Management of Water Quality Issues in Urban Stormwater Ponds

Transcription

1 Management of Water Quality Issues in Urban Stormwater Ponds Ryan Xu and Ricky Kwan Storm Consulting October 2018 Bringing engineering to life

2 Presentation Outline 1.0 Background 2.0 Project Description 3.0 Problem Analysis 4.0 Proposed Solutions

3 Background

4 Urban Lakes Drivers Provide Amenity Balance Cut and Fill Aesthetics Expectations Passive Recreation Aquatic Habitat Sightly No Odour Issues Algal Blooms Extensive Aquatic Weed Coverage

5 Urban Lake Health Floating Plants High nutrients High Shading Provide decomposition of organic carbon that can deplete oxygen within water column Macrophyte dominated Low turbidity Low nutrients Low algal counts Phytoplankton dominated High turbidity High nutrients Macrophytes eliminated Cyanobacteria dominated High turbidity High nutrients Cyanobacteria frequently bloom

6 Macrophyte Phytoplankton - Cyanobacteria Healthy Macrophyte dominated Phytoplankton dominated Cyanobacteria dominated Unhealthy Conceptual model illustrating the three possible equilibrium states in shallow lakes (Health Waterways Limited, 2012)

7 Dynamics of Suspended Solids Sedimentation and Resuspension Macrophyte Dominated Lakes Sedimentation Phytoplankton Dominated Lakes Resuspension Inappropriate conditions in the sediments for germination and root anchorage

8 Project Description

9 Project Description Two connected ponds with total footprint of over 8,000m 2 Shallow lakes depth less than 3m Stormwater runoff from urban catchments 85Ha

10 Project Description Phytoplankton dominated High turbidity High algal counts

11 Project Description Existing Development Limited footprint for proposed structural treatment measures Short timeframe to improve the situation

12 Project Description Design for the Development Aerator pumps and recirculation system

13 Problem Analysis

14 Approaches Nutrients Analysis Excessive Nutrients Suspended Solids Nutrients Uptake High nutrients concentration from urban stormwater runoff Both Nitrogen and Phosphorus stick to the surface of suspended solids Cyanobacteria can uptake Nitrogen from the atmosphere Waverly Oval Reduce Phosphorus and Suspended Solids

15 Approaches Retention Time Analysis High Retention Time Low Flushing Approximately 60 days (above the low risk threshold - 20 days for 80% of the time) The lakes are not connected to natural stream low regular flushing to dilute algae biomass Waverly Oval Reduce Retention Time

16 Approaches Other Factors Temperature (Peak Season) High Overwintering cell populations Waverly Oval Over 20 days Growth curves indicating times for cyanobacterial populations to reach bloom proportions based upon partially mixed conditions and starting concentration of 50 cells/ml (solid lines and 250 cells/ml (dashed line). (Health Waterways Limited 2012)

17 Solutions Short-term and Long-term

18 Solutions Short-term Solutions Short-term Solutions Algaecide Weedkiller Phoslock Flocculant Physical Removal

19 Short-term Solutions Results Results 95% of algae was removed within 4 days of the application Successfully removed most of the aquatic weeds Pre-treatment Post-treatment

20 Long-term Options Long-term Options Water Level Management Floating Wetland Recirculated Bio-retention System

21 Long-term Option Water Level Management Reduce the permanent water level Reduce the water retention time

22 Long-term Option Floating Wetland Floating Wetland Flexibility in applying to the targeted area Floating Wetland in Central Victoria ( 2018)

23 Long-term Option- Recirculation System Design Criteria 20 days retention time for 80% of the time Reduce the nutrients Sizing the Bioretention Basin and Pump System MUSIC modelling Adopt filter media with high hydraulic conductivity rates (Organic Filter) Storm s Water Quality Model to Simulate the Recirculation Process Handles recirculated system

24 Long-term Option Recirculation System Recirculated Bioretention System Adopt rainfall data and treatment effectiveness from MUSIC model Inflow and direct rainfall to Lakes Evaporation outflow from lakes Outflow from Filter Media Outflow from Filter Media Infiltration (no infiltration as the base is lined)

25 Long-term Option Recirculation System MUSIC Model Flux Files Storm s Water Quality Model Recirculate Remove Nutrients at each time step Background Concentration Results Calculate the nutrient concentration Residual nutrients Water volume

26 Costing Option Items Ball park estimates Bio-retention Preliminaries/ preparation Bioretention filter Pumps Sub-total Contingencies Total $ 381, $ 169, $ 37, $ 588, $ 117, $ 706, High infiltration bio-retention Preliminaries Bio-retention filter Pumps Sub-total Contingencies Total $ 221, $ 121, $ 19, $ 362, $ 72, $ 434, Floating Wetland Floating wetland supply installation with 12 months maintenance Contingencies Total $ 385, $ 77, $ 462, Water level management Not evaluated pending discussions with Council

27 Proposed Solution

28 Proposed Solution Short-term Solution Implement short-term water management strategies whenever necessary (hot summer months) Algaecide Weedkiller Phoslock Flocculent Physical Removal Intermediate Solution Regular maintenance of GPT units and removal of sediment Undertake regular water quality monitoring of ponds, including TP, TN, SS, ph, temperature, BOD and sediment deposition. Long-term Solution Depending on the outcome of the proposed short-term solutions, the long-term solution may or may not be required. Potential long-term solutions include recirculated bio-retention system and floating wetland

29 Effectiveness of Short-term Solutions

30 Acknowledgements Inputs from the following parties are gratefully acknowledged: Frasers Property for their assistance during the project Hawkesbury Council for harvesting and disposal of the weeds Aquatic Technologies for supply and application of the chemicals Storm and Craig & Rhodes design team for their contribution to the project

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