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On the Design of Percolation Facilities Communication of the Department of Sanitary Engineering and VI/C'ltflr Management May 1996 Fr

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On the Design of Percolation Facilities Communication of the Department of Sanitary Engineering and VI/C'ltflr Management May 1996 Fr c:>soo ISSN 0169-6246 On the Design of Percolation Facilities Communication of the Department of Sanitary Engineering and VI/C'ltflr Management May 1996 fr. N.J. Monster I ir. M.J. Leeflang Ni Faculty of Civil Engineering and Geosciences Department of Water Management, Environmental and Sanitary Engineering TU Delft Land- and Water Management Section Delft University of Technology On the Design of Percolation Facilities Communication nr.: 66 M.J. Leetlang N.J. Monster Delft University of Technology May 1996 Faculty of Civil Engineering Department of Water Management, Environmental and Sanitary Engineering Contents Contents Contents Summary Samenvatting V 1. Introduction 2. Applicable disconnection methods and their design 2.1 Introduction 3 2.2 EvaluatiOn of the various disconnection methods 3 2.3 Evaluatton of the design procedures 3 2.4 Shortcomings and proposed improvements for the design procedure 5 2.5 Proposed disconnection methods 5 3. A design graph based on simulations 3.1 Introduction 7 3.2 The traditional Storage-Design Discharge-Frequency curve 7 3.3 Development of the new des1gn graph 10 4. The modelling of the percolation trench, calculations and results 4. 1 Introduction 13 4.2 The reservoir model 13 4.3 Four different scenarios for the percolation rate from the reservoir 15 4.4 Simulations with the four scenarios 1 7 4.5 S1mulations with other scenarios 19 4.6 A percolatiOn trench with a drain on the bottom 20 5. Design graphs percolation trench 23 6. The Mulden-Rigole system 6.1 Introduction 33 6.2 The model 33 6.3 The optimal use of the system 34 6.4 The design graphs of the Mulden-Rigole system 35 7. Discussion, Conclusions and Recommendations 7.1 Discussion 37 7.2 Conclusions 38 7. 3 Recommendations 38 References 41 The Destqn of lnflltrauon and Percolauon F;~cthltes Appendix A. Simulation of the precipitation-runoff process A.1 Introduction 43 A.2 RestrictiOns and Requtrements 43 A.3 The loss process 44 A 4 The delay process 46 A.5 The combtned loss and delay model 49 A.6 Output 50 A. 7 Runoff coefficient 52 Appendtx B. Extreme value analysis B. 1 Introduction 55 B.2 Theoretical background 55 B.3 The probabtlity dtstributton of partial duratton senes 55 B.4 The use of the parttal duration serres tn our model 56 Appendix C. Tundern 59 Appendtx D. The design of the Ruwenbos MR-system 65 Summary Summary The disconnection of impervious areas is a measure to reduce the overflow fre­ quency and hydraulic load of combined and improved separate sewer systems. Disconnection can be realized by guiding the storm water runoff to an mfiltrat1on and/or percolation facility. The des1gn of such facilities has been examined in this study. Design standards and criteria from various countries have been studied and com­ pared. This showed that the current des1gn procedures ( = the determination of the requ1red storage volume of a facility) exh1b1t a number of spurious assump­ tions. Examples of these are: an mcorrect use of Intensity-Duration-Frequency curves; neglect of the loss and delay processes which occur during storm water runoff; assumption that discharge from a facility is constant in time. The objective of this study is to remove the mentioned weak points from the current design procedures in order to design disconnection facilities in a more realistic way. A computer program has been written which simulates the infiltration-percolation process of a facility. The results of several simulations have been used for the construction of a design graph. Storage-Design Discharge-Frequency curves are often used in the present design procedures. If the discharge from the facility is considered as a dynamical variable, the construction of this graph is no longer possible. A new design graph has been developed in order to solve this problem. In this new graph the permeability of the soil (K-value) is plotted against the required construction length (L-value). scaled to the amount of connected impervious area. For each graph the width and depth of a disconnection facility are chosen beforehand. For a known K-value and amount of impervious area the required construction length can be assessed directly with this graph. A reservoir model was used to simulate the rainfall-percolation process. The input of the model exists of a data series of five-minutes-depths of precipitation over the period 1970 - 1984 as recorded in Lelystad. A separate loss and delay model IS applied to transform series of ra1n f1gures mto inflow figures. The calculated storage amounts are submitted to an extreme value analysis (part1al duration). To determine the most suitable simulation model for the design graph, various alternative simulation models have been tested. The results have been compared in order to analyse the influence of each alternative on the required dimensions of the facility. F1rstly, it