed as well. In total, seven rainfall events were analyzed, and few significant differences could be found between the competitors, stress tolerators, ruderals and the vegetation mix. A conclusion is that stress tolerators may help to increase green roof performance the most, but due to the relatively short study period, continued measurements are recommended to draw further conclusions. The survival strategies did improve retention and detention relatively to using sedum vegetation and substrate only. The extent of vegetation coverage does not affect the retention or detention from the green roofs. The competitors, stress tolerators, ruderals and vegetation mix had larger plant mass than the sedum and the increased plant mass is probably the reason for their improved retention.

Stormwater impoundments are one of many types of best management practices (BMP) designed and implemented to regulate water quantity and improve the quality of runoff from urban areas. Studies of water quality in urban impoundments have indicated that conventional designs are however, not very effective at removing solids and associated pollutants. Accordingly, many urban impoundments are being re-designed to improve downstream water quality. However, few studies have systematically monitored and quantified post-design water quality improvements of urban impoundments. This thesis examines changes in the water quality performance of an urban impoundment (Columbia Lake) in Waterloo, Ontario resulting from redesign of the lake for the pre-design period (2003 and 2004) and the post-design period (2006 and 2007). To achieve this goal, four years of water quality data collected at the inlet and outlet of Columbia Lake as part of the Laurel Creek Monitoring Program was measured. Water chemistry parameters included total phosphorus (TP), soluble reactive phosphorus (SRP), suspended solids (SS), dissolved oxygen (DO), pH and total dissolved solids (TDS). Inlet and outlet discharge (Q) were measured to determine the water retention time in the lake. Concentrations and loads of TP and SS for the post-design period (2006 and 2007) were compared to those for the pre-design period (2003 and 2004). During the pre-design period (2003 and 2004), inflow TP concentrations ranged from 18 to 372 µg L-1 with an average (mean ± standard error) of 56±7 µg L-1, while outflow TP concentrations ranged from 37 to 266 µg L-1 with an average of 116±6 µg L-1. Post-design TP concentrations ranged from 10 to 124 µg L-1 with an average of 53±5 µg L-1 and from 14 to 147 µg L-1 with an average of 44±3 µg L-1 at the inflow and outflow, respectively. Pre-design SS concentrations ranged from 1.8 to 168.5 mg L-1 with a mean of 19.0±3.2 mg L-1 and from 4.0 to 194.7 mg L-1 with a mean of 66.6±4.7 mg L-1 at the inflow and outflow, respectively. Post-design SS concentrations varied from < 0.1 to 25.8 mg L-1 with an average of 8.5±0.8 mg L-1 and from < 0.1 to 42.5 mg L-1 with an average of 14.5±0.8 mg L-1 at the inflow and outflow, respectively. Sedimentation/resuspension dominated the TP and SS transfer via Columbia Lake. Pre-design TP loads (log-transformed) strongly correlated with SS loads at the inflow and outflow (r = 0.661 and 0.777, p = 0.0001). These parameters were more strongly correlated during the post-design period (r = 0.794 and 0.915, r = 0.0001), which indicates that particulate P (PP) was a dominant fraction of TP and that the release of dissolved phosphorus (DP) from bottom sediments was considerably decreased following the redesign. No significant difference was observed between inflow and outflow SRP concentrations. Discharge strongly affected TP and SS loads at the inflow and outflow during the pre- and post-design periods (r > 0.79, p = 0.000 for all). After the redesign of Columbia Lake, the average net internal P loading rate decreased from 198% to 22% for TP. The primary factor influencing the observed decreased post-design TP and SS outputs was the removal of sediment from the lake. Bottom sediment removal and changes to the lake bathymetry reduced sediment resuspension and P desorption, which decreased the average net internal SS loading rate from 828% to 154%. The Columbia Lake Water Quality Model developed by Stantec Consulting Ltd. (2004) underestimated the post-design outflow TP and SS concentrations mainly because it did not include terms that account for factors such as bioturbation, wave induced resuspension and biological activity.