turned out to be save to construct the design graphs using a clogged bottom scenario. Secondly, it is recommended to use inflow data as input, and to cons1der the percolation d1scharge from the facility as a dynamical variable. As the effect of 'consecutive storms' results in an increase of the re- iii The Oes•gn of lnflltrauon and Percotauon Fac•llt•es qu~red d1mens1ons, 1t IS recommended to mclude th•s effect m the s•mulations as well. This is done by the use of a continuous mflow series rather than applymg des1gn storms. Next. the effect of a discharge drain on the bottom of a percolatiOn trench is invest•gated. lt appeared that, compared to a fac•lity w•thout a dram, the reqUired dimens•ons of a facility with a drain decrease s•gnif•cantly. If the capac1ty of the drain increases, the permeability shows less influences on the requJred dimen­ Sions. Consequently so1ls w•th low permeability allow the applicatiOn of percola­ tion fac•hues, prov1ded a discharge drain is installed at the bottom of the facility. The Mulden-Rigole system is a combination of a percolation trench w1th a drain and a surface storage. Simulations with this facility have been performed in order to examine the optimum use of the storage above and beneath the surface. lt turned out that the storage above surface is decisive for the design. The perme­ ability of the surroundmg soil appeared to have no significant influence on the required dimensions. The construction of relevant design graphs (KL-graphs) is based on the results of the modelling as mentioned above. Various graphs have been constructed for the design of the following facilities each with different cross sect1onal dimensions: percolatiOn trenches w1th and without a dram. and the Mulden-Rigole system. These graphs are applicable to Dutch climate and soil cond•tions. provided the followmg aspects are considered. Firstly. the geo-hydrological aspects were s•mulated in a s1mphfied way. Secondly, the graphs are developed from a 'dis­ charge' point of v1ew. iv Samenvattmg Samenvatting Het afkoppelen van verharde oppervlakken is een maatregel om de overstortings­ frequentie van een gemengd en een verbeterd gescheiden rioolstelsel te reduceren en de hydraulische belasting van het rioolstelsel te beperken. Door het afstromen­ de regenwater te leiden naar een infiltratie- en/of percolatievoorziening - al dan niet voorzien van een drain onderin - kan de afkoppeling warden gerealiseerd. Het ontwerp en de dimensionering van dit soort voorzieningen is m dat onderzoek nader bekeken. In een literatuurstudie zijn ontwerpnormen en -regels uit binnen- en buitenland bestudeerd en vergeleken. Uit deze studie bleek dat de dimensioneringsprocedure ( = het bepalen van de benodigde berging van een voorziening) een aantal zwakke punten vertoont. In dit verband warden genoemd: Een onjuist gebruik van regen­ duurlijnen; het verwaarlozen van optredende verliezen en vertragingen bij het afstromingsproces; het constant veronderstellen van de afvoer uit de voorziening. Doel van dit onderzoek is om genoemde zwakke punten te ondervangen en zo tot een meer realistische dimensionenng van afkoppelingsvoorzieningen te komen. Daartoe is een computer programma geschreven waaran het neerslag-infiltratie­ percolatie proces van een voorzaening wordt gesamuleerd. Het programma is gebaseerd op een bakmodel. Als invoer wordt een 15-jaar lange reeks vtjf-minuten sommen van de neerslag gebruikt. Een apart verlies- en vertragingsmodel vormt de regencijfers om tot inloopgegevens. De berekende geborgen hoeveelheden warden onderworpen aan een extreme waarden analyse. Met de resultaten van verschillen­ de simulatie's wordt vervolgens een ontwerpgrafiek geconstrueerd. In de huidige procedures wordt veelal gebruik gemaakt van bergings-afvoer-grafie­ ken. Wordt de afvoer echter als dynamische variabele beschouwd. is de construe­ tie van deze grafiek niet meer mogelijk. Een nieuwe ontwerpgrafiek is ontwikkeld om dit probleem te verhelpen. In deze grafiek is de doorlatendheid van de bodem (K-waarde) uitgezet tegen de benodigde constructielengte (L-waarde) per hoeveel­ heid aangesloten verhard oppervlak. De breedte en diepte van een afkoppelings­ voorziening zijn voor iedere grafiek van te voren gekozen. Zijn de doorlatendheid en de hoeveelheid aan te sluiten verhard oppervlak bekend, kan de benodigde lengte van de voorziening direct met de grafiek warden bepaald. Om het beste rekenmodel voor de dimensionering te bepalen, zijn met een aantal verschillende modellen simulaties gemaakt en zijn de resultaten met elkaar vergele­ ken. Er is geanalyseerd wat de invloed is van het betreffende rekenmodel op de benodigde afmetingen van de voorziening. Om aan de veilige kant te blijven war­ den ontwerpgrafieken geconstrueerd voor een percolatiesleuf waarvan de bodem is dichtgeslibd. Bovendien wordt aanbevolen om met inloop gegevens te rekenen en de afvoer als een dynamische variabele te beschouwen. Als wordt gerekend V The Desagn ot lnhltrauon and Percolation Facohtael> met een continue reeks kan het voorkomen dat de bergmg van een voorzienmg nog niet geheel leeg as op het moment dat de volgende bui begint.
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